Flood Risk ISBN: 978-0-7277-4156-1 ICE Publishing: All rights reserved http://dx.doi.org/10.1680/fr.41561.333 Chapter 14 Design of an environmentally aligned flood alleviation scheme: the Burn of Mosset,Moray,Scotland Colin R. Thorne,* Matthew Hardwick,† Paul Winfield† and Carina Oliver† * University of Nottingham, Nottingham, UK † Royal Haskoning, Peterborough, UK Forres, Scotland, has a long history of flooding from the Burn of Mosset, and it was forecast that, without investment in flood risk management, flood damages were likely to exceed £43 million over the next 50 years. The decision was taken to implement a flood alleviation scheme (FAS) featuring an earth-fill dam upstream of Forres, capable of storing up to 3.8 million m3 of water and limiting peak discharges into the urban area to 8.5 m3/s. It was recognised that sediment- and debris-related risks to the per- formance of the structure had to be managed and Burn Management Works were designed using cutting-edge river restoration techniques for channel rehabilitation and habitat creation to enhance the capacity of the flood basin to store sediment and debris. The FAS was opened on 28 August 2009 and the dam impounded flood water for the first time on 4 September 2009, avoiding approximately £9 million of flood damages to the town of Forres. The Burn Management Works have evolved mostly as expected, although some features have not functioned as intended, and lessons have been learnt concerning the need to allow for unexpected developments when using ‘prompted recovery’ in flood risk management and river rehabilitation. 14.1. Introduction Forres is a market town (population 9000) located 16 km west of the city of Elgin in Morayshire, Scotland (Figure 14.1). The Burn of Mosset is a small but geomorphologi- cally active, gravel-bed stream that drains an area of 49 km2. It flows northwards through the centre of Forres, before confluencing with the River Findhorn in Findhorn Bay. Forres has a long history of flooding from the Burn of Mosset, with six events causing serious property damage and disruption of daily life within the last 50 years. 14.2. Background The most recent severe flooding occurred in 1997, but significant events also occurred in 2000 and 2002. The 1997, 2000 and 2002 events had estimated return periods of 50, 5 and somewhat less than 5 years, respectively. Base flow in the Burn of Mosset at Forres is less than 1 m3/s, and the 1 in 100-year flow is estimated to be 68 m3/s, making a 15% 333 Design of an environmentally aligned flood alleviation scheme Figure 14.1 Forres location map allowance for climate change up to 2080. Peak flow during the 1997 event was estimated at 48 m3/s. During that event, approximately 430 homes and 27 commercial properties were inundated (Figure 14.2) and regional communications were seriously disrupted, with both the A96 trunk road and the Inverness to Aberdeen mainline railway being closed due to flooding. Economic losses caused by the 1997 flood alone were of the order of £3.7 million, and it was forecast that without investment in a flood alleviation scheme (FAS), flood damages in Forres would be likely to exceed £43 million over the next 50 years. The economic case for a FAS for Forres was, therefore, clear. However, geomorphological reconnaissance surveys (performed using methods described by Thorne et al. (2010)) performed as part of feasibility studies for a FAS revealed that the Burn of Mosset is highly active geomorphologically, and that it transports substantial quantities of sediment ranging in size from cobbles to sand and silt-sized material. Most of the cobble and gravel-sized bedload is deposited in the channel upstream of Forres, in the form of both shifting gravel bars and semi- stationary riffles, which have historically been managed for agricultural land drainage by dredging, with the dredge spoil used to build flood embankments on an ad hoc basis (Figure 14.3). Substantial quantities of suspended sand/silt load, derived from field erosion associated with agricultural practices in the upper catchment, are also transported as wash load, but 334 Design of an environmentally aligned flood alleviation scheme Figure 14.2 Flooding in Forres during the 1997 event these are carried into urban area of Forres, where they are mostly deposited in Sanquhar Loch, which is an artificially impounded water body located on the southern edge of the town The field survey further revealed that the Burn carries considerable quantities of organic debris, including leaves, twigs and large wood. The findings of the geomorpho- logical assessment indicated that, to reduce the unacceptably high flood risk in Forres sustainably, it was essential to design a scheme that accounted for the dynamics of sediment and organic debris, as well as those of the floodwaters. 