3_6_Rivers2

Rivers and Sediment

Channel Forming Discharge

  • Discharge Measurement: The flow rate of a river is commonly measured in cubic feet per second (ft³/s).

  • Variability in Channel Forming Discharge: Channel forming discharge (the flow that shapes river channels) varies between different rivers due to a variety of factors, including local geology, gradient, sediment supply, and climate conditions.

Definitions of Discharge Types

  • Hypothesis on Bankfull Discharge:

    • The hypothesis posits that the channel looks as it does because the flow carves out the channel. Therefore, bankfull discharge is seen as channel forming discharge.

    • Bankfull discharge is defined as the maximum discharge that a river can handle before spilling over its banks, effectively maintaining the channel shape.

  • Effective Discharge:

    • Defined as bankfull discharge corresponding to a flow that transports the most suspended sediment and bed material.

Effective Discharge and Suspended Sediment

  • Breakdown of Effective Discharge:

    • Effective discharge relates directly to:

    • A: Discharge Frequency

    • B: Suspended Sediment Discharge

    • C: Where C = A x B (total suspended sediment transport)

    • The effective discharge for suspended sediment typically occurs once every 1 to 2 years, aligning with bankfull discharge, which reflects the channel's conveyance capacity.

Relation of Sediment Supply to Effective Discharge

  • Incision and Aggradation:

    • Incision is defined as erosion; it takes place when the sediment supply decreases (e.g., from S2 to S1), necessitating a lower effective discharge.

    • Aggradation is sediment deposition; it occurs if sediment supply increases from S2 to S3, requiring a higher effective discharge.

  • Effective discharge changes with sediment supply, indicating that river systems must adjust to variations in sediment input to maintain their morphology.

Types of Rivers

Alluvial vs Bedrock Rivers

  • Alluvial Rivers: These rivers' forms are determined by sediment deposition and transportation.

  • Bedrock Rivers: These rivers incise directly into bedrock, and deposition along their path does not significantly alter their form.

Effective Discharge in Bedrock Channels

  • Effective Discharge Applicability: Effective discharge does not directly apply to bedrock rivers due to differing geomorphic processes.

  • Processes Shaping Bedrock Rivers:

    • Low frequency, high magnitude events influence their morphology.

    • Main mechanisms for shaping include:

    1. Abrasion: Sandblasting effects caused by sediment hitting rock, creating features like potholes.

    2. Hydraulic Plucking: The removal of loosened, fractured rock blocks.

    3. Chemical Processes: Dissolution of rock at channel boundaries through chemical means.

  • Bedrock rivers operate outside the typical 1.5 - 2-year effective discharge interval.

Meandering vs Braided Rivers

  • Classification:

    • Meandering Rivers: Form in stable environments and are characterized by sinuous shapes.

    • Braided Rivers: Form in areas with high sediment supply and have multiple channels separated by bars of sediment.

  • Factors Influencing River Type:

    • Sediment Load:

    • Generally, meandering rivers have more suspended load, whilst braided rivers carry more bedload.

    • Grain Size:

    • Meandering rivers accommodate finer grained sediment while braided rivers have coarser sediment.

    • Bank Stability:

    • Bank stability decreases in braided rivers due to stronger hydrodynamics and sediment transport.

    • Discharge Variability and Slope:

    • High discharge variability is associated with braided streams, whilst meandering rivers develop in low variability sediment supply conditions.

River Evolution

  • Evolution of river patterns depends on sediment supply and bank materials; cohesive soil types lead to more stable, meandering stream forms as opposed to dynamic, braided patterns in sediment-rich environments.

Meander Dynamics

  • Flow in Meander Bends: Flow velocity is typically faster on the outside bend of a meander due to water dynamics; this increased flow can lead to erosion.

  • Meander Migration: Over time, meanders shift across their floodplain, which can affect surrounding ecosystems and human infrastructures.

Human Impacts on Rivers

  • Control Measures: Levees are constructed to control meander migration and protect human developments.

  • Dam Construction:

    • Dams serve several purposes, including flood control, irrigation, drinking water supply, energy generation, and navigation assistance.

    • Implications include limiting sediment supply downstream and potentially changing river morphology.

  • Static and Dynamic River States:

    • Only 37% of rivers longer than 1,000 km remain free-flowing, indicating significant loss due to human intervention.

Dams and Sediment Management

  • Strategies for Sediment Transport Management: Dams can impede natural sediment flow, which can lead to sediment starvation downstream.

  • Case Study: The Glen Canyon Dam in Arizona showcases how controlled floods (High Flow Experiments or HFE) can be used to enhance sediment transport and replenish downstream environments but may not be a long-term solution if water supplies are limited.