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FS8601-SY 860515DENTON CREEK FLOODPLAIN STUDY VOLUME I of II Prepared for the City of Lou Duggan, Mayor Frank A. Proctor, Jr., City Manager Dallas & Denton Counties, Texas by Anderson Engineers, Inc. Dallas, Texas Copyright 1986 13740 Midway Road · Suite 608 · Dallas Texas 75244 · 214-960-9977 VOLUME ONE TABLE OF CONTENTS SECTION PAGE INTRODUCTION .............. I 1 STUDY OBJECTIVES & SCOPE .................... 1-1 1.1.1 REPORT ORGANIZATION ............... 1-3 1.2 STUDY APPROACH & METHODS .................... 1-4 1.3 STUDY AREA .................................. 1-6 1.3.1 1.3.2 1.3.3 1.3.4 REGIONAL SETTING .................. 1-6 CLIMATOLOGY ....................... 1-7 LOCAL CONCERNS .................... 1-8 PROJECTS UNDER DEVELOPMENT ........ 1-9 DENTON CREEK FLOODPLAIN ............... -- ... . 2-1 FLOODPLAIN HYDROLOGY ........................ 2-1 2.1,1 2.1.2 2.1,3 2,1.4 HISTORICAL STREAMFLOW ............. 2-1 REGIONAL REGRESSION EQUATIONS ..... 2-7 SYNTHETIC HYDROGRAPHS ............. 2-7 DESIGN DISCHARGES ................. 2-8 2.2 GROUNDWATER OCCURRENCE AND MOVEMENT ......... 2-1~ FLOODPLAIN HYDRAULICS ....................... 2-11 2.3.1 2.3.2 2.3.3 EVALUATION TECHNIQUES ............ 2-11 EXISTING CHANNEL HYDRAULICS ...... 2-12 POST-DEVELOPMENT CHANNEL HYDRAULICS ............... 2-2~ 2.4 PARKS OF COPPELL ............................ 2-24 SECTION VOLUME ONE TABLE OF CONTENTS (contfnued) PAGE EVALUATION OF DENTON CREEK ENVIRONS ........... 3-1 INTRODUCTION ................................. 5-1 3.2 DESCRIPTION OF THE EXISTING ENVIRONMENT ...... 3-2 5.2.1 5.2.2 5.2.3 5.2.~ 5.2.5 5.2.6 SOILS ............................. 5-2 TERRESTRIAL ECOLOGY ............... 5-5 AQUATIC ECOLOGY ................... 5-5 SCENIC, AESTHETIC & RECREATIONAL CHARACTERISTICS ................... 5-6 CULTURAL RESOURCES ................ 5-7 ENVIRONMENTALLY SENSITIVE AREAS .,. 5-8 5.5 ENVIRONMENTAL EFFECTS OF PROJECT DEVELOPMENT IN FLOODPLAINS ............................... 3.3.1 3.3.2 CHANNELIZATION PROJECTS ........... LAND CREATION PROJECTS ............ ENVIRONMENTAL EFFECTS IN DENTON CREEK PROJECT STUDY AREA ........................... 3-11 STUDY FZNDINGS .............................. 4-1 GENERAL CONCLUSIONS .......................... q-1 RECOF~IENDATIONS .............................. q-q q.2.1 GENERAL RECOMMENDATIONS ........... ~-q ~.2,2 STUDY AREA RECOF~IENDATIONS ........ ~-5 REFERENCES CZTED ............................. 5-1 11 VOLUME ONE TABLES TABLE 1.2-1 1.5-1 PAGE PROJECTS UNDER DEVELOPMENT ..................... 1-5 INCHES OF RAINFALL DALLAS COUNTY ............... 1-7 2.1-1 2.5-1 DENTON CREEK DZSCHARGES ........................ 2-6 DENTON CREEK WATER SURFACE PROF[LES ............ 2-25 4.1-1 GENERAL FINDINGS & CONCLUSZONS ............ 4-2 & 4-$ FIGURES FIGURE 1-1 PAGE STUDY AREA ..................................... 1-2 2-1 2-2 2-5 2-4 2-5 2-6 2-7 2-8 2-9 2-1~ WATERSHED MAP .................................. 2-2 ANNUAL STREAMFLOW - DENTON CREEK ............... 2-3 ANNUAL STREAMFLOW - ELM FORK ................... 2-4 SEASONAL STREAMFLOW - DENTON CREEK - 1966 ...... 2-5 DENTON CREEK - STUDY AREA WORKMAP .............. 2-13 DENTON CREEK - STATIONS 18~+~ TO 268+$~ ....... 2-14 DENTON CREEK -STAT[ONS 28~+8~ TO 5~7+7~ ....... 2-15 DENTON CREEK - STATIONS 3~5+7~ TO 3~+70 ....... 2-16 DENTON CREEK - STATIONS 5~2+6~ TO 369+7~ ....... 2-17 DENTON CREEK - STAT[ON $42+6~ .................. 2-18 VOLUME T~/O TABLE OF CONTENTS APPENDZX TIME SERIES ANALYSES ................................... 1 HEC-1 COMPUTER MODEL ................................... 2 EXISTING CONDITIONS COMPUTER MODELS .................... POST-CONSTRUCTION COMPUTER MODELS ...................... WATER SURFACE PROFILES ................................. 5 CHANNEL CROSS-SECTIONS ................................. 6 FLOODPLAIN MAPS ........................................ 7 LAKE GRAPEVINE DISCHARGE DATA .......................... ENVIRONMENTAL DATA ..................................... 9 INTRODUCTION 1.~ INTRODUCTION 1.1 STUDY OBJECTIVES & SCOPE The City of Coppell, Texas, desires to manage the rapid development occurring and proposed in the floodplains of the major creeks in the City. To prepare o guideline for management of further development anticipated in these floodplains, the City authorized Anderson Engineers, Inc., a water resources consulting firm of Dallas, to complete a study of a segment of Denton Creek. This is a report on the Phase I and Phase II investigations of the three-port study of Denton Creek authorized by the City of Coppell, December 9, 1985, in the study area shown In Figure 1-1, generaliy described as extending from the confluence of Denton Creek with the Elm Fork, upstream to Denton Top Road in the Denton Creek watershed of the Eim Fork of the Trinity River in the Trinity River Basin, DalIas and Denton Counties, Texas. Phase I is a floodplain study to update the effective flood insurance study of Denton Creek to include projects currently under construction or planned for construction in the study reach. Phase II is an environmentai review of the Denton Creek floodplain ecology within the study reach to ldentlfy sensitive areas which may be affected by the rapid development occurring in this segment of Denton Creek. The third part of the authorized study Is a document Of proposed guldeIlnes for the City of CoppeIi's management of development within designated floodplain lands in the City. Other than deflnlng work tasks os described in thls introductory material, the Phase I and Phase I! designations are not otherwise referenced in this report. The Flood Insurance Study (FIS) prepared by the Fort Worth District US Army Corps of Engineers was prepared to delineate flood hazard areas throughout the City based on storm runoff and stream channel conditions ex/sting at the time of the fleld reconnaissance for that study. The floodplains identified in the FI$ are mapped on a single map of February 15, 198~. The purpose of Phase I, the floodplain FIS update, 1s to delineate the l~-¥ear floodplain In the study reach based on fully urbanized conditions incorporating the plans and studies available within the study reach as wei! as to provide information on the stream hydrauiics likely to be experienced as a result of development within this study reach in other recurrence Interval flood events. This floodplain study phase odditionaI1y provides the general background information needed for eIements of the environmental review of Phase II. 1-1 I.dz n,-o Z <I:~ 0 ~ Z J--. >. < Z ,-', Z 1-2 The environmental overview, Phase [l of the authorized study, has been conducted utilizing information available in the literature and from brlef slte reconnaissance vlslts to evaluate the existing environmental characteristics of the Denton Creek floodplain consideration of the current goals, guidelines, and master plans of the City of Coppell. The study has not incorporated field sampllng surveys. Emphasis has been placed on identifying soil, vegetation, wildlife, aquatic, scenic, aesthetic, recreational, and cultural characteristics which may be classified as environmentally sensitive resources. The results of these characterizations have been evaluated in consideration of plans for the Parks of Coppell development project and channeltzatlon of Denton Creek. 1.1.1 Report Organization Thls report ts organized into four major sections and a fifth reference section. Section 1.~ presents descriptions of the project scope and provides information on the study approach and methodology; provldes background descriptions of the site and regional setting Including general climatology, existing conditions and local concerns; and describes the current projects under development or having current plans under review by the City of Coppell. Section 2.~ presents the floodplain study update including s%reomflow, reglona! regression equations and synthetic hydrograph modeling, FIS discharge comparisons, groundwater occurrence and movement, and the floodplain hydraulics of various streamflow and channel conditions. Section 5.~ Illustrates the characteristics of the various environmenta! factors evaluated and presents a discussion of the effects of project development on these environmental resources. Section k.~ highlights some of the study findings and makes recommendations for future consideration. References cited In the text are listed in Section 5.~. The nine appendices to this text are included as a separate volume of detailed computer models and drawings. 1.2 STUDY APPROACH & METHODS The Investigative approach to thls study includes data gathering consisting of Ilterature searches, ilmtted fleid reconnaissance including meetings with long-time residents, reviews of plans and studies, acquisition of historical data where available, and compilations of various computer models of the Denton Creek study oreo hydrology and hydraulics. The final presentations of this study ore based on detailed assessments of the data thus obtained. The methods used in thls investigation are generally considered standard proctlce for studies of this type. The hydrology is analyzed using several tools available for comparison: first, using a computer model for statistical evaluations of historical data (HEC, 1982o); second, by making comparisons with regional regression equations (USGS, 1977); and, third, through the development of an HEC-1 computer modei of the watershed to generate synthetic hydrographs based on techniques of the Soil Conservation Service (HEC, 1985) (SCS, 196~). Hydraulics are evaluated using the backwater computer models HEC-2 and LRD1 developed by the Corps of Engineers (HEC, 19B2b) (LRD, 1985). Hydrology models ore included as Appendices 1 and 2 for historical data and synthetic data respectively; hydraulics models ore included os Appendices 3 and ~ for pre-development and post- development conditions respectively. Land use zoning maps for the cities of Coppell, Grapevine, Flower Mound, and Lewisvllle, and the master plan land use map of Carrollton were used in the HEC-1 synthetic hydrograph modeling to provide data on a fully developed watershed. Zoning Indicating agricultural land use was assumed to be rezoned to the nearest adjacent non-agricultural land use. Land use mopping is included in Appendix 2. No field surveys were performed for the hydraulic modeling. Hydraulic models ore based on a combination of the effective FIS, computer models, and topographic plans submitted by consulting firms for projects in the study reach as shown in Table 1.2-t. Among the agencies contacted as a part of the literature search for the envlronmentol overvlew sectlon of this study were the following: Dallas Hlstoricoi Commission, Dallas Dallas Museum of Natural Hlstory, Dallas EPA Region VI, Dollos Texas Archaeological Research Lob, Austin Texas Porks & wildllfe Deportment, Austin Texas Water Commission, Austin Trinity River Authority, Arlington US Army Corps of Engineers, Ft. Worth US Fish & Wildlife Service, Ft. Worth US Geological Survey, Austin & Dallas Written responses from the agencies contacted ore included in Appendix 9 of thls study. Several agencies responded by telephone and by sending materials which proved very useful in the preparation of this report. These and oddtttono! related references ore lncZuded in Sect/on 5.