San Francisco — Last week's devastating rainstorm in the Bay Area brought tragic loss to many families and businesses. But it also instilled determination not to be taken by surprise should such a natural event occur again.
Geologists and other specialists contacted by the Monitor made clear that enough knowledge already existed to prevent much of the misery and property loss , if not the inconvenience, brought on Jan. 4 by the combination of instable ground and excess moisture. Studies being made right now of what occurred last week - and what is still occurring under the surface - will add to the ability to pinpoint problem areas and advise residents about what precautions they should take against similar future situations.
Some of the physical features that make this area spectacularly attractive can cause trouble under certain conditions. There is great aesthetic temptation to build on precipitous hillsides and near canyon mouths. Besides, it is not easy to find housing sites in the densely populated San Francisco area that are not on steep slopes or in flood plains.
Evidence of the toll in life and property in a six-county area around San Francisco Bay continued to mount last week. As of this writing 27 fatalities haved been attributed to the great rainstorm, and damage estimates total some $ 280 million.
President Reagan declared Contra Costa, Marin, San Mateo, Santa Cruz, Solano, and Sonoma Counties major disaster areas, making residents and businesses eligible for aid, loans, and grants from a number of federal agencies. Centers where people can apply for help opened Jan. 11 in each of the counties.
Meanwhile, the American Red Cross was designated as the official volunteer agency to administer aid to storm victims. It announced an immediate drive to raise at least $250,000 to be used in helping victims of last week's storm.
Hardest hit by the storm's effects was Santa Cruz County. Fourteen fatalities were known, and crews removing mud and debris in one area were forced to stop work Saturday because of unsafe conditions. Seventeen residents were still missing. The county's total losses are estimated at some $100 million.
By Friday morning the Golden Gate Bridge and US 101 north to Marin and Sonoma Counties were open, but not all lanes on either were being used. A particularly sensitive slide area known as the Waldo Grade was being carefully monitored.
As is often the case in such events, people in other parts of the nation seemed to get the impression that the damage and the danger were more widespread than was actually the case. In most areas, and particularly Contra Costa County, flooding - with mud-laden water - was the chief problem. But there was no denying the costly and sometimes tragic impact on families and merchants hit by mudflows or high water.
Geologists point out that the mudflows (they prefer not to call them slides) from Monday's storm are over. Now, they caution, residents should be aware that as moisture sinks deeper into the hills, deeper-seated landslides are likely to occur. Alert homeowners and local officials should have sufficient foreknowledge of such slides, however, for timely evacuation.
A staff member of the Association of Bay Area Governments, a planning agency, pointed out that many of the homes destroyed or damaged last week were constructed more than 10 years ago. Houses, and developments, built in the last eight or 10 years, said Kate Jackson, were put up under more stringent regulations and are safer.
Counties and cities in the area, she said, have been upgrading building code provisions dealing with land stability.
Flooding problems have increased in recent years, Miss Jackson noted, as more residential and commercial developments are built and more streets and highways are added. There is a move on the county and local level toward requiring ''downstream assurances'' by those who build in watersheds.
Dr. Desiree Stewart-Alexander, branch chief for the US Geological Survey's (USGS) Western Region, points out that survey maps are available to the public that show degrees of slope stability. Zones where builders should have specific surveys done are indicated in red.
Survey teams of both the USGS and the state Division of Mines and Geology have been conducting aerial and ground surveys over the entire area since the end of Monday's storm. They are pinpointing sites where mudflows or landslides might occur within hours, days, or weeks. At the same time they are using the current situation as a ''laboratory'' for learning more about how such conditions occur and can be guarded against. (Geologists try to avoid the term ''mudslide,'' they say, because the phenomenon involved is a massive movement of water. At the same time, they point out that a ''mudflow'' may contain a lot of other material - rocks, trees, and other debris.
Dr. Stewart-Alexander - noting that California is not the only state that experiences mudflow, landslide, and flood problems - said the USGS has been conducting a number of studies on land stability in recent months. One, dealing with Marin County, was ready to be published at the time the big rainstorm occurred. It ''predicts,'' she says, many of the slides that took place in Marin last week. Another USGS report cites the number of destructive landslides that occurred in two previous Bay Area ''wet seasons'' - 335 in 1968-69, and 411 in 1972-73.
Geologists also warn that unless those who live on the land are required to utilize the information available - or do so on their own initiative - natural occurrences such as took place in the Bay Area last week will continue to take an unnecessary and tragic toll.