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Physical Reasons for River Flooding

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
Words to use: Sandstone. Rainfall.  Temperature. Flooding. Relief. Flood risk. Through flow. Deforestation.  Rock type.  Evaporation. Quickly. Forests.

Precipitation type, amount and duration is the most obvious reason for river flooding. Long steady prolonged
will produce rivers which rise slowly but can flood, whilst heavy short showers can cause rivers to rise and burst their banks.

Snowfall is another factor to take into account, river levels fall in the UK as precipitation is often stored as snow during cold snaps. However, when
warms and that snow melts many days worth of precipitation can end up in rivers and cause .

The
of the land can also have an impact. Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow. In addition, steep slopes also cause more within the soil. Both can raise river levels. Gentle slopes or flat land allow water to penetrate into the soil and increase lag times.
Vegetation type and coverage plays a big role, with
intercepting more rainfall than grasses. This interception increases lag time and reduces the risk of a flood. Indeed, (the removal of trees) can increase soil erosion, reduce interception and increase . Afforestation, where trees are [planted, can have the opposite effect.

Soil and
can also influence what happens to precipitation when it reaches the ground. Impermeable soils and rocks such as clay or shale do not allow water to infiltrate, this forces water to run off reducing river lag times and increasing flood risk. Permeable rocks allow water to infiltrate into them. If permeable rocks allow water in through cracks, fissures and bedding planes but not through their pores they are said to be pervious (such as limestone). Porous rocks allow water to penetrate into their pores such as .

Temperature can also have an impact as it governs evaporation. Hot days allow for more
, reducing the risk of a flood.