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CF-Andrew Park FS-SY000114 DENTON CREEK AND COTTONWOOD BRANCH CONFLUENCE HYDRAULIC STUDY EXISTING CONDITIONS BASED ON FULLY DEVELOPED OR ULTIMATE LAND USE In the City of Coppell, Dallas County, Texas Prepared for: THE CITY OF COPPELL PARKS DEPARTMENT By: Halff Associates, Inc. 8616 Northwest Plaza Drive -' Dallas, Texas 75225 MV ! 60146Se lei I ) ( This document is for Permitting purposes only and is not intended for Bidding or Construction. AVO 18109 January 2000 lll -,. Halff Associates Ill ENGINEERS · ARCHITECTS · SCIENTISTS PLANNERS · SURVEYORS 5616 NORTHWEST PLAZA DRIVE DALLAS, TEXAS 75225 (214) 346-6200 FAX (214) 739-0095 January 07, 1999 AVO 18109 Mr. Gary Sims, Director, Leisure Services Department City of Coppell 255 Parkway Blvd. Coppell, Texas 75019 Re: Denton Creek and Cottonwood Branch Confluence Hydraulic Study Based on Fully Developed or Ultimate Landuse Conditions. Dear Mr. Sims: Enclosed you will find a brief technical report summarizing the methods and procedures used to prepare hydraulic modeling to define existing conditions at the confluence of Denton Creek and Cottonwood Branch in the City of Coppell, Texas. This study analyzes flow patterns based on fully developed or ultimate landuse conditions. Plotted cross-sections, floodplain mapping, profiles, a hard copy of the HEC-2 models and a disk containing the computer input files are included with this submittal. The Cottonwood Branch modeling is based on the Coppell City-Wide Storm Water Management Study Hydraulic ultimate landuse condition model. The Denton Creek analysis was prepared using the ultimate landuse conditions model developed for the Andrew Brown Jr. Community Park (East) Conditional Letter of Map Revision (CLOMR) submittal to the Federal Emergency Management Agency (FEMA). The hydraulic models were modified to include recent improvements to the park area, such as the construction of ball fields in Andrew Brown Jr. Community Park (Central), and the construction of a new Denton Tap Road bridge across Denton Creek in 1999. Not modeled are the Andrew Brown Jr. Community Park (East) improvements proposed in the December 1992 CLOMR and the Coppell Aquatic and Recreation Center proposed in January 1999, as neither of them are entirely constructed in December 1999. The proposed pedestrian bridge across Cottonwood Branch is modeled separately and is being forwarded to the City of Coppell Parks Department in the form of a Flood Plain Development Permit Application. If you have any question regarding this report, please call me at 214-346-6226. You may also reach Mr. Walter Skipwith at 214-346-6220. Sincerely, HALFF ASSOCIATES, INC. Sabine Borgnet-Hamis, EIT enclosures cc Mr. Kenneth Griffin, P.E., City Engineer, Assistant City Manager & Director of Public Works DALLAS · FORT WORTH · HOUSTON · McALLEN TRANSPORTATION · WATER RESOURCES · LAND DEVELOPMENT · MUNICIPAL · ENVIRONMENTAL, STRUCTURAL MECHANICAL · ELECTRICAL · SURVEYING · GEOGRAPHIC INFORMATION SYSTEMS ARCHITECTURE * LANDSCAPE ARCHITECTURE · PLANNING Ill · -- Halff Associates Ill ENGINEERS · ARCHITECTS · SCIENTISTS PLANNERS · SURVEYORS 8616 NORTHWEST PLAZA DRIVE DALLAS, TEXAS 75225 (214) 346-6200 FAX (214) 739-0095 January 3 I, 1999 AVO 18109 AVO 18107 Mr. Gary Sims, Director, Leisure Services Department City of Coppell 255 Parkway Blvd. Coppell, Texas 75019 Re: Revised Schedule Andrew Brown Jr. Community Park Existing Condition Hydraulic Study Denton Creek and Cottonwood Branch Confluence Dear Mr. Sims: Attached you will find a revised delivery schedule for the Andrew Brown Jr. Community Park Existing Condition Hydraulic Study that HalffAssociates, Inc. is currently preparing for the City of Coppell Parks Department. We apologize for the delay but it became necessary to perform additional tasks to insure greater accuracy of the hydraulic modeling. During the development of the confluence analysis, we realized that some properties along Cottonwood Branch upstream of Denton Tap Road appeared flooded by the base design flood, the 100-year flood based on a fully developed (ultimate) condition watershed. We therefore attempted to obtain from the City of Coppell the Parkway Boulevard as-built construction plans and digital files of the Denton Creek watershed hydrologic study. We were able to obtain the as-built bridge plans and modify the Cottonwood Branch hydraulic models accordingly. This effort confirmed that properties upstream of Parkway Boulevard were only marginally flooded by the base flood: although computed base flood levels are slightly greater than those computed by older models, this increase does not appear to affect residences or insurable properlies. Please refer to the Denton Creek and Cottonwood Branch Confluence Hydraulic Study, Existing; Conditions Based on Fully Developed or Ultimate Land Use report submitted