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Villages C LOMR-SY 860510 JOB NUMBER 85048 DATE: 5/10/86 A FLOOD INSURANCE STUDY REVISION OF ELM FORK OF THE TRINITY RIVER IN COPPELL, TEXAS FOR MR. DAVID WETH VISTA PLEX DEVELOPMENT, INC. S1Sl BELTLINE ROAD, SUITE 1016 DALLAS, TEXAS 75240 BY CAFFEY AND MORRISON, INC. 2218 ARLINGTON DOWNS ROAD, SUITE 105 ARLINGTON, TEXAS 76011 & ,, INC. CAFFEY AND MORRISON, INC. ENGINEERS -- HYDROLOGISTS May 10, 1986 Mr. David Heth Vista Plax Development, Inc. 5t5! Beltline Road, Suite 1016 Dallas, Texas 75840 Re: Flood Insurance Study Revision of Elm Fork of the Trinity River in Coppell, Texas/Job No, 85048 Dear Mr. Hath: In late August, 1985, we discussed the flood plain/floodway on your property in Coppell, Texas. This flood plain involved Denton Creek and the Elm Fork of the Trinity River. At that time, we had two options: begin a,study to redefine the flood plain or wait for the Corps of Engineers to finish their ongoing study of the area. To help with this decision, we contacted the Corps and got an estimated completion date for their study. The Corps told us that they would be finishing the study by September. Me took them at their word and recommended that we wait until the Corps finished this study so that we would have solid data to use as a baseline. As you are well aware, the Corps moved their completion dates from September to October to November to December to January to February, and finally our last conversation indicated possibly another two or three more months. Conversations with the Federal Emergency Management Agency indicated that they don't expect results for possibly six months. In light of this schedule slippage by the Corps, we felt that the only way to obtain a project completion within a reasonable time frame was to complete the study through the area ourselves. This was a formidable task since a number of projects are underway or completed for the area. We first tried to obtain data regarding completed projects from the cities in the area, but they could provide very little help. Finally, with the aid o¢ Mr. Bill Black of the Fort Worth District Corps, we were able to obtain a11 of the needed data. Before we began the modeling procedure, we contacted Mr. Matt Miller of FEMA and Mr. Mark Headly with FEMA's technical consultant, Dewberry and Davis, to determine FEMA requirements. We then completed a flood plain and floodway model that matched Corps' data; both the present data and the recently completed Corps' Environmental Impact Statement (EIS). On May 6, we were ready to present this data to the 2212ARLINGTONDOWNSRD.,SUITE105 · ARLINGTON, TEXAS76011 · (817) 649-8110 · METRO 640-8101 Caffey and Morrison, Inc. Elm Fork/Trinity River/Coppell, Tx. Hay 10, 1986 Page -2- Corps. However, as a last check to verify the data, we contacted Mr. Bernard Massey of the United States Geological Survey (USGS), Hr. Hassey told us of data that the USGS had that indicated the flood stage elevations to be higher than we had previously believed. This USGS data was based on a measured gage height in the Flood of November, 19Bt. This information sent us back to square one. ~e had to develop a new model which corrected the previous data and correlated with the results of the USGS gage. This brings us to our present study. This study shows that your property is subject to Flooding at an elevation slightly above ~5 feet NGVD. However, the correct Floodway delineation is outside the area of your proposed development. This means that upon FEHA and City of Coppell approval of this flood study, the area could be developed by filling to an elevation above the lO0-year flood levels. Even though the City of Coppell and FEHA may approve this study, we do not recommend completion of any superstructure work beyond filling until the Corps of Engineers Finishes their Flood study. Their work should be speeded some since we provided a copy of our results to them, coded on Floppy disks that can interface with their computer systems. The specific fill work required on your property would require approximately 30,000 cubic yards of Fill to provide two to three Feet elevation of the proposed structures above the lO0-year Flood. Since Denton Creek is under the backwater influence of the Elm Fork of the Trinity River, any excavation work in Denton Creek would have no effect on the lO0-year flood elevations in the area. ~e have had some preliminary discussions with FEMA concerning this. Our determination is that fill can be taken from Denton Creek with no adverse impact on the tOO-year Frequency water surface elevation. Our only concern regarding the removal of fill material from Denton Creek is that this action might involve a determination by the Corps of Engineers under their ~04 permitting process. It is our understanding that they have no Jurisdiction unless fill is being discharged into water ways. This case involves only removal of material. However, we don't know For sure if any wetlands exist on your property. That determination must be made by the Corps. ~e will contact them and obtain their determination upon your authorization. CaFFey and Morrison, Inc. Elm Fork/Trinity River/Coppell, Tx. May I0, 1986 Page -3- We appreciate this opportunity to work with you on this project and look forward to our continued association, Sincerely, Caffey and Morrison, Inc. Ronald W. Morrison, P.E. Vice President TABLE OF CONTENTS PAGE NO, A. Introduction ........................................... A.1. Scope of Work A,2, Data Assembly and Coordination B. Hydrology .............................................. 7 C, Hydraulics ............................................. 8 C.I. Model Configuration C,2. Cross Sections 0.3. Flood Delineations C.~. Water Surface Profiles C.5. Manning's Coefficient C.6. Floodways and Encroachments C.7, Structures C.B. Velocities D, Conclusions ............................................ Appendices 1. Flood Profiles from the City of Coppell FIS 2. FEMA Floodway Maps/City of Carrollton and Coppell and Corps Worn Map 3, Comparison Flood Profiles for 1981 and Present Conditions 4. Flood Profiles for Present Conditions 5. Floodway Delineations for Comparison 6. Floodway/Flood Plain Delineations for Present Conditions 7. t981 Conditions Cross Sections B. Present Conditions Cross Sections 9. USGS Gage Calibration HEC-2 Model 10, Baseline HEC-8 (1981 Conditions) Model 11, Present Conditions HEC-~ Flood Plain and Floodway Model Reference Models Obtained from Fort North District Corps of Engineers A. Introduction The Elm Fork of the Trinity River Flows From Lake Lewisville, a Corps of Engineers discharge regulated reservoir, southward. It Flows through Lewisville, Hebron, Carrollton, Coppell, Farmers Branch and Dallas, Texas. It Finally joins with the West Fork of the Trinity River in Dallas, Texas, VICINITYMAP ~ 0 2 4 6 8 10 12 14 16 me. OF Mlbg~ -------- , i o 4 8 12 16 20 The area through this portion of the metroplex is experiencing an unusually high growth rate. ProJects are projected in a number of areas along this watershed. In August of 1985, we were asked to evaluate the Flood plain and Floodway along the Elm Fork of the Trinity at Sandy Lake Road. At that time, the Corps of Engineers was under contract to the Federal Emergency Management Agency (FEMA), completing a Special Problem Report as part of the Lewisville Flood Insurance Study. This project would define the Flood plain and Floodway through the area. The Corps had been working on this project since )ate 1984. Our position at that time was to wait until the Corps completed this study and define the area based on the Corps' work. However, we waited nine months and saw no evidence that the work was close to completion, Conversations with FEMA, Region 6 and National clearly indicated that the project might take another six months or more. We Felt that we had taken reasonable steps to obtain an official study and must now proceed on our own. First, we contacted FEMA headquarters to determine their advice on how to proceed. We were told that in order to obtain an accurate study, we must determine all the projects that might affect the study through our area. Our model should include those 'that have been completed or will be completed shortiy. Following this line of reasoning, we contacted the Cities Coppeil and Carrollton in order to obtain this information. These cities could not provide the data we needed, Me then contacted the Corps of Engineers and they provided the data we needed. Using data on proJects that the Corps agrees should be considered, we developed a Flood plain and Floodway through the area. In order to benefit the Corps in their study and to provide a consistent model, we started our model upstream Beltline Road-in Dallas and carried the model through Interstate Highway 35 in Leuisville. We agreed to provide our model to the Fort Worth District Corps in the form of 360 K floppy disks that will interface with their IBM AT computers. A.I. Scope o¢ Work The purpoge o¢ thi9 study i9 to obtain conditional approval of our Floodway determination in order to provide an area available For development. The general area of our concern is shown below. The procedure followed is outlined below: 1, Obtain the models available for the area between Beltline Road in Dallas and Interstate Highway 35 in Lewisviile. These models include the Corps- Engineers Flood Insurance Study, Albert H. Halff and Associates model, two models by Nathan O. Maiers Consultant Engineers and a model developed by Freeze Nichols and Associates. 2. Develop a calibration model for conditions which existed in 1981 in order to match the latest United States Geological Survey (USGS) gage data for the flood which occurred in that year. 3. Input lO0-year discharges into this calibration model with necessary modifications to develop an appropriate lO0-year frequency baseline model. 4. Input the changes made by the projects completed through the area into the baseline model to obtain an existing condition model. 5. Develop a floodway for this existing condition model. This floodway matches the floodway alignment established by the earlier flood insurance study model everywhere except where the flood plain changes sufficiently to make this floodway alignment inappropriate, A.2, Data Assembly and Coordination This study involved a substantial effort in the area of data assembly and coordination. The Corps of Engineers is in the process of restudying this area under contract to FEMA. This restudy was a result of the recalibration the USGS gage at Sandy Lake Road. This recalibration was a result of a flood that occurred in November of 198t. Because of this new USGS data, the Corps and FEHA agreed that the present FEHA base flood elevations through this area are not correct, and they are probably wrong on the nonconservative side. For this reason, we could not use the FIS as a baseline model. The following is a short chronology of our efforts at obtaining data: Date Action August 1985 Contracted with Vista Plex to determine flood plain and floodway boundaries. August 1985 Contacted the Fort Worth District Corps Engineers to determine Elm Fork Trinity (Lewisville FIS) study completion. They advised their completion would be late September. 3 October 1985 Contacted the Corps regarding completion schedule of Elm Fork Trinity study. They advised their completion would be late December. December 11/85 Contacted Matthew Miller, FEMA Project Officer, regarding the Elm Fork study. Mr. Hiller was not aware of any projected completion dates for the Corps' Elm Fork study. He suggested that FEMA would not approve any floodway revisions on the Trinity unless the model incorporates all FEHA approved revisions and appropriate conditional revisions. Ne also said that the latest preliminary Corps discharges should be used in the model. December 13/85 Contacted Mr. Bill MayField, Nydraulics Branch, Fort Morth District Corps, and obtained latest Corps Elm Fork model, January 1986 Contacted the Corps regarding completion of Elm Fork study. They advised their completion would be late February. January 14/86 Contacted Hattheu Hiller, FEHA Project Officer, regarding the Elm Fork study. Ne had no information about completion dates. Me discussed the requirements to obtain FEHA approval of the study if we completed it ourselves. He suggested the following approach: 1, Obtain best Corps data regarding the Elm Fork model. 