~, "References C/ted." TABLE 1.2-1 PROJECTS UNDER DEVELOPMENT ZN DENTON CREEK FLOODPLAZN FZRM (1) PROJECT NA~E MODEL TYPE (DATE) Donnenboum Denton Co. Levee Imp. HEC-2 (1/85) & District No. I (4/86) Ltncoln/Coppe/1 HEC-2 (1/86) Keas Construction Tract LRD (7/85) Porks of Coppel! Plans (1~/85) City of Coppel! Aerio! Topo (85) J.R. Davis N.D. Maier (2) ThreodgiJl-Dowdy Glnn 1-5 1.3 STUDY AREA The study area, as noted earlier, is generally that area currently mopped as Denton Creek floodplain extending from its confluence with the Elm Fork of the Trinity River upstream to Denton Tap Road. The site, as shown in Figure 1-1, is in the City of Coppell located to the north of Sandy Lake Road, to the south and west of Interstate Highway 35E, to the east of Denton Top Road, and includes the Denton Creek channel and right overbank. "Left" and "right" designations, following general hydrology practice, are os facing downstream. This study includes detailed investigations within the 52,2~± ft (6.1 mi) stream segment of Denton Creek from the Elm Fork to Denton Tap Road and additionally includes hydrologic and hydraulic modeling upstream of Denton Tap Road for approximately 24,7~0 ft (4.7 mi), a total study reach of approximately 1~.8 mi. Cottonwood Branch Joins Denton Creek at the upper limits of the study reach downstream of Denton Tap Road. Elevations in the study reach range from 445 ft National Geodetic Vertical Datum (NGVD) near Sandy Lake Road to 466 ft NGVD near Denton Tap Road with a high point of 475 ft NGVD along DeForest Road. 1.3.1 Regiona! Setting This general location in north-central Texas is characterized as being in o transition zone between the major vegetational areas of the Cross-Timbers area to the west and the Blacklond Prairies area to the east having, in generai, a temperate climatology with extremes in variation. The Trinity River Basin is principally in two geographic provinces, Central Texas and Gulf Coastal Plain, with portions of the headwaters extending into the Central Lowland province. As noted by the Texas Water Commission (1965) the headwaters are in two of the principal subdivisions, the Grand Prairie and Osage Plains regions. With a drainage pattern generally in a southeasterly direction, Dallas County is in the upper Gulf Coastal Plain principal physiographic province while Denton County is in both the Central Lowland and the Gulf Coastal Plain physiographic provinces. The Elm Fork of the Trinity River, with streamflow records from 19~7 to the present at the CarroIiton gauge on Sandy Lake Road, has an average annual runoff of approximately 4.~ in (USGS, 1985). Denton Creek, with streomflow records from 1g~7 to the present at the Grapevine gauge on State Highway 121, has on overage onnuai runoff of approximately ~.~ in (USGS, 1985). Both streams are regulated: Elm Fork by both Lake Lewlsville, and Lake Grapevine and Denton Creek by Lake Grapevine. Lake Grapevine is upstream of the study area on Denton Creek in Tarrant and Denton Counties. At the Grapevine gauge, minimum flows ore maintained, as shown in Appendix 8, by releases from the lake at 10 cfs in the May through September season and at 5 cfs in the September through April season (Corps of Engineers, 1975). 1-6 1.3.2 Climatology The climatology of this region of Texas includes large variations in rainfall and runoff which characterize the usual hydrologic conditions in the Trinity River Basin. The mean annual rainfall varies considerably fram year to year, ranging from less than 2~ to mare than 5~ inches. A large portion of the annual rainfall results fram thunderstorm activity, characterized by heavy precipitation occurring in brief periods of time. The United States Natlonal Oceanic and Atmospheric Administration, "Rainfall Frequency Atlas of the United States," Indicates o l~-yr 24-hr storm in northwestern Dallas County of approximately 9.5 inches and a probable maximum 6-hr precipitation (PMP) of 5~.8 inches (Hershfleld, 1961). Snowfall and snowmelt also vary greatly from year to year, but, with an average annual seasonal snowfall of 5.1 inches, is not considered a significant source of runoff. Other expected rainfall events for various durations and mean return intervals ore given in Table 1.5-1. Temperatures range from extreme highs in the lffi~'s in the summer months to extreme lows of near zero in mid-winter. TABLE 1.3-1 INCHES OF RAINFALL DALLAS COUNTY, TEXAS FOR DURATIONS OF SS MIN TO 1~ DAYS AND RETURN PERIODS FROf*I I TO 1~ YEARS Duration Return Period {yrs) (hfs) I 2 5 1~ 25 5~ 1~ 12 48 96 168 2~-e ~.5 I .2 1 1.6 2 1.8 3 2.l~ 6 2.4 2.8 5.2 1.5 2.~ 2.3 2.7 3.1 3.4 1.9 2.5 2.9 5.4 5.8 4.5 2.2 5.~ 3.6 4.2 4.7 5.2 2.5 3.3 3.9 4.6 5.2 5.7 3.~ 4.~ 4.7 5.5 6.2 7.~ 5.5 4.7 5.5 6.5 7.~ 8.4 q.~ 5.4 6.5 7.6 8.5 9.5 ~.6 6.~ 7.2 8.5 9.7 11.~ 5.4 7.~ 8.2 9.8 11.2 12.7 6.2 8.1 9.5 11.2 12.7 14.1 6.9 9.~ 1~.6 12.5 14.~ 15.7 99.9~ 5~ 2e~ le~ ~ 2~ Chance of occurrence in any given year SOURCE: Hershfteld (1961) and Miller (1964). 1-7 1.5.$ Loca~ Concerns At the request of the Hayor of the City of Coppei1, three long-time cltlzens of the area were contacted In regard to their concerns over the rapid changes taklng place In the Denton Creek study oreo. Two families were visited at the project site and a third was interviewed by telephone. A discussion of resoIutlon of the local concerns is lncluded tn the conclusions and recommendations of Sectlon 4.~. The general concern ts that the development may leave present properties in a situation of worsening flooding conditions than have been experienced historically (Warren, 1988); among the strongest concerns are the following: The development generally to the south of DeForest Rood includes fill which Is perceived [correctly] to be interrupting the continuity of natural overland drainage from the northwest to the southeast within and along the overbonk areas of Denton Creek. Temporary ditching along DeForest Road is not oilevtotlng the problem since tncrensed runoff from the development is also being concentrated, by the present grades draining to the some ditch, thereby aggravating what is seen at the site during a typical runoff event. Water now stands at an existing culvert which used to drain freely. In addition, the temporary ditch is eroding deeply at its confluence with Oenton Creek (Burns, 1988) (Mclnnish, Site drainage into and through the new Parks of Coppell city park area is being questioned. The large excavations providing the fill to bring the remainder of development up out of the floodplain are perceived os future problem areas (Burns, 1986). Site drainage through the various ponds of the Lakes of Coppell development is being questioned. Failure of some of the retaining walls is perceived as o continuing problem without an agency to make repairs once the land is fuliy developed (Burns, 1986). Impacts of the development on local groundwater occurrence and movement is being questioned. The observed removal of gravel and replacement of the naturally occurring material with less pervious and uncompocted material is presented as an example (Burns, 19B6). Other helpful comments noted In the site visit with Hr. Burns (1986) are o number of recollections of observed flooding events tn Denton Creek and the Elm Fork during his 47 years of living in the area which include: 1-8 1. Floodwater elevations to near the present MacArthur and Samuel Boulevards [approximate most recently in 1981; intersection of elevation 558], 2. Observed overtopping of the banks along Denton Creek occurs with a frequency varying from ~ to 8 years and may take from two to three weeks to return to within the banks; 3. FIoodwater eievotions to near the intersection of MacArthur Boulevard and Sandy Lake Rood [approximate elevation ~5~], most recentIy in 1981, inundating Bonhard's Nursery and a residence now serving as a temporary poilce station; 4. Floodwaters occasionally back into Denton Creek from Elm Fork; 5. Flood in late 5~'s or early 6~'s included drowning of many cattle (Mr. Mclnnish also mentioned this flood event); and 6. High water mark of 1981 flood on buliding near right bank of creek to the east of Burns house at approximately 1.5 ft above grade [approximate elevation q~9.5]. The dotes and approximate recurrence intervals of some of these observations are discussed in Section 2.~ in the presentation on streamflow records, 1.$.~ Projects Under Development Major projects under construction or planned for development in the study oreo include those previously presented in Table 1.2-1 among others, including the "Lakes of Coppell," "Parks of Coppell," "Llncoln/Coppell," the lakes and canals of the "Gateway Reclamation Project," and the "Denton County Levee Improvement District No. 1." Within each of the major projects ore the individual elements of roadways, utilities, storm sewer systems, and structures. Proposed future structures include o crossing of Denton Creek by MocArthur Boulevard. Two of the projects having o great impact on the study area of the Denton Creek floodplain are the "Levee Improvements" and "Porks of Coppe)l" projects which ore discussed in detai! in this report. Detailed evaluations of lndividuo! design elements ore beyond the scope of this report; although, where data are available, results of reviews of design elements in the study reach ore included in this discussion. 1-9 In his letter of February 26th, L.H. Hawkins~ Jr. (1986) of the Ft. Worth District Corps of Engineers, indicates that the Corps has processed two permit requests in thls oreo of Denton Creek, but that the "Levee Improvements" and "Porks of Coppell" projects as presented In the available drawings may not have been reviewed and approved. Of the two permit requests reviewed, the first is a temporary rood crossing covered under the nationwide permit provisions for bridges and the other, determined not to require a permit, for an area of channelization, bridge improvements, and levee generalIy along the left bank area of the Denton Creek channei from approximate stream stations 2B~+80 to 522+~ in the upper end of the study area. The letter and attachments from the Corps are Included in Appendix 9. The US Fish and Wildlife Service ($ohnson, 1986) has reviewed the proposed Lincoln/CoppeI1 project but ts not aware of the other projects completed or under construction tn the study area. The letter from the US Fish and WlidIife Service is oiso lncIuded in Appendix 9. Comments from both agencies are further eiaborated upon in Section $.~. The preliminary engineering report and preliminary plans for the Levee Improvement District No. 1 project hove been approved by the Texas Water Comznission (1985) subject to requirements which include remedlai action, as required, to assure no adverse impact during low fiows. A copy of the TWO approval order ls included in Appendix g. Denton County Levee Improvement District No. I The Denton County Levee Improvement District No. I project includes channelization aIong approxlmateiy 2.~ mtIes of Denton Creek from approximate stream station 18~+~ in the common floodplain of the Elm Fork of the Trinity River, upstream to approximate stream station 512+30, Just downstream of Denton Tap Road near the confluence with Cottonwood Branch. The new channel wti1, in generoi, paralleI and occoslonaIiy intersect the existing ailgnment of Denton Creek In a straightened and widened channei having either fli1 or a Ievee aiong the Ieft bank to reciaim virtuaii¥ aii of the left overbank of the existing floodplain. Levee and fill siopes ore proposed at ~:1 (H:V). AS presently planned, the flow 1/ne of the new channel will match that of the exlstlng Denton Creek flow 1/ne at the points of Intersection. The proposed channel will be trapezoidal in section without a piIot channel, having 5:1 side slopes and a bottom width which varies from 3~ ft tO 18~ ft. Improvements wlI! be mode at the Denton Top Road brldge to minimize damage from anticipated velocity increases. The majority of the project 1/es tn the Clty of Lewlsvtlle w/th much of the creek-side channel work in Coppell. 