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ST9401-CS 950328WIER & ASSOCIATES, INC, March 28, 1995 Mr. Kenneth Griffin, P.E. City Engineer Engineering Department City of Coppell 255 Parkway Boulevard Post Office Box 478 Coppell, TX 75019 214/462-0022 RE: W&A # - 94093, DENTON TAP ROAD; Coppell, Texas DeaF: In our meeting last Friday we reviewed the drainage patterns and possible culvert construction options for proposed Denton Tap Road north of Denton Creek. The improvement options were based on extending the proposed 3-6' x 5' culvert preliminarily planned for construction by the Texas Department of Transportation. In that meeting we indicated that extending the culvert along Denton Tap Road to Denton Creek would involve the following: There is limited horizontal space available between the existing roadway surface and existing utilities on both sides of the roadway. A wide structure (20' out to out for culvert) would be difficult to install while keeping traffic in service and only one lane of the road would be open at the point of installation. A typical storm drain pipe could more easily be installed and would not require closing lanes. The culvert would take up considerable space limiting opportunities fo~ installation of other utilities. The culvert extension improvements would be extremely expensive due to the construction difficulty, and large size caused by a high downstream tailwater. We estimated the cost to be on the order of $600,000. Do The outlet of the structure would be near or at the Denton Creek bridge crossing, which would increase erosion potential at this critical location. In our meeting, I reminded you that we were of the opinion that the water course has been diverted by adjacent property owner reclamation activities. The diversion if honored potentially transfers substantial drainage improvement costs to the city. In our Mr. Kenneth Griffin, P.E. W&A #94-093 March 28, 1995 Page 2 of 4 meeting you directed us to research and document as best we could the history of alterations to the drainage patterns. Also, you asked that we provide our opinion of responsibilities for drainage costs and recommendations. The remainder of this letter will provide our findings, opinions, and recommendations. The watershed of main consideration is referred to as Watershed "A" on the attached exhibits and in the discussion as follows: I. PRE-RECLAMATION WATERSHED PATTERNS: We obtained several topographic maps applicable at different points in time on the tract. We have copied portions of maps at previous points in time which are attached and are described as follows: A. Portion of 1959 USGS map marked Exhibit "A". B. Portion of February 9, 1987 Vista Ridge Area Plan by Carter and Burgess, Inc. provided by the city and marked as Exhibit "C". C. Portion of cities topographic mapping dated February 29, 1992 and marked as Exhibit "E". For purposes of discussion, the property west of Denton Tap Road will be referred to as the Coppell 200 JV tract and the east property as Vista Ridge. The February 9, 1987 Vista Ridge Area Plan is the most revealing map. One foot interval contouring is shown on this map which appears to be the effective condition prior to reclamation activities by the property owners east and west of Denton Tap Road (refer to Exhibits "C" & "D"). This map also indicates the proposed reclamation contours on the Vista Ridge tract. This map indicates the area west of Denton Tap Road indicated as Watershed "A" to drain south within the Coppell 200 JV tract in generally a well defined channel to the south side of proposed Spur 553. The channel becomes less defined south of Spur 553, but clearly flows southeast through the Coppell 200 JV tract to the two existing culvert crossings on Denton Tap Road. The flow crosses under Denton Tap Road then drains east within the Vista Ridge tract approximately 2400' to an existing creek. The creek then drains south to Denton Creek. The 1959 USGS map (refer to Exhibits "A" & "B") indicates most of the creek channel east of Denton Tap Road. Our conclusion of the pre-reclamation drainage pattern is that Watershed "A" drained through the Coppell 200 JV tract to Denton Tap Road at the existing culverts then drained east through the Vista Ridge tract. The existing culverts on Denton Tap Road provided cross drainage (only) between the Coppell 200 JV tract and the Vista Ridge tract. It is apparent from the 1987 topography indicated on Exhibit "C" (prior to reclamation), the ridge along the west side of Watershed "A" on the Coppell 200 JV tract was low vertically in relationship to the channel low and is close to the channel. Due to the shallow channel depth, we believe in a flood event a large portion of Watershed °'A" would in fact have bypassed and drained to the south across the watershed divide thence directly into Denton Creek. In one location the difference in the top of the divide and the low channel is only 0.8' according to the spot elevation shots indicated on the map. We made a rough calculation Mr. Kenneth Griffin, P.E. W&A #94-093 March 28, 1995 Page 3 of 4 II. III. assuming inlet flow control on the culverts with the head on the culvert at the divide elevation (459.5) and estimated the most capacity the culvert could have passed before bypassing to be 372 cfs approximately. The culverts would probably not be discharging under inlet control and much of the flow would have bypassed across the ridge upstream of the