M&E Consultants was engaged by Texas State Soil and Water Conservation Board (TSSWCB) and Natural Resources Conservation Service (NRCS) to prepare the detailed design for the rehabilitation of Lower Running Water Draw (LRWD), Site 4 originally constructed in 1976 with a height of 28 ft, a dam length of 3,444 ft, and is comprised of about 101,280 cubic yards of earth and rock fill. The dam is in Hale County, Texas approximately 5 miles west northwest of downtown Plainview, Texas. Originally designed for floodwater detention and sediment retention, the dam was built as a significant hazard potential structure in an agricultural area. There has been some development downstream of FRS No. 4. The Texas Commission on Environmental Quality (TCEQ) and the United States Department of Agriculture (USDA), the NRCS (formally Soil Conservation Service (SCS)), both concur that LRWD Watershed FRS No. 4 is a high hazard potential structure based on current criteria.
The selected alternative in the supplemental work plan for rehabilitation included:
- Removing the existing principal spillway (PS) inlet and installing a new standard inlet tower with the crest elevation at the existing low-stage crest, El. 3408.0. The current principal spillway crest is El. 3411.2. The new inlet will connect to the existing conduit.
- Lowering the existing earthen auxiliary spillway (AS) crest from El. 3419.8 to El. 3418.1 and re-grade the inlet and outlet channels.
- Installing a 340 feet wide roller compacted concrete (RCC) AS over the existing embankment with crest El. 3415.7. The existing 30-inch conduit will empty into the RCC stilling basin.
- Lower the top of dam (TOD) crest from El. 3424.1 to El. 3424.0, re-grade the upstream slope to original slope dimensions, and flatten the downstream slope to 3H:1V.
The NRCS conducted a topographic survey in December 2014 and M&E performed an additional topographic survey in January 2017 to develop current contours and elevations of FRS No. 4. SITES was utilized to evaluate the existing and alternative dam/spillway configurations for capacity, conformance to TR 60 requirements, and to assess the stability and integrity of the existing vegetated auxiliary spillway. Stage-area relationships were determined from available LIDAR data combined with a bathymetric survey of the pool area conducted by M&E. Modifications of the existing vegetated spillway in combination with various structural (RCC) spillway were compared using SITES to identify the most economical rehabilitation alternative. After the planning process was completed, The National Oceanic and Atmospheric Administration (NOAA) updated the rainfall data as published in Atlas 14, Volume 11, Version 2.0. The changes for Lower Running Water Draw, Site 4 indicates the 100-year, 24-hour rainfall decreased from 6.7 inches to 6.4 inches and the 100-year, 10-day rainfall decreased from 5.2 inches to 4.99 inches. There were minor changes to the preferred alternative because of the new rainfall data.
The design included construction drawings and specifications, engineering cost estimate, construction performance time, SWPPP, QA Plan, O&M plan, and design report. M&E’s support of the ongoing construction phase has been to address design questions, provide engineering support for interpreting drawings and specifications or assist with modifications, and provide a construction engineer to assist the NRCS construction inspector. This project illustrates M&E’s commitment to NRCS and the firm’s knowledge of NRCS policies, procedures and methodology for dam assessments, planning, design, and construction.