Hydrology, River Processes, and Hazards Study Notes

Introduction to Hydrology and River Systems

• Fundamental Role of Rivers: Rivers are essential components of the global water cycle, responsible for shaping landscapes, providing freshwater for drinking, irrigation, and industrial use, and serving as vital transportation routes for trade.
• Key Study Concepts:
  • Spatial Variation: Understanding how rivers differ across locations and identifying similarities and differences between drainage basins to understand discharge and flooding.
  • System Interaction: Fluviological environments are understood through the interaction of inputs, processes, and outputs within the hydrological cycle.
  • Environmental Interactions: Recognition of the relationship between rivers and human activity, necessitating management strategies.
  • Change Over Time: Rivers evolve over various timescales. While flood events shown on storm hydrographs may take a few hours, the formation of landforms and landscapes can span hundreds of years.

The Global and Local Hydrological Systems

• Global Hydrological Cycle: This is a closed system with no external inputs or outputs. Water cycles continually between the atmosphere, lithosphere, and biosphere.
• Drainage Basin System: This is an open, local system characterized by the input, output, transfer, and storage of energy and matter. It comprises an area of land drained by a river and its tributaries (also known as a catchment).
• Watershed: The boundary of a drainage basin, typically defined by the highest contour of surrounding land. Precipitation falling beyond the watershed enters a different basin.

Case Study: The Colorado River

• Location: Southwestern United States.
• Source: Originates in the Rocky Mountains of Colorado.
• Length: Approximately $2250\,\text{km}$.
• Course: Flows through seven US states: Wyoming, Colorado, Utah, New Mexico, Arizona, California, and Nevada, before reaching the Gulf of California in Mexico.
• Drainage Basin Area: Roughly $400,000\,\text{sq km}$, representing approximately one-twelfth of the contiguous United States.
• Human Impact: Serves as a critical water source for around $40\,\text{million}$ people and supports extensive agriculture. The system is heavily regulated via dams, reservoirs, and water allocation agreements.

Drainage Basin Dynamics as an Open System

• Scale of Basins:
  • Macro-scale: Example: The Congo Basin, which covers $13\%$ of Africa ($4,000,000\,\text{sq km}$) and contains $40$ major tributaries.
  • Micro-scale: May consist of just a single stream.
• Inputs: Primarily precipitation ($P_{pt}$, including rain, snow, hail, sleet, dew, or frost). It represents the conversion and transfer of moisture from the atmosphere to land.
• Outputs:
  • Evapotranspiration ($E_{T}$): The combined effect of evaporation and transpiration. In humid areas, $75\%$ of moisture may be lost this way; in arid areas, it can reach $100\%$.
    • Evaporation: Conversion of liquid to water vapor from surfaces (puddles, streams). Rates increase with temperature, wind, low humidity, and surface area.
    • Transpiration: Water drawn from soil by plants, released as vapor through leaf pores (stomata).
  • Potential Evapotranspiration ($P_{ET}$): The amount of evaporation that would occur if water sources were unlimited.
  • River Discharge ($Q$): Water flowing out of the basin into the sea or other channels. Calculated as:         $Q = AV$         where $Q$ is discharge (measured in $m^3/s$ or cumecs), $A$ is the cross-sectional area, and $V$ is velocity.
• Stores:
  • Interception Storage: Precipitation caught by vegetation. Broadleaved trees have higher interception potential. Urban and cleared areas have significantly lower rates.
  • Surface Water Storage: Water on the surface (puddles/turloughs as temporary; lakes/wetlands as permanent).
  • Soil Moisture Storage: Water held in subsurface soil pores. Sand is permeable with large pores; clay is hygroscopic (minerals swell), making it impermeable.
  • Groundwater Storage: Water percolated into bedrock pores and cracks. It accounts for almost $97\%$ of Earth's freshwater. Residence time can reach $20,000\,\text{years}$.
  • Channel Storage: Water currently held within river or stream channels.
• Transfers (Above Ground):
  • Throughfall: Water dripping from saturated leaves/twigs or falling through gaps.
  • Stemflow: Water running down branches and main trunks.
  • Overland Flow (Surface Runoff): Occurs when soil is saturated or precipitation exceeds the infiltration rate.
  • Hortonian Flow (Infiltration Excess): Fast, shallow laminar flow occurruing when rainfall exceeds infiltration and soil depression capacity; leads to severe erosion.
  • Channel Flow: Movement of water within defined stream/river banks.
• Transfers (Below Ground):
  • Infiltration: Absorption of water into the soil. Infiltration capacity is the maximum rate of absorption.
  • Percolation: Vertical movement of water through soil into bedrock under gravity. Fast in Carboniferous Limestone; slow in non-porous rocks.
  • Throughflow: Water moving downslope through soil via natural pipes or percolines.
  • Groundwater Flow/Baseflow: Slow upward or lateral seepage of groundwater from the phreatic zone into the river bed, maintaining flow during dry periods.

