‘RIVERS OF CONCRETE’ (Creating Ecological Value in Concrete Rivers)

PETER WORRALL Technical Director, Penny Anderson Associates

DAVID PALMER Principal Engineer, Black & Veatch

The River Restoration Centre 6th Annual Network Conference 2005

Total channel length lost DoNR 2,224m (989m siphon) LR 2,211m (1,103m siphon)

New Channel DoNR 3,219m (171m siphon) LR 2,560m (171m siphon)

Total Structural Losses (LR + DoNR) • 3138m of concrete lined channel

edge • 1548m of ‘naturalised’ channel edge • 2092m siphon Total Extent of replacement structures (LR + DoNR) • 2309m of ‘naturalised’ channel edge of which 1455m is targeted for water vole habitat • 6100m of ecologically enhanced concrete lined channel edge • 2469m of concrete channel without enhancements • 342m of siphon and culvert (130m being siphon)

Achieving Biodiversity Aspirations - 1

Key Constraints

Low Mean Flows – LR 0.55m3/sec - DoNR 0.78m3/sec

Low flow velocities LR – 0.09m/sec to 0.33m/sec DoNR 0.13m/sec to 0.59m/sec

Conveyance requirements

Concrete channel context

Bird exclusion netting

Shading and temperature regulation

Achieving Biodiversity Aspirations - 2

1. Changes in micro-velocities – by creating sinuous flow path (‘meander’ wavelengths based on 10 to 14 times channel width) - by constricting the channel width at flow cross-over points (increasing flow velocities between 15 and 33%) - by installing larger gravels/cobbles in channel constriction zones

2. Habitat opportunities – different media - stone, gravel, clay, wood and coir, for bed and banks - different levels in relation to mean flow (+100mm, -100mm, -200mm) - backwater and on-line channel features - roughened concrete wall structures

3. Plant diversity – submerged, emergent and marsh plants together with flower rich grasslands

4. Upstream and downstream connectivity within the system – mammal shelves and passage ways

‘Naturalised’ banks for water vole LR 570m DoNR 885m

A berm provides opportunities for growth of

emergent and wet grassland plants, which in

turn provide excellent cover and foraging for

water voles (berm width can be variable)

A bank >0.5-1m in height allows

water voles to construct nest

chambers above water level,

reducing the risk of drowning

during flood events

For illustrative purposes only

Bank height

>0.5-1m

Substrate of bank is

sympathetic to water

vole burrowing (i.e. no

gravely/stony soil or

very loose sand

A shallow profile

(55-60o) to the bank

top will allow

management of

plant growth

A steep bank profile (75-85o)

provides excellent

opportunities for water voles to

construct burrow systems

(subsoil or faced with coir roll)

Marginal shelf planted seeded with

suitable for water voles such as

rushes, sedges and wetland herbs

Minimum requirement for this specification

should be 50m per 150m of naturalised bank

T5 Twin Rivers – Habitat Creation for Water Vole