Rational drainage design for the desert Southwest.

Abstract

Drainage systems for the desert Southwest are currently designed without much consideration for the climatological or surficial conditions of the region. The "100 year" flood has become the design standard throughout the United States due to misunderstandings about requirements of the National Flood Insurance Program. The effect of larger floods is virtually ignored, seasonal variations of rainfall patterns and intensities are neglected, and hydrologic data collection is extremely limited in watersheds of the urbanizing Southwest. The laws of nature are obscured by the rules of man during the planning and design of desert drainageways. Procedures for extrapolating runoff records and estimating the magnitude of the 100-year flood, including the LP III probability density function, the NOAA Atlas, and HEC-1, have been widely adopted in the arid regions as part of local drainage regulations. Plans are normally not approved unless the basis of design complies with the regulations. Assumptions inherent in the methods are questionable and data to verify the assumptions are limited. Drainage design can be improved by using available field data and a simple method--based on the Rational Method--is developed. Benefit-cost analysis is a valuable tool for establishing project alternatives, project size, and cost/benefit allocation. An equitability index is defined for evaluating fairness, and it is combined with the benefit-cost ratio for refining and selecting project design. Estimates of flood peaks can be improved by considering channel abstractions as "negative base flow"; by recognizing the presence of the n-value paradox; by extending flood records through paleohydrologic study; by monitoring rainfall, runoff, and the effectiveness of design strategies in urban catchments; and by using more suitable rainfall estimates. Drainage design can be made more rational by also considering sediment transport; by including nonstructural design alternatives; and by evaluating a range of flood magnitudes, not just the 100-year flood. A conceptual drainage ordinance not based on the NFIP is presented.