Calleguas & Conejo Creek Floodplain Analysis
Two-dimensional numerical models were generated for the Calleguas and Conejo Creek floodplains upstream from the Ventura (101) freeway, including their confluence to approximately 1.5 miles downstream of their confluence. Previous one-dimensional HEC-RAS numerical modeling suggested that large areas of shallow flooding existed on the overbanks of the creeks and illustrated the need to prepare a two-dimensional model which would serve as the primary baseline hydraulic model. Two-dimensional modeling, however, successfully models channel and shallow floodplain systems, and can accurately model the confluencing streams.
A FLO-2D two-dimensional model including Conejo and Calleguas Creeks was developed and included an embedded representation of PACE’s previously completed detailed HEC-RAS model. This FLO-2D model encompassed the same area evaluated as part of the HEC-RAS model but expanded to the limits of the published FEMA FIRM to accurately assess the limits of the shallow flooding. The FLO-2D model evaluated the two-dimensional variation of velocity and water surface elevation for overland flow. Special attention was paid to develop an accurate representation of the confluence of the two streams and any subsequent, combined overland flooding.
The final model extended from upstream of HWY 101 to approximately 1.5 miles downstream of the Calleguas-Conejo confluence, and from west of Lewis Road to the easterly extent of the floodplain. These model extents were chosen to eliminate any issues associated with the arbitrary location of the model boundary influencing the hydraulics, particularly at the stream confluence. In addition, extending the model downstream from the original HEC-RAS limits provides the ability to establish a detailed baseline model that can quantify changes to the downstream channel conditions as a result of the proposed development. Initial FLO-2D model runs suggested that the boundary at the Lewis drain may be influencing the floodplain so the model western boundary was extended westward in order to eliminate these issues.