Understanding Fluvial Processes: Supporting River Restoration

Understanding Fluvial Processes: supporting River Restoration

The River Restoration Centre therrc.co.uk Understanding your catchment

Hydrology Sedimentology Energy associated with the flow Relationship between silt, sand of water affects the system and gravel size/position to hydrological regime

River Processes Adjustment in shape and size of the river and topographical changes in the floodplain in relation to climatic conditions and geology

DRIVERS = Natural and Anthropogenic Affect the relationship between river and floodplain features, habitats and their quality? Sediment: river and floodplain processes...

The study of sediment sources, fluxes and storages within the river channel over short, medium and longer timescales and of the resultant floodplain morphology (Sear and Newson, 1993) the pattern of which is determined by the hydrological regime in a natural system.

Helps to identify appropriate mitigation measure. Catchment – the hydrology

• Streams receive water from their drainage basin • Discharge: related to catchment area plus other influences

Discharge

• Small flow is exceeded a lot of the

time, a given high flow is rarely

exceeded

• Discharge has a magnitude- frequency Discharge relationship – both the very high and very low flows are rarer, the middle 0 99 % of time equalled or exceeded size flow more common

• Important because impact on how much sediment and associated pollutants will move in the system and where they will be transported to within the context of the catchment characteristic. Effects of hydrology • Catchments receive inputs (precipitation) • Transformed to outputs (streamflow and evaporation) • Input = output (allow for storage) • Output is continuous. • Inputs (precipitation) are discrete (i.e. separated by time/space). Note: – Visible and invisible inputs (e.g. seepage) – Some river flow in summer maybe sustained by the flow of groundwater

Types of flow to the river

• Base or steady flow: Slower route: rate depends on geology and landuse and flows through rather than over land.

• Flashy or overland: Catchments respond very quickly: depends on geology and human intervention.

Affects how quickly sediment, water and pollutants enter the river. Impact on aquatic habitat distribution and quality. Impacted by human intervention in the catchment

River

Pontbren Catchment – Field site for Flood Risk Management Research Consortium (FRMRC)… what characteristics can you see?

Mechanisms of sediment movement Understanding connectivity in the sediment transfer system

Where are the sediment ‘sources’, ‘transfers’ and ‘sinks’?.. Helps to identify risk Gradient or long profile (the energy curve) Catchments naturally saucer shaped • Steeper at the top - lots of energy but less catchment area so less discharge Long profile • Middle reaches - less gradient, more

discharge from larger catchment

(m)

• Lower reaches- lower gradient but Height highest discharge.

So where you are in the catchment Distance downstream (m) effects erosion/deposition characteristics

Summary of controls Human intervention and disruption to natural sediment sources The case of realignment

The effect of a sediment pulse from upstream Erosion and sedimentation in the middle course a) Lateral channel shift b) Increase in sediment supply

Adjustment to disturbances Discharge

Velocity distribution in channel velocity profile bed roughness

Channel Bed material Bed shear geometry or characteristics stress habitats availability selective transport of sediment competence & capacity erosion & Bedload deposition transport

Sediment supply after Ashworth and Ferguson (1986) Minimising risk and uncertainty: understanding the catchment context

Human intervention: sediment and pollutant sources and sinks

River habitats What do we want to achieve?

Full of native wildlife

Water low in pollution

Natural structure and Sufficient water flow connectivity

From RRC’s PRAGMO document (Judy England) A range of habitats

From RRC’s PRAGMO document (Judy England) What level of detail do we need to consider?

Frissell et al. (1986) Processes influencing ecology • Longitudinal processes: – Hydrology – Fluvial geomorphology • Lateral processes: – Interactions with riparian zones & floodplains • Internal processes: – Predation, competition, life cycle etc. Habitats for invertebrates - vegetation

Bank full water level terrestrial vegetation Low water level emergent tree semi –terrestrial broad leaf roots vegetation vegetation emergent narrow leaf vegetation floating leaf Submerged vegetation vegetation Habitats for invertebrates - physical Riffle Exposed riverine Glide sediments

Marginal Large woody dead-water debris Pool

Water surface Bank

Marginal Water table damp areas

Hyporheos Groundwater Case studies Norman Park, Ravensbourne, London ( from the MOT) Works undertaken • New open course, culvert buried; • Spoil used to create ‘topography’; • Planting with trees, shrubs, wildflowers & grass; • Footbridge and stepping stones; • Gravel ‘beach’; • Sediment ‘fan’ at lower end; • Gravel introduced into the bed; • Safe & shallow bank slopes; • Allows flooding to spill into the park and not the housing downstream.

River Bure, Norfolk: before

During

After

Hawley manor, Darent 2006

After (design N Holmes)

Midsomer Norton stream (Nr Bristol – before)

After Babingly brook, Norfolk an ornamental pond

During

After