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Chapter II, The Physical and Types of Environment Flood Proofing Buildings on flood plains can be damaged by seepage of flood waters though walls and foundations, by ground-water pressure on floor slabs and walls, by the backup of water through sewerage systems, by the entrance of flood waters which overtop channel banks and inundate flood plains, and by ice jams. The physical environment is an important fac- tor in determining which sources of flood waters are present at a particular location, and thus will influence the nature and design of flood proofing measures. The permeability of the earth materials, the stability of flood plain slopes, and the interconnection of flooding with the ground water resource are significant environmental elements to be considered. Under some conditions, these environmental elements could rule against the use of flood proofing, while in others the condition could promote the use of flood proofing. Permeability of Site Materials The permeability of the materials at a building site largely determines the movement of ground water aroup.d and under the foundation walls and floor slab. In some environ- ments, ground-water pressure will build up against such buildings. In these situations a drainage system can be considered as a means to relieve the ground-water pressure by creating a cone of depression (pumping cone) under the building. Permeability, in part, determines the level of the zone of saturation (commonly known as the water table). Beneath this level, problems of underground seepage control are most likely to be encountered. .However, such problems can also be encountered above this level in areas which have localized "perched" water table conditions. The range in permeability of some common types of earth material is presented in the following table. In materials of low permeability, such as clay, the quantity of ground wa- ter flow into underground openings is likely to be small even at "1'elatively high gradients and could be drained only with a close network of tiles. In materials of high permeability, TABLE 1: RANGE 1N PERMEABILlTY FOR SELECTED MATERIALS Selected Materials Range in Permeability Low Unweathered clays Very fine sands, silts; mixtures of sand, silt, and clay; (Common Flood Plain Deposits) Clean sands, mixtures of clean sands and gravels (Course textured alluvial fan, glacial ouiwash) Clean gravel Intermediate High Very High Source: Based on Todd,-Ground Water HydroloJiIT (1959),'p. 53. 7