An investigation of flows and losses within the alluvial sands of ephemeral rivers in Zimbabwe

Abstract

Ephemeral rivers are common hydrological features in Southern Africa where periods of up to 9 months occur with no significant rainfall. In many cases, they have well-defined channels in-filled with alluvial sands in which significant quantities of water flow, even when the surface flow has ceased. This water resource is commonly exploited either by shallow pits dug in the sand or by well-points, infiltration galleries or collector wells in the river bank and provides a valuable, readily available supply for local people [Hussey, S.W., 1997. Small-scale sand abstraction systems, 23rd WEDC Conference Water and Sanitation for All, Durban, WEDC]. The processes involved in the flow in these so-called ‘sand rivers’ present interesting hydrological and hydraulic questions. From a hydrological perspective, it could be argued that the flow in the alluvium is an extension of the surface flow and could be estimated by extrapolating the surface flow recession curve. Hydraulically, the flow can be visualised as a closed conduit, which has a slope to its base approximating to the sand surface but more irregular. Although the flow may follow the well-known Darcy formula, the conventional methods of analysis of groundwater systems may not apply as they normally assume horizontally bedded deposits of infinite lateral extent. This paper presents the results of a study of four sites in the Matabeleland Province of Zimbabwe over a period of about 3 years. At each site, water level readings were recorded from a number of piezometers in the alluvium together with an estimation of the bedrock profile. The velocity of flow was estimated from the slope of the phreatic surface as well as from salt dilution experiments. A simple model has been developed based on a ‘closed conduit’ concept, which estimates the recession of the water levels and flow rates in the alluvium, including the effects of evaporation from the sand surface as well as recharge by intermittent rainfall during the dry season. This model has been calibrated and validated against the field observations from the four sites.