MODULE 1 _______________________________________________ RAINFALL - RUNOFF Introduction In order to understand and design erosion control BMPs and structures a background in applied hydrology is needed. Knowing how to compute or determine volumes and peak rates of runoff in the context of the recurrence interval or return period are important skills. Conceptually, the precipitation - runoff process can be visualized as shown in Figure 1-1. At the start of a runoff producing precipitation event, the surface runoff is zero. As the rain continues, the areas located nearest the point-of-interest or the stream will slowly begin to yield some runoff water, causing runoff to begin at a low rate at the point-of-interest, see time t1. As the rain continues, larger and more remote areas of the watershed begin to contribute runoff, see time t2. When rain has occurred long enough for the total watershed to be contributing runoff, the time of concentration, tc, the peak rate of runoff occurs. The peak rate of runoff is the parameter needed for the design of most water carrying structures, such as channels, culverts, slope drains, temporary diversions, etc. In the cases where water storage structures, such as silt and skimmer basins, it is necessary to have a measure of the total volume of runoff. This is the area under the runoff rate versus time plot (called a hydrograph).
Figure 1-1. Conceptualization of the hydrologic processes.
Rain, in addition to causing or driving the runoff process, also drives the erosion process. The commonly used RUSLE erosion model is based on the rainfall energy needed to detach soil particles. In order to properly plan and correctly size most erosion control structures, it is necessary to predict peak rates and/or volumes of surface runoff as a function of the probability that the design storm will be exceeded during the life of the structure. The ability to predict runoff characteristics requires a great deal of information about time of the year, geographic location, size of contributing watershed, duration of the storm, time required for all areas of the watershed to contribute runoff water to the outlet, rainfall intensity, soil moisture conditions, and the degree or magnitude of flooding being considered. The purpose of this module is to explain how to quantify the runoff volume and the peak rate of runoff expected from small watersheds (