1-1~1 Parks of Coppel! The Parks of Coppel! are being formed by excavations wtthln the right overbank of the Denton Creek floodplain, providing much of the fill for reclamation of remaining floodplain areas surrounding the Parks development. The Parks site is generaily to the north of Parkway Boulevard and along the right bank of Denton Creek a distance downstream from Denton Tap Road of approximately 660~ ft (from stream station 233+7~ upstream to stream statlon 321+5~±). The excavations w111 become lakes within the Parks area, which when landscaped and combined with proposed playing fields, will become a new large streams/de park. The bank areas are generally belng kept in-place to provide levees whlch will lessen the frequency of inundation of the Parks by Denton Creek flooding. Gated outlet structures wlll provide inter/or dralnage from the park lakes into Denton Creek. Cottonwood Branch flows through o port/on of the western project limits and when site work ts completed, will complete the low-levee protection of the Parks from Denton Creek flooding with its high right bank. Floodlng in Cottonwood Branch was not studied in this investigation, ~acArthur Boulevard Br~d~e The design of the crossing of Denton Creek by ~acArthur Boulevard has not yet been prepared for review. It is assumed, however, that the crossing will be near approximate stream station 255+5~ since this will be a narrow segment of the new channel and floodway which will minimize the required bridge size. 1-11 DENTON CREEK FLOODPLAIN 2.~ DENTON CREEK FLOODPLAZN 2.1 FLOODPLAZN HYDROLOGY Denton Creek, at the study oreo confluence with the Elm Fork of the Trinity River, has a contributing watershed of approximately 725 sq mi extending west-northwest of the site to lnciude Lake Grapevine, whlch itself has a watershed subarea of approximately 695 sq mi (Dowell, 196~). The 28! sq mi of contributing suboreas downstream of Lake Grapevine are generaliy fram the Baker Branch and Cottonwood Branch tributaries of Denton Creek os shown in the watershed mop Figure 2-1. The most accurate method of determining runoff peaks and frequencies is to review o long historical record of measured flows. Rarely is there cny such record; however, ~ust upstream at State Highway 121 end Denton Creek, records ore ovolioble for USGS gaging station ~8~55~ near Grapevine, Texas, from 1947 to the present, for 7~5 sq mi of the watershed, including the regulated flows from Lake Grapevine. Downstream of the study site, on the Elm Fork of the Trinity River at Sandy Lake Rood, records are available for USGS gaging station ~80555~ near Carrollton, Texas, from 19~7 to the present, including the regulated fiows of Lewlsville Lake and Lake Grapevine. To determine discharges likeiy to occur in Denton Creek at the study oreo, three approaches were utilized: first, o review was mode of historical events; second, predictions from regional regression equations were made; and flnolly, a synthetic hydrogroph model of the watershed was mode. Comparison of results ore shown in Table 2.1-1. 2.1.1 Historical Streomflo~ The historical discharges were obtolned from USGS Water Resources Data Reports for the period of record at the Grapevine gaging station, ct the Corroliton gaging station for the same time period, end time- series analyses mode of the peak discharges on both Denton Creek and Elm Fork. The Elm Fork gauge readings on the same dates os those of the Denton Creek peaks were used to help in establishing simultaneous discharges and starting water surface elevations for the hydraulic models. The Grapevine d0to were adjusted to account for the downstream location using the equations proposed by Creoger, Oustln and Hines (1972). Annual maximum, minimum, and average discharges at the Grapevine and Corrollton gaging stations ore shown in Figures 2-2 and 2-3, while seasonal variations of flow in Denton Creek ore represented by the data of water year 1966, as shown in Figure Appendix 1 includes the flood flow frequency analysis computer program data for both locations. There are severoI recorded no-flow days. 2-1 OENTDN TAP R9 / % % 2-2 Z W X 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O0 ~0 ~ ~ (s;o) 3C)21'VHS)SIO 69 ~ ~9 /9 - 99 g9 ~ O~ Lg 9g gg ~g OC~ 'q 0 0 0 0 0 0 0 0 0 0 0 0 %--- -:IC)~IVHOSIC] - O~ - 6L - 8L - L~ - 9L - - gl o; [@ O~ C~ c L- (s,~0) 30~IYHOSIC] I×t I I ' ×1 I ×1 I I I I I I 0 bJ Z 0 U 0 TABLE 2.1-1 DENTON CREEK DZSCHARGES (cfs) NEAR STREAM STATZON 184+~6 CONFLUENCE WZTN ELM FORK Ifil[AN RECURRENCE ZNTERVAL DATA SOURCE Historicel(1) Regression HEC-1 F/S 1 58 - 2 1676 2166 7725 5 4766 4325 11768 16 8120 6215 15~68 25 14286 8885 56 2~476 11~95 21517 1~ 28~5~ 1349~ 25772 5~(2) 362~ 362~0 362~ 128~0 182~ 2~6~ 362~ NOTES: (1) At o 95~ confidence level of being equaled 0¢ exceeded. (2) 5B~-yr discharge ts do~lnoted by disch0rges fr~ Lake 6rapevine. Mean recurrence intervals, tt should be pointed out, while usefu! in conceptualizing the expected frequency of recurrence of o given magnitude event, ore not necessarily representative of any actual event. Much os o room full of people mny provide on overage height for the group of 5 ft 9 in, no one in the room has to be 5 toll for the statement to be accurate. Thus. e discharge may be shown OS having o 2-yr mean recurrence interval, even though o discharge of this particular magnitude may never occur. The data indicate that Denton Creek has overflowed its natural banks at least eight times In the 36 years of record, most recently in water year 1982 (November 1, 1981) with o discharge at the Grapevine gauge o¢ 97~ cfs, o predicted mean recurrence interval of between 25- and 33-yrs. Dates indicating overtopping include water years 1948. 1982, 195~, 1949, 1964, 1961, 1958, and 1957, listed in order of decreasing discharge. Exceedonce Intervals for these events ore estimated to be 2-6 12~-, 25-, 15-, 15-, 5-, ~-, ~-, and 5-yr events, respectively. The observations of residents (Burns, 1986) (Hclnnlsh, 1986) confirm that the discharges and associated floodwater elevations predicted by the statistical analyses of Appendix 1 and by the existing conditions backwater model of Appendlx 3 and are within o range of acceptable occurocy. 2.1.2 Regional Regression Equations The regional regression equations of Schroeder and Massey (1977) were established for rural sites on streams in Texas where unregulated flow conditions prevail. Much of the streamflow downstream of Lake Grapevine has been from largely rural unregulated areas, although, the watershed is urbanizing rapidly. Further, as a result of the attenuation by Lake Grapevine, the discharges at the study area con be expected to be dominated by runoff from the subareos downstream of Lake Grapevine for all but very rare events (Estep, 1985). Therefore, the use of regression equations should be helpful in the evaluation of expected discharges when compared to historical events. Runoff from the subwotershed downstream of Lake Grapevine is largely controlled by the Gottonwood Branch subarea. Using the regression formulas of Schroeder and Mossey (1977) for flood-frequency region 2 (upper Trinity River Basin}, the average watershed slope of Cottonwood Branch of 18 fi/mi, and total subarea of 28 sq mi, the resulting discharges ore estimated with standard error estimates of ~ to 55 percent as shown in Table 2.1-1. Within the error limits, the regression equations are felt to closely predict runoff from the undeveloped watershed, particularly for the more frequent (2-yr throu9h l~-yr) events. 2.1.$ Synthetic Hydrographs Analyses of streomflow data ore complicated by the fact that watershed conditions ore seldom constant during the period of record. Changing land use, channel modifications, reservoir construction, and land treatment all contribute to changes in the hydrologic responses of a watershed. Predictions of future discharges are, therefore, entangled in the interpretations of historical changes, the prediction of future changes, and attempts to draw conclusions from the past variability of nature. The SCS-based methodology of generating synthetic hydrogrophs iS frequently used for estimating future events based on representative characteristics of the watershed and the resulting rainfall/runoff relationships. For ease of application, the SCS methodology utilized in this study has been formulated into an HEC-1 generalized computer model, "Flood Hydrogroph Package," (HEC, 1981). The full model is presented in Appendix 1 for the l~-yr, and the 2-, 5-, lffi-, and 5~-yr 2~-hr precipitation events distributed as SCS Type 2-7 ZI storms with and without areal adjustment of point rainfall. In Table 2.1-1, the discharge values shown include areal adjustment in the frequent (2- through 5~-yr) events and no areal adjustment in the l~-yr event. Stream routing is by normal depth modified Puls routing assuming a channelized stream similar to that being developed in the study area. The subwatershed downstream of Lake Grapevine has been further divided into seven subareas as shown in Figure 2-1. Soils of the subwotershed are shown in the Dallas, Denton and Tarrant County soils surveys (USDA 198~a, 1980b, 1981) to be principaily in hydroiogic group D with decreasing areas of land in groups B, C, and A; meaning, in generai, that high to moderate rates of runoff can be expected. Land use of the fully developed watershed will consist of extensive areas in light industrial, commercial, and multi-famiiy properties, as presentIy zoned, with smaller areas of parks and slngle family residentloi land use. These characteristics combine to indicate SCS Curve Numbers ranging from 82 to 92 in the suboreos. Log times, the t/me from the center of moss of the rainfall excess and the resultant peak of the SCS hydrograph, range from ~.6 to 2.1 hours for the suboreas. The resulting predictions of discharges from a fully developed watershed range from higher than hlstoricoi events to approximately equal to the effective FIS discharges for the various frequency events evaluated, as can be seen in Table 2.1-1. 2.1.~ Deslgn Discharges The variation in the tabulated discharges shown in Table 2.1-1 serves more to provide on indication of the results of urbanization than to note the accuracies of any given methodology. A comparison of the results further points to the difficulty of making such predictions, As noted earlier, the historical data ore the most useful source of information for a relatively static watershed. With a data base of some 56 years, the most accurate predictions will be in the 2- to 25- yr frequency events since the curve-fit to available data will not hove to be extrapolated; thus, predictions of discharges in short return interval events are likely to be relatively accurate while the less frequent (long return interval) event predictions will likely overestimate the actual discharge. The regiona! regression equations simply lndicate relative agreement wlth the historical data. Since they are based on regional data which ore not site-specific, the regional equations of Schroeder and Massey (1977) should be used in the Denton Creek study area with caution, n0tlng the original limitations of rural and unregulated watersheds. Finally, the HEC-1 model is on exompie of a hypothetical watershed clearly revealing the results of urbanization of the watershed, with a notably more pronounced effect on discharges in the frequent flood events than on discharges in the rare events. Rossmlller (198~) 2-8 indicates that increases in runoff from an urbanized watershed over that from the natural watershed of 6~e% in o 2-yr event end of 2e~ in a 1~-¥r event ore not unusual. Hampton (1975) shows that runoff from an actual l~-yr order-of-magnitude event in on urbanized 29.4 sq-mi portion of the White Rock Creek watershed in Collin and Dallas Counties is approximately 129~ csm. For comparison, this some event in the Denton Creek subarea downstream of Lake Grapevine would produce o flood in excess of 35,~ cfs. It is concluded, therefore, that the design discharges to be assumed for design of structures in the Denton Creek study area be based on the design life of the structure. If the development is to be a "permanent" improvement or may impact other permanent improvements, as will the flood control channel proposed in the study area~ discharges shall be from the fully developed watershed using the HEC-1 model or similar synthetic method. If a facility is to be built with o shorter design life of, soy 2~-yrs~ more or less, and will not otherwise affect permanent improvements, the design discharge may be based on the adjusted historical data of the Grapevine gouge with the knowledge that when the structure is replaced, new discharge determinations will be required. The 5~-yr event should remain at the 36,200 cfs of the effective FIS. The present bose-flood (l~0-yr mean recurrence interval event) discharges of the effective FIS ore felt to be too low to use for design of permanent structures. 