Task 5: shoreline change memo

Town of Winthrop · from the Town's website · 12 pages · 3.0 MB
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81 Technology Park Dr., East Falmouth, MA 02536 
Direct Extension: (508) 495-6221 
Main Telephone: (508) 540-8080 
FAX: (508) 540-1001 
e-mail:lweishar@whgrp.com 
www.woodsholegroup.com 
MEMORANDUM 
Date: May 16,2016 
To: Mr. Art Leventis 
From: Lee Weishar, Ph.D.; PWS 
Re: Task 5A Shoreline Change and Memorandum 
Introduction 
A computer-based shoreline mapping methodology, within a Geographic Information System
(GIS) framework, was used to compile and analyze changes in historical shoreline position in the
vicinity of Coughlin Park in the Town of Winthrop, Massachusetts. The purpose of this task was
to quantify changes in shoreline and coastal bank positions using the most accurate data sources
and compilation procedures available, and to evaluate long-term rates of shoreline and bank
change. Because this information is critical to most coastal zone management decisions, emphasis
has been placed on data accuracy and clear presentation of results. The following section provides
a detailed description of the methods and data sources used in this study. 
Data Sources 
Shoreline change can be determined from accurately overlying historical vertical aerial
photography with recent digital orthoimagery. The shoreline, as defined by the high water line,
can then be extracted from each of the photos; the high water line is evidenced by a change in gray
tone on black and white and color aerial photographs, and the position of this wetted boundary is
identifiable on most air photos (Stafford and Langfelder, 1971). 
Given the timeframe of interest, this study used only digital orthoimagery (DOQQs) to evaluate
historical changes in shoreline (Table 1). There was shoreline data from the Office of Coastal Zone
Management (CZM) starting as early as 1839, but only years present in this analysis were used.
We started with 1995 to avoid any misinterpretation of shoreline change rates due to the Boston
Harbor dredging activities that artificially extended the land on which Coughlin Park is located.
Since the primary focus of this project is to mitigate erosion along the western shoreline of
Coughlin Park, the more recent data was more appropriate and would provide a more accurate
representation of the conditions that exist at the park. 
Shoreline position information was obtained from a series of large-scale digital aerial photographs
for the following time periods: 1995, 2001, 2005, 2008, and 2013 (Table 1). Photographs from
these years were selected based on availability of orthoimagery from the MassGIS data base. An
additional year of aerial imagery was also obtained from Google Earth for 2015. The time series

of photographs selected for this study represents the highest quality and most reasonable spaced
photographic data available. 
Table 1. Summary of Shoreline Source Data Characteristics for Study Area 
Data Compilation and Analysis Methods 
The aerial photographs shown in Table 1 provide a synoptic view of the Coughlin Park coastline
with a temporal coverage ranging from 2 to 6 years. Orthoimagery from 1995-2013 were retrieved
with completed geo-referenced spatial information. The georeferencing tool available within
ESRI’s ArcMap was used to transform, resample and rectify the 2015 image retrieved from Google
Earth against the 2013 MassGIS orthoimagery. 
The next analysis step involved identification of a reference shoreline for each of the aerial- and
orthophoto years. Air photo interpretation along a shoreline is an art based on science, supported
by familiarity with the area and its processes, and includes a certain amount of error and
interpretative subjectivity. Delineation of the reference shoreline is the most important and most
subjective part of shoreline change analysis, particularly in areas where relief distortion can
compound problems. The horizontal position of the high water shoreline as recognized on the
beach and on photography was determined using a hierarchy of criteria dependent on morphologic
features present on the subaerial beach. The primary criterion was a well-marked limit of uprush
by waves associated with the most recent high tide. This generally was recognized as a beach
scarp or debris line, marking the upper limit of the foreshore. If a scarp could not be identified, a
debris line usually was identified. Reference shorelines for each of the photographs were captured
through heads-up digitizing, and stored digitally in GIS. 
A second reference line representing the top of bank at the subject property (the west side of
Coughlin Park) was also captured through heads-up digitizing. The top of bank is easily visible
on most aerial photographs. This additional data was useful in comparing with the high water
shoreline to verify temporal changes in bank erosion. For coastal management purposes, the
erosion rate of the top of bank also represents the factor most important in determining short-term
risk to the park infrastructure. 
Once the shoreline and bank data were compiled spatial and temporal changes in the data were
quantified. This was accomplished by identifying a series of shore normal transects along the
coastline where discrete measurements of shoreline and top of the coastal bank change could be
made and could be computed to calculate rates of change. A total of 50 shore normal transects
were established at 25 foot evenly-spaced intervals along the shoreline which covered
approximately a quarter mile (Figure 1A). The area directly west of the tennis and basketball courts
is the immediate area of concern and incorporates the area that was used for the shoreline change 
Date Data Source Comments 
1995 MassGIS 1.6 ft/pixel resolution 
2001 MassGIS 1.6 ft/pixel resolution 
2005 MassGIS 1.6 ft/pixel resolution 
2008 MassGIS 1 ft/pixel resolution 
2013 MassGIS 1 ft/pixel resolution 
2015 Google Earth 1 ft/pixel resolution

