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| Titel |
Assessing the continuity of the upland sediment cascade, fluvial geomorphic
response of an upland river to an extreme flood event: Storm Desmond,
Cumbria, UK. |
| VerfasserIn |
Hannah Joyce, Richard Hardy, Jeff Warburton |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141384
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| Publikation (Nr.) |
 EGU/EGU2017-4891.pdf |
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| Zusammenfassung |
| Hillslope erosion and accelerated lake sedimentation are often viewed as the source and main
storage elements in the upland sediment cascade. However, the continuity of sediment
transfer through intervening valley systems has rarely been evaluated during extreme events.
Storm Desmond (4th – 6th December, 2015) produced record-breaking rainfall
maximums in the UK: 341.4 mm rainfall was recorded in a 24 hour period at Honister
Pass, Western Lake District, and 405 mm of rainfall was recorded in a 38 hour
period at Thirlmere, central Lake District. The storm was the largest in a 150 year
local rainfall series, and exceeded previous new records set in the 2005 and 2009
floods. During this exceptional event, rivers over topped flood defences, and caused
damage to over 257 bridges, flooded over 5000 homes and businesses, and caused
substantial geomorphic change along upland rivers. This research quantifies the
geomorphic and sedimentary response to Storm Desmond along a regulated gravel-bed
river: St John’s Beck. St John’s Beck (length 7.8 km) is a channelised low gradient
river (0.005) downstream of Thirlmere Reservoir, which joins the River Greta, and
flows through Keswick, where major flooding has occurred, before discharging into
Bassenthwaite Lake. St John’s Beck has a history of chronic sediment aggradation,
erosion and reports of historic flooding date back to 1750. During Storm Desmond,
riverbanks were eroded, coarse sediment was deposited across valuable farmland and
access routes were destroyed, including a bridge and footpaths, disrupting local
business. A sediment budget framework has been used to quantify geomorphic
change and sedimentary characteristics of the event along St John’s Beck. The
volume and sediment size distribution of flood deposits, channel bars, tributary
deposits, floodplain scour, riverbank erosion and in-channel bars were measured
directly in the field and converted to mass using local estimates of coarse and fine
sediment bulk densities. During the event 5000 tonnes of sediment was deposited on
floodplains surrounding St John’s Beck; 65% of this sediment was deposited in the
first 3 km of the reach downstream of Thirlmere Reservoir where the channel is
unconfined and channel slope and capacity rapidly decrease. Flood sediment deposits
were composed of a single layer of sediment of a similar grain size distribution
(mean D90 116 mm), with fines generally sparse. The main source of sediment
deposited during the event originated from the channel bed and banks; 1500 tonnes of
sediment was stored within channel bars. Approximately 2000 tonnes of sediment was
eroded from the riverbanks during the event; with local lateral riverbank recession
exceeding 12 m. An estimated 500 tonnes of sediment was scoured from the floodplains
along the first 3 km of the reach downstream of Thirlmere Reservoir, with local
floodplain scour around a bridge estimated at 300 tonnes. Overall, this sediment budget
study demonstrates the importance of valley systems as a major source and sink
of sediment along the upland sediment cascade during an extreme flood event. |
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