2-9 2.2 GROUNDWATER OCCURRENCE AND [~:)VEMENT Groundwater in the Denton Creek study oreo and at depths of interest generally occurs In the Woodbine Group of the Gulf Series of Cretaceous age aquifers in north-central Texas (Know[es, 1986). The Woodbine Group outcrop is characterized by low, rounded, wooded htlIs along the western margin of the Eastern Cross Timbers physiographic subdivision of Texas, and by gentle slopes along the eastern mergln. The soil is reddish sand with iron concretions end some cloy. The surface supports o dense growth of timber, consisting chiefly of post oak and biockJock Nordstrom (1982) reports in his reglonoI study that the Woodbine Group ts divided into three water-bearing ports: the upper, middle, and lower, copabIe of yielding smoli to large amounts of water. The upper part is of concern in this study. In generel, groundwater tn the upper port is of extremely poor quoilty for domestic use since It is very high In iron content. Total thickness of the Woodbine Group is approximately 25~ ft near the outcrop. The average specific capacity calculated from production tests of wells in the Woodbine Group was 2.9 gpm/fi; transmisstbllity values overage 47~ gpd/ft; and permeability values average 44 gpd/ft and higher, ranging from 84 to 167 gpd/ft In the more permeable sands of the outcrop (Nordstrom). Water measurements in a well near the Denton Creek USGS gaging station ~8~55~ on SH 121 hove been reported by Nordstrom. Other wells exist near the project study oreo, but ali hove been finished in the deeper Twin Mountains or Poluxy formations or have no recorded measurements. The primary source of the groundwater is rainfall on the outcrop area In addition to seepage from oreo reservoirs and from the Trinity River tributaries (Nordstrom). Groundwater occurs in water-table and artesian conditions, with water-table conditions prevailing In the outcrop near the study oreo. The overage rote of movement of groundwater Is estimated by Baker (196~) to be approximately 15 ft per year downdip towards the east-southeast. Natural discharge from the Woodbine occurs through springs, seeps and evopotronsplrotion. Artificial discharge was estimated In 1976 to be about 2~,5~0 ac-fi from wells. The water levels tn the outcrop fluctuate seasonally, but with measured levels at the USGS well of from 24 to 38 ft below the surface, do not seem to be declining os are levels in the lower port. It can be concluded that the work In the Parks and Lakes of Coppell ls unlikely to have on appreciable Impact on the local groundwater levels. The lakes will help to boionce recharge lost by Increased lmpermeobie land cover. The only oreo of concern may be local lowering of the groundwater levels adjacent to the Creek if the channelizatton to full tholweg depth Is permitted. This increased bank-seep loss can be avoided by maintaining the low-flow discharge profile os recommended in Section 4 of this study. 2.~ FLOODPLAZN HYDRAULZCS 2.$.1 Evaluation Techniques The study segment extends, in general, from the confluence with the Elm Fork upstream to above Denton Tap Road. More specifically, the Elm Fork floodplain dominates Denton Creek from Denton Creek stream station ~+~ to approximate stream station 184+~ which, in this study, is tn both the Elm Fork and Denton Creek floodplains. From stream station 18~+~ to approximate stream station $25+05, Just upstream of Denton Top Rood, detailed investigations hove been performed for this report, Beginning w/th the natural conditions modeled in the effective Denton Creek FIS and adding the further detail of the existing channel, including: right overbank fill of the KeDs Tract in the common floodplain of the Elm Fork; other on-slte fill as obtained from iapc maps and ex/sting condition models of project submittals; lake excavations; and additional cross-sections where available, a detailed existing conditions model of the stream segment in the study reach was developed using LRD (1985) and HEC (1982b) computer modeling procedures. Upstream of Elm Fork stream station 1¢5~+2~ (approximate Denton Creek stream stotlon 125+6~) and more particularly, upstream of Denton Creek stream station 1B~+4~, the Denton Creek hydraulics are felt to be representatively modeled uslng the HEC methodology exclusively with starting conditions the result of both HEC and LRD modeling at Elm Fork stream station 1~3~+20. Upstream o? the Llncoln/Coppell project west of Denton Tap Road, the model is that of the most recently available FIS model of Denton Creek. The adjusted sections showing the additional floodplain ?111, levees, and channellzatlon have been added to the detailed existing conditions model where appropriate, to develop a post-construction model. Both the ex/sting conditions and post-construct/on conditions models are run several times with various discharges to demonstrate and evaluate the impacts of projects being developed in the study oreo. The existing conditions and post-construction computer models are presented in Appendices 3 and ~ respectively. Water surface profiles at several locations along the stream and for the various discharges evaluated are presented in Appendlx 5. Cross-sections of the full channel reach are displayed in Appendix 6, Including the channellzatlon belng developed by the Denton County Levee Improvement District No. 1. These modeling techniques accomplish the following: 1. Better deflne the model and assure compatibility with the effective FIS; 2. Illustrate the input data-base and model output for use In floodway definition; and 2-11 Demonstrate the impact of development of the proposed projects on the Denton Creek hydraulics upstream and downstream of the project reach. The work mops showing the floodp)ain and floodwoys in the study area and through the full Denton Creek study segment ore displayed as Figures 2-5 through 2-1~ of this section of Volume ! and again in Appendix 7 of Volume II of this report; further, reproducible mylar drawings of these figures have been submitted to the City with this report. 2.3.2 Existing Channel Hydraulics In the reach evaluated, Denton Creek is currently part of an effective flood insurance study. The existing channel has a deep and well defined, sinuous channel having slopes both covered with trees and brush and with exposed raw earth banks. The channel itself generally appears to be natural in the study reach with meanders and deep pools. The thalweg slope is very flat, no riffle areas were observed. The top of bank areas are tree-lined immediately adjacent to the channel. Virtually all the overbonk areas are disturbed either by previous agricultural activities or by the present development activities. Plonnin~'$ "n" Values The effective FIS uses Monning's "n" values in the Denton Creek study reach from ~+~ to 125+6~ (approximate Elm Fork station 1~$~+2~) of ~.~5 in the channel and ranging from ~.~6 to ~.~85 in the overbanks. Upstream to Denton Tap Rood, these values are generally modeled as ~.~5 in the channel and ~.~55 in the overbonks, with Isolated overbonk areas ranging from ~.~5 to ~.~6. Chow (1959) indicates that for major streams with wide floodplains at flood stage, natural streams of irregular and rough section may have Monning's "n" values ranging from ~.~35 to ~.1; therefore, the roughness coefficients of the effective FIS were not changed in the existing conditions model. Structures The only structure crossing Denton Creek in existing conditions within the study reach is the bridge at Denton Top Rood in the 5~ ft reach from approximate stream stations 322+~0 to ~22+5~. The stream stationing at the bridge should include the following station equation: Station 522+5~(F]S) Downstreom = 522+50(Actuol) Upstream 2-12 At stream station ~69+7~ the channel distance (XLCH) has been corrected in this report to be 2~2~ ft from stream station ~9+5~ to avoid stationing conflicts with the effective FIS. The maximum existing bose-flood velocity through the Denton Tap Rood bridge is 7.7 fps. Velocities through the bridge in other events range from 1.2 fps to 8.6 fps tn the historical 1-yr to l~-yr event discharges respectively, The bridge will not overtop in the FIS nor in the historical l~-yr event, Velocities Existing conditions bose-flood velocities wlthtn the study reach range from a low of 2.~ fps at stream station 238+~ to a high of 7.5 fps ct stream station 288+7~, with velocities of 5.9 and 7.7 at the general detail study limits of the common Elm Fork floodplain and downstream side of Denton Top Road respectively. Velocities at specific cross- section locations of interest in existing conditions ore tabulated in the summary printout tables of Appendix 5 under the headings of SECNO (SECtion NO.) and VCH (Velocity in the CHannel). Yate~ Surface Profiles the existing conditions channel ore tabulated in Appendix $ (CNSEL for Computed Noter Surface ELevation) and ore displayed In Appendix 5 of Volume II of this report. In general, the profiles lndlcote that the bridge at Denton Top Road does not overtop, as noted earlier, while the Parks of Coppell oreo will experience overtopping from flooding in once every 1~ years. Encroachments Flooch.oy Determinations and Hazards The floodway In the effective FIS from stream station 18~+~ to Denton Top Rood at stream statlon ~22+~ ranges from approximately 21Q~ ft wlde to $6~ ft wide with the most narrow point being at stream stotlon 28B+7~ at 1~5~ ft wide. The flood hazard factor (FHF) in this reach ts ~15, meaning that the rounded 0veroge difference in flood elevation between the 1Q-yr event and the 1QD-yr event can be expected to be 1.5 fi, designated os o flood Zone A~. Upstream of Denton Tap Rood the floodway generally narrows upstream of the influence of the bridge to less than 1~0 fi; the FHF remains at A3 for the several thousand feet evaluated upstream of the bridge. 2-19 Peet-Development Channel Hydraulice In the reach evaluated, Denton Creek ts port of the Denton County Levee Improvement District No. I (DCLD) which ls currently under construction. The project is to consist of channeltzotion of the stream along approximately 2.~ miles of Denton Creek, os noted earlier, from approximate stream station 18~+~ to epprox/mate stream station $12+5~ downstream of Denton Top Rood and ts to include channel protection through the Denton Top Rood bridge. The new channel wll! generally parallel the ex/sting stream along the left bank tn o straightened alignment, intersecting the existing channel meanders at several locations, replacing the full extstlng stream segment from etreom etotton lg3+~ to stream stotlon 223+7~ and relocating and replactng the extsttng channel from approximate stream stations 225+70 to 255+3~. The floodplain tn the left overbank wtll be reclaimed by ftlllng ar levee protection to within o few hundred feet of the existing top of bank. No right overbonk reclamation ts proposed by the Levee Improvement District although the abandonment of stream segments and the new floodway are modeled by DCLD to effectively include additional right overbonk reclamation from approximate stream stations 18~+~ to 255+5~. This study includes on evaluation of both scenarios, with and without channel and overbank reclamation between stations 18~+~ and 255+5~. The right overbonk has been reclaimed with fill tn the floodplain frlnge areas to the current floodway ltmtts from approximate stream station 255+5~ upstream to Denton Top Road. Since the f111 was to be placed only in fringe areas, evidently no study was mode or ts presently available regarding the tmpoc%s of this right overbonk ~eclamotton. From approximate stream stations ~5+~ to 85+~, a project study by Nathan D. Molar Consulting Engineers (1985) has shown that the right-bank floodway may be revised in this stream segment along the stream segment dominated by the Elm Fork without adversely impacting upstream or downstream floodplain hydraulics. Other areas of the right overbonk ore not mopped os floodplain, although detailed topographic maps and resident observations indicate elevations which 2nclude additional right overbank flood prone areas G[ong Sandy Lake Road to the west of Denton Creek to the approximate intersection of the present ~lacArthur Boulevard. I~ann~ng's "n" Values The proposed channe! section includes both the existing stream and the new channel at several [ocations and at others only the new channel where the new chonne! has replaced or realigned the existing stream. The Honnlng's "n" values used tn the post-development Denton Creek etudy reach from ~+~e to 125+6~ (approximate Elm Fork station 1~+2~) remain os In existing conditions. Upstream to Denton Top Road, these values ore generally modeled os ~.