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I I I I I I I I I I I I I I I I I I I 4.1 SECTION 4 CHANNEL STABILITY GENERAL The evaluation of channel stability is dependent on several factors: (a) geologic setting of the channel and locations of bedrock outcroppings or other natural features providing grade control; (b) historical lateral and vertical migration of the channel in response to water and sediment delivery and conveyance; (c) erosion processes and failure mechanisms associated with existing bank stability problems; (d) existing man-made structures providing grade control; and (e) development of the basin resulting in potential increases in stormwater and sediment runoff. Urbanization will typically increase peak discharge and runoff volumes. Development may also result in a temporary increase in sediment discharge during construction. Following construction, sediment delivered to the channel can often decrease resulting in a sediment deficit condition that may induce channel and bank instability Natural streams tend to adjust to an equilibrium condition where the channel has the ability to transport water and sediment proportional to amount of water and sediment delivered from upstream and lateral sources. This characteristic is governed primarily by parameters such as slope, depth of flow, effective flow width, and the size of the available bed material. . The Elk River is a complex system influenced by a number of the aforementioned factors. The nature of the bed and bank material, existing natural and artificial grade control structures, the potential for lateral and vertical migration of the channel, and surrounding land use characteristics all affect the existing system. Man-made and natural features appear to be limiting bed lowering along the existing Elk River at several locations within the study reach. The man-made controls include: . The bridge at U.S. 40 . The bridge at the Denver and Rio Grande Western Railroad In addition to these man-made features, the confluence with the Yampa River influences bed levels at the lower portion of the Elk River. These existing controls have the effect of artificially holding the channel bed at grade and could increase the potential for bank erosion and channel widening. There is evidence at some locations that these controls have created areas of sediment deposition upstream while promoting channel scour and bank erosion at locations immediately downstream. With the aforementioned bed level considerations, bank erosion and lateral migration have a strong influence on the stability of the existing Elk River channel. For the majority of the study reach, bank erosion is primarily a function of local flow-induced shear stresses where flow impingement causes under-mining of the banks and results in mass failure of the overlying bank material. Detachment and removal of individual particles appears to playa minor role in significant bank movement. Lateral adjustments occur primarily as a result of mass failure and the subsequent transport of the failed material. 23