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<br />___--I <br />:::I <br />t <br />~ <br />t <br />t <br />\ <br />1 <br />~ <br />( <br />( <br />i <br />~ <br />\ <br />\ <br />I <br />( <br />( <br />t <br />, <br />\ <br />> <br />i <br />\ <br />f <br />i <br />t <br />( <br />( <br />\ <br />I <br />i <br />( <br />( <br />( <br />\ <br />( <br />( <br />t <br />( <br />\ <br />\ <br />~ <br />( <br />, <br />f <br />I <br />( <br />, <br />i <br />,. <br />( <br />( <br />i <br />f <br />i <br />I <br />i <br />" <br />" <br />( <br />r <br />( <br />, <br />f <br />r <br />,. <br />I <br />" <br />I <br />1 <br />1 <br /> <br />of attractive building materials, and levees landscaped. Soil conditions must be capable of <br />supporting loads transmitted to the foundation. While both walls and levees can be designed for <br />marginal soil conditions, the cost of such measures may be prohibitive. <br /> <br />The nature of flooding is important in determining feasibility in several ways. High velocity <br />flows cause erosion which could endanger a wall or levee unless protected, and erosion <br />protection adds to the cost. In addition, velocity adds a dynamic pressure to the design which <br />may further increase cost. The location of flood waters relative to the ~tructure is also important. <br />If only the backside of a structure need be protected the selection of means and number of <br />openings required may be different than if the entire structure or front is protected. When the <br />depth of flooding is greater than that for which protection can be provided it precludes use of <br />the measure or requires adopting a different level of protection. This, of course, is true of any <br />flood proofing measure. Generally, for a small wall or levee, six feet is the practical limit <br />although designs are feasible for greater heights. If access openings are necessary automatic <br />closures should be used or ample warning time should be available to install shields and gates. <br />Warning times vary greatly for different hydrologic and local community conditions. Both <br />daytime and nighttime operation should be planned for when selecting the method of closure. <br /> <br />Costs <br />The principal base cost items for small walls and levees are, <br /> <br />. Construction of wall or levee. <br /> <br />. Drainage for the interior, enclosed area. <br /> <br />. Protection against sewer back-up. <br /> <br />The principal variables in estimating the costs of the first item is the length and height of wall <br />or levee. Generally, the nature of the flood hazard will determine the length and height. <br />Structures built on topography sloping up from a river or creek can often be protected by <br />providing a wall or levee on the backside only. Costs in this situation will be considerably less <br />than if the entire structure must be protected. Interior drainage can usually be handled by <br />installing a sump pump and sewer backup by a gate valve. Engineer's estimates of these basic <br />cost items are presented in Table 5-1. For a $30,000 structure the costs as a percentage of <br />structure value range from 0.5 to 1.6 percent depending upon whether a wall or levee is used <br />and its height. <br /> <br />32 <br />