~55 tn the channel and ~.~55 tn the 2-2~ overbanks with segments of ~.~$5 in the left overbank to represent the new and maintained levee. The lakes in the Parks of Coppell are included In the sections and ore modeled as ineffective flow areas untll overtopping of the right bank occurs; "n" values of the lakes ore osslgned os ~.~55. Based on the values Chow (1959) assigns, as noted earlier, for major streams with wide floodplains ot flood stage these values are felt to be acceptable for modeling expected flood conditions. Zt is imperative that the levees and new channel be maintained with on onnucl mowing program for these #n" values to remain applicable. In addition to the levees and new channel, o new bridge ls proposed to extend the alignment of MocArthur Boulevard northward across Denton Creek. The plans for the exact alignment of this structure ore unknown at present; however, it is expected that the most likely location will be near stream station 255+~ in order to minimize the size of the structure and its impact on the upstream floodplain. The floodway at 255+5~ with the new channel and levees in-place is approximately 225 fi; thus, the new bridge will likely be o three-span structure approximately $~ ft long (allowing for slopes). It ts possible to design this structure for minimal lmpact on the floodplain hydraulics. The exlstlng bridge at Denton Tap Road may be adversely impacted by the chonnellzotlon; therefore, scour protection is required at the brldge to prevent structural damage or fo/lure. Velocities in the 25- yr, 5~-yr and l~-yr historical events are greater than 8 fps downstream and through the structure with the lQ- and 25-yr events additionally ot erosive velocities upstream of the structure. The highest velocity of those discharges evaluated, will be 15.5 fps at the downstream face of the bridge in the 5~-yr historical event of the post-scenario without fill in the channel or right overbonk segment from stream stations 225+7~ to 255+$~. Protection must extend from approximately 5~ ft downstream of the bridge to approximately 1~ ft upstream of the brldge as currently modeled. Velocities reach range from o low of 2.5 fps at stream station 195+$~ to o high of ?.8 fps at stream station 228+~ with velocities of 7.2 fps at the downstream extremity of the detoll study limits at the common Elm Fork floodplain and 14.2 fps at the upstream detail study limits Ot Denton Tap Rood. The channel must be protected at the downstream limits of the chonnelizotton as well as that previously noted at Denton Top Rood. Additional information Is requlred to determine the extent of 2-21 protection called for at the downstream limits of channelizatton. As currently modeled, hundreds of feet of protection are required to protect against possible downstream damage and headcutting caused by velocities In excess of 8 fps in the 5-, 25-, 5~-, and l~-yr recurrence interval historic event discharges, with both the 5-yr and the l~-yr events having velocities tn excess of 11.~ fps. Bose-flood velocities are generally non-erosive w/thin the Levee District project; however, in addition to the high velocities at the downstream project i/m/ts, the historic l~-yr event discharge velocities ore greater than B-fps (o theoretical threshold eroslve velocity) at stream stations 258+~, 2~7+~, and 255+5~ in the DCLD channel model. Except for the protection through the bridge at Denton Top Rood noted earlier, velocities upstream of the DCLD improvements can be generally considered non-erosive. Veloclt/es at specific cross-sect/on locations of interest In post-development conditions ore tabulated in the summary printout tables of Appendix 4 under the headings of SECNO and VCH, /nd/coting section number and charms! veloclty respectively. ~ate~ Surface ProftZes the post-development conditions channel ore tabulated tn Appendix ~ (CWIEL) and ore d/splayed graphically in Appendix 5 of Volume I1 of this report. In general, the profiles indicate that, as In ex/sting conditions, the Denton Top Rood bridge will not overtop. The Parks of Coppell area will experience overtopping from flooding in Denton Creek in a historical mean recurrence lntervol of approximately once every 55 years. Nater surface profiles tn the post-development channel ore both higher and lower than tn existing conditions, with elevations tn the more frequent events significantly lower than in exlstlng in historical discharges of the 25-, 5~-, and l~-yr flood events through the reach from stream stations 255+$~ to 522+5~, generally the segment along the Porks of Coppell upstream to Denton Top Rood, as would occur in the channel os currently modeled by the DCLD. Encroachments~ Floodway Determinations and Hazards The floodway of the new levee dlstrlct from stream stotlon 18k+~ to Denton Top Rood at stream station $22+~ will be narrowed to a range of between 25~ to 5~ ft wide from stream stations 199+5~ to Increase briefly to approximately 85~ ft at stream stat/on 268+3~, then return to approximately 5~ ft from stream stations 2B~+~± to 512+5~± and, finally, narrow uniformly to the brldge at Denton Top Road. The flood hazard factor (FHF) tn thls reach will change to os a result of the increased efficiency of the chonnellzed stream in 2-22 the more ??equent flood events, meonlng that the rounded ave?age difference in flood elevation between the 9~-y? event and the l~-yr event in the full study ?each from the confluence with the Elm Fork floodplain to the study limits con be expected to be 2 ft, designated as a flood Zone A4. $TATZON TABLE 2.$-1 DENTON CREEK NATER SURFACE PROFZLES PRE-DEVELOPMENT vs. POST-DEVELOPMENT ELEVATZON (ft NGVD) MEAN RECURRENCE /NTERVAL - HZSTORZCAL EVENTS l~8-yr 58-yr 25-yr PRE- POST- PRE- POST- PRE- POST- 255+$~ 455.74 455.~5 455.91 455.25 454.$1 451.65 268+5~ 456.77 ~57.~0 ~55.98 ~5~.65 ~55.19 ~52.6~ 28~+8~ ~57.61 ~57.88 ~56.86 ~55.5~ ~56.~7 ~55.28 288+7~ ~59.~2 458.18 ~58.24 455.97 457.59 455.74 296+1~ ~6~.~2 458.7~ 459.25 456.46 458.4~ ~5~.16 3~5+7~ 465.81 k59.45 46~.~ 457.1~ ~59.16 ~54.71 $12+$~ 461.56 ~6~.19 46~.85 457.63 ~59.99 ~55.16 521+5~ k62.58 ~6~.59 ~61.65 ~57.96 ~6~.85 ~55.57 $22+~ ~62.11 ~59.65 461.~5 k57.~1 ~6~.71 ~55.1~ 522+$~ ~62.52 46~.~1 ~61.65 ~57.58 46~.8~ ~55.22 2-23 2.& PARKS OF COPPELL The post-development floodplain of Denton Creek will include the Parks of Coppell; however, as noted earlier, the frequency of flooding by overtopping of Denton Creek ts expected to occur less often. That le, the mean recurrence interval of floods equaling or exceeding the top- of-bank stage in existing conditions is approximately a l~-yr Interval; the post-development conditions which include the chonnelizotion of Denton Creek will provide a mean recurrence Interval of $$-yr for floods equaling or exceeding the top-of-bank stage. As the ~atershed becomes more urbanized cnd peak discharges Increase, this overtopping will occur more often. Floodlng from interior drainage into the porks oreo from the south, based on recent evaluations of the proposed lake elevations and drains, w111 frequently Inundate the park lands for short durations. A rainfall having a 5~ probability of being equaled or exceeded in any given year will inundate the pork ploylng flelds for at least 6 hfs; indicating that. on the overage, the fields con be expected to flood at least every other year from interior drainage. 2-2. EVALUATION OF DENTON CREEK ENVIRONS ~.~ EVALUATZON OF DENTON CREEK ENVZRON$ 3.1 ZNTRODUCTZON The environmental overview of this study has been conducted utilizing Information oval/able in the literature and from brief site reconnaissance vlslt$ to evaluate the exist{rig envlronmentaI characteristics of the Denton Creek floodplain In consideration of the current goals, guidelines, and master plans of the City of Coppell. The study has not incorporated field sampling surveys. Emphasis has been placed on Identifying soll, vegetation, wildlife, aquatic, scenic, aesthetic, recreational, Qnd cultural characteristics thQt may be classified os environmentally sensitive resources. The results of these characterizations hove been evaluated In consideration of plans for the Parks of Coppell development project and channellzatton of Denton Creek. The study area ~s shown in Figure 1-1. The area may be generalZy described as the Denton Creek floodplain east of Denton Tap Rood and west of the confluence of Denton Creek with the Elm Fork of the Trinity River. This area Includes portions of both Dallas County and Denton County, Texas, This section of the report is organized Into two major sub-sections, Section 5.2 ond Section 5.5. Section 5.2 presents descriptions of the existing choracterlstlcs of the vorlous envtronmento! factors evoluated. A discussion of the effects of project development on these envlronmentol resources is presented in Section 5.$. Study conclusions and recommendotlons, including those of ~hls section, are summarized In Section 4.~. References cited In the text are llsted in Section 5.~. :5-1 3.2 DESCRZPTZON OF THE EXZSTZNG ENVZROMflENT $.2.1 $otle According to the published Soil Survey for Dallas County (USDA, 198~). the floodplain of Denton Creek consists primarily of two major soil types (mop units). These soil types include (1) Frio silty cloy, occasionally flooded, ~-2~ slopes (immediately adjacent to the creek); and (2) Trinity clay, occasionally flooded, less that 1~ slopes (immediately adjacent to the Frio soils). Both the Trinity and Frio soils ore deep, nearly level, clayey soils on floodplains. Frio soils ore well-drained and are typically found in broad bottomlonds along the larger streams. The floodwaters ere shallow, and the floods are of brief duration. Permeability is moderately slow, and the available water capacity is high, Runoff is stow, and the hezard of erosion is slight. This soil is well-suited to improved Bermuda gross. Trinity soils are somewhat poorly drained and are typically found on brood bottomlands along the Trinity River and 1ts larger tributaries. Like the Frlo soils, permeability is slow, and water capacity is high. Runoff is very slow, and the hazard of erosion is slight. Trinity soils also ore well-suited to improved Bermuda gross. The flood hazard, clayey texture, and very-slow and moderately-slow permeability of both soils and the very high shrink-swell potential of the Trinity soils ore the major limitations to urban and recreational uses. The suitability of the soils for development of recreational sites in both Frlo and Trinity soils in the form of camp areas, picnic areas, playgrounds, and paths and trails is rated os "severe" in the soil survey (USDA, 198~). This is due primarily to the flooding potential, wetness, and clayey characteristics of the soils. The soil survey also describes the wildlife habitat potentials for Frio ond Trinity soils. Frio soils ore described os "good" for grasses and hardwood trees, "fair" for wild herbaceous plants, and "poor" for wetland plants. These classifications for Trinity soil vegetation types indicate that overall wildlife potential is "good" ~or woodland wildlife. "?elm" for open land wildlife, and "poor" for wetland wildlife. The Dallas County soil survey lists several selected species of shrubs and trees which are suitable for growth on Frlo and Trinity soils. Shrubs listed os suitable for both soils include pittosporum, loquat, oleander, Texas sage, Texas laurel, obelio, acubo, mahonio, fotsia, and pomegranate. Trees listed os suitable for both soils lnclude pecan, sweetgum, Japanese block pine, oaks, cedar elm, redbud, crobapple, hackberry, ginkgo, and Chinese pistoche. Existing vegetation in the study oreo will be discussed in Section 3.2.2. 