to the City of Coppell in January 2000 for further information. The City of Coppell was unable to locate a digital version of the hydrologic model. We also contacted Kimley-Hom and Associates, Inc. who performed the study in 1989, and Mr. Ron Mon-ison, with Motrison Hydrology Engineering, Inc., who was in charge of the study at the time. Both attempted to retrieve the digital files but were unable to do so. The combined effort delayed us by several weeks. The original project scope of work consisted off DALLAS · FORT WORTH · HOUSTON · McALLEN TRANSPORTATION · WATER RESOURCES · LAND DEVELOPMENT · MUNICIPAL · ENVIRONMENTAL, STRUCTURAL MECHANICAL · ELECTRICAL · SURVEYING · GEOGRAPHIC INFORMATION SYSTEMS ARCHITECTURE · LANDSCAPE ARCHITECTURE * PLANNING · ,- Halff Associates lll ENGINEERS · ARCHITECTS · SCIENTISTS PLANNERS ° SURVEYORS 1. a hydraulic study of the Denton Creek and Cottonwood Branch confluence based on a fully developed (ultimate) condition watershed, 2. a hydraulic study of the same confluence using the Flood Insurance Study (FIS) discharges, which is an analysis of the existing condition watershed, 3. a Fill Permit submittal for a proposed pedestrian bridge across Cottonwood Branch in Andrew Brown Jr. Community Park (Central), and 4. a Conditional Letter of Map Revision (CLOMR) Request to the Federal Emergency Management Agency (FEMA) for proposed Park facilities in Andrew Brown Jr. Community Park (West). This effort necessitates a hydraulic study of the impact of the proposed improvements. Items 1 and 3 are now complete and have been submitted to the City of Coppell Parks Department. We propose the following schedule of delivery for Items 2 and 4: · Confluence study based on FIS conditions: February 29, 2000 · Hydraulic Study of the proposed park facilities: May 1st, 2000 · CLOMR submittal to the City of Coppell for review: May 22, 2000 After completion of the City review, the CLOMR will be forwarded to the FEMA offices in Washington, DC. FEMA takes in general 3 months to review a CLOMR request. Please call me at 214-346-6226 if you have any question regarding this revised schedule. You may also reach Mr. Walter Skipwith at 214-346-6220. Sincerely, HALFF ASSOCIATES, 1NC. Sabine Borgnet-Harris, EIT CC Mr. Kenneth Griffin, P.E., City Engineer, Assistant City Manager & Director of Public Works Page 2 lmm · ,, Halff Associates ENGINEERS ° ARCHITECTS ° SCIENTISTS PLANNERS ° SURVEYORS 8616 NORTHWEST pLAZA DRIVE DALLAS, TEXAS 75225 (214) 346-6200 FAX (214) 739-0095 March 20, 2000 AVO 18107 Mr. Gary Sims, Director, Leisure Services Department 732 Deforest Road (or PO Box 478) Coppell, Texas 75019 Re: Proposed Tennis Courts in Andrew Brown Jr. Community Park (West). Dear Mr. Sims: Per our March 20, 2000 telephone conversation, we are investigating the possibility to reduce to twelve (12) the number of tennis courts that the City of Coppell proposes to build in Andrew Brown Jr. Community Park (West). Halff Associates, Inc. was originally retained by the City of Coppell to study the impact of the proposed construction of eighteen (18) tennis courts in Andrew Brown Jr. Community Park (West), in the City of Coppell, Texas, and to submit a request for a Conditional Letter of Map Re-¢islbn (CLOMR) to the Federal Emergency Management Agency (FEMA). We cannot ascertain that any number of tennis courts can be built on the site without first performing an in-depth hydraulic study of the proposed improvements. Tennis court construction requires enclosing the area with fences. It is the impact of the fences in the flood plain that must be mitigated with compensating excavation. As explained in our March 1st, 2000 letter to Mr. Brad Reid, City of Coppell Park Planning and Landscape Manager, "Construction of this nature in the flood plain generally requires mitigating excavation to achieve the City's requirement of no rise in base flood levels, the l O0-year flood based on fully developed or ultimate landuse conditions. FEMA also requires a no rise condition if construction occurs in its regulatory floodways. Approximately 50 percent of Andrew Brown Jr. Community Park (WesO is situated in the floodway of either Denton Creek or Cottonwood Branch, and roughly 90percent of the park is in the flood plain ". We will go ahead and analyze the impact of constructing a reduced number of tennis courts in the park. The area available for mitigation is very limited and we cannot guarantee that any number of tennis courts will give satisfactory results. However, if the reduced number of tennis courts does not meet the City's needs, one may want to DALLAS · FORT WORTH · HOUSTON · McALLEN TRANSPORTATION ° WATER RESOURCES ° LAND DEVELOPMENT ° MUNICIPAL · ENVIRONMENTAL · STRUCTURAL MECHANICAL · ELECTRICAL ° SURVEYING · GEOGRAPHIC INFORMATION SYSTEMS ARCHITECTURE ° LANDSCAPE ARCHITECTURE ° PLANNING · -- Halff Associates ENGINEERS · ARCHITECTS ° SCIENTISTS PLANNERS · SURVEYORS consider the construction of another type of recreational facility that would not involve fences. At this point, we recommend a meeting w/th the City of Coppell Parks Planning Depmtment to discuss options for mitigation, the excavation of areas to offset encroachments caused by the t~nnis courts and fences. Please call me at 214-346-6226 if you have any further questions regarding the park improvements. You may also reach Mr. Walter Skipwith at 214-346-6220. Sincerely, HAL7 ASSOCIATES, INC. Sabine Borgnet-Hards, EIT CC Mr. Kenneth Griffin, P.E., City Engineer, Assistant City Manager & Director of Public Works file: 18109\wp~ltO32000.doc Page 2 DENTON CREEK AND COTTONWOOD BRANCH CONFLUENCE HYDRAULIC STUDY EXISTING CONDITIONS BASED ON FULLY DEVELOPED OR ULTIMATE LAND USE TABLE OF CONTENTS Project Description B. C. D. E. F. G. Introduction and Site Description Mapping Base Models Confluence Relocation Revised Existing Condition Models Results Conclusions and Recommendations Page # 1 1 1 2 2 4 5 ii. Tables Table 1 Table 2 Table 3 Table 4 Comparison Table of Hydraulic Results - Denton Creek - 100-yr Comparison Table of Hydraulic Results - Denton Creek - 2-yr Comparison Table of Hydraulic Results - Cottonwood Branch - 100-yr Comparison Table of Hydraulic Results - Cottonwood Branch - 2-yr III. Figures and Exhibits Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Cottonwood Branch at Denton Tap Road Water Surface Profile - Denton Creek Water Surface Profile - Cottonwood Branch Cross-sections - Denton Creek Cross-sections - Cottonwood Branch Exhibit 1 Existing Flood Plain IV. Appendices Appendix A Appendix B Appendix C Denton Creek - Revised Existing Condition HEC-2 Model Cottonwood Branch - Revised Existing Condition HEC-2 Model Digital Input Data Files on Disk I. Project Description DENTON CREEK AND COTTONWOOD BRANCH CONFLUENCE HYDRAULIC STUDY EXISTING CONDITIONS BASED ON FULLY DEVELOPED OR ULTIMATE LAND USE PREPARED FOR THE CITY OF COPPELL PARKS DEPARTMENT JANUARY 2000 AVO 18109 ,4. Introduction and Site Description. Halff Associates, Inc. was retained by the City of Coppell Parks Department to investigate existing conditions at the confluence of Denton Creek and Cottonwood Branch in the City of Coppell, Texas. This report provides technical data in support of a hydraulic engineering analysis of the confluence based on fully developed or ultimate landuse conditions. The confluence is located downstream or East of Denton Tap Road and north of Parkway Boulevard within Andrew Brown Jr. Community Park (East) in the City of Coppell, Dallas County, Texas, as shown on Figure 1, Existing Flood Plain. This hydraulic analysis extends upstream and investigates flood patterns in Andrew Brown Jr. Community Park (West), West of Denton Tap Road. Cottonwood Branch is a tributary of Denton Creek, which is itself a tributary of the Elm Fork of the Trinity River and flows in an easterly direction near the northern corporate limits of the City of Coppell. Denton Creek drains a watershed area of 719 square miles which includes 695 square miles of drainage area for Lake Grapevine and Cottonwood Branch. The study area is located approximately 5 river miles upstream of the confluence of Denton Creek with the Elm Fork of the Trinity River. B. Mapping. The revised floodplain delineations shown on Figure 1, Existing Flood Plain, were drawn against the City of Coppell 1998 aerial topographic mapping by Teague Nall& Perkins and Dallas Aerial Surveys, Inc. (Texas State Coordinate System, North Central Zone - NAD '83 - Photo date: March 18, 1998). The aerial topography was modified per the January 1999 Denton Tap Road bridge construction plans by Wier and Associates, Inc., and per the April 1998 Andrew Brown Jr. Community Park (Central) ball fields construction plans by Schrickel, Rollins and Associates, Inc. (SRA). At the time of the present study's field surveys, construction of the ball fields was complete while construction of the bridge was almost complete. As-built plans were not yet available in both cases. C. Base Models. The US Army Corps of Engineers hydraulic computer program HEC-2 was utilized to determine water surface elevations along both creeks. The discharges in the models are based on hydrology for ultimate landuse as developed by Kimley-Hom and Associates, Inc. in 1989. Page 1 The Cottonwood Branch hydraulic modeling was prepared using the Coppell City-Wide Storm Water Management Study hydraulic model (CWEXULT1) developed in January 1991 by Halff Associates, Inc. (then Albert H. Halff Associates, Inc.) for the City of Coppell. The original Cottonwood Branch 1988 Flood Insurance Re-Study (FIS) model was updated by Kimley-Horn and Associates, Inc. in 1989. This updated model, with minor revisions, was then used in the 1991 Coppell City-Wide Storm Water Management Study. The