2. Obtain copies of all revisions and appropriate conditional revisions that might affect the project, 3. Put together a model with all of these. 4. Submit the model through Coppell to FEHA. 5, FEMA will review and issue comments, If they are favorable, the results will have to be compared to the Corps' study when it comes out before Final map revisions are made. January 14/86 Contacted Mr. Mark Headly, Director of Revisions for Dewberry and Davis, FEMA's technical consultant. We wanted to know how to obtain data submitted regarding projects along Elm Fork Trinity, Mr. Neadly said that we must contact Region 6 and request data. The process would probably be lengthy. We also discussed the requirements addressed by Mr. Miller. Mr, Headly concurred with everything Mr. Miller had said. January 22/86 Met with Diane Leatherwood of FEMA Region 6 regarding revisions and conditional revisions approved by FEMA. She provided general information about projects submitted to FEMA, but could not provide any specific information regarding hydraulic models. March 3/86 Contacted Mr. Dell Greer, Director of Natural and Technological Hazards Division, regarding the Elm Fork study. He provided general information. However, we were unable to obtain any specific information regarding status of the study or hydraulic data. March 26/86 Met with Mr. Bill Black of the Fort Worth District Corps. Me received needed information and direction (see copy of letters at the end of this section). April 3/86 Sent a letter to Mr. Matthew Miller, 'FEMA Project Officer, explaining our approach and asking for guidance if our approach was not correct (see copy of letter at the end of this section). May 7/86 Met with North Texas Council of Governments and the Fort Worth District Corps of Engineers in a meeting about the Trinity River Environmental Impact Study. Obtained copies of the report to use as comparison of our results. May 13/86 Contacted Mr. Bernard Massey and Mr. Walter Lear of the USGS regarding gage records at the Sandy Lake gage. They confirmed gage data and provided a copy of the gage rating curve. Mr. Lear also said that USGS observed that the weir at the Carroilton dam ISandy Lake Roadl was flooded above 12000 ors and had no effect on the water surface elevation at that discharge. 5 May 21/86 Presented the Elm Fork study results to Mr. Bill Black, Fort ~orth District Corps. ~e also provided all data on 360 K floppy disks that would interface with the Corps' IBM AT computers. May 21/86 Appointment made with Mr. Ed Powell, City Engineer, City of Coppell, Texas, to explain our report and ask for their review. May 23/86 In anticipation of a favorable response from the City of Coppeli, we sent final copies of our Elm Fork study to FEMA Region 6, FEMA Headquarters, Corps of Engineers, the United States Geological Survey and the North Texas Council of Governments. 6 CAFFEY AND MORRISON, INC. ENGINEERS -- HYDROLOGISTS March 31, 1986 U.S. Army Corps of Engineers Fort Worth District Flood Plain Management Section 819 Taylor Street Fort Worth, Texas 76102 Attn: Mr. Bill Black Dear Mr. Black: We are writing this letter to thank you for the help you provided in obtaining data for the Elm Fork of the Trinity River in the area shown on the attached map. We have been asked to develop a flood plain and floodway model through this area. As you are well aware, the Corps is in the process of developing a flood insurance study (FIS) revision that covers the area in question. Our objective in this project is to develop a study that is as close to the final Corps study as possible. At our meeting of March 26, it was our intention to determine the information which you are Using to develop the Corps model of this area and to follow, as closely as possible, your modeling procedure. To the best of our knowledge, we have obtained a base model and all of the projects that are to be included in the FIS to update that base model to the current conditions. The updated model will then match, as closely as possible, the final model which the Corps will develop for the FIS. The following is a listing of all the models to be considered in this project. It should be noted that we are only concerned with the Elm Fork of the Trinity River between Beltline Road and Interstate Highway 35. We are planning to develop a floodway model for an area just upstream and downstream of Sandy Lake Road: 1. Corps of Engineers Preliminary Flood Insurance Study Model for Lewisville, Texas. Title: Executed 6 Feb. 84 7:41:51 DD Elm Gage Q's Low Flows (10 & 50 Yr.) Elm Fork Between Lewisville Dam and R.M. 23 Elm Fork X Sections Survey Mar 1983 D.R. Gregory 2212 ARLINGTON DOWNS RD., SUITE 105 · ARLINGTON, TEXAS 76011 · (817) 649-8110 ' METRO 640-8101 Caffey and Morrison, Inc. Elm Fork/Trinity River March 31, 1986 Pa~e -2- 2. Corps of Engineers Elm Fork FIA Study. Title: a. "ACOE Ft. Worth" Printer: 9 -D-Gregory 19 Aug. 85 11:23:38 Terminal: 73 Elm Fork FIA Study Natural Conditions Sta 32.70 To Sta 342.60 X-Section Data: USGS Quads & Surveyed Chan & Valley Sec Levees Reflected On "D" Lines See File EFlVS For Upstream Comps File Created Jan. 77 -- Mayfield Revised 10-4-79 b. "ACOE Ft. Worth" Printer: 9 D-Gregory 19 Aug. 85 13:45:28 Terminal: 73 Elm Fork FIA Study Natural Conditions Sta 966.70 To Sta 1090.00 X-Section Data: USGS Quads & Surveyed Chan Sections This File Based On Revised USGS Gage Data (Carrollton NP) See File EF3VS For Downstream Comps File Created 9/23/83 Mayfield Modified For Regional EIS Jun. 7 85 D.R. Gregory Discharges For 10, 50, 100 & 500 Yr. Freq. c. "ACOE Ft. Worth" Printer: 9 D-Gregory 19 Aug. 85 11:24:42 Terminal: 73 Sections Added Or Modified For Regional EIS -- 895.9, 905.3, 908.0, 911.3, 930, 937.8, 952.7, 966.2 See File EF2VS For Downstream Comps See File EF4VS For Upstream Comps File Created Jan. 77 -- Mayfield Caffey and Morrison, Inc. Elm Fork/Trinity River March 31, 1986 Paqe -3- 3. FEMA Approved FIS Revision by Albert Halff and Associates. Title: a. 21 Jun. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "ELMBLTR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 06-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 10-Yr. Flood b. 21 Jun. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "ELMBLFWR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 06-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 100-Yr. Flood c. This model has not been approved by FEMA. However, it was suggested that we incorporate the data. 18 Sep. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "EMBLFWRR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 09-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 100-Yr. Flood 4. Engineering Report to the Dallas County Commissioners Court for the Proposed Creation of Dallas County Levee Improvement District No. 18. October, 1984 by Nathan D. Maier, Consulting Engineers, Inc. 5. Flood Plain Reclamation Study/Lakes of Coppell/Coppe!l, Texas. Prepared for Triland Investment Group. December, 1984 by Nathan D. Maier, Consulting Engineers, Inc. Caffey and Morrison, Inc. Elm Fork/Trinity River March 31, 1986 Page -4- 6. Flood Plain Reclamation Study/Keas Construction Tract/ Coppell and Carrollton, Texas. Prepared for Veenker Investments, Inc. July, 1985 by Nathan D. Maier, Consulting Engineers, Inc. 7. Flood Plain Study of Elm Fork, Trinity, at Interstate Highway 35 by Freese and Nichols, Consulting Engineers. The title of the run was: Carrollton, Texas HUN85107 Hunter.Dat;2 100-Yr. Discharge Elm Fork - Trinity River (109000 To 121540) F & N Encroachment Run June 24, 1985 - MJD The discharge values to be used in this final model will conform to those shown on the attached table, which we have obtained from the Corps. Again, we wish to express our gratitude for the help we received in piecing together this potential model. Without the CorDs' help, it would be impossible to coordinate all the considerations for this area. If we have left out anything which needs to be considered, or if any additional information becomes available, we would appreciate it if you would let us know. Sincerely, Caffey and Morrison, Inc. Ronald W. Morrison, P.E. Vice President cc: David Heth CAFFEY AND MORRISON, INC. "',.1 '~'-,- 13ENERRL LOCRTION i~RP I +~ ,~-~;~ ~' TABLE 5 (CONT' D) %~ /~US~E~T Ta . 1 ..... .~'-.:~ Co~D E~OB~ILI~ DISC~GE 0PT~0N ~ " I~ L0~TION . ' DA-SQ.~ 10 ~ 50 ~ I00 ~ 500 ~ 10~ ~0U~ T~ BA~ ~ - 11.59 ~1169. i718~. 20208. 25363. 34150. S~TING E~. 438.1 FT ~L 103 ~0U~ FROM BAC~ ~ D~ 12.59 ~575. 6897. 8163. ll0~A. 14379. TO E~ F0~ C0~. NO~: ~ FO~O~NG ~ ~OP~ DISHES FO~ ~ I~IC~ POI~ 0F I~ST 0N ~ (1) ELH FO~ D. A. BE~ %.Z7 6300.* 9000.* 21000.* 55000.* 62000.* D~ 3 g~ ~0~ BELOW C0~. ~ 7.82 6300.* 9000.* 21000.* 55000.* 62000.* 8 E~H FO~ BEL~ CO~. ~ ' 13.00 7125. 1~300. 21000.* 53000.* 62000.* S~T C~ (S.H. 121) 13 E~ FO~ B~0W ~ B~ 17.09 6300.* 9000.* 2!~00.* 55000.* 62000.* C0~. 2Z E~H FO~ B~OW CO~. ~ 36.25 142'02. 20469. 23507. 55000.* 62000.* 26 E~H FO~ ~0~ IH 35 g ~1.50 ~I335. 16055. 21000.* 55000.* 62000.* 33 E~ PO~ B~OW T~ER C~. 66.67 20575. 29687. 3~522. 55000.* 62606. CO~ (AT IH 35 E CROSSI~G) ~4 E~H FO~ B~OW C~OLLTON !04.00 2~317. 37075. ~A359. 60394. 84185. 50 E~H FO~ BELO~ H~ON B~CH 118.9~ 23~65. 35916. 42893. 59155, 82321. ~0~ · 54 E~M FO~ BELOW G~I~ ~32.17 23288. 36571. a4~86. 62061. 861S5. EXCEED LOC~ FLOWS. .. ,..:,.~ TABLE, 5 (CONT'D) 'y ' F..'~CU'~D oN 2.5 0C% 85 A~ ~'L:40:14 L EWI SV ILLE FLOOD INSUP. ANCE STUDY EL~H FOB~ TKINZT~ KIVEK LE%TISVILLE FLOOD INSUEANCo- SI~DI Co~D pKOB~ZLI~ DISC~E OPTION 1 I~ LOCATION DA-SQ.~ 10 ~ 50 ~ 10O ~ ~00 ~ S~F 65 E~H FO~ BELOW CO0~ B~ ~9.08 ~.~3080. 36924. 4~625. 62675. 86760. ' 63716 · 8803~. 80 E~ FO~ ~EL0W F~ S~ 158.06 ~3056. 37382. ~5292. 88 E~ FO~ BELOW ~S~2 ~- ~78.52 2Z123. 36821. 453a9. 66~85. 92754. CO~ · 97 ~ EO~ BELOW JOES ~ ~00.78 20253. 34174. 42665. 62076. 87822- CO~ 104 E~O~ BELO~ BA~ B~ 214.93 19867. 34362. 43216. 63477. 897~1- 107 ~OU~ ~0~ E~H FO~ AT 220.00 - 19497. 33838. 42678. 63039- 89367 · CM,, INC. ENGINEERS -- HYDROLOGISTS April 3, 1986 Mr. Matt Miller Federal Emergency Management Agency Federal Insurance Administration Office of Risk Management Risk Studies Division 500 C. Street $.W. Washington, D.C. 20472 Re: FEMA Map Revision for Tract of Land in Coppell, Texas Adjacent Denton Creek/Job No. 85048 Dear Mr. Miller: We have been asked to develop a flood plain and floodway model through the subject property (see attached map). The Fort Worth District Corps of Engineers is in the process of developing a flood insurance study (FIS) update of this area. Our objective in this project is to develop a study that is as close to the final Corps study as possible. On March 26, 1986, we met with Mr. Bill Black of the Fort Worth District Corps Flood Plain Management Services Branch. At that meeting, it was our intention to determine the information which was being used to develop the Corps model of this area and to follow, as closely as possible, the Corps' modeling procedure. To the best of our knowledge, we have obtained a base model and all of the projects that are to be included in the FIS to update that base model to the current conditions. The updated model will then match, as closely as possible, the final model which the Corps will develop for the FIS. The following is a listing of all the models to be considered in ~his project. It should be noted that we are only concerned with the Elm Fork of the Trini=y River between Beltline Road and Interstate Highway 35. We are planning to develop a floodway model for an area just upstream and downstream of Sandy Lake Road: 1. Corps of Engineers Preliminary Flood Insurance Study Model for Lewisville, Texas. Title: Executed 6 Feb. 84 7:41:51 DD Elm Gage Q's Low Flows (10 & 50 Yr.) Elm Fork Between Lewisville Dam and R.M. 23 Elm Fork X Sections Survey Mar 1983 D.R. Gregory 2212 ARLINGTON DOWNS RD., SUITE 105 · ARLINGTON, TEXAS 76011 · (817) 649-8110 · METRO 640-8101 Caffey and Morrison, Inc. Land Adjacent Denton Creek, Coppell, Texas April 3, 1986 Page -2- 2. Corps of Engineers Elm Fork FIA Study. Title: a. "ACOE Ft. Worth" Printer: 9 D-Gregory 19 Aug. 85 11:23:38 Terminal: 73 Elm Fork FIA Study Natural Conditions Sta 32.70 To Sta 342.60 X-Section Data: USGS Quads & Surveyed Chan & Valley Sec Levees Reflected On "D" Lines See File EF1VS For Upstream Comps File Created Jan. 77 -- Mayfield Revised 10-4-79 b. "ACOE Ft. Worth" Printer: 9 D-Gregory 19 Aug. 85 13:45:28 Terminal: 73 Elm Fork FIA Study Natural Conditions Sta 966.70 To Sta 1090.00 X-Section Data: USGS Quads & Surveyed Chan Sections This File Based On Revised USGS Gage Data (Carrollton NP) See File EF3VS For Downstream Comps File Created 9/23/83 Mayfie!d Modified For Regional EIS Jun. 7 85 D.R. Gregory Discharges For 10, 50, 100 & 500 Yr. Freq. c. "ACOE Ft. Worth" Printer: 9 D-Gregory 19 Aug. 85 11:24:42 Terminal: 73 Sections Added Or Modified For Regional EIS -- 895.9, 905.3, 908.0, 911.3, 930, 937.8, 952.7, 966.2 See File EF2VS For Downstream Comps See File EF4VS For Upstream Comps File Created Jan. 77 -- Mayfield Caffey and Morrison, Inc. 'Land Adjacent Denton Creek, Coppell, Texas April 3, 1986 Page -3- 3. FEMA Approved FIS Revision by Albert Halff and Associates. Title: a. 21 Jun. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "ELMBLTR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 06-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 10-Yr. Flood b. 21 Jun. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "ELMBLFWR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 06-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 100-Yr. Flood c. This model has not been approved by FEMA. However, it was suggested that we incorporate the data. 18 Sep. 85 Elm Fork Model Prepared By Albert H. Halff and Associates, Inc. "EMBLFWRR"RJ From X-Sec 680.0 (COE) U.S. Of IH 635 To Carrollton Dam (968.1) 09-85 Elm Fork Of Trinity River With IFCD Levee No Floodway 100-Yr. Flood 4. Engineering Report to the Dallas County Commissioners Court for the Proposed Creation of Dallas County Levee Improvement District No. 18. October, 1984 by Nathan D. Maier, Consulting Engineers, Inc. 5. Flood Plain Reclamation Study/Lakes of Coppell/CoDDell, Texas. Prepared for Triland Investment Group. December, 1984 by Nathan D. Maier, Consulting Engineers, Inc. Caffey and Morrison, Inc. Land Adjacent Denton Creek, Coppel!, Texas April 3, 1986 Page -4- 6. Flood Plain Reclamation Study/Keas Construction Tract/ Coppell and Carrollton, Texas. Prepared for Veenker Investments, Inc. July, 1985 by Nathan D. Maier, Consulting Engineers, Inc. 7. Flood Plain Study of Elm Fork, Trinity, at Interstate Highway 35 by Freese and Nichols, Consulting Engineers. The title of the run was: Carrollton, Texas HUN85107 Hunter. Dar; 2 100-Yr. Discharge Elm Fork - Trinity River (109000 To 121540) F & N Encroachment Run June 24, 1985 - MJD The discharge values to be used in this final model will conform to those shown on the attached table, which we have obtained from the Corps. Our purpose in sending this information to FEMA is to request that any information or assumptions which we have made that are incorrect or need to be modified, be corrected early in the game. If any modifications are needed, please call me at telephone (817) 640-8101 as early as possible. Your help is greatly appreciated. We will also provide a copy of our work to the Corps on floppy disk in case they want to use it in their FIS determination. Sincerely, ~rrison, Inc. Ronald W. Morrison, P.E. Vice President cc: Mr. R. Dell Greer, Chief Natural and Technological Hazards Division Federal Emergency Management Agency, Region VI 800 N. Loop 288, Federal Center Denton, Texas 76201-3698 Attn: Bert Carr Mr. David Heth CAFFEY AND MORRISON, INC. \ GENERRL LOCRTION COL~UTED PROBAI~ILIT~ DISCRAI%Gll; OPTION 1. .... IND~'Y LOCATION . ' DA-iQ.HI 10 TEA~ 50 YEAR I00 ~ 500 TEA~ .S~F 102 ~ T~ BA~ ~ - 12.59 ~1169. 17184. 20208. 25363. 34150. S~=!NG E~. t38.1 YT ~L TO E~H FO~ ¢0~. 1 (1) ~ FO~ D. A. BE~ 1.17 6300.* 9000.* ~I000.* 55000.* 62000.* D~ 3 l~ FO~ B~ CO~. ~ 7.82 6300.* 9000.* 21000.* 55000.* 62000.* 8 E~H FO~ B~ CO~. ~ ' 13.00 7~5. l~]O0. 11000.* 55000.* ' 61000.* 13 ~H FO~ B~O~ ~ B~ ~7.0g 6300.* g000.* 2!~00.* 55000.* 62000.* ~1 ELH ~0~ S~0~ CO~. ~ 36.~6 14Z'O~. Z0469. 23507. 55000.* ~1000.* Z6 E~ FO~ ~O~ IH 35 E &[.50 [~35. ~6055. 21a00.* 55000.* 33 ~H EO~ S~O~ T~E~ Cl. 66.67 20575. 29687. 345Z/. 55000.* CO~ (AT ~H 35 E 44 ~H FO~ B~O~ C~O~TON 104.00 24317. 37075. 44359. 60394. 84185. )0 E~ F0~[ B~0W E~OH B~CH 118.94 23165. 35916. 42893. 59155. 82321. 54 E~H FO~ BELO~ G~ 132.L7 23288. 36571. 44L86. 6206~. 86~85. *DISC~CE - F~q~llCf ~L~ES FROM g~E LE'~ILLE CONS~E~D 'C~TZC,~ FLO'JS' ~ THeY RICED LOC~ FLOWS* LE~rls~ILL~ FLOOD INSUI~/~ STUDI ~ LO~O~ DA-SQ.~ 10 ~ 50 ~ 100 ~ 500 65 E~ FO~ B~OW CO0~ B~ L39.08 · 23080- 36924. 446Z5. 62675. 86760. co~. ' :ii'::; ;;> ' O~ ~ 63716 80 ~ ~0~ B~ ?~ S~ L58.06 ~3056. 37382. 45292. GO~. '. 88 E~ FO~ B~ ~~ ~' 178.52 22123. 36821. ~53&9. 66~85. 9275~. 97 ~ ~O~ B~W JOES ~ 200.78 20~3. 3417t. ~2665. 62076. 87822. 10a ~'FO~ BELOW B~ B~ 2L~.93 19867. 3~362. 43116- 63~77. 8971L. ~07 ~0~ ~H E~ FO~ ~ ilO.00 ~.L9~97. 33838. il678. 63039. 89367. ~0~ B. Hydrology All discharge values used in this report for the tOO-year frequency were obtained from the Fort Worth District Corps of Engineers and represent the latest information available to date. A copy of the discharge values is shown on the following page. In addition to these discharge values presented by the Corps, we also obtained discharge values used in the current Draft Environmental Impact Statement (see following pages). These values compare favorably with those presented earlier by the Corps. In order to match the USGS gage station at Sandy Lake Road, we obtained the 198t discharge value of 21100 cfs at a recorded gage height (plus datum) of ~4~ feet NGMD. We also obtained the' mean annual flow for the 1981 water year of 4300 cfs, This was used to determine the normal flow elevation. f ''[iL "' "" L~~ ~OOD ~SU~ S~O~ ..- CO~D PROB~L~ DISC~GE OPTION I ' " ~ LO~TION. DA-MQ.~ 10 ~ 50 ~ I00 ~ 500 ~ .S2F 102 ~OU~ ~U BA~ ~ - 12.59 11169. 17184. 20Z08. 25363. 3~150. 103 RO~ FROM BAC~ ~ D~ 12.59 4575. 6897. 8163. 11044. 14379. [O~: ~ FO~O~M~ ~ ~O~ DISHES FOR ~ %~%~D POI~ OF I~ST ON ~ E~M FO~ % (1) Z~ YO~ D. A. Bg~N 1.27 6300.* 9000.* Zl000.* 55000.* 62000.* D~ 3 ~ FO~ B~OW CO~. ~ 7.82 $300.* 9000.* 21000.* 55000.* 62000.* 13 E~ ~O~ BELOW ~ B~ 17.09 6300.* 9000.* zl6oo.* 55000.* 62000.* co~ · zl E~M FO~ B~OW CO~. ~ 36.26 142'0~. 20469, 23507. 55000.* 62000.* 2~ E~M FO~ ~0~ IE 35 E ~1.50 11335, 16055. 21000.* 55000.* 62000.* 33 E~M FO~ B~OW TL~E~ C~. 66.67 20~75. 29687. 34~2~. 55000.* 62606. CO~ (AT IE 35 E CROSSZMG) 44 E~M F0~ B~0W C~0LLTON 104.00 243~7. 37075. 44359. 60394. 50 g~ FO~ B~OW EU~N B~CE 118.94 23165. 35916. &2893. 5915~. 82321. C0~ · 54 E~ FO~ BELOW G~I~ !32.~7 23288. 36571. 44186. 6206~. · DISC~GZ - F~Q~ICf ~LZASES FROM L~E LE~!LLE CO:[SIDE~D "C~ITIC2~ FLCWS' '~E~ TF~Y EMCEED LOC~ FLOWS. E.XiI~UTED ON Z~ OCT 8§ AT 1!:40:i~ LEW"I fig' ILLE FLOOD I~N-~URANCE $~DY ET.H FOF, K TtI.-"tlT'~ 11~1 LEWIMVILLE FLOOD INSURANCE STUDY I~ LO~TION DA-SQ.~ 10 ~ 50 ~ 100 ~ 500 ~ S2F 65 E~ FO~ BELOW COO~ B~ : i39.08 ~ 23080. 3692~. 44625. 62675. 86760. CO~. '~ .'~:- i~,'. · 75 EUM Y0= B~OW ~ A SLUICE "';:~i43-~g::::f'~'~1661 37381. 45077. 63089. 86889. 80 E~ EO~ S~OW F~ S~ 158.06 ~056. 37382. 45292. 63715. 88032. 88 ~ ~0~ S[LOW ~~ ~- ~78.5Z 22~. 3682~. ~53~9. 66485. 9275~- CO~ · - -. g7 [~ EO~ B~OW JO~S C~ 200.78 20253. 34~74. 42665. 62076. 87822. C0~ ~04 S~'?O~ ~ZLOW BA~ S~ 2~.93 ~9867. 34362. 432~6. 63~77. 897~. ~07 ~0~ ~OM ~ ~0~ A~ 2~0.00~.~9~97. 33838. ~678. 63039. 89367. ' IJ~ TABLE OF CONTENTS HYDROLOGY AND HYDRAULICS HYDROLOGY APPENDIX ~'] TRINITY RI~R REGIONAL ElS I X~ I. Hydrologic Analysis ~ A. Scope of Work - Purpose and Goals B. Drainage Area C. Geosraphical Limits of Channel Modification D. Model Development' 1. Callbrated Model '"~ 2. Hypothetical Precipitation ~, ~ a.~ Precipitation Source b. Precipitation Duration c. Precipitation Distribution d. Time to Peak 3.Precipitation Losses 4Urbanization Estimates g. Scenarios Investigated ] A-1 Sub-Area Division of Drainage Area A-2 Urbanization Curves 4 A-3 Urbanization Cu~es APPENDIX A - HYDROLOGY AND HYDRAULICS I. Hydrologic Analysis A. Scope of Work - Purpose and Goals The purpose of the Hydrology portion of the Trinity River Regional Environmental Impact Study (EIS) was to evaluate the magnitude of flows in the Trinity River above its confluence with Five Mile Creek and portions of two of its major tributaries, the West Fork and the Elm Fork. Ultimate general watershed urbanization and alternative flood, plain development and channelizatlon options were considered. B. Drainage Area The area modeled in this study consisted of the entire drainage area of the Trinity River upstream of the point where Five Mile Creek flows / into the Trinity River near the intersection of the Trinity River and Interstate Highway 20 (about 10 miles southeast of downtown-Dallas). The total drainage area at that point of the Trinity River is' approximately 6275 square miles. The total drainage area of the Trinity River above the Dallas gage is 6106 square miles and at the Confluence of the Elm Fork and the West Fork is 6061 square miles. The terrain elevation varies from 1200 feet NGVD at the headwaters of the West Fork of the Trinity River approximately 35 miles south-southwest of Wichita Falls, Texas to 380 feet NGVD at the confluence of Five Mile Creek and the Trinity River. The Trinity River Watershed as described above is located in a region of temperate mean climatological conditions, experiencing occasional extremes of temperature and rainfall of relatively short duration. The National Oceanic and Atmospheric Administration Station at Fort Worth, Texas shows an average annual rainfall of 32.3 inches during a recent ten year period. The extreme annual rainfall values since 1887 are a maximum of 51.03 inches occurring in 1932 and a mimimum of 17.91 inches occurring in 1921. The mean relative humidity is 65 percent and the average temperature is 65.8 degrees. Generally the major storms experienced in the Trinity River Watershed above the Five Mile Creek confluence are produced by heavy rainfall from frontal-type storms which occur in the spring and summer months, but major flooding can also. be produced by intense rainfall associated with localized thunderstorms. These thunderstorms may occur at any time during, the year but are more prevalent in spring and summer months. C. Geographical Limits of Channel Modification The study area of interest extends from the downstream end of the proposed Dallas Floodway Extension (approx. the confluence of Five Mile Creek and the Trinity River) upstream on the West Fork of the Trinity River to Riverside Drive in Fort Worth, and upstream on the Elm Fork of the Trinity River to Lewisville and Grapevine Lakes. The river channel between these geographical locations was conceptually modified to reflect the conditions of each scenario to be evaluated. D. Model Development The area modeled was divided into 108 sub-areas in order to be responsive to the timing of each major tributary's runoff contribution and also to obtain detailed flow information (flood hydrographs) at all major points of interest on the West Fork, Elm Fork, and. the Dallas Floodway and Floodway Extension of the Trinity River. Figure A-1 shows the sub-area arrangement. The computer program used to develop the hydrologic model for this study was the HEC-1 Program 723-X6-L2010 developed by the Corps of Engineers' Hydrologic Engineering Center at Davis, California. This HEC-1 model is designed to simulate the surface runoff response of a river basin to precipitation by representing the basin as an interconnected system of hydrologic and hydraulic components. 