Geog350_Ho_Project

Title 1997 New Year's Storm Event
Author David Ho American River College, Geography 350: Data Acquisition in GIS; Fall 2009 Contact Information (daveho48@yahoo.com)
Abstract The 1997 storm event occurred on New Years day with high rainfall and runoff along the Sierra Nevada. Precipitation values ranged from 2 inches up to 9 inches. Inflows into Folsom Dam reached up to 209,860 cfs and had outflows of 109,971 cfs.
Introduction The New Year’s Day flood of 1997 was caused by one of the largest storms in Northern California this century. The storm represented a classic orographic event with warm winds from the southwest blowing over the Sierra Nevada and dropping enormous amounts of rain at the middle and high elevations. The watershed was already saturated from previous storms thus having a high runoff rate. The volume of runoff surpassed previously recorded volumes in the Sierra streams that flowed west. Many of the flood control reservoirs, such as Folsom Dam, received these historic volumes and caused the Dam to fill at a unprecedented rate. The reservoirs had made record downstream releases to keep the lake from overtopping. The project that will be presented will be on the 1997 New Year’s storm event in Sacramento on Folsom Dam and the American River. The storm event will be recreated by observing how much precipitation fell and the resulting flow in the American River. The River is divided into four segments: American River, North Fork, Middle Fork, and South Fork. All four segments will be used in the analysis.
Background The American River is divided into four segments: American River, North Fork, Middle Fork, and South Fork. Folsom Dam is located on the American River approximately 26 miles upstream from the confluence with the Sacramento River. The lake spans three counties: Sacramento, Placer, and El Dorado. The Dam provides flood protection, domestic and municipal water supply, irrigation, recreational activities, and hydropower to the city of Sacramento. Documented large storms are important for creating flood frequency graphs and calibrating hydrographs for watershed assessments. These graphs, in turn, are vital information prior to designing levees and dams in order to provide adequate protection to the surrounding city. One cannot design a levee or a dam based on one single event. There needs to be a way to quantify large storm events and design a system that can protect against future events. Flood frequency graphs are flow values with a corresponding exceedance value that is statistically calculated using historical flows. Using historical records of flow data, one can generate the probability of a certain flow occurring or exceeding. This is where the term “100 year flood” appears; the 100 year flood is a statistically derived flood event that would occur once every hundred years. What is important to keep in mind is that the 100 year flood is a statistically derived value and has no bearing to what actually happens. Since the weather and how much it rains is not something that can be predicted, one cannot assume that a 100 year flood occurs every 100 years. A hydrograph depicts the discharge value over a period of time. When it rains, the water will flow either underground or on the surface on the Earth. Eventually, most of the water will end up in a creek or a river. If there was no rain, the hydrograph would be straight line with minor variations; however, when the water reaches the river, one will see spikes in the graph indicating how much water had traveled through. Hydrographs are important tools for observing how much flow is traversing through the river.
Methods The objective of this project is to observe rainfall and associated flow to answer the following questions: when did the highest flow event occur, how much rain fell on that day, where did the most rainfall occur, and how much flow occurred in each river. ArcGIS will be used to map out locations of rain and flow gages as well as illustrate the analysis that goes into watershed assessment. Rainfall interpolations, image georeferencing, and subbasin digitizing can also be performed in GIS. The analysis will be broken up into four portions: Gathering, sorting, interpreting, and creating data. To determine the day the flood event occurred, precipitation and flow data from December 1996 to February 1997 will be examined. Microsoft Excel, a spreadsheet program, is used to sort through precipitation and flow information. Rain data typically is presented in three forms: incremental, accumulated, and tipping bucket. The data needs to be in incremental form before any analysis can be done. Data can be collected at the California Data Exchange Center (CDEC, 2009) online. Runoff values can be found on the USGS waterdata website (Waterdata, 2009) . Folsom Dam information can be found at the Army Corp of Engineers intranet website (not open to public access). The study area is approximately 70 sq miles and consists of the entire American river including the North, Middle, and South Fork. There is an extensive gage network within the area. After locating the gages, quality control for each of the gages will need to be determined to see if it’s usable. Problems with gages can range from: offline, data does not extend far back enough, negative values, no values, or abnormally high values.
