The permeability of riverbed sediment samples

Abstract

The Canterbury Plain stretches roughly 120 km along the Pacific coast and from the eastern foothills of the Southern Alps, at some 300m elevation, it stretches roughly 60km to the coast. A depth of sediment of several hundred metres has been measured near the coast. The sediments consist mainly of gravel and sand mixtures, lenses of gravelfree sand and layers of wind-blown and water-borne glacial silt. The three major rivers on the plain, the Waimakariri, the Rakaia and the Rangitata remain incised upstream over roughly half their length from the foothills to the coast. According to information collected by the N. Z. Geological Survey of the Department of Scientific and Industrial Research , by the North and South Canterbury Catchment Boards and the Ministry of Works and Development on ground- water contours, supported by field observations on seepage patterns along the river banks, groundwater in this upper section tends to move towards the rivers. Farther downstream the ground-water contours indicate a movement of water from the riverbeds to ground-water. Closer to the coast the levels of riverbeds, ground water and ground surface tend to intersect as evidenced by groundwater- fed streams and drainage problems. As far as the Waimakariri river is concerned, the river most studied because of its significance for water supply to the city of Christchurch, the contours indicate that the ground-water level dips below the river level a short distance downstream from where river incisement ceases, probably over a distance of 4km. On the basis of river-flow measurements, recharge of ground water from the riverbed takes place in this section. Using the area-velocity method, a number of measurements have been made on river losses in this sector (summarized by Mandel, 1974), but owing to the difficulty that exists in measuring flow in braided rivers, a certain reservation still persists as to the reliability of the informationo Therefore, there remains an interest in obtaining corroboration and further information on the magnitude of the recharge and the factors that govern it. In the present work an attempt has been made to understand some of the basic factors involved in this recharge by studying the hydraulic characteristics of riverbed sediments. In a separate study the movement of fine particles within these sediments is studied and another study aims at improving on the measurement of river flow in braided channels by attempting to refine a dye-dilation method for this purpose.