DEVELOPMENT OF TWO-DIMENSIONAL WATER QUALITY MODEL FOR WIDE RIVERS AND ITS CALIBRATION

Abstract

Rivers and streams support a multitude of uses including municipal, industrial and agricultural water supply; recreational opportunities; and a means of wastes disposal. With the growing urban and industrial requirements, large amounts of organic and inorganic wastes are spewed back into streams. This inevitably results in deterioration of the quality of water in rivers. Also, massive applications of agricultural chemicals and fertilizers pollute natural water resources leading to immense ecological imbalances. In this direction, water quality modelling provides a rational framework for the analysis of problems to delineate pragmatic solutions. River water quality models attempt to simulate temporal and spatial changes in water quality with recourse to mathematical representation of various phenomenon occurring in aquatic ecosystem. Simulation models, either near-field or far-field may be classified as steady state or dynamic, stochastic or deterministic and numerical or analytical as per the method of solving governing equation. Literature reveals that, the predicted concentration distributions from one-dimensional water quality models are often used in decision making, such as waste- load allocation in rivers, to achieve designated water quality objectives: However, wide rivers classified on the basis of aspect ratio (width/depth) and existence of secondary currents, exhibit transverse variations in water quality arising out of transverse dispersion for several kilometers downstream of the wastewater discharge. Therefore, one-dimensional models based on the completely mixed system assumption are unlikely to provide true representation of hydraulic regime in wide rivers. Thus, the decision making capability in water quality management necessitates the development of practicable framework for wide rivers. In view of this, the objectives of the present research are : • Development of two-dimensional water quality model for wide rivers • Calibration of model tlu•ough field study • Sensitivity analysis for the developed model The system of biotic and abiotic components selected in this research, includes nine water quality parameters of biochemical oxygen demand (BOD) dissolved oxygen (DO); algae. as chlorophyll...a; three forms of nitorgen (organic, ammonia and nitrate nitrogen); two forms of phosphorus (organic and inorganic phosphorus) and one conservative parameter (chloride). Conceptualization of the whole structure and the interfaces between the individuals of the system in an integrated framework considers the basic structures of two- dimensional flow field and the submodels of ecosystem processes. The submodels governing the phenomenon have been selected in accordance with the problem definition in order to focus the baic kinetic interactions among the abiotic and biotic components of the system, viz. decay, transformation and ii consumption by living organisms; and growth, death; sinking and predation by higher trophic level organisms, respectively. Finally, the relevant submodels of select ecosystem processes have been coupled with the streamtube model, in orthogonal curvilinear coordinate system to define the complete model. The solution methodology for the complex integrated framework of multiparameter system in this research, utilises an implicit finite difference scheme based on Crank-Nicolson approximation, unequal intervals; and method of back substitution to solve the tridiagonal system of equations. A generalized framework has been designed on computer environment (HP-UX machine) to facilitate the testing of modelling scenarios that, supports following configurations : simulation of conservative parameter simulation of DO-BOD system simulation of multiparameter system...