Denton Creek hydraulic analysis was prepared using the revised existing condition model (DENCR-DW) developed for the Andrew Brown Jr. Community Park (East) Conditional Letter of Map Revision (CLOMR) submittal to the Federal Emergency Management Agency (FEMA). The original Denton Creek model updated by Kimley-Horn and Associates, Inc. in 1989 was used, once again with minor revisions, in the 1991 Coppell City-Wide Storm Water Management Study, and subsequently revised in the Andrew Brown Jr. Community Park (East) CLOMR. Please note that the downstream end of the Cottonwood Branch channel has been analyzed in the various Denton Creek models (downstream of Denton Creek cross-section 32725) up to the present analysis. The Cottonwood Branch model CWEXULT1 starts with Denton Creek cross-section 32725 (See Exhibit 1 - Existing Flood Plain). D. Confluence Relocation. The confluence of Denton Creek and its tributary Cottonwood Branch was relocated in the hydraulic models of both streams from about 145 feet downstream of Parkway blvd. to about 1450 feet downstream of Denton Tap Road. The effective confluence occurs directly downstream of the ball fields designed by Schrickel, Rollins and Associates, Inc. (SRA) at Andrew Brown Jr. Park Central. In the Denton Creek model, the confluence is now modeled upstream of section 30384, and the discharge change takes effect at the next section, 30604 (Denton Creek flows only at section 30604) (See Exhibit 1 - Existing Flood Plain). E. Revised Existing Condition Models. The revised models reflect the following changes: Insertion of a widened Denton Tap Road bridge over Denton Creek, under construction but essentially complete by the end of 1999. The bridge design was prepared by Wier and Associates, Inc. in January 1999, while the bridge hydraulic modeling was performed by Morrison Hydrology Engineering, Inc. in February 1996. The bridge was first inserted in the Denton Creek 1992 CLOMR model per the Morrison 1996 model, then revised per Halff Associates, Inc. 1999 field surveys. Insertion of the bridge resulted in a 2.00 ft rise at the downstream face 0fthe new bridge. This 2.00 foot rise is consistent with the 1996 modeling of the then proposed bridge by Morfison, which computed a 1.24 foot rise at that location (458.09-456.85 = 1.24 ft). The same proposed bridge inserted in the 1992 CLOMR model resulted in a 1.66 ft rise at the downstream face and a 1.83 ft rise at the location of the upstream face. Results are different from those in the Morrison model (1.66 ft versus 1.24 ft) because Morrison Page 2 used the FEMA model instead of the 1991 City-Wide Stormwater Management Study model. ~lhe remainder of the 2.00 rise can be attributed to various causes, such as improved overbank ground data definition per 1999 field surveys and per the 1998 City of Coppell topographic maps, and slightly revised Cottonwood Branch bridge data, also per 1999 field surveys (at this preliminary stage, the downstream end of Cottonwood Branch was still analyzed in the Denton Creek model, which is consistent with the CLOMR and 1991 City-Wide Stormwater Management Study modeling). Insertion of the SRA ball fields and parking lots along Denton Tap Road at Andrew Brown Jr. Community Park (Central) (per construction plans, as-built plans not yet available). Flood levels increased by no more than 0.01 foot, if at all. The model, still preliminary at this point, was not fine-tuned to within 0.01 ft. Revisions to ineffective flow limits upstream and downstream of Denton Tap Road. The flow path was narrowed down from that used in the CLOMR model because at some point in the past, a culvert was removed from underneath Denton Tap Road, between the two streams. The flow is now restricted only to the two bridge openings. Extension of the Cottonwood Branch model to downstream of the SRA ball fields. 1595 feet of the Cottonwood Branch channel that were modeled as part of Denton Creek are now modeled with Cottonwood Branch. The present Denton Creek model no longer includes Cottonwood Branch flows between cross-sections 30604 and 32725. The new Cottonwood Branch cross-sections reflect all the changes listed above except for the those due to the Denton Tap Road bridge. The revised Cottonwood Branch model takes into consideration the fact that flood waters may spill over the SRA ball fields downstream of Denton Tap Road. This confluence hydraulic analysis was prepared at the same time as another hydraulic study on Denton Creek and Cottonwood Branch, also performed by Halff Associates, Inc., which evaluated the impact of the construction of a proposed pedestrian bridge on Cottonwood Branch directly downstream of the SRA ball fields. The proposed pedestrian bridge study extended the Cottomvood Branch model downstream of the effective confluence point for reasons that are explained in depth in the January 2000 Pedestrian