1. Calibrated Model The hydrologic model was first developed for calibration purposes to reflect the.1985 urbanized conditions of the drainage area without Ray Roberts and Joe Pool Lakes. The model was calibrated by adjusting hydrologic parameters such as "time to peak" and "precipitation loss rates" within reasonable limits in order to match as closely as possible the peak values of five different frequency-floods at several streamflow gaging stations. The values of the peak flows of frequency-floods at any particular gage was determined by performing a frequency-flow analysis from the record of flows at that gage. The time period covered by the gage record of flows was selected to start in 1953, since all major reservoirs were in place by 1952, and to continue to the latest published flow. This allowed the recorded flows to generally reflect watershed conditions as they were in 1985. The only reservoirs that were activated after 1952 were Lake Arlington in 1957, Lake Amon Carter in 1956, and Lake Weatherford in 1957. Ail three are small structures with no flood control storage and have a minor influence on the flow gages downstream. Streamflow gages in the study area are shown in Figure A-1. As can be seen from Figure A-I, two stream flow gages were deactivated by the establishment of Ray Roberts and Joe Pool Lakes were considered existing Roberts Lake. Ray for 1985 and ultimate development EIS scenario evaluations. 2. Hypothetical Precipitation ao Precipitation Source The hypothetical precipitation for each frequency-flood was developed using data from National Weather Service Technical Paper 40 (TP40), National Oceanic and Atmospheric Administration Technical Memorandum NWS Hydro-35, and Corps of Engineers Civil Engineer Bulletin No. 52-8 ("Standard Project Flood Determination"). Figure 15 of TP40, depth-area-duration curves, was used to adjust the rainfall for watershed size. b. Precipitation Duration A 24 hour storm duration was useH for the frequency storms (10 year, 25 year, 50 year, and 100 year storms). The duration of the Standard Project Storm was adopted as 96 hours. c. Precipitation Distribution The rate (or intensity) of precipitation for all storms was assumed to remain constant throughout any one-hour time interval although it changes from one time interval to another. The rate (or intensity) of precipitation for the frequency storms (10 year through 100 year) was assumed to be evenly distributed over the entire drainage area, but the rate of precipitation for the Standard Project Storm was assumed to be distributed according to an elliptical pattern where the rate of precipitation is inversely porportional to the distance from the elliptical center. Only one elliptical storm center, which was the estimated critical center for the Dallas gage, was evaluated. Figure A-1 shows the elliptical pattern. d. Time to Peak Time .to peak was developed for each sub-area using methodology described in "Synthetic Hydrograph Relationships, Trinity River Tributaries, Fort Worth - Dallas Urban Area" by T. L. Nelson, 1970. An urbanization curve indicates elapsed time (time to peak) from the midpoint of a unit duration of rainfall to maximum runoff for a given sub-area. The geographical characteristics of the sub-area such as length of major stream (L), the location of the center of gravity (Lca), percent urbanization, and the overall slope (S) of the major stream determine the entering arguments for the urbanization curve from which "time-to-peak" for the sub-area is extracted. The "time-to-peak" used for each sub-area was generated from the Fort Worth-Dallas East-West Cross Timbers Curve and Blackland Curve by interpolating between them, based on the percentage of each soil type within the sub-area. The percentage of soil type was derived from a Soil Survey report for each county published by the U.S. Department of Agriculture. The East-West Cross Timbers and the Blackland Urbanization Curves are shown in Figures A-2 and A-3. 3. Precipitation Losses The block loss method of estimating precipitation losses was used in this study. Two different loss rates were used: (1) the initial loss which must be satisfied before any runoff occurs and (2) a constant loss in inches-per-hour which continues after the initial loss has been satisfied. The values of both losses vary with the return. frequency of the storm. The standard values of both loss rates were initially used in the computer model. Comparisons were made between "frequency analyses peak flows" at the major stream gages in the area and the "model computed value of peak flow" at the same gages. Adjustments were made to the loss rates to improve the comparisons of peak flows at the gages. The adjusted values were used in this study. This was the method of calibrating the model to produce results that agreed with peak flow values determined from gage frequency analyses. 4. Urbanization Estimates Estimates of the amount of urbanization for each sub-area in this study for 1985 Existing Conditions were made by referring to the most recent~ maps and charts available. In an effort to be as accurate as possible in estimating "percent urbanization" expected to exist as ultimate development (Future Conditions), a request was made by the Corps of Engineers, through the North Central Texas Council of Governments, for information from the' major cities in and around the Metroplex as to their projected future growth rates. Thirty-two cities responded to the request for future growth estimates in some fashion, and that information was considered in the estimate of ultimate urbanization percentage ior each sub-area in the computer model. These urbanization estimates for ultimate development were used to evaluate all scenarios except the 1985 Existing Conditions scenario. E. Scenarios Investigated Ray Roberts and Joe Pool Lakes were considered in Place for 1985 and ultimate development conditions. Comparative discharge-frequency data were developed for "Future Without" considering ultimate Watershed urbanization with storage-discharge data based on 1985 river channel conditions. Modified storage-discharge data for routing reaches for other scenarios and channel and overbank assumptions were. developed based on backwater computations. These modified storage-discharge data were input to the data file for the HEC-1 hydrology model and new discharge-frequency data were computed. Assumptions involved with each scenario are described in the Hydraulic Analysis portion of this Appendix. Computed frequency discharges are presented in Tables A-1 to A-21. F. Application of Results Frequency discharges generated for this study are comparative (for comparing impacts of various alternatives) and are not always critical for each discharge location. The storm center for the standard project storm and the rainfall area-reduction factors for frequency storms for this study have been set up to generate critical discharges for the Trinity River Dallas Floodway. Discharges generated for the West Fork Trinity River do not supersede the existing Flood Insurance Studies (FIS) for the West Fork. However, the new HEC-1 one-hour model does consider storage-discharge changes which have occurred due to recent projects along the West Fork. If the new HEC-1 model were used with critical centering and area reduction factors for storms along the the West Fork, critical frequency discharges could be generated for an updated FIS. Adopted discharges for the Elm'Fork were .developed using the NUDALLAS computer program. Reductions in loss rates (increases in runoff volume) due to increased future imperviousness were considered in the NUDALLAS analysis. The Elm Fork discharges are based on critical storm centerings and the peak flows for existing conditions will be utilized for new Flood Insurance Studies. storms for this s~udy have been set up to generate critical discharges for the Trinity River Dallas Floodway. Discharges generated for the West Fork Trinity River do no~ supersede the existing Flood Insurance Studies (FIS) for the Wes~ Fork. However, the new HEC-1 one-hour model does consider storage-discharge changes which have occurred due to recent projects along the West Fork. Adopted discharges for the Elm Fork were developed using the NUDALLAS computer program. Reductions in loss rates (increases in runoff volume) due ~o increased future imperviousness were considered in the NUDALLA$ analysis. The Elm Eork discharges are based on critical storm centerings and the peak flows for existing conditions will be utilized for new Flood Insurance Studies. TABLE A-24 EXISTING CI)NDITIONS DISCHARf~S (cfs) Station (ft) Station (ft) 10-Yr,, 25-Y~ 50-Y~ 100-Yr SP~ 0000* - 3750 19500 26000 33800 42700 89400 3750 - 6850 19500 26600 33800 42700 89400 6850 - 18450 19900 27000 34400 43200 89700 18450 - 34210 20300 27100 34200 42700 87800 34210 - 48700 21000 28000 35000 43500 90000 48700 - 55300 22160 29400 36800 45300 92800 55300 - 67800 23100 30400 37400 45300 88000 67800 - 71150 23200 30500 37400 45100 86900 71150 - 76000 23100 30400 36900 44600 86800 44200 86200 76000 - 86430 23300 30200 36600 86430 - 91130 23200 __ 30000 35900 429OO 8230O ~ 91130 - 96620 ~300 31300 37100 44400' 84200 ~96620 109000 23000 28000 35000 41000 79000 109000 - 113720 20600 25400 29700 34500 62600 113720 - 116050 11300 13800 16100 21000 62000 116050 - 141000 14200 17600 20500 23500 62000 141000 - 150350 6300 6300 9000 21000 62000 *West Fork/Elm Fork confluence C. Hydraulics This study was the most difficult we have encountered to date. The hydraulics were very complicated and to make matters worse, the USGS gage data didn't seem to match with the data developed by the Corps. It was not until we had spent considerable hours correlating runs that we began to get a clear picture of what was happening with the hydraulics. We will present a detailed analysis of our model in the paragraphs that follow, In order to further evaluate our model, we also reviewed the results of the Corps' recently released Draft Environmental Impact Statement (see sheets at the end of this section). Our final elevations were slightly higher than the results of the Corps' EIS study, For example, at Sandy Lake Road the following comparison is made for the existing condition: Corps lO0-year Frequency CafFey & Morrison tOO-year elevation, ft., NGVD frequency elevation, ft., NGVD 4~3.1 444.73 This variation will be discussed in later paragraphs. 0.1. Model Configuration The HEC-2 computer model was used for this study. The version used was a release dated November, 1976, updated May, 1984. Three models were developed for this study. They were: 1. Calibration model (ELMF.NV). This model matched 1981 condition5 using the discharge value measured in November of 1981 by the USGS at the Sandy Lake Road gage station. 2. Mean daily Flow determination (ELMF.NV). This model was used to determine the elevation of the normal Flow line. A smoother channel was assumed below elevations determined in this model. This will be discussed later. 3. Baseline model (ELMFIOO.EXT). This model matched 19BI conditions using lO0-year Frequency discharges developed by the Corps. It represents conditions that existed before revisions were made and most nearly approximates FIS conditions if they had been corrected with the USGS gage data of 1981. The purpose of this model was to determine the effect of the many changes to the Elm Fork through the area of our study. B 4. Improved model (ELMFIOi.IMP). This model is the existing conditions model, It includes all of the floodway revisions and appropriate conditional revisions. We considered "appropriate conditional revisions" as those which Mr. Bill Black considers appropriate in his development of the Elm Fork Flood Insurance Study. 