3-2 S.2.2 Terres~rtal Ecology Vegetation Hays et al (1972) characterized the environmental features of the 5~- mlie reach of the Elm Fork of the Trinity River between the Lewlsvllle Dom end the confluence with the West Fork of the Trinity. Their study area included a portion of the Denton Creek floodplain. The study area identified earlier in Figure 1-1 is located in a transition zone between major Texas vegetational areas. The Cross Timbers area extends to the west of the study oreo, and Blackland Prairies area extends to the east. The Cross Timbers area is dominated by Black~ock Oak and Postoak trees with Little Blue Stem grass es the chief understory species. The Blacklond Prairie once supported a tall gross community but is now in a degree of stress with domination by Little Blue Stem, Buffalo grass, and Curly ~esquite. The most noteworthy vegetative system tn the Elm Fork floodplains is the mesophytic forest of hardwood trees which llne the immediate banks and lowlands of the Trinity and 1ts tributaries. These riparian forest areas are diverse and include o rich variety of tree species such os Cedar Elm, Hackberry, Bumelta, Box Elder, Ash, and Hickory. Sub-canopy trees include Hawthorne, Osage-Orange, and Creeper with an understory dominated by grass, Greenbrier, and Coralberry. The most mesic (moist) ZOne at bankslde includes the tree species Cottonwood, Box Elder, Red Ash, and Willow. The Denton Creek floodplain tn the study area ls characterized by these mesophytic wooded areas in strips of varying widths along the creek banks and tn the immediate lowland areas. Huch o~ the floodplain land on the south side of the creek tn the Parks of Coppell development area has already been cleared. The remaining strip Of trees, shrubs, and understory vegetation along the creek bank serves as a buffer zone between the development area and the creek. Thls remaining strip of woody vegetation along the creek banks is similar tn character to the Elm Fork mesophytic forest areas described by Hays et al (1972). There are no listed rare or endangered plant species that are known to occur in the study area (Potter, 1986). Johnson (1986) of the US Fish and Wildlife Service reports numerous small areas of typical wetland vegetation and morphology scattered throughout the study oreo based on photo-interpretation of topographic and aerial photo mopping of the area, The mopping of the these areas provided by the USFWS agrees well with the areas described earlier, but, iS outdated as o result of the development occurring in the area. 3-5 Wildlife Hays et al (1972) reported that a variety of mammals occur within the Elm Fork floodplain area, The Cotton Rat occurs where grassy cover Is available, while the Norway Rat and House Mouse dwell near human habitatione, The Harvest Mouse lives In both grassland and open forest (especially savannahs), while the Deer Mouse is primarily an inhabitant of dense forest. Blair (195~) and Burl and Grossenhetder (196~) reported that about ~5 species of mammals occur in the Elm Fork region. Because of human actlvlty in the oreo, probably less than ~ spec/es remain. Typical mammal Inhabitants of the mesophytlc forest described above include Beaver, Muskrat, Armadillo, Raccoon, Opossum, Swamp Rabbit, Striped Skunk, two species of squirrels, and the Grey Fox. Conant (1958) reported that about 12 specles of amphibians hove been reported for the Elm Fork region. Two of the most Interesting species Include the Green Treefrog and the Gray Treefrog. All of these amphibian species depend on mesophytlc (mo/st) habitats for compietlon of their life cycles. The mesophytlc forest areas adjacent to Denton Creek and the Elm Fork region are typical of these types of moist habitats. Conant (1958) also stated that approximately ~ species of reptlles have been reported for the Elm Fork area. These specles lnclude the Cottonmouth, Pigmy Rattlesnake, Garter Snake, Diamond-Backed Water Snake, Blotched Water Snake, Broad-Headed Sklnk, Five-Lined Sktnk, and the Ground Sklnko These specles along wlth eight of the nine turtle specles occurring in the oreo are all hlgh humldlty water and forest species. Approximately 32~ bird species were reported for the Elm Fork area by Pullch (1961). Over ~ of these bird species are migratory species which "stop over" in the mesophyttc forest areas during the spring and fall. Spectacular species which are sometimes observed in the area Include the Palnted Bunting, Promonothory Warbler, Red-Toil Hawk, Belted Kingfisher, and the Great Blue Heron. The Texas Porks and Wildlife Deportment (1986) has listed a number of wlldllfe species which occur in Dallas and Denton Counties os endangered or threatened. Endangered species which have been confirmed in both counties include the B01d Bogle, the Interior Least Tern, and the Arctlc Peregrine Falcon. The Whooping Crane has been confirmed for Dallas County but ts listed os probable for Denton County. Threatened species which have been confirmed for both counties include the White-Faced [bis, Osprey, Wood Stork, Texas Horned Lizard, and Louisiana Milk Snake. The American Swallow-Tolled Kite has been confirmed for Denton County but is listed as probable for Dallas County. The Golden-Cheeked Warbler is listed as possible for Dallas County but is not listed for Denton County. Hays et al (1972) reported that no wildlife species were endemic to the Elm Fork area, Further, the US Fish and Wildlife Service (Johnson, 1986) indicates that no Federally listed threatened or endangered species are likely to be impacted by work In project study area, However, In addition to those species identified by the Texas Parks and Wildlife Department, the USFWS lists several additional endangered or threatened (or endangered status pending) species which may migrate through and breed in the area. These include the American Peregrine Falcon, the Brown Pelican, the Piping Plover, and the Black-Copped Vireo. Detailed field surveys and somplin9 would be required to determine if any of the endangered or threatened species listed above occur in the Denton Creek floodplain area. If detailed investigations ore made, particular attention should be given to the potential for block-copped vireos and whooping cranes. AS described in Sectlon 3.2.2, the mesophytlc forest areas which serve as wildlife habitat in the Denton Creek floodplain occur in bands or strips of varying widths along the creek banks and in the immediate lowland areas. Because clearing act/v/ties for development hove already occurred on both the north and the south sides of the creek, a substantial portlon of available wildlife habitat has been removed from the area. The remaining strips of forest along the banks of the creek serve to limit the available wildlife habltat to the creek banks and near-bank areas, The wildlife communities found in the Denton Creek floodplain are slmllar in character to those described for the Elm Fork mesophytlc forest areas by Hays et al (1972). $.2.$ Aquatic Ecology Hays et al (1972) conducted fleld flsh collections at a number of locations in the Elm Fork oreo. One of these sampling stations was located on Denton Creek upstream from its confluence with the Trinity. The varieties of fish species found in the Elm Fork and its tributaries are Impacted significantly by releases of water from Grapevine Reservoir and Lake Lewlsvllle. Hubbs (1961) listed 52 fish species which are expected to occur in the Elm Fork area. Hays et al (1972) collected 25 species durlng their studies. The Elm Fork and 1ts tributaries are unlque in that they drain both the Eastern Cross Timbers and Blacklond Prairies vegetation areas. Since the Elm Fork area is o unlque transitional area between two major vegetation areas, o large number of fish species ore found in the area. The 25 species collected by Hays et al (1972) are as follows: Longnose Gar Threadfln Shad Gizzard Shad Golden Shiner 3-5 Blacktail Shiner Red Shiner Blackspot Shiner Bullhead Minnow Channel Catfish Block Bullhead Freckled MaCram Blackstripe Topminnow Mosquitofish Mississippi Silverside White Bass Spotted Bass Largemouth Bass Warmouth Bluegill Longear Sunfish White Crappie Dusky Darter Logperch Orange Throat Darter Freshwater Drum The reach of Denton Creek between Denton Top Road and the confluence with the Elm Fork of the Trlnlty may be described as a meandering, deep channel with a relatively low velocity. The decrease in elevation from Denton Tap Rood to the Trinity is only four feet. Thus the creek has o much greater abundance of "pool" or standing water habitats rather than "riffle" or flowing water habitats. The fish community present in Denton Creek is influenced substantially by water releases and species contributions from Grapevine Reservoir. The Elm Fork system also is a source of fish species influx to Denton Creek. It ls expected that the fish communities found in Denton Creek are similar in character to those described for the Elm Fork area by Hays et al (1972). No fish species previously reported for the Elm Fork area may be considered endemic, and no species are listed as endangered or threatened. $.2.~ Scenic, Aesthetic, and Recreational Characteristice Although the Elm Fork area has been described os unique in that two major vegetation areas ore drained, the Denton Creek portion of the oreo is not particularly unique as compared to the remainder of the Elm Fork system between Lake Lewlsvllle and the confluence with the West Fork. From a scenic or aesthetic standpoint, the Denton Creek floodplain Is very eimllar in character to other Elm Fork areae. The oreo has an inherent scenic and aesthetic beauty that is enjoyed by residents and visitors. However. the Denton Creek floodplain is located near Coppell which is o rapidly growing community due to its 3-6 proximity to major business centers in Dallas and Ft. Worth, DFW Airport, and major transportation arteries. Commercial and residential developments are expanding rapidly tn the oreo to provide services and housing for the influx of people. This type of urbanization brings with it clearing, grading, and construction activities which will be carried out in previously undisturbed forest, pasture, farming, and stream areas. Although the existing scenic and aesthetic character of the oreo will be modified, a comprehensive landscaping plan which includes porks development and flood control can maintain scenic qualities in the developed area. Existing recreational facilities in the Denton Creek and Elm Fork floodplain oreo include Hclnnlsh Pork near the Sandy Lake Road bridge over the Elm Fork and o pork of sports fields west of the Denton Tap Road crossings over Cottonwood Branch and Denton Creek. The Mclnnlsh Park ls outside of the Porks of Coppell and Lakes of Coppell project areas; the sports pork (recently named Andrew Brown Park) ls within the Parks of Coppell oreo but not within the current construction oreo. Neither pork will be directly affected by project development. However, the Parks of Coppell project plan includes the creation of parks on the south side of Denton Creek in the floodplain between the creek and residential developments. Although porks do not exist in this oreo presently, considerable acreages of pork land will hove been created when all development plans hove been implemented. 