Estimating fluvial wood discharge using time-lapse photography with varying sampling intervals

Monitoring large wood (LW: width > 10 cm, length > 1 m) in transport within rivers is a necessary next step in the development and refinement of wood budgets and is essential to a better understanding of basin-wide controls and patterns of LW flux and loads. Monitoring LW transport with coarse interval (>= 1 min) time-lapse photography enables the deployment of monitoring cameras at large spatial and long temporal scales. Although less precise than continuous sampling with video, it allows investigators to answer broad questions about basin connectivity, compare drainages and years,and identify transport relationships and thresholds. This paper describes methods to: (i) construct fluvial wood flux curves; (ii) analyze the effects of sample interval lengths on transport estimates; and (iii) estimate total wood loads within a specified time period using coarse-interval time-lapse photography. Applying these methods to the Slave River, a large-volume (10(3) m(3) s(-1)), low-gradient (10(- 2) m km(- 1)) river in the subarctic (60(circle) N), yielded the following results. A threshold relationship for wood mobility was located around 4500 m(3) s(-1). More wood is transported on the rising limb of the hydrograph because wood flux declines rapidly on the falling limb. Five- and ten-minute sampling intervals provided unbiased equal variance estimates of 1 min sampling, whereas 15 min intervals were biased towards underestimation by 5-6%, possibly due to periodicity in wood flux. Total LW loads estimated from the 1 min dataset and adjusted for a 15% misdetection rate from 13 July to 13 August are: 1600 +/- 200 # pieces, 600 +/- 200 m(3) and of the order of 1.3 x 10(5) kg carbon. The total wood load for the entire summer season is probably at least double this estimate because only the second half of the summer was monitored and a large early summer peak freshet was missed. Copyright (c) 2014 John Wiley & Sons, Ltd.