1. www.hoddereducation.co.uk/geographyreview Water-cycle management Assessing the value of ‘soft’ management strategies Simon Oakes Hodder & Stoughton © 2016

2. Water-cycle management This presentation supports the article by Stuart Lane in G EOGRAPHY R EVIEW Vol. 30, No. 1, pages 35–39 which looks at the effectiveness of natural flood management in a catchment. Hodder & Stoughton © 2016

3. Key terms Drainage basin water cycle The drainage basin is a subsystem within the global hydrological cycle. It is an open system as it has external inputs and outputs that cause the amount of water in the basin to vary over time. Bankfull dischargeDischarge is the rate of flow of the river in cumecs. Bankfull discharge is the maximum rate of flow prior to the overtopping of the river bank and the inundation of the flood plain with water. Natural flood management, or soft management An alternative to hard flood defences. It includes a wide range of measures involving land-use practices, storing rainfall in the landscape and slowing river flow. It also has ecological benefits and can reduce sediment erosion. Hodder & Stoughton © 2016 Do you know what these terms mean? Click to reveal the definitions Do you know what these terms mean? Click to reveal the definitions

4. The water cycle The drainage-basin water cycle is an open system. Inputs of precipitation eventually exit the basin as outputs of river discharge or as evaporation and transpiration. Overland flow and throughflow are key processes that transfer water through the system. They are complex processes which can affect river basin populations in adverse ways. The focus of this presentation is ways of reducing flooding that results from the inability of the river channel to store all the arriving runoff during a rainfall event. Hodder & Stoughton © 2016

5. The storm event During a storm event, large amounts of overland flow can lead to a rapid rise in discharge. If bankfull discharge is exceeded in a populated area, the resulting flood can threaten lives and property During a storm event, large amounts of overland flow can lead to a rapid rise in discharge. If bankfull discharge is exceeded in a populated area, the resulting flood can threaten lives and property Hodder & Stoughton © 2016

6. Floodplain actions: an overview Hodder & Stoughton © 2016 Embankments raise the level of the river and increase channel storage capacity. This means more runoff can enter the river channel before bankfull discharge is reached This low-value rural land has been used to create an off-line flood storage zone on the River Tone in Somerset. By allowing the river to flood naturally in some areas, water can be temporarily stored on the floodplain. This reduces the flood risk for areas further downstream This low-value rural land has been used to create an off-line flood storage zone on the River Tone in Somerset. By allowing the river to flood naturally in some areas, water can be temporarily stored on the floodplain. This reduces the flood risk for areas further downstream Because the river meanders naturally across its floodplain, the length of the river is greater than would be the case for a straight channel. For this reason, it may be unwise to straighten a river channel. Although its gradient is increased, the net effect could be an increased flood risk Robert Harding/ Alamy

7. Natural flood management Natural flood management (NFM) is a type of catchment-based flood management (CBFM). In addition to enhanced floodplain storage, more ‘soft’ measures are introduced upstream to reduce flooding downstream. Particular attention is paid to how land use management changes can help reduce runoff. Central to NFM is the notion of attenuation. This means reducing the size of the peak flood discharge by trying to reduce the amount of runoff that enters the river channel in a short period of time. Soft measures often have other benefits. They may reduce soil erosion or they may benefit the environment. These kinds of benefits are often overlooked because they can’t be quantified in in monetary terms, unlike the avoidance of property damage caused by flooding. The following series of slides provides an assessment of the value of several soft NFM approaches. Hodder & Stoughton © 2016

8. Soft NFM techniques (1) Hodder & Stoughton © 2016 Floodplain storage Water falling during a rainstorm may be stored so that it does not contribute to downstream flooding. Storage might be in small ponds and ditches or in larger areas such as washlands. Changing arable farming techniques Farming that leaves the soil surface bare for long periods after crops are harvested may increase runoff during storm events. Changing the land use to grassland for cattle or recreation can encourage infiltration into the soil. Assessment: The storage is only effective if it is empty at the right time during the rainstorm. If it has been filled early on, it won’t reduce the discharge peak. Assessment: The storage is only effective if it is empty at the right time during the rainstorm. If it has been filled early on, it won’t reduce the discharge peak. Assessment: Test cases in the South Downs show that replacement with grassland has reduced local flood risk in some cases. But many famers will resist change because they have an established business. Assessment: Test cases in the South Downs show that replacement with grassland has reduced local flood risk in some cases. But many famers will resist change because they have an established business.

9. Soft NFM techniques (2) Reducing livestock density Cattle above certain densities can lead to soil compaction. Grazing can reduce biomass and reduce water loss to evapotranspiration. Both of these can increase runoff. Basically, sheep can cause floods. Buffer strips and zones Buffer strips or zones, such as woodland beside rivers, may have high infiltration rates and may help to slow runoff entering rivers. Assessment: Small-scale studies confirm livestock can affect flooding. However, it is possible some of the areas studied would flood irrespective of cattle, especially low relief areas on floodplains. Changing rainfall patterns may mean flooding will increase even if livestock numbers are reduced. Assessment: Small-scale studies confirm livestock can affect flooding. However, it is possible some of the areas studied would flood irrespective of cattle, especially low relief areas on floodplains. Changing rainfall patterns may mean flooding will increase even if livestock numbers are reduced. Assessment: Their effects on catchment- scale flood risk have not been demonstrated and so cannot be adequately assessed yet. Assessment: Their effects on catchment- scale flood risk have not been demonstrated and so cannot be adequately assessed yet. Hodder & Stoughton © 2016

10. Soft NFM techniques (3) Hodder & Stoughton © 2016 Improved field drainage Field drains (networks of drainage ditches) dry out the land surface more quickly, in theory increasing water storage when storms are approaching in fast succession: antecedent conditions become less of an issue. This may delay the onset of rapid runoff and a flashy hydrograph. Afforestation Trees may increase water loss by evapotranspiration and increase infiltration through their root networks. Both processes reduce overland flow. Assessment: Studies, notably in peaty soils, have confirmed that drainage does dry out the land surface. But this effect may be lost after long periods of prolonged rainfall, which is when flooding is most likely. Assessment: Studies, notably in peaty soils, have confirmed that drainage does dry out the land surface. But this effect may be lost after long periods of prolonged rainfall, which is when flooding is most likely. Assessment: Afforestation is a major land- use change which many people will object to, for a range of economic and environmental reasons. Assessment: Afforestation is a major land- use change which many people will object to, for a range of economic and environmental reasons.

11. Arriving at a judgement 1.Having reviewed all the possible measures, what were their strengths and weaknesses? 2.What, if any, recommendations would you make if you were put in charge of attempts to manage flood risk along a river? 3.Can soft approaches ever be a substitute for hard engineering when it comes to providing proper security and risk management for home owners? Hodder & Stoughton © 2016 In particular, according to Professor Stuart Lane, there is the problem of scale to consider. Many of the techniques described have been piloted or demonstrated only at small scales (catchments up to tens of square kilometres). They have reduced localised flooding but their impacts at larger scales have not been demonstrated. Homeowners along major rivers remain anxious about flooding; many believe hard engineering such as dams and river diversions are the only way to safeguard their homes reliably.

12. Hodder & Stoughton © 2016 This resource is part of G EOGRAPHY R EVIEW, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to: http://www.hoddereducation.co.uk/geographyreviewhttp://www.hoddereducation.co.uk/geographyreview