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ST9401WA-CS 951106WIER & ASSOCIATES, INC. November 6, 1995 Mr. Ken Griffin, P.E. Assistant City ManagedCity Engineer City of Coppell 255 Parkway Boulevard P.O. Box 478 Coppell, TX 75019 214/304-3683 · FAX 214-304-3673 Re: W&A #94-093; Denton Tapp Road Reconstruction; Coppell, Texas Dear Ken: Several weeks ago we met regarding the city review comments on the Conceptual Plans for proposed Denton Tap Road. At that meeting we learned that the city council had decided to delay construction of the roadway project but would proceed with the 16" water line construction ahead of the bridge construction. The original design concept would provide placement of the 16" water line on the proposed bridge crossing. An alternate method of installing the water line across the creek would need to be established. Also, several alterations were discussed in the bridge crossing design to accommodate expanded hike and bike trail capabilities. We were asked to prepare revised conceptual designs for the waterline crossing ahead of bridge construction and revised bridge details for the hike and bike trail additions. This letter report and accompanying revised conceptual plans include our submittal for the water line and bridge changes. The conceptual plans attached are discussed in the following: I. 16" WATER LINE CROSSING AT DENTON CREEK: We have evaluated two options including a below grade crossing at the creek and an elevated crossing. The feasibility of a below grade crossing is discussed first below: A. BELOW GRADE CROSSING: The water line crossing options at the creek are impacted by the flow conditions established by water released from Mr. Ken Griffin, P.E. W&A #94-093 November 6, 1995 Page 2 of 6 Lake Grapevine. Currently flow is released from the lake continuously even in dry periods. We do not believe a below grade crossing should be attempted unless flow can be stopped or reduced to a very low flow during construction. Construction options with flow in the creek would probably require an individual Section 404 permit to dam up one side of the creek and lay pipe from side to side. This construction procedure would be difficult. The flow releases from the lake when above normal pool are regulated by the Corp. of Engineers Lake Control Division out of the Forth Worth District Office. The flood flow releases from area lakes are monitored at the Elm Street Bridge Crossing in downtown Dallas. Once the lake is at or below normal pool level the flow releases are considered to be low flow conditions and the volume is set by the City of Dallas and Lake Cities Municipal Utility District downstream to meet raw water usage demands. Flood flows are always higher than low flows. Low flows from Lake Grapevine and Lake Lewisville are combined to provide the required low flow volume to the cities downstream. Low flows from Lake Grapevine can be as low as 14cfs up to about 70cfs with typical low flows at 40cfs. Low flow periods generally occur in late summer and mid to late winter. We contacted the Corp of Engineers Lake Control Division by telephone and obtain a summary of flows for the past two years which is attached to this letter report. I discussed with the Corp of Engineers staff the feasibility of transferring the low flow requirement from Lake Grapevine to Lake Lewisville for a short period of time to allow downstream construction in the creek. We were advised that the City of Dallas could be requested to reduce low flows down to as low as zero flow. The Corp staff indicated this has happened before for short periods for construction on the creek. We contacted John Wilson of the City of Dallas Water Utilities Department to confirm the procedure for requesting zero low flows. Official approval will be required but the City of Dallas will probably agree to a zero flow condition. We contacted Mr. Bill White of Park Cities Municipal Utility District regarding permission for zero flow. Mr. White indicated Park Cities would be agreeable but would also need an official written request. If zero low flow can be obtained from the city of Dallas and Park Cities for a week, we believe a crossing can be installed below the creek. The benefits of a low water crossing constructed with no flow are as follows: The crossing will be less costly. The trench will be located upstream of the proposed bridge crossing and if backfilled with lean concrete, it can assist to protect the upstream bridge embankments from scour. The negative aspects of