Natural Flood management
NATURAL FLOOD MANAGEMENT
NSEPA describe Natural flood management as a technique that aims to work with natural hydrological and morphological processes, features and characteristics to manage the sources and pathways of flood waters. These techniques include the restoration, enhancement and alteration of natural features and characteristics.Natural flood management describes a range of techniques and through promoting changes in land management, restoring habitats and natural processes to the River Annan, the RAW project will attempt to implement and assess their success.
Many river channels within the Annan Catchment have been historically straightened or canalised as part of flood defences and land drainage. The effect of this is to reduce the length of the channel to increase the efficiency of the channel in moving water. This often results in flooding and an increase in sediment build up downstream. Re-meandering river channels attempts to restore a river channel to its natural shape, increasing its length and slowing the energy. This decreases the flow of water, effectively storing water in the river channel. This can reduce the risk of flooding further downstream. The new form of the river channel creates a variety of flow conditions which have a positive impact on sedimentation and biodiversity while providing habitats for a range of aquatic and land species of flora and fauna. Re-meandering straightened channels can help deliver some of the objective of the Water Frame Work Directive by increasing morphological and flow diversity in a straightened channel, creating natural conditions that provide better habitats for macrophytes and benthic invertebrates and improving habitats for fish.
Left a straightened part of the Annan water as it runs towards Moffat. Straight sections discharge rapidly, increasing river erosion and sediment build up.
Right a more natural river channel, the Water of Milk, another of the River Annans tributaries. Meanders reduce energy in stream, reducing erosion and downstream transport of materials.
Riparian Fencing & Native Tree Planting
Riparian strips of native trees and herbaceous vegetation cover protected from livestock grazing, natural drainage through well-structured soil, areas retained as wetland – all of these will help to intercept rainfall and aid absorption into the ground from where it will be released more slowly into rivers. As a result, flows are ‘buffered’; peaks are lower during floods, and retained at a higher level during droughts. Soil and nutrients are better retained in the catchment. The result is a much healthier river and with lower flood risk too (Wild Trout Trust).
Cattle trampling causes compaction of the soil, while grazing allows only shallow routed plants to grow, sealing the ground causing rain water to run straight into the river causing it to rise quickly. Excluding cattle though riparian fencing will allow the soil to absorb more water. The effect is magnified by tree roots. This video by the Wild Trout Trust demonstrates the benefit of riparian fencing.
SEPA have produced a Natural Flood Management Handbook which provides practical guidance on implementation of such schemes.