FLOOD CONTROL ECONOMICS
8.8 FLOOD CONTROL ECONOMICS
In a flood control project, the degree of flood protection (i.e., to the required stage) should be justified by an economical analysis of the costs involved in raising the structure to the required heights (say, the height of spillway) and the direct and indirect benefits obtained by flood
HYDROLOGY
protection up to that stage. Generally, the flood stage for which the ratio annual benefits to cost is a maximum is adopted for the design of the flood protection works (Fig. 8.19). Protection against floods of rare occurrence is uneconomical because of the large investment (for a small increase in the benefits) and hence there is always a certain amount of flood risk involved.
Land slope
0.6 m
A 0.9 m 0.9 m
Top of bund Retaining wall
b. c/s of bund on AA
Waste weir a. Plan
Fig. 8.18 Contour bund
Benefits =1
Total benefits 2
Costs Marginal increase Marginal increase
in benefit in benefit
crores
Max.
crores in .
benefits
in
1 Total costs
. Rs Rs , , costs benefits
T otal otal T
Design stage of Design stage of
1 2 flood protection flood protection
Q D Q = Max. feasible flood P peak reduction cumec
Reduction in flood peak Q (cumec)
Fig. 8.19 Cost-benefit analysis for design stage of flood protection
The flood control costs include: (i) Capital costs involved in the construction of the structure to the required flood height
(i.e., to offer the required degree of flood protection, say, by a combination of dam- spillway, levees and channel improvement)
(ii) Interest cost on capital expenditure (iii) Sinking fund, depreciation and taxes (iv) Operational expenses and maintenance cost The benefits of flood control include
(i) Direct or primary benefits accruing from prevention of flood damages to structures downstream, losses arising from disruption of communication, (and business), loss of life and property, damage to crops, etc.
FLOODS—ESTIMATION AND CONTROL
(ii) Indirect benefits resulting from the money saved under insurance and Workmen’s compensation laws, higher yields from intensive cultivation of protected lands and reduction in flood-prone epidemic diseases, etc.
Assessment of potential flood damage is an important requirement in any organised flood control programme. Generally stage-damage curves are plotted to show that damage in a particular region in relation to the rising flood stages in that reach. Indirect damage by floods deals with the loss of business and services to community. This is a socioeconomic loss and is difficult to assess because of the various economic and physical factors involved whereas the direct damages can be estimated in terms of money value. An assessment of the damages caused by floods can be made when the data is collected and presented in the following form:
(i) Area affected (km 2 )
(ii) Population affected (lakhs) (iii) Crops affected (a) in hectares (b) value in rupees
(iv) Damage to property (a) number of houses damaged (b) cost of replacing or repairing in rupees
(v) Loss of livestock (a) category number (b) value in rupees
(vi) Human lives lost (vii) Damage to public works in rupees Suitable flood indices have to be developed from year to year to enable comparison to be
made of the various flood damages.
Combination of Flood Control Measures
The object of a flood-control study is to decide which of the flood control measures, singly or in combination, are most suited and their location, size, design and costs. The design of flood control works should be closely related to the hydrological features and economic justification of the project.
Suppose the flood control project involves the construction of a reservoir, levees and channel improvement works, the factors governing the final selection of various combinations are illustrated in Fig. 8.20. The cost of construction of reservoir to reduce the design flood to various lower peaks is indicated by curve (1). The cost of construction of levees and other channel improvement works for protection against various flood peaks is indicated by curve (2). The sum of the ordinates of the two curves indicates the combined cost of flood protection by reservoirs, levees and channel improvement works and is given by the curve (3). The minimum point P of the combined cost curve represents the most economic combination. The reduced flood peak and the total cost of works can be read off at this point.
See also Appendix-C: Reservoir Design Studies.
3 Combined cost
. in
crores) . crores) (Rs in
Cost of
in
reservoir
2 Cost of
Total cost of
3 1 2 flood peak Design
Q D Peak flood Q (1000 cumec)
Fig. 8.20 Cost analysis for combination of flood control measures
Example 8.8 The costs of construction of levees for flood protection for various flood peaks are given below. From this and other data given, make an economic analysis of the flood control project and determine the flood peak for which the levees have to be designed.
Annual project (1000 cumec)
Flood peak
Total damage
Recurrence interval
under the flood
of flood peak
cost up to the
peak
(yr)
flood peak
(Rs. in crores)
(Rs. in crores) 10 0 2 0.2
2.4 Solution The economic analysis is made as shown in Table 8.5 on the basis of benefit-cost
ratio. The ratio of benefit to cost is a maximum of 1.39 when the levees are constructed to
safely pass a flood peak of 40000 cumec (Fig. 8.21). Hence, the levees designed for this flood peak will be most economical.
FLOODS—ESTIMATION AND CONTROL
Flood protection
4.0 by levees
3.5 Benefit Benefit Cost ratio = 1.37
Annual Annual
benefits benefits
2 Annual 1.5 Annual
costs costs
Design flood Design flood
Ann
Ann 0.5 peak peak
Flood peak Q, cumec
Fig. 8.21 Benefit-cost analysis (Example 8.8)