GEO1020 - River Geomorphology Study Notes

GEO1020 - River Geomorphology

Definitions and Overview

• Fluvial: Pertaining to rivers or produced by rivers.

• Geo: Related to the surface of the Earth.

• Morphology: The scientific study of form and structure.

• Fluvial Geomorphology: The study of sediment sources, fluxes, and storage within the river and floodplain channel over short, medium, and long timescales (Sear and Newson, 1993).

Importance of Studying Fluvial Geomorphology

• Understanding Contemporary Landscapes: Essential for grasping how current river landscapes have developed and will change in the future.

• Processes of Erosion and Transport: Involves the erosion, transfer, and deposition of materials from immediate events to geological time frames (millennia).

• Support for Sustainable Management: Heavily supports the agenda of sustainable river basin management.

• Underpins Environmental Policies: Essential for framing environmental policy, law, and land management decisions, such as the EU Water Framework Directive.

River Basin Evolution

• Time-Scale of Processes: River systems are affected by processes occurring over very short to very long time periods; longer spatial scales correspond with longer time scales of adjustment.

• Autocyclic Controls (Internal): These include drainage network growth/change, progressive fan or river aggradation, and channel behavior.

• Allocyclic Controls (External): This involves external factors such as climate (storm seasonality and temperature), tectonics (uplift, subsidence, faulting), and eustatic changes (sea level fluctuations).

• Evolution Perspectives:

  • Early advances focused on erosion cycles (Davis, 1899) but did not account for factors like climatic variation and differential uplift rates.

  • Initial assessments were made regarding how rivers may transport materials.

Characteristics of Pool and Riffle Sequences

• Spacing: Average spacing between pools in straight and meandering rivers is about 5 to 7 channel widths, roughly half the average meandering wavelength.

• Riffle Heights: Generally, riffle heights maintain a constant proportion relative to channel depth.

• Formation of Pools: Pools can form through scouring around Large Woody Debris (LWD), which are stable features maintained by dynamic flow changes as water levels rise and fall.

• Lateral Migration: Rivers migrate laterally through erosion along cut banks and deposition on point bars, which are crucial for this process.

• Wavelength Control in Meandering Rivers:

  1. Neck Cutoff: Occurs when meanders intersect and both necks come together.

  2. Chute Cutoff: Involves downstream elongation at the bend apex, leading to headward incision where erosion facilitates overland flow to the downstream bend.

Characteristics of Braided Rivers

• Flow Dynamics: Braided rivers have shallow flows with multiple channels across a floodplain.

• Channel Width: For the same discharge, the total channel width of a braided river can be about 2-3 times greater than that of a single-thread channel.

• Channel Depth: Average depth in braided rivers is typically 2-3 times less than that of a single-thread channel.

• Bank Material: Requires erodible bank material, characterized by low vegetation and non-cohesive coarse sediments (e.g., sands).

• Sediment Load: High sediment loads are necessary for maintaining braided channel morphology.

• Bar Lengths: Bar lengths are generally five times the width of the bars.

• Controls on Braided River Formation:

  • Erosion of banks occurs due to meandering flow through central bar units, promoting mild sinuosity.

  • Enhanced erosion of banks can also be accompanied by chute cutoff and lateral accretion of exposed bars, resulting in further bank erosion.

River Basin Morphometry

• Influence of Bedrock Structure on Drainage Patterns: Dominant controls include:

  1. Dendritic: Arises in relatively uniform bedrock, typically consisting of flat-lying sedimentary rock or massive igneous rock.

  2. Trellis: Formed by alternating bands of weak and resistant bedrock.

  3. Radial: Rivers diverge from a central elevated point, commonly found in isolated volcanic cones or domes.

  4. Centripetal, Rectangular, Deranged patterns.

• Research Insights: Hack and Montgomery and Dietrich illustrated a consistent relationship between river basin size and length; river catchments exhibit self-similar patterns as the scale increases. The morphology is adapted to minimize energy expenditure at both local and global scales.

• Climate Influence on Drainage Network Development:

  • Climate controls the regional water cycle, affecting streamflow generation, channel erosion regimes, and vegetative resistance.

  • Basin morphology varies according to climate, with elongate shapes in drier regions compared to dendritic forms in humid climates.

  • In arid basins, significant erosion events are infrequent, leading to closely spaced downstream extensions.

Geomorphic Processes and River Channel Evolution

• Steeper Rivers and Incision Rates: Steeper rivers tend to incise soil and rock faster. Generally, rivers at higher uplift rates tend to have evolved into higher and steeper landscapes.

• Pre-Rupture vs. Post-Rupture Processes: Evolution of river basins over time reflects shifts in understanding and measurement that occur across various spatial and temporal scales, greatly facilitated by new technologies such as:

  • Isotopic and radiometric analysis for erosion rate quantification over millennia.

  • High-resolution imaging of Earth's surfaces and subsurfaces using geophysical methods (e.g., LiDAR, Ground Penetrating Radar).

  • Remote sensing techniques and digital technologies that enable real-time monitoring of processes and forms.

Planform Controls and Channel Form Relationships

• Headwaters vs. Large River Channels: Headwater streams are typically narrower and deeper than larger river channels. Channel width increases downstream more rapidly than flow depth, enhancing the width-to-depth ratio as it flows downstream.

• Discharge Effects on Channel Variables: Increases in channel width, mean depth, and velocity occur downstream correlated with mean annual discharge.

• Impact of Bed Material Type:

  • For the same discharge and channel slope, sand-bed channels tend to be wider and shallower than gravel channels, exhibiting faster rates of morphological change.

River Morphology Classifications

Straight Channels (Sinuosity S < 1.3)

• Characterized by a single channel with pools, riffles, and slight meandering. These channels often represent transient forms or occur in areas with strong structural controls limiting lateral migration.

• Exhibits minor channel widening and incision, with a width-to-depth ratio of $<1.3$.

Meandering Channels (Sinuosity S $\geq 1.3$)

• Defined by a single channel with pronounced inner point bar channels.

• Wide-ranging sediment behavior including suspension or mixed bedload, with common channel incision and widening. The width-to-depth ratio is typically $>1.5$.

Braided Channels

• Features two or more channels with bars of sand and small islands.

• Typically seen with a width-to-depth ratio of $>1.3$, predominantly consisting of bedload with occurrences of channel widening and mid-channel bar formation.

Anabranching Channels

• Characterized by two or more channels with significant stable islands, usually with a width-to-depth ratio $>2.0$ and consists of suspended load leading to slow meander widening and bank accretion.

Importance and Control of Meandering Rivers

• Significance of Meandering Rivers:

  • Act as drivers of modern landscape evolution, facilitating sediment transport, ecosystem diversity, and serving as critical resources for water management.

  • Generate hazards for infrastructure and human populations related to land loss.

• Reasons for River Meandering:

  • Occurs to minimize energy expenditures as water flows downslope, making straight configurations unstable due to turbulence.

  • Mobile and erodible banks create fundamental channel instabilities.

• Characteristics of Meandering Channels:

  • Average meander wavelength is approximately twelve times the channel width, maintaining a relatively constant ratio that does not vary significantly with channel width.

  • The sinuosity is influenced by riverbank materials; channels with higher clay and silt content are often more sinuous.

• Control Mechanisms of Meandering:

  • Alternate bars (depositional units) affect flows in deeper environments, leading to a complex downstream flow pattern and increased elevation in pools.

  • Maximum velocity shifts laterally in meandering patterns as water courses through the succession of bar units, resulting in regular bank erosion along straight sections.