Subsurface Zones and the Water Table

• Water Table: The upper limit of the saturated zone (phreatic zone). It fluctuates with topography, season (higher in winter), and climate.
• Phreatic Zone: The permanently saturated zone where all rock/sediment pores are filled with water.
• Vadose Zone: The zone of temporary or intermittent saturation above the water table.
• Aquifer: An underground layer of permeable rock from which water can be extracted.
  • Perched Water Table: An aquifer formed above an impermeable layer (aquiclude) situated above the regional water table.
• Groundwater Recharge: The refilling of rock pores by downward-moving water. Areas with low recharge consider groundwater a non-renewable resource.

Drainage Basin Characteristics Influencing Hydrology

• Size: Larger basins have higher potential discharge but longer lag times.
• Shape:
  • Circular: Produce a "flashy" response as water reaches the channel from all points at similar times.
  • Linear/Elongated: Spread runoff over a longer duration, reducing peak discharge.
• Drainage Density: The number of streams per unit area. High density results in faster drainage, reduced lag time, and higher flood risk.
• Slope (Relief): Steeper slopes increase the speed of overland flow via gravity, leading to short lag times.
• Soil and Rock Type:
  • Permeable/Porous: (e.g., chalk, sandstone, gravel) allow infiltration, reducing surface runoff.
  • Impermeable: (e.g., clay, granite) restrict infiltration, increasing overland flow and peak discharge.
• Vegetation: Dense forests (especially tropical rainforests intercepting up to $80\%$ of rain) increase lag time and reduce flood risk. Deforestation significantly speeds up water flow into channels.
• Land Use: Urbanization (tarmac, concrete, drains) creates impermeable surfaces that channel water to rivers at high speeds, increasing peak flow and reducing lag time.
• Climate Factors:
  • Intensity/Duration: High intensity exceeds infiltration capacity even in permeable soils.
  • Temperature: Higher temperatures increase evapotranspiration and can cause rapid snowmelt, altering runoff timing.
  • Humidity/Wind: Low humidity and high wind speed increase evapotranspiration rates.
  • Antecedent Moisture: Previous rainfall that saturates the soil before a new event, making surface runoff more likely.

Drainage Patterns

• Dendritic: Tree-like pattern where water converges from various directions; typically high density.
• Rectangular: Streams follow geological weaknesses and gaps in blocky bedrock.
• Radial: Water drains away from a central high point, such as a mountain or hill, into separate channels.
• Trellised: Streams follow slopes downhill and converge along areas of eroded rock.

River Channel Processes

• Erosion Mechanisms:
  • Hydraulic Action: The force of moving water compressing air in bank cracks (cavitation), creating shock waves that weaken the channel.
  • Corrasion (Abrasion): Mechanical wearing of bedrock by sediment. Corrasion specifically refers to excavating or digging out (e.g., potholes), whereas abrasion deflates or wears down the surface like sandpaper.
  • Attrition: Collision of transported particles with each other, making them smaller, smoother, and rounder downstream.
  • Corrosion (Solution): Chemical dissolution of rocks (e.g., limestone) by slightly acidic water (pH $< 7$).
• Transport Mechanisms:
  • Traction: Rolling of large boulders/cobbles along the bed by high-energy flow.
  • Saltation: Bouncing or hopping of smaller pebbles/gravel.
  • Suspension: Fine sand and silt carried within the water body; common in lower reaches.
  • Solution: Dissolved minerals carried invisibly.
• Energy and Transport Capacity:
  • Capacity: The total mass of material a river can transport.
  • Competence: The maximum particle size a river can transport.
• Hjulström’s Curve: Graph showing the relationship between velocity and particle size for erosion, transport, and deposition.
  • Clay requires high velocity for erosion due to particle cohesion.
  • Gravel/boulders require high velocity due to weight.
  • Small particles can remain in transport at very low velocities once entrained.

