👋 Welcome to 'The Human Impact' Study Notes!

Hey Geographers! This chapter is incredibly important because it connects everything you learn in Physical Geography to the real world—how *we*, as humans, change the environment.

You will be looking at three main environments: rivers, the atmosphere, and slopes. Don't worry if some of the science seems complex; we'll break it down using simple analogies and clear steps. This topic is essential for linking processes (AO1) to real-world management and evaluation (AO4).

1. Human Impact on Hydrology and Fluvial Systems (Syllabus 1.4)

1.1 Modifications to Catchment Flows and Stores

Human land use significantly alters how water moves (flows) and is held (stores) within a drainage basin. Remember, a natural system tries to balance inputs (rain) and outputs (river discharge). We disrupt that balance.

Key Land-Use Changes and Their Impacts:
  • Deforestation (Removing trees):
    Impact: Reduces interception (water caught by leaves) and evapotranspiration (water released by plants). Less water is stored in the vegetation. Water hits the ground faster, increasing overland flow and flood risk.
  • Afforestation (Planting trees):
    Impact: The opposite of deforestation. Increases interception, reducing the speed at which water reaches the ground. This slows down the movement of water, helping to reduce immediate flood peaks.
  • Urbanisation (Building cities/roads):
    Impact: Creates large areas of impermeable surfaces (tarmac, concrete). This stops infiltration and percolation. Water runs directly into drains and rivers, increasing the speed and volume of flow, leading to a very short lag time (the time between peak rainfall and peak river discharge).
  • Abstraction and Water Storage (Dams, reservoirs):
    Abstraction: Taking water out (e.g., for irrigation or drinking). This reduces the total volume of channel flow downstream, which can stress ecosystems and limit water availability for others. Water Storage (Dams): Creates massive artificial stores. They regulate flow (often releasing steady water for HEP or supply) but can significantly reduce peak flows downstream, altering natural flood regimes.

1.2 The Causes and Impacts of River Floods

Floods are natural, but human activity often makes them more frequent and severe.

Causes of Floods:
  • Physical Causes: Intense precipitation (heavy rain/snowmelt), saturated soil (antecedent moisture), or steep slopes.
  • Human Causes:
    • Urbanisation (as discussed above, speeding up runoff).
    • Deforestation (reducing storage).
    • Inadequate flood management structures (e.g., failed levées or poorly maintained drains).
Prediction of Flood Risk: Recurrence Intervals

Flood risk is predicted using historical data to calculate the probability of a flood event occurring in any given year.

The recurrence interval (or return period, $R$) is the average time between floods of a certain magnitude (size).

The probability ($P$) is calculated by:

$$P = \frac{1}{R}$$

Example: A '100-year flood' has a recurrence interval (R) of 100 years. The annual probability (P) of this flood happening is \( \frac{1}{100} \) or 1%.

⚠️ Common Mistake: A 100-year flood can happen next year, or twice in the same decade! It simply means there is a 1% chance *every year*. It does not mean it only happens once per century.

1.3 Prevention and Amelioration of River Floods

We manage flood risk through two main approaches: Hard Engineering (building structures) and Soft Engineering (working with nature).

A. Forecasts and Warnings (Non-Structural)

These focus on reducing the impact on people, not stopping the water itself.

  • Forecasts: Predicting when and where a flood will occur (using weather radar, river level gauges).
  • Warnings: Communicating the risk to the public so they can prepare (evacuation, moving valuables).
B. Hard Engineering (Structural)

These are usually expensive, involve construction, and often have environmental drawbacks.

  • Dams and Reservoirs: Huge stores that can hold back floodwater and release it slowly. (Benefit: Highly effective protection. Drawback: Massive social/environmental disruption, siltation.)
  • Straightening and Deepening Channels: Cuts off meanders to speed up flow and increases the channel capacity. (Benefit: Fast removal of water. Drawback: Increases flood risk downstream, destroys habitats.)
  • Levées (Embankments): Raised banks along the river, increasing the channel's height capacity. (Benefit: Protects the immediate area. Drawback: Can fail suddenly, causing catastrophic flooding, and separates the river from its natural floodplain.)
  • Diversion Spillways/Flood Relief Channels: Artificial channels built parallel to the main river to divert excess water during a flood. (Benefit: Reduces pressure on urban areas. Drawback: Expensive, requires lots of land.)
C. Soft Engineering (Environmental)

These methods are often cheaper, more sustainable, and work by enhancing the natural processes of the drainage basin.

