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1653.3 F.L. Ogdm e./ "I./ JOllfMI of Hydr%fY 228 (2000J 82-100 90 g: 1653.2 . ~ 1653.\ W e ~ 1653.0 ~ . ~ -Observed S!mulated Ra,non lake Only ...----nn ~ .' 1652.9 ~-~ 1652.6 14151617 18 19 20 21 22 23 24 ~me (h) MDT. 28 July 1997 Fig. 4. Ob~e!'\led and simulated rise in Horsetoolh reservoir. 28 July .991 values for each different soil type were varied within pre-specified ranges during the calibration. Overland flow retention depths were also varied, but of second- ary importance. The CASC2D model of the Horse- tooth catchment is quite insensitive to overland flow and channel roughness coefficients because of the steep slopes in the Horsetooth catchment. The calibra- tion was performed manually until the model satisfac- torily simulated the rise in Horsetooth reservoir. The calibrated model was able to simulate the lake level rise quite accurately, as shown in Fig. 4. This figure also shows the ponioo of the simulated lake level rise due solely to rainfall directly on the lake. As the figure indicates, approximately 70% of the rise in the lake is due to runoff flowing into the lake. Simulation results reveal that the runoff production efficiency for the evening stonn on Horsetooth Reser- Table 1 Calibrated ~ilhydraulic paramelers fordifferenltCllluralclJ.sslficalions voir was 70%, which confirms the assumption of high initial soil water content The satmated hydraulic conductivity values identified through calibration on the djfferent sojj types are listed in Table 1, and are assumed to be the best estimates for those soil textmes. 10, Modeling of Spring Creek The hydrologic model CASC2D was used because it simulates spatially-varied hydrology and because it is possible to modify the model to include the influ- ence of the relevant hydraulic structures. Three deten- tion basins were include in this modeling study, as shown in Fig. 1. CASC2D was modified to jnclude perennial and ephemeral (e.g. detention basins) water bodies. The 0.21 km2 Dixon reservoir at the very west edge of the Spring Creek watershed is the only peren- niallake of appreciable size in the watershed. There are, however, a number of ephemeral lakes that fonn in detention basins or behind channel constrictions during significant runoff events. Simulation of ephem- erallakes is complicated because they can change size appreciably during runoff events. As lakes grow in size some model grids are inundated, and channel reaches become shorter due 10 inundation. CASC2D was modified to calculate the change in surface area and volume of each lake, identify inundated grids, and update the channel lengths correspondingly. The same process is used in reverse when the lake volume decreases. Detention basin outlets were modeled using circular and box culverts, and free weirs. It is thought that essentially all of the runoff from this stonn was generated by the Hortonian mechan- ism. Lateral subsurface flow and saturation from below because of rising water tables is insignificant Soillypt Percentcover&ge Saturatcdbydraulic ElfectiVCpot"D$;ly Gn::enandAmplcJlpilJary conduclivily (cmlb) budp.ararneter(cm) Sandy loam ]1.18 1.00 0.4]2 ]1.0] Lo~ 42.27 0,60 0.434 8.89 Sill loam 264 0,34 0.486 16.68 Claylo:un 20.52 0.20 0.390 20_88 Clay 0,08 0,(16 0.385 31.63 Impervious 23.31 0,00 0,000 0,00 \~ . ~'fi Table 2 Appli~d land-us.elland-cover $ellsilive model paramelers F.L Ogden el "I./ Journal ofHydr%gy 228 (2000) 82-100 91 Land-use type Percent coverage ]mpe!'\liousareas Indusmal Public areas Residenllal(/awlls) AgricuJturalarea~{primariJypilStu(e) 23,31 18.88 0.30 J.UJ 22.38 for this event given the high-intensifY and short dura- tion of rainfall, large depth to groundwater table (>5 m), and low soil hydraulic conductivities. The role of the city slOrm sewer system in draining over- land runoff is neglected in the modeling because the system was completely overwhelmed by the magni- tude of ruooff. Additionally, the flow conveyance fric- tional slope in the slOrm sewers was limiled by the land surface slope, because most, if not all, of the stonn sewer inlets were completely filled. The soil saturated hydraulic conductivity values identified in the Horsetooeh watershed caljhra!ion are used in the Spring Creek simulations. All land surface characteristics (e.g. impervious areas, spatially-varied soils) are considered in assigning model inputs. Values for overland flow roughness coefficients and other Iand-use/land-cover sensitive parameter were derived from published sources (HEC, 1985; USDA, 1986) and are listed in Table 2. Retention depth is assumed to be a function of land- use, and magnitudes were assigned similarly to Tholin and Keifer (1960). Simulation results are verified againsl USGS indirece peak discharge measurements at two locations (stations VI and U3 on Fig. I). After verification, the hydrograph resulting from this simu- lation, which includes all relevant land-surface detail, is considered as the "reference hydrograph ". The reference hydrograph is assumed the "best estimate" of the actual runoff hydrograph because it was derived from simulations incorporating all watershed detail and the best estimates of all model parameter values and rainfall. 11. The reference simulation All the available details of land-use patterns are Manning'srougbness coelftcient R~[Cnljon storagc (mm) om 0.15 0,2 0,2 0,2 13 " I' 5,0 50 induded in the assignment of model input parameters. Watershed characteristicS input to CASC2D include spatially varied maps at 30 m resolution of: topogra- phy, soil hydraulic conductivity, Green and Ampt capillary head parameter, soil initial water COofent, soil porosity, overland flow Manning's n, and over- land flow retention depth. Impervious areas were assigned soil saturated hydraulic conductivity values of 0.0. Each of the land-use types is associated with an overland roughness coefficient and a certain amount of retention deplh (i.e. the depth of water held in microtopography). The polarimetric radar estimates were used as model input. n is worth mentioning that no calibration was performed at this stage because all soil, roughness, and retention parameters for pervious areas were derived either fcom the Horse- looth watershed calibration (e.g. soil salUrated hydraulic conductivity) or literature values where applicable (e.g. retention depth). The only runoff data to compare againsl are the USGS indirect peak discharge measurements down- stream from Shields Street (denoted as station UI on Fig. J): 234 m'ls (8250 ft.1/s). with an uncertainlY of :t25%, time unknown (Smith, 1997). The high uncer- tainty (:t25%) at this location is Ihe result of signifi. cant flows down a city street. Downstream from Riverside Avenue (denoted as U3 on Fig. I) near the watershed outlet the indirecl peak discharge measurement is 166 lOlls (5860ftl/s), with an uncer- tainty of :t15%, time later than 22:32 MDT (Smith, 1997). The stage at the watershed outlet was recorded until this time, when the stage recorder electronics were inundated by rising floodwaters. Information from post-flood photographs. high water marks, and personal communications helped in forming a general idea about the timing of the flood. The single most significant watershed feature that