CORE UNIT: PATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT
2. Improve flood defences The OPW proposed to widen and deepen the river, build flood defence walls and build earthen flood defence banks (levees) to prevent flood water reaching the flood plain. A plan has been put in place and is being developed.
Fig. 51 Old Bridge weir covered during flood 8.7 River rejuvenation
Landforms represent a balance between the action of internal and surface processes. The balance may change over time.
Rejuvenation means ‘to make young again’. For a river this means it will begin to flow faster with renewed energy to reach sea level (known as a river’s base level), even though it has flowed through its entire course. Rejuvenation happens when the base level either rises or falls. This can happen in two ways.
1. Sea levels may drop during ice ages when water is locked away as ice, causing a lowering of the sea level across large areas. When ice melts, sea levels rise over large areas (called eustatic sea level changes).
2. The land may rise out of the sea due to tectonic activity raising the crust (called isostatic uplift). Isostatic uplift also occurs when a great weight is removed from the land (e.g. melting of an ice cap). The land gradually rises in response to the removal of the weight, much like a block of wood rises out of water when a weight holding it down is removed. This happens because the crust floats on the mantle below.
In both cases there is a drop in the sea level and the river’s mouth is now high above it. This steeper gradient to the new base level gives the river extra energy and allows the river to renew the process of vertical erosion. Erosional processes start to reshape the landscape in an attempt to form a new long profile.
The following distinctive landforms show rejuvenation has occurred: