Allan Water Natural Flood Management Techniques and Scoping Study Document: R01 Version: 2 Scottish Environment Protection Agency September 2011 Allan Water Natural Flood Management Techniques and Scoping Study Scoping Report Scottish Environment Protection Agency September 2011 Halcrow Group Limited 16 Abercromby Place, Edinburgh EH3 6LB tel 0131 272 3300 fax 0131 272 3301 halcrow.com Halcrow Group Limited and CRESS have prepared this report in accordance with the instructions of client Scottish Environment Protection Agency for the client’s sole and specific use. Any other persons who use any information contained herein do so at their own risk. © Halcrow Group Limited 2011 Allan Water Natural Flood Management Techniques and Scoping Study Document history Allan Water Natural Flood Management Techniques and Scoping Study Scottish Environment Protection Agency This document has been issued and amended as follows: Version Date Description Created by Verified by Approved by 0.1 09/05/11 Interim draft to inform steering Neil Nutt J Drake J Drake group and assist pilot project Charles Perfect selection process. 0.9 23/06/11 Interim draft of first issue ahead Neil Nutt J Drake J Drake of June steering group meeting Charles Perfect 1.0 29/06/11 First issue, for steering group Neil Nutt J Drake J Drake comment Charles Perfect 2.0 06/09/11 Final issue Neil Nutt J Drake J Drake Charles Perfect Allan Water Natural Flood Management Techniques and Scoping Study Executive Summary The Allan Water is a tributary of the River Forth and has a catchment area of approximately 216km 2. The catchment is predominately upland with notable areas of improved agricultural land, moorland and forestry. In the lower reaches the Allan Water passes through the towns of Dunblane, Bridge of Allan and a number of villages. Parts of these towns and villages are thought to be at flood risk as highlighted by flooding in 2006. As part of this study which has been led by SEPA, Halcrow-CRESS have investigated the factors which would influence the implementation of natural flood management measures within the Allan Water catchment with a view to the development of a natural flood management strategy for the catchment. The development of the project has been overseen by a Steering Group with representatives from: • Forestry Commission Scotland & Forest Research • Perth & Kinross Council • RSPB • Scottish Government • SEPA • SNH • Stirling Council A comprehensive catchment reconnaissance survey has been undertaken to provide a detailed baseline of the ecological and geomorphological conditions within the catchment. This initial data collection has also included meetings with land managers to gather their experiences of how the catchment operates in flood events but also to get insights into how they manage the land and what they would like to see in an eventual natural flood management strategy. The consultation process has also been extended to include a website and mail drop to the identified landowners. A broad scale catchment restoration strategy has been developed for the Allan Water catchment based on the findings of the catchment reconnaissance and a desk based GIS analysis. This strategy identifies priority areas for catchment restoration activities such as landuse change, reach restoration and flood plain reconnection. The strategy is intended to form the basis of guidance which can be provided to land managers, Local Authorities, community groups and charities. A detailed option appraisal process has been undertaken to identify which natural flood management measures will be most effective within the Allan Water catchment. This option appraisal has been conducted following the principles of Flood and Coastal Erosion Risk Management – Appraisal Guidance (Environment Agency, 2010) which forms current flood risk management best practice. The option appraisal was divided into three phases starting with a broad scale judgement based Allan Water Natural Flood Management Techniques and Scoping Study assessment of 19 options across the catchment and culminating in a detailed quantitative assessment of the four most favourable options. The hydrological benefits of each option have been tested using a specially developed methodology based on the Soil Conservation Service’s Runoff Curve Number hydrological method as presented in TR-55 (USDA, 1986). This methodology has allowed a detailed insight into how the catchment operates in flood events and includes provision to assess the benefits of typical natural flood management measures such as landuse changes and channel restoration. The final stage of the option appraisal process identified that, as could be anticipated, none of the considered measures have a robust case if only the economics of flood mitigation are considered. However on consideration of the wider social, environmental and economic context in which the measures would sit, the case for the implementation of natural flood management is much improved. A summary of the ranking process undertaken in the final phase of the option appraisal process is shown in Table 1. Option Environmental and Economic Overall ranking social benefits ranking ranking 1. Muckle 3 4 4 Burn headwaters 2. Danny 4 1 3 Burn headwaters 8. River 2 2 2 Knaik riparian corridor 17. Allan 1 3 1 reduced maintenance Table 1: Summary of the social, environmental and economic benefits and disbenefits of the considered options. Ranking, 1 is highest and 4 is lowest. It was decided by the Steering Group that the restoration of the River Knaik riparian corridor should be adopted as the pilot project for the following reasons: • Local perception is that the River Knaik is the dominating contributor to flood risk within the Allan Water catchment. This perception has been supported by the hydrological analysis undertaken as part of this study; • Early consultation with the main landowners has identified that the landowners are amenable to improvements to the riparian corridor; Allan Water Natural Flood Management Techniques and Scoping Study • The riparian planting would be consistent with Local Biodiveristy Action Plans and the Forestry Commissions Potential Native Woodland Network. These consistencies would improve the prospect of securing funding; • It is a sub-catchment scale project; • The River Knaik and its riparian corridor is visible from the B827 therefore offering increased public exposure; • The River Knaik contains good riparian reaches alongside poor reaches that will be valuable for demonstrating what is being proposed to land managers and other stakeholders. It will also demonstrate that what is being proposed is feasible; • There are a number of similar sub-catchment reaches within the wider Allan Water that could benefit from similar riparian improvement projects such as the Danny Burn, Ogilvie Burn, Muckle Burn, Keir Burn, Millstone Burn and Wharry Burn. The restoration of the River Knaik would be a good demonstration site to these land managers; and • The presence of cultural heritage sites in the lower catchment and the scenic Braco Falls all within walking distance of Braco offers the opportunity for significant amenity improvements through the extension of the existing path network. The restoration of the Allan Water between Greenloaning and Blackford was highlighted by the Steering Group as a secondary pilot or flagship project. Additional scoping has been undertaken to further quantify the key constraints in regards to progressing this option. At this time the primary constraints are understood to be: • The proximity of the reach to the Shelforkie Moss SAC, South Tayside Goose Roosts SPA and the Carsebreck & Rhynd Lochs SSSI is likely to trigger the need for an Appropriate Assessment; • The proximity of the reach to the Stirling – Perth railway line; • The watercourse presents a boundary between two landowners. Modifying the planform of the watercourse will have legal implications for landownership; and • The interest of local fisheries as the reach is understood to be utilised by salmon for spawning. It is proposed that a concept design for the restoration of this reach will be prepared by Halcrow-CRESS in August 2011 to assist in consultation. In conjunction with the Steering Group it has been identified that the preferred pilot demonstration site for the Allan Water Natural Flood Management Programme is the restoration of the River Knaik’s riparian corridor. This can be divided into two main elements: Allan Water Natural Flood Management Techniques and Scoping Study i) Extensive riparian planting in the upper catchment in the vicinity of the confluence of the Corriebeagh Burn and Arrevore Burn with the River Knaik. ii) Improvements in riparian fencing with scattered infill riparian planting opportunities. To facilitate the rapid progression of this programme additional design work has been undertaken. This design work is such that focused and meaningful consultation can take place with landowners, funding bodies, regulators and other stakeholders. The following activities have been undertaken: • Identification of riparian planting opportunities by; - Flood mapping of the upper River Knaik - Site based catchment reconnaissance • Preparation of a range of tree planting specifications (using species of local provenance); • Identification of riparian planting opportunities by site based reconnaissance; • Preparation of fencing specifications to protect the proposed tree planting and allow existing vegetation to increase its extent; and • Identification of physical constraints