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14 . Vegetation is also important along the banks of a river or a stream. Along th bank, good vegetation adds to the stability of the bank and thus reduces the bank erosio and lateral movement of the channel. . A major concern when developing in the flood plain is removing the natural egetation and the protection it provides, When natural vegetation is removed, measures sho Id be taken to replace it immediately or provide stability for the resulting unprotected areas. 3,4 Soil Characteristics . Soil characteristics are important in erosion, sedimentation, infiltratib"nand seepage. Discus- sion of erosion and sedimentation is given in the next section. One of the m st important soil characteristics is permeability, a measure of the ability of water to flow th ough a par- ticular soil. Permeability, to some degree, controls the amount and rate of sur ace water infiltration, and the zone of saturation. A saturated s9il condition occurs when water occupys all the available spaces, or voids, between individual'soil grains. Soils with hig permeability such as sands and gravels have higher infiltration rates then clayey soils. This implies a faster runoff from clay soils, whereas sandy soils may not permit surface runoff until saturated. However, more importantly, ground water and seepage are more active in san y soils because of their rapid drainage characteristics. . Groundwater and seepage are major concerns when floodproofil}g and/or de elopment in the flood plain is being considered. Groundwater level is a significant compon nt of the hyd- rostatic forces that develop during flooding and must be considered in the str ctural design of footings, foundations and basement walls. Structures below the ground lev I must also prevent seepage into the building, thus, the seepage characteristics of the soil are an impor- tant consideration. . . Another concern in regard to soil characteristics that spould not be overlook d in areas with steep slopes is the possibility of landslides, land slumping or debris flow. As s i1s become saturated, they can loose their slope stability resulting in landslides or mudflo situations. Mudflow evaluation requires detailed analysis, however, a qualitative evaluati n of an area can provide insight into the potential for mud or debris flow. In general, mud lows can occur where an accumulation of loose soil, rock and other debris on steep slopes bcome entrained (saturated) with water. The first characteristic to look for is a history of mudflows and how often have they occurred in the area. Areas where mudflows are for ed are charac- terized by quite definite geological, geomorphological and climatic conditions of erosion. pro- cesses and by the presence of mudflow forming centers. Specific characteristi s include badly sheared and shattered bedrock which yields fine and poorly sorted mic ceous fines upon weathering and some type of narrow rock gorge or ravine where the de risand weathered material can collect. . . 3.5 Erosion and Sedimentation Erosion and sedimentation are processes that occur naturally during flooding. As thedis- charge and velocity of surface runoff increase, the erosional forces also increa e. All fine material such as silts and sands become suspended in the flow. Larger rocks a d boulders are rolled along the bottom of the main channel. The main sources of sedime t being transported by the surface runoff are from the channel bottom and bank erosi n. Sedimenta- tion or deposition of the sediments being transported begins when the veloci y of the.sur- face runoff decreases to the point where the sediments can no longer remain in suspension. During flooding, much of the sediment is deposited on the floodplain, due to slower velocities as water leaves the main channel. . . Sedimentation can cause several problems. Low lying areas in the flood plain can become completely filled over with sediment. Sediment increases abrasive action and dds to the work and cost of cleanup. Sediment can also cause blockage of existing chan els diverting water to new channels. .