Gennaro Cioffi

Field evidence for rapid downstream fining of river gravels through selective transport Ferguson, R.I, Hoey, T., Wathen, S. and Werrity, A Field evidence for rapid downstream fining of river gravels through selective transport. Geology, 24,

Introduction Downstream fining of bed material Methods The Scottish Highlands (Study Area) Results Downstream Fining Selective Movement of Tracer Pebbles Selective Transport at Bed-load Trap Discussion/Interpretation Question/Answer

What happens to Grain size properties downstream? Grain size decreases approximately exponentially with distance if there are no lateral inputs of coarse sediment from tributaries or valley sides Abrasion vs. Sorting Study area Allt Dubhaig in the Scottish Highlands, Scotland

Abrasion – is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport (in this case running water) Sorting - refers to selective transport; size selection during entrainment, transport and deposition

A stream found in the central Scottish highlands No significant complication by lateral water/sediment inputs or human disturbance

This graph (Figure 2) illustrates the long profile of the river bed, clearly outlining the decrease in slope along the reach (+20 m)

Study was conducted in a stream reach approximately 2.5 km in length This area is frequently flooded due to seasonal affects/heavy rainfalls 100 pebbles counted at 120 cross sections Figure 1: Representation of river reach studied

This graph (figure 2) illustrates the long profile of the river bed, clearly outlining the decrease in slope along the reach (+20 m)

Bulk samples selected from 11 sites 1460 tracer pebbles painted and fitted with magnets released at 6 sites

Figure 2 (from article) shows results of sorting process where sediment size clearly decreases with distance downstream [Doc Cam] Notice gap in data where curves change from concave to convex This represents the transition of gravel bed material to sand bed material This is a result of the natural size selective sorting during the entrainment of sediment

After 2 years, 25% of tracer pebbles were remaining on the bed’s surface Results showed that distances travelled by tracer pebbles were right skewed Coarser pebbles travelled less, on average, than finer particles (Figure 3) [Doc Cam] However, less noticeable due to the plot being non-linear

Bed-load trap was emptied after each major flow occurrence Catches ranged from 14 kg to 1200 kg and were sieved to allow analysis of changes in transport rate and grain size distribution

Figure 3 [Doc Cam] shows that tracers (of similar size) in the upstream region (T1-T4) travelled much farther than those in the downstream end (T5-T6) Possible linkage to equation: Q S /Q~ S/D Transport of sediment is proportional to stream power Q S ~ QS/D

Furthermore, Figure 3 also illustrates, by negative sloping, that coarser material travelled much shorter distances than the finer material Pebble counts at the 120 sites along with the bulk samples showed an approximate 80% reduction in D 50 This is many magnitudes larger than the lab tests of abrasion (0.1% reduction) with similar sediments over similar distances travelled

Bed-load trap analysis also showed that selective transport (sorting) was evident across the full spectrum of flood magnitudes Since tracers in the upstream region travelled much farther, one can conclude that the shear stress decreases downstream Maximum bed load diameter was seen to increase with shear stress.

The study of tracers provides sufficient evidence that downstream sorting is the main cause of river gravel fining within the reach Hence, measured abrasion rates are far too small to explain observed downstream fining, but bed-load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport

Questions?