U.S. Army Fort Shafter Flats Flood Mitigation, Honolulu
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The U.S. Army Fort Shafter Flood Mitigation Project involves a 1,700 linear-foot levee wall, high-capacity flood control pumps, and a modified bridge opening to increase flow capacity.

The project objective was to protect a 30-acre, highly used section of the Fort Shafter Flats area. The area has been plagued by frequent floods caused by overflow from the Kahauiki Stream. Water caused damage to warehouse structures and motor pools.

Geosyntec's Scope of Services

The flood mitigation project protects the military installation for the 1% annual exceedance probability event (100-year storm), ensuring the protection of military assets and continued operation following storm events. The project included the preparation of a Conditional Letter of Map Revision (CLOMR) and the preparation and completion of a Letter of Map Revision (LOMR) to revise the Flood Insurance Rate Map (FIRM). The levee design and remapping are consistent with FEMA 44 CFR 60.15 requirements for mapping areas protected by levee systems. The LOMR was approved in 2020.

Geosyntec worked closely with the local engineering firm Sato & Associates, Inc. and the USACE to develop a hydraulic analysis of the Kahauiki Stream modifications, including the levee wall design and modified bridge opening. Along with the detailed hydraulic analysis, Geosyntec conducted a scour analysis and prepared GIS maps of the modified floodplain and floodway. Following construction, Geosyntec worked closely with Pacific Geotechnical Engineers and the USACE to prepare the Letter of Map Revision (LOMR) and the supporting documentation to seek and obtain levee certification. A detailed hydrologic analysis was conducted to determine the extent of residual flooding behind the levee system and to optimize the high-capacity pump system's configuration to provide flood protection for the 1% annual exceedance storm event.

Notable Accomplishments

Geosyntec maintained close coordination with the USACE and the local lead engineer to complete the comprehensive analysis and the project’s design, secure acceptance of the CLOMR, and subsequent acceptance of the LOMR. The Geosyntec analysis and final pump control configuration and sizing resulted in significant savings in construction costs.