Rancho San Antonio County Park Detention Basin

Peranente Creek ood protection project The Permanente Creek Flood Protection Project extends 10.6 miles through the cities of Mountain View and Los Altos.

San Francisco Bay D

Project elements

A Floodwalls

McKelvey Park detention

C Channel widening Palo N Alto Shoreline Park D New diversion structure

Rancho San Antonio Park detention

101 k

Middle eld Rd e

Central Expy A

e California St r C El Camino Real e t n e n a

m r e Rancho San Antonio County Park P

Castro St DShoreline Blvd detention237 basin San Antonio Rd PermanenteCalderon Ave Creek Flood Protection Project

k e e r To keep excess flows of water from spilling out of C

e l a Cuesta Dr Permanente Creek, the Santa Clara Valley Water H District will construct two flood-storage areas off of Permanente Creek in Mountain Rancho San Antonio County Park. The detention areas will temporarily View Sunnyvale store floodwaters until peak flows pass, reducing the need for more flood protection elements downstream.

The detention areas will be approximately 12 acres in size and

Grant Rd 15 feet deep with mild side slopes, contoured to the surrounding area,

Los Altos and replanted with native trees and grass. Flood flows would inundate 85 Los Altos the site very rarely and quickly drain away. A 25-year flood, which has Hills Foothill Expy a 4 percent chance of occurring in any given year, would result in about one foot of water in the detention area that would drain away in hours. A 100-year storm, which has a 1 percent chance of occurring in any given 280 year, would fill the area, but it would drain out in one to four days. D Features Benefits Cupertino • A 15-foot deep depression to • Potentially enhances many acres Flood-prone area collect peak storm flows from of wildlife habitat Permanente Creek • Provides flood protection for The Permanente Creek Flood • Planting of native trees thousands of homes and businesses Protection Project will provide natural in Mountain View and Los Altos flood protection for Mountain View • Removal of non-native trees that and Los Altos, create recreational compete with native species • Reduces construction impacts opportunities and enhance the • Replacement of existing to downstream residential and environment. This project is funded maintenance bridge business areas by the countywide Safe, Clean Water • New bathroom facilities • Reduces flow rates, allowing and Natural Flood Protection for potentialos riparian Atos restoration • New,Saint Joseph enlarged Ave paved parking area Program parcel tax passed by downstream

voters in November 2012. Peranente Creek

Mora Dr The four project elements: Cristo Rey Dr A Floodwalls and levees north 280 of U.S. Highway 101 B Flood detention area at N City of Mountain View’s Rancho

McKelvey Park San Antonio aond ay C Channel widening and Park deepening of existing channels along Permanente Creek D Flood detention areas at Cuertino County of Santa Clara’s Rancho San Antonio Park Canyon ak ay Questions? PG&E valleywater.org/services/ PermanenteCreek.aspx (408) 755-0333 Detention area continued on back...

Permanente Rd Stevens Creek Blvd D Rancho San Antonio County Park detention basin What to expect during construction Construction of the Rancho San Antonio County Park detention basins is expected to begin in late 2016 and take approximately two years to complete. The Santa Clara Valley Water District and Santa Clara County Parks are working together to minimize construction impacts as much as practical and in accordance with county ordinances. We appreciate your understanding and cooperation as we embark on this important flood protection project. • The main detention basin will be constructed between the main park entrance and the Gate of Heaven Cemetery. • The South Meadow parking lot will be open throughout construction. The new, expanded parking lot and bathroom will be built first, slightly north of the existing equestrian parking lot. Upon completion of the new facilities, the existing parking lot will be closed. • A visual screening fence will be installed around the construction areas. • Various parts of the South Meadow and Hammond- Snyder Loop Trails will be closed during construction. Trail accessibility signage will be installed at least two weeks in advance of any closures. • Regular construction work hours are 8 a.m. to 5 p.m. Work is scheduled Monday through Friday, excluding legal holidays. • Safety of the community and our employees is our priority. Barricades, railings, lights, fences and Top: Existing view of Rancho San Antonio County Park. other warning devices will be used for the greatest Bottom: Photo rendering of future flow detention design at Rancho San Antonio County Park. public safety and convenience.

100-year event runoff Peak flow from the 100-year event What is a flood detention area? stored in the detention basin Flood protection methods can include berms around Current level of buildings, widening channels, raising floodwalls, flood protection elevating structures and roadways, and/or constructing runoff water Water in the detention basin a bypass channel. In areas where development is released back limits widening the creek or raising floodwalls, flood- into the creek storage basins are used to temporarily divert and store 1hr 2hr 3hr 4hr 5hr 6hr floodwaters until a major storm passes. storm hours

Above: The graph illustrates a sample hydrograph timeline of a 100-year flood event on Permanente Creek.

Left: During a flood event, excess water stored peak runoff from the creek flows into detention Flood event creek drainage outlet and detention basin water that would water elevation pipe structure basins. The basin protects downstream have gone over homes and properties by holding the the bank stored water will return to creek after flood event water until the flood passes.