Webinar: Woodlands & NFM

Dr Tom Nisbet from Forest Research, working on the Landwise NFM project, presented a webinar today on Woodlands and NFM. Tom went through the base mechanisms of how woodlands can effect flood conveyance.

Reducing the volume of flood water at source by increasing evaporation

The amount of interception annually changes with rainfall, however on average conifers intercept 35% and broadleaf about 17% of rainfall. Tom also mentioned the different soil water storage of different plants such as grass, health, oak and pine.

Slowing the rate of runoff from the land by increasing soil infiltration

The open structure and high organic content of woodland soil aids water infiltration and storage, reducing the risk of rapid surface runoff.

Enhancing floodplain storage and delaying flood peaks by increasing hydraulic roughness

Hydraulic roughness creates a barrier effect, slowing river flows, pushing water onto/across floodplains and temporarily increasing flood storage. Tom referred to Manning’s terms of roughness, in regards to reducing flood runoff.

Reducing sediment loss and downstream siltation

Soil runoff can be reduced by providing physical shelter, reducing water transport that carries soil in runoff, by improving soil strength & stability, and protecting river banks.

Headline finding:

“There is a broad support for the conclusion that increased tree cover in catchments results in decreasing flood peaks, while decreased tree cover results in increasing flood peaks.

While there is strong evidence of an influence on small floods, only a few observational studies have assessed large floods and the majority of these found no influence on peak floods.”

Tom outlined the reasons for mixed results for large floods:

• Challenge of detecting impacts on relatively rare events, e.g. flows exceeding structures
• Lack of experimental studies due to timescale, controlling for background changes, cost etc.
• Scale and location issues
• Role of existing land use and management practices
• Woodland design factor e.g. in terms of type, age, shape and structure
• Woodland management factors including scale and timing of practices such as felling

One example Tom mentioned was Coalburn in Northern England which assessed the impacts of conifer afforestation on peak flows. This was a long-term case study from 1967, with catchment planning in 1973. It looked at the effect of forest growth development over time, on discharge. The study found evidence of a reduction in discharge, however rainfall changes overtime need to be considered and incorporated into these types of studies.

Tom also mentioned the role of Leaky Woody Structures. There is evidence that these can contribute towards reducing flood risk, in particular, the measures installed at Pickering. Modelling showed what flooding would be predicted if woodland measures had not been put in place.

It can be difficult to measure the effect of NFM interventions on flood peak events. Although we can observe effects, it can be difficult to quantify. It is important to monitor these features in order to record if any get washed out by high flows and could lead to blockages.

Moreover, models allow us to assess the impact of woodland on large peak flows, and be adjusted to our understanding of hydrological processes. Tom mentioned how the evidence is strong enough to inform forest policy and practice. We can target communications and assets at significant flood risk, and prioritise catchments where there is most need and scope to make a difference. By joining up strategies and plans we can achieve longer term goals and greater resilience.

Tom finally mentioned how we should take care in how we manage our woodlands, and avoid large-scale felling. We should minimise the fallow period between felling and restocking, and consider the impact of planned forest clearance. Overall, we should follow good practice, such as restoring riparian zones and wetlands.

The final session was a question and answer session. The first question was regarding beavers in NFM. Tom mentioned how these animals can be seen as catchment engineers, and their practices increase the hydraulic roughness element. There is a direct relation to how you would expect beavers to contribute to reducing flood flows.

Another question was regarding local decision making in a world of opportunity maps. Tom mentioned the importance of the balancing act between the type of tree and how it is managed in a particular location.

Thanks to Tom and everyone involved in running this webinar. Great to hear interesting examples and studies about the effect of woodlands on flood risk impacts.