Geomorphology, river hydrology and natural processes€¦ · Geomorphology, river hydrology and natural processes ... stabilised by large ‘key-stones ... rarities Purple Iris and

Geomorphology, river hydrology and natural processes

Steve Addy

SNH Sharing Good Practice Event: Identifying and planning river restoration projects

Strathpeffer, 24th September 2013

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

Width

Depth Roughness

(sediment and

bedforms)

Water and sediment inputs

Valley width

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, non-

cohesive alluvial banks

Alluvial bed

Dense vegetation, cohesive

alluvial banks

Alluvial bed

Bedrock banks

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

Alluvial banks Glacigenic landforms

Tributaries Hillslopes

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)

Source: http://www.environment-agency.gov.uk/cy/hiflows/

Old Bridge, Perth

River Dee at Woodend

River flow in open channels

Area (A)

Bankfull

Wetted perimeter

(WP) R = A/WP

Q = A x V

V = (1/n) x R0.66 x S0.5

Where S slope (m/m) and ‘n’ Manning’s roughness

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:

Depth

(stage)

Velocity (V, m/s) is given by the Manning equation:

Hydraulic radius (R) a measure of channel shape is given by the formula:

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):

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.

gdS

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

Bed surface

Turbulence

Dissolved load

Rolling

Sliding

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

Imbrication

Bedform and grain roughness

Packing, protrusion and hiding effects

Armouring

Sub-surface layer

Armour layer Flow direction

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.

~Bankfull flow Low 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

(Winterbottom and Gilvear, 2000)

(Hooke, 1977)

Channel variability in space

Stored sediment

Stream flow

Stream power

Bed material size

Channel gradient

Drainage area, distance downstream

Ma

gn

itu

de

(adapted from Church, 2002)

Alluvial reach

Bedrock reach

Local ‘step’ changes

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

10 years before dam completion

(RCAHMS, 2010)

15 years after dam construction

(RCAHMS, 2010)

48 years after dam construction

Get Mapping, 2010)

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

Water quality,

temperature and

species pool

Channel geomorphology

Physical habitat

Biota

Catchment and river

processes

(Gilvear, 2011)

(Scottish Rivers Handbook, 2013)

Connectivity and complexity in river systems

“The biophysical complexity

(heterogeneity) of rivers underpins their

long-term vitality”

Naiman, 2006

(from Naiman et al., 2006)

River Nethy

(Gilvear, 2011) (Gilvear, 2011)

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.

Ballinluig Island (SSSI), River Tummel 1946

1989

1994

1999

(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