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?
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