Grade 12

Advanced

10 min

Lesson 7 of 12

Environmental Risk Assessment

Not Started

Applying GIS to identify areas vulnerable to natural hazards like floods, wildfires, and erosion.

Imagine a city where the mayor knows exactly which house will flood before the first raindrop falls. How do we turn raw satellite data into a life-saving crystal ball?

Learning Objectives

1.Synthesize multi-criteria data (elevation, soil, and precipitation) to map flood susceptibility using weighted overlays.
2.Analyze the intersection of human development and critical habitats using spatial buffer analysis.
3.Model wildfire propagation vectors based on fuel load, slope aspect, and wind patterns.

The Power of Weighted Overlay

To predict environmental risk, GIS professionals use Multi-Criteria Decision Analysis (MCDA). Instead of looking at one map, we stack multiple layers—like elevation, soil drainage, and rainfall—and assign each a 'weight' based on its importance. For flood mapping, a Digital Elevation Model (DEM) is often the most critical layer. We calculate a total risk score

R

using the formula:

\begin{gathered}R = \\sum_{i=1}^n w_i c_i\end{gathered}

where

w_i

is the weight of the factor and

c_i

is the scaled value of the criteria. By reclassifying these values into a common scale (e.g., 1 to 10), we can identify high-risk zones where multiple dangerous conditions overlap.

Let's calculate the risk score for a single pixel in a raster map. 1. Assign weights: Elevation ( $w_1 = 0.6$ ), Soil Permeability ( $w_2 = 0.4$ ). 2. Get pixel values (scaled 1-10): Elevation score $c_1 = 2$ (low land), Soil score $c_2 = 8$ (clay/non-porous). 3. Apply the formula: $R = (0.6 \\times 2) + (0.4 \\times 8) = 1.2 + 3.2 = 4.4$ . 4. Result: On a scale of 10, this area has a moderate risk.

Quick Check

In a flood model, why might we give 'Elevation' a higher weight than 'Soil Type'?

Think about which factor physically forces water to move.

Answer

Gravity dictates that water primarily flows to the lowest point regardless of soil, making elevation the dominant physical driver of flooding.

Habitat Fragmentation and Encroachment

Environmental risk isn't just about disasters; it's about the risk humans pose to nature. We use Buffer Analysis to identify 'Edge Effects'—the zones where human activity (roads, noise, pollution) bleeds into wild spaces. If a critical habitat is smaller than its required Core Area after buffering for human encroachment, the species within are at high risk of extinction. GIS helps us calculate the Connectivity Index, ensuring that 'wildlife corridors' remain open between fragmented patches of forest.

A new highway is planned through a forest. 1. Create a 500m buffer on both sides of the highway vector to represent the 'noise disturbance zone'. 2. Use the Clip tool to remove this buffer from the total forest polygon. 3. Calculate the remaining 'Core Area'. If the area drops below $50\%$ of the original, the project fails the sustainability check.

Quick Check

If a forest is cut in half by a road, why is the total 'Core Area' loss usually greater than just the area of the road itself?

Consider the area surrounding the road, not just the pavement.

Answer

Because of 'Edge Effects'—the disturbance zone (noise, light, invasive species) extends deep into the forest from the road's edge.

Modeling Wildfire Propagation

Wildfire modeling is dynamic, meaning it changes over time. We use Raster Surface Analysis to look at Aspect (the direction a slope faces) and Fuel Load (vegetation density). In the Southern Hemisphere, north-facing slopes receive more sun, drying out vegetation and increasing risk. We model fire spread as a vector where the rate of spread

S

is influenced by wind speed

W

and slope gradient

\\theta

\begin{gathered}S \\propto W \\cdot \\cos(\\theta)\end{gathered}

This allows emergency services to predict where a fire will be in 6 hours and evacuate residents proactively.

Predicting fire movement on a 3D surface: 1. Identify the ignition point on a north-facing slope ( $30^\circ$ incline). 2. Layer a Wind Vector map showing 40km/h gusts from the West. 3. Use the Cost-Distance tool in GIS to calculate the 'least resistance' path for the fire. 4. The fire will move fastest where the wind vector aligns with the steepest upward slope.

Key Takeaways

1Weighted Overlay ( $R = \\sum w_i c_i$ ) is the standard method for combining diverse environmental datasets.
2Buffer Analysis is essential for measuring human encroachment and protecting core habitat areas.
3Wildfire modeling requires integrating static data (slope/aspect) with dynamic data (wind/moisture).
4GIS turns reactive disaster management into proactive risk mitigation.

Quick Check

Multiple Choice

Which GIS tool is best for determining the 'noise pollution zone' around a new airport?

Multiple Choice

In a Weighted Overlay for wildfire risk, which slope aspect is generally highest risk in the Northern Hemisphere?

True/False

A Weighted Overlay requires all input layers to be reclassified to the same numeric scale before calculation.

What's Next?

Review Tomorrow

In 24 hours, try to sketch the formula for Weighted Overlay and explain how 'Edge Effects' impact habitat size.

Practice Activity

Open a free GIS tool (like QGIS or ArcGIS Online) and try to create a 1km buffer around a local river to see which buildings fall within a potential flood zone.

Browse

Browse

Environmental Risk Assessment

Learning Objectives

The Power of Weighted Overlay

Habitat Fragmentation and Encroachment

Modeling Wildfire Propagation

Key Takeaways

Quick Check

What's Next?

Environmental Risk Assessment

Learning Objectives

The Power of Weighted Overlay

Habitat Fragmentation and Encroachment

Modeling Wildfire Propagation

Key Takeaways

Quick Check

What's Next?