a below grade crossing are as follows: Mr. Ken Griffin, P.E. W&A #94-093 November 6, 1995 Page 3 of 6 It will be necessary to coordinate the construction along with Lake Grapevine seasonal low flow conditions. It will be necessary to obtain a zero low flow permit from the City of Dallas, Lake Cities and the Corps of Engineers. The pipe will not be accessible for maintenance or for inspection. Other public utilities could utilize an above grade elevated crossing. We believe an elevated crossing is more desirable since weather conditions and approvals from other agencies will not be required and the pipe will be accessible for maintenance. B. ELEVATED CROSSING: We evaluated the design concept for an elevated crossing and promptly determined that a preliminary design of the proposed bridge crossings would be necessary in order to locate the elevated bridge crossing horizontally, vertically, establish toe abutment location, and to locate the piers. The elevated crossing must fit into the future phased bridge construction process and the piers should be in line with the proposed bridge piers. Piers out of line with the future bridge would increase the probability of hang-up of trees and large debris unless the elevated crossing is located away from the proposed bridge. The attached typical bridge cross section and bridge plan indicates the location we recommend for an elevated pipe crossing. The crossing would be west and slightly upstream of the existing bridge. In the future the pipe crossing would be between the separated bridges for the main lanes of the divided roadway. In this location, the pipe crossing would be hidden from view for most vehicles by the future bridge railing. Vehicles with high seats such as trucks and vans could see the pipe crossing. The pipe crossing could be used for other public utilities. If an elevated crossing is installed, we would suggest requiring GTE transfer the existing five 4" fiberglass conduits on the existing bridge to the pipe crossing ahead of the future roadway construction. Other public utilities could attach to the elevated crossing clearing their underground facilities ahead of the future roadway construction. The elevated crossing would be accomplished by two wide flange beams supporting the pipe for the long spans with cross angels for lateral support. We expect the elevated crossing to cost approximately $75,000. It is our recommendation to construct the elevated pipe crossing which will require preliminary designing the bridge to confirm the pier locations, horizontal pipe crossing location abutment location and column cap elevation. We have prepared the attached preliminary elevations and plans for the bridge to reflect our preliminary bridge design efforts to date. The bridge design is discussed in more detail later on in this lette~report. Mr. Ken Griffin, P.E. W&A #94-093 November 6, 1995 Page 4 of 6 II. HIKE AND BIKE TRAIL MODIFICATIONS: In our last meeting you advised us the city would require 18' wide hike and bike tails under the bridge on both sides and a crossing over the east future bridge. We contacted Michael Cart regarding the bridge sections he prefers and incorporated his recommendations into the bridge sections attached. We can discuss any alterations you prefer to Mr. Carr's recommendations in a future meeting. III. PROPOSED BRIDGE CROSSING PRELIMINARY DESIGN: We performed a preliminary hydraulic study on the proposed bridge section after completion of the conceptual plans. In our last meeting we advised you that the bridge width proposed in the conceptual plans (which matches the existing bridge opening) would result in high velocities exceeding 12 feet per second for the fully developed 100 year flood event. We believe the bridge section area should be increased to reduce velocities at the bridge location. The anticipated scour depths for channel contraction, piers and abutments are increased proportional to the velocity. The hike and bike trail modifications will increase the bridge section and reduce velocities somewhat. The attached plan indicates the bridge section we recommend. We have performed considerable effort and research into evaluation methods for protection of the bridge from scour. Widening the bridge section will decrease velocities and reduce potential scour depths. The borings at the site performed by EmCON after completion of the conceptual plans are indicated on the attached bridge elevations. The borings at the bridge location indicate the materials along the creek