Permanente Creek Flood Protection Project history and background

The Permanente Creek watershed encompasses 28 square miles, including portions of the cities of Los Altos, Mountain View, Cupertino, and Los Altos Hills. The creek originates in the Santa Cruz Mountains and discharges into San Francisco Bay through Mountain View Slough. Hale Creek is the major tributary to Permanente Creek. The majority of flows from the upper Permanente watershed are directed to Stevens Creek through the Permanente Diversion.

The total project longitudinal distance includes approximately six miles of Permanente Creek, two miles of Hale Creek, and one mile of the Permanente Diversion. Of this total length, approximately half is in a “natural” or unlined state, while the other half is concrete-lined.

The Permanente Creek watershed has had a history of recurring floods which have adversely impacted the safety and economic stability of residents and businesses in Los Altos and Mountain View. Major flooding occurred in 1862, 1911, 1940, 1950, 1952, 1955, 1958, 1963, 1968, 1983, 1995 and 1998. In December 1955, the so-called “Christmas Storm” inundated approximately 770 acres in the lower reaches of Permanente Creek. Homes, businesses and agricultural land in Mountain View and Los Altos sustained losses. Bridges and culverts in Mountain View were extensively damaged. One hundred people were evacuated from their homes for a two-week period. In 1958, flooding occurred along both the upper and lower reaches of Permanente Creek. In response to these floods, the District and other agencies have, at various times, improved several sections of the creeks. The major portion of channel lining and the construction of the Diversion were conducted in the 1960’s, and there was significant follow-up work in the 1980’s. Recently, it has become clear that further improvements are necessary.

The Permanente Diversion was designed to divert the majority of storm runoff from the upper Permanente watershed to Stevens Creek. The diversion structure is currently not functioning properly and diverts almost all the flows to Stevens Creek. This project will identify opportunities and constraints associated with both watersheds to mitigate the flooding problems in the Permanente Creek watershed, while not exacerbating the flooding condition of Stevens Creek. Any problems identified in the Stevens Creek watershed will be addressed in a separate project.

The current efforts of flood control improvements for Permanente began in 1997. Much useful information and data has been gathered, such as the structural integrity analysis report completed in 2000. A fully staffed project team began work in July 2001 and is expected to complete the planning and design phases by March 2010. Construction will be performed between 2010 and 2016, under a separate project number funded by the Clean Safe Creeks Program.

The majority of the civil planning and design work will be conducted by in-house staff. Some of the specialty tasks, such as hazardous materials investigations, environmental assessment and permitting per CEQA, and geotechnical investigations will be contracted to outside consultants.