Geomorphology, river hydrology
and natural processes
SNH Sharing Good Practice Event: Identifying and
planning river restoration projects
Strathpeffer, 24th September 2013
Steve Addy
Outline
Controls on channel morphology
Fluvial processes
Channel morphology variability
River types
Links to river ecology
Some definitions
Fluvial geomorphology
Fluvial: rivers and streams
Geomorphology: the study of landforms and the processes
that form them
Hydromorphology
WFD leglislative term that encompasses fluvial
geomorphology and hydrology (i.e. the physical factors that
govern river ecosystems)
Alluvial
Sediment moved and deposited by rivers
Why is fluvial geomorphology important?
• Fluvial forms and processes are recognised as a key
component of river systems in EU WFD and national
legislation
• River landforms and flows constitute the basic ‘physical
template’ that influences riverine biota
• Fluvial geomorphology input is needed for sustainable river
management and restoration
• For mitigating flood risk and geomorphic hazards
Channel morphology
Consists of 5 variables that are inter-dependent i.e.
change of one variable leads to change in others in
response to alteration of the fundamental drivers
Water and sediment inputs
Valley width
Width
Roughness
(sediment and
bedforms)
Depth
Channel pattern
(Church, 2006)
Scales of channel adjustment
(Buffington, 2012)
Boundary conditions: inherited topography,
geology and sediment sources
• Topography reflects millions of years of landscape
development
• Scottish landscapes have been conditioned by glaciation
and rivers are still adjusting to this legacy.
Boundary conditions: bed and bank material
Sparse vegetation, noncohesive alluvial banks
Dense vegetation, cohesive Bedrock banks
alluvial banks
Alluvial bed
Alluvial bed
Bedrock bed
Increasing channel boundary resistance
Boundary conditions: large dead wood
• Wood is an important component of natural river ecosystems
• Can create significant flow resistance (hydraulic roughness)
• Influence on sediment transport and river bed habitats
(Soulsby, 2006)
Drivers: sediment supply
Tributaries
Hillslopes
Alluvial banks
Glacigenic landforms
Drivers: hydrology
• Determines channel size and the movement of material
• How catchments respond to precipitation and snowmelt
and in turn generate river flow depends on:
- Climate
- Topography
- Land use
- Vegetation
- Soil
- Geology
- Superficial drift cover
(Source: Metoffice, 2012, Open Government License)
River flows over time
Understanding how flows change (magnitude and frequency) over
time is important for understanding channel changes, predicting
and managing flood risk
Sources of information:
Flow gauging network (~100 years)
Historical record (centuries)
Geomorphic record (1000s of years)
Old Bridge, Perth
River Dee at Woodend
Source: http://www.environment-agency.gov.uk/cy/hiflows/
River flow in open channels
Bankfull
Wetted perimeter
(WP)
Area (A)
Hydraulic radius (R) a
measure of channel shape
Depth is given by the formula:
(stage)
R = A/WP
Discharge (Q, m/s3) or flow, is defined as the rate at which a volume of water
travels through a cross-section per unit of time. Given by the formula:
Q=AxV
Velocity (V, m/s) is given by the Manning equation:
V = (1/n) x R0.66 x S0.5
Where S slope (m/m) and ‘n’ Manning’s roughness
Energy
• Stream power and boundary shear stress are often
calculated to give a measure of the energy available to do
geomorphic work in a river.
• du Boys (1879) boundary shear stress equation (τ, N/m2):
  gdS
where ρ is the water density (1000 kg/m3), g is gravitational
acceleration (9.81 m/s 2), d (m) is the average channel depth
and S (m/m) is the channel slope.
Variability of shear stress
Determined by differences of flow, channel geometry (depth)
and slope
Higher shear stress
Lower shear stress
Same discharge and
channel geometry
Same channel slope and
geometry
Same discharge and
channel slope
Fluvial processes: sediment transport
Dissolved load
Turbulence
Rolling
Sliding
Bed surface
Fluvial processes: sediment transport
• Washload or suspended load is the movement of fine
(generally < 2 mm) material in suspension
• Bedload movement is the movement of coarse sediment
along the bed; most important in terms of shaping
channel morphology
• Transport capacity – the volume of sediment that can be
transported
• Competence – the maximum size of sediment that can
be transported
Predicting sediment entrainment
When shear stress applied by a flow exceeds the critical
shear stress (i.e. the resistive force) required to mobilise a
particle, entrainment occurs
Critical shear stress (τc, N/m2) can be calculated by using the
Shields (1936) equation:
τ = ρsτ*Dig
where ρs is the specific density of sediment set to 2650
kg/m3, τ* is the Shields dimensionless critical shear stress
value, Di is the particle size of interest and g is gravitational
acceleration (9.81 m/s 2)
Further controls on sediment entrainment
Armour layer
Flow direction
Sub-surface
layer
Armouring
Imbrication
Bedform and
grain roughness
Packing, protrusion and
hiding effects
Fluvial processes: sediment deposition
• Occurs when sediment in transport falls below a
threshold velocity
• Controlled by channel geometry, roughness and changes
of discharge
Washload deposition
Bedload deposition
Dominant discharge
• Exceptional flood events that exceed bankfull can
significantly alter channel morphology through sediment
transport and have a long lasting morphological impact
• However smaller flows, dominant discharge (~bankfull)
flows that occur every 1-2 years (~median annual flood)
may have a greater control on average channel size,
sediment characteristics and pattern.
