|dc.description.abstract||Sand extraction from the bed of the lower Waikato River has occurred over the past three decades, primarily to meet the demands of the Auckland construction industry. The extraction has affected bed levels over large areas of the river.
From analysis of the Waikato Catchment Board's periodic river channel cross sectional surveys degradation of river bed levels is seen to be occurring in the Puni to Huntly reach as a result sand extraction and river training, and in the Karapiro to Hamilton reach as a result of construction of the Karapiro dam. Noticeable degradation has occurred since 1964, and the rate has increased between the most recent bed level surveys, 1981 to 1987. The river level has lowered to an extent where further lowering will hinder the operation of water intake structures at the Huntly Power Station, and promote drainage of the Whangamarino wetlands.
From the interpretation of echo sounding traces, aerial photographs and field observations it is seen that sand bar, dune and ripple bedforms exist in the lower Waikato River. Two distinct dune populations exist, a short steep group with a ripple index (λ/h) of about 24 and a long flat group with a ripple index of about 40. Sand bar bedforms, which have previously been loosely referred to as 'dunes', dominate the Huntly to Mercer reach and are related to a sinuous thalweg pattern. The bars also exist in two forms, with straight or strongly curved crests. A relationship between flow discharge and the rate of bar migration exists. However, at high flows(~> 600 m³/s) the bars become rounded and the crests undefineable. The river bed is devoid of detectable bedform (h < 0.1 m) immediately downstream of sand extraction sites.
Using the relationship between bedload transport and discharge determined from bar migration in the present study, and a similar relationship developed previously (Finley, 1974), an estimate of medium term (for the period 1975 - 1989) bedload transport is 180 000 m³/yr. Several bedload transport functions were applied to the lower Waikato River. Colby's Relations (1964) was found to be the most applicable mathematical approach as it provided estimates of bedload transport comparable to those estimated from bar migration. The Karapiro dam has blocked the major supply of bedload, so the bedload transport rate can be expected to drop in future.
The bottom sediments of the lower Waikato River are predominantly coarse poorly sorted pumiceous sands. There is no significant difference in textural statistics between the Mercer and Puni areas. The Puni area has a shallow pre-fluvial basal unit, thereby limiting the useful resource to 7- 9 m³/ m² of river bed. No pre-fluvial unit was found in the Mercer area. A sedimentation rate of 2.8 mm/yr since the Taupo eruption was inferred from stratigraphic data.
From the results and interpretations in this study, continued sand extraction at the present rate at Mercer will promote further bed level lowering, and operations would be better concentrated in the tidally influenced areas around Puni. It is recommended that future cross sectional surveys should be conducted more regularly (2-3 yrs) and each survey should be completed over a shorter period (4-6 weeks), and regular future assessment of bedload transport should be made. A need exists for further research into the effects of sand extraction and management of the lower Waikato River.||