Quantifying the morphological effects of gravel extraction on the Motueka River

Abstract

The Motueka River is a wandering gravel bed river located in the Nelson region, South Island, New Zealand. The removal of gravel from the riverbed for commercial and local uses has been carried out for at least the last fifty years. However, in recent times, concerns have arisen over the influence of gravel extraction on changes in mean bed levels and gravel storage within the riverbed.

This study employs the end-area method to measure morphological change in the Upper Motueka riverbed between 1960 and 2000. This consists of comparing repeat cross-section surveys and calculating the mean bed level change for the time period between measurement dates. From this, gravel volume changes can be interpolated. This study attempts to improve on previous estimates of gravel storage changes in the Upper Motueka by using a variable active channel width for calculation of mean bed level changes.

Digitising the river planform demonstrated that the form of the Upper Motueka River has altered quite dramatically over the 40-year study period. The results of the end-area calculations show a mean bed level degradation (-0.3m) and a net decrease in gravel storage (-642000m³ ) in the Upper Motueka River over the 40-year study period. The total amount of gravel extracted from the study reach over the same time period is a substantial proportion of the total gravel loss (87%). The use of a variable active channel width for each successive pair of surveys should represent gravel volume changes better than the previous analysis. However, there is no statistically significant difference apparent between the results produced using a fixed active channel width and those produced using a variable active channel width.

A significant problem with using the end-area method is the reliance on the cross-section survey data. There is no indication of morphological change between the measured cross-sections. Although the use of aerial photograph analysis can account for this to some extent by allowing determination of an average active channel width, changes in the bed level between cross-sections cannot easily be quantified. Furthermore, the end-area method fails to establish the full extent of aggradational and degradational change that has occurred between the survey dates. In summary, the use of the end-area method for gravel bed-rivers, which are both spatially and temporally dynamic, can only provide a coarse estimate of gravel volume changes.