Bed‐material transport estimated from channel surveys: Vedder River, British Columbia

Abstract

This study investigates the possibility to estimate bed-material transfer in gravel-bed rivers by analysis of morphological changes along Vedder River, British Columbia. Data from repeated cross-section surveys are used to estimate volume changes along the length of an 8 km reach. Gravel budgets are based on a continuity approach. An error analysis is performed to evaluate the uncertainty in the best estimate of transport rates. The mean annual gravel transport into the reach over a 9 year period was estimated to be 36600f 5600m3yr-'. The sediment transport regime along the length of the river is evaluated and examined in relation to peak flood flows. Significant spatial and temporal variability in transport rates is demonstrated, making dubious the generalization of transport estimates from hydraulic calculations, or from sample measurements at a single cross-section. The assumptions, procedures and limitations of the 'morphological approach' to sediment transport analysis are discussed. It is concluded that this approach provides information of quality comparable or superior to that of direct measurements of transport, yet requires less field effort. It also provides additional information about river morphological changes, making it a preferred method for geomorphlogical investigations and for many river management concerns.

Channel change and bed-material transport in the Lower Chetco River, Oregon

“…This type of "morphology-based" approach (Popov, 1962;Martin and Church, 1995) relates volumetric change within a reach to assumptions regarding storage, annual transport lengths, or independent boundary conditions to provide annual estimates of bed-material flux. Morphology-based approaches to estimating sediment budgets have been applied to numerous gravel-bed rivers throughout the world, including many rivers in similar environments as the Chetco River (Collins and Dunne, 1989;Martin and Church, 1995;McLean and Church, 1999;Ham and Church, 2000;Gaeuman and others, 2003;Martin and Ham, 2005;Surian and Cisotto, 2007). In proper settings, this approach has the advantage of (1) being based on actual measurements of observed channel change, (2) being potentially applied for multiple periods and in the absence of flow data, and (3) integrating multiple transport events in determining bed-material fluxes, thereby avoiding the uncertainties in predicting transport from applying strongly nonlinear transport relations to highly variable flows.…”

“…For the Chetco River, most bed material is stored in the bars flanking the low-flow channel, so this analysis focused on estimating changes in bar volume. Estimates of volumetric change are best acquired from repeat high-resolution topographic surveys (Martin and Church, 1995;McLean and Church, 1999), but in the absence of such surveys, they are commonly obtained by mapping planview changes between sequential sets of aerial photographs and estimating the thickness of bed material involved in the mapped changes (Collins and Dunne, 1989;Gaeuman and others, 2003). Short analysis periods are preferable to reduce the negative bias in calculated volumetric change introduced by possible repeated erosion and deposition at the same location by multiple events.…”

“…The simplest situation and the one applied in this study is to assume no gravel transport from the river to the ocean, and to consider the net changes to represent bedmaterial influx rates for the entire lower Chetco River. This approach has been applied to several of the British Columbia studies, in which bed-material fines downstream and the channels transition from gravel to sand bed (Martin and Church, 1995;McLean and Church, 1999;Ham and Church, 2000). This assumption may not apply perfectly to the Chetco River because of the historical presence of isolated gravel bars downstream to FPkm 1.…”

“…Accordingly, the net volume accumulated in the study reach is a minimum indication of bed-material flux at the upstream end of the study reach. A more complete assessment includes the volume removed by gravel extraction (Martin and Church, 1995), thereby implicitly assuming that the mining volumes have been replenished without substantially affecting bar and channel boundaries. This assumption is approximately correct for the Chetco River where recent gravel extraction has been by bar skimming at locations away from the low channel, and repeat surveys show substantial replenishment most years (Ted Freeman, Freeman Rock Inc. and Robert Elayer, Tidewater Contractors Inc., written commun., 2008).…”

“…Accounting for the 62,000 m 3 /yr removed by gravel mining during 2005-07 (the LIDAR topography of 2008 was acquired before that year's gravel mining) results in an estimated total gravel influx into the lower Chetco River of 113,000 m 3 /yr for the 2005-08 analysis period. This value is probably best considered a minimum value as a consequence of (1) the negative biases inherent in the method, especially for periods spanning multiple transport events (Martin and Church, 1995), (2) the assumption that little bed-material leaves the river, and (3) our selection of a thickness scenario that minimizes positive volumes; although incomplete replenishment of mined areas would bias this value positively. Similar calculations for the other two analysis periods with gravel extraction data indicate annual bed-material influxes ranging from 3,000 m 3 /yr during 1995-2000 up to the 45,000 m 3 /yr measured for 2000-05.…”