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<br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />To more accurately reflect the quickly changing rainfall patterns caused by orographic impacts in the <br />Ralston Creek basin, the watershed was divided into three rainfall regions - western (upper), central, and <br />eastern (lower). The western (upper) region consists only of the upper, mountainous region of the Ralston <br />Creek watershed. The central region consists of the middle reaches of the Ralston Creek watershed and the <br />highest regions of the Van Bibber watershed. The eastern (lower) region consists of the lower reaches of <br />the Ralston and Van Bibber watersheds, and the entire Leyden Creek watershed. All areas below the <br />reservoirs are in the eastern region. <br /> <br />Using section comers as reference points, the delineated watersheds were superimposed onto the depth- <br />duration-frequency figures given in the District's Criteria Manual (Figures RA-l through RA-12 in the <br />Manual). Point rainfall depths were then estimated for the 1O-year, SO-year, and 100-year events for I-hour <br />and 6-hour durations for the western, central, and eastern regions. Point rainfall depths for the 500-year <br />event were estimated using log-probability plots. <br /> <br />Because the Ralston Creek watershed is large, rainfall patterns needed to be adjusted according to aerial <br />correction factors. There were a number of different scenarios that could produce peak events at various <br />locations in the watershed. An uncorrected rainfall pattern is valid anywhere in the watershed with total <br />contributing drainage area less than 10 square miles. A basinwide correction factor for 92 square miles <br />could produce results valid anywhere in the basin. However, additional considerations must be made for <br />localized analysis. For example, a more intense storm falling just on the eastern region (which is generally <br />urban in nature) could produce higher flow rates at downstream design points than a Jess intense storm <br />covering the entire watershed. In addition, to analyze operations of the reservoirs located on the Ralston <br />Creek mainstem, a rainfall pattern considering a storm raining on areas contributing just to these reservoirs <br />was needed. <br /> <br />As such, a total of five correction scenarios were created and modeled: <br /> <br />1. Uncorrected - For design points with total contributing drainage area less than 10 square miles (2- <br />hour rainfall event) <br />2. 92 square mile correction - Considered a storm event falling uniformly over the entire basin (6-hour <br />rainfall event) <br />3. 49 square mile correction - Considered a storm event in the areas above Arvada/Blunn Reservoir <br />only to analyze its operations (6-hour rainfall event) <br />4. 40 square mile correction - Considered a storm event in the eastern region of all three watersheds <br />only (6-hour rainfall event) <br />5. 10-20 square mile correction - Used for reaches of Leyden and Van Bibber Creeks with total <br />drainage area greater than 10 square miles (3-hour rainfall event) <br /> <br />For any design point of interest, results from two models should be considered. The reported flow rate <br />would be the larger of the 92 square mile correction model and the appropriate other model for the location <br />of interest. <br /> <br />Rainfall patterns were developed using spreadsheet templates developed by UDFCD during the update of its <br />Criteria Manual. These spreadsheets only considered areas up to 75 square miles, so the 92 square mile <br />correction hyeto'graph was developed separately using methods outlined in the Criteria Manual. It should <br />also be noted that the spreadsheets do not automatically develop 500-year rainfall. However, the same <br />procedure to develop the 100-year pattern is used, but with 500-year point depths replacing the 100-year <br />point depths. <br />To summarize, all five rainfall patterns were utilized for the eastern region and three patterns (uncorrected, <br />48 square mile correction, and 92 square mile correction) were utilized for the central and western regions. <br />In addition, the 10-20 square mile correction was developed for the central region because it included a <br /> <br />portion of Van Bibber Creek. This means a total of 12 rainfall patterns were developed for each design <br />event, resulting in a total of 48 rainfall patterns developed and used in this study. <br /> <br />3.7 Regional Reservoirs <br /> <br />Four reservoirs were included in the Ralston Creek watershed model. These include Ralston Reservoir, <br />ArvadalBlunn Reservoir, Leyden Lake, and Hayes Lake. Storage-discharge parameters used in the <br />UDSWMM hydrologic routing model were provided by the City of Arvada for Ralston and Arvada/Blunn <br />Reservoirs and Leyden Lake. UDFCD provided stage-storage information for Hayes Lake. <br /> <br />For the initial calibration model, Arvada/Blunn Reservoir, Leyden Lake, and Hayes Lake were removed as <br />storage elements from the model. They were replaced with I-foot direct conveyance links. <br /> <br />Boyle also used a recent study of flood control improvements to Leyden Dam (Reference 8) to determine <br />modeling parameters in the hydrologic analysis. Stage and storage data used for modeling for <br />ArvadalBlunn Reservoir was provided by the City of Arvada. <br /> <br />A labyrinth weir spillway was being designed for the ArvadalBlunn Reservoir at the time of this report and <br />is anticipated for construction in late 2004. The weir will raise the overflow spillway elevation by 5 feet. <br />The routing characteristics of the Arvada/Blunn Reservoir used in the 2003 hydrology model assume the <br />construction of this labyrinth weir spillway. The new spillway will increase the routed 100-year discharge <br />compared to the discharge from the current spillway configuration. <br /> <br />3.8 Peak Rows <br /> <br />Peak flows were produced by analyzing the watershed for each rainfall scenario and each design frequency. <br />In order for the initial calibration model to be considered consistent with past published results, the peak <br />flows at critical design points must be within 10% of those values reported in past studies. In order to make <br />the values consistent, the following procedures were performed: <br /> <br />1. For all reaches basinwide for which Manning's 'n' values greater than 0.07 were computed, a <br />reduced value between 0.06 and 0.07 was substituted. For example, if a value of 0.096 or 0.106 was <br />computed, it was replaced with 0.066. The assumption was that despite the results of Equation RO- <br />W, a value higher than 0.07 was not consistent with channels in this basin. The Manning's 'n' <br />coefficient was also judged to be the most justifiably altered physical parameter for calibration <br />purposes. <br />2. Leyden Creek, which was initially modeled as a broad, relatively flat floodplain, was replaced with a <br />more-defined channel. <br />3. The Manning's 'n' values of the Leyden Creek mainstem were replaced with a value of 0.040 below <br />Leyden Lake. This was needed to speed up the response of the Leyden Creek watershed to better <br />match past published results. <br />4. The Manning's 'n' value for two upper reaches of Ralston Creek were increased to 0.070. This was <br />needed to slow down the response of the Ralston Creek watershed to better match past results. <br /> <br />After each of the above procedures was executed, peak 100-year flow results were within 10% of previously <br />published values. The model was then considered to be calibrated. A separate model was then created <br />which added-in Arvada/Blunn Reservoir, Leyden Lake, and Hayes Lake. Peak discharge profiles for <br />Ralston and Leyden Creeks are shown on Figures 2 and 3, respectively. Design Peak flows, compared to <br />the Flood Insurance Study (PIS) published data, are shown in Table 1. <br /> <br />6 <br />