Stochastic modeling of urban drainage systems
During wet weather flow events, bypasses and overflows of raw sewage may occur in many municipalities. It is generally accepted that combined sewer overflows will cause water quality problems such as low dissolved oxygen, lake eutrophication, beach closures, fish kills, and impaired aesthetics. To abate these problems from the existing wastewater collection and treatment facilities, it is worthwhile to consider a computerized control scheme within a wastewater collection and treatment system. The majority of water quantity-quality models describing urban drainage systems developed to date are deterministic. They require considerable data for calibration, verification, and sensitivity analysis. This will limit their applications since most of urban watersheds do not have enough data available. Stochastic models of sewer flows and quality parameters have also been researched. Most of the efforts were based on the application of Box-Jenkins models, which require a tedious three-stage indentification process. This makes the models unsuitable for automatic control. Furthermore the transfer function model identification procedure suggested by Box and Jenkins are not feasible since the rainfall series containing many zeros, can not be "prewhitened". The objective of this study is to develop stochastic models for sewer flow and quality series, which are more suitable for automatic control schemes of urban drainage systems. Both univariate and transfer function type models are investigated for actual flow and quality series collected from Fusina, Kenosha, and Green Bay treatment plants. A new identification procedure of transfer function models is developed. By using Green's function and inverse function, the original nonlinear estimation problem is converted into a linear model so that it can be easily estimated using a linear multiple regression technique. A new method for determining the unit hydrograph of an urban drainage system is also developed. The stochastic sewer flow and quality models developed in this study can predict flow and quality values adequately, and can be easily implemented on computers with less external interference. The methodology developed for the unit hydrograph determination may have a wide range of applications such as infiltration-inflow analyses, dry-weather wet-weather flow separation, real time flow forecasting, quality analyses, and classic hydrologic design of storage and conveyance systems.
"Stochastic modeling of urban drainage systems"
(January 1, 1989).
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