analysis (Transects 29-47). This area west of the tennis courts was of high concern because of the
client’s observations of rapid erosion of this area. For the rate of bank change analysis, a total of
19 shore normal transects were established at 25 foot evenly-spaced intervals at the area of concern
(Figure 1B) which runs approximately 455 feet. 
Figure 1. Area in of Coughlin Park, Town of Winthrop, MA selected for analysis of
historical shoreline change (A). A small area pertinent to the study area was selected for
analysis of historical shoreline change. (B). Evenly spaced transects represent points where
bank change statistics were computed. 
At each transect, distances of shoreline movement were calculated, and annual rates of shoreline
change were determined using the various time intervals between shorelines. A matrix of long-
term and incremental shoreline change rates was developed using all of the available shoreline
data. The rates of shoreline change were calculated using two different methods. The shoreline
change rates were calculated using the end point method, which uses the distance over which the
shoreline position changed (measured along the transect), divided by the number of years over
which the change occurred. This method was used to compare successive shoreline dates.
Shoreline change rates were also calculated using linear regression. The linear regression method
was used to determine the rates of change for the entire data set (1995 to 2015) as well as two
shorter time periods, 1995 to 2005, and 2005 to 2015. In this method, an average rate of change
is based on a best-fit line to a series of points representing shoreline position over a period of time.
The linear regression method is most applicable when looking at long-term averages in the rate of
shoreline change, and is the method most often used for planning purposes and for management
decisions. 
A B

Synthesis of Changes in Shorelines and Top of Bank 
To evaluate the trends in shoreline change at Coughlin Park, a variety of graphical representations
have been developed. Three time periods were chosen to address specific questions concerning
the shoreline change. The longer term shoreline change period (1995-2015) was chosen to show
the rate of change over the entire period of data available. The longer term data provides the most
reliable quantification of the shoreline response because it smooths out the effects of large erosion
and accretion events. The shorter time periods analyzed reflect a greater influence of either a large
accretion or erosion event may have on the shoreline change rate and are instructive when
determining the effect that large storm events have on the shoreline. To analyze whether recent
rates of change have been different or similar to longer range trends, two additional periods were
also selected (1995 to 2005, and 2005 to 2015). Figures 2, 3 and 4 show the historical shoreline
positions from 1995 to 2015, 1995 to 2005, and 2005 to 2015. Graphs showing rates of shoreline
change, calculated using both linear regression and end point methods, are displayed along with
the shoreline position data in Figures 2, 3, and 4. Thus, trends of shoreline change at any given
point along the coast can be visualized and quantified by comparing the three graphs shown in
Figures 2, 3, and 4. 
For the bank analysis, Figure 5 shows the historical bank positions from 1995 to 2015, and Figures
6 and 7 show the bank positions between two shorter time periods, 1995 to 2005 and 2005 to 2015.
The graph showing rates of bank change, calculated using both linear regression and end point
methods are displayed next to the bank position data in Figures 5, 6, and 7. 
Shoreline Change Results 
The data sets in Figures 2 - 4 shows the important longer term trends in shoreline change for this
section of coast of Coughlin Park. The rates presented on the map itself are linear regression
results in ft/yr, while both linear regression and end point results are shown on the graphs beside
each map. Longer term accretion rates between 1995 and 2015 are highest (+0.8 ft/yr) at Transect
5 (approximately 620 feet north of the tennis courts), while erosion rates were highest (-1.7ft/yr)
at Transects 32-34 (approximately 220 feet north-west of the tennis courts). Longer term trends
at the area of concern (Transects 29 through 47) show that the shoreline was eroding at an average
of -1.2 ft/yr (-0.4 ft/yr using endpoint analysis). 
To see whether recent shoreline change rates have followed longer term trends, shoreline change
rates were also analyzed for 1995 to 2005 and 2005 to 2015. Between 1995 and 2005, the entire
shoreline was accreting at an average of 0.1 ft/yr differing from the longer term average erosion
rate of -0.3 ft/yr as noted in Table 2 1 . An interesting observation was seen between transects 13
to 26, where the shorter term trends (1995-2005) showed average accretion rates of 1.2 ft/yr which
differs from long term erosion rates of -0.2 ft/yr for the same transects (13-26) (Figure 1A)
indicating a transition from accretion to erosion. Erosion rates between 1995 and 2005 were
highest (-1.8 ft/yr) at Transects 33 and 41. The observed highest erosion rates at Transect 41 was
different from the longer term rates where highest erosion rates were located between Transects
32 and 34. Additionally, for the 1995 to 2005 time frame, the highest accretion rates (+1.5 ft/yr)
were seen at Transect 16 (approximately 600 feet north of the tennis courts); again, this was a
different area where highest accretion rates were observed in the longer term analysis (1995 to
2015). In fact, the long-term trends at Transect 16 showed erosion of -0.4 ft/yr. 
1 The erosion and accretion rates were averaged across all transects.

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Figure 2. Historical shoreline (high water line) positions from 1995 to 2015 and long-term rates of shoreline change computed
using linear regression and end point methods. Red lines denote area of concern. 
Transect Number 
Rate of Shoreline Change (ft/yr) 
Linear
Regression
to 2015 
Endpoint to 2015

Figure 3. Historical shoreline (high water line) positions from 1995 to 2005 and shorter term rates of shoreline change
computed using linear regression and end point methods. Red lines denote the area of concern. 
Transect Number 
Rate of Shoreline Change (ft/yr) 
Linear
Regression
to 2005 
Endpoint to 2005

Figure 4. Historical shoreline (high water line) positions from 2005 to 2015 and shorter term rates of shoreline change
computed using linear regression and end point methods. Red lines denote area of concern. 
Transect Number 
Rate of Shoreline Change (ft/yr) 
Linear
Regression
to 2015 
Endpoint to 2015

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Table 2. Average shoreline accretion and erosion rates (ft/yr) (Linear Regression) 
Shorter and longer term erosion rates for the park shoreline (Transects 27-47) were -1.0 ft/yr and
-1.2 ft/yr respectively (Table 2), indicating the erosion rates at the area of concern have been
steadily increasing. On the contrary, the area in the northern section of the shoreline (Transects 1-
12) showed the same accretion rate of 0.5 ft/yr for both the longer term and shorter term analysis
(1995-2005), indicating this area was stable and accreting sand. 
Evaluating the recent shoreline change rates from 2005 and 2015, however, showed a transition to
a more erosional phase throughout almost the entire study area with the exception of the most
northern section of the shoreline (Transects 1 to 12). Between 2005 and 2015, the area between
Transects 1 and 12 showed an accretion rate of 0.7 ft/yr (Table 2). Compared to the longer term
accretion rate of 0.5 ft/yr for Transects 1 to 12, this area, in recent years, has been accreting sand
at a faster rate. In the recent shorter term analysis (2005-2015), the northwest section of the
shoreline (Transects 13 to 26) was eroding at a rate of -0.3 ft/yr which differed from the accretion
phase this area was enduring during the 1995 to 2005 time period, indicating a change from
accretion to erosion from the early to recent time period. This erosion rate of -0.3ft/yr for the 2005-
2015 time period was similar to the long-term trend of -0.2 ft/yr for the same area (Transects 13-
26). The area of concern (Transects 27-47) had an erosion rate of -1.4 ft/yr for the time period
between 2005 and 2015, which was greater than the longer term average of -1.2 ft/yr and the 1995-
2005 time period average of -1.0 ft/yr, indicating that in recent years, erosion of the area of concern
has increased. 
End point rates, for both the shorter and longer term analyses, were generally lower in magnitude
compared to the linear regression rates throughout the site (Figure 2, 3, and 4). This indicated that
larger morphological changes were occurring between the endpoint year limits (1995 and 2015). 
Long-Term Top of Bank Changes 
As discussed earlier in this section, in addition to the shoreline change analysis, we also evaluated
the rate of change of the top of the bank in the area of concern (west side of Coughlin Park). The
locations of the top of bank as it was delineated from each of the data sources and rate of change
data are pictured in Figure 5 -7. Like the shoreline change analysis, the rate of bank change was
quantified along shore perpendicular transects. Figure 5 shows the linear regression rates of
change of the top of the bank from 1995 to 2015 for the 19 transects at the area of concern. The
longer term trend for the entire length of the bank showed the area eroding at an average rate of -
1.5 ft/yr (-0.5 ft/y using end point analysis). 
Transects 1995-2015 1995-2005 2005-2015 
1 to 12 0.5 0.5 0.7
13 to 26 -0.2 1.2 -0.3
27-47 -1.2 -1.0 -1.4
All -0.3 0.1 -0.5

