• Revision:River Derwent Case Study


River Derwent is located in North Yorkshire, which is situated in north east England. March 1999, River Derwent and its tributaries experienced their worst floods in 100 years. It was the only area in UK to suffer flood problems of at the time.

Causes of the flood:

 North Yorkshire received a large amount of rainfall from a low pressure area over the North Sea. Some of this rain fell on to snow causing it to melt.

 At Church House on the North York Moors, 229 mm of rain fell between 1 and 9 March 1999, this is more than three times the average rainfall for the whole of March which is 75 mm.

 Run-off from the North York Moors was very high, however, some of the water from the Upper Derwent was diverted along the Scarborough Cut and out to sea, overland flow from the saturated catchment soon caused the river to rise downstream of the cut.

 The river collected water from other tributaries in addition to the rainfall, causing river levels to rise 4.38 m above its normal level.

Effects of the flood:

 Many roads in the area were flooded, causing railway lines to close.

 198 homes were flooded, 103 of them at Norton were worst affected settlements.

 Low-lying agricultural land throughout the floodplain was flooded, although this helped to reduce flood peaks downstream- acted as water storage zones.

Environment Agency:

 Worked 24 hours a day during the flood.

 First warnings of an expected flood even were given to North Yorkshire County Council on 5th march 1999 based on rainfall predictions.

 9 formal flood warnings were issued to the public by media, the emergency services, the AA, the local authority and the Environment Agency itself.

 Provided help to operate pumps, supplied 13,000 sand bags to local authorities and monitored river levels.

How did the features of the River Derwent drainage basin contribute to the March 1999 flood?

The river discharge was high due to the steep slopes, and the bedrock was made up of mainly shale thus water ran quickly into rivers. The Sea Cut was dug to reduce the amount of water in the Upper Derwent, therefore the water could not be contained and reached 51m3/sec in march from the normal flow of 1m3/sec.

The flood hydrograph of the selected rivers during the March 1999 flood shows that the Buttercrambe discharged the most amount of water, with levels reaching 160m3/sec on the 9th of March.

Explain why people believed that they were not at risk of flooding?

 People believed that’s they were not at risks of flood because an extreme flood did not occur in such a long time.

 Some people may not have had any knowledge of the altitude of land that they live on, thus may have believed that they were in no risk of flood water reaching them.

Suggest why so many people did not receive a flood warning?

 The flood warnings may not have been formal

 People may have not understood or heard the flood warnings

 Flood warnings may have not reached some people due to power failure, thus they did not receive any alert through media or automatic voice message over the telephone.

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