California Conditional Letter of Map Revision
Trihydro provided hydraulic modeling and floodplain permitting services for a remediation project to increase wetland area and habitat diversity along the East Fork San Luis Obispo Creek.
A confidential client contracted Trihydro to provide hydraulic modeling and floodplain permitting services for an ongoing remediation project in San Luis Obispo County, California.
The existing East Fork San Luis Obispo (SLO) Creek is highly channelized, with high channel bank berms functioning as non-levee features that would limit active flow conveyance to the main channel. Since the berms were not engineered or designed for flood prevention, flood waters would likely cause washouts and inundate adjacent oxbows and detention ponds along the channel. As part of the larger remediation project, proposed grading would remove sections of the existing berm and reconnect historic oxbow areas along the right bank of East Fork SLO Creek, increasing wetland area and habitat diversity with slower-moving waters off the main channel.
Hydraulic Modeling
Such complex hydraulic scenarios are well suited for 2D hydraulic modeling, which removes many modeling assumptions required in 1D hydraulic modeling. Trihydro used the US Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) version 6.3.1 software package to develop a 2D hydraulic model for this study. We used the 2D hydraulic model to analyze existing condition hydraulics with berms intact, assess the larger valley-wide floodplain without berms, and evaluate proposed grading to check that the historic oxbows would be reconnected at desired flood frequencies.
While 2D hydraulic modeling has many advantages, 1D hydraulic modeling is still preferable for floodplain permitting applications. For the Conditional Letter of Map Revision (CLOMR) submittal package associated with this project, Trihydro used 2D modeling results to inform the development of a 1D hydraulic model in HEC-RAS version 6.3.1. The 1D hydraulic model features the main channel of East Fork SLO Creek, along with four junctions and two split flow reaches to model reconnected sections of the right bank oxbow.
Developing a CLOMR Submittal Package
Trihydro completed a CLOMR submittal package that SLO County floodplain management staff and the Federal Emergency Management Agency (FEMA) reviewed and approved. Floodplain permitting for this project was complex due to dated Zone A effective mapping and several recent restudies within the study area. A preliminary effective FEMA study significantly revised the effective floodplain in the downstream portion of the study area. Additionally, SLO County recently submitted an appeal request and restudy for the same study reach. Trihydro coordinated extensively with SLO County floodplain management staff to verify compatibility between the county’s restudy appeal and Trihydro’s successful oxbow grading CLOMR submittal.
