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HomeMy WebLinkAboutCC AG PKT 2010-03-22 #NAGENDA STAFF REPORT DATE: March 22, 2010 TO: Honorable Mayor and City Council THRU: David Carmany, City Manager FROM: Terrence L. Belanger, Interim Public Works Director SUBJECT: JANUARY 19, 2010 WEATHER EVENT FOLLOW -UP SUMMARY OF REQUEST: On Tuesday January 19, 2010, the Bridgeport neighborhood, in the City of Seal Beach, experienced extremely high levels of rainfall, within a very compressed timeframe. The quantity of rain which fell in about a hour time period overwhelmed the storm drainage system, which caused neighborhood flooding. This report answers several questions that were posed, by neighborhood residents and business owners. BACKGROUND: We st End Pump Station Watershed Area: The West End Pump Station is located between the east levee of the San Gabriel River and First Street, northerly of Welcome lane. West End Pump Station provides flood control service to a 165 acre watershed in the westerly portion of Seal Beach, covering portions of Marina Hill North, Bridgeport and Old Town. The original pump station was built in 1959. At the time of its construction, the pump station contained two pumps that had a total pumping capacity of 111.4 cfs, which was less than the peak flow from a five -year storm. A five -year storm pumping capacity is 157 csf. The pumps discharge water into the San Gabriel River through two, 30 inch diameter, 70 foot long reinforced concrete pipes. In 2007, the West End Pump Station had its two pumps upgraded from 111.4 cfs to 200 cfs (two 100 cfs pumps). The 200 cfs pumping capacity is designed to handle the flood drainage of a 10 year storm. In order for the West End Pump Station to have the pumping capacity to handle a 25 year storm, the pumps would have to be increased to a total of 250 csf. There are constraints associated with the West End Pump Station that impinge upon pumping efficiency and capacity. More on that later. Agenda Item N Page 2 The storm drainage system consists of several parts. Each street in the watershed area that has curbs on both sides of the street is essentially an open storm water culvert. The streets convey storm water to the various underground storm drain inlets (catch basins). The underground storm drains move the captured storm water through the system to West End Pump Station wet well (forebay). The maximum capacity of water in the wet well is constrained: by 1) the storm conduit from the adjacent trailer park (6' above the bottom of the wet well), 2) the footprint (length and width) and depth of the wet well, 3) the pump station land locked by the San Gabriel River, the trailer park and the Oakwood Apartments. Because of these wet well constraints, the maximum efficient capacity of the pump station is approximately 180 cfs (7 -8 year storm). Tuesday January 19, 2010 Storm: The weather events that impacted Seal Beach, on January 19, 2010, were extraordinary. The weather was severe enough to cause the activation of the City's Emergency Operations Center (EOC). The weather radar showed that intensity of rainfall over Seal Beach was the highest the weather service measures. When one viewed the radar information (realtime) the colors over the City, they were reddish - purple. The weather in the Seal Beach was so chaotic that the EOC received a tornado warning. In fact, a tornado did touch down at Pacific Coast Highway and west of Warner Avenue, in the Sunset Beach area. The West End Pump Station watershed area received heavy rainfall in fairly short timeframe, on January 19, 2010. The question that has been asked is "what year storm did the West End watershed area experience "? The answer is that we really do not know, definitively. There were no monitored rain gauges in the area. But, we can offer a range, based on conversations with meteorological experts, storm system engineers, the County of Orange and others. The estimates range between a 5- year storm day, at the Seal Beach Blvd. sewer pump station to a 300 -year half - hour to hour event, measured in west Long Beach. To give additional perspective, the SBB sewer pump station pumps about 600,000 gallons of effluent per day. On January 19 , the sewer pump station pumped 2,700,000 gallons of fluid. 2.1 million gallons of what was pumped by the sewer pumps was likely storm water. The West End Pump Station was fully operational, January 19 Contrary to rumors, the catch basins were clear and operating. Water was being conveyed through the streets to the storm drain inlets. Each element of the storm water drainage system was operational. Then, the heavy rain deluge occurred. Whether the deluge was a 30 -year event which was the estimated storm's intensity or a larger storm event, whatever the storm's volume, the West End Page 3 Pump station and the City's tributary storm drain system (surface and underground) were overwhelmed. The flooding occurred because there was more water than the system could handle. Once the rain slowed in its volume and the storm drainage system "caught up ", by pumping water through the pump station thereby reducing water volume in the streets, the storm drainage system has performed flawlessly. The City has had a very wet winter. Yet, the only time this season the system was overwhelmed was January 19 It seems clear that the West End watershed area was inundated by rain storm of near biblical proportions, on Tuesday, January 19, 2010. Storm Drain Monitoring: The City Maintenance Division checks and cleans the storm drainage systems catch basins twice per year; before rainy season and after rainy