River Harbourne

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'

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'

A381 Kingsbridge

not to scale

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

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

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