0,2, Cross Sections Appendix 7 shows the cross sections as they existed in 1981. We attempted to create 1981 conditions by removing all post 1981 development which affects our project from these cross sections. We have also plotted the calibration elevation on the cross sections to demonstrate the shallow overbank conditions which will be discussed later. Appendix 8 shows existing condition cross sections. This represents conditions as they exist with all of the floodway revisions and appropriate conditional revisions in place. 0.3. Flood Delineations Appendix 2 is a copy of the FIS delineation from the Corps' work map. We have also included copies of applicable sections of the Carrollton and Coppeil FIS floodway maps. In order to show the floodway changes, we have included in Appendix 5, a map which shows the FIS floodway as it appears on the original FIS maps. Aisc shown are the floodway changes approved by FEMA by final revision or conditional revision. We have also delineated a floodway change at Sandy Lake Road which ue feel is the most technically accurate floodway through this area. Finally, we have included in Appendix 6, a flood plain and floodway delineation that incorporates all of the changes and represents present conditions. C.4. Water Surface Profiles Appendix 1 contains water surface profiles from the Coppell Flood Insurance Study. These are no longer appropriate mince the gage at Sandy Lake Road has made recalibration necessary. They are included for information and comparison. 9 Me have also included, in Appendix 3, water surface profiles which show a comparison between 1981 conditions connected to reflect the Sandy Lake gage and the existing conditions which include all revisions and conditional revisions. This profile shows that water surface elevations below the Sandy Lake gage have increased since 1981 while the elevations at and above the gage are approximately the same. Finally, in Appendix 4, we have drawn the present condition water surface profile for the lO0-year frequency flood. In order to demonstrate the changes, we have included the following table. ]O0-YEAR FREQUENCY MATER SURFACE ELEVATIONS 198l Conditions Considering 1981 Present Cross Section Coppell FIS* USGS Gage Record Conditions 930 440.9 440.66 440,66 937.8 441,2 441,93 442,19. 950,97 441.~ 442.84 443.09 952.7 441.5 442,95 443.22 966.2 443 444,72 444,76 988,7 443,7 444,77 444,81 967,6 444,1 444,72 444,73 968,1 444,3 445,12 q45.14 1006 445,5 446,28 446,30 1014 446,7 446,45 446,47 1022 445,9 446,87 446,68 1030,2 446,2 446,87 448,88 1090 448,5 448.88 448,86 Measured from flood profiles 10 C.5. Hanning's Ooefficient The roughness coefficients used in this model hold the key to describing the hydraulic conditions that exist. Our first attempt at this model considered roughness values as they were developed in the Flood Insurance Study and in all the subsequent revisions since then. We used these values directly from cup usual source, "Open Channel Hydraulics" by Ven Te Chow. However, we were unable to match the gage records at the Sandy Lake gage. We were considerably low even though we were using the same discharge value measured at the gage in November o¢ 198] (see following page for gage record). This was perplexing until ue looked closely at the cross sections with the gage discharge plotted. The example cross section below points out a typical situation: CROSS SECTZON 950.97 EL~4 FORK 'TR];NTY RZVER DALLAS COUNTY 460 450 '~ 44O 430 I I 420 , 800 o~n~ 3800 5300 6800 8300 9800 11300 12800 14300 BZSTANCE. FEET 11 400 TRINITY RIVER BASlN 08055500 EI.H FORK TRINITY RIVER NF. AR CARROLLTON, TX LOCATION.--Let 32°57'57'', long 96'56'39", Dallas County, Hydrologic Unit 12030103, near left bank at do~natrea~ side of bridge on Sandy Lake Road, 40 ft (12 m) upstream from Ca~ollton D~m, 0.3 mi (0.5 ks) do~nstrenm.from Denton Creek. 1.0 mi (1.6 ks) upstream from .et. Louis Southwestern Railway 1.tnes bridge. 2.3 mi (3.7 k~) northwest of Cat-rollton, and 18.2 mi (29.3 ks) upstream from mouth. DRAINAru~' ARKA.--2,459 mis (6,369 lanZ). · PERIOD OF RECORD.--Jantmz~ 1907 to current year. Honthly discharge only for some periods, published in WSP 1312. Prior to Hov~ssber 1923, published as "near REVISED RECORDS.--WSP 788: 1924. WSP 1148: Drainage erea at former site. WSP 1632: 1908(H). ~SP 1922: Drainage CAGE.--Water-ataHe recorder and concrete control. Datum of sage £s 433.40 ft (132.100 m) National Geodetic Vertical Da~ of 1929. Prior to November 1923, nonrecord£ng sage at sics 15.5 mi (26.9 k~) dovnstrea~ at different dat~. Nov. 1. 1923. to ~k~v. 13, 1934. nonrecordinS gage, and Nov. 14, 1936. to July 6. 1938, water-stage recorder at present site end dates. July 7. 1938. to Apr. 14, 1939, nonrecordtng g~e at site 9.3 mi (15.0 k~) downstream at datu~ 22.94 ft (6.992 m) lover. Apr. 15. 1939, to Sept. 30, 1955, water-stage recorder at site 8.5 mi (13.7 dovns~rea~ at datum 22.94 ft (6.992 m) REHAR~S.--Records good. Flow is largely regulated by Levisville Lake (station 08D52800) since November 1954 end by Grapevine Lake (station 08054500) since July 1952. Records furnished by .the city of Dallas show ~hat during the year 129.100 acre-ft (159 hms) was diverted from pool at gage and 41,670 acre-it (51.6 hms) vas diverted from rive channel 16 mi (23 .ks) downstream for unmicipal use. Also, 2,350 acreoft (2.90 hms) vas returned from a water treat- sent pleat Co tbs river below chis station. Records fut-nished by the Dallas Power end Light Co. show that during the year 6,510 acze-ft (8.03 hms) was diverted fr~n pool at gage into North Lake for cnol£ng water at electr£c generatinA plant. Several observations of water temperature were made du~in~ the year. Gage-height telemeters located at station. AV~P~O!~DISCRARGE.--~7 years (vater yea~e 1908-54), prior to regulation by L~via~ille and Grapevi~e Lakes, 818 (23.1~ m~/e). 592,600 acre-fc/~ (731 hm~/y~); 28 years (rater years 1955-82), regulated, tmadJuaced, 769 ft$/s (21.78 mS/e), 557,100 acre-ft/yr (687 ~J~£KEHES FOR PDIOD OF RECORD.--~--~ gage height, about 17 ft (5.2 m) _~, 25. 1908, present site and datum, from information by local resident, eet4~nated discharge, 145,000 ~tS/s (4.110 m /s), at site 8.5 mi (13.7 'ks) downstream (from information by Corps of Engineers).; max .t~um ~a.s.e h. eigh_c su. bsequ.e, nt.to 1908_, 14.5 ft (4.12 m) Apr. 26. 1942, present site and dattm, from obse~V&tiou oy National weather uervLce; a~acnarEe a~ site 8.5 mi (13.7 km) downstream, 90,700 fts/e (2.570 mS/s); no flow et Flood in 1866 ~eached about ~he $a---e sta~e es flood of Nay 25, 1908. EXTRD~ES FOR CURRENT ~J~.--Naxi~um discharge, 21,100 fts/s (598 mS/a) Nov. 2 'at 1200 hours, ga~e height;' 10.65 (3.246 u); no flow Oct. 8. DISCHARGe, ]~q CUBIC FEE~ PER SECOND, WATER YEAR OCTOBER 1981 ~O SEPTEMBER 1982 DAY OCT NOV DEC J~J~ FEB HAR APR HAY ~ JUL AUG SEP ' 1 204 12700. 6980 5350 3150' 33/,..0" 596 250 7530 6580 6580 3950 2 172 20500 ' 7050 5360 3210 4180 596 322 6660 6530 6540 ~020 3 214 19000 7060 5350 3680 4320 580 291 5650 6380 64.90 /,,030 4 176 15700 ' 7050 5300 3550 /,..190 580' 315 4960 6500 64.70 4.040 5 152 12400 7050 5280 3540 3050 564 342 /,,~70 6610 66,90 6010 6 190 10~00 7040 5270 3490 1010 556 691 4.600 6650 6510 3850 7 321, 9000 7010 5260 3550 902 540 5410 7100 6520 3780 8 3.9 8310 7020 5260 3520 646 56..8 455 6200 5630 . 6530 3500 9 79 8700 7010 5250 356,0 161 54-8 4~3 6500 5840 6530 849 10 66 8690 7000 5200 3530 152 629 623 6440 6/+40 6510 151 11 63 8110 6970 5190 3470 136 621 649 6380 2960 6500 289 12 1290 7260 6960 5170 3290 140 629 2250 6510 988 6500 285 13 1550 6~50 6960 5160 3510 152 723 10900 6750 6630 6460 310 14 176.0 6510 6920 5170 3340 185 731 3390 6510 6270 6440 308 15 119 6830 6910 5160 2820 185 723 476 6190 6640 6430 294 16 872 6930 6890 516.0 2980 348 76.8 361 61&0 6720 - 642.0 362 17 3140 6880 6880 5130 3750 355 754 730 1810 6670 6420 301 1 8 6560 6990 6880 5060 3910 368 588 1880 1090 6480 6410 319 19 7&30 6760 6870 &200 3850 ~,57 564 3350 5160 5860 6320 322 20 7910 6730 6860 3750 3820 450 564 4950 6320 5660 5730 238 21 7830 7010 681'0 3790 3800 672 556 5470 6460 5620 '5.590 145 22 8920 7050 6750 4010 3800 4/,,3 4.95 5350 6590 6300 55/,..0 114 23 8090 7030 6740 3750 3800 443 852 5160 6280 6540 5520 · ' ~ 53 26 7230 7010 6690 3720 3780 450 895 5390 7060 6640 5440 199 26 6070 7O00 6680 '3~00 844 6600 6900 6680 5190 215 3910 650 27 600o 700o · 6620 352o3290 588 6~2 6810685067,o.5,6o2o9 28 6230 6990-659o352o3150 .510408 67205380673o5,~0 ,82 29 66~0 6980 6.0 3400 --- 495 328 716o 4950 6650 5, 60 ,76 30 6.,8o 699o 5590 394o --- 495 ~04 77oo 6030 657o 4990 ~70 31 7390 --- 5360 3840 --- 525 --- 7810 --- 6570 4230 --- TOTAL 110181.9 264910 210350 142790 ~8980 30012 18550 103132 176700 18886,8 186080 36995 ~ 3556 8830 6785 4606 3535 968 618 3327 5823 6092 6003 1233 NAX 8920 20500 7060 5360 3950 4320 895 10900 7530 7100 6580 4040 N~N ' 3.9 64.50 5360 3400 2820 136 204 154 1090 988 4.230 11 4 AC-FT 218500 525400 417200 283200 196300 59530 36790 206600 346500 374600 369100 73380 CA1.. YR 1981 TC'TAL 643767.33 NE.AN 1764 I, tAX 20500 l'ilN .00 AC-FT 1277000 rarER YR 1982 TOTAl, 1565528.90 Hr_,AN . 4289 iv, AX 20500 NIN 3.9 AC-ri' 3105000 As can be seen, the majority oF the overbank Flow is very shallow, often less than one foot. Chow states that, "on Flood plains, the value oF 'n' usually varies with the stage of submergence oF the vegetation at low stages." Chow presented 'n' values in the overbanks oF the Nishnabotna River in Iowa. Although it would be impossible to draw definite numbers From this small sample, a range oF changes in 'n' values might be a good indication. For a meadow, in Chow's example, the 'n' value doubled when water surface elevations decreased From Four Feet to one Foot. The conclusion we drew From this comparison was that 'n' values in the overbank oF the Elm Fork oF the Trinity might be considerably higher than expected when low Flood Flow conditions exist, perhaps on the order oF as much as twice the expected value. With this thought in mind, we again modeled the Elm Fork using higher overbank 'n' values. Even though we were closer, we still Felt that the 'n' values we had to use were excessive, so we considered another option. The main channel o¢ the Elm Fork is relatively smooth under normal Flow. We used an 'n' value oF 0.06 For this channel. However, we had set the top oF bank stations according to the cross section, A closer look at the channel, as shown below, shows clearly that the area above the normal Flow level is virtually choaked with trees and brush. This heavy growth thins out some after moving some distance away from the banks. It didn't seem appropriate to model the channel with a roughness of 0.06 with this heavy growth in the upper portion. We elected to model the channel with a roughness of 0.06 up to the elevation of the mean flow conditions. We took this to be 4300 ors. A run was made which set the elevations of the mean flow. The water surface elevations are shown below. These elevations were used on "NV" cards to model the channel, and everything in the channel above this elevation was considered as very heavy trees. In = 0.16} Cross Sect. lO0-Yr. No. Q ~ater Surface 930.00 4300,00 419.93 937.80 4300.00 423.97 950.97 4300.00 428.43 952.70 4300.00 428.93 966.20 4300.00 431.66 966.70 4300.00 437.26 967,60 4300.00 438.50 Considering these two approaches, we were able to obtain a good calibration with the USGS gage. USGS gage records for a discharge of 21100 cfs yield a reading of 444 feet NGVD. Our model obtained a water surface of 443.55 feet NGVD for the same discharge. This range is well within acceptable limits. Next, we tried running the' tOO-year discharge in this calibrated model. The results we got were plotted on the cross sections and we then discovered that we were no longer at extreme shallow flow conditions. Instead, we find that depths have increased substantially. Considering the same cross section seen earlier with the lO0-year discharge, this becomes obvious: t3 CROSS SECTION 950.97 ELM FORK TRINTY RIVER DALLAS COUNTY 460 450 Z ~-- 440 LL i Z' 430 I 0 I ~- 420 l __1 u~ 4~o [ 800 2300 3800 5300 8800 8300 9800 11300 12800 ~.4300 DISTANCE, FEET So, now we had to determine a new 'n' value in order to obtain an appropriate model. We elected to reduce the 'n' value based on engineering Judgement and a review of an aerial photo we obtained for 1981 conditions (see next sheet). The resulting baseline model does not match the USGS rating curve (see next sheet) for the Sandy Lake gage. However, this USGS rating curve is extrapolated past the 21100 ors discharge. We firmly believe that this extrapolation is based on conditions that do not consider variable 'n' values. Based on good and reasonable backwater analysis methods, the 'n' values required to reach the USGS projected water surface elevation at the tOO-year frequency event are not reasonable. We believe that the method presented in this report is much more accurate. After all, both the USGS and we are guessing past the recorded data. The following table presents the 'n' values used for the calibration model, baseline model and present conditions model through the Sandy Lake gage. 