Natural flood protection
A history of flooding
Long before the Santa Clara Valley was developed, streams naturally
flooded with great regularity. Floods cleared streams of debris, and
silt flowed from the streambeds to adjacent land, replenishing the nutrients
in the soil. Far from a disaster, flooding benefited the environment.
Today, flooding in Santa Clara Valley is far from innocuous.
Communities can suffer millions of dollars in property damages and vital
roadways can become impassable. The vast majority of homes and commercial
buildings were built on lands that had flooded for eons. In the 1950s,
the water district began addressing flood problems in earnest.
Early flood protection efforts
In the early days, flood protection meant straightening creeks and lining
them with concrete. While these channels are great for moving water
quickly, they are not much to look at. We now have new flood protection
methods that are not only more scenic, they foster natural habitats,
and often include trails and interpretive benches.
A more natural approach
The water district balances the need to provide flood
protection with the need to protect streams and natural resources. Various
factors are considered in determining the best methods to improve a
creek's capacity for flood water while minimizing impacts to the ecosystem.
A few examples of these methods are:
The water district employs numerous flood protection
technologies that help keep creeks as natural and beautiful as possible.
Water district goals have kept pace with changing community values.
New multipurpose flood protection projects protect property while preserving
habitat, improving water quality and providing creekside trails.
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| A gabion wall |
Gabions
In some creeks, gabion walls, rocks enclosed in wire baskets, are arranged
like a staircase up a creek bank. Native vegetation can grow in the
crevices of these walls while keeping the bank from eroding. The example
pictured here is Los Gatos Creek, taken 18 months after construction.
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| A crib wall during construction |
Crib
walls
Another technique to strengthening creek banks is to use crib walls--dead
trees arranged in a Lincoln-log fashion. After the logs are buried,
natural vegetation can flourish. Over time, the logs will decompose,
and the root structures of newly planted trees and shrubs will eventually
provide protection from erosion.
The photo above, taken during construction of a
crib wall, shows how high waters have eroded the banks.
The second picture below shows how another bank,
along Bodfish Creek near Gilroy, looked one year after installation,
with native plants already beginning to establish themselves.
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| A cribwall, one year after installation |
Another technique to increase a creek's capacity
for flood waters is to build an overflow channel parallel to a creek.
This channel is only used when water flows are at their highest. This
way, the natural creek bed can be left alone, preserving wildlife habitat.
Bypass channels
The Guadalupe River runs right through downtown San Jose and has caused
millions of dollars in flood damages. To increase protection from flooding,
a system of bypass channels is in the works. But you will never see
them. They will be under ground. When the river rises above a given
level, water will begin flowing through massive culverts that will run
under downtown streets. These culverts divert the excess water around
the developed areas of downtown where local development has been built
right up to the narrow portions of the river.
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| Matadero Creek in Palo Alto |
Last resort
Unfortunately, housing
developments built very close to creeks over the years limit the options
for restoring these creeks. Often, there is just no space left to create
a natural creekbed. In those areas, concrete channels with steep sides
are often the only way to protect homes and businesses from floods.
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