More realistic and accurate estimates of historical floods and more complete representation of flood processes and drivers in regions having mixed forms of precipitation (e.g., rain and snow) are necessary for improved installation risk and vulnerability assessments. The significance of improved flood assessments in regions of mixed precipitation forms was initially established by the successful earlier SERDP project RC-2546 for which the next generation intensity duration frequency (NG-IDF) methods were developed and applied. The new NG-IDF curves account not only for missing precipitation falling as snow, but also for climate-changed precipitation amounts where hourly extreme rainfall rates over the U.S. may increase by as much as 400%. The advancements here will further increase the realism and accuracy of flood estimates for such mixed precipitation regions, improving climate vulnerability assessments, for example, built partly with projected future floods. As such, the primary objective of this project is to demonstrate new technologies that
- Improve hydrologic design under a nonstationary climate,
- Can be rapidly transferred into improvements for installation risk analysis and decision-making, and
- Ultimately improve the resilience of the Department of Defense's (DoD) infrastructure.
Building from the previous SERDP work on NG-IDF curves under a nonstationary climate, the project team will refine and validate the following two methods to meet the project objective: 1) enhance NG-IDF-based hydrologic design by constructing a suite of NG-IDF curves for all land cover types under historical and future climates; and 2) use timeseries of water available for runoff used to generate the NG-IDF curves with a digital elevation model based flow routing approach to estimate streamflow at each channel reach in the widely used National Hydrography Dataset Plus stream network database. These tools will be demonstrated at the local to regional scale within two or more subdomains that include DoD facilities. The subdomains will be selected through collaboration with U.S. Army Corps of Engineers (USACE) field offices to identify the most significant and useful locations for test cases. Estimated flood frequencies and seasonal flow volumes created for NG-IDF will be compared with observations. The technical readiness of the NGIDF method and the enhancements made through this project will be evaluated and improved through active collaborations with operational staff in the selected contiguous United States subdomains.
A significant limitation in current installation vulnerability and risk analyses in regions having mixed precipitation forms is the exclusion of snow from current IDF representations for those regions. Hence, the products here would increase the realism and accuracy of flood estimates for such mixed precipitation regions and reduce the likelihood of installation over or under protection with respect to climate vulnerability. The NG-IDF Advisory Group will include USACE and DoD installation water managers who can help the project team select the most meaningful sites for applying the new methods and making comparisons of new and extant methods. The project team expects the subdomains selected for this study to represent a range of different precipitation regions and to include DoD installations where there has already been evident flood risk, especially for installations in regions of mixed forms of precipitation that will benefit most from using the NG-IDF curves. Through these subdomain studies, the project team will be able to demonstrate improved risk assessments by having a more complete representation of flood processes in regions of mixed precipitation. The NG-Advisory Group will help ensure that the transfer of knowledge and early products from this project can have direct relevance and utility to actual on-the-ground water management problems like vulnerability to extremes that can be addressed with improved IDF curves.