M&E developed a Supplemental Watershed Plan and Environmental Assessment (EA) for the rehabilitation of Lower Running Water Draw 4. The purpose of this supplemental watershed plan is to meet current safety and performance standards for a high hazard dam and to provide continued flood protection for downstream properties for a 100-year 24-hour flood event. The dam currently provides flood protection benefits to about 1,307 acres of agricultural and urban land, 117 properties (48 residences, 59 commercial, and 10 public), one interstate highway, 2 major state highways, 9 city streets, and 4 unpaved roads. However, if the dam were to fail, there are 66 residences, 58 commercial properties, 12 public properties, and 16 roads located downstream that would be impacted. The risk of loss of life and property damage due to potential over-topping breach of the dam during an extreme flood event (probable maximum flood, PMF) is the reason that FRS No. 4 needs to be rehabilitated.
The preferred alternative removes the existing principal spillway (PS) inlet and installs a new standard inlet tower with the crest elevation at the existing low-stage crest elevation. The new inlet will connect to the existing conduit. Lower the existing earthen auxiliary spillway (AS) crest elevation by 2.1 feet and re-grade the inlet and outlet channels. Install a 340 feet wide roller compacted concrete (RCC) AS over the existing embankment with the crest elevation 1.6 feet lower than the earthen AS. Lower the top of dam (TOD) crest elevation 0.1 feet, re-grade the upstream slope to original slope dimensions, and flatten the downstream slope to 3:1. The existing 30-inch conduit remains in place and will empty into the RCC stilling basin. Total required acreage for easements for this alternative is 305 acres and covered by the current easements of the sponsors. The RCC structure and extension of the downstream toe for flattening the embankment slope will extend beyond the existing fences (about 1.2 acres total) but remain within the current easement area. Best management practices during construction will be utilized to avoid and minimize any potential adverse impacts. Construction activities will result in the disturbance of approximately 27 acres (confined to embankment and auxiliary spillway) and will require that a Storm Water Pollution Prevention Plan (SWP3) be in effect. All disturbed area will be re-vegetated using adapted and/or non-invasive native species. Planting equipment will be cleaned and certified seed will be used to prevent the spread of invasive species. No compensatory mitigation will be required as a result of implementing this alternative.