Bridge Flood Plain Development Permit Application. The present Cottonwood Branch model starts with the slope/area method using data generated by the proposed pedestrian bridge hydraulic model. A weir flow analysis was performed to model flood waters lost to Denton Creek. The hydraulic analysis also determined that flood waters spill across the Andrew Brown Jr. Park West ball fields upstream of Denton Tap Road from Cottonwood Branch towards Denton Creek even in the event of a lesser flood such as the 2-year flood. Before the models were modified for the weir flow conditions, water levels in Cottonwood Branch appeared to be 6.6 feet higher than in Denton Creek at Andrew Brown Jr. Park West, in the event of a 100-year probability flood. Page 3 Both the 100-year flood and the 2-year flood discharges were modified to model the weir flow or spill condition, in both the Denton Creek and the Cottonwood Branch models. For the 100-year probability flood, Cottonwood Branch discharges were reduced from 8390 cfs to 5502 cfs downstream of Parkway Boulevard, a change of 2888 cfs. Denton Creek discharges were increased between sections 30604 and 32725 to reflect the flow captured from Cottonwood Branch. The Denton Creek 100-year probability flood discharges are increased from 16681 cfs to 19569 cfs, a similar change of 2888 cfs. The Denton Creek effective flow was restricted within the Denton Tap Road bridge due to obstructions downstream at cross-sections 31380 and 32150, directly downstream or east of the pedestrian/bicycle path. F. Results. In general, computed water levels are now greater along Cottonwood Branch than along Denton Creek at common locations, which corroborates the weir flow modeling. 1. Along Cottonwood Branch, computed flood levels in the event of a 100-year storm (fully developed discharges) are higher than the ones originally computed in the 1991 models. Downstream of Denton Tap Road, water levels are computed bet~veen 0.4 feet and 1.0 feet higher than in 1991, when they were calculated in the Denton Creek model rather than the Cottonwood Creek model. · At the location of the upstream face of Denton Tap Road, ~vater levels are computed 3.1 feet higher than in 1991. However, the bridge is not overtopped by the 100-year flood: the model computes 2.8 ft of freeboard to the low chord elevation. · at section 3321, about 50 feet upstream of Denton Tap Road, computed water levels are as much as 5.5 ft higher than those computed in 1991. Two commercial properties situated at sections 3495 and 3695 (south-west comer of the Denton Tap Road bridge over Cottonwood Creek), appear to be partially flooded in the event of a 100-year storm (fully developed landuse or ultimate discharges) based on the 1998 city of Coppell topographic map contours. However, further inspection of the digital topographic file seem to indicate that the buildings may be out of the flood plain (see Figure 2 below that shows top of curb elevations near the buildings). Building finished floors were not field surveyed in the course of this confluence hydraulic study. · Increased flood levels are computed through Andrew Brown Jr. Park West as well. · At the Parkway Boulevard bridge, flood levels are computed about 1.1 foot higher than in 1991. · Computed water surface elevations equal those in the City-wide model at and upstream of section 6665. Along Denton Creek, computed flood levels in the event of a 100-year storm (fully developed discharges) are lower than those computed in 1991 at cross-sections 30604 through 31335, downstream of Denton Tap Road, and are greater at cross-sections 31380 through 40650, at and upstream of the bridge. · downstream of Denton Tap Road, 100-year flood levels are computed 0.1 to 0.3 feet lower than in 1991. Page 4 · At the do~vnstream face of the Denton Tap Road bridge, water levels are computed 2.8 feet higher than in 1991. However, the bridge is not overtopped by the 100-year flood: the model computes 1.3 ft of freeboard to the low chord elevation. · increased water levels occur at most cross-sections upstream of the bridge up to section 40650. The increase varies between 2.8 feet and 0.03 feet. · Computed water surface elevations equal those in the City-wide model at and upstream of section 40650. Cottonwood Branch existing flow velocities are generally moderate except in the vicinity of the existing Denton Tap Road budge: velocities vary between 5.61 fi/sec and 13.12 fi/sec at and downstream of the bridge (100-year storm, fully developed discharges), at locations where the channel is not armored. Velocities are still high but have decreased compared to those computed in 1991 near the Parkway Boulevard bridge. Denton Creek existing flow velocities downstream of the Denton Tap Road bridge are critically high at cross-sections 31335 through 32150. Velocities vary between 12.40 ft/sec and 13.97 fi/sec at locations