CONTINUOUS SIMULATION vs. DESIGN STORMS COMPARISON WITH WET WEATHER FLOW PREDICTION METHODS

Many collection systems and wastewater treatment facilities are designed and analyzed using the Rational Method and Intensity-Duration Frequency (IDF) curves. It is well known that the accuracy of this method is highly variable and it includes many implicit assumptions that may dramatically affect modeling results. The implications of inaccurate results coupled with unrealistic assumptions may result in unnecessary costs, ineffective permits or system failure.This paper compares the advantages and disadvantages of design storm and continuous simulation techniques. A comparison of these techniques is then made by applying them to the rational method, regression method, synthetic unit hydrograph method, and the non-linear reservoir method.There are several issues that design storm techniques can not easily accommodate, including variable rainfall durations, back-to-back storms, different antecedent conditions, and statistical methods used to generate flow frequencies. These differences have a direct effect on the risk that a municipality accepts in defining a critical measure of performance.Examples are included from several municipal master-planning studies that illustrate how results can vary from one approach to another. These differences are taken to the extreme when storage facilities are necessary. However, each modeling technique needs to be examined in light of the specific purpose for which it was designed.