Weir removal

Hydromorphological benefits

Duncan Wishart – Principal Geomorphologist YksNE/NW

Structure

Hydromorphology

Weirs

Impacts of weirs

Removal

Geomorphological adjustments

Case study

Benefits

Considerations

Hydromorphology → flow and form

Structure and

substrate of bed

Structure of

riparian zone

Structure of

riparian zone

Depth and width

variation

Connection to

groundwater bodies

Quantity and

dynamics of flow

River

continuity

Spot the weir

Weirs

First weirs in Britain - Neolithic (Brown, 1997)

Wide variation in age

Range of types and purposes

Different settings

Often associated with other forms of channel

modification

Type + Age + Setting = Impact

> Impact = > Benefit of removal

> Impact = > Risk of removal?

All shapes and sizes

Types → end members

Bed check Impounding

Impacts of weirs

Upstream flow ponding

• Reduced flow diversity

• Interrupts sediment transport

• Uniform bed (physical habitat)

Across

• Increased flow energy

• Physical barrier

•

Downstream

• Scour

• Sediment deposition

• Island formation

• Incision?

• Armouring

Impacts

Sediment accumulation

- wedge

Sediment accumulation

- Riffle / Island

Toe scour

- Pool

Incision / Armouring

Removal

Removal or collapse

Triggers geomorphological adjustments

Change in channel morphology

Upstream dominated by erosion

Downstream dominated by deposition

Move toward new equilibrium forms

Change in habitats

Adjustment Upstream

Weir Downstream

Removal of bed fill by migrating

headcut (knick point) - incision

Reduction in water level Undermining of banks

Deposition of sediment released

from upstream – aggradation?

Hydromorphological benefits Before After

Deposition of sediment –

channel narrowing

(previously over-wide)

Lowering of water level to

limit of backwater

Exposure and re-

establishment of former

pool/riffle bed structure

Improved sediment

transport –creation of new

channel deposits

Gigg Weir – River Roch

Exposure of river banks

and creation of equilibrium

slope

Considerations

Weirs provide habitat

local variation, water retention

Removal can expose other features

older weirs, bridge piers

Adjustment impacts can be adverse

erosion, channel capacity

Other modifications important

realignment, width, structures

Benefits

River continuity reinstated

Depth and width variation increased

Dynamics of flow more natural

Structure river bed more varied

Substrate more varied