HR: 08:00h
AN: H51L-01    [Abstracts]
TI: Using Remote-sensing to Survey Topography and Morphologic Change on Large Braided River Beds
AU: Maurice, D
EM: m.duncan@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8011, New Zealand
AU: * Hicks, M
EM: m.hicks@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8011, New Zealand
AU: Shankar, U
EM: u.shankar@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8011, New Zealand
AB: Since 1999 we have made extensive use of a variety of remote-sensing technologies to survey bed topography over reaches of large braided gravel-bed rivers on the east coast of New Zealand's South Island. The motivations have been (i) to collect input and validation data for 2-d hydrodynamic models for quantifying in-stream physical habitat and for predicting flood levels and (ii) to survey spatially-distributed riverbed erosion and deposition in order to estimate bedload fluxes by the ‘morphological' method. Typical applications have been to river reaches 3-4 km long and 1 km wide, with grid cells from 1-5 m. We use different techniques to survey dry and wet areas of braided riverbed. For dry areas, we have used digital photogrammetry and infra-red airborne LiDAR. For wetted channels, we have generally used ortho-rectified colour imagery or multi-spectral scanning to map water depth, then we map bed topography by subtracting the water depth from a DEM of the water surface obtained from photogrammetry or LiDAR. The imagery is calibrated to water depth using field measurements on the day of imagery acquisition. Surveys are undertaken during low flows to maximise bed exposure. We use ground-based RTK-GPS and echo-sounding to collect calibration and validation data, and sometimes simply use these methods to survey the wetted areas. Orthoimagery at multiple river flows is used to validate 2-d model results. We have been able to achieve elevation accuracies at interpolated points of the order of 10-15 cm for dry areas. This accuracy typically degrades to 20-30 cm for wetted areas. Our experience has exposed a number of issues relating to survey accuracy and practicality at large river scales. These include: changing geoidal models between surveys; local systematic error with photogrammetric model mosaics; geospatial synchronisation of multi-platform data; time-synchronisation of LiDAR and imagery- collecting aeroplanes and suitable weather and river conditions; confusions in water depth mapping; and the critical importance of good data at key hydraulic controls for eco-hydrologic applications. We suggest that high resolution bathymetric LiDAR offers the best potential for future surveys in large river reaches. While the current bathymetry LiDAR systems do not appear to deliver a significantly better accuracy of submerged bed elevations than we have achieved with mixed-technology approaches for dry and wet areas, and their cost remains high, a one-stop package is hard to beat in terms of practicality and data synchronisation.
DE: 1805 Computational hydrology
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting