Merren A. Jones

Relationship between sediment supply and avulsion frequency in braided rivers

The interplay between sediment supply ( S s ), sedimentation rate ( S r ), and the frequency of channel avulsion ( A f ) exerts a primary control on alluvial architecture. In order to investigate the effect of sediment supply on avulsion frequency, four Froude-scale model experiments of an aggrading braided river were undertaken in which the magnitude of S s was progressively increased over an eightfold range. The value of A f increases at a rate slower than the increase in S s , contrary to the trend previously reported by Bryant, Falk, and Paola in their experimental study on alluvial-fan dynamics. These results suggest that the relationship between A f and S s is dependent upon bed slope and that the response of A f to an increase in S s in unconfined braided rivers may be different than that on steep alluvial fans.

Drainage capture and discharge variations driven by glaciation in the Southern Alps, New Zealand

Sediment flux in proglacial fluvial settings is primarily controlled by discharge, which usually varies predictably over a glacial–interglacial cycle. However, glaciers can flow against the topographic gradient to cross drainage divides, reshaping fluvial drainage networks and dramatically altering discharge. In turn, these variations in discharge will be recorded by proglacial stratigraphy. Glacial-drainage capture often occurs in alpine environments where ice caps straddle range divides, and more subtly where shallow drainage divides cross valley floors. We investigate discharge variations resulting from glacial-drainage capture over the past 40 k.y. for the adjacent Ashburton, Rangitata, and Rakaia basins in the Southern Alps, New Zealand. Although glacial-drainage capture has previously been inferred in the range, our numerical glacier model provides the first quantitative demonstration that this process drives larger variations in discharge for a longer duration than those that occur due to climate change alone. During the Last Glacial Maximum, the effective drainage area of the Ashburton catchment increased to 160% of the interglacial value with drainage capture, driving an increase in discharge exceeding that resulting from glacier recession. Glacial-drainage capture is distinct from traditional (base level–driven) drainage capture and is often unrecognized in proglacial deposits, complicating interpretation of the sedimentary record of climate change.

Study of connectivity of open framework gravel facies in the Canterbury Plains aquifer using smoke as a tracer

Abstract Open framework gravels (OFGs) are an inherent textural component of alluvial gravel outwash deposited by braided river systems. Being exceptionally permeable, they play an important part in facilitating the transmission of water and contaminants through alluvial gravel aquifers. Understanding how connected OFG facies are is helpful in making informed predictions about groundwater flow and contaminant transport through such aquifer systems. This work examined a section of the Rakaia fan, Canterbury, New Zealand. A 3 × 3 grid of large diameter auger holes was drilled in close proximity to a sea cliff, which provided very good three-dimensional exposure of the fan architecture. A novel smoke tracing experiment and water tracing field tests were conducted to measure the dynamic connectivity of the OFG facies. Smoke proved to be an effective tracer for measuring the interconnectedness of OFGs over set distances of 5 m. The water tracing tests confirmed that OFGs are connected across much longer distances – in excess of 18 m. Results from both tests revealed how rapid, and non-uniform, aqueous transport can be through alluvial outwash materials. The connectivity information will be used to improve realizations of the heterogeneity of the Canterbury Plains aquifer and inform future hydrogeological modelling.