'n' values are similar upstream. 14 VALUES (LEFT/RIGHT/CHANNEL) Cross Calibration Baseline Present Notes: Section Model Model Condition L = Le~t, R = Right, C = Channel 930.0 L - 0.13 .095 .095 L - cultivated, gravel pits, construction R - 0.15 .1 .06 ~ R - cultivated, heavy trees C - 0.12 .13 .13 C - 0.06 below tree line, 0.16 above 937.8 L - 0.13 .095 .095 L - same R - 0.15 .1 .06 ~ R - cultivated, heavy trees C - 0.14 .15 .15 C - same 950.97 L - 0.13 .09 .09 L - trees, brush, gravel pits R - 0.13 .085 .06 ~ R - cultivated, construction, trees C - 0.14 .15 .13 C - sa~e 952.7 L - 0.14 .09 .09 L - same R - 0.13 .085 .06 * R - same C - 0.15 .13 .13 C - same 966.2 L - 0.14 .07 .07 L - construction, land~ilt, commercial R - 0.13 .1 .06 * R - construction, trees, brush C - 0.15 .1~ .13 C - same 966.Z L - 0.13 .07 .07 L - same except includes road R - 0.1~ .1 .1 R - construction, trees, brush, road C - 0.06 .06 .06 C - no trees on upper banks 967.6 L - 0.12 .07 .07 L - so~e trees, brush R - 0.1~ .1 .1 R - brush, trees C - 0.06 .06 .06 C - n,) trees on upper banks ~ Improved with ~ill and overbank swale Note: "same" re,ers to same overbank or channel in previous cross section C.6. Floodways and Encroachments We used the same encroachment approved by FEMA for projects considered in the present condition model. We did not reduce the floodway anywhere e×oept at Sandy Lake Road. The reason we reduced the floodway at this location was because we obtained data from field surveys Isee following pagesl that showed Sandy Lake Road to be above the lO0-year frequency water surface elevation calculated at the road. This means that the assumption previously made that water was overtopping Sandy Lake Road in the right overbank was not correct. Since the "flood plain" flow is almost directly south at the road (perpendicular to the road), it must be assumed that this flow is ineffective. We modeled it this way. Further, our understanding is that floodways should not be defined in ineffective flow areas. Also, the water levels just upstream of Sandy Lake Road are about one to two feet in depth, not an area in which we would normally define a floodway. Because of this, we elected to draw the floodway along the banks of Denton Creek. 0.7. Structures An important structure in this model is the Carrollton dam, just downstream of the Sandy Lake gage station. This dam is a fast flowing effective structure at Iow to normal flows. 16 However, at flows in excess of 12000 cfs, this dam is flooded and has almost no effect on the water surface elevation. This was confirmed in the 1981 Flood by records of the USGS (conFirmed by Walter Lear, USGS, Austinl. Thus, we didn't include an overflow weir computation. The other bridges along this river are above the tOO-year flood levels, so they were not modeled as a special bridge. 0.8. Velocities Channel velocities and overbank velocities are associated with this massive river and thus are generally slow moving, even in the lO0-year event. In our field investigations, we saw no signs oF erosion. D. Conclusions We did a comprehensive model for this reach oF the river and considered projects deemed appropriate by the Corps. There was some initial concern about the high measurements by the USGS at the Sandy Lake Road gage. However, when the variables are considered, we feel that the values found in the HEC-2 backwater model are consistent with gage readings. Care must be taken in modeling a river of this magnitude. It contains a tremendous potential for disaster. Small streams can be modeled independently without a great potential For concern, but not the Trinity. Because of this, we have made our model available to the North Texas Council of Governments. We support their efforts to coordinate actions taken in regard to this great Texas river. This flood study was based on conditions shown in this report. Any changes to these conditions, or any modification oF this report, would change the results of this report and thereby nullify it. 17 hYDRAULIC ANALYSIS TRINITY RIVER REGIONAL ElS I. Hydraulic Analysis A. Scope of Work - Purpose and Goals B. Location of Limit of Study C. Origin of Data Base II. Approach - Methodology A. Development of Models 1. Existing Conditions (1985) 2. Future Without 3. 404 Permits in Floodway Fringe Only Maximum Development 1 (MD1) 5. Maximum Development 2 (MD2) B. Computation of Water Surface Elevations With Hydrology Revised for Each Scenario. III. Summary List of Figures A-4 - A-9 Trinity River Existing Conditions Water Surface Profiles A-10 - A-15 Elm Fork Existing Conditions Water Surface Profiles A-16 - A-23 West Fork Existing Conditions Water Surface Profiles List of Tables A-1 - A-22 Flood Frequency/Water Surface Elevation for Each Scenario A-23 Trinity River Existing Conditions Discharges A-24 West Fork Existing Conditions Discharges A-25 Elm Fork Existing Conditions Discharges I. Hydraulic Analysis A. Scope of Work - Purpose and Goals The purpose of the hydraulics study is to develop hydraulic models necessary to assess the immediate and potential impacts of development and construction along the West Fork and Elm Fork of the Trinity River. The goal throughout this study has been to provide technical data used in making an evaluation of these impacts on river hydraulics, flood protection, fish and wildlife habitats, and water quality. B. Location and Limits of Study The hydraulic models developed for this study include a portion of the Trinity River and portions of its two main tributaries, the West Fork and Elm Fork. Limits of the Trinity River model are from Malloy Road, southeast of downtown Dallas in Seagoville, upstream to the West Fork/Elm Fork confluence. The West Fork model extends from its confluence with the Elm Fork upstream to Riverside Drive in Fort Worth. The Elm Fork model extends from its confluence with the West Fork to Hwy 121 in Lewisville. C. Origin of Data Base Hydraulic models developed for the 1978-1981 Dallas and Tarrant County FEMA studies (Flood Insurance or FIS) were the primary data base used in the Trinity River Regional EIS. Data files developed for the 1979 study of the Dallas floodway extension were used in modeling scenarios which included the Dallas floodway extension. The Dallas County FIS did not include a model of the Elm Fork to the limits defined above, therefore, a file, 'modeled by the HEC-2 Water Surface Profiles computer program and developed for the 1982 Lewisville FIS, was needed to complete the Elm Fork model from river section 1090+00 to Hwy 121 (section 1503+10). Input data for each scenario are coded in format for the LRD-1 water surface profiles computer program, except for the above HEC-2 file and the MD1 and MD2 scenarios described in paragraphs II.A.4 and II.A.5. II. Approach-Methodology A. Development of Models 1. Existing Conditions (1985) The "existing conditions" hydraulic models are updated versions of the hydraulic models used in the FEMA studies of Dallas and Tarrant Counties completed in 1981. The FEMA data files were modified to reflect recent changes in the conveyance and/or storage properties of the river system caused by development, landfills, levees and channel modifications. For purposes of the Trinity River Regional EIS, the "existing conditions" model includes additions to the 1978-1981 study in place or permitted prior to July 1985. New cross sections were added where necessary. Data generated from this model serves as a basis for comparison with the possible or hypothetical scenarios addressed later in this report. 2. Future Without The hydraulic model is the same as the "existing conditions" model except for a portion of the Elm Fork below Royal Lane. Existing fill in the area east of Luna Road blocks conveyance and precludes valley storage. Although a levee has not been built, permit activity in this area has been high and complete development is anticipated in the near future. Consequently the area east of Luna Road, from Royal Lane south to Northwest Highway parallelling the Elm Fork as it bends eastward to the I.H. 35 embankment, was treated as a nonconveying nonstorage area. Conveyance and storage were not allowed in the other portions of the area where levees exist. No changes were made to the hydraulic models of the Trinity River or the West Fork. 3. 404 Permits in Floodway Fringe Only Under this scenario all floodway fringe areas defined by the FEMA study were assumed to be fully developed. The West Fork model was developed by imposing the FEMA encroachment limits on the "existing conditions" model. The differences in water surface elevations on the West Fork produced by this study and the 1982 Tarrant County FIA. study are caused by the addition of development, such as new bridges and landfills, which has occurred or has been permitted for construction inside the floodway fringe combined with increased flood frequency discharges. In the Elm Fork hydraulic model, the reaches upstream of Sandy Lake Road were modeled by blocking flow outside the floodway fringe. Development and levee construction below Sandy Lake Road closely agrees with the FEMA floodway alignment; therefore, encroachments were not made beyond the encroachments in the "existing conditions" model. Water surface elevations were computed for the Trinity River, West Fork and Elm Fork with the existing Dallas floodway channel. In addition four options were studied for the Trinity River utilizing a 100 foot and a 200 foot floodway channel with and without the Dallas floodway extension (see chapter II, section D). The hydrology for the West Fork and Elm Fork is the same for the four options, therefore the water surface elevations in those portions of the model are the same for each option. 4. Maximum Development 1 (MD1) The purpose of this scenario is to approximate the impacts of maximum land useage without control of development in the floodplain. The analysis assumed a narrowly leveeded floodway with a maximum size channel, thereby allowing the maximum developable land area within the West Fork and Elm Fork flood plains (from Sandy Lake Road to the upper limit of the study area). In the West Fork and Elm Fork models, artificial cross sections were created using fixed channel depths and fixed overbank areas while varying the channel width. The depth of Channel was fixed at 20 feet, which approximates the depth of the existing channel. The distance from top of channel to toe of levee was set at 50 feet on each side, which is the minimum distance for construction and maintenance. Channel and levee side slopes were fixed at 3 horizontal to 1 vertical. Since the overbank areas were fixed, the distance between levees varied with the changes in channel width. Conveyances at ~alected cross sections were determined from the existing conditions SPF discharge and the slope of the hydraulic grade line. The depth of flow was also noted. Channel sizes in the artificial sections were utlimately determined by varying iteratively the channel bottom width until the conveyance and depth in the artificial channel section matched the existing conditions SPF conveyance and depth at the same location. Iterations were made one section at a time. Subsequent increases in the discharges, after the hydrology was recomputed to account for the loss of valley storage, caused the water surface elevations as published for the MD1 scenario to exceed the existing conditions SPF elevations by significant amounts. Roughness coefficients used in conveyance computations were 0.040 for the channel and 0.070 for the overbank areas. The physical dimensions of this system are identified in appendix B, table B-29. 