This project examines the rate of erosion in Little Harbour, on the south-east coast of the Northumberland Strait. Coastlines were digitized using a series of airphoto mosaics from the 1970s to the present. The rate of change between digitized lines is measured using a script developed at the AGRG. Attributes are added to the data, classifying it by landform, waterbody, and angle. Results are examined to determine the overall rate of erosion, as well as to determine areas of increased vulnerability.

1. Coastal Erosion
in
Little Harbour, NS
Centre of Geographic Sciences, NSCC
Advanced Diploma in Geographic Sciences, Capstone Project
By Michael Hannon
May, 2016

2. Definitions
• Coastline
• Landward extent of the influence of the ocean
• Shoreline
• Specific water level (e.g. high tide line)
• Erosion
• Gradual & Long Term Removal of Land
• Accretion
• Gradual & Long Term Addition of Land
• Avulsion
• Significant Short Term Removal or Addition of Land
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3. Little Harbour, Nova Scotia
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4. Dynamic Environment
4
1971 to 2007

5. Source Data
• Mosaics from 1954, 1971, 1979, 1990, 2003, 2007,
and 2014
• 1954 image not used due to limited extent, poor
resolution & large warping in areas
• All orthorectified and georeferenced at AGRG
5
Year Date
Original
Scale Bands
Mosaic
Resolution Min Tide Max Tide Source
1971 June 15th 1:16,800 RGB 50cm 2.41 2.51NS Gov
1979 June 20th 1:10,000 RGB 50cm n/a n/a NS Gov
1990 July 12th 1:10,000 RGB 50cm 1.15 1.67NS Gov
2003 May 16th 1:18,000 RGB 50cm 0.41 0.54NS Gov
2007 July 28 - Aug 2nd 1:12,500 RGB 50cm 2.51 2.52NS Gov
2014 September 15th RGB + NIR 20cm AGRG

6. 2014 Mosaic
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7. Mosaics
7
1954
1971
1979
1990
2003
2007

8. Registration
• Source mosaics well georeferenced, but small
errors existed, especially in 1971 mosaic
• Largest errors were on Roy Island
• Attempt to improve accuracy along coast by
georeferencing (registering) all mosaics to 2014
• Accuracy checks
• Polynomial 1 RMS values
• Digitized roads
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9. Thin Plate Spline
• Polynomial method distributes error across entire
image, including at control points
• Thin Plate Spline has ~0 error at control points; error is
spread through the rest of the image
• Better results in practice for this exercise than the
polynomial n method
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10. GCPs
• 48 Ground Control Points (GCPs)
collected
• Generally collected along
coastline
• All GCPs tied to identifiable
features on the reference mosaic
from 2014
• Feature types
• Road intersections
• Building corners
• One prominent rock formation
used as a GCP on Roy Island due to
lack of other identifiable features
10

11. 11

12. 12

13. Registration: Results
• RMS values from
Polynomial 1 formula
used as a guide for
accuracy
• Alignment with roads
digitized off of 2014
mosaic used as well
• Error generally
decreased with newer
mosaics
0
5
10
15
20
25
1954 1971 1979 1990 2003 2007
Mean RMS (metres)
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14. Coastline Digitizing
• Digitized at various scales from 1:250 to 1:1000,
but most lines were digitized at 1:500
• Difficult to automate due to wide variety of
features / patterns at coastline
• General Rule: Digitize along limit of contiguous
vegetation
• Two passes:
• First pass for an approximate fit
• Second pass for adjustments, in context of other lines
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15. Marsh
• Large changes from year to
year
• Low relief makes avoiding
areas submerged at low
tide difficult
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2003
2007
1971

16. Beach
• High degree of change
& ambiguous coastline
• ‘Land’ was area with
~>50% vegetation cover,
contiguous with the
main body of vegetation
on land
16
2007 1979
2014

17. Engineered (Roadside)
• Controlled coastlines along
roadways
• Change in shore due to new
construction or error in
registration
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2014
19711990

18. 2014
18
Bank
19712003
• Vegetation edge sometimes
clearly visible due to erosion
• Other times, overhanging
trees obscure the coastline

19. Seawalls
• Artificial structures designed to slow erosion
• Shorelines drawn along landward edge of seawalls
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20. Attributes
• Landform, adjacent waterbody, and direction
• Spatially joined (using the NNJoin plugin) to the
output of the erosion measurement script
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21. 21
Landform Length (m)
Bank 18,701
Beach 7,199
Marsh 4,404
Engineered 2,104
Attributes: Landform

22. 22
Waterbody Length (m)
Strait 5,134
Outer Harbour 3,801
Inner Harbour 13,996
Cove 9,477
Attributes: Waterbody

23. Attributes: Direction
• Calculated using attribute table field calculator
• Using assumption that water was always on left-
hand side (along direction of travel) of the line, the
direction to the water was found by subtracting 90°
from the line direction
• Directions were classified into 15-degree
increments
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24. 24
Attributes: Direction

25. Erosion Calculation
• Based on script from AGRG
• Input: Folder of shapefiles, 1 line feature/file
• Year of data contained in filename
• Output: 1 output file per input file
• Erosion negative, Accretion positive
• Lines split into segments of user-defined length (value
used: 1 m)
• Attributes added by script giving amount and rate of
change relative to the other lines
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26. Erosion Calculation
• Calculates distance & rate of change
from each input line to every other
input line
• On each iteration, a reference line is
compared against all other lines
• Reference lines are split into 1 m
segments
• Comparison line converted into points
• Distance is measured using Near
function
• Type of change (erosion / accretion)
determined with a selection using a
polygon version of comparison line
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27. Clipping
• Large gaps in airphoto mosaic coverage, especially
in northern areas, leading to many nodata values
• Nodata values removed by clipping lines to the
extent common to all mosaics
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28. Closing
• Coastlines must be closed to obtain correct results
• As noted, the erosion calculator script creates a polygon
from the input line and makes a selection based on
intersection with that polygon
• Performance improved greatly after problem was corrected
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29. Change by Landform
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• Accretion & Erosion in
all landforms
• Long tails for Beach &
Marsh
• Most frequent: small
amount of erosion of
banks

30. Change by Waterbody Class
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• Again, accretion &
erosion in all categories
• Long tails for Harbour
classes
• Beaches, Marshes
more common
• Little change in coves

31. Change by Direction
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- No obviously
abnormal patterns
- Northumberland
Strait runs NW/SE
- Would appear
as high erosion
on 0-45 & 46-90

32. Waterbody vs Landform
• Generally uniform erosion of banks, except in coves
• Large movements of sandbars / marshes in Outer
Harbour
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Waterbody Class Bank Beach Engineered Marsh
Cove -0.70 -1.78 -1.64 -0.47
Inner Harbour -5.40 -2.31 1.14 -0.87
Outer Harbour -5.39 30.21 10.64
Strait -4.65 -4.36
Erosion (-ve) & Accretion (+ve) between 1979 and 2014

33. Sea Level Rise vs Rate of Erosion
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1.6
1.7
1.8
1.9-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
1970 1980 1990 2000 2010
AverageTideHeight(m)
RateofChangein
Coastline(m)
Bank Beach Engineered Marsh Avg Yearly Tide
Tide levels from the Charlottetown station (#1700)
- Rate of erosion is amount of erosion per year between 2014 and the
comparison line
- Rate of erosion increases with time, at roughly the same pace as the
increase in average high tide

34. Conclusions
• Generally small movements in the coastline
• Large movements in sandbars
• Moderate erosion of banks, consistent across most
years
• Trend of increasing rate of erosion is apparent
34 / 33

35. Questions?