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Managing Floods And Delivering Benefits To The Water Environment

Congress: 2015
Author(s): Tim Ellingham (Peterborough, UK)

Tim Ellingham



Keyword(s): Sub-theme 11: Key vulnerabilities and security risks,
Oral:
Abstract

Introduction
Moray Flood Alleviation is a £170m programme of 5 schemes providing protection to 4 communities in Morayshire, Scotland. The schemes have been delivered for and within built communities recognising a number of important historic and natural interests. Throughout the design and construction of these schemes, in addition to reducing flood risk, wider benefits to the water environment have been delivered. These benefits, which are described in this paper, are:
* sustainable solutions working with natural processes;
* optimising solutions for high and low flows (floods and droughts);
* habitat improvement and diversity;
* enhanced use and environment for amenity and quality of life; and
* managing the groundwater impact (contamination and recharge).

The Schemes
Moray Flood Alleviation consists of 5 schemes that reduce flood risk to 2000 properties in the communities of Elgin, Forres, Lhanbryde and Rothes. The schemes differ significantly in both their scale and solution. The community of Lhanbryde is protected by a small flood storage reservoir whereas Elgin benefits from a £86m scheme that through setting back defences creates a blue corridor providing attenuation for flood waters. Forres benefits from two schemes that provide protection from the Burn of Mosset and the River Findhorn. The Burn of Mosset scheme was delivered for £21M and is principally a flood storage reservoir that includes an innovative baffle crump weir within the control structure that optimises the storage by passing a consistent flow under different heads of water. The £45m Findhorn scheme that involves a significant increase in channel capacity of the river also includes measures to address local flooding in the Pilmuir Area. At £22M the Rothes scheme addresses flood risk from three watercourses through containment defences within the town.

It is important to consider that the operation of this infrastructure is not a common occurrence and furthermore when it is required the schemes may be operating for a period of only 2 or 3 days. The watercourses are therefore in their 'normal' state for a significant proportion of the time and it is during these periods that the wider water environment benefits are important.

Wider Benefits
The schemes mitigate environmental impacts and raise the environmental quality of the watercourses to comply with legislation such as the Water Framework Directive. On the Burn of Mosset the innovative Burn Management Works have created an area of wet woodland. By dividing the channel upstream, creating environmental ponds and planting wetland trees new habitats were created. The schemes have been designed considering fish spawning and fish migration, for example the Rothes Scheme includes three fish passes.

The schemes have been designed so that they work with natural processes within the river basin and restore the watercourses to a more natural profile. Geomorphological understanding has been a key aspect of the schemes, particularly for the River Findhorn where the scheme has involved removal of significant quantities of bed material.

Recognising the proximity of the watercourses to urban areas there has been a significant effort made to integrate recreational and amenity value into the schemes. Increasing access to the river corridor and creating infrastructure that benefits the community have been at the heart of the design process.

Due to previous land uses along the riparian corridor a number of the sites were found to be contaminated. Significant work was undertaken to understand the levels of contamination and groundwater flow regime. These investigations enabled the conceptual understanding of the sites and risk assessment models to be created. For the Burn of Mosset an assessment of risks to the water environment from contamination present in deep groundwater were undertaken. The results showed that the groundwater contamination did not present unacceptable risks during the construction of the scheme and that the scheme itself would not increase the contamination risk to the aquifer at depth beneath the site.

Results & Discussions
Since the first scheme was completed nearly ten years ago the schemes have protected the communities during a series of flood events. Over this period, other aspects of the schemes designed to enhance the water environment have evolved during high and normal flows. These have improved habitats, managed groundwater risks, increased the amenity value and shaped the channels beyond their design profiles. For example the event in August 2014 significantly changes occurred to the bed profile of the River Findhorn.

The five schemes in Moray demonstrate how the delivery of flood protection can lead to benefits to the wider water environment. The lesson that can be applied elsewhere is that although there is often a primary driver for infrastructure there are a number of secondary benefits that can be delivered alongside.

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