Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

The trend of catchment urbanisation not only changes the rainfall-runoff processes but also has significant effects on the quality of runoff. Nonpoint source of pollution in the form of runoff from urban areas has contributed greatly to degradation in receiving waters. The fact that stormwater runoff from urban areas carries pollutants is well documented in the literature review. An investigation has been undertaken into the nature of nonpoint source pollution export from urban catchments in Australia and the United States of America.

Urban stormwater runoff quality is a function of a number of variables, including rainfall-runoff processes, nature and characteristics of pollutants on the ground surface. It is generally recognised that the simulation of urban runoff quantity in relation to rainfall and catchment characteristics has been a relative success. However, due to lack of understanding of the processes involved in the accumulation and washoff of contaminants, and also inaccuracy in data collection, urban runoff quality models are poor. This fact calls for a basic analysis of the urban runoff quality processes.

Because of the complex and dynamic nature of pollutants in urban areas, some investigators suggest local data collection to establish site-specific stormwater quality models, however, this model is not applicable when urbanisation proceeds and changes occur in landuse characteristics of the catchment. Hence there is a need to develop a deterministic model that is compatible in accuracy with a quantity model.

The present work provides an in depth study of the behaviour of pollutant accumulation and washoff in urban areas in order to have a better understanding of these processes. The main objectives of the present research can be summarised as follows: i- To identify the problems associated with the quality of urban storm runoff. ii- To better understand the dominant mechanisms of the origin and transport of water pollutants in urban areas. iii- To develop a new stormwater quality model for predicting pollutant washoff from urban catchments.

To achieve the objectives of the project, the research has been conducted in four major parts. In the first part, major pollutants, sources, and problems associated with quality of urban runoff are investigated. The major water quality parameters in urban areas are: nutrients, suspended solids, heavy metals and toxic materials. These pollutants are accumulated during the dry period between storms. A number of buildup models have been reviewed and for a selected few, technical comments are provided. From the measured runoff quality data, a number of exponential buildup equations for different landuses are derived. Wide variation in buildup estimation for similar landuses indicates that identifying a buildup equation for a specific landuse is not appropriate.

In the second part of this study, an extensive review of existing washoff algorithms undertaken. It was found that the majority of the developed methods are empirical with little understanding of the physical processes. Most existing experimental investigations do not sufficiently explain the important mechanisms. Hence an attempt has been made to formulate the basic physics of the washoff process. Based on the assumption that the transport rate (washoff pollutant rate) is proportional to distribution and availability of pollutants, a new method for the estimation of total pollutant washoff is proposed. The parameters involved are initial pollutant load, cumulative runoff volume and washoff coefficients. The advantages of this formulation, in comparison to other complex washoff algorithms, are simplicity and process-orientated features. A set of new equations have been developed for predicting Event Mean Concentration (EMC), pollutograph and loadograph.

The third part of the study involved the calibration and verification of new proposed models. This was undertaken for data collected from 170 storm events in 15 urban catchments. The catchments used for the analysis are located in Australia and the United States of America. The pollutional parameters used in the analysis are non-filterable residue (NFR), total phosphorus (TP), total nitrogen (TN), oxidised nitrogen (NOx-N), and ammonia nitrogen (NH3-N). The calibration and verification of the proposed total pollutant washoff model were also compared with the existing washoff algorithms (SWMM4 and Nakamura). In general, the predictability of the proposed washoff algorithm has been shown to be consistently better than the other two washoff models. The better results can be related to the simple structure of the proposed equations and their close relation to the physics of the pollutant washoff phenomenon.

Field investigations of stormwater pollution are tedious and expensive. The fourth this study focused on the applicability of the proposed relationships to ungauged catchments. It has been found that the variation in the coefficients used in the proposed total pollutant washoff equation is mostly affected by catchment characteristics, water quality parameters, hydrological conditions and antecedent dry periods. An investigation has been made to determine these factors which have the greatest influence on the coefficients. From the results of this analysis a computer program has been developed for ungauged urban catchments. A set of data from impervious plots in a US urban catchment was used for testing the newly developed computer program. The results confirm that the proposed method can satisfactorily reproduce the pollutographs and loadographs.

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