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I I I I I I I I I I I I I I I I I I I B. III. HYDROLOGIC ANALYSIS A. GENERAL The hydrologic analysis was performed to define the storm runoff peak flows for the various subwatersheds of the Lower First Creek study area. This infonnation was then used to evaluate the existing drainage facilities, identify any problem areas, develop alternative drainage improvements, and in the development of the Flood Hazard Area Delineation (FHAD) and preliminary design of improvements. The study area is divided into five (5) major watersheds. Besides the main First Creek watershed, the DF A 0055 watershed is subdivided into the Arsenal North Drainage watershed, with four (4) Tributaries. These are Rolla Tributary, Union Pacific Tributary, Elm Drainage and Birch Run. In addition, the watershed upstream ofthc study area was modeled in ordcr to provide representative inflow hydrographs at the upstream study limit. Each watershed in the study area was divided into subwatersheds as shown in Figure III-I. The sub watersheds were generally numbered consecutively from west to east and limited to a maximum area of 717 acres. The study area was divided into a total of 52 subwatersheds with an average area of 260 acres, the upstream watershed divided into 115 subwatcrsheds with an average of 149 acres. PREVIOUS STUDIES A number ofUOFCD hydrologic evaluations have bcen completed for the First Creek watershed. TC&B uscd these evaluations as the foundation to dcvelop a complete hydrologic model for the watershed. Wright Water Engineers (WWE) completed an Outfall Systems Planning study, including a hydrologic analysis, in 1990 for UOFCD. The hydrologic study included Colorado Urban Hydrograph Procedure (CUHP) and Urban Drainage Stormwater Management Model (UDSWM) hydrologic modeling of the entire watershed. Previous work by Simons Li and Associates (SLA), in thc upper portion of the watershed, was integrated into the modeling analysis by WWE by directly inputting the SLA hydrograph into thcir UDS WM model. As a result, the WWE model did not include any subwatersheds and routing elements upstream of Pica dilly Road. Subsequent to the WWE study, UOFCD contracted with Taggart Engineering Associates to modify a portion of the First Crcck watershed modeling. Taggart Engineering rcmoved the direct input hydro graph from the WWE UOSWM modcl, and revised the model to include the neccssary subwatershed and conveyancc routing elements upstream of Pica dilly Road. In 1998, MartinlMartin Inc. used the hydrologic models developed by WWE (refined by Taggart Engineering) to assess regional drainage improvements in the Gateway Area, south of Den vcr International Airport. The Martin/Martin study evaluated a different detention basin sccnario than originally proposcd by WWE in the outfall systems plan. MartinlMartin's work resulted in a revision to the outfall systems plan. The MartinlMartin plan included a number of small detention facilities within the Gateway Area in lieu of the single large detention basin (Bald Eagle Shallows) that was proposcd by WWE within the RMA site. Implemcntation of a number of small detention basins within the Gateway Area was deemed to be more feasible with the Gateway dcvelopment schedule than was the implementation ofthc RMA Bald Eagle Shallows detention basin. TC&B used the MartinlMartin results as tbe foundation for the analysis to identify the hydrology within the lower First Creek watershed. There were some minor modifications to the MartinlMartin hydrologic analysis after their final report was published in November 1998. Based on UOFCD direction, TC&B used these revised files as the basis to start this work. TC&B also revised the CUHP and UDSWM datasets to integrate DFA 0055 subwatersheds and conveyance elements directly into the First Creek models. Runoff from DFA 0055 had becn coded as an inflow hydrograph in the WWE and later in the MartinlMartin UOSWM file that was provided to TC&B. Based on information in the WWE report, TC&B revised tbe First Creek CUHP datasets to include DF A 0055 subwatersheds and their characteristics. The First Creek UOSWM datasets were also modified to include the necessary routing elements from DF A 0055. c. DESIGN RAlNFALL The I-and 6-hour rainfall depths (depth.duration frequency information) for the 2., 5-, 10.,50. and toO-ycar rcturn period rainstorms were obtained from the Urban Storm Drainage Criteria Manual (USDCM) Volume I Rainfall Section (Reference 4). Following the procedure in USDCM Volume I Rainfall Section, the 2- and 3-hour rainfall depths were determined from the 1- and 6.hour rainfalls. The I-hour rainfall depth is shown in Table III-I. A summary ofthc 1-,2-,3-, and 6-hour depths is shown in Table III-2a. Tables III.2b through 111-2g present the adjusted and unadjustcd dcsign storms used in the hydrologic modeling. The 1-,3- and 6-hour rainfall depths for the 500-year storm event were determined by plotting the 2-,5-,10-,50. and tOO-year depths on log.probability paper and extrapolating. The 6-hour rainfall duration was uscd for both the adjusted and unadjusted modeling. This rainfall duration is consistent with the duration used by Martin/Martin. Table 111-1: One-Hour Rainfall Depths Return Period Rainfall (inches) 2-Year 1.00 5-Year 1.38 10- Year 1.63 50-Year 2.25 100-Year 2.65 500.Year 3.29 .10 -