MODELING THE TRANSIENT IMPACT OF RAIN EVENTS ON THE OXYGEN-CONTENT OFA SMALL CREEK

Oxygen has been monitored in a small urban creek for a two months peri od. The purpose of the data collection is to use statistical modelling to obtain a better understanding of the phenomena governing the oxyge n concentration. The long term goal is to influence the process of urb an runoff to achieve oxygen concentration suitable for fish. Oxygen fl uctuates significantly on a diurnal basis. Furthermore the purpose is to establish a model by which we are able to use rain data to estimate extreme values for comparison with standards for minimum oxygen conce ntration. The experience is that the fluctuations cannot be explained adequately on a deterministic basis alone. Significant stochastic vari ation has to be accounted for. Data from a small stream is used to ide ntify a dynamic model of the oxygen level as a function of solar radia tion, water depth, and rain. The model is formulated in continuous tim e as two coupled stochastic differential equations. The continuous tim e formulation makes it possible directly to interpret the parameters o f the model. Hence the model is useful for monitoring the actual state of the stream. In this paper a grey box modelling approach, which is a statistical method taking the known physical relations into account, is used. This approach is closely related to the continuous time form ulation. The parameters of the model are estimated using discrete time data and a maximum likelihood method. In evaluating the likelihood fu nction a Kalman filter is used. The dynamic model makes it possible to assess the transient impact of the urban runoff due to rain events, a s well as the effect due to solar radiation. The ultimate outcome of t he analysis is to determine the required size and location of storage basins to be constructed in the sewer system, in order to decrease com bined sewer outflows and extreme oxygen depletion during rain. Copyrig ht (C) 1996 IAWQ. Published by Elsevier Science Ltd.