Harbertonford Flood Defence Scheme: the benefits of an integrated approach to flood defence design
Matt Jones BSc MSc MIEEM LMIFM
Presentation
Context of scheme
Selected flood defence design
Approach to flood design
Benefits of the approach
Catchment overview
River Harbourne (River Dart Catchment) Relatively small, agricultural lowland catchment Dynamic, meandering watercourse Gravel bed with medium mobility of bed materials Agriculture dominates land use
Pasture 80%
Arable 15%
Woodland and urban 5%
Harbertonford
Typical picturesque South Devon village
Dominated by residential
Pub, post office, restaurant Harbertonford
Flood problem
Properties in village have flooded 21 times in past 60 years (six times since 1998)
Flood defence scheme given ‘accelerated status’ by DEFRA (one of only nine such schemes in the UK)
The Scheme A381 Totnes
Harberton Stream
Flood storage New overflow pipe reservoir New Culvert - Autumn 2000 (by Devon County Council) Crowdy Mill Weir Woodland to be replaced Road by 'fixed riffle'
new Bow Road wetland area Tor View Rolster River River Bridge Harbourne Harbourne Playing field Village Hall Lowering using riffle Yeolands Stream pool sequence (already improved) Mill Leat Earth Dam with Bridge Works - Summer 2001 Control Structure (by Devon County Council) Enlarged channel
A381 Kingsbridge
not to scale Summary scheme stats!
40 year standard of protection
Benefit:cost ratio of 2.3:1
Total scheme cost is £2.25M (£2.03M received through DEFRA grant aid)
Statutory permissions and consents granted
Construction began in February 2002
Construction to be complete in October 2002 The Scheme A381 Totnes
Harberton Stream
Flood storage New overflow pipe reservoir New Culvert - Autumn 2000 (by Devon County Council) Crowdy Mill Weir Woodland to be replaced Road by 'fixed riffle'
new Bow Road wetland area Tor View Rolster River River Bridge Harbourne Harbourne Playing field Village Hall Lowering using riffle Yeolands Stream pool sequence (already improved) Mill Leat Earth Dam with Bridge Works - Summer 2001 Control Structure (by Devon County Council) Enlarged channel
A381 Kingsbridge
not to scale Summary scheme stats!
40 year standard of protection
Benefit:cost ratio of 2.3:1
Total scheme cost is £2.25M (£2.03M received through DEFRA grant aid)
Statutory permissions and consents granted
Construction began in February 2002
Flood storage area (1)
Dam will cover up to 20 m length of channel;
Approximately 4 m in height;
Intercept flows of >1:10 year flood;
Maximum capacity: 1:40 year flood;
Varying topography of dam (steeper upstream slope, shallow downstream slope);
Planted with trees on upstream slope. Limited on downstream slope;
Flood storage area (2)
Culvert to mimic natural channel size;
Pools created upstream and downstream of structure;
Culvert designed with respect to Scottish Executive Guidance on migratory fish;
Creation of 4.1 hectares of wet grassland/ woodland;
Gain in terms of DEFRA High Level Target 9
Informal recreational area;
Creation of bat roost within service hut
The Scheme A381 Totnes
Harberton Stream
Flood storage New overflow pipe reservoir New Culvert - Autumn 2000 (by Devon County Council) Crowdy Mill Weir Woodland to be replaced Road by 'fixed riffle'
new Bow Road wetland area Tor View Rolster River River Bridge Harbourne Harbourne Playing field Village Hall Lowering using riffle Yeolands Stream pool sequence (already improved) Mill Leat Earth Dam with Bridge Works - Summer 2001 Control Structure (by Devon County Council) Enlarged channel
A381 Kingsbridge
not to scale
B
B
Lowering of river channel in village
Proposed creation of fixed riffle pool sequences from immediately upstream of
road bridge to below the weir (in association with RRC)
Channel capacity increased to 1:10 year flood
Designed from a geomorphological basis in order to maintain structure and minimise maintenance (in association with RRC)
The Scheme A381 Totnes
Harberton Stream
Flood storage New overflow pipe reservoir New Culvert - Autumn 2000 (by Devon County Council) Crowdy Mill Weir Woodland to be replaced Road by 'fixed riffle' new Bow Road wetland area Tor View Rolster River River Bridge Harbourne Harbourne Playing field Village Hall Lowering using riffle Yeolands Stream pool sequence (already improved) Mill Leat Earth Dam with Bridge Works - Summer 2001 Control Structure (by Devon County Council) Enlarged channel
A381 Kingsbridge
not to scale Lowering of leat
Low flow channel to be created within leat
Maintenance of abstraction licence volume
Management required due to siltation
The Scheme A381 Totnes
Harberton Stream
Flood storage New overflow pipe reservoir New Culvert - Autumn 2000 (by Devon County Council) Crowdy Mill Weir Woodland to be replaced Road by 'fixed riffle' new Bow Road wetland area Tor View Rolster River River Bridge Harbourne Harbourne Playing field Village Hall Lowering using riffle Yeolands Stream pool sequence (already improved) Mill Leat Earth Dam with Bridge Works - Summer 2001 Control Structure (by Devon County Council) Enlarged channel
A381 Kingsbridge
not to scale
Widening along Bow Road
• Removal of embankment along left bank • Creation of meandering channel within widened area • Geomorphological design (RRC advice) • Construction of a wall
Widening along Bow Road
• Removal of embankment along left bank • Loss of trees although some retained • Creation of meandering channel within widened area • Geomorphological design (RRC advice) • Construction of a wall Widening along Bow Road
• Removal of embankment along left bank • Loss of trees although some retained • Creation of meandering channel within widened area • Geomorphological design (RRC advice) • Construction of a wall
Approach to flood defence design
Key concepts
Multi-disciplinary team formed from the outset - ensures an integrated approach from the start of the project. Core Design Team
Project Manager
Civil engineers
Hydrologist
Geotechnical engineer
Environmental scientist
Landscape architect
Fluvial geomorphologist Key concepts
Multi-disciplinary team formed from the outset - ensures an integrated approach from the start of the project.
Engineering, environmental and cost objectives established at inception Engineering/cost objectives
Examples: – to provide flood alleviation to property – to be cost-effective – to have limited construction programme constraints – to have an acceptably low construction and maintenance hazard – to maximise sustainability Environmental objectives (1)
Examples: – to integrate with the geomorphological processes of the River Harbourne – to maintain, or where appropriate, enhance suitable habitat conditions and movement corridors for all fish species – to have no adverse effect on licensed abstractions Environmental objectives (2)
Examples: – to maintain, or where appropriate enhance, biodiversity (DEFRA Target 9) – to design a scheme in-keeping with the Conservation Area – to minimise the effects on the local community – to maintain or enhance the water quality of the River Harbourne Key concepts
Multi-disciplinary team formed from the outset - ensures an integrated approach from the start of the project.
Establish engineering, environmental and cost objectives for the scheme.
Consultation from project inception Consultation • Planned and began at project inception • Statutory bodies and non-statutory organisations • General public • Benefits: • Consents • Information retrieval • Ownership of ideas and scheme Key concepts
Multi-disciplinary team formed from the outset - ensures an integrated approach from the start of the project.
Establish engineering, environmental and cost objectives for the scheme.
Consultation from project inception.
Design progressed as an iterative process as respective assessments/consultations progressed. Design
Multi-disciplinary core design team
Consideration of wide range of options (Scoping):
Land management;
Catchment storage; and
In situ defence.
Specialists brought in at appropriate design stages
Engaging contractors at an early stage - facilitated by EA Contractor’s Framework
Benefits of the approach
• A cost-effective scheme has been developed which seeks to work with the geomorphology of the river whilst also leading to ‘environmental gain’; • Minimises future maintenance; • Facilitates permissions and consents • ‘Ownership’ by stakeholders • Core team skills enhanced by inputs from specialists • Contractor familiarity with scheme concepts prior to start
The future?
“Engineers, environmental scientists and river restoration experts have worked together to provide a truly sustainable flood defence solution for Harbertonford. The challenge for our industry is to build on the successes of schemes such as this and maximise the benefits of fully integrated project teams”.
John Taberham, EA NCPMS SW Region Projects Team Manager The future?
“It is a very beautiful river. I have always learned something from it. One can learn much from a river.” Siddhartha, Herman Hesse
Monitoring to inform future management
Monitoring to inform future scheme design