Laserfiche WebLink
<br />It is necessary to position the downstream end of the model where <br />there is a stable rating curve or known water surface elevation. <br />In degradation studies this may be several miles downstream from <br />the dam at a rock outcrop or concrete weir. For studies in reservoirs <br />the operating policy will establish the reservoir level for the water <br />surface profile computations and the program will adjust the bed <br />according to calculated results. The overbanK subsections do not have <br />a movable bed. <br /> <br />h. Dredging. Part or all of the movable bed portion of a cross section <br />can be dredged at the beginning of each computation cycle or at selected <br />points in time. The bottom elevation, lateral limits and depths of <br />overdredging are specified on H-cards. When bottom elevations are <br />higher than that specified for the dredged channel bottom, all points <br />are lowered to the depth of overdredging. Outside of the dredged <br />channel the points are not changed. Sediment material is assumed <br />to be removed from the channel, but does not decrease the cross <br />sectional area of the overbank as an actual spoil pile would. <br /> <br />The elevation of channel bottom is calculated at the end of each <br />computation cycle (W-card). The total volume of deposits is <br />calculated and dredging is done after this calculation is made. <br />Two options are available for dredging. The $DREOGE card triggers <br />a detailed calculation for the amount of material to be dredged. <br />This material is removed from the section and coordinate points <br />are modified. This option is expensive to run and can be turned <br />off with a $NO DREDGE card. An alternative option is to emulate <br />dredging by changing cross section coordinates but not the actual <br />volume of deposits. Therefore, the volume of deposits continues <br />to increase even though, at the time a water surface profile is <br />calculated, the proper dredged channel is used. <br /> <br />This is a computation expedient to save computer time--not a short <br />cut in the calculations. As long as dredging is requested for each <br />computation cycle, (col. 7 on card *, Hydrologic Data) results will <br />be correct. However, if this emulation is stopped by the "N" opt <br />(*-card, col. 7), subsequent results will be completely erroneous <br />unless the $DREDGE option was requested just prior to that *-card. <br />The $DREDGE option is required to reshape each cross section and <br />calculate the correct volume of deposits. <br /> <br />i. Ineffective Area. When high ground or some other obstruction <br />prevents water from flowing into a subsection, the area up to that <br />point is ineffective for conveying flow and should be suppressed <br />until the water surface exceeds the top elevation of the obstruction. <br />An elevation can be established to identify the point below which <br />overbank area is ineffective. <br /> <br />11 <br />