Low flow
~Bankfull flow
Fluvial processes: bank erosion
•
Common mode of adjustment in
coarse bedded rivers.
•
Rate controlled by material
properties, vegetation, weather
and degree of scour by flowing
water
•
Allows accommodation of
prevailing flow regime and
creates habitat
Fluvial processes: channel migration
(Hooke, 1977)
(Winterbottom and Gilvear, 2000)
Channel variability in space
Magnitude
Stored sediment
Bed material size
Channel
gradient
Stream flow
Stream power
Drainage area, distance downstream
(adapted from Church, 2002)
Local ‘step’ changes
Alluvial reach
Bedrock reach
Channel morphology equilibrium and
variability over time
(from Buffington, 2012)
Channel morphology equilibrium and
variability over time
• Channels in perfect equilibrium: sediment input =
sediment output
• Seldom occurs in reality, channels tend to be in a state of
dynamic- or quasi-equilibrium
• Channel equilibrium can be perturbed but will evolve
towards a new state of equilibrium
Quasi
Dynamic
Response to a major disturbance
(Knighton, 1998)
Channel degradation
Incised channel
Unaffected section
A useful link: ‘River Geomorphology Videos’
http://serc.carleton.edu/NAGTWorkshops/geomoph/emriver/i
ndex.html
Sudden channel change
Jan 2007
March
2009
30 m
Channel response to impoundment:
upper River Moriston
(RCAHMS, 2010)
10 years before dam completion
(RCAHMS, 2010)
15 years after dam construction
Get Mapping, 2010)
48 years after dam construction
Channel classification
(Montgomery and Buffington, 1998)
Bedrock channels
 High gradient for a given drainage area and high transport
capacity relative to sediment supply
 Limited sediment storage and highly stable
 Often form ‘knick-points’ in river profiles and base levels
Boulder-bed channels
 High gradient, confined channels dominated by boulder
and cobble substrate
 High transport capacity relative to sediment supply
 Exhibit cascade or step-pool morphologies that are
stabilised by large ‘key-stones’
Plane-bed and plane-riffle channels
 Medium to low channel gradient characterised by a
relatively featureless bed topography
 Transport capacity to sediment supply ratios are in
balance
 Low sinuosity and variable floodplain extent
Meandering channels
 Low channel gradient characterised by an undulating bed
topography
 Tend to have pool-riffle sequences and extensive floodplain
 Sediment deposition over point bars balanced by erosion on
outside of meanders
(SNH/Aerographica)
Wandering channels
 Low gradient channels with locally braided planform and extensive
sediment storage
 Characterised by a high sediment supply to transport capacity ratio
 Associated with rapid channel migration and avulsion (channel switching)
Braided channels
 Low gradient channels with mutliple channels and a wide active
channel width
 Characterised by a high sediment supply to transport capacity ratio due to
coarse sediment inputs from upstream and weak banks
 Associated with the highest rates of channel migration
(SNH)
(Scottish Rivers Handbook, 2013)
Fluvial geomorphology and lotic habitats
Catchment and river
processes
Channel geomorphology
Physical habitat
Biota
Water quality,
temperature and
species pool
(Gilvear, 2011)
(Scottish Rivers Handbook, 2013)
Connectivity and complexity in river systems
(from Naiman et al., 2006)
“The biophysical complexity
(heterogeneity) of rivers underpins their
long-term vitality”
Naiman, 2006
River Nethy
(Gilvear, 2011)
(Gilvear, 2011)
Ballinluig Island (SSSI), River Tummel
1946
1989
1994
1999
150 plant species: two national
rarities Purple Iris and Jacobs
Ladder (both introduced) and the
native nationally rare plants
Maiden Pink and Shady Horsetail,
plus 31 locally rare higher plants.
(Gilvear and Wilby, 2006)
Distribution of channel types and spawning
habitat
Riffle units in
low gradient
alluvial reaches
preferred.
Freshwater pearl mussel habitat
 Physical habitat quality dictated by the ability of mussels
to burrow into bed sediments and the stability of the
substrate over time
 Survival also depends on a healthy salmon population and
the habitat to support it
Summary
• For a given river section, its morphology is characterised by five
variables: cross-sectional shape, sediment size, slope, bedforms
and planform
• Rivers naturally adjust these variables to the imposed natural
drivers and ultimately evolve towards the most stable and
efficient condition for transporting water and sediment
• Artificial alteration of controlling factors may cause a departure
from its natural adjustment or equilibrium and in turn have
undesired consequences
• Different river types and features will play a role in maintaining
differing species and lifestyles. River complexity, heterogeneity
and a shifting habitat mosaic is important