FLOOD FORECASTING PRACTICE IN NORTHERN CALIFORNIA

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1 FLOOD FORECASTING PRACTICE IN NORTHERN CALIFORNIA California Department of Water Resources Post Office Box , Sacramento, California USA By Maurice Roos, Chief Hydrologist ABSTRACT Although often considered a dry climate, northern California is actually quite flood prone. Large winter Pacific storms can cause rapid rises on major rivers. Flood forecasting is a major tool to warn people and to aid in proper operation of the large foothill reservoirs. Real time telemetry information of precipitation and stream stages, plus 18 to 24 hours of quantitative precipitation forecasts (QPF) including projected snow elevation levels, are used in forecasting flows and river stages. To help emergency personnel and reservoir operators with the more uncertain outlook beyond the immediate forecast range, projected QPF out to 5 days is used to develop a guidance forecast for the extended period. INTRODUCTION The purpose of this paper is to describe flood forecasting practice on major rivers in northern and central California. The risk of a large flood is seasonal; the region is vulnerable during the November through April rainy season but not during the dry season. Rivers can rise from low flow levels to damaging floods within a couple of days. Some measure of the size of the threat can be gained from the design flow of the lower Sacramento River floodway system which is 17,300 cubic meters per second. Peak historical flood flows on the Eel River of the north coast region were 21,500 cms from an 8100 square km watershed. Flood defense measures include levees, a bypass system for excessive river flows, and flood detention behind foothill dams. Flood forecasting is a major tool to warn people and to aid in proper operation of the large foothill reservoirs. Flood forecasting relies on incoming data from an extensive radio telemetry system of real time measurements of precipitation and stream stage. Lead time between rainfall and resulting peak runoff at most major dams and headwater forecasting points ranges from 5 to 16 hours. Telemetry information, plus 18 to 24 hours of quantitative precipitation forecasts, QPF, including projected snow elevation levels, are used in providing forecasted flows and river stages. Emergency service people want and need more lead time, up to 36 or more hours. Often there is no way to make forecasts beyond a day with any confidence. Our solution at this point is to issue river stage forecasts based on current conditions and the 18 to 24 hours of QPF; then use projected QPF out to 5 days to develop a guidance hydrograph to 5 days in the future. Thus emergency service entities can see what the immediate short range forecast is and get a sense of what flood forecasters see as a likely scenario further out in time. The guidance forecasts do require caution on rivers below major reservoirs because the flood forecasters only include scheduled release changes by operators and do not try to anticipate what they might do later in the storm. As weather forecasts get better, we will likely extend the firm forecast time a bit more. We may also shift to ensemble forecasts for the guidance hydrograph. 1

2 BACKGROUND Flood management strategy in California is a multipronged approach depending on the area and local geography. One element is designating areas of zones subject to flooding and restricting development therein. Another important element consists of physical works, levees, floodways, and flood storage behind dams in foothill reservoirs. Flood warning, including flood forecasts and notification of people at risk, is part of the strategy, as well as providing technical advice and assistance in flood fighting once high water arrives. The river flood forecasting is a joint program of the federal government (National Weather Service) and the State of California (Department of Water Resources). As noted earlier, major flood threats are seasonal. Most of California has a Mediterranean climate with wet winters and dry summers. Figure 1 shows the average monthly precipitation pattern for the Sierra Nevada and, indeed, for much of California. Note that half the annual precipitation is in the three months of December through February and about three fourths during the five month November through March period. Much of the year, California is under the influence of a high pressure area, which accounts for fair weather and lack of precipitation during the summer, much like Spain. During the winter season, the storm belt shifts southward and occasionally places the State under the influence of Pacific storms, which bring vitally needed rain and snow. Figure 1. Monthly Precipitation Distribution in the Sierra Nevada 2

