Improved Flow Frequency Techniques with Frequency-Based Storms and Bulletin 17C

Authors

Katherine M. Werner, United States Army Corps of Engineers, Omaha, NebraskaFollow

Date of this Version

2019

Citation

Proceedings of the SEDHYD 2019 Conference on Sedimentation and Hydrologic Modeling, June 24-28, 2019, Reno, Nevada, USA

Comments

United States government work

Abstract

This paper summarizes the hydrologic evaluation performed by the United States Army Corps of Engineers (USACE), Omaha District as part of the Elkhorn Basin Flood Plain Management Services (FPMS) Flood Risk Identification Study. The Elkhorn Basin FPMS Study is a collaborative effort between the Nebraska Department of Natural Resources (NeDNR), the USACE, and the Federal Emergency Management Agency (FEMA) Region VII. The Elkhorn River is located in northeastern Nebraska, covers approximately 7,000 square miles, and includes 11 Federally Constructed Levees, 5 high hazard dams, and dozens of NFIP participating communities. Previous flood risk information in the basin was developed specific to different communities or decisions, such as National Flood Insurance Program (NFIP) Mapping and levee management (O&M, risk screening, NFIP certification, and 408 modifications). Unfortunately these different studies used various methods which lead to inconsistent data. One of the reasons for no previous basin wide evaluation is the basin itself, with a large area and precipitation gradient, basin modeling is a challenge. With efforts to remap the area in the NFIP, expected levee certification efforts, transportation improvement efforts, and recent flooding in 2010, it was recognized that there was a need for updated, consistent information.

The purpose of the hydrologic analysis was to develop peak flow frequencies (2-, 5-, 10-, 25-, 50- , 100-, 200-, 500-year, and 100-plus) and corresponding hydrographs throughout the Elkhorn River Basin. Peak flow frequencies were estimated at over a dozen gages throughout the watershed using Bulletin 17C guidelines. Additionally, a gridded hydrologic model of the Elkhorn watershed using Hydrologic Engineering Center Hydrologic Modeling System (HECHMS) was developed, calibrated, and simulated with frequency-based design storms to estimate peak flow frequencies and associated hydrographs at 372 computation points throughout the watershed. This paper is based on a more detailed 2018 report (USACE 2018).