High Waters Along the San Joaquin

A heavy downpour began on New year’s Day 1997 in much of Northern and Central California. More than 30 inches fell during a three-day storm. The deluge created a volume runoff that quickly overwhelmed reservoirs and levees along the San Joaquin River. The flood of 1997 were considered the largest in the 90-year record of Northern California floods.

In January 1997 floods vividly pointed out the importance of floodplain management especially in the San Joaquin River basin where much of the floodplain is still relatively undeveloped. DWR’s San Joaquin District is helping to produce new floodplain maps of the area along the San Joaquin River between Friant Dam to Highway 99.

Such disasters provide valuable learning opportunities for flood management. This year, in response to a new directive from the State Legislature, DWR has begun remapping the reach of the San Joaquin River between Friant Dam and Highway 99. This mandate grew from a recommendation in the State’s May 1997 FEAT (Flood Emergency Action Team) report that DWR improve floodplain maps. In all, the Legislature appropriated $1.4 million on 1997-98 to accomplish this re-mapping. Of this amount, $130,000 was designated for the San Joaquin River project.

Residents of the Fresno region are curious about the San Joaquin River mapping study, about its likely findings and what it may portend for new floodplain boundaries. They also have questions about a revised water flow estimate made by the U.S. Army Corps of Engineers that DWR is using to establish new floodplain boundaries.

Earlier this year, the Corps more than tripled its estimate of San Joaquin River flood discharges expected to occur below Friant Dam after a 100-year storm. In 1981, its estimate was 19,800 cubic feet per second. This year, after assessing data gathered during a downpour on January 3, 1997, and evaluating other data, the Corps increased that to 71,000 cfs.


COE Procedure and DWR Study

Meanwhile, a Corps’ report explaining the procedure used to develop these estimates, and detailing estimated Friant Dam discharges resulting form 10-, 50-, 100-, and 500-year storms, is being circulated for public review and comment. This August the 90-day review period ended. DWR has forwarded all comments received to the Federal Emergency Management Agency. “At that point,” says Paul Romero, a DWR associate engineer working on the river mapping study, “FEMA consultants will review the comments, as well as the Corps’ report, and decide on a base flood discharge figure for the mapping study.”

DWR Will than use this figure to determine new floodplain boundaries, and FEMA will prepare and distribute new floodplain boundary maps.

“Up to now,” says DWR associate Engineer Kevin Faulkenberry, who is working with Romero on the mapping study, “DWR has been collecting data on terrain and historical flood evens in the flood-impact area... It has also been developing and calibrating the computer models it needs to evaluate these data and waiting for high flows on the river to diminish so the surveying can begin.”

Impact on Insurance Rates and Property Values

What all these preliminaries are leading up to is determination of what parts of the riverfront area will be included inside the new floodplain boundaries on revised FEMA flood insurance rate maps. Lenders, real estate agents, and insurers use these maps to establish the value and insurability of property. Since being in or out of a floodplain affects and value, building requirements, and insurance costs significantly, local interest in this mapping study is strong.

“The area of influence is impossible to predict right now,” says DWR’s San Joaquin District Floodplain Coordinator Ed Perez. “But once the designated floodplain expands, the cost of building and insuring dwellings in the added flood zone is apt to increase.”

Perez adds that some undeveloped property merged into the expanded flood zone will likely decline in value as increased costs and risks dampen its appeal. “This sort of fallout has occurred in other riverfront areas,” he notes. “That’s the big reason why local interest is so intense in the maps DWR is developing and the use FEMA will make of them... The existing San Joaquin River floodplain map is based on the Corps’ 1981 hydrologic estimate; the new map will be based on a much larger projection. For property owners, especially, there’s lots at stake.”

Uncertain Future
For months now, people have been asking Romero and Faulkenberry to estimate where the new floodplain will fall.

“One person wanted me to go up with him in a plane and point out where the new 100-year flood lines will be,” say Faulkenberry. “There’s no way to do that with certainty; we haven’t done enough surveying yet. It’ll be awhile before we’ll be able to approximate what the new lines will be and awhile longer before their impacts are fully apparent.”

Between now and then, the Department is responding to inquiries on its mapping effort and staying on schedule to meet its self-imposed April 1999 deadline to complete its floodplain study and forward it to FEMA.

Over the next decade, FEMA is committed to reducing the threat that flooding poses to life and limb, and to decreasing flood losses to property ;and the economy by at least 15 percent.

These are serious commitments, and DWR’s efforts to produce current and reliable floodplain maps of the San Joaquin river will help FEMA achieve them.

DWR’s involvement is also closely linked to its own public effort “to protect, restore, and enhance the natural and human environments, in cooperation with other agencies, to benefit the State’s people.”

The importance of this effort is unquestionable, and it’s driving the completion of this study.

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Floodplain Mapping Area
DWR is remapping the floodplain along the San Joaquin River between Friant Dam and Highway 99. This 25-mile reach has one major tributary (Little Dry Creek) and no flood diversion facilities. It runs through a wide and sparsely populated gorge and is fringed by parks and a parkway, several golfing complexes, a pistol range, crop land, exotic fish farms, gravel-mining operations, occasional structures, and a great deal of open land.

Following the January 3, 1997 storm, many of these fringe areas were ravaged by floodflows resulting from sudden and mandatory releases from Friant Dam. A trailer park near the junction of the river and Highway 41 were virtually destroyed. Many acres of riverfront land were eroded by the large and fast-moving flows. In all, property damage totaled many millions of dollars and led to the decision to remap the floodplain to better anticipate and plan for the effects of future flows and continued development.

Another golf course is presently under construction in the riverfront area, and plans are on the drawing board for new homes that would overlook the course.