5. Maximum Development 2 (MD2) As in MD1 this scenario represents a condition in which all flood flows are contained within a levee system. This scenario emulates the Dallas floodway/levee system with the width between levees arbitrarily fixed at 1300 feet. The channel depth is 20 feet with side slopes 3 horizontal to 1 vertical. In the West Fork and Elm Fork models, the channel width was varied iteratively until conveyance and depth in the artificial section matched the existing conditions SPF conveyance and depth at the same location. Roughness coefficients used in the conveyance computations were 0.040 for the channel and 0.070 for the overbank areas. The physical dimensions of this system are shown in appendix B, table B-54. o B. Computation of Water Surface Elevations With Hydrology Revised for Each Scenario Valley storage data were developed for each scenario with a range of 20 discharges from 200 cfs to over 400,000 cfs. Water surface elevations, using revised hydrology for the 10 yr, 25 yr, 50 yr, 100 yr, and SPF discharges, were computed for the following scenarios: 1. Existing Conditions (1985) a. Trinity River b. West Fork c. Elm Fork 2. Future Without a. Trinity River b. West Fork c. Elm Fork 3. 404 Permits in Floodway Fringe Only - Existing Dallas Floodway a. Trinity River b. West Fork c. Elm Fork 4. 404 Permits in Floodway Fringe Only a. Trinity River (1). 200 ft channel Dallas floodway - No floodway extension (2). 100 ft channel Dallas floodway - No floodway extension (3). 200 ft channel Dallas floodway - With floodway extension (4). 100 ft channel Dallas floodway - With floodway extension 5. Maximum Development 1 - MD1 a. Trinity River (1). 200 ft channel Dallas floodway - No floodway extension (2). 100 ft channel Dallas floodway - No floodway extension h. West Fork (artificial sections) c. Elm Fork (artificial sections upstream of Sandy Lake Road) 6. Maximum Development 2 - MD2 a. Trinity River (1). 200 ft channel Dallas floodway - No floodway extension (2). 100 ft channel Dallas floodway - No floodway extension b. West Fork (artificial sections) c. Elm Fork (artificial sections upstream of Sandy Lake Road) Ail hydrology beyond existing conditions was computed assuming ultimate urbanization conditions. The Dallas floodway and floodway extension levees were assumed to extend vertically to contain all flows for each scenario in terms of computation of valley storage and water surface elevations. A hydraulic computer model was not developed for the MD1 and MD2 scenarios for the West Fork or Elm Fork due to time and cost constraints, therefore, storage data were determined graphically. A set of conveyance curves was developed, one curve for each optimum channel configuration, with conveyance, K, plotted against depth of flow. Revised reach lengths and a separate curve showing the relationship of area to discharge at each artificial section were provided to the hydrologists for the determination of valley storage and flood frequency discharges. Once revised hydrology was computed for the MD1 and MD2 scenarios, conveyance was computed by the relationship Q=KS 1/2, where Q is the discharge at an artificial section, and S is the slope of the hydraulic grade line at the same section. The depth of flow corresponding to each K value was read directly from the curve unique to a particular section. Water surface elevations were determined by adding the depth of flow read from the curve to the thalweg elevation at the corresponding section. The results of each scenario, indicating the flood frequency and corresponding discharges and water surface elevations at selected locations along the Trinity River, West Fork and Elm Fork, are provided in tables A-1 - A-21 at the end of the appendix. Tables A-22 - A-24 indicate peak discharges and reaches for the Trinity River, West Fork, and Elm Fork. Existing conditions water surface profiles for the Trinity River, West Fork and Elm Fork are shown in figures A-4 - A-23. III. Summary The results of the hydraulic analysis indicate continued development of the West Fork and Elm Fork valleys, until the available valley storage is progressively reduced and a substantial length of the total reach is channelized, will produce a discharge from a flood event the order of magnitude of the design flood which will exceed the capacity of the existing Dallas floodway system (per original design parameters and contract plans). It should be noted that significant settlement of the levees has occurred in some locations, therefore, the full four not universally available at the present time. feet of freeboard is Some restoration of these settlement areas has been completed by the City of Dallas, however, a complete evaluation of the system has not been made. The effects of channelization and loss of valley storage is most significanlty illustrated in the MD1 and MD2 scenarios which produce large contrasts in flood discharges and water surface elevations for the Trinity River, West Fork and Elm Fork as compared to the existing conditions.. The analyses of existing conditions indicate, with Joe Pool Lake on line, the discharges produced by a design flood are about 6 percent greater than the original design discharge of the Dallas floodway system. About 3 feet of the original 4 feet of freeboard of the The analysis also indicates ultimate Dallas floodway would remain. urbanization of the watershed (exclusive of further floodplain development in the West Fork and Elm Fork) will produce a discharge for a design flood in the Dallas floodway (Trinity River) about 7 percent greater than the design flood. When ultimate urbanization is combined with a level of flood plain development in the West Fork and Elm Fork valleys in accordance to the FEMA floodway plan, the discharge from a design flood would be about 25 percent greater than the original design discharge. Such a discharge would reduce the existing freeboard of the Dallas floodway system to l~ss than 1 foot. The hydraulic and hydrologic portions of this report were completed with the purpose of providing hydraulic and hydrologic information for the Trinity River Regional EIS. The results of this study should not be considered conclusive, but rather the data should be considered a basis for predicting comparative impacts to the flood plain and adjacent wetland areas of the Trinity River, West Fork and Elm Fork. Data generated by this study should be helpful in focusing attention to areas of potential flood damage which impose threats to the environment and human safety. The results of this study should not be used beyond the intended scope of the Trinity River Regional EIS and do not supersede the Dallas FIS. ~ TABLE A-3 ELM FORK EXISTING CONDITIONS 1985 Flood Water Levee Channel Flood Discharge Surface Crest Flow Line Location ~requency (cfs) Elevation Elevation Elevation Loop 12, 10-Yr 20300 417.8 * 434.0 389.2 Sta 294+60 25-Yr 27100 420.3 * 50-Yr 34200 422.1 * lO0-Yr 42700 424.9 * SPF 87800 434.3 * Royal Lane, 10-Yr 23100 427.1 -- 401.4 Sta 592+20 25-Yr 30400 427.9 50-Yr 37400 429.0 100-Yr 45300 430.3 SPF 88000 437.1 Valley View Lane., 10-Yr 23100 431.2 441.0 402.0 Sta 711+50 25-Yr 30500 432.4 50-Yr 36900 433.3 100-Yr 44600 434.2 SPF 86800 439.2 Sandy Lake Road, 10-Yr 23000 441.3 -- 414.5 Sta 967+60 25-Yr 28000 441.5 50-Yr 35000 442.3 100-Yr 41000 443.1 SPF 79000 447.4 IH 35E, 10-Yr 11300 449.3 -- 424.0 Sta 1138+20 25-Yr 13800 449.7 50-Yr 16100 450.1 100-Yr 21000 450.6 SPF 62000 452.8 Hwy 121, 10-Yr 6300 456.9 -- 430.0 Sta 1503+10 25-Yr 6300 457.2 50~Yr 9000 458.3 100-Yr 21000 461.5 SPF 62000 468.4 * Water surface elevations produced by West Fork peak discharges and Elm Fork coincident discharges. CORPS OF ENGINEERS U.S. ARMY o THAL.[O-~~ 430 DISTANCE IN FEET/lO0 Coppetl, Texas 75019 214 - 462 - 0022 The City With A Beautiful Future July 3, 1986 Federal Emergency Management Agency Federal Insurance Administration Office of Risk Assessment Risk Studies Division 500 C Street S.W. Washington, D.C. 20472 Re: Flood Insurance Study Revision along the Elm Fork Trinity River in Dallas, County, Coppell, Texas (Vistoplex Development on Sandy Lake Rd.) Attention: Mr. Matt Miller Dear Mr. Miller: The City of Coppell has received a Technical Report from the consulting engineering firm of Caffey and Morrison, Inc., on the existing and proposed flood plain and floodway conditions along the Elm Fork Trinity River, adjacent to Sandy Lake Rd., Coppell, Texas. This report, prepared by Caffey and Morrison, Inc., includes a detailed hydraulic analysis of the existing channel and overbank conditions, as well as proposed improve- ments. The report also includes the HEC-Z computer printout, flood plain and flood way maps and possible revisions of the existing flood insurance maps. It is our under- standing that this same information has already been submitted to your office for review. The City of Coppell has reviewed this material and agrees with its content. If you have technical questions about this report, please contact Mr. Ron Morrison, P.E., with Caffey and Morrison, Inc. (1-817-649-8110). Any administrative questions should be directed to the office (1-214-462- 0022). Thank you for your consideration of this very important request. Sincerely, "~i~tE~ Powell, P.E. Engineer cc: Mr. Wayne Ginn, P.E., Ginn, Inc. Mr. Ron Morrison, P.E., Caffey and Morrison, Inc. Mr. Bill Kuno, Federal Emergency Mangement Agency, Region IV NET ACREAGE 12.6 AC .. -.