Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods controlled to meet FEMA mapping standards. These data were assimilated together with the best available bathymetric datasets, including USACE hydrographic LiDAR, into high- resolution seamless digital elevation models. The hazard analysis considered the combination of storm surge and hurricane-induced wave hazards. This included independent analysis and/or modeling of storm surge, return frequency flood elevations, wave setup, overland wave hazards, and wave runup. The ADvanced CIRCulation model for coastal ocean hydrodynamics (ADCIRC) was selected to develop the stillwater elevations or storm surge for the study area. The Empirical Simulation Technique (EST), also developed by the USACE, was used to determine the stillwater frequency curves for the 10-, 2-, 1-, and 0.2-percent annual chance stillwater elevations. Deepwater wave conditions were determined using the Shore Protection Manual (SPM) prediction technique and limited fetch analyses were performed in harbor and sheltered areas. Wave setup was differentiated and evaluated for areas with and without fronting reefs. Areas of primary frontal dune were identified, delineated, and eroded. Overland wave propagation hazards were evaluated using the WHAFIS model. Wave obstructions were verified at representative transect locations by field reconnaissance. Wave run-up was evaluated using the RUNUP 2.0 and TAW methodologies, depending of the presence of reefs and local steepness of the bathymetry. Wave hazard analyses were conducted at FIS transect locations, in addition to more tightly- spaced"mapping transacts." These additional transacts were facilitated by the application of an integrated GIS toolset that automates repetitive modeling tasks, and enables a more detailed analysis than typical coastal FIS studies. Wave setup values were assigned to these transects according to analysis at adjacent FTS transects. Otherwise, the mapping transects were treated with the same overland wave propagation and wave runup assessments as the FIS transects. The coastal hazards determined from the above analyses were synthesized in the form of the standard FEMA special flood hazard boundaries for the Zone VE, Zone AE, Zone AO, and Zone X hazard areas. These are presented in the TSDN as workmaps produced at a scale of 1':500". The workmaps also include stillwater stations, topographic elevation contours, FIS and mapping transect locations, and the shoreline. Wave analysis for the 0.2%annual chance event was not included in the scope of the study. The 0.2% return frequency stillwater elevation was exceeded by cumulative flood elevation from the 1% stillwater elevation and wave setup, therefore, the boundary of the 0.2% annual chance event was not delineated. To steeper areas where mapping scale limits the gutter placement, the SFHAs are only identified by the position of the 100-yr flood boundary. Mapped Base Flood Elevations (BFEs) are considerably dependent to the topographic representation at each transect. As a result, localized variations in the topography at other locations may not be fully reflected in the mapped SFHAs and BFEs. A Technical Support Data Notebook (TSDN) was compiled for each county in the study area. Storm surge and return frequency elevation analyses were inclusive of all counties, and thus all materials pertaining to those analyses, including model input, output, and documentation are included in each county TSDN. The remainder of the data, including wave modeling, mapping, workmaps, topography, etc., are island and county specific. Therefore,these data are only presented in the appropriate countywide TSDN. 10-34 Hawaii County Multi-Hazard Mitigation Plan