Los Angeles receives an annual average volume of 831,400 acre-ft of runoff,
which originates from rainfall within
the city, irrigation activities within the
city, and flows from areas upstream of
the city, according to the master plan.
Of this total, 89 percent is lost to evapotranspiration or to surface discharge to
the ocean, while the remaining 11 percent, or approximately 92,000 acre-ft, enters the city’s aquifers. Of this
amount, 29,000 acre-ft enters by virtue of the city’s and county’s centralized
recharge facilities, which capture flows
and direct them into water supply aquifers. Of the remaining 63,000 acre-ft
that infiltrates in pervious areas within the city, 35,000 acre-ft enters water
supply aquifers, while 28,000 acre-ft
percolates to perched aquifers that are
not used by the city for water supply.
All told, the 64,000 acre-ft of runoff annually entering the water supply aquifers represents more than 10 percent of
the city’s yearly water demand of approximately 550,000 acre-ft.
However, Los Angeles could markedly increase its ability to capture and
use stormwater. Using a mix of centralized and distributed facilities, the city
could begin capturing an additional
68,000 to 114,000 acre-ft of runoff an-
nually within the next 20 years, depend-
ing on how aggressively it acts, accord-
ing to the LADWP’s master plan. The
report defines centralized facilities as
those that “capture generally more than
100 acre-feet per year and are unique to
a specific location and opportunity,” including reservoirs with large surface areas and spreading grounds, according
to the plan. By contrast, distributed facilities capture less than 100 acre-ft per
year and “have similar designs, allowing
them to be implemented programmati-cally” across the city, the report states.
Examples of distributed facilities include
rain gardens, “green” streets, cisterns, areas designed to facilitate infiltration, and
projects for the direct use of stormwater.
By itself, capturing stormwater will
not address all future water needs in Los
Angeles. However, doubling or nearly
tripling the city’s capacity for capturing
stormwater for water supply will help
Los Angeles in the coming decades, says
Mark Hanna, Ph.D., P.E., a principal
water resources engineer in the Los Angeles office of Geosyntec Consultants.
The firm served as the prime consultant and technical leader in developing
the master plan. “In combination with
increased efforts in conservation, reuse,
and groundwater management, this
represents a significant dent in meeting
future demands,” Hanna says.
In assessing the potential for captur-
ing stormwater in Los Angeles, the team
led by Geosyntec Consultants first eval-
uated prevailing conditions, including
existing regulations, incentives, devel-
opment patterns, and flood control fa-
cilities, Hanna says. Modeling was then
conducted using two approaches orig-
inally developed for assessing the Los
Angeles basin: the watershed modeling
system known as the Loading Simula-
tion Program in C++, or LSPC, and the
Ground Water Augmentation Model, or
GWAM, developed by the U.S. Geologi-
cal Survey in partnership with the Coun-
cil for Watershed Health, of Los Angeles.
The modeling results determined the
“current and realistic potential” regard-
ing the amount of stormwater available
for capture and use for water supply pur-
poses in Los Angeles, Hanna says.
The project team then developed potential alternatives ranging from large
centralized facilities for the infiltration of
significant volumes of runoff to smaller
“programmatic solutions” that could be
implemented throughout the Los Angeles basin, Hanna notes. After working with the LADWP to conduct an extensive public outreach campaign, the
project team carried out further hydrologic modeling using 27 years of continuous local rainfall data. In this way the
team was able to estimate the amount
of stormwater that could be captured by
various practices over the next 20 years.
Potential projects also were evaluated for their cost-effectiveness and the
extent to which they could realistically
be deployed throughout the city. Although centralized facilities tend to
have lower life-cycle costs per acre-foot
captured, they are limited to locations
having significant volumes of runoff.
Furthermore, the soil above the water
supply aquifer must lend itself to water infiltration. Distributed facilities,
on the other hand, can be installed in a
broader swath of the city. Among such
facilities, subregional infiltration and
green street programs were found to
have the lowest life-cycle costs per acre-foot captured, according to the report.
Projects for the direct use of stormwater were found to have the highest life-cycle costs, in part because they tend
to require distribution and treatment
systems. However, such systems can be
employed in areas that are not condu-
cive to infiltration. —JAY LANDERS © C
The 150-acre Tujunga Spreading Grounds, in Sun Valley, California, is one of the Los Angeles Department of Water and Power’s key facilities for recharging groundwater supplies.