The Allan Water scoping study report is now available online | Main Report | Appendices a b c d | Drawings

   

Catchment characteristics

The Allan Water drains an area of approximately 210 square kilometers. A range of habitats exist including upland heath, native woodland and raised bog. The rivers flow through wide floodplains and steep gorges and a number of distinct river types exist. As is the case for much of the region, extensive agricultural production, forestry and the construction of urban zones has stimulated many alterations to the landscape which influence biodiversity and the patterns of flooding.

Flooding on the Allan

A broad flat valley, together with steep tributaries along its length, makes the Allan Water particularly prone to flooding. In recent years, floods in the region have become increasingly common, a trend that is predicted to continue under most climate change scenarios. Flooding causes damage to property and infrastructure and disruption to services.

A recent analysis of the flood risk in Bridge of Allan estimated that 340 residential and commercial properties would be at risk from flooding by a 1 in 200 year event with estimated damage in the region of £12.5 million. Currently flood defences are expected to struggle to protect properties from a 1 in 50 year flood.

Biodiversity

Alterations to the course and morphology of the River Allan and its tributaries have changed its character.

These have changed the natural dynamics of the watercourse, in many cases affecting the ecological processes important for sustaining biodiversity.

In places, alterations to the catchment and watercourses have increase the rate at which rainfall reaches the river by increasing the speed of overland flow. This is a common aim of land drainage practises. However this results in higher river levels. Additionally straightening and dredging of channels associated with improvements to agricultural production on floodplains and the construction and upgrade of transport links. Combined, these changes result in fast accumulation of water in low gradient areas or where flow is constricted including both rural and urban areas such as Green Loaning, Dunblane and Bridge of Allan.

Habitat restoration

Opportunities for restoring the natural processes can often bring a range of benefits including reductions in flood risk. These benefits, increasingly referred to as ‘ecosystem services’, are difficult to measure both interms of value and effectiveness. Obvious examples include Salmon fisheries and tourism and Flood management. Others such as health benefits have been shown but are less easy to quantify.

Natural flood management

The value of restoration as part of a flood management strategy is widely proposed and examples exist where it has been implemented as part of a wider sustainable flood management strategy. Natural Flood Management ranges from work in the upper catchment to slow the flow into rivers such as blocking ditches and planting woodlands, to restoration of channel meanders to encourage restore the occurrence of flooding in areas where it is more manageable.

Scope of study

The objectives of the scoping study are:

  • To identify the potential for NFM measures in the Allan Water catchment to reduce the severity of flooding for affected communities
  • To identify measures within the strategy which will improve the ecological status of water bodies within the catchment
  • To build on existing knowledge of NFM and catchment restoration
  • To develop a methodology which is applicable for use in catchments across Scotland
  • To identify how the strategy can be moved from a plan into reality through a phased delivery of practical restoration works

Further information

1. Information for the SEPA gauging station at Bridge of Allan

2. Exploring the Social Impacts of Flood Risk and Flooding in Scotland. Scottish Government. ISBN 978 0 7559 659
3. The Flood Risk Management (Scotland) Act 2009 / The Water Environment and Water Services (Scotland) Act
4. Climate change: flooding occurrences review. Scottish Executive Central Research Unit, 2002
5. Flood Planner: A Manual for the Natural Management of River Floods. WWF Scotland