GJN3890 Pacific Water Resources Contract PA

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Lower Amazon Floodplain Mapping Project Page 6 Pacific Water Resources, Inc. 6/16/04 • Flood hydrographs in electronic format for each basin and rainfall event modeled • HEC-HMS models for previously unmodeled areas • Submittal of hydrologic model files and backup data to NSP for review • FIS certification for hydrology modeling TASK 6. HYDRAULIC ANALYSES This task involves the creation of an unsteady flow hydraulic flood profile model using HEC-RAS Version 3.1.2 for existing conditions of urbanization. The model will be used to develop flood elevation profiles for each of the five flood events being evaluated. This task will also develop the 100-year FEMA floodways under existing conditions of urbanization. The FIS hydrographs and flows accepted by FEMA as part of their review of the Task 5 submittal will be used for xese models. The results of this task will be submitted to the City and FEMA for approval prior to completing the mapping (Task 7) and FIS submittal package (Task 8). Create FIS Hydraulic Model. PWR will create a HEC-RAS 3.1.2 hydraulic model using the ground surface TIN (Provided by the City in Task 4) and the available ground survey data (also provided by the City in Task 4). PWR will endeavor to reuse as much data as possible from the various UNET models created by URS and the Corps of Engineers that were obtained from the City. Working with the observations made in Task 3 and survey data provided by the City in Task 4, checks of critical hydraulic dimensions included in these previous UNET models will be performed. The storage versus elevation relationships for flood storage areas included in the HEC-RAS model will be spot checked and the geometry of the critical inflow and outflow hydraulics that control the filling of these areas will be verified. PWR will run the models for the five flood events evaluated by this project (i.e. 24-hr 100-yr, 24-hr 500-yr, 72-hr 100-yr, 72-hr 500-yr, and Feb `96). Results will be tabulated to identify the higher elevation for the 100-yr and 500 yr events. Model FIS Floodway. PWR will model the 100-year existing floodway following FEMA Guidelines fora standard 1-foot-rise floodway. This involves narrowing the effective flow width at each section using FEMA approved conveyance reduction from each side of the overbanks until no further narrowing is possible without exceeding the allowable rise at that section or, more often, at another section further upstream. Within the upstream storage area created by the "1135 Levee" project, the floodway boundary will be identical to the 100- year floodplain boundary (i.e. storage floodway) so no encroachment will occur. Prepare Hydraulic Submittal. Upon completion of the activities outlined in this task, PWR shall submit the hydraulic modeling to the NSP for an independent QA/QC review. The MAS (see Task 1) will specify the items that need to be submitted to the NSP. This submittal will include documentation of the RAS Unsteady hydraulic model suitable for FEMA review purposes. The time consuming activity of this task involves assembling the required backup data used in the analysis, as specified in the MAS and FEMA standazds. Deliverables: • I-iEC-RAS 3.1.2 hydraulic models for the five flood events and floodway runs • Submittal of all hydraulic model files and backup data to NSP for review • FIS certification for hydraulic modeling TASK 7. FLOODPLAIN AND FLOODWAY MAPPING This task will prepare fmal FIS formatted flood profiles and floodplain and floodway boundazies using the results of the hydraulic modeling (Task 6). The process will develop GIS map layers for submittal to the City for review prior to submitting to FEMA as part of the FIS package (Task 8). Plot Flood Profiles. PWR will tabulate and graph in FEMA format flood profiles for the controlling profile (higher of the 72- or 24-hour events) for the 100- and 500-year events, including downstream backwater effects.