Geomorphic Response Of Fall River To The 2013 Flood
Schutte, Mark 1
1 Olsson Associates
In Rocky Mountain National Park, the 2013 flood destabilized segments of Roaring River and deposited an unusually large amount of sand- and gravel-sized sediment near the confluence with Fall River. We initiated field studies of these two rivers in May 2014 to investigate the geomorphic response of Fall River to an increase in sediment supply. Measurements of water discharge and bed load were taken from May through August at three different locations to capture variations in sediment transport rates.
Peak transport rates coincided with the peak discharge at the upstream sampling site (FR 1), but lagged behind the peak in discharge at the lower site (FR 2) by about three weeks, which is consistent with diffusive movement of sediment as observed in earlier studies. On average, 2014 transport rates were 0.015 kg/m/s and 0.035 kg/m/s at FR-1 and FR-2, respectively. Bankfull Shields stress calculations showed a nearly constant value across the study area, about 1.56 A combination of changing slope, depth, and grain size throughout the study area resulted in the same estimated value of reference Shields stress at each site, 0.028.
Annual sediment loads estimated at each site, as well as erosion and deposition tracked by comparing cross sections measurements, both indicate a majority of the sediment deposited by the flood was eroded and transported out of the Fall River channel in 2014. Annual sediment loads were estimated from discharge- and shear stress-based empirical relations and a time series of discharge scaled to the continuous discharge of the Big Thompson River from a nearby USGS gage in Moraine Park. Annual loads estimated at FR 1 and FR 2 were about 3,000 Mg and 4,600 Mg, respectively. Relative to the estimates following the 1982 Lawn Lake flood, annual loads were lower than the three years immediately following the flood, but comparable to the loads estimated four to five years after the flood.
Analysis of the results of the field data collected indicate that Fall River experienced some erosion in 2014, transporting sediment which had been stored in its channel from the flood. Fall River was primarily able to accommodate the increase in sediment supply by adjusting its bed texture, and secondarily due well-vegetated banks which prevented channel widening. Annual sediment loads estimated from all approaches indicate that Fall River was able to transport a majority of the sediment supplied to it in 2014.
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