Bedrock erosion and canyon formation during extreme floods have dramatically altered landscapes on Earth and Mars. Grand Coulee was carved by outburst floods from Pleistocene glacial Lake Missoula and is the largest canyon in the Channeled Scabland, a megaflood‐scoured landscape in the northwestern USA. Quantifying paleo‐discharge is required to understand how landscapes evolve in response to extreme events, but there are few constraints on the magnitude of the floods that incised Grand Coulee; hence, we used hydraulic modeling and geologic evidence to quantify paleo‐flood discharges during different phases of canyon incision. When upper Grand Coulee was incising by headward waterfall retreat, the paleo‐discharge was 2.6 × 106 m3s−1, which produced shear stresses great enough to cause the waterfall to retreat via toppling of basalt columns. The largest possible flood through upper Grand Coulee, a Missoula flood which raised glacial Lake Columbia to a stage of 750 m, produced a modeled discharge of 7.6 × 106 m3s−1. The discharges associated with waterfall retreat and drainage of glacial Lake Columbia are >80% and ∼50% lower, respectively, than the 14–17 × 106 m3s−1discharge predicted by assuming the present‐day topography was inundated to the elevation of high‐water marks. Due to bedrock incision, high‐water marks may overestimate paleo‐flow depth in canyons carved by floods, hence bedrock erosion should be considered when estimating paleo‐discharge in flood‐carved canyons. Our results indicate that outburst floods with discharges and flow depths much lower than those required to inundate high‐water marks are capable of carving deep canyons.
In May 2012, a sediment‐laden flood along the Seti Khola (= river) caused 72 fatalities and widespread devastation for > 40 km in Pokhara, Nepal's second largest city. The flood was the terminal phase of a hazard cascade that likely began with a major rock‐slope collapse in the Annapurna Massif upstream, followed by intermittent ponding of meltwater and subsequent outburst flooding. Similar hazard cascades have been reported in other mountain belts, but peak discharges for these events have rarely been quantified. We use two hydrodynamic models to simulate the extent and geomorphic impacts of the 2012 flood and attempt to reconstruct the likely water discharge linked to even larger medieval sediment pulses. The latter are reported to have deposited several cubic kilometres of sediment in the Pokhara Valley. The process behind these sediment pulses is debated. We traced evidence of aggradation along the Seti Khola during field surveys and from RapidEye satellite images. We use two steady‐state flood models, HEC‐RAS and ANUGA, and high‐resolution topographic data, to constrain the initial flood discharge with the lowest mismatch between observed and predicted flood extents. We explore the physically plausible range of simplified flood scenarios, from meteorological (1000 m3 s−1) to cataclysmic outburst floods (600,000 m3 s−1). We find that the 2012 flood most likely had a peak discharge of 3700 m3 s−1in the upper Seti Khola and attenuated to 500 m3s−1when arriving in Pokhara city. Simulations of larger outburst floods produce extensive backwater effects in tributary valleys that match with the locations of upstream‐dipping medieval‐age slackwater sediments in several tributaries of the Seti Khola. Our findings are consistent with the notion that the medieval sediment pulses were linked to outburst floods with peak discharges of >50,000 m3 s−1, though discharge may have been an order of magnitude higher.
more » « less- NSF-PAR ID:
- 10392432
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 48
- Issue:
- 6
- ISSN:
- 0197-9337
- Page Range / eLocation ID:
- p. 1135-1151
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Profound effects of episodic megafloods (≥106 m3/s) have been observed on Earth and Mars. Quaternary megafloods sourced from valley‐blocking glaciers on the Tibetan Plateau likely play an important role in the geomorphic evolution of the Yarlung‐Tsangpo Gorge and mountain landscape of the eastern Himalaya. We use the first 2D numerical simulation of a megaflood sourced from a reconstructed 81 km3Tibetan lake to analyze flood hydraulics and examine the erosional and depositional potential of megafloods in mountain landscapes. The simulated flood has a duration >60 hr and a peak discharge of 3.1 × 106 m3/s. We find that the extent of inundated features like terraces, narrow valley sections, tight meander bends, and overtopped ridges influences locations of observed maximum depth (370 m), speed (76 m/s), and bed shear stress (>100 kPa), creating dynamic patterns of erosive potential. Consequently, it is difficult to predict local (≤1 km) patterns of megaflood erosional potential from either unit stream power or flood power from smaller magnitude outburst floods. However, both are useful when predicting regional (≥25 km) order‐of‐magnitude shifts in megaflood flood power. Portions of the flood domain downstream of the Gorge experience lower bed shear stresses and flood power <5 kW/m2, indicating potential for significant deposition. We suggest widespread deposition of boulders within the modern channel and fine‐grained particles on hillslopes during a megaflood likely impedes subsequent erosion and affects channel width and longitudinal form throughout the flood pathway. Our findings show the legacy of megaflooding in mountainous terrain includes both extensive erosion and deposition.
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In the mid-ninth century, an earthquake triggered a landslide that blocked the narrow gorge of the Jhelum River where it exits the Kashmir Valley. The landslide impounded a lake that extended ≈100 km along the floor of the valley, implying an impounded volume of ≤21 km 3 , flooding the capital, Srinagar, and much agricultural land. An engineered breach of the landslide was contrived by a Medieval engineer resulting in the catastrophic release of flood waters. Using reasonable assumptions we calculate the probable minimum drainage time of this Medieval flood (<4 days) and maximum downstream surge velocities (≈12 m/s). These would have been sufficient to transport boulders in the bed of the Jhelum with dimensions of ≈6 m, consistent with those currently present in some reaches of the river. Given the morphology of the Jhelum gorge we consider that landslide outburst floods may have been common in Kashmir’s history. Ancient shorelines indicate that paleo-lake volumes in the Kashmir Valley may have exceeded 400 km 3 which, were they released in catastrophic floods, would have been associated with potential downstream outburst velocities >32 m/s, able to transport boulders with dimensions ≈40 m, far in excess of any found in the course of the Jhelum or in the Punjab plains. Their absence suggests that Kashmir’s ancient lakes were not lowered by outburst mechanisms much exceeding those associated with Suyya’s flood. Present-day floods have been many tens of meters shallower than those impounded by landslides in the Jhelum in the past several thousands of years. A challenge for future study will be to date Kashmir’s ancient shorelines to learn how often landslides and major impoundment events may have occurred in the valley.more » « less
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