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<br />The Manning equation was developed for uniform flow in which the water-surface <br />profile and energy gradient are parallel to the streambed, and the area, <br />hydraulic radius, and depth remain constant throughout the reach. It is <br />assumed that the equation also is valid for the nonuniform reaches usually <br />found in flood plains. The energy equation for a reach of nonuniform open- <br />channel between sections 1 and 2 shown on figure I is <br /> <br />(h+h )1 = (h+h )z + (hf)l-Z + k(~ )l-Z, <br />v V V <br /> <br />(2) <br /> <br />where <br /> <br />h <br />v <br /> <br />h = elevation of the <br />above a common <br />= velocity head at <br /> <br />water surface at the respective sections <br />datum, <br />the respective section = ~VZ/2g, <br /> <br />~ = the velocity head coefficient, <br />hf = energy loss due to boundary friction in the reach, <br /> <br />~ = upstream velocity head minus the downstream velocity head, <br />v <br /> <br />k(~ ) <br />v <br /> <br />= energy loss due to acceleration or deceleration in a <br />contracting or expanding reach, and <br />k = a coefficient, generally taken to be 0.0 for contracting <br />reaches and 0.5 for expanding reaches. <br /> <br />LIMITATIONS <br /> <br />Several limitations of the Manning equation and hydraulic calculations <br />include streams subject to debris flow, very high-gradient streams, and <br />modification of the channel during a flood. <br /> <br />Debris flows, mudflows, and debris and alluvial fans are common through- <br />out Colorado mountain regions. The hydraulic characteristics of debris flows <br />and mudflows are such that the selection of n values for them and subsequent <br />conventional hydraulic analyses probably are not applicable because of the <br />large sediment load, channel scour and deposition, and a lack of a well <br />defined channel. These hazard areas can be identified from geomorphic and <br />sedimentologic evidence that remains in the flood plain and generally are <br />found in small, steep watersheds and at the confluence of these watersheds <br />with larger streams (Costa and Jarrett, 1981). <br /> <br />A debris flow is a heterogeneous mixture of water and sediment of differ- <br />ent sizes and has a high degree of fluidity. Evidence of past debris flows <br />may consist of levees of poorly sorted debris that border the channel. <br />Debris-flow deposits are more poorly sorted than water-flood deposits, and the <br />largest rocks are concentrated near the surface and edges of the deposits. <br />There is no evidence of excessive discharge downstream from the deposits. A <br />mudflow is similar to a debris flow except the material is predominantly fine <br />grained. An indication of a mudflow is a coating of mud on the ground <br />surfaces, obstruction, and vegetation. <br /> <br />3 <br />