Glendale (PDF) - Hazard Mitigation Web Portal - State of California

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 8 – FloodsFlooding due to Failure of Above-Ground Water Storage Tanks:Seismically induced inundation can also occur if strong ground shaking causes structuraldamage to above-ground water tanks. If a tank is not adequately braced and baffled, sloshingwater can lift a water tank off its foundation, splitting the shell, damaging the roof, and bulgingthe bottom of the tank (elephants foot) (EERI, 1992). Movement can also shear off the pipesleading to the tank, releasing water through the broken pipes. These types of damage occurredduring southern California’s 1992 Landers, 1992 Big Bear, and 1994 Northridge earthquakes.The Northridge earthquake alone rendered about 40 steel tanks non-functional (EERI, 1995),including a tank in the Santa Clarita area that failed and inundated several houses below. As aBaldwin Hills Dam - Dark spot in upper right hand quadrantshows the beginning of the break in the dam.result of lessons learned from recent earthquakes, new standards for design of steel water tankswere adopted in 1994 (Lund, 1994). The new tank design includes flexible joints at theinlet/outlet connections to accommodate movement in any direction. All of Glendale’s watersteel tanks have been retroffited with flexible expansion joints to allow for movement duringearthquakes.Water lost from tanks during an earthquake can significantly reduce the water resourcesavailable to suppress earthquake-induced fires. Damaged tanks and water mains can also limitthe amount of water available to residents. Furthermore, groundwater wells can be damagedduring an earthquake, also limiting the water available to the community after an earthquake.Therefore, it is of paramount importance that the water storage tanks in the area retain theirstructural integrity during an earthquake, so water demands after an earthquake can be met. Inaddition to evaluating and retrofitting to meet current standards, this also requires that thetanks be kept at near full capacity as much as practical.The Effect of Development on Floods:When structures or fill are placed in the floodway or floodplain, water is displaced.Development raises the river levels by forcing the river to compensate for the flow spaceobstructed by the inserted structures and/or fill. When structures or materials are added to the2006 PAGE 8- 15
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 8 – Floodsfloodway or floodplain and no fill is removed to compensate, serious problems can arise. Floodwaters may be forced away from historic floodplain areas. As a result, other existing floodplainareas may experience flood waters that rise above historic levels. Local governments must requireengineer certification to ensure that proposed developments will not adversely affect the flood carryingcapacity of the Special Flood Hazard Area (SFHA). Displacement of only a few inches of water canmean the difference between no structural damage occurring in a given flood event, and theinundation of many homes, businesses, and other facilities. Careful attention should be given todevelopment that occurs within the floodway to ensure that structures are prepared towithstand base flood events.In highly urbanized areas, increased paving can lead to an increase in volume and velocity ofrunoff after a rainfall event, exacerbating the potential flood hazards. Care should be taken inthe development and implementation of storm water management systems to ensure that theserunoff waters are dealt with effectively.How Building Codes Address Building In Known Flood Prone Areas:Flood Insurance Rate Maps (FIRM) and Flood Insurance Studies (FIS) Floodplain maps are thebasis for implementing floodplain regulations and for delineating flood insurance purchaserequirements. A Flood Insurance Rate Map (FIRM) is the official map produced by FEMAwhich delineates SFHA in communities where NFIP regulations apply. FIRMs are also used byinsurance agents and mortgage lenders to determine if flood insurance is required and whatinsurance rates should apply.Water surface elevations are combined with topographic data to develop FIRMs. FIRMsillustrate areas that would be inundated during a 100-year flood, floodway areas, and elevationsmarking the 100-year-flood level. In some cases they also include base flood elevations (BFEs)and areas located within the 500-year floodplain. Flood Insurance Studies and FIRMs producedfor the NFIP provide assessments of the probability of flooding at a given location. FEMAconducted many Flood Insurance Studies in the late 1970s and early 1980s. These studies andmaps represent flood risk at the point in time when FEMA completed the studies. However, itis important to note that not all 100-year or 500-year floodplains have been mapped by FEMA.FEMA flood maps are not entirely accurate. These studies and maps represent flood risk at thepoint in time when FEMA completed the studies, and does not incorporate planning forfloodplain changes in the future due to new development. Although FEMA is consideringchanging that policy, it is optional for local communities. There is no FIRM map for the City ofGlendale.Flood Mapping Methods and Techniques:Although many communities rely exclusively on FIRMs to characterize the risk of flooding intheir area, there are some flood-prone areas that are not mapped but remain susceptible toflooding. These areas include locations next to small creeks, local drainage areas, and areassusceptible to man-made flooding.In order to address this lack of data, jurisdictions can take efforts to develop more localizedflood hazard maps. One method that has been employed includes using high-water marks fromflood events or aerial photos, in conjunction with the FEMA maps, to better reflect the trueflood risk. The use of GIS (Geographic Information System) is becoming an important tool forflood hazard mapping. FIRM maps can be imported directly into GIS, which allows for GISanalysis of flood hazard areas.Communities find it particularly useful to overlay flood hazard areas on tax assessment parcelmaps. This allows a community to evaluate the flood hazard risk for a specific parcel during2006 PAGE 8- 16
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City CouncilDave Weaver, MayorRafi
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Natural Hazards Mitigation PlanCity
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Table 4-2: STAPLEE Ranking of Actio
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NOTES:This map is intended for gene
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City of GlendaleUpdateHazard Mitiga
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Similar to Safety ElementIncludesEa
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HospitalsPartnershipsWithin City an
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Impact on CityMost significant area
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