Several methods for synthetic unit hydrographs are available in the literature. Most of these methods involve the hand fitting of a curve over a set of a few hydrograph points, which can sometimes be a subjective task. Besides, the user often finds it difficult or simply neglects to adjust the generated unit graph to a runoff volume of one unit (inch, cm, or mm). It is the purpose of this paper to present to the design hydrologist a simple method to fit a smooth gamma distribution over a single point specified by the unit hydrograph peak and the time to peak with a guaranteed unit depth of runoff.

A statistical analysis of all available continuous hourly and 15‐minute duration rainfall records for Pennsylvania was performed to develop an updated procedure to estimate design storms. As a resuit of this study, Pennsylvania was divided into five homogeneous rainfall regions and a set of rainfall intensity‐duration curves developed for each region, for return periods of 1 to 100 years and durations ranging from 5 minutes to 24 hours. The PDT‐IDF curves were judged to be a better representation of Pennsylvania rainfall than the nationwide TP‐40 maps, particularly for storm events of 10‐years and lower return periods. The average time distribution of 24‐hour storms in Pennsylvania was found to be well represented by the SCS Type II distribution. The Corps of Engineers SPS 24‐hour distribution was found to differ appreciably from both the SCS Type H and the Pennsylvania 24‐hour storm distribution. For storm durations between 15 and 90 minutes the standard Yarnell intensity‐duration curves closely resemble Pennsylvania storm distributions.

No abstract

Basin parameters such as drainage density, channel slope, shape factors, and a geometric factor were used in a factor analysis of 112 basins in Pennsylvania and surrounding states. Three distinct homogeneous definable groups resulted. Group I basins were those with an area on the order of 100 square miles, low channel slopes, and/or located within the western glaciated portion of Pennsylvania. Watersheds underlain by carbonate rocks or glacial tills constituted Group 11. A main channel slope of greater than 0.017 in a high precipitation area or a small drainage area were the criteria for Group III basins. There exists a continuous range from Group I basins, where absorption was high to the Group III basins where either the storm pattern or the absorbing character of the soils gave a very low absorption.