Challenge

A 14 mile-section of Upper Llagas Creek that runs southwest toward the Pacific Ocean south of San Jose, California, had narrow channels that frequently flooded, causing devastating destruction and threatening the communities of Morgan Hill, Gilroy, and San Martin and nearby agricultural areas. Flooding not only caused destruction of property but also endangered the safety of the public water supply.

Solution

The Santa Clara Valley Water District (SCVWD) and the City of Morgan Hill worked together to find a solution that would protect the watershed and area residents. Many years in development, the project was supported by team members from GLA who first performed a geotechnical investigation starting in 2005 to help inform how best to address problem flooding. The geotechnical investigation included more than 130 borings and 20 piezometers, completed on a fast-track schedule in less than 6 weeks utilizing multiple drilling crews. GLA’s geologic scope included photogeologic analysis of Quaternary alluvial surfaces and deformation patterns, lineament analysis, integration of subsurface data and mapping into geologic model for liquefaction evaluation, evaluation of potential seismic sources, and preparation of geologic portions of the final report.

To prevent future flooding, narrow channels are being widened to expand the flow capacity of the creek. A tunnel is also being constructed to safely redirect stormwater underground through the communities. As part of the project team, GLA performed a supplemental geotechnical investigation, using conventional and vibrating wire piezometers in selected borings for monitoring of groundwater levels. We also provided laboratory testing of rock samples, and geotechnical recommendations to the tunnel designer.

Results

This long-planned project by SCVWD, scheduled for completion in 2027, will help protect Morgan Hill, and other area communities from the destruction of repeated flooding.