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I I I I I I I I I I I I I I I I I I I Structural Alternatives Alternative B: A Natural Type Waterway Following-the General Historic Channel Deter- mined from Old Aerial Photog-raphs, Old Mapping, and Local Records Alternative B is a slight variation of Alternative A. It is possible that during periods of land development changes and alterations were made on the historic channel. It is further possible that thes.e__ch8,ng,E!s and alterations now are_~Qunter-productive to the goal of eliminating or reducing flood dimaJi;e:--'------------- --'------------.--~ '---- .- ~._~--~.-.~-~."._--------"._. " Where it is feasible to allow the waterway to revert to its historic channel it will be necessary to review old aerial photographs, maps and records in order to compare the location of the historic waterway to the location of the present waterway. By so doing, it will be possible to determine how much right-of-way will have to be purchased to allow this reversion to occur. In the case of DFA 0054, this alternative will be feasible where there is sufficient open space in developed areas and where the channel and floodplain are not presently developed and already in a natural state. In the latter case, this alternative is the equivalent of Alternative A. In the developed areas of the DF A 0054 floodplain, there are few places where it is economically feasible to implement this plan because of the extensive right-of-way requirements. This..--l!HJ!l'I1a1~ th~r_ef()J'E'l,basbeen ,eliminated frgroJurther consideration in tJ:lis.,~ll:lysis. ,-- Alternative C: The Installation of Major Underg-round Conduits along- the General Course of the Historic Stream Channel, USing- Local Rights-of-Way Whenever Possible Closed conduits are considered as a viable alternative under three conditions: (1) When it is feasible for an open, lined channel to be constructed, but the dangers of a large body of fast moving, relatively deep water make it more practical to enclose the channel. (2) Where closed conduits already exist that are capable or nearly capable of transporting the design flood. (3) In highly developed areas where the closed conduits are used as part of the storm water collection system. The principle disadvantage of utilizing closed conduits is that they are generally substan- tially more expensive to construct than equivalent open channels, which usually results in a box culvert or pipe being designed for a smaller design flood. The overall runoff interception of close conduits is not as effective as with an open channel and is further aggravated by the fact that debris tends to collect and effectively block the entrances to closed conduits further reducing the capacity of the structure. Since the conduit is underground, little, if any, residual flood carrying capacity above the design flood is achieved. Larger floods can, and ~ occur. When an unusual event does occur, the lack of an adequate surface flood route can be disastrous. For these reasons, underground conduits have been considered for implementation on DFA 0054 only where space or right- of-way restrictions preclude the construction of adequate open channel facilities. V-5