HomeMy Public PortalAboutAssessment of the Century Scale Sediment Budget of the Brewster CoastCenter for Coastal Studies
115 Bradford Street
Provincetown, Massachusetts 02657
www.coastalstudies.org
Assessment of the Century Scale Sediment
Budget of the Brewster Coast
A Report Submitted to the Town of Brewster
Graham S. Giese
Mark Borrelli
Stephen T. Mague
Theresa L. Smith
Patrick Barger
Center for Coastal Studies
June 2015
1
INTRODUCTION
In 2005 the Center for Coastal Studies (CCS) began developing and evaluating a sediment
budget-based geomorphic model to determine long-term volumetric coastal change and
longshore sediment transport along outer Cape Cod (Giese, et al., 2011). The methodology
developed as part of this work was subsequently applied to the Cape Cod Bay coast, and between
2012 and 2014 CCS completed work on assessments of the coastal sediment budget between
Provincetown Harbor and Jeremy Point in Wellfleet. These studies demonstrated that
comparisons of contemporary bathymetric and terrestrial lidar with high quality 1930s
hydrographic and terrestrial data along evenly spaced cross-shore transects provide an effective
means of estimating century-scale sediment budgets along Cape Cod Bay shores. The results of
these assessments were documented in two technical reports funded by the Island Foundation
(IF) (Giese et al., 2012; Giese et al., 2013), and another funded by the Massachusetts Bays
Program (MBP) (Giese et al., 2014).
Figure 1: A) Study Area. B) Coverage of 2010 bathymetric lidar within study area. Note area with no
bathymetric lidar coverage. C) Study area with transects. D) Red transects in area with no lidar coverage.
Bathymetric data was collected via vessel-based acoustic surveys along these transects.
As shown in Figure 1(A), the present study conducted for the Town of Brewster and funded by
the Coastal Resiliency Grants Program of the Massachusetts Office of Coastal Zone
Management (CZM) extends east from the Nobscusset Point/Chapin Beach area of East Dennis
to Rock Harbor in Orleans. While the focus of this work is the Brewster shoreline, it was
2
necessary to extend the analysis slightly to the east and west in order to develop a preliminary
sediment budget for the Brewster shoreline. Notwithstanding the need to expand the scope of
inquiry, the results of this study provide a quantitative assessment of sediment transport and
sediment budget for approximately 15.5 km (9.3 miles) of the southerly Cape Cod Bay coast.
This information is vital to an understanding of the historical conditions that contributed to the
present position, shape and size of the coastline, and will contribute to estimating future changes.
Accordingly, these data can be used to reduce the vulnerability of communities and ecological
systems to the impacts of a changing climate and rising sea levels.
METHODOLOGY
As discussed above, the present study represents an extension of work completed previously by
CCS for the northerly coast of Cape Cod Bay. Like the earlier work, it is a contribution to the
Center’s long-term goal to define longshore sediment transport processes and “littoral cells” for
the entire shore of Cape Cod Bay that have been discussed and described quantitatively by
Berman (2011). The geomorphic model employed has been discussed in previous reports (e.g.,
Giese et al., 2011) and is introduced here as a framework for presenting the project’s historical
data compilation and processing.
Theoretical Model Framework
The sediment budget-based geomorphic model applied to the Cape Cod coast in this study is
based on the conservation of mass, coastal wave mechanics, and the coastal morphodynamic
concept of transport within littoral cells. It can be used to quantify the longshore sediment
transport rates, sediment sources and sinks, and the boundaries between littoral cells. The model
depends upon two fundamental principles: 1) the smooth, regular form of most exposed sandy
coasts is primarily the product of wave action and 2) waves striking the coast at an angle produce
a flow of sediment along the shore in the direction of wave travel.
The net flow of sediment along the coast over an extended time period, generally annualized, is
termed littoral drift or (net) longshore sediment transport. This transport is quantified in the
model as the volume rate (e.g., cubic meters per year) of sediment crossing a shore-perpendicular
transect that extends across the active coast from the landward limit of wave-produced sediment
transport, and is designated, Q.
Coastal erosion and deposition do not depend directly on the magnitude of Q, but rather on its
rate of change alongshore, dQ/dy (cubic meters per meter per year), that is, the slope of Q when
it is plotted against alongshore distance, “y”. Erosion results when transport, Q, increases
alongshore (i.e., dQ/dy is positive); deposition results when Q decreases alongshore (negative
dQ/dy). This relationship can be expressed explicitly as
dA/dt = - dQ/dy
3
where “dA/dt” (square meters per year) is the time (“t”) rate of change in cross-sectional area
(“A”) between two cross-shore transects at a single location.
In addition to the role of sediment transport change along the shore, a shore-perpendicular
transect typically gains or loses area due to (net) cross-shore transport of sediment such as wind-
transported sand exchange between a beach and coastal dunes, tidal inlet losses, or offshore
transport of very fine sediment by turbulent seas during storms. These gains or losses are
designated by q, defined as the net cross-shore transport per unit shoreline distance (square
meters per year). The change in cross-sectional area at any point along the shore depends upon
the total contributions of longshore and cross-shore sediment transport at that location:
dA/dt = - dQ/dy – q.
To simplify this relationship, we introduce the symbol, E, to represent the negative of “dA/dt”,
the volume rate of coastal change per unit shoreline distance, i.e., erosion. Substituting, this gives
E = dQ/dy + q.
Application of this expression along a coastal segment enables a volumetric analysis of shoreline
change, a 3-dimensional estimate of change as opposed to the more common 2-dimensional view
that results from a linear analysis of shoreline advance or retreat. If the segment is sufficiently
large to contain an entire littoral cell including all source regions, transportation paths and sinks,
then integration of dQ/dy can yield the total values of Q at each point along the shore. At the
updrift and downdrift cell boundaries are points where Q equals zero; these are termed “null
points” (Dean and Dalrymple, 2002), and their location is required for a meaningful evaluation
of Q at other locations.
