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accomplished in many cases, however, by coating them, preferably on the exterior, with hydraulic cement, epoxy paint, or other similar waterproofing materials. It must be recognized that sealing and waterproofing of walls increases the hydraulic forces acting on the walls unless the drainage through the walls which is afforded by the cracks and crevices prior to sealing is provided by other means. Sometimes the wisest course would be to permit the seepage through the wall and then control it by a floor drain and sump pump. Existing cracks and leaks in walls sometimes can be the most practical form of drainage to relieve pressure. In some cases this drainage can be supplemented by holes drilled through the walls. Structural and hydraulic analyses of alternative designs and associated cost estimates will enable the designer to choose the most suitable means of controlling seepage at a given building. Sewage Backup.-Most existing subdrains, whether connected to sewerage systems or not, are subject to backflow and high pressures during floods. Since these high pres- sures could burst the usually encountered clay pipe subdrains and endanger basement walls and floors, some device such as a gate valve must be provided for protecting or isolating the subdrains around the building from these high pressures. There are several alternative methods for controlling backflow through sewers. One method would be to install a main valve at a location where the sewer is strong enough to resist the flood induced pressure and where all possible reverse flows can be stopped. See locations "A" and "B" in Figure 20. This valve should be designed to accommodate grit and other materials which could lodge in it. If the pipe is of sufficient strength, an alternative would be to install separate valves on all basement fixtures and floor drains (Figure 20). These valves could be inflatable rubber plugs or a similar type of mechanically expandable rubber plug. Valves designed for low pressure (20 psi and less) could be installed in drain lines of fixtures which are below design water levels. In either of the above alternatives, it would be necessary to provide adequate sump pumps to handle any leakage. Figure 21 presents another alternative for controlling sewer backup. This alterna- tive provides for outletting all floor drainage, appliance drainage, drain tile flow, and any seepage that might enter the building, to a sump pump. The pump would lift the drainage up to an elevation above the design flood on a permanent basis. By thus eliminating all gravity sewer drains, the problem of flooding backflow can be eliminated and a subsurface area permitted to function during floods. Structural Engineering Observations The highly technical and thorough nature of the investigations and analyses required in the design of effective, safe, and reliable flood proofing measures for both new and ex- isting facilities cannot be over stressed. Construction of new, or modification of existing sutxlrainage systems without such investigation and analysis can result in a situation poM tentially more dangerous to life and property than no flood proofing program at all The large number of factors and the potential magnitude of the forces involved make it impossible to design flood proofing measures by intuition. Such an approach can lead to loss of property and even life during a flooo, with all the attendant legal problems in addition to the disruption and misery. 29