culverts. POST RECLAMATION WATERSHED PATTERNS: In the late 1980's reclamation was performed on portions of the Coppell 200 JV tract and the Vista Ridge Property. Exhibits "E" & "F" are duplicated from the cities current topographic map which reflect the results of the reclamation projects. Exhibit "E" is a copy without comments and Exhibit "F" reflects information added to demonstrate the impacts of the reclamation. Reclamation activities on both the Coppell 200 JV tract and the Vista Ridge Tract diverted the channel. On the Coppell 200 JV tract the drainage channel was diverted at point 1 to drain to Denton Tap Road at point 2. The flow then drains south to the two culverts and crosses under the roadway to point 5. From point 5 the channel was diverted by construction of a new channel to Point 6 where a riprap structure was constructed at the drop into Denton Creek. The existing channel was filled on both properties. Also, the filling on the Vista Ridge tract directed some area east of the roadway to drain toward the roadway. On the Coppell 200 JV tract, the reclamation and filling extends to the south culvert crossing on Denton Tap Road (Culvert B). West of Denton Tap Road Downstream of the culvert storm runoff will divert from the watershed over the drainage divide once the flow depth exceeds 459.5 The impacts by the Coppell 200 JV tract include diversion of the channel, directing additional watershed area to Denton Tap Road and altering the bypass flow conditions. The impacts by the Vista Ridge reclamation include diversion of the channel, directing additional watershed area to Denton Tap Road and probably increasing the downstream tailwater elevation at Denton Tap Road. The tailwater condition results from the filling of the wide flat lower downstream channel which was replaced by a narrow ditch redirected to Denton Creek. OPINION OF CHANNEL MODIFICATIONS: We believe the channel modifications substantially benefit both of the adjacent tracts by creating more usable property less hindered by cross drainage ways by directing the drainage channels to Denton Tap Road. We are not aware of any previous agreements made between the property owners and the city which placed cost responsibility for the diversion of the channels on the city. We cannot see why the city would be a party to a diversion agreement without commitments by the property owners to participate in the additional drainage improvement costs. If the diverted drainage channels are honored in the proposed improvements to Spur 553 and Denton Tap Road, substantial additional drainage improvement costs will be bore by the city of Coppell. It is our opinion that the city and the state should not bare any additional costs caused by the diversion of the channels. The city and the state should only be willing to install the drainage in the diverted locations if the net cost difference is bore by the two adjacent property owners. Mr. Kenneth Griffin, P.E. W&A #94-093 March 28, 1995 Page 4 of 4 IV. RECOMMENDATION: We suggest the city inform the TxDOT, and the two adjacent property owners of our findings and arrange a meeting to discuss possible solutions. We believe a solution that considers the drainage along the pre-reclamation alignment of the channel and the bypass of part of Watershed "A" to Denton Creek would be a reasonable solution. Another solution could include directing the runoff through the Coppell 200 JV tract to the Denton Creek flood plain. The watershed partially bypassed to this route prior to reclamation activities and this is the shortest most direct method to direct the runoff to Denton Creek. The diverted watershed "B" is insignificant in comparison to the total watershed of Denton Creek. If a solution cannot be reached satisfactory to the parties, we suggest the city take the action as follows: Direct the TXDOT in their design of Spur 553 to design systems that convey the watershed to at least point 3 at an elevation at the existing culvert flow line of 455.3. If the state wishes to direct the runoff along Denton Tap Road, it would be in an enclosed system meeting the 100 year design criteria and would need to be below subgrade line of the proposed pavement. The city would provide a proposed pavement profile. Based on the 100 year flood plain upstream of Denton Creek at elevation 460.31, low points along Denton Tap Road will be at elevation 462.31. The profile of the roadway will need to contain a series of local low points and high points to maintain the minimum low point elevation at 462.31. Advise the adjacent property owners that the city will design cross drainage at Denton Tap Road from points 3 & 4 to point 5 and will release the flow at this location at the existing culvert flow line. The city will also design parallel drainage along Denton Tap Road north of Culvert A to collect a short distance off the roadway. The adjacent property owners will be responsible for improved condition cross drainage through their tracts when developed. We are available to meet with you or others as you direct to resolve the drainage design concept. Please call me if you have any comments or questions. Very truly yours, ULYS LANE III, P.E., R.P.L.S. Executive Vice President WIER & ASSOCIATES, INC. UTL:ckw Enclosures s6 5 1 ooom. N. 3650 L / ]. G Ill }: ,.,----__~ %~}:':"' ] h " "' } ;j )))) f 9, 3649 ~,~ / ,~X/,'/z~ / r ~ " --_ ;,)o _== I,,..: lOoa~ 36 5 1 ooom. N. 451 ....... - ============================ :.: