| OCR Text |
Show cal subdivision as a condition of local cooperation imposed by the Federal Government as prerequisite to the provision of the improvement. The primary advantage of flood plain zoning is its low initial cost as compared to cost of positive measures for providing against once-in-lifetime floods. Losses from flood plain zoning may occur, such as exclusion of more intensive uses which, even after allowing for occasional damage or destruction by floods, may still yield a greater return than the most profitable use under zoning restrictions alone. It is desirable to evaluate benefits and costs of a zoning program for comparison with alternatives of providing flood protection and combination of zoning and works. The choice between the three methods of preventing flood damages then turns on the relative economic advantages with due re- gard for related intangible considerations. Zoning restricts future uses and conforms to normal be- havior of a river. It will be most readily applied in unsettled flood plains and should also be eval- uated and integrated with works designed to con- trol and detain flood stages. There are four general applications of flood plain zoning: (1) Prohibition of encroachment upon an estab- lished floodway to insure that the designed hy- draulic capacity of the floodway is not reduced by obstructions and to prevent or minimize future flood damage in the unprotected area of the flood- way. (2) Restrictions on development in the pro- tected or unprotected flood plain where mainte- nance of the flow capacity is not a controlling consideration. (3) Restrictions on improvements in areas sub- ject to inundation by impounded waters of reser- voirs or sumps necessary for the proper functioning of interior drainage facilities landward of levees. (4) A combination of zoning with levees and other works to achieve adequate protection at lowest total cost. As a condition precedent to Federal participation in building floodways, the Corps of Engineers re- quires that local interests shall maintain the flood- way area free of encroachments. A case in point is the Rio Grande Floodway, authorized in 1948. Generally it is the Corps of Engineers' practice to provide a 3-foot freeboard for federally improved levees protecting agricultural lands above a "design flood." This provides protection generally against all floods except the kind that might occur once in 100 years. That degree of protection (3-foot free- 322 board above design flood) is not undertaken unless the cost will be fully amortized by the flood damage savings generally within 50 years. The foregoing applies to the Rio Grande, with the added feature that the lands to receive protection in the middle Rio Grande under the authorized Federal project are long-established irrigated tracts. Accordingly, there is little if any need for local authorities to zone such agricultural lands. Dan- ger of undermining levee stability from river scour is more serious than overtopping with the 3-foot freeboard. Flood plains should be posted to indi- cate possible flood stages, and zoning should deny permits for buildings and other structures that might be damaged by inundation. In the case of unleveed agricultural lands, local authorities may well apply flood zoning. They might well consider the advisability of any of the following: prohibiting any nonagricultural im- provements in the flood plain and if practicable requiring relocation of existing structures elsewhere; limiting development to types negligibly affected by flood occurrences; or apprising all concerned that a flood hazard exists. Posting of certain areas as flood hazard zones will be desirable where scattered urban development already has begun and it is not feasible on economic grounds to prohibit additional development. An area along Dark Canyon adjacent to Carls- bad presents an example of an undeveloped, fre- quently inundated zone adjacent to an urban area. In the interest of safety and public health in this zone, it is desirable to prevent residential develop- ment. A slum-type community may spring up in this area because of the low cost of land and its accessibility. The area does not now deserve pro- tection because of availability of suitable flood- free residential land on higher ground in and near Carlsbad. Proper flood plain zoning by local authorities eliminates the need for flood protection works. Detailed study may reveal areas in the vicinity of Albuquerque for which zoning should be con- sidered. Irrigated lands of the Middle Rio Grande Conservancy District adjoining the city to the north are now given limited protection by levees. In the authorized Rio Grande floodway, it may be eco- nomical to provide a higher degree of protection for Albuquerque than for these adjacent farm lands. In this event, at some point upstream from the city it would probably be necessary to reconstruct or otherwise modify an existing auxiliary tie-back levee for the protection of the city in the event the |
