Multi-Hazard Mitigation Plan 6.2 City of Sacramento Community Element
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Multi-Hazard Mitigation Plan 6.2 City of Sacramento Community Element CITY OF SACRAMENTO COMMUNITY PROFILE The City of Sacramento was founded in 1849 and was the first incorporated city in the State of California. The City was founded at the confluence of the American and Sacramento rivers, two of the largest rivers in the state. As California’s capitol city, Sacramento is the center of governmental policy for the entire state, as well as the cultural, educational and business center of a four-county metropolitan region. The City is run by a City Council-City Manager form of government under guidance of a City Charter that was adopted by voters in 1920. Sacramento is known as the “City of Trees” and includes about 165,000 trees in its urban forest, second only to Paris for the most number of street trees. Source: www.cityofsacramento.org The City of Sacramento covers 96.34 square miles and has a population of 411,200. This equates to a population density of 4,268 persons per square mile. Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-1 December 2004 TOTAL VALUES AT RISK FROM HAZARDS The total values at risk from all hazards are presented as a worst-case baseline. Like other values presented in this plan, these are deceptively low because they do not include the values of infrastructure, government and church facilities, or the local economy. Additionally, Assessed Values in California are lower than actual because they are frozen to only reflect the value at the time of the last sale. Source: Sacramento County Assessor's Office, Assessments by Land Use; 9-10-04 The Risk Assessment portion of this plan indicates that the City of Sacramento does not face a catastrophic natural disaster, so damage during any given disaster event would be less than the $28.6 Billion displayed. However, the risk for the City varies from the rest of the County within the mapped floodplain due its different size, and the varying numbers of structures and varying values of those structures. Therefore, the following section takes a closer look at the City of Sacramento’s vulnerability to flooding. FLOOD PROBLEM IDENTIFICATION In the City of Sacramento, the overlapping American and Sacramento River floodplains encompass a landmass of more than 100,000 acres. According to SAFCA, Sacramento’s risk of flooding is the greatest of any major city in the country. A major flood could result in significant loss of life, property damage, and economic disruption. This section follows on to the historic flood problems listed in the Sacramento County Flood History section of this plan and provides a more detailed perspective of the flood hazards and risks within the City of Sacramento. Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-2 December 2004 SACRAMENTO FLOOD HAZARD MAP-AMERICAN/SACRAMENTO FLOODPLAIN Four types of floods can impact the City of Sacramento: localized street flooding, river flooding, levee overtopping/failure, and dam failure. Certain health hazards are common to all of these events. Standing water and wet materials in structures can become a breeding ground for microorganisms such as bacteria, mold, and viruses. This can cause disease, trigger allergic reactions, and damage materials long after the flood. When floodwaters contain sewage or decaying animal carcasses, infectious disease is of concern. Direct impacts such as drowning can be limited with adequate warning and public education about what to do during floods. Due to the large amount of population at risk, warning and evacuation will be paramount to reduce life and safety impacts with any of these types of flood events. Localized Street Flooding. The City’s local drainage system services 100 square miles and is handled by 123 storm drainage basins. Since the City is typically lower than the elevated rivers, the majority of local drainage must be collected and pumped into the rivers. The City operates 102 storm water-pumping plants to keep the drainage pumped down. A major power outage during a rain event could contribute to significant flooding. Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-3 December 2004 River Flooding. River flooding can result from either flash or slow rise flooding in any of the four stream groups that affect the City. Many areas in Sacramento are subject to sheetflow, which is broad shallow, overland flooding generally less than 2 feet deep and characterized by unpredictable flow paths. The warning time associated with slow rise floods will enable life and property protection. Flash floods, however, usually require immediate evacuation within the hour. Once flooding begins, personnel will be needed to assist in rescuing persons trapped by floodwaters, securing utilities, cordoning off flooded areas, and controlling traffic. This could overtax local response capabilities and require outside mutual aid. Levee Failure/Overtopping. Generally, levees fail due to overtopping or collapse due to seepage, subsidence, erosion, or any combination thereof. A catastrophic failure resulting from collapse can occur very quickly with relatively little warning. Levee failure usually occurs when the levee is saturated from high flows or there is an inherent defect in the levee. Floodwater will flow in a relatively shallow path and collect in low-lying areas. The maximum flood pool could exceed 15 feet in depth, which could result in serious life and safety impacts. Levee failure scenario maps have been developed as part of the 1996 City of Sacramento Comprehensive Flood Management Plan. The 21 maps show where the water would go, how long it would take to get there, and how deep it would be in various parts of the City. Historically, levees have been the answer to flood control as the City grew and developed over the years. The cores of today’s levees are often the levees built by farmers and settlers as much as 150 years ago. Early levees were not constructed to current engineering standards, and little care was given to the suitability of foundation soils. It was believed prior to 1986 that the levees containing the Sacramento River and the American River were of sufficient height and stability to protect the county from 100-year or greater storms. The storms that occurred in February 1986 demonstrated that the levees were not sufficient. The USACE also concluded that the levees along the Natomas East Main Drainage Canal which protect Natomas and the Dry Creek area to the east, were too low to safely contain the flows produced by the coincidence of peak discharges in Dry and Arcade Creeks and maximum flood releases from Folsom. As a result of these findings, FEMA reassessed the 100-year floodplain in the Sacramento area and issued new Flood Insurance Rate Maps. These maps, which became effective in November 1989, mandated the purchase of flood insurance by all residents and businesses within the 100-year floodplain and caused the City of Sacramento to impose severe restrictions on new residential development in the Natomas area. Source: City of Sacramento Comprehensive Flood Management Plan (CFM), 1996. In May 22, 2000 the AR Zone was re-designated A99 after substantial improvements were made to the levee system that brought the level of protection back to the 100-year flood. This removed the AR development requirements, while maintaining the insurance requirements. Additional levee work and erosion control efforts were completed in 2004 that will change the A99 zone to X zone. The City submitted a Letter of Map Revision to FEMA in October 2004 requesting the change in flood zone and was awaiting response during the final draft of this plan. The change of the City’s floodplain zones over time is displayed in the series of maps below, followed by a larger version of the recent proposed changes. Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-4 December 2004 Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-5 December 2004 Sacramento County City of Sacramento Community Element Multi-Hazard Mitigation Plan Page 6.2-6 December 2004 Dam Failure. The remote possibility of dam failure flooding also exists from Folsom and Nimbus Dams. A catastrophic dam failure could easily overwhelm local response capabilities and require mass evacuations to save lives. Impacts to life safety will depend on the warning time available and the resources to notify and evacuate the public. Major loss of life could result and there would be associated health concerns as well as problems with the identification and burial of the deceased. City of Sacramento Drainage Areas This section documents the stream groups and drainage areas that pose flood problems to the City of Sacramento. The drainage patterns affecting the City of Sacramento can be divided into the following groups and geographic areas: • Natomas Area Stream Group (Northwest Sacramento) • American River Stream Group (Downtown and Northeast Sacramento) • Morrison Creek Stream Group (South Sacramento) • Sacramento River (Western edge of Sacramento) Slopes of the main drainage ways in these areas increase from west to east, thereby reflecting the general topographic pattern of the surrounding region. Stream gradients vary from 0.5 foot per mile to 10 feet per mile and average approximately two feet per mile. Natomas Area Stream Group. In 1911 the Reclamation District No. 1000 was created and approximately 43 miles of levees were constructed around approximately 55,000 acres. This area came to be known as the Natomas Basin. It is bounded on the west by the Sacramento River, on the north by the Natomas Cross Canal, on the east by the Pleasant Grove Creek and Natomas East Main Drainage Canals and on the south by the American and Sacramento Rivers.