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For Flood Plains
© 1997, 1998 Streamline Publications
The natural erosion of semiarid lands,
including much of California's central and southern regions, is a far greater
concern than in desert or high-rainfall areas. This intrinsic erosion problem develops
further as increased population—drawn by the pleasant climate and a
resurgent economy—and the resultant additional building leave their marks
upon the land.
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Vegetation density and rainfall
intensity in semiarid regions underlie this situation. Vegetation over broad
non-urban watershed areas is typically sparse. In locales where trees and bushes
occur naturally, they are widespread survivors in the competition for the 10 to
16 inches of annual precipitation these regions can expect. The spaces between
the trees and bushes are populated by scattered, low-growing, drought-tolerant
plants. Such vegetation offers little protection for the soils around them from
rain that often comes in disproportionately large amounts during a short season.
Compare this to wetter regions where
abundant, rainfall supports vigorous plant growth that in turn produces a matrix
of organic material from dead plants—a giant sponge that overlays, protects
and enriches the soil. Conversely, in rain-shadowed, arid lands where plant
cover is much less than that of semiarid regions, it is the very scarcity of
rainfall that protects the soil from water erosion. Rain that does fall on rare
occasions on the parched, often porous soil is absorbed—though flash floods
can occur. Semiarid region soil pores can be
clogged by raindrop impact causing low penetration and absorption and a high
rate of runoff.
NPDES Phase II
According to a recent report by the
Center for Continuing Study of the California Economy, the state could see a
population growth of 6 million—an 18 percent increase—by the year
2005. Much of this growth will be in the south-central and southern coastal
regions of the state between Ventura and San Diego Counties. The U.S.
Environmental Protection Agency has begun its National Pollutant Discharge
Elimination System (NPDES) program Phase II, which is placing greater emphasis
on erosion control—the antithesis of population growth. Pressures on
municipal and regional governments to deal with "nonpoint" sources of
runoff have increased. It is no longer a matter of a local jurisdictions being
able to accept environmental impact reports (EIRs) that consider only a segment
of the wider watershed area. Downstream problems are now viewed as upstream
responsibilities. Erosion control standards for
stormwater runoff will now have to be enforced more strictly or be developed or
modified. Stormwater will require treatment to remove sediments and pollutants.
Special attention will be given to agriculture, construction, forest management,
range use and municipal storm drains. Runoff from developed lands may commonly
carry with it entrained salts, nutrients, pesticides, organic matter, bacteria,
animal wastes, sewage effluent, toxic metals, hydrocarbons, automotive and
household hazardous waste. As construction "paves" the
land with impermeable cover—buildings, roads, parking lots—stormwater
runoff is increased and concentrated, leading to increased downstream sediment
and decreased absorption. Streams are scoured of riparian vegetation and
streambeds are eroded and undercut. Rivers and pools become hostile environments
for aquatic animals and fish as smothering sediment builds and fills pools and
tidal basins. Wetlands are changed forever.
Lessons of the Past
In the mid-1800's the Army Corps of
Engineers was charged with protecting surrounding lands from the Mississippi
River—fertile lands that are natural flood plains—and with keeping the
river navigable. The main weapon employed by the Corps was the levee. By any
assessment, the unintended consequences of the battle with the river have
developed into a nightmare. Levee-protected cities along the river are today
tens of feet below river bottom level, its bed having filled with
sediment. Continued protection means building the levees higher and higher.
There will be a limit. Yet, in the face of such overwhelming
evidence of what has happened on the Mississippi with its levee-fortressed
towns, levees are still in prevalent use across the country—except where
rivers, such as the Los Angeles, are converted to concrete channels. Nature will ultimately have her way. If
we stand in her path we can expect her to ignore us.
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