Nimpact Academy - Module 11: Coastal Dynamics & Beach Width

Beach Width Variability: The Hidden Story of Erosion

While the Shoreline Risk Proxy measures *where* the waterline moves, Beach Width Variability measures how the physical beach itself is changing—growing wider through accretion or narrowing through erosion.

The NDWI Method: Tracking Water's Edge

Nimpact uses the Normalized Difference Water Index (NDWI) to distinguish water from land across 25+ years of satellite history. By tracking how NDWI values change at your beach location, we can estimate whether the beach is growing or shrinking.

# NDWI Calculation (Sentinel-2)
NDWI = (Green - NIR) / (Green + NIR)
NDWI = (Band3 - Band8) / (Band3 + Band8)

# For Landsat 7
NDWI = (Band2 - Band4) / (Band2 + Band4)

Values range from -1 to +1:
  NDWI > 0.3:  Open water
  NDWI 0 to 0.3: Wet sand, shoreline zone
  NDWI < 0: Dry land, vegetation

Three-Period Analysis

Nimpact compares three time periods to measure long-term trends:

Period	Years	Satellite	Purpose
Baseline	2000-2009	Landsat 7	Historical reference
Mid-Period	2015-2019	Sentinel-2	Transition assessment
Recent	2020-2025	Sentinel-2	Current conditions

Why Multi-Period Analysis?

A single 5-year period might capture a temporary anomaly (like a major storm or exceptional drought). By looking at 3 periods over 25 years, we can distinguish true long-term trends from short-term fluctuations.

Converting NDWI to Beach Width

Here's the clever part: NDWI changes correlate with physical beach width changes. Through extensive calibration, Nimpact has determined that:

Calibration Factor: NDWI change of -0.1 ≈ 10 meters of beach narrowing

Formula:
Estimated Width Change (m) = NDWI Change × -100

Example:
If NDWI decreased from 0.15 to 0.11 (change = -0.04):
Width Change = -0.04 × -100 = +4 meters (beach widened!)

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Interpreting Beach Width Results

Trend Classification

Width Change	Trend	Interpretation
> +5 meters	Widening (Accretion)	Beach is growing—sediment deposition exceeds erosion
-5 to +5 meters	Stable	Minor fluctuations within natural variability
< -5 meters	Narrowing (Erosion)	Beach losing width—erosion exceeds deposition

Annual Rate Calculation

The report also calculates rate of change per year:

Rate (m/year) = Total Width Change / Years Elapsed

Example from Report:
  Width Change: -4.1 meters (narrowing)
  Period: 2000 to 2020 (20 years)
  Rate: -4.1m / 20 years = -0.21 m/year
  
Interpretation: Beach losing about 20cm per year

⚠️ Critical Interpretation Rules

Rates < 0.5 m/year: Typically within natural variability—not automatically a concern
Rates 0.5-1.0 m/year: Moderate change—worth monitoring and investigating causes
Rates > 1.0 m/year: Significant change—likely requires professional geotechnical assessment

Real-World Example

Looking at the McGregor West report:

Width Change: -4.1 meters
Annual Rate: -0.21 m/year
Trend: Stable

Analysis: While technically narrowing, the rate is so slow (21 cm/year) that it's within the normal variability for a lake beach. This is NOT a red flag for erosion—it's just documenting that the beach hasn't significantly changed in 20+ years.

What Causes Beach Width Changes?

Accretion (Widening):

Upstream sediment supply from rivers
Longshore drift depositing sand
Storm events transporting beach material onshore
Human interventions (beach nourishment projects)

Erosion (Narrowing):

Wave action removing sediment
Reduced sediment supply (dams blocking rivers)
Sea level or lake level changes
Coastal development disrupting natural processes
Removal of protective vegetation

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Suspended Sediment & Water Quality

Beyond Clarity: What's IN the Water?

While Secchi depth tells you how far you can see, suspended sediment analysis tells you what's making the water murky. This matters because:

Mineral sediment (clay, silt) indicates erosion or runoff—potential instability
Organic turbidity (algae, tannins) is often natural—not necessarily a problem

The Sediment Ratio Method

Nimpact uses Sentinel-2's multiple bands to distinguish sediment types:

# Sediment Ratio (B4/B3)
Sediment_Ratio = Red_Band / Green_Band
Sediment_Ratio = Band4 / Band3

# Classification
Ratio < 1.0:  Low sediment (organic turbidity)
Ratio 1.0-1.2: Moderate sediment  
Ratio > 1.2:  High sediment (mineral-laden water)

Why This Works

Mineral sediment (clay/silt) reflects red light strongly → high B4/B3 ratio

Organic material (algae/tannins) absorbs red light → low B4/B3 ratio

This lets us distinguish erosion-sourced turbidity from natural organic turbidity!

Absolute Turbidity Measurement

The report also includes absolute turbidity index—a measure of total water clarity regardless of source:

Turbidity_Index = (B4 × 0.3 + B3 × 0.7) / 1000

Classification:
  < 1.0:  Excellent (clear water)
  1.0-2.0: Good (typical lake/coastal)
  2.0-3.0: Fair (murky, elevated turbidity)
  > 3.0:  Poor (very turbid, impaired)

Water Quality Overall Score

Nimpact combines turbidity and chlorophyll into an overall quality score:

Scoring Formula:

Turbidity Score: 95 (Excellent) down to 25 (Poor)
Chlorophyll Score: 95 (Low algae) down to 25 (High algae)

Overall Score = (Turbidity Score + Chlorophyll Score) / 2

Example (McGregor West):
Turbidity: Good (75 points)
Chlorophyll: Good (75 points)
Overall: 75/100 (Good)

Marine Debris Assessment

Advanced reports may include a debris index that uses optical signatures to estimate plastic and trash accumulation potential. This is experimental—values are relative indicators, not absolute measurements. Always verify with field observations!

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📏 Module 11: Coastal Dynamics & Beach Width