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<br />APPENDIX I <br />CALCULATION OF ENCROACHMENT STATIONS BY <br />METHOD 4 <br /> <br />~ <br /> <br />The following example is provided to illustrate the calculation <br />procedure for determining encroachment stations for a cross section by <br />Method 4. First, the conveyance and water surface elevation is computed <br />for the natural conditions. These computations would be made in calcu- <br />lating the first profile of a multiple profile run using HEC-2. Next, <br />the water elevation is increased an incremental amount (one foot for the <br />example) and the resulting conveyance is computed. To determine the <br />required encroachment, the increase in conveyance is expressed as a <br />proportion of the "natural" conveyance. Half of the proportional <br />increase in conveyance is eliminated from the left overbank and half <br />is eliminated from the right overbank, if possible. To determine the <br />stations of encroachment, the program assumes that conveyance varies <br />linearly with horizontal distance between cross section stations (from <br />GR card data). For a horizontal ground line, the assumption is correct, <br />and for small horizontal distances between stations, the assumption is <br />satisfactory. If there is a large distance between adjacent stations <br />and the ground profile is sloping, the computed encroachment stations will <br />not provide for proper reduction of conveyance. The program user can <br />check the calculations by reviewing the flow distribution printout for <br />the natural profile. If the computed encroachment station is between <br /> <br />. <br /> <br />1-1 <br /> <br />APPENDIX I <br />