Assessment of Temporal Trend-Dynamics of Water Flow and Sediment Flux for Chókwè Irrigation Scheme in Gaza Province, Mozambique

Abstract

Sedimentation of irrigation canals is one of the major challenge that affects their hydraulic control and operation. The sedimentation reduces water conveyance capacity and may lead to overtopping in low land areas. A case in point is the Chókwè Irrigation Scheme (CIS) which has experienced sedimentation problems for decades. There is scanty information on water flow and sedimentation within the canals in the CIS which is crucial for proper management and operation of the scheme. To address this trend-dynamics of water flow and sediment flux for CIS was assessed. The study involved fieldwork at 9 stations during the dry (DS) and wet (WS) seasons, laboratory analysis and scenarios predictions. Bathymetric data were obtained for 2001, 2016 and 2019. Flow measurements and collection of the bed and suspended loads, were carried out using respective devices. The laboratory analysis for water and sediment’s pH, turbidity, Total Dissolved Solids (TDS), Atterberg test and sodium, calcium, magnesium, and potassium concentrations analysis, were performed. Forecasting and trend analysis of water flow and sediment flux was conducted using ARIMA, Mann-Kendall and Fourier Transform tests. Simulation of best scenarios was achieved by use of Ackers and White, Brownlie, Engelund-Hansen, Van Rijn and Yang models. Results indicated significant differences in canal banks and centre canal bed bathymetric profile between 2001, 2016 and 2019. The inflow physico-chemical factors explained their effects on sediment influx and deposition. The concentration of TDS was found to be around 250-300 ppm and 380-500 ppm, for DS and WS, respectively. Sodium was the most predominant element in water and sediment samples, reaching 0.285 ppm. The Liquid Limit, Plasticity Limit and Plasticity Index, were 52%, 20% and 32%, respectively. A mean of 2.96 N/m2 was obtained for the critical shear stress of erosion (τc). The main particle size in bed load sediments which were classified as saline were silt, fine sand and clay at 52%, 39% and 9%, respectively. The temporal water flow was found out to have a positive trend in majority of the stations. Sediment discharge recurrence was found out to be in every 2-4 and 1-2 years for DS and WS, respectively. At the offtake, the water flow and sediment flux magnitude was much lower. The results of this study show that there was a positive influence of water flow velocity on sediments settling time, which varied with time, grain sizes and canal depth. This study provides trends and scenarios of water flow and sediment flux that could be used for better management and operation of the CIS.