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Last modified
11/23/2009 10:51:29 AM
Creation date
10/4/2006 9:42:32 PM
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Fields
Template:
Floodplain Documents
County
Statewide
Title
Floodway Determination Using Computer Program HEC-2
Date
5/1/1974
Prepared For
US
Prepared By
COE
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />~ <br />, <br /> <br />program. Additional computer runs may be made to improve the accepta- <br /> <br /> <br />bility of the floodway. The computerized procedure is discussed here <br /> <br /> <br />and illustrated in the example problem. <br /> <br /> <br />The computer procedure is based on calculating a natural pro- <br /> <br />file as the first profile in a multiple profile run. Other profiles <br /> <br />, <br /> <br />in tile run can be calculated using various encroachment options as <br />desired. The five encroachment methods available are described in <br />Exhibit 9A of the Users Manual. The input requirements and associated <br />output statements are also presented. The HEC-2 Standard Test T. <br />shown in Exhibit 8 of the Users Manual, provides an example of ap- <br />plication of Methods 2. 3. and 4 of the encroachment options. The <br />procedure described herein is based on the use of Method 4. which <br />provides the most direct means for delineating a floodway. <br />Method 4 establishes encroachment limits at a cross section on <br />the basis of natural (i.e.. prior to encroachment) flow conditions. <br />As part of program input. the user specifies a target incremental <br />increase in water surface elevation (normally one foot). The program <br />increases the computer "natural" water surface elevation by the incremen- <br />tal amount and computes the corresponding increase in the computer con- <br />1 <br />veyance over the natural conveyance for each cross section. The pro- <br />gram then sets encroachMent limits at each cross section by eliminat- <br />ing the increased conveyance from the extremities of the overbank <br />areas. The program removes an equal percent of conveyance from each <br /> <br />, <br /> <br />l~onveyance is defined as K. 1.49 AR2/3 where: n. Hanning's rough- <br />n <br />ness coefficient. A . Flow area. and R . Hydraulic radius. <br /> <br />5 <br />
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