Laserfiche WebLink
<br />4.2.4 IrriKation Diversions <br /> <br />The irrigation diversions divert sediment as well as water. Attempts are <br />made to minimize the volume of sediment diverted, particularly of the coarser <br />size fractions. No data were available on the relationship between diverted <br />water and diverted sediment. HEC-6 does not use a load curve to determine <br />quantities of sediment diverted; rather a relation between the concentration <br />of sediment in the diverted water and that in the main stem is used. For this <br />modeling effort, it was assumed that all the diversions divert silt at a <br />concentration equal to the ambient silt concentration in the main stem. The <br />concentration of all sand fractions in the diverted water was assumed to be <br />75~ of the corresponding main stem concentration. (75~ was used as the ratio <br />of diversion to main stem concentration in the John Kartin sedimentation <br />report). It was assumed that none of the gravels would be diverted. These <br />factors can be quite easily varied for refinement of the data set as necessary. <br /> <br />4.3 Development of Hydrologic Data <br /> <br />To simulate the behavior of a stream for a 100-year period with a movable <br />boundary model such as HEC-6, one needs a continuous flow record for a <br />100-year period. Typically, flow data are obtained as mean daily discharges <br />and then aggregated into longer-period, variable time steps to minimize the <br />computational effort. As continuous flow records rarely exist for 100 years <br />or more, the modeler must assemble, constt'Uct, or synthesize the record <br />somehow. Development of appropriate long-term flow sequences for sediment <br />routing is an important research topic yet to be addressed. The procedure <br />described below is reasonable and has been used on previous studies. <br />Development of the flow record proved to be the most difficult and time <br />consuming aspect of this study. <br /> <br />Daily stream flow data were obtained for eight gages (Arkansas River and <br />tributaries between the towns of Avondale and Las Animas) from U.S. Geological <br />Survey records. Monthly flow volumes for the eleven diversion structures <br />within the study reach were obtained from the State of Colorado. <br /> <br />A base time period of 1941 to 1980 had the most overlapping, continuous, <br />daily data for all the necessary gages (Fig. 4.9). After development of a <br />discharge hydrograph for the 40-year period as described below, that flow <br />record was repeated 2.5 times to produce a 100-year flow record. <br /> <br />Missing tributary records were intet'polated on a monthly basis using HEC-4 <br />(8). The interpolated monthly values were then converted to daily flow <br />values. This was accomplished by taking the interpolated missing monthly <br />volume and dividing it by the observed monthly volume of a nearby tributary <br />stream gage. This produced a ratio which was used to multiply the nearby <br />gage's daily flow values. Chico Creek's missing daily flow values were <br />patterned after the Huerfano Gage. Timpa Creek's missing daily flow values <br />were patterned after the Apishapa Gage. <br /> <br />The first adjustment made to the tributary flows was to modify them for <br />ungaged and non-contributing drainage areas. This was necessary to provide an <br />estimate of sediment being delivered from adjacent ungaged areas (through the <br />tributary sediment load curve). <br /> <br />24 <br />