INTRODUCTION

Drainage systems are divided into two categories: minor and major. The minor system, which consists of swales, small ditches, gutters, small pipes, and the other various types of inlets and catch basins, collects and conveys runoff to a discharge area or impoundment. Components in the minor system are sized to carry runoff generated by the more frequent, short-duration storm events. The major drainage system includes natural streams, channels, ponds, lakes, retention and detention facilities, large pipes, and culverts. Design criteria for the major system are based on significant amounts of rainfall produced by the less frequent, long-duration storms and are further governed by the hydraulic concepts related to bridges and large conveyance structures.

Storm drain design requires two basic types of analyses: the hydrologic aspect of estimating runoff and the hydraulic aspect of sizing the components. Although there are numerous techniques to estimate runoff, the one selected for design of a particular component depends on the following factors:

• What is being designed

• Type of input data available

• Type of output data required

• Cost effectiveness of the technique

• Required accuracy

• Size of watershed

• Accepted design standards of the approving agency(ies)

Generally, the design of minor-system components requires only the determination of peak runoff discharges, whereas major-system components require not only the peak discharges but the time variation of runoff for an effective design. Analysis of major systems may include detailed hydrology and hydraulic modeling using one- and two-dimensional modeling software.