  • Floodplain and Drainage Basin Management: Managing land use across the entire catchment area (e.g., limiting building on high-risk floodplains).
  • Wetland and River Bank Conservation: Restoring marshes and wetlands, which act as natural sponges, storing water and slowing flow.
  • River Restoration: Allowing the river to return to a more natural, meandering course, which increases friction and reduces flow velocity.

💡 Key Takeaway for 1.4: Human activity generally reduces the capacity of the drainage basin to store water (less interception, less infiltration), leading to faster runoff and higher flood peaks. Management involves balancing expensive hard engineering with sustainable soft engineering.

Case Study Reminder: You MUST study a recent river flood event (e.g., Pakistan 2010 or UK floods 2015/2020) focusing on causes, impacts (people/environment), and evaluating the success/failure of management attempts.

2. Human Impact on Atmosphere and Climate (Syllabus 2.4)

Humans impact climate at two main scales: globally (through the atmosphere) and locally (in cities).

2.1 The Enhanced Greenhouse Effect (EGE) and Global Warming

The Earth naturally has a Greenhouse Effect—this is good! Gases like water vapour and CO2 trap some outgoing longwave radiation, keeping the planet warm enough for life.

The Enhanced Greenhouse Effect (EGE)

The EGE is the additional warming caused by human activities (anthropogenic factors) increasing the concentration of Greenhouse Gases (GHGs) in the atmosphere.

Evidence of Global Warming
  • Rising global average temperatures (records show a clear upward trend since the industrial revolution).
  • Melting glaciers and ice caps.
  • Sea level rise (due to thermal expansion and meltwater).
  • Changes in the distribution and frequency of extreme weather events.
Possible Causes (Human Activity)

The main human cause is the increased burning of fossil fuels (coal, oil, gas) for energy, industry, and transport, releasing massive amounts of CO2.

  • CO2: From burning fossil fuels and deforestation (less CO2 uptake by plants).
  • Methane (CH4): From farming (livestock and rice paddies) and decaying waste in landfills.
  • Nitrous Oxide (N2O): From industrial processes and nitrogen fertilisers.
Atmospheric Impacts
  • Increased global average temperature (Global Warming).
  • Altered Precipitation Patterns: Some areas become wetter, others drier, increasing risks of drought or flooding.
  • Changes to wind and pressure systems (e.g., affecting the frequency or intensity of tropical storms).
  • Ocean Acidification: Oceans absorb excess CO2, changing their pH level, which harms marine life, especially coral reefs.

2.2 Urban Climate: The Urban Heat Island (UHI)

Cities are often significantly warmer than the surrounding rural areas—this is called the Urban Heat Island (UHI) effect.

Effects of Human Activity on Urban Climate:
1. Temperature (Heat Island)
  • Cause: Building Materials: Concrete, bricks, and tarmac have a high heat capacity and low albedo (they absorb solar radiation during the day and release this heat slowly at night). (Analogy: A dark asphalt road acts like a radiator.)
  • Cause: Anthropogenic Heat: Heat generated by human activities, such as cars, air conditioning units, and industrial processes.
  • Cause: Pollution Dome: Airborne pollutants trap outgoing longwave radiation, enhancing the local greenhouse effect within the city boundary.
2. Humidity
  • Lower Humidity: Water is quickly removed via drains and pipes, reducing evaporation. Less vegetation also means less transpiration. The air is generally drier than rural areas.
3. Precipitation
  • Higher Precipitation: The warmer air above the UHI is more unstable and rises easily. City pollution provides more condensation nuclei (tiny particles for water droplets to form around). These two factors increase the chance of rainfall, often slightly downwind of the city centre.
4. Winds
  • Wind Speed Reduction: Tall buildings create friction, slowing down overall wind speeds (known as the 'blanket' effect).
  • Local Turbulence: Streets act as 'urban canyons', funnelling wind and causing swirling eddies of turbulence at street level.

