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I I I I I I I I I I I I I I II I. I I I Alternative D: The Use of Lined Flood Channels in Order to Reduce Right-of-Way Requirements The linings for channel alternatives considered in this study consist of either concrete or grass. All channel sections considered are trapezoidal and have been sized according to the design flood. Cost estimates provide for a maintenance road to be con- structed adjacent to the channel. The primary reason for using lined channels is to confine the design flood in as small an area as possible. This has the effect of reducing the area of the floodplain and consequently, the right-of-way requirements. Besides the obvious benefits of reduced floodplain area, there are many secondary benefits associated with the use of lined channels such as increased property values and deceased insurance requirements for property adjacent to the floodplain. The choice between a grass-lined and a concrete channel is a matter of aesthetics and economics. Concrete channels transport storm water more efficiently and generally have smaller operation and maintenance requirements, but have higher initial construction costs and are less pleasing aesthetically than grass-lined channels. Greater right-of-way areas are required by grass-lined channels due to low velocity requirements, flattened side slopes to facilitate maintenance, and relatively high side and bottom friction. While concrete channels require less right-of-way than grass channels, the higher velocities associated with lower friction factors and increased gradients pose problems to public safety and also have a tendancy to increase capacity requirements of the downstream channel reaches by decreasing the time of concentration and channel storage. Alternative E: Selected or Limited Structure and Channel Improvements Limited structure or channel improvements will be considered for those cases where flood damages are localized or where damage occurs as the result of an individual structure or portion of channel that is not, capable of transporting the design flood. Consideration to the improvement of structures will be focused on road and irri- gation canal crossings. In many cases, structures under roads on DF A 0054 are sized to handle only low flows. As a result of development of land in the drainage basins, the volume of runoff to be transported has increased beyond the capacities of many of the structures resulting in flooding caused by backwater and the overtopping of roads. Selected channel improvements will be considered where the natural channel or floodplain in general is capable of transporting the design flood except for short, limited reaches, without excessive property damage. In these cases, minor improvements to the deficient reaches can result in significant reduction in damages. Improvements can be provided in the form of lined channels, embankments, berms, closed conduits, enlarged culverts or bridges, drop structures, diversion structures, etc. V-6