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3.3 Stable Channel 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. We are fortunate in that we have a very similar project recently completed. We have observed the behavior of the channel closely and have adapted our grading plan to geometries that are more natural and provide more consistent connection to the adjacent floodplain. Representative and typical cross sections are shown on Sheet 3. 4.0 DESIGN CONSIDERATIONS AND METHODOLOGIES The following discussion outlines the primary design parameters that were considered and incorporated into the project design. 4.1 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 ichannel.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 South 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 South 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 through a series of diversion and gates. Released flow is often intended for irrigation supply and/or flood control risk management.As such the flows in the channel do necessarily follow typical storm water runoff hydrographs. Moderate flows may be sustained for a periods of weeks rather than days. Often,and especially during drought years,there was no release from Farmington Reservoir and little in creek other than agricultural tailwater. 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 5 270124CDR/November 16, 2007