Nimpact Academy - Module 7: Terrain Analysis & Vegetation

The Physical "Bones" of a Shoreline

While water quality metrics assess current conditions, terrain analysis evaluates the permanent physical structure that determines long-term stability and erosion risk. Nimpact uses NASA's Shuttle Radar Topography Mission (SRTM) elevation data and Landsat Normalized Difference Vegetation Index (NDVI) to assess shoreline resilience.

SRTM Elevation Data

The SRTM dataset was collected during an 11-day Space Shuttle mission in February 2000, using radar interferometry to measure Earth's topography:

SRTM Technical Specifications

Spatial Resolution: 30 meters (1 arc-second) for North America, 90m globally
Vertical Accuracy: ±6 meters (90% confidence)
Coverage: 80% of Earth's land surface (60°N to 56°S)
Measurement Method: C-band and X-band radar penetrates vegetation to measure ground surface

SRTM's key advantage over optical satellites is vegetation penetration—it measures true ground elevation, not tree canopy height.

Slope Analysis: The Critical 15% Threshold

Nimpact calculates slope (rate of elevation change) within 100m of the shoreline. Slope expressed as percent rise:

# Slope Calculation Slope = (Vertical Rise / Horizontal Distance) × 100 Example 1: 5m rise over 100m distance Slope = (5/100) × 100 = 5% (gentle, stable) Example 2: 15m rise over 100m distance Slope = (15/100) × 100 = 15% (steep, caution threshold) Example 3: 25m rise over 100m distance Slope = (25/100) × 100 = 25% (very steep, high risk)

Why 15% is the Action Threshold

Engineering and geotechnical standards identify 15% (8.5° angle) as the point where:

Natural slope stability begins to decrease significantly
Erosion rates increase exponentially
Landslide and bluff collapse risk becomes elevated
Foundation engineering becomes more complex and expensive
Local building codes typically require geotechnical assessment

Bluff Height and Collapse Risk

Coastal bluffs (steep slopes > 10m height) present special hazards:

Wave Undercutting: Water erodes base, creating overhang that eventually collapses
Freeze-Thaw Cycles: Water infiltrates cracks, freezes, expands, and breaks apart rock/soil
Groundwater Seepage: Subsurface water reduces soil strength and triggers slumping
Vegetation Loss: Tree removal or death eliminates root stabilization

# Bluff Height Risk Classification Height < 5m: Low risk - typical bank erosion Height 5-15m: Moderate risk - monitor for undercutting Height 15-30m: Elevated risk - periodic inspection needed Height > 30m: High risk - professional geotechnical assessment required

NDVI: Vegetation as Natural Armor

The Normalized Difference Vegetation Index quantifies photosynthetically active vegetation by measuring the difference between near-infrared (NIR) and red light reflectance:

# NDVI Formula NDVI = (NIR - Red) / (NIR + Red) Where: NIR = Near-Infrared reflectance (Band 5 in Landsat 8/9) Red = Red reflectance (Band 4 in Landsat 8/9) Values range from -1 to +1: NDVI < 0: Water, bare rock, snow NDVI 0-0.2: Bare soil, dead vegetation NDVI 0.2-0.4: Sparse vegetation (grassland, crops) NDVI 0.4-0.6: Moderate vegetation (shrubland, agriculture) NDVI 0.6-0.8: Dense vegetation (forests, wetlands) NDVI > 0.8: Very dense tropical forests

Why Vegetation Matters for Erosion Control

Vegetation protects shorelines through multiple mechanisms:

Root Systems: Bind soil particles, increase shear strength, resist mass wasting
Canopy Interception: Reduces raindrop impact energy, minimizes surface erosion
Water Uptake: Removes soil moisture, increases stability, reduces pore pressure
Wind Buffer: Reduces wind erosion and wave energy at shoreline
Energy Dissipation: Stems and roots slow water flow, trap sediments

    Critical Insight: A 25% slope with dense forest (NDVI = 0.7) is often more stable than a 10% slope with bare soil (NDVI = 0.1). Vegetation can compensate for steep terrain—until that vegetation is lost.

Temporal NDVI Analysis

Nimpact doesn't just measure current NDVI—it tracks changes over time (2015-2024) to identify vegetation decline:

# Vegetation Health Trend Analysis Year 2015: NDVI = 0.68 (healthy forest) Year 2020: NDVI = 0.71 (stable) Year 2024: NDVI = 0.45 (concerning decline) Interpretation: 34% NDVI loss over 9 years Likely causes: - Invasive species (emerald ash borer, bark beetles) - Disease (oak wilt, sudden oak death) - Development/clearing - Drought stress - Storm damage Action: Investigate cause, consider vegetation restoration

NDVI Decline Alert Thresholds

> 10% NDVI loss in 5 years: Normal variation, monitor
> 20% NDVI loss in 5 years: Significant decline, investigate causes
> 30% NDVI loss in 5 years: Critical decline, "natural armor" failing, erosion risk elevated

Combining Slope and NDVI: Stability Matrix

Nimpact evaluates shoreline stability using a matrix approach:

Slope	NDVI	Stability	Recommendation
< 10%	> 0.5	Excellent	Standard monitoring
10-15%	0.3-0.5	Moderate	Maintain vegetation
> 15%	< 0.3	Poor	Geotechnical assessment required

Limitations of Terrain Analysis

SRTM and NDVI have important constraints:

SRTM Age: Data from 2000—doesn't capture recent development or erosion
Vertical Accuracy: ±6m error means subtle slope changes may be missed
Resolution: 30m pixels can't detect small-scale features like individual trees or small gullies
NDVI Ambiguity: Can't distinguish forest types or detect subsurface root health
No Soil Data: Clay, sand, and rock have very different stability despite similar slopes

Professional Assessment Still Needed: Nimpact terrain analysis is a screening tool. Sites with steep slopes, high bluffs, or declining vegetation should receive on-ground geotechnical investigation before development.

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Module 7: Terrain Analysis & Vegetation