Elevated post-event sediment yield extends the impact of catastrophic flooding
Abstract
The long-term effects of catastrophic flooding remain poorly known, due to 1) the rarity of high-magnitude events, 2) geomorphological tools insufficient to detect change with adequate spatial resolution and coverage, and 3) studies too short in duration to capture post-flood river dynamics. We address these shortcomings using a combination of traditional and cutting-edge tools—channel cross-section surveys, suspended sediment sampling, and multi-temporal airborne and terrestrial lidar—to study rivers in New England affected by a >100-yr recurrence interval flood. In particular, we seek to answer the research question: in the years after a flood, how do transient processes—i.e., hillslope revegetation, changing hillslope-channel connectivity, sediment-source exhaustion—contribute to the overall impact of the event? A suite of measurements taken in a 7-yr post-flood study indicates a lack of recovery in sediment delivery and transport. Detailed annual terrestrial lidar surveys of 15 landslides show that these landslides are eroding a depth of 5 - 20 cm/yr (avg. 14 +/- 5 cm/yr std. error) with no sign of slowing. These results align with regional airborne lidar analyses that indicate similar erosion rates (avg. 9 cm/yr +/- 0.1 cm/yr std. error) on 83 landslides in the region. The landslides in both these surveys are a statistically representative sample of an inventory of 995 flood-generated landslides mapped following the event. When measured erosion rates are applied to the landslide inventory, the post-flood erosion is equal to >60% of the flood contribution, or 15 - 30 years of regional background sediment yield, despite the landslides comprising <0.1% of the total land area. Elevated landslide erosion rates have produced suspended sediment concentrations an order-of-magnitude higher than pre-flood for equivalent discharges, as well as rapidly changing channel geometries and bed sediment compositions. This study illustrates the importance of considering transient post-flood recovery, or lack thereof, which may be a critical contributor to total event impacts.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP22A..06D
- Keywords:
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- 1815 Erosion;
- HYDROLOGYDE: 1817 Extreme events;
- HYDROLOGYDE: 1824 Geomorphology: general;
- HYDROLOGYDE: 1861 Sedimentation;
- HYDROLOGY