ARCHIVED REPORTS_2013_4 - Laserfiche WebLink

Home Browse Search

® 2.2 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. 2.3 Stable Chatinel Design The new channel realignment needs to be stable and function as a natural system. Excessive bank erosion and meandering has to be minimized. The design needs to anticipate the vegetative succession and perform without flooding or sustaining excessive bank erosion under those conditions. 3.0 DESIGN CONSIDERATIONS AND METHODOLOGIES The following discussion outlines the primary design parameters that were considered and incorporated into the project design. 3.1 Hydrologic Considerations One of the basic design parameters of a channel is the amount of flow the channel must carry. Typically only the maximum flow is considered.However,in order to design a natural stable channel a full spectrum of flows must be taken into account. Though large magnitude flows are important from a flood control standpoint they typically do not occur at frequent intervals and thus do not contribute to the year in and year out forces acting on the channel, which really shape the channel. This typically is referred to as the channel forming discharge.Large magnitude discharges typically cause catastrophic bank failures and play a role in the molding the general channel configuration,at least locally. However,it is the typical winter flows that occur on the order of every one to two years that drive general channel shape and morphology through erosion and sediment transport The hydrologic regime of the North Branch of the South Fork of Littlejohn's Creek is somewhat unusual. Farmington Reservoir is a flood control reservoir operated by the U.S. Army of Corps of Engineers located upstream,that controls stormwater flows in the North Branch of the South Fork of Littlejohn's Creek. A review of average daily releases from Farmington Reservoir for 21 water years (1979 to 1999)shows the maximum release was 2,420 cubic feet per second(cfs).This stormwater is divided several tributaries. Often, and especially during drought years, there was no release from Farmington Reservoir. FEMA Flood Flows The Federal Emergency Management Agency (FEMA) routinely publishes stream flows associated with return interval(years)for flood insurance purposes.Table 1 below indicates the magnitude and recurrence interval for peak events at the Austin Road Landfill. Questa Engineering Corporation 2 99279CMApril b, 2000