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<br />LOCAL FLOOD WARNINGS
<br />
<br />In regions more susceptible to flash tlooding,
<br />local entities often take over flood warning respon~
<br />sibilities, They work closely with the River Forecast
<br />Center or the nearby NWS otfice, In the past
<br />decade, many areas in southern California and
<br />along the central and north coasts developed
<br />cooperative programs called ALERT (Automated
<br />Local Evaluation in Real Time), Under the ALERT
<br />programs, precipitation is measured by gauges
<br />in the watershed linked to computer models adapted
<br />to local situations, which determine expected runoff.
<br />
<br />With adequate calibration and distribution of rain
<br />gauges within the watershed, an ALERT system
<br />provides timely information to help determine
<br />whether to evacuate. It also pinpoints areas of
<br />greatest concern, allowing more effective use of
<br />emergency personnel, the probable extent at the
<br />flooding and a response plan.
<br />
<br />The system has been credited with saving lives and
<br />property during floods in California communities.
<br />When a storm struck the city of Petaluma on
<br />Feb, 14, 1983, city officials were told at 4 a,m, that
<br />flooding was imminent according to the ALERT
<br />system. Officials contacted emergency crews and
<br />by 5:15 a.m. an emergency center was set up and
<br />evacuation vehicles were dispatched. Less than 40
<br />minutes later, police and firefighters were going door-
<br />to-door to 321 residences in low.lying areas to deliver
<br />evacuation warnings. In less than an hour, flood
<br />waters raced through the residential streets and
<br />poured into the houses in the evacuated areas.
<br />Residents who did not immediately evacuate after
<br />notification were rescued by boats. No loss of life
<br />was reported, and the ALERT system was given
<br />much at the credit.
<br />
<br />However, the Petaluma system consists of only one
<br />simple watershed, Los Angeles County's ALERT
<br />system, on the other hand, consists of 33 watersheds
<br />which contain 15 dams and extensive downstream
<br />channel systems. Forecasting with this system is
<br />extremely complex, requiring extensive monitoring
<br />of the flows going into the dams and heading
<br />downstream.
<br />
<br />An essential element of coastal weather forecasting
<br />is monitoring of high tides and waves. Flood damage
<br />along the coast is caused primarily by storm driven
<br />waves and high tides, which erode cliffs and cause
<br />loss of beach sand,
<br />
<br />
<br />A significant factor
<br />in spring flooding
<br />of southern Sierra
<br />rivers is snow melt.
<br />Through the Cali,
<br />fornia Cooperative
<br />Snow Survey Pro-
<br />gram, each year the
<br />state measures how
<br />much water content
<br />is in the Sierra
<br />Nevada snowpack.
<br />Forecasts of spring snow
<br />melt begin on Feb, 1, when an
<br />average at two-thirds at the normal
<br />snowpack has usually accumulated,
<br />(The "water year" runs from Oct. 1 to Sept.
<br />30,) Forecasts are issued in February, March,
<br />April and May, The April 1 forecast, which reflects
<br />the normal peak snow accumulation, is mostly
<br />used by water planners.
<br />
<br />FIRES AND FLOODS
<br />
<br />Fires and floods are a volatile mix. Fires, which are
<br />common throughout the semi-arid West, can
<br />radically alter the environment. They can affect the
<br />physical and chemical makeup of the soil, disrupt
<br />vegetative cycles and strip away natural barriers to
<br />erosion.
<br />
<br />On steep slopes, gravity pulls ash, soil and debris
<br />into drainages, choking channels and threatening
<br />the area's ability to absorb runoff, High temperature
<br />fires sometimes form "hydrophobic soil," which
<br />repels water. When winter rains fall on the burned
<br />area, rainwater cannot percolate to deeper layers
<br />of the soil, increasing the amount of rainwater
<br />runoff that carries the top few inches of soil and
<br />rock.
<br />
<br />Seeding burnt areas with annual grass and other
<br />quick.growing vegetation can help decrease erosion,
<br />but often there is not enough time for the plants to
<br />take hold before storms hit.
<br />
<br />Strenuous efforts averted mudslides in Berkeley after
<br />the Oakland Hills fire at 1992, Mudslides were a
<br />problem in Malibu and other southern Calitornia
<br />communities following the fires in 1993.
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