Previous studies of channel morphological change have often relied upon sparse spatial and temporal data-sets, resulting in a degree of uncertainty in their conclusions. Furthermore, previous methodologies have been unable to quantify small-scale changes in channels composed of sand and fine gravel due to inadequate data density. This study reports the use of a high-resolution 3D laser scanner (LMS-Z210) in the assessment of erosion and deposition volumes in the proglacial zone of Glacier du Ferpècle and Mont Miné, Switzerland. Highresolution (> > > > >500 points/m 2 ) data obtained across a 5881 m 2 area of braid plain during the early part of the melt-water season permitted digital elevation models (DEMs) of bar surfaces and channels to be produced. The approach negates the need for complex surface interpolation algorithms required in surveys with lower point density, e.g. those obtained from global positioning systems or total stations. Furthermore, the technique has the advantage of being able to cover a large area over a comparatively short time period. Laser scanner returns revealed high vertical precision, ± ± ± ± ±0·02 m, for dry bar surfaces; however, submerged areas (< < < < <0·2 m depth) returned lower precision, with a range of − − − − −0·15 to + + + + +0·06 m. To account for this, different levels of detection (LoD) were applied to the dry and wet parts of the study reach during DEM subtraction. Subtraction of successive DEMs revealed two short depositional episodes over the ten-day study, with erosion dominating in between. Three episodes of avulsion were identified, two of which appeared to be triggered by constriction avulsion, whilst the other was initiated by bar-edge erosion. Transient lobes and sediment sheets could also be identified at the downstream end of the reach, as could lobe progradation. Relatively minor, but common, changes in channel morphology could also be detected, such as bar edge accretion, bank erosion and chute development. In an analysis of the effects of survey frequency upon volumes calculated following DEM subtraction, daily surveys were found to increase erosion volumes by 67 per cent and deposition volumes by 14 per cent when compared with an 8-day survey interval.

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This paper compares two approaches used to derive measures of annual sediment transfers within a 1 km long piedmont reach of the gravel-bed River Coquet in Northumberland, northern England. The techniques utilize: (i) channel planform and cross-section surveys based on a theodolite/electronic distance measurement (EDM) survey of 21 monumented channel cross-sections and channel and gravel bar margins; and (ii) theodolite-EDM survey generating a series of x,y,z coordinates, from which digital elevation models (DEMs) of the reach were constructed. Calculating the difference between DEM surfaces provided a measure of volumetric change between surveys carried out during the spring of 1999 and 2000. The use of kriging in DEM generation and differencing permits computation of estimate variances and confidence intervals for sediment transfer. Error analysis, validating the DEMs using surveyed cross-sections, indicated a mean error between surveyed and DEM-generated cross-sections of around twice the value of the D 50 of the surface sediment in the reach. Comparison of sediment volumes derived from the two approaches suggests that, compared with the DEM method, monumented cross-sections underestimate the magnitude of volumetric changes that occur within the reach. The cross-section approach relies on a simplistic integration of the volumes, whereas DEM differencing provides an estimate at a resolution under the control of the analyst. Furthermore, the cross-section approach does not permit a reliable estimate of the uncertainty of the volumes calculated. In addition, the DEM methodology based on the morphological unit scale provides an explicit identification of spatial patterns of erosion and deposition, a feature that cross-section-based approaches may fail to include.

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