Laserfiche WebLink
<br />. <br /> <br />. <br /> <br />Approximate Studies , <br />These studies are based on a minimal amount of technical data and should <br />be used only under the following conditions: <br />. for a limited rural area where only one or two single family residences or <br />other structures will be constructed: and <br />. for a river reach extending up to several miles where there is no imme- <br />diate need to delineate the detailed 1 DO-year floodplain but where <br />the local government wishes to alert potential developers to the flood <br />hazard. <br />Approximate procedures allow the engineer to estimate the area that would <br />be affected by a 1 OO'year flood based solely on existing information. <br />Researching the area's flood history. Interpreting topographic features. using <br />handbooks to estimate hydrologic and hydraulic characteristics. and obtain' <br />ing the opinions of Qualified professionals (hydraulic engineers. hydrolo- <br />gists. etc.) are methods used for approximate studies. The intent of such <br />studies is to: <br />. provide a reasonable estimate of the potential flood hazard at any pro- <br />posed building site; <br />. minimize the engineering and planning costs to those using the study; <br />and <br />. fulfill the regulatory requirements for local governments in the National <br />Flood Insurance Program (NFIP). <br />The types of approximate floodpialn studies which may be available to local <br />officials are: <br />. U.S, Geoiogical Survey's (U.S.G.S.) Floodprone Area Maps - a 7.5 minute <br />Quadrangle ('" 2.000') which show the 1 DD'year flood outline. <br />. Federal Emergency Management Agency's (FEMA) Flood Hazard Boun. <br />dary Maps (FHBM's) which show special Flood Hazard Areas in a <br />community, <br />. FEMA's Flood Insurance Rate Maps (FIRM's) which show approximate <br />areas as indicated by un.numbered "A" zones in those portions of the <br />community where detailed Information was not generated. <br /> <br />. <br /> <br />Detailed Studies <br /> <br />There are two types of detailed floodplain studies. floodplain information <br />studies a~d master dralnageway planning studies. Floodplain information <br />studies delineate floodplains; master dralnageway planning studies <br />delineate floodplains. consider alternative approaches to problems. and <br />recommend preferred alternatives. Another category of study incorporates <br />features of both approximate and detailed studies: mapping of dam failure <br />flood zones is discussed after the steps in floodplain Information studies <br />have been explained. <br /> <br />FlOOdplain Information Studies <br />Delineation of floodplains through detailed methods includes flood history <br />research. review of watershed development patterns. hydrologic evaluation <br />of the watershed to develop streamflow rates (discharges for the study <br />streams). hydraulic computations to obtain the floodwater surface elevations. <br />and a portrayal of flood outlines on detailed topographic mapping. <br />There are four baSIC steps that should be followed when obtaining flood- <br />plain information and data through detailed engineering procedures: <br />1. base mapping and field surveys should be performed; <br />2. the proper hydrologic method for a particular drainage basin should b6 <br />selected and followed; <br />3. the water surface elevations (flood levels) which are representative of a <br />projected event should be computed; <br />4. the water surface elevation data should be transferred to a flood outline <br />map assuring that datum consistency between the f100dDlaln <br />delineations and on.the-ground conditions IS maintained. <br />