Simulation of catchment runoff, erosion and sediment transport using a transient numerical model for Mlalazi catchment
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Date
2019
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University of Zululand
Abstract
Catchments and estuaries are fragile systems that are prone to serious degradation from many different anthropogenic impacts. Much research has been conducted on developing an understanding of the fluvial processes in river catchments and in estuary dynamics. Many of the anthropogenic impacts on estuarine systems are generally derived from subjective expert opinion in South Africa. The Mlalazi Estuary is one of the best conserved estuaries in KZN. As a result of the changing state of the marine and fluvial conditions there is a possibility that such changes may trigger management interventions. There is therefore a need to derive reliable flows from the Mlalazi Catchment as it is a driver of sediment deposition and erosion processes which may impact on the opening or closing of the Mlalazi Estuary. The latest Reserve Determination study for Mlalazi Estuary was based on rapid assessment with low confidence (<40%) in simulated monthly streamflow. The study illustrated a need for further detailed assessment of catchment hydrology using appropriate and calibrated models. In an ungauged catchment where there is limited observed data, numerical models are useful tools to derive best estimates of flow, erosion and sediment transport. In this study the HEC-HMS hydrological model was developed, calibrated, validated and applied for simulation of runoff, erosion and sediment transport from the Mlalazi Catchment into the estuary. Calibration and validation was done at delineated sub-catchments with observed flow records. An event based calibration approach and a continuous approach were used in the development of the model. For the event model the initial and constant loss method was used for simulating rainfall loss from the catchment surface, while the Soil Moisture Accounting (SMA) Model was employed for the continuous simulations. The event calibration was based on two selected extreme storm events, namely the Domoina (31 Jan 1984) and Imboa (17 Feb 1984) cyclonic events; and the validation was done on two storm events in February 1985 and September 1987. The calibration and validation for continuous simulations of flows were from 1977-1986 and 1986-1999 respectively. The Nash-Sutcliffe Efficiency (NSE) and overall Root Mean Square Error (RMSE) were used to evaluate the model performance. The continuous flows for the catchment were then simulated from 1950 to 2017 incorporating erosion and sediment transport. The erosion was simulated using the Modified Universal Soil Loss Equation (MUSLE), and the Ackers-White method was chosen for sediment transport potential. The erosion and sediment transport models
SIMULATION OF CATCHMENT RUNOFF, EROSION AND SEDIMENT TRANSPORT USING
A TRANSIENT NUMERICAL MODEL FOR MLALAZI CATCHMENT
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were not calibrated due to limitations of observed data, but parameter values were estimated from other studies available in literature for this region. The simulated sediment yield from the catchment was evaluated by comparison to sediments yield found by other studies in this region.
It was concluded that a physically based, numerical simulation model provides a pragmatic method for the derivation of reliable hydrodynamic data and information in catchments with limited observed data like the Mlalazi Catchment. Furthermore, this study allowed a smooth linkage with the study of the Mlalazi Estuary that employed the HEC-RAS model.
Description
Submitted in fulfilment of the requirements for the Degree of Master of Science in the Department of Hydrology in the Faculty of Science and Agriculture at the University of Zululand, 2019.
Keywords
River catchments, Estuaries