ARCHIVED REPORTS_2013_4 - Laserfiche WebLink

Home Browse Search

2.2 Jurisdictional Areas A wetland delineation completed by Monk and Associates August 2007(Appendix A),the existing creek channel to be relocated has approximately 1.25 acres of open water habitat and wetlands. The width between the Ordinary High Water Marks (OHWM) varied between 16 to 20 feet. 2.3 Flood Control Conditions The existing channel performs relatively well from a flood control perspective. According applicable existing flood plain mapping the channel generally has the capacity to convey the 100- year flow throughout the relocated portion. However,the channel upstream and downstream of the relocation reach does not have adequate capacity. This portion of the creek generally flows through agricultural areas. Flows exceeding the 50-year event spill from the channel and flood the adjacent farm fields. It is questionable that all of the predicted 100-year flow reaches the channel along the Forward Landfill. Figures 1 and 2 show the applicable FEMA flood insurance panels for the area. As can be seen on these maps large amounts of flow leave the channel upstream of the proposed reach and flood agricultural land to the south. 3.0 DESIGN GOALS AND OBJECTIVES The goals and objectives of the relocation project can be divided into three general categories: (1) provide riparian habitat restoration/creation to mitigate the filling a portion of the South Branch of ® the South Fork Littlejohn's Creek that exceeds current value and area of the project reach; (2) provide adequate flood control in the realigned section of the creek;and(3)provide a stable channel design that meets or exceeds the functions and values of the existing creek. 3.1 Habitat Recreation The proposed relocation concept is to create a channel realignment that recreates the function and habitat value of a natural creek system in this region.The concept is to create a riparian zone that has an appropriate mix of vegetation types and functions naturally with little maintenance and intervention. The plans call for recreating approximately 4 acres of jurisdictional wetlands (3:1 mitigation ratio) and 9 acres of riparian zone (2:1 mitigation ratio) as shown on Sheet 2. The long-term goal is to create a natural,self-sustaining ecosystem that provides high quality aquatic and terrestrial habitat, with minimum maintenance requirements, while at the same time meeting flood management objectives. The project will provide a variety of habitats; regularly inundated wetlands and emergent marsh, riparian corridor, and upland oak savannah. These habitats can be viewed from the previously completed north branch restoration project. These areas will provide resident and foraging habitat for both terrestrial and aquatic species. 3.2 Flood Control The realigned channel needs to carry the 100-year flood flows within its banks. Erosion protection should be provided in areas with high velocities or sharp bends. A U.S. Army Corps of Engineers HEC-RAS hydraulic model was used to determine design water surface elevations and estimate channel velocities and other pertinent flow parameters for stable channel design. Questa Engineering Corporation 4 270124CDR/November 16, 2007