20201213_svlevee-52-3.jpg

EXISTING LEVEE
AND TRB CONDITION

Behind homes on Vendola Drive sits a levee and timber-reinforced berm (TRB) which was installed in 1983 by the Marin County Flood Control & Water Conservation District – Zone 7. TRB installation was completed, largely on private residential properties, following two years in a row of devastating and widespread floods in Santa Venetia, to prevent flooding in the community when the tides in Las Gallinas Creek reach high levels. The TRB consists of two vertical panels of wood filled with soil and installed on top of the levee. Typically, the wood panels must be replaced every 20 years due to deterioration. While the old TRB system has worked so far for high tides, the deterioration of the timber, compounded with the rising sea levels and sinking land elevations, threatens its ability to protect Santa Venetia from flooding.

The TRB and levee system is currently maintained by Zone 7 only where property owners provide permission through temporary rights-to-enter.  This process is not cost-effective and historically there have been a handful of property owners who deny Zone 7 staff and contractors permission to enter and maintain the TRB.  Therefore, maintenance is not comprehensive and potential weak spots remain along the system.

In 2019 the Zone completed a topographic survey of the project area using an unmanned aerial vehicle (aka a drone) equipped with a LiDAR 3D laser scanner to have current elevation information. Using this information in combination with a current county-wide dataset provides a detailed picture of existing elevations. The colored digital elevation model below gives a sense of levee location, variability, and the interior ground surface.

Ten years of prior work has pointed to the levees as the weakest link in the flood protection system for Santa Venetia including interior drainage and watershed-scale studies. These can be found on the Gallinas Watershed page under "Resources"

An analysis completed by a geotechnical consultant as part of a comprehensive levee analysis in partnership with the U.S. Army Corps of Engineers (USACE), indicated that high tides would likely damage the TRB and flood the community before they were high enough to reach the top of the TRB. Additionally, the analysis revealed that some sections of the earthen levee without the TRB are not high enough to hold back expected high tides over the next few decades. Click here to access the 2014 Levee Evaluation Reports.

Elevation Map of Zone 7 Floodplain.jpg