Risk-cost approach to interim drainage structure design

An interim drainage structure provides short service during the construction months of the permanent drainage structure. This study presents a risk-cost approach by which the design flood for an interim drainage structure can be derived from the capacity of the permanent structure, cost-capacity ratio, cost-damage ratio, and construction months during a year. To accommodate various construction periods in a year, the design capacity for such a temporary drainage structure can be adjusted to the monthly rainfall or runoff distribution at the project site. Design charts were produced for the break-even economic condition using the Gumbel distribution for annual maximum analysis. Similar solutions were also achieved using the Exponential distribution for annual exceedance analysis. Design methodology presented in this paper provides an objective basis for selecting construction months and sizing the interim drainage structures.