Communities often rely on floodplain maps based on historic flood frequency information that may no longer be accurate. Hydrologic variation due to land use and climate changes may cause greater flood risk than identified in currently available floodplain mapping products. New mapping and modeling with revised hydrology and hydraulics for evaluating flood risk often take considerable time and resources to develop, which a community’s budget may not support. Consequently, communities are making land use decisions based on the best available science that may be outdated and no longer representative of the existing flood risk.
With this in mind, the Iowa Silver Jackets piloted an evaluation approach on the Cedar River Basin that can be utilized on other watersheds. It is an approach that can help identify which communities are most sensitive to changes in hydrology and provide a quick and inexpensive method for evaluating non-structural options, such as zoning and relocation and elevation of structures to reduce current and potential future flood risk.
The study team adjusted flood probabilities at designated gage locations along the Cedar River’s main stem to account for the longer period of record since many of the standing Flood Information Studies had used hydrology from the 1970s and 1980s timeframe. These new flows, which account for climate change impacts or watershed changes, serve as input for the hydraulic model.
A planning level Hydrologic Engineering Center – River Analysis System (HEC-RAS) hydraulic model, known to practitioners as an approximate channel method hydraulic model, was developed for the Cedar River main stem (338 miles) in a relatively short period of time and at a low cost and produced reasonable results. This hydraulic modeling approach modified the Manning’s ‘n’ values to compensate for the lack of channel data. This approach compared the rating curves from USGS gages to HEC-RAS computed rating curves at the same location. The hydraulic model was appropriate for planning purposes and in evaluating damages at a community level. It can provide a comparative analysis of how flood risk may have changed in a given community over the past several decades and help inform how flood risk may change over the next several decades due to projected climate and landuse changes.
To perform an economic analysis, this HEC-RAS hydraulic model was used to generate depth grids, and a Hydrologic Engineering Center’s Flood Impact Analysis (HEC-FIA) computer program was used to estimate damages by the depth of flow on the given structure. This methodology has limitations due to the lack of accurate spatial placement of the structure type on the landscape but provides a marked improvement from the census block approach that is common to HAZUS. Use of this methodology helped to identify communities that have notable changes in flood risk both in terms of total damages and the percentage change. This pilot determined that a planning level hydraulic model may be developed at a fraction of the costs of a detailed model and may provide a reasonable level of accuracy for estimating economic damages at a community level.