The abundant slackwater-deposit paleostage-indicators (SWD-PSI), particularly the driftwood lines, allow water-surface elevations of the 1911 flood to be known at many more points than are typically available in paleoflood studies on other rivers in the region. Paleoflood discharges typically are calculated using one-dimensional hydraulic models based on Manning-n roughness coefficients (Webb and Jarrett, in press). We measured 30 channel cross-sections spaced at 200-300 m (650-975 ft) intervals over a distance of 8 km (5 MI) that were designed to correspond to a surveyed water-surface profile from the SWD-PSI (Orchard, 2001). Low-water Manning-n coefficients were determined by calibrating a step-backwater model to a water-surface profile measured at a constant discharge of 113 m³/s (4,000 ft³/s). The Manning-n values were extended upwards to the SWD-PSI elevations by correlation with published photographs of channels with known roughness coefficients. Bedrock controls the entire cross section at the upstream end of the study reach, indicating that scour and fill is not significant, and an ancient debris fan and a severe constriction constrain hydraulic conditions to critical depth at the downstream end.
The abundance of SWD-PSI along the San Juan River allow the use of the slope-area method, which ordinarily is not appropriate for estimating discharges for paleofloods (Webb and Jarrett, in press). A particularly well-defined driftwood line near the center of the reach provided sufficient elevations to allow a slope-area estimate using 3 of the 30 cross sections. Using the slope-area method, the discharge of the 1911 flood was estimated to have a discharge of 4,350 m³/s (154,000 ft³/s). The step-backwater analysis used all 30 cross sections and estimated a discharge of approximately 4,200 m³/s (148,000 ft³/s) for the 1911 flood.
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Last modified: Friday, January 11 2013, 02:19:03 AM