Water quality modeling study for Umhlangane River, South Africa.

Abstract

Over the past few decades, river water quality has been a critical issue in many parts of the world due to various domestic, industrial and agricultural pollutants. The challenge lies in developing mechanisms and tools, that will assist us to mitigate, prevent or possibly reverse deteriorating river water quality. Water quality models are the most useful tools in describing river ecological conditions, assessing effects of water pollution and assisting decision makers for water quality management. They can be used to predict the changes of the water quality parameters like Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), etc. They also contribute in reducing the cost of labour and time needed to conduct field studies or experiments to some degree. One of the well-known water quality models is the Hydrologic Engineering Centre River Analysis System (HEC-RAS). This study aimed to assess pollutant transport characteristics of Umhlangane River north of Durban using the HEC-RAS model. Hydraulic outputs were produced by executing the hydraulic model for each defined point in time. The water quality simulation was obtained from the HECRAS model with modelled hydraulic data as inputs. The Hybrid Cells in Series (HCIS) model is a conceptual mixing cells based water quality model that has an advantage over the Fickian based advection dispersion equation model (ADE). An impulse response of the HCIS model matches with the same of the ADE, when the Peclet number is more than four. The HCIS model produced reasonable results in terms of percentage error when compared with actual recorded data. The simulation results of BOD and COD tend not to vary with time unlike the observed results due to average constant input of pollutants. A main advantage with this model is that it deals with first order ordinary differential equation and which can accommodate any reaction kinetics without any complexity in model equation unlike the ADE model. Thus this study aimed to derive a model component for the HCIS and investigated its ability to simulate water quality parameters such as BOD, COD and DO under predefined condition. The proposed model in this study yielded positive outcome at the upper reach of Umhlangane River with an average agreement between simulation results and the observed data. The work is concluded by rendering a future potential scope of the HCIS to incorporate nutrient dynamics and non-point source pollution.