Environmentally sensitive and sustainable river management

Overview

Sustainable river management, advantages and disadvantages of washlands and limitations of afforestation as a flood management strategy.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is meant by sustainable river management?

   Model answer

   Managing the river so that flood risk is reduced in a way that can be maintained indefinitely without excessive cost, without damaging the river ecosystem and without passing the problem on to other places or to future generations. It works with natural processes rather than against them - storing and slowing water rather than only speeding its removal - and it accepts that some flooding of the floodplain is natural and should be accommodated in the right places rather than prevented everywhere.

2. 2

   What is a washland?

   Model answer

   A washland is an area of the floodplain that is deliberately set aside to be flooded when the river exceeds a certain discharge. It is kept free of housing and permanent structures and is used instead for rough grazing, hay meadow, playing fields, wetland nature reserve or informal recreation, so that when it floods the damage is negligible. Water may be allowed to spill onto it naturally over a lowered bank or be admitted through a controlled sluice.

3. 3

   How does a washland reduce flooding downstream?

   Model answer

   It provides temporary storage that takes water out of the channel exactly when discharge is at its highest. Diverting part of the flood peak into the washland lowers the volume passing downstream at that moment, so the downstream peak discharge is reduced and, because the stored water is released slowly afterwards, it is also delayed and spread over a longer period. On a storm hydrograph the effect is to flatten and broaden the peak. Several washlands along a catchment can attenuate a flood wave substantially.

4. 4

   What are the advantages and disadvantages of washlands?

   Model answer

   The advantages are that they are cheap to create and to maintain compared with engineered defences, they store rather than transfer the water so downstream communities benefit too, they create valuable wetland habitat for wading birds and aquatic species, the deposited silt maintains soil fertility, and they can provide public open space. The disadvantages are that they need a large area of floodplain, which may not exist in a built-up catchment, that land is taken out of intensive agricultural or development use and owners must be compensated, that farming on them is restricted to low-value uses and timing is uncertain, and that in a very large event the storage fills and can offer no further protection.

5. 5

   How does afforestation in the upper catchment reduce flood risk?

   Model answer

   Trees intervene at several points in the drainage basin system. The canopy intercepts rainfall so that some of it evaporates without ever reaching the ground, and the rest is delayed as it reaches the soil by throughfall and stemflow. Transpiration continuously draws water out of the soil, keeping soil moisture storage available to absorb the next storm. Roots create channels that raise infiltration capacity, so water enters the soil and travels slowly as throughflow instead of rapidly as overland flow, and the litter layer physically obstructs any surface flow. Roots also bind the soil, reducing erosion and so limiting the sediment that would otherwise silt up the channel downstream. The overall effect is a longer lag time and a lower peak discharge.

6. 6

   What are the limitations of afforestation as a flood management strategy?

   Model answer

   It takes decades to become fully effective, since young trees intercept and transpire very little, so it does nothing about the risk facing a community today. It requires large areas of upland to be taken out of farming, which affects rural livelihoods and requires compensation or subsidy. Dense conifer plantations can acidify soils and streams, support little biodiversity and look uniform, and the ploughing and drainage used to establish them can actually increase runoff in the early years, as can clear-felling later. Most importantly, its effect is greatest on small and moderate storms; in a prolonged, extreme event the soil beneath the trees becomes saturated too, and the reduction in peak discharge is much smaller.

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