Case Study: River Exe Flashcards
Which part of the River Exe forms the case study area?
The upper catchment - which lies above the gauging station at Thoverton.
What is the total annual precipitation input?
1295mm – this is relatively high, particularly over Exmoor.
What proportion of the precipitation input leaves the catchment area as discharge?
Approximately 2/3s or 65%. This is above average for a river
What is the water balance for the River Exe? Give figures.
Precipitation (1295mm) = evaporation +/- storage (451mm) + runoff (844mm)
Spearman rank correlation coefficient can be used to test the relationship between monthly precipitation and discharge. Outline the findings.
The Rs value was 0.36 suggesting a weak positive correlation. However, at 10 degrees of freedom the confidence level was well below 5%, indicating that the result wasn’t statistically significant and could therefore have occurred by chance.
Identify and explain a physical factor that accounts for the relatively high runoff.
Geology - most (85%) of the upper catchment is underlain by impermeable bedrock (Devonian sandstones). This means that water cannot percolate so ground water storage is very limited. The peaty moorland soils quickly become saturated, so rainfall becomes surface runoff.
Describe the main land use in the upper catchment (include percentages).
• 67% agricultural grazing - grassland
• 15% woodland
• 3% moors and peat bog
Explain how the land use affects the water balance.
• The low % of woodland means less interception and less evapotranspiration so more runoff.
• Grassland transpires and intercepts less than woodland and is kept short by grazing sheep and cattle who also trample soil, so a high % of rainfall runsoff.
How many houses in the Exe catchment are at risk of a 1 in 100-year flood? Where are they located?
11,000, or approximately 10%
Mostly in towns in the lower catchment e.g., Exeter and Exmouth.
What is meant by the regime of a river?
Describe the regime of the River Exe.
Regime = The pattern of a river’s discharge over the course of a year. Data from the gauging station at Thoverton shows that the River Exe is flashy but less flashy than might be expected. There is a clear response to rainfall events (shown by a steeper rising limb), but this is not sudden, and discharge recedes relatively slowly afterwards (gently sloping falling limb).
Outline the key physical features of the drainage basin that contribute to the relative flashiness of the regime (shown by a relatively short lag time), which increases flood risk.
• Geology – upper catchment underlain by impermeable rock (85% Devonian sandstone), so precipitation runs off the surface into rivers rather than percolating to become groundwater (shortens lag time).
• High drainage density with an extensive network of tributaries – transferring rainfall quickly downstream (shortening lag time).
Identify the key physical feature of the drainage basin that helps explain why the regime is less flashy than expected.
• The rural nature of the upper catchment (82% grassland or woodland) – vegetation intercepts and slows transfer of water to rivers, it also encourages infiltration and soil moisture storage (lengthening lag time).
Identify the key human activities in the past that have contributed to the relative flashiness of the regime, which increases floor risk.
• Drainage ditches dug in the peat on Exmoor to make the land easier to use for grazing.
• Replacement of woodland with grazing land for livestock.
Explain the impact of drainage ditches on Exmoor on the regime of the river.
Drainage ditched channel precipitation more quickly off the moorland and into rivers. This reduces potential infiltration, so less water is stored on the moorland, and more is transferred downstream via channel flow, increasing the flood risk for properties in the lower lying lower catchment.
Identify a human feature on the Exe and explain how it makes the river is less flashy than expected.
The Wimbleball Reservoir (built in 1979) on the River Haddeo, an upland tributary of the Exe, regulates water flow, ensuring a steady regime throughout the year. It prevents peaks and troughs that might cause flooding or drought and is a significant water store.
