Recharge of ground water from riverbeds

Abstract

Initial observations on a relationship between the magnitude of recharge of groundwater from riverbeds and the river stage, led to a study of certain characteristics of riverbed sediments, characteristics that could possibly affect infiltration to ground water. Field surveys: (a) revealed the occurrence of stratified and of flood-dumped sediment deposits, and (b) led to a conclusion that characteristics of riverbed sediments could better be studied in the laboratory than in the field. Measurements of lateral and vertical permeabilities yielded a wide range of values. Taking into account the heads and hydraulic gradients that apply under field conditions, the results of these measurements could, in essence, explain observed magnitudes of recharge within a short stretch of river, a stretch where the level of the ground water dips below that of the riverbed. Particular interest has centred around the movement of particles into, through and out of sediment bodies which can explain a general decrease in permeability in newly laid-down deposits with time and a change-over from an initial degree of turbulent flow to laminar flow. Particle movement could also explain a suspected delineation of existing and buried riverbeds within the mass of gravel and sand in which they are embedded. A follow-up on this by geophysical means might yield important information that could guide future exploration of the ground water resource. Particle movement through sediment bodies can also assist in explaining a decreasing permeability of sediments towards the coast and the possible creation of a restriction of outflow of underground water to the ocean. It could possibly also explain the emergence of underflow as surface flow in a river such as the Selwyn. Observations have been made on significant differences in rates of permeability depending on whether a rising or a falling head is applied. Follow-up work is suggested on: (1) a laboratory study of silt behaviour at a fresh-water/salt water interface; (2) structural stability and hydraulic behaviour in flooddumped deposits; (3) refinement in methods for measuring flow in braided river channels; and (4) possible delineation of buried riverbeds (ground water veins) by geophysical means.