3.2.5 Cultural Resources Ms. Carolyn Spock, Head of Records. at the Texas Archaeological Research Laboratory, University of Texas at Austin, was contacted regarding the locations of any listed archaeological sites within or near the Porks of Coppell project oreo. She identified six sites that have been listed for the oreo. General descriptions of these sites ore provided in the following paragraphs. Two sites ore located near the intersection of Sandy Lake Rood and MocArthur Boulevard. Site ~IDL2? is the location of o former grove where skeletal remains were found during sand and grovel excavations In 19~. The remains were recovered and sent to Austin. Site ~IDL3~ ls also located in this some oreo and is known os the Wheeler Site. This site is the locotlon of a 3,500-year-old non-pottery, non- agricultural culture of hunting people who specialized in o great deal Of hide-working. Numerous projectile points and hommerstones hove been recovered from the oreo. Site ~1DL$1 is located nort~ of DeForest Rood near the Denton County line. The site is o NeD-American site, and it ts thought that prevlous 9ravel operations hove already destroyed the slte. 3-7 Two sltes ore located near the Denton Top Road bridge over Cottonwood Branch. Site ~1DL52 is east of Denton Top Rood and was exposed during borrow pit excavations. An archaic hearth was found along with bi?aces and bi?ace fragments, burned rocks, bone, shell, flakes, and chips. No further preservation or research actions ore recommended. Site ~1DL11 is located west of Denton Tap Road. The site is o surface comp that has been well-searched numerous times. Materials round include many arrowheads and other flint artifacts. Site ~1DN27~ is located north of Denton Creek and east of Denton Tap Road in Denton County. In 1971, several flakes were found on the surface of a low hill that was grass covered and sloping toward Denton Creek. Research potential is considered minimal due to the extent of plowing which has occurred at the site. 3.2.6 Environmentally Sensitive Areas characteristics that may be considered unique, o? special value or importance to the community, of other environmental qualities, or are unusually susceptible to adverse Impacts. Environmental sensitivity threatened terrestrial or aquatic species, sensitive or unique habitat In the preceeding sections, the Porks of Coppell project area has been characterized in each of these environmental disciplines. This section will serve to summarize the project oreo environmental sensitivity in these disciplines os described earlier. Previous ecological surveys and current listings of the Texas Parks and Wlldllfe Deportment and the US Fish and Wildlife Service generally indicate that no endangered or threatened terrestrial or aquatic specles have been documented for the study area although there exist several small wetlands and the potential exists for migration and breeding of several species including the black-capped vireo, a candidate species under the Endangered Species Act. None of the aquatic and terrestrial habitats described for the Denton Creek floodplain may be characterized as particularly sensitive or unique; however, the projects os proposed and completed may be under Corps of £ngineers Section ~ Permit requirements of the Cleon W0ter Act since the work Includes chonnelizotion, relocations of the main channel, and further, includes work in areas having wetland areas identified as shown in Appendix g. The developers and the Denton County Levee Control District No.1 In particular should be required to submit the project plans to the Corps for their reviews of bank stabilization, discharges o? dredged and fill material, levees, stream diversion end diversion structures, channel construction, and related development. 3-8 The previous preliminary approvals by the Corps (see Appendix 9 letter and attachments from Espey, Huston & Associates, Znc. 9/1~/85) were approvals only If discharges were not to be made into Denton Creek and adjacent wetland areas. The data obtalned in this study from both the US Flsh and Wildlife Servlce (Johnson, 1986) Grid the HEC-2 models provided by Donnenbaum Engineering Corporation lndlcote fill in both the ex/sting channel and adjacent wetland areas. The only existing recreation areas near the project area are porks tha~ are located beyond project boundaries in areas where they will not be affected by project development. Development activities will modify the existing scenic quality of the Denton Creek floodplain. However, project plans will result in the creation of parks, flood protection, landscaping, and preservation of mesophytic forest areas along the stream banks. The only listed archaeological sites that might be affected by the project are Site ~IDL$~ (the Wheeler site) and Site ~1DL52 (the hearth site). It appears that both of these sites have been previously researched and that the sites hove been destroyed through residential development, road construction, and borrow pit activity. If further activity is planned in these specific areas, prior contact with the Texas State Historic Preservation Officer is recommended. 3.3 ENVZRONIflENTAL EFFECTS OF PROJECT DEVELOPMENT ZN FLOODPLAZNS 3.3.1 ChanneXtzatton Pro~ects Channellzatton projects con affect stream flaws, water qualttv, and aquatic and terrestrial biology, causing significant environmental impacts. In a study by Erlckson, Linder and Harmon (1979), It was reported that channeltzotton Caused increases in drainage rotes of over five times greater than in unchonnellzed areas and, although the claimed benefits of chonnellzotlon were watershed protection and flood control, the channel permitted and stimulated wetland drainage deleterious to wildlife. Interestingly, Moki, Hazel and Weber (1975) concluded from o year's study that channel/zed streams matntoln perennial and clear flow during drought periods in sharp contrast to the murky water of low flows in the natural streams of their North Carolina Study area. The impacts of chonnellzotlon on the aquatic ecosystems have been reported la the literature in a number of studies. Loss of sinuosity, loss of habitat including sources of food, and reduction in water quality ore among the changes caused by chonnellzation which affect flsh populations. Recovery of the aquatic ecosystem Is reported to be more rapid in areas of chonnelizotion where bank vegetation ls left In place and spoil is spread away from the bank areas than if stream-side vegetation and snags are removed (Heodrlck, 1976). Frederlckson (1979) reported on the floral and faunal changes in lowland hardwood forests resulting from channelizotion in southeastern Missouri, Indicating that, among other things, chonnelizotion reduced or changed riparian habitat by reducing forest area by as much os 78 percent, as compared to no more than 7 percent in unchannellzed areas, and that bird populations tended to ovoid the channelized streams although, channelizotlon did reduce flooding and benefit agriculture. Documentation of the effects of chonnellzotion on songbirds and small mammals by Possardt and Dodge (1978) indicates that the impact on small mammals and songbird populations was most dramatic where streams/de vegetation hod been extensively destroyed. 3.3.2 Land Creation Projects from the filling of wetland and floodplain areas to provide residential and commercial development have been noted by Elkington (1977) and Darnell (1977) who point out that, omon9 others, impacts include loss of wildlife habitat, increases in suspended sediments in modification of stream flow regimes. S.~ ENVZROI~IENTAL EFFECTS ZN DENTON CREEK PROJECT STUDY AREA Much of the filling, clearing, grading, and smoothing activities associated with the Parks of Coppell project have already occurred. It is anticipated that the strip of tress along the southern banks and along portions of the northern bank of Denton Creek will remain as on aesthetic buffer zone between the creek and the porks and housing developments beyond. This buffer will serve os habitat for wildlife similar to that available in other Elm Fork drainage areas and should help accelerate recovery. The banks of Denton Creek in the project oreo ore fairly steep. Existing conditions ore such that o review of historical flood events indicates the banks hove on overtopping frequency of only once in approximately every 1~ years (o 1~ exceedonce probability in any given year). Following the Levee District project Implementation of chonnellzotion, flooding is estimated to overflow the banks, on the overage, of only once in every 35 years (o 3~ exceedonce probability in any given year). A l~-yr flood exceedonce frequency is not often enough to create hydric habitats sultoble for the propagation of wetland vegetation. A decrease in flood exceedonce frequency to once in 35 years should not significantly lmpoct the abundance and diversity of vegetation and wildlife species. There ore no endangered or threatened vegetation or wildlife species known to occur in the Denton Creek floodplain near Coppell. The chonneltzatlon of Denton Creek os proposed and presently under construction for flood control purposes in the reach downstream from Denton Top Rood to the Elm Fork floodplain will hove on impact on the ecology o? the existing creek channel since water surface elevations of low-flows are likely to be reduced substantially by the large channel bottom width and low thalweg. The low-flow elevations are expected to be low enough to create pools of stagnating water or even dry channel segments in the existing channel meanders durlng summer months. If possible, it is recommended that the flood control channel tholweg be rolsed to approximate the water surface profile of the overage annual peak flow (approximately equivalent to o two-year exceedonce frequency flood event), to assure that overage annual flows be maintained in the existing channel, AS o minimum, the chonneltzed segment thalweg should be raised to the water surface profile approximated in a flow of 125 cfs (the gS~ probable exceedonce discharge) through the Parks of Coppe! stream segment. Maintenance of the recommended flow water surface profiles will essentially maintain the existing channel flow characteristics and, therefore, will help to assure that no adverse impacts on the ecology of the existing channel OCCUr OS a result of completion of the flood control channel. This alternative of maintaining the low-flow channel should be included In the application for revlew under Section ~ of the Cleon Water Act. Maintenance of o law-flow has been additionally recommended by Espey, 3-11 Huston & Associates, Inc. in o letter report (Jasper, 1985). Further, the Texas Water Commission (1985) has required os port of the Commission approval, that the Levee District provide a report of analysis o~ low flow impacts of the chonnellzotion of Denton Creek wlth recommendations for mitigation if required. ~laintenonce of the scenic character of the Denton Creek floodplain area is Important to locol residents of Coppell. Although the scenic character of the area will be modified through project Implementation, core Is evidently being token to retain wooded areas adjacent to the creek; care should be token to maintain overage annual low-flow in the creek channel, as noted earlier, Landscaping of developed areas will help to provide on aesthetically pleasing environment, No existing parks ar recreational facilities will be adversely impacted by development along the floodplain in the study reach. Plans are to create new porks for the use and enjoyment of the citizens of the community. It is not anticipated that archaeological sites identified in the project vicinity will be adversely impacted by development activities. State records indicate that the sites Identified hove been either extensively studied or destroyed by previous excavation activity. The Texas State Historic Preservation Officer should be contacted if surface disturbances are planned in any areas of potential archaeological interest. 