season. Many of the catch basins have debris "catchers ", which are designed to capture leaves and other smaller debris, during periods of low water flow. These debris catchers are located on the of the lower entry point to the catch basin. When storm water is flowing with velocity and volume, storm water flows over the debris catchers into the underground drainage system. If a person is interested in reviewing the catch basin cleaning report, it is available on -line. West End Pump Station Improvements The City completed the West End Pump Station Preliminary Design Report, in 2002. That study recommended a new parallel pump station to bring the total capacity to a peak flow from a 25 -year storm. However, due to budget limitations, the City implemented an interim project to maximize the capacity at the existing pump station. The project, which was completed in 2007, replaced the top slab of the sump structure to support the larger pumps and equipment. Two new 100 cfs capacity variable frequency drive operated pumps were installed to replace the original 111.4 cfs pumps. The interim project replaced the electrical equipment, controls and telemetry, replaced a portion the 30" discharge pipes with 36" discharge pipes, constructed a new pre- fabricated building to house the mechanical and electrical equipment; and provided a 350kW natural gas engine generator, with outdoor enclosure to operate one pump, during commercial power outages. The interim project replaced a portion of the original 30" discharge pipe, with a 36" discharge pipe. The two 36" sections of the discharge pipes run from the pump station building toward the San Gabriel River. About midway to the bike trail, which runs on top of the levee, the 36" sections of pipes transition down to 30" so that pipes could be interfaced. The 30" discharge pipes that discharge Page 4 storm water into the river are sections of the original pipes. The discharge pipes are of different sized sections because of the gas lines and oil line that run parallel to the property fence /bike trail/river levee. There were time and budget constraints associated with the interim project. The various public agencies and private companies from whom the City would have had to get permission or to have underground utilities relocated would have had held up the project. It was determined that increasing the pumping capacity was the highest priority. So, the modifications to the discharge lines were constructed in the interest of time and funding. As has mentioned previously, the existing pumping capacity of 200 cfs is less than the peak flow from the 25 -year design storm. The additional capacity would be provided by a parallel pump station on the property adjacent to the existing pump station. The recommended capacity of the parallel pump station should be no less the capacity of the existing pump station so that when either pump station is taken out of service, in the future, the existing level of protection is not reduced. A 400 cfs pumping capacity is capable of pumping a 100 -year storm. However, the current surface and underground storm water drainage system, in conjunction with an undersized wet well (forebay) mitigates against the proposed additional West End pumps from operating at close to that capacity. Summary: The West End Pump Station watershed area is sump area. What is a sump area? In this instance, a sump area is an area wherein rain water does not gravity flow directly into a flood control channel; here, the San Gabriel River. The West End watershed area must have its flood water pumped into the river. The streets, catch basins, underground storm drain structures and the pump station are integral elements of the storm drainage system. The greatest majority of the time the West End storm drainage system operates flawlessly. However, every so often, there occurs a rainfall event that overwhelms the flood drainage system. On January 19, 2010, the West End watershed area was inundated with an extraordinary rainfall event. .. There was so much water in the streets and in the underground storm drains that the West End pump station was not able to pump the water out into the river fast enough. The volume of water was such that the West End watershed area lay under water, until the combination of the rainfall subsiding and the West End pumping catching up to volume flowing through streets and storm drains to wet well and the pumps. With the exception of January 19, 2010, the West End storm water system has worked as it has been designed in every other rainfall event this rainy season. The report may not have directly responded to the concern raised about pumping capacity, at the West End pump station. There are two 100 cfs pumps on -line at the West End Pump Station. The pumping capacity of 200cfs is theoretically Page 5 capable of pumping down a 10 -year storm. This translates into the West End pumps being able to pump 203 cfs, for a twenty -four hour period. The estimated 30 -year storm, which occurred on January 19, 2010, would have been characterized as needing a pumping capacity of at least 260 cfs (estimated peak discharge). The volume of rain that fell in then the short time frame in which it fell resulted in a quantity of water in the West Pump Watershed area that accumulated in the storm drainage system (streets, storm drains, wet well) In amounts that overwhelmed the entire system's capacity. FINANCIAL IMPACT: No fiscal impact. RECOMMENDATION: Receive and file. SUBMITTED BY: 4 WM0 I., IA�lt errence L. Belanger Interim Director of Public W rks NOTED AND APPROVED: -5?�n David Carm ny City Manager