where the channel is not armored. Velocities are high upstream of the bridge as well, 10.26 fi/sec at section 32355 and 10.03 fffsec at cross-section 32400, ~vhere the channel is not armored either. Velocities range from 7.39 fi/sec to 5.37 ft/sec at the same locations in the event of a 2-year probability flood event. G. Conclusions and Recommendations. The revised analysis does not indicate increased flooding of residences or buildings along the creeks in the event of a 100-year computed flood, versus the effective models prepared in 1991 and 1992, in spite of the fact that computed base flood levels have increased. Downstream of Denton Tap Road, base flood levels have generally decreased along Denton Creek, except at section 32150, and increased along Cottonwood Branch. Along Denton Creek, the increase may affect properties that have been developed since the aerial topographic mapping was flown and that are not shown on the attached Flood plain map (Figure 1). Along Cottonwood Branch, increases generally occur outside of the residential property boundaries. Two properties on Rustic Meadow are now shown barely inundated while they previously were shown outside of the 100-year (ultimate landuse conditions) flood plain. Channel flow velocities are high downstream of the Denton Tap Road bridges along both creeks, a condition that may create future erosion problems. We recommend that the City watch for the onset of erosion at areas where frequent high velocities were computed, and if erosion occurs, to armor the channel as needed to protect recently built improvements. The Denton Tap Road bridges are not overtopped, they do not run pressure flow conditions. Both streams have 100-year water surface elevations lower than their Denton Tap Road bridge low chord elevations. The Cottonwood Branch bridge causes a constriction that raises the water surface elevation upstream of Denton Tap Road (see below). However, if the constriction were removed, homes downstream on the right overbank could be flooded. Page 5 Upstream of Denton Tap Road, along Denton Creek, base flood levels have increased on undeveloped properties and park areas between cross-sections 32355 and 33470. Properties shown as undeveloped residential lots at cross-sections 34260 and 34850 sustain an increase in flood levels of 0.5 to 0.6 ft. Upstream of Denton Tap Road, along Cottonwood Branch, two commercial properties situated at sections 3495 and 3695 appear to be partially flooded in the event of a base flood, although the buildings themselves are most likely not affected. We recommend that the city field survey the buildings finished floor elevation and the adjoining area to better determine the flooding status and the need for floodproofing if any (see Figure 1, Cottonwood Branch at Denton Tap Road). We recommend that existing flow patterns, including overflow paths and the Cottonwood Branch bridge, be maintained in their current configurations. Widening the Cottonwood Branch bridge may cause water that is now diverted across Andrew Brown Jr. Park West to instead inundate properties along Cottonwood Branch and downstream of Denton Tap Road. We recommend that the overflow situation be maintained so that water keeps flowing towards Denton Creek, do not block the flow, possibly dedicate a water path to that purpose. Do not build up any further flow restriction at Andrew Brown Jr. Park West without re-running the weir flow computer analysis. Do not build along Andrew Brown Jr. Park West in a way that may cause increased flooding upstream, per the City's Floodplain Ordinance. Channel flow velocities are high upstream of the Denton Tap Road bridges as well. Once again, we recommend that the City monitor the development of erosion problems to prevent damage to existing bridge structures. Upstream of Andrew Brown Jr. Park West, along Cottonwood Branch, the 100-year flood plain now encroaches on private property at locations where it previously may have been barely outside of the property line. Residences upstream of Parkway Boulevard at Cottonwood Branch cross-sections 3694-4238 (Copperstone Rd., Martel Street) are shown on the topographic map at approximately 468 elevation, while the 100- year flood level varies between 465.3 and 466.4. At cross-sections 4753 and 5150, flood levels have increased by 0.02 ft or less. Modeling updates: The City of Coppell should seek to update the HEC-2 models to the new US Army Corps of Engineers HEC-RAS model as soon as possible. The HEC-2 computer program is DOS based and may become obsolete in the future. Also there have been cases where the different computations used in the HEC-RAS program resulted in higher computed water surface elevations. Both FIS and fully developed landuse hydrology should be updated at the same time. These revisions could be cost shared as a part of a joint FEMA/City of Coppell FIS Update. Page 6 II. Tables o.