bank are soft relatively erosive sandy clays and sand and the depth to hard shale for foundations is relatively deep. We have estimated the anticipated maximum scour depths at the pier locations and conclude that the scour must be mitigated by structural protection. The abutments outside of the piers will need to be lined and the inside of the piers protected from scour. Another consideration in the design materials utilized is the presence of ground water roughly below elevation 449. We have discussed the specific design options utilizing Gabion baskets with Craig Olden of the Olden Company and researched other options. The preliminary design concept we propose would include concrete riprap on the hike and bike trail and the abutment above the trail. Below the hike and bike trail to the Column locations 12" to 18" thick gabions or cable concrete are proposed to allow the water table to freely drain while protecting the slope. At the Column locations we propose a concrete Mr. Ken Griffin, P.E. W&A #94-093 November 6, 1995 Page 5 of 6 header at grade. Inside the columns along the main channel a means will be needed to protect the columns from scour. The constant flow in the creek channel along with the soft bank materials and ground water table will make construction of conventional measures difficult to install. We indicate sections for installation of gabion baskets cable supported concrete riprap and steel sheet piles. We estimate the costs for the three options to be as follows: Gabion Baskets - $91,000 Cable concrete - $75,000 Sheet piles with weathering steel - $102,000 Sheet piles without weathering steel - $75,000 Corrosion resistant steel and non-corrosion resistant steel costs are provided above. Attached is information regarding the corrosion resistance of steel sheets. This literature indicates that sheet piles driven in undisturbed soil and in fresh water conditions provide a long service life. Corrosion resistant metals can be used for additional safety. The sheet pile option we believe is the best option for the reasons listed below: 3. 4. 5. The work will not require major excavation of the channel bank which may require an individual Section 404 permit and permit preparation costs. Coordination with various cities to control lake low water flow will not be required. The deepest protection for maximum potential scour depth will be provided. As scour occurs, the channel section will enlarge to the greatest area reducing velocities which will reduce potential scour depth. The bank edge will need to be removed during construction and replaced. The soft bank materials will encourage erosion once replaced. Sheet piles are more structurally stable than gabions or cable concrete. The wire mesh on gabion baskets will deteriorate and rock will eventually be lost. Cable concrete can float if edges erode. A Section 404 General Permit is in place for Roadway Crossings (No. 14 Roadway Crossing) which allows up to 0.30 acres of impact on 404 jurisdictional property. We anticipate the sheet pile option will meet General Permit requirements but the other options may impact areas exceeding 0.30 acres. Considering the cost and construction aspects we believe the sheet pile method to be the best option. Mr. Ken Griffin, P.E. W&A #94-093 November 6, 1995 Page 6 of 6 We have prepared several exhibits attached to demonstrate the items discussed in this letter report. We hope to meet with you soon to discuss these items in more detail. Very truly yours, ULYS~N~E III, P.E.,'R.P.L.S. UTL:ga C: Rick Wieland, w/plan enclosures Howard Pafford, w/plan enclosures Michael Carr, w/plan enclosures Attachments: Conceptual Plan Summary of Flows Typical Bridge Cross Section SUMMARY OF FLOWS LAKE GRAPEVINE OUTLET SPILLWAY Month Sep-95 Aug-95 Jul-95 Jun-95 May-95 Apr-95 Mar-95 Feb-95 Jan-95 Dec-94 Nov-94 Oct-94 Sep-94 Aug-94 Jul-94 Jun-94 May-94 Apr-94 Mar-94 Feb-94 Jan-94 Dec-93 Nov-93 Oct-93 Sep-93 Aug-93 Jul-93 Jun-93 May-93 Apr-93 Mar-93 Feb-93 Comments 34 cfs avg. ~ 610 cfs one day 20 to 30 cfs 60 to 70 cfs Flood releases up to 500 cfs 15 cfs 13 to 17 cfs 200 to 300 cfs Flood Flow Flood Flow Flood - some days at 65 cfs Flood water most of month - 70 cfs some days 65 cfs - some Flood Flow Flood Flow - 65 cfs latter part of month Flood Release 40 cfs Flood Flow Flood Flow Flood Flow Flood Flow 44 cfs two days - rest Flood Flow 44 cfs 60 cfs to 44 cfs 60 cfs 40 to 73 cfs Flood Flow Flood Flow Flood Flow - one day 36 cfs Flood Flow Flood Flow 11/2/95 \94093\CFSCHART.XLS