Mountains, cities, and co-mingling floodplains – past and present evaluation of city drainage

The City of San Buenaventura (Ventura), California is a diverse City, having a varied mix of residential, commercial, and industrial developments. Its location at the base of the Western Traverse Mountain Range, between two major river systems (Santa Clara and Ventura), and the Pacific Ocean provides numerous recreational opportunities for its residents. At the same time, the stark transitions between steep tributary streams and coastal plains/valleys presents a significant drainage evaluation and flood hazard management challenge.

City watersheds draining to the Santa Clara and Ventura Rivers have a number of natural coalescing alluvial fan features, with flows from various intermittent streams combining to form complex floodplain patterns. Over time, these areas have been developed and runoff has been diverted into both City and County engineered channels and storm drains. Many of these facilities were designed and constructed prior to the 1990s with design levels ranging from 10 to 50-year flood events with and without additional freeboard. Due to previous cost prohibitions and model limitations, most of the tributaries and facilities have only been studied using traditional, simple one-dimensional, disjointed hydrology and hydraulic methods.

Are these estimates on the tributaries still valid and if not, do we think that the facilities will work as intended and designed? What do more recent flood events on the larger streams say and does it correlate with the function of the storm drain system? In 2018, the City of Ventura initiated a comprehensive effort using tools such as TUFLOW and PC-SWMM software to create a dynamic hydrologic and hydraulic model of the entire City that could be used to pinpoint potential drainage problems under several storm events and historical conditions.

This presentation will focus on the process embarked upon by the City and its partners, including lessons learned along the way as well as some evolving results.