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.Lower Amazon Floodplain Mapping Project Page S <br /> Pacific Water Resources, Inc. 6/16/04 <br /> SCS Type lA rainfall distribution already coded into the City's XP-SWMM model will then be used to <br /> i estimate the 500-year 24-hour rainfall event to be used in the hydrologic modeling. (See `Setup and Execute <br /> XP-SWMM Models below.) <br /> Tabulate the 72-Hour Rainfall Events. Because of the considerable amount of storage provided by the <br /> "1135 Levee" project, FEMA would like to ensure that longer duration and much higher volume runoff events <br /> are evaluated as part of the hydrologic/hydraulic analysis. Using the published results of design storm <br /> development work completed by MGS Engineering Consultants, Inc. as part of Clean Water Services' <br /> comprehensive Watersheds 2000 project, PWR will tabulate the volume-based 72-hour rainfall distribution <br /> and depths for use in hydrologic modeling of the 1-percent and 0.2-percent annual chance events. <br /> Prepare a Historic Rainfall Event -Feb `96. PWR will prepare a rainfall input file for the period <br /> surrounding the February 1996 storm event. This event is one of the largest to occur in the 20'~ century and <br /> was significant in part due to its volume. This event will be used to check that flood elevations obtained in the <br /> hydraulic analysis are reasonable and do not exceed that estimated for the 1-percent annual chance event. <br /> Prepare Letter Report for the New Storm Distributions. PWR will prepare a letter report documenting the <br /> derivation of S00-yr, 72-hr and historic rainfall events. This brief letter will describe the method used for <br /> estimating the S00-yr rainfall depth. It will support the 72-hour storm distribution by referencing and <br /> attaching the MGS work noted earlier. And, it will identify the dates and rainfall depths included in the `Feb <br /> `96' event. The purpose of this report will be to support FEMA's review of project hydrology. <br /> Execute XP-SWMM Model and Format Output for RAS Unsteady. The existing conditions XP-SWMM <br /> model for the Amazon Creek Basin (including Willow Creek), Greenhill Basin and the A3 Channel Basin will <br /> beset up and executed to develop inflow flood hydrographs for the 24- and 72-hour 100- and S00-yeaz events <br /> and the Feb `96 historic event. The resulting hydrographs from XP-SWMM will be converted to .dss files for <br /> d~ use in the HEC-RAS model developed in Task 6. PWR will only modify the rainfall input for the "existing <br /> ~~r? conditions" XP-SWMM models obtained from the City. No other modifications will be made. <br /> Develop HEC-HMS Models for Unmodeled Contributing Areas. Working with the field data obtained in <br /> Task 3 and other available resource information, PWR will develop HEC-HMS hydrologic models for the <br /> currently unmodeled Greenhill Road Ditch Drainage, the Grosbeck Channel Basin and the Dead Cow Creek <br /> Basin. The combined drainage area for these three unmodeled basins is approximately 1200 acres (slightly <br /> less than two square miles). Each of these hydrologic models will be single basin models (i.e. no subbasins) <br /> used to estimate appropriate inflow hydrographs for the Task 6 analysis. PWR will then execute the models <br /> for the 5 flood events outlined above and check the 100-yeaz peak flows per acre against published data from <br /> the City's Stormwater Master Basin Plan. <br /> Document Hydrologic Analysis. PWR will document the results obtained from each activity completed by <br /> this task and will include the letter report for the new storm distributions as an appendix. The documentation <br /> L will be in the form of a technical memorandum whose total combined tables and text is estimated at 6-10 <br /> pages with up to an additional 10 pages of figures. PWR will create a draft version of this document and <br /> submit it to the City for review. Once review comments have been received, a fmal version of the document <br /> will be prepared and submitted to the City, FEMA and FEMA's NSP for review. <br /> Prepare FEMA Hydrologic Submittal. Upon completion of the above hydrologic activities, PWR shall <br /> submit the results and documentation noted to FEMA's NSP for an independent QA/QC review. The MAS <br /> (see Task 1) will specify the items that need to be submitted to the NSP. The bulk of the effort required for <br /> this task involves assembling backup data as specified in the MAS and FEMA standards. <br /> <br /> Vii' Deliverables: <br /> ~r ' • Storm distribution letter report (approximately 1-2 pages) <br /> • Draft and Final Memorandum on the hydrologic analysis <br /> • Table of peak FIS flood inflows and azeas for FIS report <br /> <br />