Working with land managers we have been able to introduce subtle changes to current land management practices in order to slow water flow from the hills, create floodwater storage areas and reconnect the river with its floodplain. So far we have carried out the following on 24 separate farms:
- 209 ha of riparian woodland created, which will help increase rainfall interception, evapotranspiration, soil infiltration and slow overland flow
- 25 km of fencing erected and over 327,000 native trees planted
- 1 km of contour hedges planted, to slow overland flow
- 2.9 km of re-meandered channel created (this has increased river length, reduced the slope and speed of the water flow and provided more space for flood waters, as well as creating new habitats and improving the landscape)
- 2.9 km of flood embankments removed
- 136 ‘high flow restrictors’ installed that will encourage out-of-bank flow and hold back water in the headwaters
- 40 leaky ponds created (29,955 m2) – these wetland features have a good deal of ‘free board’ built in so that they will store water during intense rainfall events
Monitoring the effects of these measures is an important part of this project. A network of rain gauges, groundwater and river level gauges installed throughout the valley collects data on how the changes affect river flows and flood frequencies. Other monitoring programmes reveal what changes occur to the river’s habitats and wildlife such as fish, aquatic invertebrates and vegetation. Detailed monitoring and modelling of the groundwater has also been undertaken at a site close to Eddleston village.
Emerging results for the project show that:
- different NFM measures can reduce flood risk by slowing the flow, temporarily storing surface waters and delaying the peak floods
- appreciable flood risk reduction through NFM is likely only to be achieved by the widespread application of many NFM measures throughout the entire catchment
- NFM measures work best in smaller catchments and in response to lower-level flood events – they will not stop flooding in major events
- NFM and habitat enhancement measures provide a wide range of additional benefits
- the economic value of the flood damages avoided, and multiple benefits of restoration measures can be demonstrated
For each NFM measure the emerging results show that:
Impact of high-flow log structures in headwater streams
- ‘Lag time’ between rainfall and peak river flow has increased by up to 7 hours in headwater catchments up to 25 km² with high-flow restrictors
- In the upper catchment, the 2-year return period flood peak has reduced by ca. 30% post NFM measures
Re-meandering
- re-meandering and reconnecting the channel to the floodplain can locally improve flood storage by up to 6%
- meandering alone, staying within flood banks without temporary storage on the adjacent floodplain is less effective
- new meanders add 8-47% channel length, with increase in extent of potential spawning sites for salmon
- New meander channels have an increased diversity of habitats and are rapidly recolonized by plants and invertebrates
Temporary flood storage ponds
- Ponds in the upper catchment can readily store water, but this is only effective in small sub-catchments
- Modelling the potential impact of large floodplain ponds suggests that they could locally reduce the discharge peak by up to 20% and delay it by up to 6 hours, but such ponds occupy prime farmland
- Creating temporary flood storage ponds leads to significant enhancement of catchment biodiversity, including newts and dragonflies
Woodland planting
- Infiltration of rainfall under mature broadleaf woodland is 5-8 times that under adjacent grazed pasture, with rough grazing also good for infiltration
- Modelled impacts of planting trees at a landscape-scale show up to 40% reduction in high flows and flood peaks delayed by 45 minutes under different climate scenarios
- Recent and ongoing research shows the importance of soil type and location on the effectiveness of woodland planting
In terms of costs and benefits of NFM the results show that NFM measures already implemented show a positive present value (PV) of £950k from flood damages avoided. For a hypothetical maximum scenario this increases to £2,850k (PV) over 100 years. It has also been calculated that the total value of other benefits delivered by NFM across the catchment (£4.2m PV) are 4x larger than flood damages avoided alone. Biodiversity, carbon, water quality, access and recreation are key added NFM benefits.
All of the measures have been introduced in partnership with local landowners, farmers and foresters to ensure that the work can be successfully integrated into their existing businesses. This data is invaluable for Scottish Government and others in assessing the effectiveness of ecological restoration/natural flood management techniques.
You can find out more about the results of our monitoring programme by visiting the Eddleston Water Project database page where we have a wide range of material including project reports, academic papers and project presentations.