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Section 3.0 Baseline Hydrology Analysis 3.3 Streams 3.1 Major Watersheds Streams (major drainageways) draining 130 acres or more of subwatershed area were defmed as shown in Appendix F. To provide an orderly system of managing stream information and hydrologic results, streams (above each confluence point) were designated with an identification number, Streams were numbered consecutively from downstream to upstream, Coyote Run was designated as Stream 100. Thirty-eight tributaries were identified in the Coyote Run watershed (Streams 101-138), Box Elder Creek within the study area was designated as Stream 200, Fifty-one tributaries were identified in the Box Elder Creek portion of the study area (Streams 201-251), In all, 91 separate streams were identified within the study area, In addition, Box Elder Creek upstream of the study area was given the designation Stream 400 and one tributary upstream of the study area was identified with the designation Stream 401. The study area was divided into five major watersheds and modelled separately, to man- age the size of the models, Watersheds 1 and 2 pertain to downstream and upstream Coyote Run, Watersheds 3, 4, and 5 pertain to lower, middle, and upper Box Elder Creek within the study area. In addition, the watershed upstream of the study area was modelled in order to provide representative inflow hydrographs at the upstream study limit. This upstream watershed was designated as Watershed 6. The Watershed 6 analysis was conducted with less detail than the analysis of the five watersheds within the study area 3.2 Subwatersheds Each stream shown in Appendix F was divided into a series of adjoining conveyance elements, The conveyance elements were designated with even numbers, increasing consecutively in an upstream direction, Design points (direct routing elements) were coded into the hydrologic models between each pair of conveyance elements, The design points were designated with odd numbers that faIl appropriately between each pair of even conveyance element numbers. Each of the five watersheds in the study area were divided into subwatersheds as shown in Appendix D, The subwatersheds were numbered consecutively from north to south and limited to a maximum area of 130 acres. Watershed 6 (upstream of the study area) was divided into subwatersheds with areas no greater than 10 square miles, A work map of Watershed 6 is shown in Appendix E, As shown in Table 3-1, the study area was divided into a total of 615 subwatersheds with an average area of 78 acres and Watershed 6 was divided into 18 subwatersheds with an average area of 7 square miles. Some of the streams shown in Appendix F have been identified with names and numbers on UDFCD Drainage Basin Description Maps (UDFCD, 1990), Table 3-2 provides a cross reference between the study stream numbers and the eqnivalent UDFCD numbers and names, Table 3-1 Subwatershed Distribution Watershed Number Area I Number of (Sq. mi.) Subwatersheds 1 (Coyote Run downstream) 12 94 2 (Coyote Run upstream) 17 136 3 (Box Elder downstream) 11 98 4 (Box Elder middle) 18 156 5 (Box Elder upstream) 17 131 Subtotal, Study Area 75 615 Average Subwatershed Area 0.12 (78 acres) -- 6 (Box Elder upstream of study area) 127 18 Average Subwatershed Area (square miles) 7 Total 202 633 Variations exist between stream delineations shown in the Drainage Basin Description Maps and the streams shown in Appendix F. The streams shown in Appendix F represent major drainageways (streams draining 130 acres or more), which include streams not shown on USGS mapping, and do not include USGS stream reaches that extend far upstream into 130 acre subwatersheds, 3.4 Hydrologic Models The UDFCD's Colorado Urban Hydrograph Procedure (CUHP) (UDFCD, 1985a) model was used to develop runoff hydrographs for each subwatershed, The runoff hydro graphs were routed through the existing stream network using UDFCD's Storm Water Management Model (UDSWM2) (UDFCD, 1985b), The UDSWM2 model does not have a channel infiltration component; however, channel infiltration, even though it occurs, is not expected to significantly affect peak discharges during extreme flood events. DENlOO1768l.WP5 3-1