Hanging Levee Breach Analysis for Sespe Creek SC-2 Levee Certification

Levees are often the difference between a community in a floodplain and a community with floodplain protection. FEMA requirements for levees are continuously being refined to ensure levees meet a high standard of design. One critical levee design element is a tie-in to high ground, ensuring maintained protection and freeboard. But does a roadway count as high ground? In the case of the SC-2 Levee along Sespe Creek in Ventura County, California, the downstream tie-in location is a roadway bridge crossing. For levee certification, FEMA does not consider a roadway as able to provide a level of protection comparable to that of a levee. When a levee does not tie-in to high ground, it is considered a “hanging levee,” and requires additional analysis for floodplain mapping. After multiple discussions with FEMA and public agencies involved in the project, an assessment method was agreed upon to evaluate the hanging levee at the roadway. The approach? A breach analysis assuming the roadway is completely removed and lowered to its natural ground elevation. The existing terrain was modified in GIS to set the channel bank at the roadway to its historical elevation, resulting in a large “breach.” A two-dimensional HEC-RAS analysis was performed using this terrain and an overtopping flow hydrograph calculated using results of the post-project one-dimensional HEC-RAS model. The hanging levee analysis provided “breached” floodplain extents, which were included in the revised mapping as part of a Letter of Map Revision to certify the levee.