Natural Flood Management: the Role of Forestry. Nadeem W. Shah Thomas R. Nisbet Huw Thomas

Transcription

1 Natural Flood Management: the Role of Forestry Nadeem W. Shah Thomas R. Nisbet Huw Thomas

2 2 Deeside

3 3 Ballater

4 4 Cumbria

5 Land use Recognition that land use affects flooding Need to deal with flooding at the source and the floodplain, and not only the receptor. Natural Flood Management (NFM) alongside hard engineering. Forests can play a significant role in NFM Image credit: Dodson&Flinker

6 SEPA s NFM handbook SEPA s NFM handbook recognises woodland creation as one of the main NFM measures riparian woodlands, floodplain woodlands and catchment woodlands. our improved understanding of how to manage Scotland s diverse forests sustainably, provides an opportunity for woodland creation to play an important role in benefiting the environment. 6

7 Water use of trees Greater water use of trees reduces the volume of flood water at source Larger evaporation or interception of rainwater by conifer canopies - reduce the volume of rainfall to ground by 25-45% per annum, compared to 10-25% for broadleaves less runoff and groundwater recharge Effect reduces for shorter periods of more intense rainfall associated with flash floods - there remains a significant daily interception loss, at least for conifers (6-7 mm/day for conifers vs 1-2 mm/day for broadleaves). 7

8 Storage Greater water use over consecutive days can lead to soil moisture deficit during growing season Additional storage that can help reduce runoff during summer storms Less of an effect in winter due to rewetting of soils. 8

9 Infiltration Tree roots improve soil infiltration and infiltration rates are usually higher under woodland than other land uses such as agriculture. Helps water move downwards contributes to reducing rapid surface runoff and flood generation. Reduced erosion less sediment and nutrient transport.

10 Infiltration Infiltration rates were up to 60 times higher under young native woodland shelterbelts compared to adjacent heavily grazed pasture, with 90% of the improvement occurring within two years of stock removal and tree planting (Carroll et al., 2004). 10

11 Infiltration Infiltration rates low under 6 year old plantation and grazed grassland. Significantly higher infiltration under 48 year old plantation than 6 year old plantation and grassland. Suggests that Scots pine plantations recover root biomass relatively quickly, enhancing infiltration rates. 11 Archer, N. A. L. et al. (2015) Rainfall infiltration and soil hydrological characteristics below ancient forest, planted forest, and grassland in a temperate northern climate. Ecohydrology,

12 Hydraulic Roughness The greater hydraulic roughness exerted by trees, shrubs and large woody debris (LWD) slow down flood flows and enhance flood storage Aquatic environment is sensitive to heavy shading, therefore broadleaves are more appropriate to streamsides. 12

13 Soil protection Reduce erosion, sediment delivery and siltation, increasing conveyance and reducing need for dredging Poor forest management can disturb soil and increase sediment delivery to watercourses controlled by the Water Guidelines and Guidance 13

14 Managing drawbacks Fertiliser use is low but needs to be managed well - guidelines and working with SEPA. Acidification - restrictions on large scale new planting and restocking in worst affected areas Avoid planting in areas where maintaining water flows is an issue, e.g. drinking water catchments 14

15 15 Woodland types

16 Floodplain and riparian Riparian and floodplain planting can be particularly effective at increasing channel and floodplain hydraulic roughness, which can delay flood flows and raise upstream water levels, enhancing flood storage.

17 Farm-scale measures to improve soil infiltration and reduce rapid runoff. Farm woodland

18 Shelterbelt

19 Short Rotation Forestry

20 Upland woodland

21 Productive conifers

22 Riparian zone

23 Interception Image credit: treeutah.org Woodland buffers intercept sediment, nutrients and pesticides before they reach watercourses

24 Interception Image credit: USDA National Agroforestry Centre Woodland canopies can provide a barrier to airborne pollutants such as ammonia and pesticides. Shelterbelts can achieve reductions in spray drift of between 60 to 90% (Ucar and Hall, 2001; Lazzaro et al., 2008). Riparian woodland is efficient at both intercepting aerial drift of pesticides and trapping pesticides bound to sediment in run-off (Lowrance et al. 1984).

25 25 Multiple benefits

26 River Almond

27 Approx buffer widths: East - 100m; West - 290m; South - 30m River Tay

28 Approx 100m buffer width on West side Beaulieu

29 Buffer on East side > 2 km; Habitat connectivity Habitat connectivity

30 Opportunity mapping

31 Woodland for Water Forestry Grants Scheme higher payment rate available to create and manage woodlands for water in target areas of Scotland. Restricted to SEPA s 14 priority catchments. Does not include FRM areas. 31

32 Eddleston Water Project The main aim is to investigate if changes to land use management and the restoration of natural habitats can help improve river ecology and reduce the risk of flooding in Eddleston and Peebles. 32

33 Burnhead Farm (Eddleston) Looking at the effects of riparian plantings on flood flows the contribution to reducing flood risk through natural flood management Monitoring water levels and flows at three sites a fourth (control) site was discontinued. 2 ha of native broadleaves planted in 2011 at around 1600 stems/ha supplemented by some willow planting in

34 34 Slowing the Flow

35 35 Water management

36 Conclusions We need to consider a number of catchment-wide measures to manage flooding, at the appropriate scale and location. There is strong evidence to support woodland creation to help manage flood risk and diffuse water pollution 36