METEOROLOGICAL AND HYDROLOGICAL DRIVERS OF SEDIMENT
TRANSFER IN A STEEP ALLUVIAL FAN, WESTLAND, NEW ZEALAND.
McGill-Brown MA,1 Fitzsimons SJ,1
1
University of Otago, New Zealand
Aims
Techniques for quantifying the magnitude, frequency and patterns of sediment transfer of bedload
particles has evolved from painted rock methods to modern radiofrequency approaches. This
study has focussed on an underexamined area in the West Coast of New Zealand, Potters Creek,
which is an ephemeral alluvial fan stream that has the conveyance, stream capacity and
competency to transfer large quantities of sediment in any given flow. Using radiofrequency
methods to capture sediment transfer allows for a more intricate knowledge of the fluvialgeomogpholohy interface to be quantified and understood in this environ. Analysis of the
hydrological and meteorological patterns driving flashy and episodic flow conditions; capable of
initiating individial particles, and the active layer of sediment will be undertaken.
Methods
RFID (radiofrequency identification) tags (23 mm) were embedded in tracer particles (between 81
– 1,236 grams), and during recovery missions a transponder was used to locate dispursed
particles. RFID technology is consdiered to be a reliable and accurate method for locating
dispursed tracer particles compared to other bedload tracing studies. Multiple cross sectional
surveys were completed to capture volumetric sediment aggradation/degradation of the
streambed at various times throughout the experiment. Time lapse photography was used to
quantativley observe streamflow conditions, which was particularly useful in no-flow conditions to
analyse geomorphic change, and in high-flow conditions to visualise stream geometry, wetted
perimeter and bankfull conditions. D50 (median sediment size) analysis was completed to show
the sediment size fraction change downstream, which was used to characterise the fining process
and quntify sediment communition. Depth duration frequency analysis was undertaken to
characterise the precipitation events that occured throughout the experiment. Rainfall data was
observed at Potters Creek using a TB3 tipping bucket rain gauge, in conjunction with Franz Josef
Ewes rain gauge (NIWA). A bubbler was used to measure stage, and was selected due to the
ability for it to be fixed to a highly active bed layer was achieved. Discharge was calculated by
regression analysis, spot flow gaugings, as well as pegging high-flow events from the time lapse
photography. Vector maps were produced to show sediment transport magnitude and patterns
over various spatial and temporal extents.
Results
Over the 9-month experiment, the maximum distance a tracer particle was transferred was 2,144
m. Of a small cohort of recovered particles, evidence suggests that particles abrade, on average
7 grams per 500 meters travelled. Compared to their original mass, clasts of schist composition
experienced higher levels of abrasion (6.6%) compared to metamorphosed clasts (2%), as did
tracer particles that were transferred from the proximal zone (10.2%) compared to mid-fan (3.7%)
or distal zone (1.14%). The median sediment texture (D 50) decreases downstream from 99 mm in
the proximal zone to 39 mm in the distal zone. Sediment has accumulated in the mid-fan zone
due to a lateral injection of sediment from a tributary, increasing the D 50, which requires stream
competency and capacity to adjust for transfer to occur. Despite a D50 increase in the mid-fan
zone, sediment size does reduce throughout the catchment, showing evidence of rounding and
fining downstream. Axial commination shows that the rate of fining from the clast of each axis is
higher at the upstream sites with highest slope is, and the axis lengths are on average 2 times
higher here compared to downstream reaches. Topographic cross sections show that near the
previously mentioned tributary is an aggrading site, causing the alluvial fan to be characterised by
sediment concavity in the mid-fan zone, these cross sections also show that aggradation and
degradation zones alternate downstream. The rainfall lag on average was 100 minutes from a
large rainfall interval to the peak of the hydrograph, recession to baseflow generally took another
100 minutes. Depth-duration frequency analysis of precipitation data during this experiment
shows that there have been 113 events that have exceed return periods of between 2 – 50 years
for durations ranging from 0.5 hours – 72 hours, with one additional event exceeding a 100-year
event. Maximum discharge during this experiment was 53.54 m3s-1 with 12 other events recorded
to exceed 5 m3s-1, where it is likely that sediment has transferred during these events.