The science of hydrology is concerned with the estimation of the intensity of rainfall, the
distribution of the flow of the rainwater over the land, and the determination of the flow quan-
tity (peak and total) that eventually reaches some specified point, the “point of solution.”
Of primary concern to the highway engineer is the frequency of occurrence of the peak discharge. Although many methods for determining runoff have been proposed over the years, making an accurate prediction is difficult, because of the many and varying parameters that contribute to the complexity of the problem. These parameters include the affected drainage area, the rainfall intensity, the time of concentration of the rainfall, and the percent of the rainfall that will actually reach the point of solution. In addition to the difficulty in forecasting flows due to the inaccuracies in measuring and predicting the above parameters, different techniques that are commonly used to predict flows may produce significantly different results for a specific site and situation.
The objective of hydrologic analysis is to estimate the quantity of runoff for which a specific hydraulic structure must be designed. The magnitude of the study must be proportional to the risks involved. Those risks include the potential for damage to the roadway and adjacent property and the importance of the roadway in the transportation system.
