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<br />32.8--5 <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />1. The type and size of channel materials. <br /> <br />2. The availability of seed sources or likelihood of <br />vegetative sprouting. <br /> <br />3. The length of growing season during which this portion <br />of the channel is not inundated. <br /> <br />4. The availability of non-channel water sources capable of <br />supporting vegetation. <br /> <br />5. The water storage capacity of streamside substrate. <br /> <br />6. The shape of the channel cross-section. For example, a <br />large width/depth ratio (shallow, wide channel) has higher <br />potential than one with a small width/depth ratio (narrow, deep <br />channel) as in 1 and 2 of exhibit 3. <br /> <br />E. Common Stec 5. Survey channel cross-section to measure <br />channel area associated with potential vegetation encroachment. <br /> <br />The area to be considered for vegetation encroachment prevention <br />flows generally lies between the top of the inner berm and the <br />start of the channel banks. Survey the channel cross-sectional <br />area below the elevation defined by the start of channel banks. <br />These measurements are made at the point of quantification or at <br />the representative gage station (if sufficiently similar to <br />allow extrapolation). <br /> <br />F. Common Stec 6. Determination of vegetation encroachment <br />prevention flows (QLY)' <br /> <br />The discharge needed to prevent vegetation encroachment is <br />calculated using the cross sectional area measurements and a <br />stage-discharge relationship, if available at the quantification <br />point, or the Manning and continuity equations. An alternative <br />approach utilizes the ratio of cross-sectional areas, that is, <br /> <br />b b <br />Ay /AB = QLy/QB' and solving for QLY' <br /> <br />Note: Use b as defined in a relationship of the form Q = KAb <br />derived from gage station records or local data. For method D <br />assume b = 1. <br /> <br />If vegetation encroachment potential at the point of quantifi- <br />cation is similar to that of the representative gage station, a <br />ratio for vegetation encroachment prevention flow can be <br />developed at the representative gage station using stage- <br />discharge or hydraulic geometry relationships and applied <br />directly at the point of quantification. <br /> <br />*-FSH 4/89 AMEND 3-* <br />