Engineering Inc - July/August 2019 - 35

The Upper Llagas Creek Flood
Protection Project is employing a
variety of strategies to preserve
the established riparian ecology
while at the same time reducing
the potential for flooding.

Preserving
a Meandering
Stream
PROJECT: UPPER LLAGAS CREEK
FLOOD PROTECTION PROJECT
SANTA CLARA COUNTY, CALIFORNIA
FIRM: WOODARD & CURRAN
SACRAMENTO, CALIFORNIA

n innovative flood protection
project in California's southern
Santa Clara County is providing
improvements to several Llagas
Creek tributaries. Extending from the city
of Morgan Hill in the north to Gilroy in
the south, the project preserves the meandering quality of the streambed while also
accommodating peak flows during storm
events.
Historically, Llagas Creek and its tributaries flood during major storm events, affecting more than 1,000 homes, 500 businesses
and 1,300 acres of agricultural land that sits
within a 100-year floodplain. To provide
critical flood protection to the surrounding
area, the Santa Clara Valley Water District worked with Woodard & Curran to

Glenn Hermanson
develop approximately 13.9
downtown Morgan Hill, thus
Sr. Technical Practice Leader
miles of stable stream chanavoiding and minimizing
Woodard & Curran
nel that provides increased
impacts to the native creek
flow capacities, improved stormwater
channel, residences and businesses. Low
management, improved water quality and
flows will remain in the natural creek chanenvironmental habitat restoration.
nel through downtown, thus preserving
The design reduced the project impact
existing creek characteristics and vegetation.
by widening the creek on only one side,
"As the flow increases, the water level rises
allowing swaths of native riparian trees and
and begins to spill over a weir," says Glenn
habitats to remain. Woodard & Curran
Hermanson, senior technical practice leader
applied a modern geomorphologic analysis
for Woodard & Curran. "That takes all the
to the creek to determine a channel shape
high flow out of the creek and puts it into a
that would provide a more stable waterway
double box culvert, which then transitions
not prone to incising or depositing sediinto a tunnel under the hill." Adding the
ment. To restore the creek habitat, existing
bypass effectively removed the city of Mortopsoil and streambed material are being
gan Hill from the floodplain.
harvested and stored during the widenIn addition, the project proposes to coning phase and will be placed back into the
vert an existing rock quarry to wetlands
new channel during the latter phases of
and restore a remnant section of Llagas
construction to support native revegetation
Creek to convey low flows for enhanced
plantings. By reusing the topsoil, the natufish passage-both of which contribral nutrients and native bacteria will help
ute to a healthier watershed and overall
restore the creek to a natural state. Hunecosystem.
dreds of instream complexities-such as
The $172 million project is scheduled
logs, root wads, boulder clusters and turtle
to start construction in 2019 and be comperches-will be constructed in the creek
pleted by the fall of 2022, not including a
bed to provide habitats for endangered fish
three-year native plant establishment after
and other animals.
construction. n
A high-flow bypass including a
Tom Klemens is a freelance writer based
2,100-foot-long tunnel has also been
near Chicago and a registered Professional
Engineer in Illinois.
designed to convey high flood flows around
JULY / AUGUST 2019 ENGINEERING INC.

Engineering Inc - July/August 2019

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