-. OFFICE 711,140 SI:: . RETAIL 63,903 SF Sandy Lake Road ..... · · Preliminary Site Plan . INC. CAFFEY AND MORRISON, INC. ENGINEERS -- HYDROLOGISTS May 23, 1986 Mr. Ed Powell City Engineer City of Coppell P.O. Box 478 Coppell, Texas 75019 Re: Flood Insurance Study Revision along the Elm Fork Trinity River in Dallas County, Coppell, Texas Job No. 85048 Dear Mr. Powell: The enclosed flood study of an area adjacent to Sandy Lake Road along the Elm Fork Trinity River which is in your city has been submitted for your review with respect to its general content and concept. The study was prepared in order that a Flood Insurance Study Revision might be obtained in the project area from the Federal Emergency Management Agency. Final approval cannot be obtained until as-built plans have been submitted to FEMA. Your city has final approval authority over such matters in your jurisdiction. If the study meets with your approval in general content and concept, you are respectfully requested to write FEMA and so inform them. Also request that FEMA review the material for the purpose of issuing approval for a Flood Insurance Study Revision. In order to save a few days, we have already forwarded copies of the flood study to the FEMA Regional and Washington offices and to the Corps of Engineers for their permitting review. Initiation of the review by FEMA is dependent upon their receipt of your letter indicating the approval of the floodway revision. Should you have any questions about the study or need a better understanding of the development objectives, please call us. Sincerely yours, Ca~_gy and Morrison, Inc. Vice President 2212 ARLINGTON DOWNS RD., SUITE 105 · ARLINGTON, TEXAS 76011 · (817) 649-8110 · METRO 640-8101 Coppell, Texas 75019 214 - 462- 0022 /~ The City With A Beautiful Futu e March 21, 1986 Mr. Bill Kuno Federal Emergency Management Agency, Region IV Hazards g Mitigation Division Federal Center Denton, Texas 76201 RE: Village at Cottonwood Creek Coppell, Texas Dear Mr. Kuno: The City of Coppell has received a Technical Report on existing and proposed flood plain and floodway conditions along Cottonwood Branch within the reference subdivision. ~is report, prepared by Wier and Associates, Inc., includes a detailed hydraulic analysis of the existing channel and overbank conditions, as well as proposed improvements. The report also includes the HEC-Z computer printout, flood plain and floodway maps, and other data necessary to provide a conditional letter of map revision and possible revisions of the existing flood insurance maps. It is our understanding that this same info~ation has already been submitted to your office for review. If you have any technical questions about the report, please contact Mr. David McCa~er, P.E., with Wier and Associates, -Inc. (817-265-2006). ky administrative questions should be directed to this office (214-462-0022). Thank you for your consideration of this very important request. Sincerer, Powell, P.E. City Engineer City of Coppell cc: Mr. W. Wayne Ginn, P.E., Ginn, Inc. Mr. David McCarver, P.E., Wier & Assoc., Inc. TOTAL ACREAGE 18.3756 AC OFFICE 78,140 SF RETAIL 63,903 SF \ "" 5andy Lake ~oad ....... Prelimin~r7 Site Plan I ' I ! . INC. CAFFEY AND MORRISON, INC. ~ ENGINEERS -- HYDROLOGISTS May 23, 1986 Mr. Ed Powell City Engineer City of Coppell P.O. Box 478 Coppell, Texas 75019 Re: Flood Insurance Study Revision along the Elm Fork Trinity River in Dallas County, Coppell, Texas Job No. 85048 Dear Mr. Powell: The enclosed flood study of an area adjacent to Sandy Lake Road along the Elm Fork Trinity River which is in your city has been submitted for your review with respect to its general content and concept. The study was prepared in order that a Flood Insurance Study Revision might be obtained in the project area from the Federal Emergency Management Agency. Final approval cannot be obtained until as-built plans have been submitted to FEMA. Your city has final approval authority over such matters in your jurisdiction. If the study meets with your approval in general content and concept, you are respectfully requested to write FEMA and so inform them. Also request that FEMA review the material for the purpose of issuing approval for a Flood Insurance Study Revision. In order to save a few days, we have already forwarded copies of the flood study to the FEMA Regional and Washington offices and to the Corps of Engineers for their permitting review. Initiation of the review by FEMA is dependent upon their receipt of your letter indicating the approval of the floodway revision. Should you have any questions about the study or need a better understanding of the development objectives, please call us. Sincerely yours, Caffey and Morrison, Inc. L-~Ronald W. Morrison, P.E. Vice President 2212 ARLINGTON DOWNS RD., SUITE · ARLINGTON, TEXAS760~ · (817) 649-8110 · METRO 640-8101 105 SAMPL-E~TRANSMi TTAL LETTER November 27, 1985 Mr. Matt Miller Federal Emergency Management Agency Federal Insurance Administration Office of Risk Assessment Risk Studies Division 500 C Street S.W. Washington D.C. 20472 Dear Mr. Miller: Mr. Robert Hawrylak of Hawr¥1ak and Associates, Inc. of Arlington, Texas has submitted the attached Flood Study of the Freeport North Addition along Cottonwood Branch in Coppell, Texas. As City Engineers, we have reviewed the material in regard to general content and concept and it meets with our approval. In view of this, we request that FEMA review the material as a Conditional Revision and provide comments. Upon a favorable review by FEMA and completion of the project, we intend to submit as-built plans to obtain a final Flood Insurance Study .Revision. We have asked that the firm preparing the report, Caffey and Morrison, Inc. of Arlington, Texas, send a copy of the report to your office and'to'the FEMA regional office in Denton, Texas. We understand that in order for the channelization tb ~emain in the condition for which it was designed, it is necessary for the city to maintain the channels relatively free of heavy growth. Our city ordinance addresses this and the city accepts responsibility for maintenance .of the channels through this project. We have submitted a copy of this letter and the accompanying report to the FEMA Region Six office in Denton, Texas. We appreciate your attention to this request and look forward to your reply. Sincerely, Wayne Ginn, P.E. Ginn, Inc. Consulting Engineers 16135 Preston Road Suite 112 Dallas, Texas 75248 Region VI, Federal Center 800 North Loop 288 ~ Denton, Texas 76201-3698 APPENDIX 1 FLOOD PROFILES FROM THE CITY OF COPPELL FIS il APPENDX 2 FEMA FLOOOWAY MAPS CITY OF CARROLLTON AND COPPELL AND CORPS WORK MAP t9 APPENDIX 3 COMPARISON FLOOD PROFILES FOR 1981 AND PRESENT CONDITIONS 2O CO~4PARISON 100 YR WATER SURFACE EL~4 FORK TRINITY RIVER DALLAS COUNTY 448 ........ 446...... t ......... ' --'"""'""~ 445.... I .... ~'-"~ .... 443 -,: ~~ ~ . ', 44~ 440 t I 438 1. 1 g~O g30 g40 gso gBO g70 gBO ggo ~000 JO~O DISTANCE, KEET X ~00 APPENDIX 4 FLOOD PROFILES FOR PRESENT CONDITIONS PRESENT CONDITIONS 100 YR WATER SURFACE ' EL~ FORK TRINITY RIVER DALLAS COUNTY 455 , '..,,- · ,,, ~ ': .... ~.,~ ........... 450 ' ~' ~~:~ ~'~' > 445 .... Z -.-.. 440 ~ 435 I~ 430 . ~ 425 ..... 0 :: 420 > 4t5 ~,~ 4~0 _ _ . ................ = .... 405 _ 920 940 960 980 tO00 DISTANCE, FEET x ~00 PRESENT 1O0-YR WATER SURFACE EL~ FORK TRiNiTY RZVER 4§0I J I....... ! ..... I 455__' ................ I.... I ~ I ~. 450 .................. ~ .............. (_9 ~ ..... -_.. ~4§ t''~ ~0 Et_ q~5 ...... [. ~ 430 :' 485 ~-~ ~:~ 420 ao05 ioi5 i025 ~035 1045 i055 i0~5 i075 10~5 ~095 DISTANCE, FEET X PRESENT O0-YFi XATER SUPlFACB ELM FORK TFI]NTTY F::I_'[VFFI DALLAS COUNTY 465 ....... 4150 __ -.-.. 450 ....... ..= ............. - .... ----- ......... !-- 445 : LLi - I~. 440 ......... ~ 435 ............... ! ........... CZ) '' ! 430 ........... >. 425 _ ,.,, ~FIA ._ LL[ J! LLI 420'_ .... ~' . ............ 4t5' .J 1 1085 i095 ii05 iii5 i125 li35 i145 ii55 i165 1175 DISTANCE, FEET X 100 APPENDIX 5 FLOODWAY DELINEATIONS FOR COMPARISON APPENDIX 6 FLOODWAY/FLOOD PLAIN DELINEATIONS FOR PRESENT CONDITIONS 23 APPENDIX 7 1981 CONDITION5 CROSS SECTIONS CROSS SECTION 930 EL~ FORK TRINITY RIVER DALLAS COUNTY 455 .................................... / n x A ;> 445 ..... /5 x ....... I 435 ......... / ~--_~: gl~ ~ ..... > 415 . ................... 111 I 405 ..... 800 2400 4000 5600 7200 8800 i0400 i2000 i3600 15200 DISTASCE, FEET CROSS SECTION g 7.8 ELM FORK TRINTY RIVER DALLAS COUNTY 465 , .......................... · ..... :> 454 .............. LH 443 . , - _ .......... 0 tq~I i ::> 42't ...... ' .... I 4'1( · , , II · 950 2290 3620 4970 6310 7650 8990 iOa30 11670 `130'10 BISTANCF, FEET CROSS SECTION 950.97 ELM FORK 'TRINTY RIVER OALLAS COUNTY 460 ~. 450 LD ' ~-- 440 LIJ 1 3> 480 . ,' [j_J , LIJ " 410 800 2300 3800 5300 6800 8300 9800 i i300 12800 14300 DISTANCE,FEET CROSS SECTION 952.7 ELM FORK TRINTY RIVER DALLAS COUNTY 455 .... / :> 446 , ......................... 2~' 428 ....................... CD H I 3> 4~9 .......... LLJ 4~0 ., · .................. 950 2250 3550 4850 6:t50 7450 8750 :tO050 11350 :t2650 DISTANCE, FEET CROSS SECTION 966.2 ELM FORK TRINTY RIVER DALLAS COUNTY 460 .................. ::> 450 .......... LLJ CAt ~ATI 0¢ ~ 430 .............. CD 3>- 420 : . ....... I 300 :1800 3300 4800 6300 7800 9300 :10800 12300 :13800 DISTANCE, FEET C,qOSS SECTION 958. ELM FORK TRINTY RIVER DALLAS 60UNTY 450 F- 440 I IIJ C~]LI ~A"F'I Ot (D ~- 420 Ltl 1. 300 &800 3300 4800 6300 7800 9300 10800 &2300 ~3800 DISTANCE, FEET APPENDIX 8 PRESENT CONDITIONS CROSS SECTIONS CBOSS SECTION §30 · ELM FOBK TBINITY BIVEB DALLAS COUNTY 455 ........................................... ~. 445 ..... Z I 4~5 . . I]lllJ~l > 4t5....................... Iill 405 ...... .-, BO0 ~400 4000 5800 7800 8800 ~0400 tSO00 t~BO0 ~5~00 DISTaStE, FEET :~' CROSS SECTION g37.8 ELM FORK TRINTY RIVER DALLAS COUNTY /,1 Yr- I-.- 443 ,. ,' ~LL ., '/.",' ' ; ; ' ~ 432 ' O H /,, ~.. :>,. 42J. I 410 950 2290 3630 4970 63i0 7650 8990 &0330 ~ i670 &30 lO DISTANCE, FEET CROSS SECTION 950 . 97 ELM FORK TRINTY RIVER BALLAS COUNTY ~ , , ;J~il I;11 I ! / ~ I LI II111 IMI I,I I ' · ..... l', ." ' ,/ I / ~-- 440 II1 420[ 800 2300 3800 5300 6800 8300 9800 i laO0 12800 14300 DISTANCE, FEET CROSS SECTION 952.7 ELH FORK TRINTY RIVER DALLAS COUNTY 455 .. . .... / ~ 446 .- - ............. I 437 Ii1 : : ~. D. ~A I~ I . ~ 4~g ............... ILJ~ 4~0 ..... - ........... 950 ~250 3550 4850 6~50 7450 8750 ~0050 ~350 ~2650 DISTANCE, FEET CBOSS SBCTION 966.2 ELM FOBK TB!WTY BIV~B DALLAS COUNTY 460 ........................ > 4~_o .... 3oo ~oo ~oo ~oo ~oo ~oo ~3oo ~o~oo ~oo ~oo 0 ISTANCE, FEET CROSS S~CT~ON 966. ELH FORK TR~NTY R~VER DALLAS COUNTY 450 > 445 Z i--- 440 ' _j I 1 425 [ ~00 ~800 ~00 4800 6~00 7800 9~00 DISTANCE, FEET CROSS SBCITOW 967.6 EL~ FORK TRiNTY RIVER DALLAS COUNTY 440 ~o > 420 I~_1 _ I 1000 2400 3800 5200 6600 8000 9400 JO800 12200 13600 DISTANCE, FEET CROSS SECT.T. ON 968. t ELM FORK TRT_NTY R~.VER BALLAS COUNTY 460 C--a b'-'-'> 450 Z I---- 440 ;z~ 430 r ° ;> 420 _J I 300 ~800 ,3300 4800 6300 7800 9300 10800 ~.2300 ~.3800 DISTANCE, FEET CROSS SECTION ~.006 ELN FORK I'BINTY RIVER DALLAS COUNTY 460 444 436 428 420 J I 300 ~800 3300 4800 6300 7800 9300 ~OaO0 ~2300 ~3800 DZSTANCE, FEET CF:lOSS SECT ~[ON 'l 0 ~I 4 ELM FORK TRT_NTY R~VER DALLAS COUNTY 46§ i t-- 449 iiii iii iiiJii~, -~4~: l' l~ ,.,~ , 0 ' ! i ~ 433 ............ __~ ! i i 485 ' ~ .... ~ __~ ..... 300 ~800 3300 4800 6300 7800 9300 10800 12300 ~3800 DiSTANCX FSET CROSS SECT I ON _~ 022 ELM FORK TRINTY RIVER O~LL$S COUNTY ~. 457 Z I--. 44g --i o~ 44~t~'"~~~~ ~" II > 433 .~ ~r~ _ _11 425 __ ~ i ....... 1 0 i500 3000 4500 6000 7500 9000 i0500 12000 ~3500 DISTANCE, FEET CROSS SECT ]] ON ~L030 . 2 EI_I~ FORK TRINTY RIVER DALLAS COdNTY 457 449 , 44~ 433 BOO ~000 3200 4400 5600 6BO0 BO00 9~00 10400 11600 DISTANCE, FEET CROSS SECTION ~090 ELN FORK TRINTY RIVER BALLAS 60UNTY 465 :::> 455 ' 445 LL! 0z~'' 435 425 800 t800 2800 3800 4800 5800 6800 7800 8800 9800 DISTANCE, FEET CROSS SECTION i 136.23 ELIVl FORK TR!NTY RIVER DALLAS COUNTY 452 I--- 444I I. JJ · 436 LI.J ! I~1 3500 4500 5500 6500 7500 8500 9500 i0500 i 1500 12500 OlSTANCE, FEET CBOSS SECTION ~ ~36.73 ELM FORK TRINTY RIVER DALLAS COUNTY 465 ! ! ilI .-> 457 b,- 449 J / 0~ 44t. Ii H i ~ 433 , LL! i 't 42_5 ___._L P. 000 3000 4000 5000 6000 7000 8000 9000 t.0000 1. :1.000 DT_STANCE, FEE]- CROSS SECT.iON ~37.2 EL_~4 FORK TR_~NTY R_T_ VER DALL~,SCOUNTY 4~0~ 428 , 35oo 4500 55oo 6500 7500 a500 9500 ].o5oo ~ ~ioo ~.2~00 DISTANCE, FEET CBOSS SECTI ~LM FOBK TBI~TY DALLAS COUNTY !!! I11 ,H ' ~" 433 , ~ ' I 425 3500 4500 5500 6500 7500 8500 9500 i0500 :1. _t500 ~2500 DISTANCE, FEET CROSS SECT i ON ~ ~ 38.8 ELM FORK TRINTY RIVER OALL~S COUNTY > 457 CD 1 t.-- 449 t 0 t 425 · ..... _J L .... 3500 4500 5500 6500 7500 8500 gsoo ~0500 ~ ~500 ~2§00 DISTANCE, FEET CROSS SECTIOJ',,I _1. ~39. P. i::L~ YORK '-I-~Zt'qTY I:::I:VER DALLAS COUNTY I---444 I _J J j ; J ~ 4~8 3000 4000 5000 6000 7000 BO00 go00 lO000 i lO0012000 DISTANCE, FEET CROSS SBC'T'iON i I.~9.35 EL.M FORK-i"R~NTY RI_VE,q DALLAS COUNTY LU 441 0 ~ 433 425 ~350 3350 4350 5350 6350 7350 8350 9350 10350 11350 O I S ?ANCE, FEET CFtOo - ~S ,8~CTlOiq i!5J. 39 67 EL~ FORi( TR)iNYY ~LAS COUNTY ~ ~ I ~ ~ / 3350 4350 5350 6350 7350 8350 9350 I0350 ~1350 D ~[ S~'~'~"?' F~ ,~OSS SZC-r''~' ' _ :q .L ..'.L . ~,.. I cLNI.FORK 'l'Ri,~'~, ~, ~, R~. vEq DALLAS COUNTY 452 __ > 448 ................................... ~t'~ 440 ~ ............... LL. , ~ 434 3500 4500 5500 6500 7500 8500 9500 ~0500 ~ ~500 ~2500 APPENDIX 9 USGS GAGE CALIBRATION HEC-2 MOOEL 26 APPENDIX I0 BASELINE HEC-2 (1981 CONDITIONS) MODEL ~7 APPENDIX 11 PRESENT CONDITIONS HEC-2 FLOOD PLAIN AND FLOODWAY MODEL