Effect of urban street pattern on drainage.

Abstract

In cities, storm runoff is usually collected in the streets and conveyed to underground storm drains through inlets located at street level. Construction and maintenance of storm drain systems is always expensive. In semiarid regions with a very low frequency of storm occurrence, most cities use streets as the drainage route even though their primary function is for the movement of traffic. As a result, the shape of a runoff hydrograph at the outlet of an urban watershed will vary with the pattern of street arrangement within the watershed. The objective of this study is to investigate tradeoffs between alternative street patterns with respect to urban drainage. The study area is confined to residential subdivisions where the subdivision boundary is assumed to be the watershed boundary. Three street patterns, namely, rectangular, curvilinear and dendritic, are considered in this study. The cost-effectiveness technique is employed to give an objective evaluation for these three alternative patterns. The effectiveness is measured by three kinds of evaluators. The first evaluator is the flow depth at certain points on the streets; the second evaluator is the total time during which the streets are occupied by a certain depth of water; the third evaluator is the total cost associated with the construction of a street pattern. A framework of a distributed system model has been constructed for simulating the runoff hydrographs and flow depths at certain points on the streets. A parcel of land near Willcox, Arizona, is selected to illustrate how the model could be used. Possible subdivisions of the land for each of the three different types of street patterns are shown, and the runoff hydrographs and flow depths from each of the patterns are examined. It is found that the peak flow rate resulting from the rectangular pattern is 13 percent higher than that from the curvilinear pattern and 29 percent higher than that from the dendritic pattern. The dendritic pattern has the smallest percentage of street intersections occupied by water during a storm. Time of occupation of high water stages at street intersections is much shorter in the dendritic pattern than that in the rectangular and curvilinear patterns. The development cost for the dendritic pattern is the lowest among these three patterns. Therefore, the dendritic pattern appears to be the best type of street arrangement in terms of urban drainage.