14.3. Option development and appraisal A variety of potential solutions were considered for addressing the flooding problem. These included a number of contrasting flood risk management strategies. Options ranged from hard engineering solutions (involving construction of a diversion channel around Forres, channel enlargement, or the raising of existing flood defences within 335 Design of an environmentally aligned flood alleviation scheme Figure 14.3 Gravel removal and ad hoc embanking of the channel of the Burn of Mosset upstream of Forres the town) to flood storage, removal of properties in vulnerable areas, and changes in catchment management. The criteria by which the options were appraised were developed by the stakeholders at a value management workshop. g Environment – The FAS will aim to create sustainable development that recognises the importance of the environment, minimises construction impacts and enhances the environment where possible. The scheme will aim to preserve the local character of the town and its surroundings. g Sustainability – The FAS will aim to ensure the best achievable balance between environmental, social and economic well-being through use of the Morayshire Flood Alleviation Group’s sustainability criteria and indicators. In the absence of any locally accepted methodologies available at that time, a document entitled Rethinking Sustainability: Design Guidance and Procedures was produced in November 2002 to inform management of scheme development. g Geomorphology – The FAS will aim to minimise disturbance to natural geomorphological processes. g Stakeholder consultation – The public, statutory consultees and interest groups shall be consulted at key stages of the development of the FAS. The views and opinions of stakeholders shall be taken on board and reflected in the development process and its outcome. Community values shall be central to the scheme. g Landowners – The FAS will aim to minimise the impact on landowners and property occupiers. 336 Design of an environmentally aligned flood alleviation scheme g Planning development – The FAS will, where possible, accord with the Moray Development Plan (2000), planning policies and objectives. The FAS will avoid constraining future development opportunities. g Operation and maintenance – The selected option will be designed with the aim of minimising future operation and maintenance requirements. The design shall accommodate safe access and egress for all operations and maintenance activities. g Opportunities – The FAS design shall fully consider opportunities for improving recreation and amenity values. Three options judged to be technically feasible and capable of delivering a minimum flood defence standard of service of 1 in 100-years were appraised according to these criteria. 14.3.1 Option 1: Channel diversion The principal works would include the following. g Construction of a diversion channel from the Burn of Mosset upstream of Forres to the River Findhorn, including: – 1.9 km long diversion channel – culvert or bridge under the A940 road – cascade to grade the diversion channel into the Findhorn valley. g Works through Forres, including: – rehabilitation of floodwalls and embankments – removal of existing weirs and channel bed reprofiling – flood-plain lowering. 14.3.2 Option 2: Floodwalls and embankments through Forres The principal works would include g raised floodwalls and embankments throughout the urban area of Forres g modifications to seven bridges (including the A96 trunk road bridge and the Inverness to Aberdeen mainline railway bridge) g removal of existing weirs and channel bed reprofiling g flood-plain lowering at selected locations g raising the dam at Sanquhar Loch upstream of the town centre. 14.3.2 Option 3: Flood storage dam at Chapelton The principal works would include g construction of an earth-fill dam situated on the Burn of Mosset at Chapelton and capable of storing up to 3.8 million m3 of water, including a flow-control structure to limit peak discharges into the urban area to a maximum of 8.5 m3/s g construction of a sediment and debris trap upstream of the dam g earth embankments to protect properties close to the perimeter of the storage area from flooding. 337 Design of an environmentally aligned flood alleviation scheme 14.4. Preferred option The preferred approach for the FAS was identified by evaluating the three options using the sustainability criteria outlined above, while also considering the benefit–cost ratio for each option. As noted earlier, sustainability was evaluated with reference to a guidance document Rethinking Sustainability, which provided a framework for assessing sustain- ability based on economic, social and environmental factors. This appraisal resulted in option 3 (flood storage dam at Chapelton) emerging as the preferred option. The main structural element in this option is a dam at Chapelton (Figure 14.4), which restricts peak flows passing through it during flood events to a maximum of 8.5 m3/s, with the excess flow being stored temporarily in a flood reservoir upstream of the dam, to provide a standard of service to Forres of at least 1% annual exceedance probability (i.e.