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Module 8: Shoreline Risk Proxy

Statistical Variability Analysis and the Master Stability Metric

The Master Metric: What Is Shoreline Risk Proxy?

The Shoreline Risk Proxy is Nimpact's single most important stability metric. Unlike temperature or clarity (which measure current conditions), the risk proxy quantifies the historical variability of waterline position over 10 years (2014-2024)—essentially measuring how "unstable" or "mobile" a shoreline has been.

Key Concept: The risk proxy does NOT measure "how many meters of land were lost"—it measures the statistical variability (standard deviation) of waterline position. High variability = unpredictable behavior = elevated risk = professional assessment needed.

The Data Source: JRC Global Surface Water

Nimpact uses the European Commission's Joint Research Centre (JRC) Global Surface Water dataset, which analyzed 5 million Landsat images from 1984-2024 to map where water was detected in every 30m pixel over time:

JRC Dataset Specifications

  • Temporal Resolution: Monthly water presence/absence (1984-2024)
  • Spatial Resolution: 30 meters (Landsat native)
  • Global Coverage: Every location on Earth
  • Validation: 99.7% accuracy in water/land classification
  • Algorithm: Expert system analyzing all Landsat bands to detect water

For each pixel, JRC records: "In how many images was this pixel classified as water?" A pixel that's water 100% of the time is permanent water. A pixel that alternates between water and land is the active shoreline zone.

How the Risk Proxy Is Calculated

# Shoreline Risk Proxy Calculation # Step 1: Extract 10-year water frequency history (2014-2024) # For each 30m pixel in a 500m radius around the pin Pixel A: Water in 0% of images → Always land Pixel B: Water in 15% of images → Usually land, occasionally wet Pixel C: Water in 50% of images → THE SHORELINE ZONE (active boundary) Pixel D: Water in 85% of images → Usually water, occasionally exposed Pixel E: Water in 100% of images → Always water # Step 2: Calculate standard deviation of water presence # Pixels with 30-70% water frequency have highest variability # Step 3: Risk Proxy = Standard Deviation of water presence # Values typically range 0.01 (stable) to 0.30+ (highly variable) # Step 4: Compare to water-type-specific thresholds TIDAL_THRESHOLD = 0.18 # Coastal areas naturally more dynamic LAKE_THRESHOLD = 0.15 # Lakes more stable than tidal RIVER_THRESHOLD = 0.30 # Rivers most dynamic

Interpreting Risk Proxy Values

The risk proxy is NOT a linear measurement of land loss. It's a statistical measure of shoreline behavior consistency:

Risk Score Interpretation Action
< 0.10 Very Stable - waterline barely moves Standard monitoring sufficient
0.10-0.15 Moderately Stable - seasonal variation normal Follow local building setbacks
> 0.15 (lakes) Elevated Variability - movement exceeds seasonal norms P.Eng assessment for structures < 30m from water
> 0.18 (tidal) Elevated Variability - active coastal processes Coastal engineering assessment required
> 0.30 (rivers) High Variability - channel migration or bank erosion Fluvial geomorphology study required

Water-Type-Specific Thresholds

Different waterbody types have different "normal" variability ranges:

Why Different Thresholds?

  • Lakes: Enclosed systems with stable water levels (except during major droughts/floods). Threshold = 0.15. High scores indicate unusual instability.
  • Tidal/Coastal: Daily tidal cycles cause normal intertidal zone movement. Threshold = 0.18. Accounts for natural tidal variability.
  • Rivers: Flowing systems with natural channel migration and seasonal flood stages. Threshold = 0.30. Rivers are inherently more dynamic.

What Causes High Risk Scores?

Several physical processes can drive elevated shoreline variability:

  1. Wave Action: Persistent wave energy erodes banks and transports sediment
    • Dominant in large lakes and exposed coasts
  2. Water Level Fluctuations: Multi-year wet/dry cycles expose different shoreline positions
    • Example: Great Lakes 2013 (record low) to 2020 (record high) = 2m swing
  3. Groundwater Seepage: Subsurface water saturates banks, causing slumping
    • Common in bluff areas with artesian aquifers
  4. Channel Migration: Rivers naturally meander, cutting new channels
    • Normal for alluvial rivers, problematic for infrastructure
  5. Ice Push: Spring ice breakup pushes shoreline materials inland
    • Significant in northern lakes with 4+ months ice cover
  6. Human Modification: Upstream dams, dredging, hardened structures alter sediment transport
    • Can accelerate erosion downstream of intervention

Local Comparison: Ranking Against Nearby Beaches

Nimpact ranks your beach's risk proxy against the 5 nearest beaches with similar water body type:

# Example Local Comparison Your Beach: Risk Proxy = 0.22 Nearest 5 Lakes: 1. Lake A (8km away): 0.08 (very stable) 2. Lake B (12km away): 0.14 (moderate) 3. Lake C (15km away): 0.19 (elevated) 4. YOUR BEACH: 0.22 (elevated) 5. Lake D (18km away): 0.25 (high) 6. Lake E (22km away): 0.31 (very high) Ranking: #4 out of 6 (67th percentile) Interpretation: "More variable than 2 of 5 nearest lakes— elevated risk relative to immediate area"

This local context is critical—a score of 0.22 might be normal in a wave-exposed region but exceptional in a sheltered bay.

Limitations and Cautions

What the Risk Proxy Cannot Do:
  • Predict Future: Historical variability doesn't guarantee future behavior (climate change, development)
  • Measure Rate: Doesn't tell you "2 meters/year erosion"—only that movement is occurring
  • Identify Cause: Can't distinguish between wave action, groundwater, or ice push without field investigation
  • Replace Engineering: Screening tool only—P.Eng required for design of shoreline protection

When Professional Assessment Is Mandatory

Nimpact flags for engineering review when:

Value Proposition: By pre-screening ALL potential sites and identifying the 10-20% that actually need engineering review, Nimpact can save $50,000-100,000+ in unnecessary assessments while ensuring high-risk sites get proper professional attention.
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