3-12 STUDY FINDINGS STUDY FZNDZNGS · ,1 GENERAL CONCLUSZONS This study has been conducted, os previously noted, utilizing data gathering conslst/ng of literature searches, 1/m/ted field reconno/ssance Including meetings with long-time residents, reviews of plans and studles, ocqulsltlon of historical dato where available, and compilations and development of various computer models of the Denton Creek hydrology and hydraulics to determine and evaluate exlstlng conditions and to predict post-development conditions regarding the envlronmento! and floodplain characteristics of the Denton Creek study oreo generally from upstream of Sandy Lake Rood to upstream of Denton Top Rood in consideration of the current goals, guidelines, and master plans of the City of Coppell. Emphas/s has been placed on identifying potent/aZ problem areas wlth/n the study reach. The dora submitted by the engineering firms to the Clty of Coppell for their review hove, In generol, conformed to stondord recommended engineering proctlce for the pro~ects reviewed in this study. Upon sotlsfocto~y project completion, it ts generolly concluded thor none of the work proposed or under construct/on, with the exception of the chonnellzotton being performed by the Denton County Levee Control District No. 1, oppeors to hove the potentlol to provide ony slgnlflcont losttng odverse lmpoct tn the study reoch of Denton Creek. If the proposed guidelines ore odopted ond implemented, It Is further concluded thor the City of Coppell should not experience ony worsening flooding conditions. Summorlzed tn Toble ~.1-1 ore generol conclusions Identified wl%hln this study with references to the sectlons of the report whlch more fully discuss portlcular findings. TABLE ~.1-1 OENTON CREEK STUDY AREA GENERAL FZNDINGS & CONCLUSZON$ F/NDZNGS/CONCLUSIONS REFERENCE SECTZON (Page No.) 1. Floodplain maps are developed showing the post-development floodpIaln with all presently planned construction complete. 2. Cottonwood Branch - closure of opening into Parks of Coppell Excavation No. 3 required for overtopping conclusions to be valid. 2.3.1 (2-13 to 2-18) Appendix 7 3. Historical discharges verify local 1.3.3 (1-9) observations and indicate design 2.1.1 (2-6) discharges must be revised to account for 2.5.2 (2-19) urbanizing of watershed. 4. No change requlred in discharges for 2.1.1 (2-6) 5~-yr event. 2.1.4 (2-9) 5. All necessary permit applications may not have been submitted for ~ Jurisdictional determinations on portions of projects in the study oreo and may be required. 1.3.4 (1-1~) 3.2.6 (3-e) 6. Chonnellzatlon within Denton County Levee 1.3.4 (1-1~) Improvement District No. I os presently 2.3.3 (2-21) proposed and under construction, may cause 3.3.1 (3-1~) adverse impacts on the floodplain hydraulics 3.4 (5-11) and environment within, downstream, and upstream of the study oreo. 7. The Porks of Coppell will flood less frequently in post-development conditions than in existin9 conditions. 8. The Porks of Coppell will experience flooding from interior drainage on a recurrence interval of, on the overage, biennially. 2.3.2 (2-19) 2.3.3 (2-22) 3.z, (3-1~) 1.3.3 (1-8) 2.3.3 (2-22) 2.~, (2-2~) TABLE ~.1-1 (continued) DENTON CREEK STUDY AREA GENERAL FZNDZNGS & CONCLUSZON$ FZNDZNGS/CONCLUSZONS REFERENCE SECTZON (Page No.) 9. The MacArthur Road bridge should be located near stream stotlon 255+$ffi and could be a $~ ft long 5-span bridge. 14. Environmental impacts of the Levee District chonneltzotlon include adverse impacts on the low-flow channel of Denton Creek. (5-11) 11. No particularly environmentally sensitive 5.2.6 (5-8) areas identified within study area, although 3.2.2 (3-5) wetlands areas exist and the potential exists for loss of nesting hob/tat for the Block Copped Vireo. 12. The work generally to the south of DeForest Road ls Interrupting the natural overland flow as well as increasing the local runoff. 15. Local groundwater levels are not expected to be adversely lmpacted by work proposed in the Parks and Lakes of Coppell, but may be affected in the bank areas by the proposed stream chonnellzotion. ~.3.3 (t-s) 2.2 ~.2 RECOF~ENDAT/ONS · .2.1 General Reco~endattons The City of Coppell should adopt o set of floodplain management guidelines which incorporate the following provisions, some of whlch should be Immediately applied to projects currently being developed in the Denton Creek study reach because of the likelihood o? creating permanent adverse impacts: Design discharges for evaluation of streams In floodplains in the City of Coppell shall be based on two discharges at the design point: A. The Low-Flow discharge; defined as the discharge having o 95~ probability of bein9 equaled or exceeded (the 1.~5-yr mean recurrence interval discharge) in a given year os determined by historical data or regional regression equations; and B. The Base-Flood discharge; defined as the l~O-yr mean recurrence interval discharge (1~ probability of being equaled or exceeded) from a fully developed watershed determined by hydrogroph methods. Alterations of the channel or adjacent floodplain shall not result in any increase in water surface elevation, on site, upstream nor downstream of the project site in the Bose Flood discharge. Alterations of the channel or adjacent floodplain shall not result in any changes in the Low-Flow channel hydraulics upstream nor downstream of the project site. No alterations shall occur to the Low-Flow channel except as provided for in an approved stream master-plan. Alterations of the channel or adjacent floodplain shall not result in eroslve velocities on site, upstream or downstream of the project, except that, if velocities in the natural channel ore erosive, no increase in velocity shall occur upstream or downstream of the project site. Alterations of the floodplain shall occur only if it can be shown that equal conveyance alterations can occur on both sides of the channel and oll other criteria ore met. Application for alterations of the channel or adjacent floodplain shall include: A. Hydrologic and hydraulic analyses clearly definlng existing conditions, proposed conditions and impacts of the project, including work maps and stream profiles upstream and downstream of the site for sufficient distances to demonstrate o match to existing conditions for at least 3 consecutive cross-sections; B. A site grading and drainage plan showing cut slopes, fill slopes, on-site contours, returns to adjacent property contours, and l~-yr flood Inundation lines; C. A landscape plan; D. An erosion control plan or narrative describing temporary and permanent erosion controls; and E. Other such materials as may be required to clearly define the project location, scope, and intended use. · .2.2 Study Area Reco~mendations In addition to implementing the general recommendations above, it is recommended that the City of Coppell toke the following steps as soon as possible; particular attention is called to items 2, 3, and 5 which may have irreversible impacts if not corrected soon: Close the openlng from Cottonwood Branch 1nra the Parks area near "Excavation 3" to prevent flooding of the park from either Cottonwood Branch or Denton Creek. Determine if Corps of Engineers ~ Permits ore required in the project area, particularly for the Denton County Levee Control District No. 1 (DCLCD) project channelization including reviews of wetland areas shown by the US Fish & Wildlife Service and a determination of the potential for effects on the black-copped vireo. not excavated to below the natural Denton Creek low-flow profile or that "low-flow" diversions are provided. Reevaluate the normal pool elevations of the interior lakes of the Porks of Coppell to determine if frequency of interior flooding is acceptable or can easily be altered. 5o Take steps to require the developers to correct or mlnlmlze the adverse lmpacts on upstream and downstream channel velocities which will occur with the channelizatlon of the DCLCD project Including an evaluation of Cottonwood Branch which is not included in the backwater analysis of the DCLCD channel but which may be adversely Impacted as a result of the DCLCD channelizotlon. Requlre the developers along DeForest Rood to provide an adequate storm drainage System to collect and drain the storm runoff from their site Including run-on to their site from the north. The concern expressed by local residents regarding the Lakes of Coppel! retaining walls may be related to a lack of or an insufficient drainage system behind the walls to relieve groundwater pressures. The project deslgn engineer should be required to address this concern. [nterlor dralnoge systems in the Lakes of Coppell project ore felt to be adequate; no further evaluations are warranted. REFERENCES CITED B.~ REFERENCES CITED Baker, E.T. (196~). Geology and Ground-water Resources of Grayson County, Texas. Texas Board Engineers Bulletin 6015, as reported in Nordstrom. Blair, W. F. (195~). The Biotic Provlnce of Texas. Texas Journal of Science, Vol. 2, No. 1, pp 95-117, as reported in Hays et al. Burns, John (1986). PersonaI communication at site visit; Coppell, Burr, W. H. and R.P. Grossenhelder (1964). A Field Guide to the Mammals. Houghton Mifflin Company, 284 pp, os reported in Hays et ol. Canter, Lorry W. (1985). Environmental Impact of Water Resources Projects. Lewis Publishers, Inc., Chelsea, MI. 552 pp. Chow, Yen Te (1959). Open Channel Hydraulics. Company, New York. 6B~ pp. McGraw Hlll Book Conant, R. (1958). Reptiles and Amphibians of Greet Island Refuge, Tortoni County, adopted from A Field Bulde to Reptiles and Amphibians. Houghton Mifflin Company, os reported in Hays et ol. Creoger, Justin and Hines (1972). Engineering for Dams. Vol 1, p 125, tn National Engineering Handbook, Section 4, Hydrology. USDA Sail Conservation Service. NEH Notice ~-1~2, Washington, DC. August. Wetlands, June R.M. (1977). Overview of Major Development Impacts on Proceedings of the National Wetland Protection Symposium, Reston Virginia, as reported in Canter. Dowell, C,L. (196~). Dams and Reservoirs in Texas, Historical and Descriptive Information. Bulletin 6q~8. Texas Water Commission, Austln. July. Elklngton, J.B. (1977). The Impact of Development Estuorlne and Other Wetland Ecosystems. Environmental Vol k, NO2, pp 195-144, as reported In Canter. Projects on Conservation, Erickson, R.E., R.L. Linder and K.W. Harmon (1979). Stream Chonnellzotton (PL 83-566) Increased Wetland Losses in the Dakotas. Wildlife Society Bulletin, Vol 7, No 2, pp 71-7B, os reported in Canter. 5-1 Estep, Gregg (1985). Personal communication with Gregg Estep of the US Army Corps of Engineers District Ft. Worth, Hydrology Section, 11119/85. Fredertckson, L.H. (1979). Floral and Faunal Changes in Low Land Hardwood Forests in Hlssouri Resulting from Channellzation, Drainage, and Zmpoundment. FWS/OBS-78-91, US Fish & Wildlife Service, Washington, DC, January, as reported in Canter. Hampton, B.B. (1975). Hydrologic Data for Urban Studies in the Dallas, Texas Hetropollton Area, 1975. USDI, US Geological Survey. Austln. Hawkins, L.H., Jr. (1986). Letter in response to request for information on permits issued and pending in Denton Creek study area. Chief, Office Operations Branch, Department of the Army, Fort Worth District, Corps of Engineers, Fort Worth, TX February 26. Hays, T. R., T.R. Hellier, and T. E. Kennerly (1972). Environmental [mpact Study of the Elm Fork Region of the Trinity River. Dept. of Biology, University of Texas at Arlington. Prepared for the U.S. Army Corps of Engineers, Ft. Worth District, April 14. Heedrick, H.R. (1976). Effects of Stream Channelization on Fish Populations in the Buena Vista Harsh, Portage County, Wisconsin. September,. US Fish & Wildlife Service, Stevens Point, WI, os reported in Canter. HEC (1981). HEC-1 Flood Hydrogroph Package. Generalized Computer Program 725-X6-L2¢l~, Hydrologic Engineering Center, US Army Corps of Engineers, Davis, CA. September. Revised January, 1985. October, 1984 update. HEC (1982o). Flood Program 723-X6-L755~, Engineers, Davis, CA. Flow Frequency Analysis. Generalized Computer Hydrologic Engineering Center, US Army Corps of February. HEC (1982b). HEC-2 Program 725-X6-L2~2A, Engineers, Davis, CA. Water Surface Profiles. Generalized Computer Hydrologic Engineering Center, US Army Corps of September. May, 1984 update. Hershfield, David M, (1961}. 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