3 Most of California s moisture originates in the Pacific Ocean to the west and southwest. Storms with a long southwesterly fetch generally produce more precipitation, sometimes floods, because these storms tap air with a higher moisture content originating over warmer water. As moisture laden air is blown over mountain barriers, such as the Sierra Nevada, the air is lifted and cooled, and drops additional rain or snow in the high country, especially on the western slopes. Typically, the orographic area precipitation is three to four times the amount in the lowlands. For example, the 1600 m elevation Blue Canyon weather station northeast of Sacramento averages about 1600 mm per year, about 3.5 times the 450 mm expected at Sacramento in the middle of the Central Valley. The direction of orographic wind flow is important. The greatest amount of water is extracted when the wind flow is at right angles to the mountain range, or from the southwest for the Sierra Nevada. A southerly wind direction does not produce large amounts in the Sierra, but often concentrates precipitation at the north end of the Sacramento Valley near and above Shasta Reservoir. The most feared storms are slow moving, with a long southwesterly fetch extending from Hawaii, the so called pineapple connection. Often there is a near balance between a high pressure area to the south of California and a strong low pressure system off the northern California or Oregon coast. The greater the pressure difference the stronger the southwestern winds, which can reach speeds over 100 km/hr at 3000 m over the San Francisco Bay area. This warm, moist southwesterly flow pattern was evident in all the big Northern and Central California floods, such as those in December 1964, February 1986, and the New Year s flood of Figure 2 shows the location of major northern California rivers. In the Central Valley the Sacramento River drains the northern part of the basin, flowing southward past the city of Sacramento to be joined by the northerly flowing San Joaquin River, then turning westward in the Delta to flow out to the ocean via San Francisco Bay. The southern end of the Central Valley, south of Fresno, is an area of interior drainage; rivers from the southern end of the Sierra would naturally flow out into a couple of intermittent lakes on the Valley floor to eventually dry up. The Sacramento River floodway system (Figure 3) is designed to carry very large winter season rainfall floods, culminating in a combined flow of 17,300 m 3 / s in the river and bypass past Sacramento. The largest known flow of record was in February 1986 at about 17,500 m 3 / s. The San Joaquin River is smaller and drains the higher elevation southern Sierra Nevada. Its flood carrying capacity is about one tenth that of the Sacramento River system at about 1500 m 3 / s. The peak historic lower San Joaquin river flow reported was 2200 m 3 / s in December The San Joaquin River has another distinction; it can also generate a snowmelt flood problem in the late spring or early summer in about one year in 10, even with the present reservoir storage. The largest flood ever measured in California was the huge North Coast region flood of December 1964, on the Eel River at Scotia. This flood peaked at 21,300 m 3 / s and caused tremendous damage in the northwestern part of the State as well as on rivers in the Central Valley, Western Nevada, and San Francisco Bay area. 3

4 REGULATED WATERWAYS There is a wide range of degrees of flood control regulation on California rivers. At one end of the scale are most of the rivers in the State s North Coast region, including the Eel, Klamath, and Smith Rivers which are mostly free flowing. Residents have to rely on watching the weather and the flood forecasts when the streams are rising to get themselves, transportable property, and livestock out of the way of approaching floodwaters. The flood forecasting program does enable a viable dairy and livestock economy on the limited area of flat ground in the lower Eel River Valley and its delta near 4

5 Eureka. At the other end of the scale are the major federal flood control projects in the Central Valley drainage basin on the Sacramento and San Joaquin Rivers. Flood management here is provided by a combination of foothill reservoir storage and valley levees and floodways. 5

