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13 II. PHYSIOGRAPHIC CONSIDERATIONS FOR FLOODPROOFING ACTI VITI E S 3.1 General . Flooding and subsequent flood plains are a natural aspect of the hydrologic ecosystem. Flooding plays a positive role in the renewal of the earth's resources by replenishing soil moisture and depositing fertile silt from the river to the flood plain. Floods become a pro- blem when combined with human development. Prior to any human development, the dura- tion, magnitude, and frequency of flooding is based on the dynamic interaction between. water runoff, and the physical characteristics of the watershed that convey the runoff. Thus, prior to the implementation of f1oodproofing measures enabling development in the flood plain, it is important to understand the physio- graphic characteristics that should be considered. These characteristics include channel cross section and slope, vegetation. soil characteristics, erosion and sedimentation, and debris. 3.2 Channel Cross Section and Slope . The channel cross section and slope of both the channel and the watershed have a signifi- cant affect on the type and extent of flooding that occurs. Steeply sloping areas and narrow, v-shaped channels are typically associated with the mountainous regions of the state. The type of run-off often associa~d with steep-gradient areas is short term with a rapid rise of water and high-velocity flows. With high-velocity flows, it only requires several inches of depth to sweep individuals off their feet. High velocities also produce the forces that enable the movement downstream of cars, structures, and debris. High velocities can also cause severe erosional damage. Flooding along larger rivers or the flat, downstream areas of rivers that originate in the mountains occurs at a slower rate and at a lower velocity than steep- sloped areas. Flooding in these areas is often predictable days to weeks ahead of time, and ample warning can be given. However, the duration of the flooding is much longer, and the extent of flooding is spread much wider. . . In relation to the applicability of flood proofing measures, velocity is a significant component in the forces that must be considered in structural design. The duration of flooding is a con- cern in that access must be maintained for certain public services, and protection against seepage must be provided. . Another characteristic that is associated with the gradient of the channel is the formation of deltas and alluvial fans. They occur wherever there is a change from a steep to a flat gradient such as from a steep mountain stream to a flat open valley or where a steep tributary enters a main channel. As the sediment, bed material (larger stones and boulders) and water reaches the flatter section of the stream, the coarser material can no longer be transported because of the sudden reduction in both slope and velocity. Consequently, a cone, fan or delta builds out as the material is deposited. In these areas, the channel is very unstable and the potential for lateral movement of the channel is great. Thus, development in or near these areas should be avoided. . 3,3 Vegetation . Vegetation plays an important role in several aspects of flooding and the flood plain. On the watershed, good vegetation cover, in the form of brush and trees, reduces the amount of runoff by intercepting rainfall, allows increased infiltration, and provides storage by reducing the rate of overland flow., Vegetation cover also reduces the amount of erosion from the watershed. . As flood waters exceed the channel capacity, they extend into the flood plain. In the flood plain, vegetation tends to reduce the velocity resulting in storage within the flood plain and reduction in the peak discharge.