Manhattan, Kansas, also known as “the Little Apple”, has been struggling with a significant flood problem that impacts approximately 250 residential structures located along the Big Blue River and Wildcat Creek. The city is revising two floodplain management plans to determine how to best address the flooding for areas not provided with a high level of risk reduction from structural measures, such as the Tuttle Creek Dam and the Manhattan Federal Levee.
The National Nonstructural Flood Proofing Committee (NFPC) was contacted to provide technical assistance, as the Kansas City District conducted a sampling of 49 structures to determine the feasibility of implementing nonstructural measures such as elevation, buyouts, relocation, and basement fill. After depth and velocity of flooding were leveraged from recent FEMA studies, the assessment team developed an inventory of attributes for each of the structures to enable a building-by-building evaluation of relevant and feasible nonstructural measures.
A NFPC Structure Inventory Attribute Table was populated. The table is a one-page document that illustrates essential structure data, such as address, first floor elevation, structure type, foundation perimeter distance, number of structural corners, exterior wall construction, and whether or not there is a basement or attached garage. These data are critical for determining the type of adaptive measures conducive to the flood risk and for developing a cost estimate. Much of the data, as the study team found, are readily available from county assessor databases, while other necessary data came from combining LiDAR and rough first floor elevations from GoogleStreetView.
A Flood Damage Reduction Matrix was used to determine which nonstructural measures might be appropriate for further consideration, such as elevation, relocation, acquisition, or wet and dry flood proofing. The matrix is also helpful in expanding nonphysical approaches, such as flood warning and flood insurance. The matrix links the individual building’s attributes with characteristics that could potentially impact the building, such as flooding (depth, velocity, ice and debris) and site location (coastal, riverine, and soil types). The matrix helped to identify the scope and budget for plan formulation and prioritize the measures the city was most interested in pursuing with the public. The results of the benefit-cost analyses were part of the application to the Housing and Urban Development National Disaster Resilience Competition.
Of the 49 sample structures, 9 are feasible for buyout, 23 for relocation, 35 for basement fill, and 13 for elevation. Each measure by itself could save at least $100,000 in expected annual reduced damages. The results demonstrated that a minimal budget can go a long way in evaluating nonstructural measures as feasible flood risk reduction opportunities.