Changing River Environments » Overland Flow and Channel Flow

What you'll learn this session

Study time: 30 minutes

The difference between overland flow and channel flow
Factors affecting overland flow including infiltration and runoff
How water moves through river channels
The impact of velocity and discharge on river systems
How human activities affect river flow patterns
Case studies of river management and flooding events

Understanding River Flow Systems

Rivers are dynamic systems that transport water and sediment across the Earth's surface. How water moves both over land and within river channels is crucial to understanding river environments and their changes over time.

Key Definitions:

Overland flow: The movement of water over the land surface before it enters a channel.
Channel flow: The movement of water within a defined river channel.
Discharge: The volume of water flowing through a river channel per unit of time (measured in cumecs - cubic metres per second).
Velocity: The speed at which water flows (measured in metres per second).

💧 Overland Flow

Overland flow occurs when rainfall cannot infiltrate into the soil and instead moves across the land surface. This happens when:

Rainfall intensity exceeds infiltration capacity
The ground is already saturated
The surface is impermeable (e.g., concrete, compacted soil)
Vegetation cover is sparse or absent

🌊 Channel Flow

Channel flow is the movement of water within the river channel itself. It is affected by:

Channel shape and size (cross-section)
Channel roughness (friction)
Gradient (steepness) of the river
Volume of water entering the channel

Factors Affecting Overland Flow

Several factors determine whether rainfall becomes overland flow or infiltrates into the ground:

🌧️ Rainfall Characteristics

Intensity: Heavy rainfall overwhelms infiltration capacity.

Duration: Prolonged rainfall saturates soil, increasing runoff.

Distribution: Where and when rain falls affects flow patterns.

🌱 Surface Conditions

Vegetation: Plants slow runoff and increase infiltration.

Soil type: Sandy soils absorb water faster than clay soils.

Slope: Steeper slopes increase runoff speed and volume.

🏙️ Human Factors

Urbanisation: Concrete surfaces prevent infiltration.

Deforestation: Removing trees increases runoff.

Land management: Farming practices affect water movement.

The Infiltration Process

Infiltration is the process by which water enters the soil surface. The rate at which this happens is crucial for determining how much rainfall becomes overland flow.

⬇️ Infiltration Capacity

This is the maximum rate at which water can enter the soil. It is affected by:

Soil porosity and permeability
Soil moisture content
Presence of organic matter
Soil compaction

When rainfall intensity exceeds infiltration capacity, overland flow occurs.

📊 Infiltration Rates

Different surfaces have vastly different infiltration rates:

Forest floor: 10-15 mm/hour
Grassland: 5-10 mm/hour
Bare soil: 0-5 mm/hour
Urban surfaces: 0-1 mm/hour

These rates explain why flooding is more common in urban areas.

Understanding Channel Flow

Once water enters a river channel, its movement is governed by different factors than overland flow. Channel flow is the primary way that water moves through a drainage basin.

Factors Affecting Channel Flow

The speed and volume of water in a channel depends on several key factors:

📏 Channel Geometry

Width and depth: Wider, deeper channels can carry more water.

Cross-sectional shape: Affects the wetted perimeter and hydraulic radius.

Sinuosity: Meandering channels slow flow compared to straight channels.

🧱 Channel Roughness

Bed material: Smooth bedrock creates less friction than boulders.

Vegetation: Plants in the channel increase roughness.

Obstacles: Debris and human structures affect flow patterns.

📉 Channel Gradient

Slope: Steeper channels have faster flow.

Profile changes: Gradient typically decreases downstream.

Local variations: Rapids, waterfalls and pools create flow diversity.

Velocity and Discharge

Two key measurements help us understand river flow: velocity and discharge.

🏃 Velocity

Velocity is the speed at which water flows, measured in metres per second (m/s).

Velocity varies:

Across the channel (faster in the centre, slower at edges)
Vertically (faster at surface, slower near bed)
Along the course (generally increases downstream despite gradient decrease)

Typical velocities range from 0.1 m/s in small streams to over 3 m/s in large rivers during floods.

🌊 Discharge

Discharge is the volume of water passing a point in a given time, measured in cubic metres per second (cumecs).

Discharge formula: Q = A × V

Where:

Q = Discharge (m³/s)
A = Cross-sectional area (m²)
V = Velocity (m/s)

Discharge typically increases downstream as tributaries join the main channel.

Human Impacts on River Flow

Human activities significantly alter both overland flow and channel flow patterns:

Urbanisation Effects

Urban development dramatically changes how water moves across landscapes:

Increased impermeable surfaces reduce infiltration and increase overland flow
Storm drains and sewers speed up the movement of water to channels
Channel modifications (straightening, concrete lining) increase flow velocity
Reduced lag time between rainfall and peak discharge
Higher peak flows increasing flood risk

Case Study Focus: River Tees Flooding, UK

The River Tees in northeast England demonstrates how land use changes affect river flow:

Urbanisation in Middlesbrough and Stockton has increased impermeable surfaces
Agricultural practices in the upper catchment have compacted soils
December 2015 flooding saw record discharge levels following Storm Desmond
Peak discharge reached 900 cumecs (normal levels are around 20 cumecs)
Flood management now includes both hard engineering (flood walls) and soft approaches (upstream woodland planting to slow runoff)

Flow Hydrographs

Hydrographs are graphs showing how river discharge changes over time, usually following a rainfall event.

📈 Natural Catchment Hydrograph

In natural, forested catchments:

Longer lag time between peak rainfall and peak discharge
Lower peak discharge
More gradual rising and falling limbs
Higher baseflow during dry periods

This pattern reduces flood risk and maintains river health.

📉 Urban Catchment Hydrograph

In urbanised catchments:

Shorter lag time (sometimes just 1-2 hours)
Higher peak discharge
Steeper rising and falling limbs
Lower baseflow during dry periods

This "flashy" response increases flood risk downstream.

Managing River Flow

Understanding overland and channel flow is essential for effective river management:

Sustainable Management Approaches

Modern river management focuses on working with natural processes:

Afforestation: Planting trees in upland areas to increase interception and slow runoff
Sustainable Urban Drainage Systems (SUDS): Permeable paving, rain gardens and detention basins to mimic natural drainage
River restoration: Removing concrete channels and re-introducing meanders to slow flow
Floodplain reconnection: Allowing rivers to access their natural floodplains during high flows
Leaky dams: Small woody barriers in headwater streams that slow flow during floods but allow normal flow at other times

Case Study Focus: Pickering "Slowing the Flow" Project

Pickering in North Yorkshire has pioneered natural flood management:

The town experienced severe flooding in 2007 with damages over £7 million
Traditional flood defences were deemed too expensive
Instead, 167 woody dams were installed in upstream tributaries
35,000 trees were planted in the catchment
A small bund was created to temporarily store floodwater
During the December 2015 floods, Pickering remained dry while nearby towns flooded
The scheme cost £2 million, far less than traditional defences

This demonstrates how understanding and working with natural flow processes can create effective, sustainable solutions.

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