💡 Key Takeaway for 2.4: Globally, human GHGs drive climate change; locally, the structure of cities creates unique microclimates, most notably the UHI, which alters temperature, wind, and rainfall patterns.

Case Study Reminder: You MUST study a named urban area (e.g., London, UK or Phoenix, USA) showing the effects on temperature, humidity, precipitation, and winds.

3. Human Impact on Slopes and Mass Movement (Syllabus 3.4)

Slope stability is a delicate balance between forces trying to hold the slope up (shear strength) and forces trying to pull it down (shear stress, usually gravity). Human activities can change this balance, making slopes more stable or less stable.

3.1 The Impact of Human Activities on Slope Stability

A. Decreasing Slope Stability (Increasing Mass Movement Risk)

These activities reduce shear strength or increase shear stress:

  • Deforestation/Vegetation Removal: Tree roots bind the soil together. Removing them drastically reduces the soil's shear strength, making shallow slips and slides more likely.
  • Overloading the Slope: Building large structures, waste tips, or road infrastructure on the top or middle of a slope increases the load, thereby increasing shear stress (gravity's pull).
  • Excavation and Steepening: Human activity (like road building or quarrying) can undercut the base (toe) of the slope, increasing the slope angle and removing natural support, increasing shear stress.
  • Vibrations: Blasting for construction or heavy traffic can cause small earthquakes that weaken the slope material.
  • Adding Water: Leaking pipes, faulty drainage systems, or irrigation can saturate the soil. Water adds weight (increasing shear stress) and acts as a lubricant (reducing shear strength).
B. Increasing Slope Stability (Reducing Mass Movement Risk)

These activities aim to increase shear strength or decrease shear stress (often through modification strategies).

3.2 Strategies to Modify Slopes to Reduce Mass Movements

These are structural and environmental methods used to protect communities from landslides, rockfalls, and flows.

  • Pinning (Rock Bolts): Long steel bars drilled deep into the rock face to secure unstable joints and fissures, effectively increasing the internal cohesion and strength of the rock mass.
  • Netting and Fences: Steel mesh or wire netting draped over steep rock faces to catch smaller falling debris (rockfalls) and prevent them from reaching roads or homes below. Barriers can also be built at the base.
  • Grading (or Regrading/Battering): Reducing the slope angle, often by digging material from the top and placing it at the bottom. This is highly effective as it reduces shear stress. It can be expensive and requires a large area of land.
  • Afforestation/Planting Vegetation: Using plant roots to bind shallow soil layers, preventing shallow flows and slides. This is a cost-effective, soft engineering method.
  • Drainage Control: Installing trenches, pipes, and diversion channels to ensure water is quickly removed from the slope material and the surface. This is vital because water is a major trigger for mass movement.

Did you know? The stability of a slope is often modelled using the Factor of Safety (FoS). If FoS > 1, the slope is stable. If FoS < 1, failure is likely. Human interference often pushes the FoS below 1.

💡 Key Takeaway for 3.4: Slopes are unstable when humans either remove the natural support (deforestation, excavation) or add too much weight and water (building, leaky pipes). Management focuses on reinforcing the slope (pinning/netting) or reducing the angle (grading).

Case Study Reminder: You MUST study the impacts of human activity on slope stability (e.g., Hong Kong or La Conchita, California) and evaluate the attempts to reduce mass movement.

🌟 Quick Review Box: Three Human Impacts

To ace this chapter, remember the three physical environments and the key human actions:

1. Rivers (Hydrology):
Human Action: Urbanisation, Deforestation, Dams.
Key Result: Reduced lag time, increased flood peaks.
Management: Hard (dams, levées) vs. Soft (wetland restoration).

2. Climate (Atmosphere):
Human Action: Fossil fuels (Global); Buildings/Materials (Local).
Key Result: Global Warming (EGE); Urban Heat Island (UHI).
Management: Mitigation (reducing GHG emissions) and Adaptation (e.g., green roofs in cities).

3. Slopes (Geomorphology):
Human Action: Deforestation, Overloading, Excavation, Adding water.
Key Result: Decreased shear strength, increased shear stress, resulting in mass movement.
Management: Pinning, Netting, Grading, Drainage.