Laserfiche WebLink
<br /> <br />APPENDIX C <br /> <br />Sources of Flood Information <br /> <br />To develop an effective manufactured home in- <br />stallation strategy applicable to a flood hazard <br />area, several hydrologic factors must be <br />evaluated. These include the regulatory floodplain <br />boundaries and the anticipated flooding <br />characteristics for the site such as depth, veloc- <br />ity, duration, rate-of-rise, and frequency. Various <br />sources of this information are available. Also, <br />there are methods by which portions of such <br />information can be individually developed. <br />As part of its National Flood Insurance Program <br />(NFIP), the Federal Emergency Management <br />Agency (FEMA) develops Flood Insurance Studies <br />(FISs) which will often contain the most current and <br />detailed information that is available for a com- <br />munity. Such studies can include a Flood In- <br />surance Rate Map (FIRM), a Flood Boundary and <br />Floodway Map (FBFM) and a FIS report, from <br />which the following information can be obtained: <br /> <br />. Floodplain and floodway boundaries <br /> <br />. Stream profiles that show the elevations of <br />the 100-year or "base" flood (the flood that <br />has a one-percent chance of being equalled <br />or exceeded in any given year) and other <br />flood events <br /> <br />· Mapped base flood elevations which, in <br />combination with other data, can be used <br />to develop flood depths for a specific site <br /> <br />· Flood velocity data <br /> <br />. Flood frequency data <br /> <br />· Flood discharge data <br /> <br />· Historical flood information <br /> <br />FISs can be extremely valuable sources of por- <br />tions of the information necessary to evaluate a <br />potential manufactured home site. The specific <br />hydrologic data elements, and ways to obtain <br />them, are described below. <br /> <br />Flood Hazard Boundaries-Boundaries for the <br />different degrees and types of flooding, in- <br />cluding floodways, f100dway fringe, coastal high <br />hazard areas, coastal fringe, and shallow <br />flooding areas, must be identified. Flood hazard <br />boundaries are significant because they deter- <br />mine the specific flood hazard zones that are <br />part of a proposed development site or that will <br /> <br />influence development on the site. In addition, <br />boundaries indicate where floodplain manage- <br />ment regulations and flood insurance <br />requirements, apply to the site. Flood hazard <br />boundary data can be obtained from FBFMs, <br />FIRMs, and floodplain maps or' can be <br />developed from topographic maps, zoning <br />maps, aerial photographs, and related <br />hydrologic data. <br /> <br />Flood Depths-Flood depths are determined <br />by the difference between water surface eleva- <br />tion at times of flooding and normal ground sur- <br />face elevations. This information is important <br />both in determining the elevations at which flood <br />damage is likely to occur and in defining the ap- <br />propriate elevations for flood insurance and <br />floodplain management regulations. Flood <br />depths also influence the hydrostatic forces that <br />are in effect during flooding, including the <br />horizontal loads that can cause lateral displace- <br />ment or overturning, and the vertical loads that <br />can cause uplift and flotation. Flood depths can <br />be derived by using a FIRM showing base flood <br />elevations in combination with a topographic <br />map depicting ground elevations for a particular <br />site. Flood depth data is also available from <br />various technical studies that include flood <br />elevations, water surface profiles, or stream and <br />coast cross-sections. In the absence of official <br />reports, information on flood depths can be ob- <br />tained from site survey and historical records. <br /> <br />Flood Water Velocity-The average and max- <br />imum velocity of floodwater is important in deter- <br />mining hydrodynamic forces, which influence <br />horizontal loads in excess of hydrostatic loads. <br />Velocity also affects the magnitude of debris im- <br />pact loads (i .e., force of floating objects carried <br />by floodwaters), and can increase erosion and <br />affect soil stability on slopes. Data on water <br />velocity is listed in Floodway Data Tables often <br />included in FIS reports and is available from the <br />various floodplain technical studies or can be <br />determined by special hydrologic studies. <br />Velocity can also be calculated by assuming <br />floodwaters to be at a uniform flow, estimating <br />some floodplain characteristics, and using Man- <br />ning's Equation: <br /> <br />85 <br />