River Flow Types

• Laminar Flow: Smooth, horizontal motion in layers; rare in natural channels except in small-scale managed sections.
• Turbulent Flow: Erratic horizontal and vertical spiral flows (eddies). Dominant in natural rivers. Intensity depends on velocity and bed roughness.
• Helicoidal Flow: Corkscrew motion as water travels around bends, moving surface water to outer banks and bottom flow to inner banks.
• Thalweg: The path of fastest flow within a channel. In straight channels, it is in the middle; on bends, it moves toward the outside bank.
• Hydraulic Radius ($HR$): A measure of channel efficiency.     $HR = \frac{\text{Cross-sectional Area}}{\text{Wetted Perimeter}}$     A higher $HR$ indicates a more efficient channel with less friction.

River Landforms and the Course Profile

• Upper Course: Steep gradient, V-shaped valleys, vertical erosion, large angular bedload, interlocking spurs, waterfalls, and gorges.
  • Waterfalls: Formed at boundaries of hard and soft rock. Differential erosion leads to a step. Undercutting and collapse cause the waterfall to retreat upstream at rates like $1\,\text{m/year}$ (Niagara), leaving a gorge.
• Middle Course: Gentler gradient, U-shaped valleys, lateral erosion begins, meanders, river cliffs (outside bend), and slip-off slopes/point bars (inside bend).
  • Meanders: Sinuosity is the ratio of channel length to straight-line distance. Meander sinuosity is $> 1.5$. Characteristics include wavelength ($10\text{-}14 \times \text{width}$) and radius of curvature ($2\text{-}3 \times \text{width}$).
  • Riffles and Pools: Riffles are shallow, turbulent sections over coarse bedload; pools are deep, slow sections over fine sediment. Typically spaced at $5\text{-}7 \times \text{channel width}$.
• Lower Course: Flat gradient, wide floodplains, lateral erosion, oxbow lakes, levees, and deltas.
  • Oxbow Lakes: Formed when a pronounced meander neck is breached during a flood, cut off from the main channel, and eventually silted up (becoming a meander scar).
  • Levees: Natural ridges of coarse sediment deposited on banks during floods when velocity drops suddenly.
  • Floodplains: Extensive flat areas of fertile alluvium (silt) deposited by repeated flooding; bordered by steep banks called bluffs.

River Flooding: Hazards and Management

• Causes of Floods:
  • Physical: Heavy/persistent rainfall, rapid snowmelt, impermeable bedrock, steep slopes, lack of winter interception by dormant trees.
  • Human: Deforestation, downslope ploughing, urbanization (impermeable surfaces), and failure of dams.
• Impacts: Loss of life (approx. $10,000$ annually), destruction of livelihoods/homes, waterborne diseases (cholera), financial burdens of reconstruction, and destruction of riparian/fluvial habitats.
• Flood Prediction:
  • Recurrence Interval ($T$): The average time between floods of a certain magnitude.         $T = \frac{n + 1}{m}$         where $n$ is the total years of record and $m$ is the rank of the event.
  • Gauging Stations: Monitor real-time discharge ($m^3/s$) and river stage (level).
  • Flood Hazard Mapping: Identifying susceptible areas based on historical data and topography for cost-benefit analysis and planning.
• Hard Engineering Strategies:
  • Dams and Reservoirs: Regulate flow; provide electricity. Cons: High cost ($7\,\text{billion}$ for Grand Ethiopian Renaissance Dam), displacement of communities, methane release from decaying drowned vegetation, sediment backup.
  • Diversion Spillways: Channels to carry excess water during peaks. Cons: May increase flooding downstream where water re-enters.
  • Embankments/Artificial Levees: Increase channel capacity. Cons: Visual pollution, increased erosion downstream, prevents water from returning to the channel once breached.
  • River Straightening/Dredging: Increases velocity and capacity. Cons: Moves flood problem downstream; requires constant maintenance.
• Soft Engineering Strategies:
  • Afforestation: Increases interception and lag time; binds soil to reduce erosion.
  • Floodplain Zoning: Restricting construction in high-risk areas to maintain infiltration surfaces.
  • Integrated Drainage Basin Management (IDBM): Holistic planning treating the whole basin as a single unit with stakeholder cooperation.
  • River Restoration: Removing human-made structures to return a river to its natural course, maintaining biodiversity and low maintenance costs.