RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 15

Table 1-6 Energy Demand of Water Supply in Southern California

Supply and conveyance 8,900 kWh/MG

Treatment and disposal 1,300 kWh/MG

Total 10,200 kWh/MG

National average 250 kWh/MG

Source: [13].

energy required to deliver imported water supplies, we will address both issues
with one set of design solutions.
In southern California, this water cycle varies considerably. For example, in
terms of annual precipitation, the total amount of rain received by the major
communities in southern California varies dramatically; relatively wet Pasadena
in the northern portion of the region receives twice the rainfall of semiarid San
Diego in the southwest (20 in Pasadena, 16 in./yr in Riverside, 15 in./yr in Los
Angeles, and 10 in./yr in San Diego). This variability is reflected in the size
of storm events, which is so important in stormwater management calculations
and therefore important to LID design. For example, the 2-year-frequency storm
varies from 1.6 in. of rain in 24 hours in San Diego to 3.5 in. in Pasadena.
This frequency rainfall comprises some 93 to 95% of all the rain that occurs in a
century, so it is a defining statistic. Within that rainfall pattern is an equally varied
difference, on both an annual and seasonal basis, with a well-defined “wet” and
“dry” season. Such natural variability in the hydrologic cycle within the region
makes stormwater management that much more challenging.
Of course, the fact that water is limited in southern California does not
preclude the occurrence of coastal storms from the Pacific causing rainfall
events that drop almost the total annual average rainfall over a period of a few
days, as occurred during mid-January 2010. From the 18th through the 22nd, a
total of 8 in. of rain fell in the Los Angeles region, causing widespread flooding,
erosion and mud slides, evacuations, and most significantly, the structural failure
of a number of residences perched precariously on hillsides. The local building
codes allow the placement of buildings in locations that would not be considered
sustainable in a less arid environment, where saturated soils are an assumed
design condition. This cycle of rainfall, runoff, and mud slides was repeated in
January 2011, and promises to occur frequently until land development planning
better manages rainfall.
The current hydrologic cycle in southern California bears little resemblance
to the natural system of a century ago, as so well described by D. Green in her
topic Managing Water: Avoiding Crisis in California [14]:
In the 1920's, roughly 95% of the rainfall falling on Los Angeles either infiltrated
into the ground or evaporated. Only 5% ran off to the sea. Today, with the extensive
development and the paving over of our urban environment (as much as 80% of the
land is now covered with roofs, roads, parking lots, patios, etc.) and the construction
of the massive storm drain channel system, about 50% of stormwater runs off in
the Los Angeles River drainage area, while 50% either infiltrates or evaporates. ...