Results After sorting through the data, over half the gage information could not be used due to missing data, negative, or unreasonable values.
Analysis After the data is sorted through, the 1997 storm event was determined by observing which day had the highest precipitation value (see Tables 3 & 4); however, there were discrepancies over when the event occurred since the data showed two days with high precipitation values. Most of the gages had the highest rain value on January 1st, but four other gages showed high ran values on January 2nd. Since most of the gages showed highs for January 1st, one would assume that the 1997 storm event occurred on New Years Day (hence, the name). Discarding the four gages would have been the most practical thing to do, but the gages were the only one of its kind within the area. Removing the gages would have left my dataset incomplete. Instead, the time stamp for the four gages may have been incorrectly shifted a day ahead. Fortunately, there were two sets of gage data from Folsom Dam (FLD) from two different sources (Army Corp database & CDEC). Both sources are considered trust worthy and commonly used by researchers. One of the sources listed the storm event occurring on January 2nd, but the other source listed the heavy rain occurring on January 1st. There is a high probability that the timestamp may have been incorrectly shifted. Since the four gages were all retrieved from the same source, it was assumed that this error occurred for the other three gages. The resulting flow occurred the next day on January 2nd, 1997. Figures 1 through 8 show hydrographs recorded from flow gages stationed along the American River. All of the stations except gage 11446500 are located above Folsom Dam and monitor the North, Middle, and South American River. Flow in rivers increase as it moves downstream which can be observed for the Middle and South Fork hydrographs (Figures 2-7). All three segments had a peak greater than 50,000 cfs. The combined flow contributed an inflow peak of 209,860 cfs (Figure 9) into Folsom Dam. Figure 11 displays the inflow, outflow, and storage levels for Folsom Dam. Folsom Dam released an outflow peak of 109,971 cfs (Figure 10). Further downstream on the American River at Fair Oaks, the peak flow stayed fairly consistent at 106,000 cfs (Figure 8). It is unclear why the flow value dropped from the Dam outlet to Fair Oaks.
Conclusion Storm events such as the 1997 New Years storm have caused the state of California to reevaluate their levee and reservoir systems. The American River currently has a channel capacity of 115,000 cfs; however, it has become clear that the 115,000 cfs capacity may not be sufficient. In addition, Folsom Dam was believed to provide approximately a one hundred year level of protection. However, primarily because of additional years of records, flows at or above the levee design capacity are now estimated to occur much more frequently thereby endangering the developed and populated areas downstream. An extra spillway is currently being added to Folsom Dam as a means to release water earlier and faster to channel capacity. Work is also being done on the American River levees to raise the channel capacity to 145,000 cfs. Levees and Dams are only temporary measures of protection against storms. This was clearly demonstrated by the devastating effects of Hurricane Katrina. An obvious but difficult solution would be to move residential and commercial housing away from the American River. But due to the economical forces that push for expansion, it is unlikely such a thing would occur. Although continual maintenance and redesign are vital to the protection of Sacramento, there is only so much humans can do before nature runs its course.
References 1. United States. Department of Water Resources. Divison of Flood Management. The Hydrology of the 1997 New Year's Flood Sacramento and San Joaquin River Basin. By Herbert Hereth, William A. Mork, Joanne Pierce, and Elle Burns. Sacramento: Department of Water Resources, 1999. Watmanweb. Web. 29 Nov. 2009. . 2. Water Control Manual, Folsom Dam and Lake, American River, California. U.S. Army Corps of Engineers, Sacramento District. December 1987. 3. California Data Exchange Center. Department of Water Resoruces. Web. 30 Nov. 2009. . 4. USGS Real Time Water Data for the Nation. USGS. Web. Oct. & Nov. 2009. .