~ 0 III. Figures X ,\o© 0 ~'~ U.I (~) uo?,eAel:t 0 (1~) uo!le^el~ o (1J) UO!leAel~ i (Ii) uo!~e^el~ 0 LU (:g) uo!le^~l:] (~) uo!le^alg (~) uo!~e^al3 I,M (~) uo!~e^al3 m o o (~J) uo!]e^al3 (1~) UOlle^el3 0 o (~) uo!]e^~l~ (lJ.) UO!le^el:~ I I I (~J) uo!le^~13 0 OJ) uo!le^Ol3 (~) uo!le^~l~ (~) uo!le^e13 (~) uo!~e^al~ (~J) uo!]e^913 (~J) UO!le^al~ (~) uo~ e,',al3 (U) uo!le^el3 (~) uo!le^el3 (1~) uo!le^el~] (:~) uo!le^al:~ OJ) uo!i~^al3 · I I~ uo!~e^~13 (1~) uo!le^al] o (~) uo!le^al3 C) (~J) uo!~e^@f3 (1J.) Uo!leAoI3 l (D) uo!~e^al3 .] (U) l! (~) uo!le,',~,13 bl,' -M F- X," Al I -" S po \ ��j � ®Ip � 11 t'�) A A VIP, in t� �: ic: b � ' 1 �� ,�jl � � Wax �i , Am" - a ��s *N p W-M-m 10, W - - movm; MOM Appendix A Denton Creek Revised Existing Condition HEC -2 Model DCexist3.out X X o .~ .... ~ogoo~og~ ~o~o~ .... u% ~ o o o o o o o o ~ o o o o o o ~o~oo~ .... o~o~ o o~oo o ~o~oo o =~oo o ~o~o o oo ooooooo ooooooooo ooooo~oo oo o o~OOOOOOO °~ o oooooooo o o o~Oo~ o~%~ ~o~ > oo~~ ......... ~ o~o o o o o 2 o o o o .... ~oo~ S ..... 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'~ o o o oO~ ~o~ ~8© o ~o o o o 0 0 OOOOO © o o o o o o 0 rd 0 o ooo og~o o ooo~o o~Ooo oo oOa o o ,, §o o o° 0 [~ . 0 HO00 o o go °o ~o~ oog o ~ooo ~o U U o ooo~ .5 o° 0 ~ ~~ ~ ~o °° ~ o~o oo o~ofio~ooo~ ~ o 008 o o o oo~ ~o 0 0 o° 0 [~ oo~ ~ o~ oOOo oo o ~ ~o o~ o o oo U o o o o o o o o o ooo~ o o o ~ o 0 > U oo~ o~o~ o o o ° 0 > o o o§ ~ o~° o© 0 o o §o;~. © o o o o oo~ ~gg g ~ o o o o o oo~ ~o ~ooo ~°~ o© 0 o oo o o oO o o o o o o 0 CD o 0 ~ooo oo ooo~ ~-ooo o o o oooo · 0 >> o© 0 ~ o~o U o8~o ooo z z~o ~2 o° 0 ~ o oo ~ o o o cD o OoOoO o zo ~ o ~ 0 Z o© 0 ~ o 7 ° 0 o oooo Z U 0 oooo ~ 2 o°°§'~. uoo o U 6 oooom o ~ ~.~ o ooo~ o§ ~ oo~ 2o2-~ - g ~oo ~oo ~o o o o §~° ~ooo oo o o . o o o o o o oo~o.; 121 0 ~° o o mOoO . 0 ~> oo~ oOOoo~ o°Om~ ~oo o o %0 0 o o o 0 ooo~ o o o o o o o o° 0 ~ o o o o o o o o o o >> o ~ ~o°° oo~ o o o o oo oo§E oo~ ......... g§E~ ~§oo o oo oo o oooo oooo oo o ..... § ......... o ooo§ g§§§ o o o o o o oOoOoo o o o oOoOoo o°o°o°o° o°°o°o°o o°°°o°oo o°o°°°oo o°o°°°oo oo°°o°°o o°o°°o°o ooo°°° o oo~ ~oo~ ~oo~ ~o ..... ~ oo~® ~oo~ oo~ oogoo oOOOoo .......... oOoO oOOOoO oo ...... 0 o o o o o o o o o o o o o o o o o o ~ ° ° ° ~ o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oooo oo oooo ooo ooo ........ goo§ goo§ §ooo 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 oo00 0000 oooo oooo oooo oooo oooo g~goo ~g~ og~ o o 0000 0000 0000 0000 O0 0000 0000 000o 000o OO o ...... §~ooo°°g §g§Oo ~ oooo oooo oooo oooo oo o oo °o gOOoO ooo ~ o o o oo o o o o ~ o o o o o o o o o o o o oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo ...... §o oo§o §oo~ ~ ........... ~§§~ ~ ooo oo o oo o oo ooo oooo oooo 0 ~ooo oooo oooo oooo oooo oooo oooo o ~ oo o o § .... gg .... §oog oo oo oooo oo o§§ §o~ gooo oooo o o ooo ooo oooo oooo oooo o~ ~ ...... ~ .... oo oo o oooo ooo E~§~ oooo g ...... oooo ooo ooo o ooo ooo oooo o ooooo ooooo oooooo ooooo gooo o o o o o o o o o o o o o o o .... § oog§ oogg .... gooo o ooo oo oo oooo ooo o oooo oooo oooo oooo ~ g§OOoo §gOgo §ooo ooo .... g~o§ ooog oooo oooo .... §§o§ oo§o .... ggoo §oo~ oooo oooo oo oo ooo oo ooo gogo ~oog goog ggoo g ..... g ..... o o oo oo oo ooo oo o oooo oooo oooo oooo oooo oo o ooo 0000 0000 0000 0o0o 000o 00~0 00o~ ooog oo ............. °© .... °o ooooo o o o o o o o o o o o o o o o o o o o o o o o oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo ooooo ooog ooooo goo .... g goog .... o o o o o o o o o o o o o o oO o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o ~ oooo o o~o§oo §o°o°g gg o o o o o o 0 o o o o o o ~ o o o o o o oo oooo oooo oooo oooo oooo oo~o oooo oooo oo oooo oooo oooo oooo oooo oooo oooo oooo oo §oo§ .... ~§oo g ........... §oog gooo oo oo oooo oo ooo oooo oooo oo ooo oo oooo oooo oooo oooo oooo oooo oooo oooo oo oooo 0ooo oooo oooo oooo oooo oooo oooo oo § ......... go § ....... §ooo § ..... oo ooo oooo oo o ooo oooo ooo ooo oo ...... §oog .... gggo gooo §ooo oo§g oooo oo oooo oo oooo o ooo ooo oo oooo oo oooo oooo oooo oooo oooo oooo oooo oooo o o oooo oooo oooo oooo oooo oooo oooo oooo oooo oo oooo oooo oooo oooo oooo oooo oooo oooo oooo oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o© 8000o ooog ? ....... oo08 .................. oo o ooo ~ o o o o ooo o oo ooo oo oo~ oo8~ .... ~oo8 ~8oo ~§o oo8~ 8oo§ §888 oo oo o oo oo .... goog §o§~ gg ..... ~ ooo8 ooog 8g ..... § oo oooo oo o oo ooo ooo ooo oo ooo oo oooo oooo oooo oooo oooo oooo oooo oooo oooo oo oooo oooo oooo oooo oooo oooo oooo oooo oooo oo o o o o o o o o o o OoO o§a ......... ~ ..... ~ ...... §a oaao o o oooo oooo oo ooo oooo oo o o .......... ~ ....... § .... ~§oo o§oo oooo o ooo oooo ooo oo o oo o §§~°o §OO§oo §§~oooo ...... oo~§ §OO§oo ~OOOooo ~OO§oo oooo oooo oooo oooo oooo oooo oooo oooo §ooo ~§ ........ § .... § ~oo§ ~ooo ~§oo ooo oo oooo oo o ooo oo ooo oo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo oooo §OO~oo §O§§o oo~ ~oo~ oo oo oooo oooo oo~o oo§ .... ~ oo o oo o ooo oooo oooo oooo oooo oooo oooo o o o o oo o o o o °o .... oOOoo oOOOO o ooo °o °o OOoO o°o°°o°o oooo oo~o oo~o oo§ .... ~ oE§o ~§§o o§oo oo§o oooo oo o oo o ooo o o o o oo oo o oO o o oOO o oO o oo°oo .... oog°o gOgo oooo oooo oooo oooo oooo oooo oooo°°°° oo°°gg §g§§ gOOOooo g§oo .... oog°o oo°°g§ §gg§ ooog ........ g§oo §§§§ ooo§ §oog §§oo og 0 ooo oooo oooo oo ooo oo oo o ~ oooo oooo oooo oooo oooo oooo oooo oooo oo oo Ooo §ooo o o o o o o o SS§~ .... §ooo §§ ...... S~oo §oos oooo ooo oo oooo oo oo o~ ....... ~ oo~o 8000 ~oo ~oo § ........... g .... §go §ooo oo~§ 0 ooo oooo oooo oo~ o o ooo oo Z oooo oooo oooo ooo U °ooo oooo oooo ~ooo oooo oooo ooo oooo oooo ......... ~o oo~§ ooo~ ~ooo § ..... oo ooso §oo~ ooo§ o ooo oooo oo o oo ooo U oooo oooo oooo oooo oooo o oo ......... ~ oooo oo~§ o § ............. g g~oo oo oooo oooo ooo oo ooo oooo oooo oooo oooo ooo oooo oooo oooo oooo ooooo OoOgO o o o o o o o o ooooo oooo ooo~o oggo §go§ oo§o oog§ ooog §oog 0 oo ooo oo ~ oooo oooo oooo ~ oooo oooo oooo oo o oooo oo oooo oooo oo oo o oo ooo oooo oo o 000 0 000 o~ o ooo & Appendix B Cottonwood Branch Revised Existing Condition HEC -2 Model Cwexist2.out H 2 Z o o 0 ~ o ~ ~ooo 0 o~ 0 © o 0 o 0 0 o ~o. o~ 0 0 ~g § ~2 '~5'g ooo~ 0 o o ~°o o o H o ~o°o 0 o E o 0 o ~? 7-' o § ~h?oo 0 E o 0 o E o 0 o oo~ o 0 0 OoO 0 0 ? ~?'~ o§o§~ o o ~o L) o o o 0 0 o° o ~ o°8 Om 0 u ~§~o~ o o 0 ~~ T~oo o o o o~ oo~ o° 0 ~ o o o o ~ o° > oo° § ~oo oooo o o o o o o.oo 21~oo 0 U o o o o o o o ~o~o oo o ° 6 ~o§ HO00 '7, o oOO o ~ o o ro o ~ o ~ ~° ~ ~ ~o~ o ~OoO o o© o & o o o o o 0 U U oo~ o ~ o o o ~ ~oo o°O~ ooo~ ~o~ > ~ ~ ooo c~ o ~ ~ o~ u o o o~ o o o o 0 o o o 0 0 o U o ~ o° 0 ~ oo~ ~ oo~ o7~oo · .S g © 0 o~¥ g 0 0 .5 o o o H 0 o o 8§ 80 08 oo oo ..... o §§ oo o o o o o o o o o o o o oo oo oo §Oo oo°° §8 ~§o°§ §§ goo ~§ oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o oo gg goo §§ gE §§ §§o°§ ~§oo°° §~ §g o o o ~ o o o o o o o o o o o o o o o o o o o o o o o o oo oo oo o oo oo oo §§ §§ §g gg o .... go oo oo §§ ~§ oo oo oo oo oo o ~ oo oo oo oo oo oo oo oo gg gg §§ gg §g gg oo g§ o§ oo°° oo°° oo°° oo°° o~ ~ ~ oo ~ oo oo oo o~ o oo ...... oo oo §§ §g gg oo §§ g§ §§ ........ oo oo oo 0 oo oo oo oo oo oo oo oo oo oo oo oo oo o Z oo oo oo oo oo oo o ..... g§ §~ §g ............ oo oo oo oo oo oo oo oo oo oo ~ o o o o o o o o o o o o o o ~ o ~ ° o o o o ~ o ~ o o o oo o oo oo o oo o oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o§ og oo gg ggo°g §g §goo°° g§ gg oo oo oo oo o gg goo ggoo°° §g §§o° oo oo §o goo §§ §g goo g .... §§ §g oo g§ ........ oo gg §§ o oo oo oo oo oo oo ~ oo o g ~ ..... ~o ~ ~ o~ ~ ~ u ....... o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo oo oo oo oo oo oo oo oo ............ ~ ~ oo oo oo o 0 o o o o o o o oo go gg o§ ~g oo go §g og oo gg §g oo o oo o o m oo o o oo o oo oo oo oo oo oo oo oo oo oo oo o oo oo oo oo oo oo oo oo oo oo oo ~ °° oo o oo oo oo oo oo oo oo oo oo oo oo oo oo ....................... ~o gg o oo oo oo oo oo oo oo oo oo oo oo oo oo o oo oo oo oo oo oo oo oo oo oo oo oo oo .... g§ ~go°§ ggo°§o°§oo°° §§ goo go o oo oo oo oo oo oo oo oo oo oo oo o~ oo M ~o oo oo oo oo oo oo oo oo oo oo oo oo oo o ~ ~o oo oo oo oo oo oo oo oo oo oo oo oo oo o ........... § §§ ~Oo o°§ g§ g§ §°o o°§ §S ....... g ..... o oo og ....... ~ oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o ~ oo oo oo oo oo oo oo oo oo oo oo oo ~ ~ o ~o°o°°o o°o° °oo .... oo o§oo oo oo o°o© o°o© o°°o ~o© o°§ o© ....... ~ oo §§ §§ §§ ........ oo°° oo°° 0 oo oo oo o oo oo oo oo oo ~ oo oo oo oo oo oo oo oo oo oo oo oo oo oo o o oo oo o ~ ~ oo oo oo oo oo oo oo oo oo oo o oo oo oo oo oo oo oo oo oo oo o oo oo oo oo oo oo oo oo oo oo o oo oo oo oo oo oo oo oo oo oo o oo g§ g§ o ............... o ..... o oo oo ~ ~ oo oo oo oo oo oo oo oo oo oo o o o o o o o o o o o o o o o oo oo oo oo oo oo ......... g~ oo o o ~ oo oo oo oo oo oo oo oo oo oo oo oo o .... §g §g oE oo o§ ....... § o§ ...... oo ~§ ~ §§ oo o~ §~ oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o oo oo oo oo oo oo oo oo oo oo oo oo ........ oo §§ §§ §~ g§ §o §g ~o o o ..... ~ ........ ~o og oo ~o ~ oo oo oo oo oo oo oo oo oo oo oo oo ~ oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o o o o o o o o o o o o o o o oo oo ~o~o oo oo oo oo oo ........ ooo o§ ~§ §§ §Oooo°° §§oo .... oo oo oo oo oo ~ ~ oo oo oo oo oo oo oo oo oo 9 U o o o o o o o o o o o o o o (DO 0 o o 0 0 0 (DO o o 0 0 0 0 0 0 ........... °o ooo oo ~§ ~ ~§oo°°o°~oo oo o §~ §~ ~o ~ §~o o~°~ §~o~°~ oo oo §§ §~oo°° ~ ~§ §§ o© ~o ~ ~ o O0 O0 O0 0 zz z zz zz z~ zz zz zzz zz zzzz zz z 0 U 0 o oo o o oo oooo oo oo oo o o oooooo oooo ooo o oo o o oo oooo oo oo oo o o oooooo oooo ooo oo °~ ~ Appendix C Digital Input Data Files on Disk