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River catchment case study

Example of a drainage basin on a local scale - the River Greta, Cumbria
The market town of Keswick is located immediately north of Derwentwater and is in the Lake District National Park. It is a popular tourist destination in Allerdale District and has a permanent resident population of 48211. The population is greatly increased by several thousand by the tourist trade, with many camping/caravanning sites occupying low lying floodplain areas.
The River Greta flows through Keswick and is fed by the River Glenderamackin, Thirlmere via St. Johns Beck and the Glenderaterra Beck. There are many other smaller rivers that feed into the system and the River basin has a high drainage density.
Below the River Glenderamackin and St. Johns Beck confluence the river becomes relatively confined within a steep incised valley before opening out into the town, where it continues to fall relatively steeply before levelling out in the Greta Bridge area and entering the River Derwent. 2
Lake levels can impede on the drainage of numerous watercourses in the catchment and the land between Derwentwater and Bassenthwaite Lake becomes inundated as a result of large floods. 2

 

Physical Factor 1 – Climate and river regime
The graph below shows the regular river regime for the river Derwent downstream of Keswick averaged over a 30-year period.  It is evident that this river has a peak of discharge throughout the winter and a drop in discharge in the summer.  This links to the rainfall distribution of this part of the UK.  Most rainfall falls in winter due to winter storms and relief rainfall hence the higher flow rates.  Contrast this with the regime during the Storm Desmond event of 2015 (and be careful of the logarithmic scale on the graph).  It is clear that 2015 had high rainfall totals and this resulted in higher than average discharge levels, which resulted in catastrophic flooding for the town of Keswick.

River Derwent Regime
The climate is also mild, as shown on the climate graph below. This limits evapotranspiration across the basin.  On the upland areas snow often falls and rests through the winter.  This temporarily stores water that is then released as snow melt throughout the spring.

Keswick annual climate

Physical factor 2 - Geological influences
The Drainage basin of the river covers a range of different geologies which can influence greatly if water is stored or transferred.  These can be seen on the sketch map below, but in summary;
The Northern part of the basin is dominated by sedimentary rocks such as Mudstone, Siltstone and Sandstone formed approximately 444 to 488 million years ago in the Ordovician Period and interbedded Sandstone with Conglomerate formed 359 to 385 million years ago in the Devonian Period. There are also some small Igneous Intrusions. At the time of sedimentary rock formation the local environment was dominated by deep seas. Most of these rocks are Permeable and porous so allow infiltration.
The Southern part of the drainage basin has Igneous Intrusions (felsic rocks) from the Ordovician to Silurian periods (416 to 488 million years ago) and Igneous Extrusive Rocks such as Lava and Mafic. The local environment at that time previously dominated by eruptions of silica-poor magma.
These igneous rocks tend to be relatively impermeable but some water might be able to get through faults. This limits ground water stores and encourages overland flow.

River Greta Geology map

Physical Factor 3 - Vegetation and relief.
There are only sparse areas of forest in the river Greta drainage basin and many of the lowland areas are used for pasture for sheep and cattle.  The upland areas are dominated by mosses and ferns.  This allows for low interception rates across the drainage basin and lots of surface runoff.  The basin is also very steep sided and has steep relief North and south of the valley in which the town of Keswick sits.  This encourages a lot of surface runoff too.


December 2015 flood event
Causes

December 2015 was the wettest calendar month on record for the UK, with much of northern England receiving double the average December rainfall. This also followed a particularly wet November and as such, much of the ground within the Cumbria catchments was already saturated. 
From the 4th to the 7th of December there was a period of prolonged, intense rainfall caused by Storm Desmond. Over this period, new 24 hour and 48 hour rainfall records were set for the UK. Both of these were within Cumbria and broke the previous records, also within Cumbria, set during the November 2009 floods. The table below shows the record levels of rainfall that fell during the flooding event.

Precipitation records2
This rainfall fell on already saturated ground following three previous storms in November, which generated more than twice the monthly average rainfall for November. The wet conditions exacerbated the runoff from Storm Desmond and produced flood levels on the Rivers Greta and Derwent that were the highest ever recorded, breaking records set during the 2009 floods. The levels of Derwentwater and Bassenthwaite Lake also significantly exceeded previous record lake levels.  2
Many locals blame the overfilling of Thirlmere reservoir for part of the flood damage, as water could not be stored in this lake, but was released.


Effects

  • 515 residential properties and businesses were affected by flooding on the 5th and 6th December 2015. The majority of this flooding can be attributed to extreme river levels in the River Greta, following extensive rainfall over the preceeding 36-hour period. There was a huge amount of flooding both in Keswick and further upstream, with areas of significant erosion at Low Briery and high flood extent at the confluence of St John’s Beck and the River Glenderamackin at New Bridge.
  • All bridges suffered damage to varying degrees. Knights Bridge was destroyed during the flood and Forge Bridge was severely damaged. The bridges contributed to the effects of the flooding by causing a constriction to the channel which affected flows under the structures. This constriction resulted in elevated river levels experienced immediately upstream and greater scour downstream. Elevated floodwater levels had a significant impact on the upstream areas above the Greta, Wivell, Calvert’s and Forge bridges.  2
  • Services were also affected with the mains water supply for parts of the town unavailable until approximately lunchtime on 8th December. Penrith Road, which forms one of the key access routes into the town, became inundated with flood water.  This is all despite the fact that Keswick had improved flood defences put in place in 2012 in a £6m flood protection scheme. 5

 

Sources
1 - UK National Statistics, Census 2011 data.
2 - Cumbria County Council, 2016, Keswick Sec 19 Flood Investigation Report. Accessible here - https://www.cumbria.gov.uk/eLibrary/Content/Internet/544/3887/6729/6730/4271394526.pdf
3 - National River Flow Archive, part of the Centre for Ecology and hydrology, retrieved May 2018 from https://nrfa.ceh.ac.uk/data/search
4 – British Geological Survey, Geology of Britain Viewer, retrieved May 2018, http://mapapps.bgs.ac.uk/geologyofbritain/home.html
5 – BBC news, 2012, Keswick flood defence gates and barriers completed – retrieved November 2018 from https://www.bbc.co.uk/news/uk-england-cumbria-19744736

 

Written by Rob Gamesby, May and November 2018

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