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Figure 5. Historical bank positions from 1995 to 2015 and long-term rates of bank change computed using 
linear regression and end point methods. 
Bank Transect 
Rate of Bank Change (ft/yr) 
Endpoint to 2015 
Linear
Regression
to 2015

Figure 6. Historical bank positions from 1995 to 2005 and short-term rates of bank change computed using 
linear regression and end point methods. 
Bank Transect 
Rate of Bank Change (ft/yr) 
Endpoint to 2005 
Linear
Regression
to 2005

Figure 7. Historical bank positions from 2005 to 2015 and short-term rates of bank change computed using 
linear regression and end point methods. 
Linear
Regression
to 2015 
Bank Transect 
Rate of Bank Change (ft/yr) 
Endpoint to 2015 
Linear
Regression
to 2015

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Again, it was important to understand whether the shorter term trends followed the longer term
trends of bank erosion. The locations of the top of bank as it was delineated from each of the data
sources and rate of change data for the two shorter time periods (1995-2005 and 2005-2015) are
shown in Figures 6 and 7. The average bank erosion rates for the time periods 1995-2005 and
2005-2015, measured -1.5 ft/yr and -1.8 ft/yr, (-0.4 ft/yr and -0.5 ft/yr using endpoint analysis)
respectively. Similar, to the shoreline change analysis, the most recent time period (2005-2015)
had a rate of bank change slightly higher than both the earlier time period (1995-2005) and longer
term average, indicating an acceleration of bank erosion in recent years. 
For both the shorter and longer term analyses, the end point rates were generally lower in
magnitude compared to the linear regression. This indicates there were large changes occurring
between the first and last years of the analysis time period (1995 and 2015). 
Conclusions 
Given the rates of shoreline and bank change resulting from these analyses, general conclusions
about the future state of the area of concern (Transects 27-47; Figure 1A) can be made. The recent
2015 shoreline is approximately 42 feet from the picnic table area, which is directly adjacent to
the basketball courts. Assuming the given rates of shoreline erosion continue (-0.4 and -1.2 ft/yr;
endpoint-linear regression), the picnic area to the left of the basketball courts could be negatively
impacted sometime between 35 to 105 years from present day. Currently (2015), the top of the
bank is 32 feet away from the picnic area. If bank erosion rates continue as they are (-0.5 and -1.5
ft/yr; endpoint-linear regression), it would be expected that in 21-64 years the picnic area could be
negatively impacted. Based on the analysis, the top of bank is eroding faster than the shoreline and
expected negative impacts will occur sooner compared to impacts to the shoreline. 
However, it should be pointed out that the area between the top of the coastal bank and the tennis
courts is the location of a scenic path that is heavily used by residents to walk their dogs, play with
their children, and/or to just enjoy the outdoors. The path and the ability to use the path for
recreational purposes is and extremely important use of the Park for the Town. Therefore, the
current erosion of the top of the coastal bank will begin to adversely affect the path in the next 2
to 4 years. 
References 
Stafford, D.G. and J. Langfelder. 1971. Air photo survey of coastal erosion. Photogramm. Eng.,
37: 565-575.