Project Overview
JLC Engineering conducted a hydrology and hydraulic study for the Southern California Edison (SCE) Valley Substation in Menifee, California. The project was initiated to evaluate stormwater flows and floodplain conditions surrounding the substation in anticipation of new security perimeter walls that could affect the coincident existing floodplain. The study aimed to ensure that the proposed construction would not exacerbate flood risks while also refining previous floodplain assessments.
The primary objectives of the study were:
- Assessing pre- and post-project floodplain conditions.
- Evaluating the impact of stormwater runoff in a 100-year storm event.
- Refining prior RCFC&WCD floodplain models for improved accuracy.
Study Area & Drainage Conditions
The SCE Valley Substation is located at the intersection of State Route 74 and Menifee Road. A portion of the site is within a Zone A Flood Hazard Area, as designated by FEMA. The existing hydroglogic/hydraulic models for the area did not account for key drainage infrastructure, particularly culverts and stormwater diversion structures, which significantly affect how water flows through the region.
Key drainage characteristics:
- The site primarily experiences sheet flow conditions, meaning stormwater disperses over wide swaths of land rather than through defined channels.
- Previous studies overestimated flood risk by assuming water accumulates at a single point instead of spreading naturally.
Analysis & Methodology
To ensure a comprehensive assessment, JLC Engineering utilized industry-standard methodologies:
- Hydrologic Analysis:
- Conducted using the Synthetic Unit Hydrograph Method following RCFC&WCD Hydrology Manual guidelines to evaluate the 100-year storm events for 3-hour, 6-hour, and 24-hour durations.
- Hydraulic Analysis (Ongoing):
- HEC-RAS 2D modeling selected to accurately simulate floodplain conditions.
- Incorporated culvert crossings at SR-74 and Menifee Road, previously excluded from floodplain modeling efforts.
- Defined mulitple watershed subareas and multiple floodplain inflow points to better represent stormwater dispersion.
Key Findings & Results
- Refined the existing floodplain model: The previous study did not account for key stormwater infrastructure, leading to overestimation of flooding risk.
- Confirmed that the proposed perimeter wall will not worsen flood conditions when properly accounted for in hydraulic modeling.
- Identified critical drainage improvements: Incorporating multiple inflow points and existing culverts provides a more accurate assessment of flood risk.
- Recommended a 100-year, 3-hour storm event for future modeling, as it produced the most conservative combination of peak flowrate and cumulative volume magnitude.
Conclusion
This study provided a critical update to prior floodplain assessments, ensuring a more accurate representation of stormwater behavior at the SCE Valley Substation. By incorporating advanced hydrologic modeling techniques and previously unaccounted infrastructure, JLC Engineering was able to deliver a refined flood risk analysis that will support the next phase of hydraulic assessment.