FLOOD00031 - Laserfiche WebLink

Home Browse Search

.1 I ::>EC'l'ION IV BENEFIT/COST ANALYSIS The Benefit/Cost Analysis approach to determining the justi- fication for Public Works projects has long been used by public agencies. The basic approach to the Benefit/Cost Analysis is that the benefits derived from a particular project must be greater than the cost of the project to justify its con- struction. Likewise, projects with a greater benefit/cost ratio are more advantageous to the public than projects with a smaller benefit/cost ratio. Unfortunately, the parameters for establishing benefit/cost ratios are such that results may be easily manipulated by the person computing the benefits to prove whatever point he desires. The approach taken in this study is to realistically establish the actual benefits derived by the public due to reduction in flood damages. The calculation procedures outlined in the following sections and utili~ed for the study are such that all damage potentials calculated and all benefits claimed are well within the actual values. THEORETICAL BASIS The theory behind the Benefit/Cost Analysis procedure and its application to urban storm drainage projects is well covered in the text Economics of Resources P1annin by L. Douglas James and Robert L. Lee, pu l-she y Me raw-Hl- in 1971. Chapter 8, dealing with the basic Benefit/Cost Analysis theory, and Chapter 10, dealing with its application to flood control projects, would be of oreat benefit to anyone involved in drainage project analysis. Chapter 10 deals not only with Benefit/Cost approach, but also with many aspects of flood control including structural and nonstructural measures and their application to the overall field of flood control. The first step toward establishing the benefits derived from a project is to determine the damage potential prior to con- struction of any improvements. The damage potential must again be determined after the improvements have been constructed. The diff~rence betwecn the damaae Dotential orior to con- struction and after-~onstruction becomes the-benefits derived from the fadli ty. -:,H- Damage potential for a flood control project is determined by developing the damage potential for various storm return frequencies and mathematically reducing this to an average annual damage potential. Figure IV-1 illustrates the graphical procedure for determining average annual damage potential. The damage potential for a particular storm, for example a 10 year return frequency, is developed for a reach of channel. This damage potential in dollars is plotted against the return freguency, as a percentage, on a damage versus return probability graph. The same process is continued for other storm frequencies. A curve is then drawn between the plotted points representing the damage potential for all storms within the time frame being analyzed. The area beneath this curve becomes the average annual damage potential. The average annual damage potential may also be determined through mathematical calculations, although far more diffi- cult and time consuming. The graphical method illustrated in Figure IV-l is certainly as accurate as the input data available. The same procedure must be followed to determine the damage potential after construction of a particular improvement. The difference in average annual damage potential between existing conditions and conditions after the improvements are in place becomes the average annual benefit to the project. To determine the benefit/cost ratio, it is necessary to compare the average annual benefit to the cost of the project. This may be done by reducing the cost of the project into annual payments or by converting the average annual benefit into a present worth of benefits. The benefit/cost ratio for various ~lternatives arp thpn compared to determine the alternative most attractive. The comparison will show that facilities should be designed for a particular storm recurrence interval and utilizing a specific tYpe of construction. CALCULATlON METHODS USED FOR. DAMAGE POTENTIAL ANALYSIS The first step toward actual determination of damage potential in this study was the establishment of design flows for six tr"'1uencies, 2, 5, 10, 25, 50 a~d 100 year. Next the area of inundation and depth of inundation were determined for each storm frequency along the channel. This necessitated the development of rating curves for each street crossing and for critical reaches of channel under eXlsting conditions of improvement. -59-