6 Figure 3 depicts the Sacramento River Flood Control Project. It was authorized by the U.S. Congress in 1917, with most of the construction taking place over the next 25 years. The project includes nearly 1800 km of levees, 4 fixed overflow weirs (Moulton, Colusa, Tisdale and Fremont) and one gated, manually operated weir (Sacramento) which divert floodwater from the main Sacramento River into a bypass system. At the latitude of Sacramento, about 80 percent of the flood flow goes into the broad bypass channel and only 20 percent remains in the river. Upstream foothill reservoir storage space reserved for winter flood control exceeds 3.5 billion m 3 in six multi purpose reservoirs. This is about 30 percent of the total 6 reservoir capacity. Each stream is different, but overall, about half the peak daily flood flow can be stored temporarily, with downstream releases limited to the safe capacity of leveed reaches. Besides the major rivers, there is much side flow coming in from uncontrolled smaller streams; this too needs to be accounted for in operations. The rain flood storage need is seasonal, with maximum reservoir space reserved during the November through March season of greatest risk. Actual space needs during the flood season are also adjusted for the wetness of the watershed in some basins. During spring the flood requirements are relaxed and operators can use the space for storage of spring snowmelt in most years. Figure 4 gives an example of a foothill reservoir flood control requirement during the season. These are multipurpose reservoirs and it is desirable to fill later in the spring for water supply and other functions. The slanting lines in February and March represent the wetness of the watershed; more storage is permitted if the watershed is drier. Figure 4. Example of foothill reservoir flood control space requirements 6

7 FLOOD FORECASTING The State Department of Water Resources and the federal National Weather Service (NWS) at the California Nevada River Forecast Center share a very active flood forecasting and warning program. Regular Sacramento River low flow and Delta estuary tide forecasts are made every weekday morning. During the flood season, normally from mid November to mid April, State and federal personnel (one from each agency) monitor the weather and major rivers on nights and weekends and produce flood forecasts if conditions warrant. If a large event is imminent, the duty monitors will alert the staff so they can come in earlier in the morning, perhaps as a double shift operation (early and late shifts). The forecasting program depends on a far reaching radio and satellite telemetry system in which remote gages automatically report precipitation, stream stages, snowpack water content, and other hydrologic variables. Additional data on reservoir storage and actual and scheduled releases are fed into the State s California Data Exchange Center (CDEC) computers along with the telemetered data for use by forecasters. Lead time between rainfall and resulting peak runoff at most major dams and headwater forecasting points ranges from 5 to 16 hours. It takes several days for water to move down into the lower Sacramento and San Joaquin Rivers. Telemetry information, plus 18 to 24 hours of quantitative precipitation forecasts, QPF, including projected snow elevation levels, are used in providing forecasted flows and river stage. Emergency service people want and need more lead time, up to 36 or more hours. Often there is no way to make forecasts beyond a day with any confidence. Projected precipitation and snow levels out to as far as 5 days are developed by NWS RFC hydrometeorology specialists and used in guidance forecasts of major reservoir inflow to help reservoir agency operators and also at major forecast points on the rivers. The quantitative precipitation forecasts (known as QPFs) add lead time, but with progressive loss in accuracy. They can sometimes be off by a factor of two. (That is why regular forecasts of river stages are normally limited to levels modeled with 18 to 24 hours of QPF.) Tools available to the hydrometeorologists are the 72 hour outlooks by the NWS HPC office in Washington and the locally run Rhea orographic precipitation model of watershed precipitation and snow levels. The Rhea model is driven by inputs from the GFS atmospheric model and provides 6 hour time step projections out to 5 days. Our solution at this point is to issue river stage forecasts based on current conditions and the 18 to 24 hours of QPF; then use projected QPF out to 5 days to develop a guidance hydrograph to 5 days in the future. (See sample chart.) Thus emergency service entities can see what the immediate short range forecast is and get a sense of what flood forecasters see as a likely scenario further out in time. The guidance forecasts do require caution on rivers below major reservoirs because the flood forecasters only include scheduled release changes by operators and do not try to anticipate what they might do later in the storm. There is a gap in that reservoir operators may change releases which will change the longer range forecasts. We are working on this by the methods described last year by Mr. David Bowles in the first workshop. 7

8 Figure 5. Sample of a guidance forecast, Smith River near Crescent City In spite of their uncertainties, these extended quantitative forecast estimates provide helpful guidance to reservoir and flood system operators and to emergency service agencies. Some gradual improvement has been noted as better weather models and observation systems (satellites, radar, profilers, etc.) are developed. As weather forecasts get better, we will likely extend the firm forecast time a bit more. We may also shift to ensemble forecasts for the guidance hydrograph. 8