The stream gauge is modest.
But the Bay Area’s vast network of gauges, strategically placed and connected to strong communication lines and computers, can provide early warning of rising water levels and help protect lives and property from flood damage.
Kevin Murray of the Palo Alto-based San Francisquito Creek Joint Powers Authority said: “That way we can give them time to get ahead of the disaster.”
As a series of storms has just passed and continues this week, experts patrolled rivers and streams, inspecting and recalibrating valuable tools to ensure the accuracy of water data. After a severe storm, they ask: Is equipment still running? Have flow conditions changed due to fallen trees, blocked bridges, eroded dikes, or blocked storm drains?
“We’re looking at flood warning systems,” said Scott Brown, a geologist at Berkeley-based Balance Hydrologics, a consulting firm that helps Bay Area agencies manage flood warning systems.
“More storms are coming and we need to mobilize quickly to get everything back up and running again,” he said.
Despite climate change increasing the risk of flooding, protection strategies are moving away from structural measures such as walls, gates and embankments. The role of prediction and warning is growing.
Gages can provide days or hours advance notice, giving residents time to move sandbags, move cars, help the homeless, lift electronics and valuable curios off the floor, or search for higher ground. can.
At 8:43 a.m. on New Year’s Eve, gauges at the Stanford campus alerted officials that the San Francisquito Creek between Santa Clara and San Mateo counties had reached the “flood watch stage” and was continuing to rise. At 9:06 am, authorities issued a flood advisory. At 9:30 a.m., residents were warned that flooding could occur at Palo Alto’s narrow Pope He Chaucer Bridge in less than 30 minutes. The water level rose to about 21 feet and terrifyingly nearly overflowed the creek bank.
Other watersheds may allow more time and provide early warning, said Jack Xu, senior engineer for the Santa Clara Valley waters. For example, the Guadalupe River in San Jose. Flooding along the coast can be predicted 2-3 hours in advance. Coyote Creek floods may take him a day. But in an urban concrete canal, it may take only 30 minutes for water to flow from the mountain to downtown.
Monitoring will begin two to three weeks before the storm, Xu said.
A mountainous region on the edge of a cold ocean, the Bay Area is notoriously unpredictable. Slow and high velocity jets of moist air are dispersed, resulting in patchwork quilt pattern precipitation patterns with varying effects on the location.
Regarding long-term weather forecasts, Xu said: Is it a dry pattern? A series of storms proves to be an operational need to plan resources and how to move water when needed. ”
Forecasts improve within 10 days of a storm’s onset, so planning becomes more targeted as the time approaches. “But if the forecast is inaccurate, everything downstream is wrong,” he says.
They use computer models to calculate how much of the projected rainfall will runoff based on past history, soil saturation, and local geology.
Big decisions start five days before the storm, Xu said. There are regional conference calls with PG&E, other utilities, and various cities where they are pondering: Should water be released from reservoirs to make room for additional runoff? should be increased? “We are weighing the consequences and the risks,” he said, Xu said.
Ground crews assess stream conditions and look for debris and other risks. This effort is intensified if winds are expected.
When a storm makes landfall, the role of rain gauges becomes important. These gauges on distant mountains are pipes with funnels, buckets and tipping mechanisms on top to measure precipitation. They tell officials what to expect.
A flow meter has a membrane that accurately measures the depth of water and converts it to flow rate. They transmit every hour, sending four 15-minute time-stamped packets. Once a certain threshold is reached, the sensor can send data to him every five minutes.
The Contra Costa County Flood Control and Water Conservation District manages 32 rain gauges and 16 river gauges. The Santa Clara Valley Water District has a network of 70 rivers and rain gauges. Alameda County has approximately 90 rain gauges and stream gauges.
Powered by solar panels, these gauges transmit electronic signals to data loggers via radio, landline, mobile phone signals, or satellites. As computers become more and more powerful, software processes many signals into computer databases and monitors the information received. Alerts are triggered when certain thresholds are reached (for example, water filling 80% of the stream’s capacity).
As water levels rise, authorities must decide whether to issue electronic flood warnings to local residents downstream. Warnings can be localized because different locations have different flood risks.
Technological revolutions have improved the accuracy of forecasts. Over the past decade, the National Weather Service has developed weather models better suited to the West Coast. Xu said the widespread availability of his service has made it easier to transmit real-time information. Supercomputing enables more advanced computation under various scenarios.
A new ‘X-band radar’ helps estimate precipitation for a given community. Contra Costa County installed a radar on Rocky Ridge near the Las Trampas Regional Reserve last month, according to hydrologist Marc Boucher. The Santa Clara Valley Water District radar is located above the Penitencia Water Treatment Plant. They are part of a regional network of future radars, including Marin, Sonoma, San Francisco, and Santa Cruz, providing local data on atmospheric rivers.
Such tools are becoming increasingly important as man-made climate change causes more powerful storms to release significantly more water into the Bay Area, according to a joint study by the City of San Francisco and Berkeley Lab. They calculated that this week’s storm rainfall was about 5% higher than normal due to climate change.
As a new storm approaches, “we are watching because it can change every day,” said Xu. “By its nature, there are many unknowns, so we need to be prepared to deal with it.”
