Bankfull Channel Adjustments to Episodic and Long-Term Changes in Sediment Supply
Abstract
The bankfull Shields value is being increasingly used as a metric of equilibrium in gravel-bedded alluvial channels, yet equilibrium Shields values vary systematically with channel slope and grain size. While these systematic shifts are well known and reasonably well quantified, recent field-based work is documenting the varying hydraulic shifts co-occurring with changes in sediment supply. Using data compiled from 341 sites with channel morphology, grain size, slope, and long-term regional erosion rates, Pfeiffer et al. (2017) show that the ratio of bankfull Shields stress to critical Shields stress reflects tectonic setting - and thus long-term sediment supply - such that tectonically active channel reaches have higher ratios of the bankfull Shields to critical Shields. We augment those results with data from the literature that represent monitored dam removal sites (East et al., 2018) and broad regional compilations (Trampush et al., 2014; n =541) along with data from our field and sediment measurements throughout New England to generate ~1,000 field-based reaches having requisite hydraulic geometry data and estimated erosion rates or sediment supply data. Accounting for grain size and slope effects, we show that for given channel slope, bankfull Shields increases with erosion rate, e (mm/yr), as e^0.2. Our extensive data set covers regions with effective erosion rates varying from 5 mm/yr to 3400 mm/yr; over this range of erosion rates, for a given slope, the bankfull Shields number increases by a factor of 3.4. These compiled data demonstrate the dynamic relationship between channel morphology and episodic and long-term increases in sediment supply and further suggest the channel morphologic mechanistic adjustments needed to maintain equilibrium to changes in sediment supply.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP51F2189M
- Keywords:
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- 1824 Geomorphology: general;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY