East Garden Grove-Wintersburg Channel Model Study

Orange County, CA


The East Garden Grove-Wintersburg (EGGW) Channel (Facility No. C05) at the I-405 Freeway is a complex flood control system composed of an array of culverts and tributary channels.  Located in the cities of Westminster and Huntington Beach, the channel system’s unique geometric configuration, multiple confluences and storage basin demonstrates that this particular reach of channel is difficult to analyze by mathematical and computer modeling alone.  Hydraulic analysis of the culverts has clarified the issue of possible flooding at the upstream vicinity of the freeway and tributary channels during the ultimate 100-year storm.  It is estimated that over $30 million in future flood control improvements are dependent upon the accuracy of the hydraulic analyses at the I-405 crossing.  A hydraulic physical model would provide the most accurate understanding of the existing and proposed channel improvements for this reach of the channel system because of the complexity of the hydraulic processes which are occurring.


Due to proposed channel improvements at two confluences upstream and downstream of the I-405 freeway, there was uncertainty about the impact these new designs would have on the undercrossing and upstream water surface.  Proposed improvements to Newland Storm Channel will discharge flow both upstream and downstream of the freeway crossing through a split flow bifurcation. The acceptable volume of water discharged upstream of the freeway will be determined by the physical model. Likewise, the proposed improvements to Edinger Storm Channel located downstream of the freeway undercrossing calls for two 66-inch RCPs in addition to the existing RCP.  In order to efficiently maximize the performance of both proposed channels, the I-405 culverts and tributaries were scrutinized by a hydraulic physical model to fully understand the channel hydraulics.


PACE performed a hydraulic model study meeting the following criteria:

  • Geometric scale 1:15
  • Minimum reach length of 800 feet
  • Existing mainstem channel is rectangular concrete with a 60-ft width
  • 100-year expected value design discharge:


Case 1: Mainstem channel discharge 5,330 cfs upstream and 5,910 cfs downstream of the I-405 freeway undercrossing, with combined 580 cfs from tributaries Newland and Edinger Channels.


Case 2:  Mainstem channel discharge 4,450 cfs upstream and 5,910 cfs downstream of the freeway undercrossing, with 1,080 cfs from Newland Channel tributary, and 380 cfs from Edinger Channel tributary.


Key Features:

  • Hydraulic physical model
  • Complex hydraulic analysis
  • Detailed and specialized hydraulic experiments / measurements
  • Alternative design evaluations
  • Customized fabrication and specialized construction techniques

Orange County Flood Control Division

Flood Control Systems, Stormwater Management / Flood Control