James Brasington

Methodological sensitivity of morphometric estimates of coarse fluvial sediment transport

Abstract The estimation of fluvial sediment transport rate from measurements of morphological change has received growing recent interest. The revival of the ‘morphological method’ reflects continuing concern over traditional methods of rate determination but also the availability of new survey methods capable of high-precision, high-resolution topographic monitoring. Remote sensing of river channels through aerial digital photogrammetry is a potentially attractive alternative to labour intensive ground surveys. However, while photogrammetry presents the opportunity to acquire survey data over large areas, data precision and accuracy, particularly in the vertical dimension are lower than in traditional ground survey methods. This paper presents results of recent research in which digital elevation models (DEMs) have been developed for a reach of a large braided gravel-bed river in Scotland using both digital photogrammetry and high-resolution RTK GPS ground surveys. A statistical level of change detection is assessed by comparing surfaces with independent check points. The methodological sensitivity of the annual channel sediment budget (1999–2000) to the threshold is presented. Results suggest that while the remote survey methods employed here can be used to develop qualitatively convincing, moderate precision DEMs of channel topography (RMSE=±0.21 m), the remaining errors imply significant limits on reliable change detection which lead to important information losses. Tests at a 95% confidence interval for change detection show that over 60% of channel deposition and 40% of erosion may be obscured by the lower level of precision associated with photogrammetric monitoring when compared to ground survey measurements. This bias reflects the difficulty of detecting the topographic signature of widespread, but shallow deposition on bar tops.

Turbidity and suspended sediment dynamics in small catchments in the Nepal Middle Hills

The Himalayan environment has until recently been perceived to be in a critical state of environmental decline resulting from rapid population growth and associated land!use change[ Recent research however has emphasized the di.culty of developing an objective appraisal of the state of the environment in a region where empirical data are scarce and unstructured and where an understanding of the spatial and temporal dynamics of natural environmental processes remains highly uncertain[ This paper presents results from an intensive three!year project designed to help address the regional empirical de_cit establish detailed baseline environmental data and to gain an insight into storm period and seasonal suspended sediment dynamics[ The instrumentation calibration and analysis of high!frequency infrared turbidimetric records from a number of small subcatchments in the Nepal Middle Hills are reported[ Storm period and seasonal variation in turbidity and suspended sediment are examined and hysteresis patterns explored and explained[ A variety of methods to estimate seasonal suspended sediment yield in a mixed land!use catchment are examined and found to vary by up to a factor of _ve[ Despite the inherent uncertainty all estimates of catchment sediment yield are found to be high with respect to erosion plot studies from the local area and this suggests the importance of riparian and channel erosion as major sediment sources a _nding consistent with other regional studies[ Copyright 1999 John Wiley + Sons Ltd[

Small-catchment perspective on Himalayan weathering fluxes

Weathering fluxes from four small catchments in the Himalayas, one forested, one agricultural, and two glacial, have been calculated by quantifying stream discharge, deposition, and biomass uptake of major ions. These fluxes were partitioned between silicate and carbonate sources by using a mineral mass-balance model. This approach provides the first well-defined estimates of Himalayan weathering fluxes from small catchments. Silicate weathering rates are highest in the favorable weathering environment of the Middle Hills catchments, despite higher dissolved fluxes in the High Himalayas attributed to high carbonate weathering. The silicate weathering fluxes for High Himalayan small catchments are similar to estimates from the chemistry of large rivers draining the same regions. Much higher silicate weathering intensity for the Ganges basin indicates that the silicate material eroded from the High Himalayas undergoes up to six times as much weathering in the Ganges Plain as in the High Himalayan mountains. Himalayan silicate weathering rates are higher than in equivalent continental settings and, on an area-normalized basis, are comparable to fluxes from weathering of basalts on ocean islands and tropical volcanic provinces.

Retrieval of vegetative fluid resistance terms for rigid stems using airborne lidar

Hydraulic resistance of riparian forests is an unknown but important term in flood conveyance modeling. Lidar has proven to be a very important new data source to physically characterize floodplain vegetation. This research outlines a recent campaign that aims to retrieve vegetation fluid resistance terms from airborne laser scanning to parameterize trunk roughness. Information on crown characteristics and vegetation spacing can be extracted for individual trees to aid in the determining of trunk stem morphology. Airborne lidar data were used to explore the potential to characterize some of the prominent tree morphometric properties from natural and planted riparian poplar zones such as tree position, tree height, trunk location, and tree spacing. Allometric equations of tree characteristics extrapolated from ground measurements were used to infer below-canopy morphometric variables. Results are presented from six riparian-forested zones on the Garonne and Allier rivers in southern and central France. The tree detection and crown segmentation (TDCS) method identified individual trees with 85% accuracy, and the TreeVaW method detected trees with 83% accuracy. Tree heights were overall estimated at both river locations with an RMSE error of around 19% for both methods, but crown diameter at the six sites produced large deviations from ground-measured values of above 40% for both methods. Total height-derived trunk diameters using the TDCS method produced the closest roughness coefficient values to the ground-derived roughness coefficients. The stem roughness values produced from this method fell within guideline values.

Sensitivity of morphometric estimates of sediment transport in large gravel-bed rivers

The estimation of fluvial sediment transport rate from measurements of morphological change has received growing recent interest in the last five years. The revival of the morphological method reflects continuing concern over traditional methods of rate determination but also the availability of new survey methods capable of high-precision, high-resolution topographic monitoring. In particular, remote sensing of river channels through aerial digital photogrammetry is a potentially attractive alternative to labor intensive ground surveys. However, while photogrammetry presents the opportunity to acquire survey data over large areas, data precision and accuracy, particularly in the vertical dimension is lower than traditional ground survey methods. This paper presents results of recent research in which DEMs have been developed for a reach of a large braided gravel bed river in Scotland using both digital photogrammetry and high resolution RTK GPS ground surveys. For both approaches, a statistical level of detection of change is assessed by intercomparing surfaces with independent check points. The sensitivity of the annual channel sediment budget to this level of detection is presented. Preliminary results suggest that as much as 60% of channel deposition and 30% of erosion may be obscured by the lower level of precision associated with photogrammetric monitoring.