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<br />e <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />, <br /> <br />e <br /> <br />4.0 SELECTION OF ALTERNATIVE DESIGNS <br /> <br />-tne-first step in the selection process is to determine the r~ngc of <br />practicable design alternatives. Constraints, in addition to engineering <br />considerations, will frequently limit the available alternatives at a site. <br />In some cases, these nonengineering constraints may severely limit the design <br />alternatives available. <br /> <br />Some items which may limit the design are: <br /> <br />(1) Prescribed minimum design flood criteria as in the case of the <br />Interstate. <br /> <br />(2) Limitations imposed by roadway geometrics such as maximum or <br />minimum grade lines, site distance, vertical curvature, etc. <br /> <br />(3) Overtopping frequency of the adjoining roadway. In particular, <br />that section of roadway involving the same watershed under <br />consideration. <br /> <br />(4) Topographical features such as stream levees, elevation of the <br />watershed divide, and clearances for highways or railroads which <br />are bridged. <br /> <br />(5) Navigation clearance requirements. <br /> <br />(6) Flood plain ordinances or other legislative mandates limiting <br />allowable backwater or encroachment on the flood plain. <br /> <br />(7) Channel stability considerations which would limit velocity or <br />the amount of constriction. <br /> <br />(8) Ecological considerations such as may exist with wetlands or in <br />other sensitive environments. <br /> <br />(9) Geological or geomorphic conditions or constraints including <br />subsurface conditions. <br /> <br />(10) Social considerations including the importance of the facility <br />as an emergency evacuation route in time of peril. <br /> <br />(11) Availability of funds to construct the facility. (This item may <br />or may not be a consideration in a first appraisal but could <br />ultimately govern the design selection.) <br /> <br />11 <br />