Cell boundaries in source regions, or null points in net longshore sediment transport, can be
located by considering the implication of our initial assumption that net longshore sediment
transport results from waves striking the coast at an angle, thereby producing a flow of sediment
along the shore in the direction of wave travel. When referring to the long-term sediment flow at
any particular coastal location (as we are in this study), the actual waves concerned are the
composite of all waves that acted on that shore over the entire time period of the study. We
replace those “actual” waves with a single “model” wave which, acting continually over that
time period, would have produced the same net sediment flow. Thus, the littoral cell boundaries
(null points) in source regions are located at those locations where the model waves approach
onshore in a direction that is at right angles to the shoreline, i.e., the angle, “θ”, between wave
approach and a line drawn perpendicular to the shore is zero.
4
This specific relationship between longshore sediment transport, Q, and wave angle, “θ”, is
consistent with the general expression between the two (e.g., Komar, 1998):
Q ~ sin 2 θ.
At the null point, “θ = 0”. Since the derivative of “sin 2 θ” is proportional to “cos 2 θ”, it follows
that
dQ/dy ~ cos 2 θ.
Thus dQ/dy is maximum at the null point (θ = 0).
Model Adjustment
Numerical integration of dQ/dy to calculate Q is valid when the transects are approximately
perpendicular to the coastline and parallel to each other, conditions not met in the eastern section
of the Brewster study area (Figure 1). Therefore for this study, Q was calculated by summing ΔQ
values derived individually for each pair of transects. ΔQ, in turn, is the annualized change in
volume between transect pairs - found from (1) the vertical change between profiles along each
of the two transects and (2) the horizontal distances separating them - reduced by the volume lost
due to cross-shore processes at each transect pair segment of the study area. Details are provided
below in “Transect Construction, Volumetric Analysis and Sediment Flow Calculation.”
Historical Data Compilation and Processing
Based on previous work of CCS in Cape Cod Bay, the historical base map for the current study
was developed from hydrographic and terrestrial data sets compiled for the period 1933 – 1940.
Four hydrographic surveys were conducted in eastern Cape Cod Bay by the USC&GS
(predecessor to NOAA’s Coast Survey) during 1933-34 (Figure 2). These surveys were
combined with adjacent terrestrial information provided on USC&GS topographic surveys (T-
sheets), U.S. Geological Survey (USGS) Quadrangles, and U.S. Department of Agriculture,
Natural Resource Conservation Service (USDA-NRCS) 1938 aerial photographs to provide a
relatively seamless, synoptic coverage of the entire Cape Cod Bay study area.
Historical hydrographic survey data were downloaded from the NOAA National Geophysical
Data Center (http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html ), including Descriptive
Reports, color image Hydrographic Smooth Sheets (H-Sheets), digital point data in ASCII XYZ
format, and metadata. Original survey data were compiled at scales of 1:10,000 (or in some cases
1:5,000) and related horizontally to the North American Datum of 1927 (NAD27) and vertically
to local mean low water (MLW) for the geographic area covered by each survey.
5
Figure 2: NOAA Hydrographic Survey Point Coverage for Cape Cod Bay, Massachusetts (Red shades denote
1940 surveys. Blue shades denote 1933-34 surveys).
The historical terrestrial data used to characterize the limited area of the land-sea interface (i.e.,
the area influenced by marine and coastal processes) consisted primarily of USC&GS 1933 and
1938 T-sheets, USGS quadrangles surveyed in 1941, and USDA-NRCS 1938 aerial photographs.
These post- “Hurricane of ’38” photographs were flown on November 21, 1938, near the time of
local high water and were used to help identify landforms such as coastal banks and dunes and to
verify changes to the terrestrial environment resulting from the record hurricane.
USC&GS T-sheets for the study area (and accompanying Descriptive Reports) were downloaded
as non-georeferenced survey scans from the NOAA NOS Special Project web site at
http://nosimagery.noaa.gov/images/shoreline_surveys/survey_scans/NOAA_Shoreline_Survey_
Scans.html. Similarly, non-georeferenced scans of USGS historical quadrangles were
downloaded from the University of New Hampshire at http://docs.unh.edu/nhtopos/nhtopos.htm.
The extent of landside topography incorporated into the historical data sets was limited to the
relatively small area of land influenced by marine and coastal processes (the land-sea interface)
necessary for the volumetric analysis. While the USGS topographic work provides broad,
synoptic coverage of topographic conditions existing at the time of the survey, there are inherent
data limitations associated with this mapping effort related generally to the relatively coarse
mapping scale and less dense elevation data for early mapping efforts. To minimize these
limitations, topographic data obtained from each Quadrangle was supplemented with additional
elevation data derived from:
1) USC&GS T- and H-Sheet Descriptive Reports.
2) The elevations of the mean low water (MLW) and mean high water (MHW) lines
as obtained from the 1930s Coast Survey T- and H-sheets.
6
3) Profiles obtained from contemporary survey work to characterize representative
beach and bluff profiles.
4) The location of natural features shown on historical T-Sheets and aerial
photographs such as the toe of coastal banks and salt marshes, the elevations of
which relative to MHW and MLW can be estimated.
5) The elevations of physical features such as road intersections, railroad centerlines,
building corners, etc., common to both historical and contemporary data sets and
not likely to have changed over time.
Elevation data from these supplemental sources were added to the historical data set and blended
with USGS topographic information to increase the reliability and density of the limited landside
topography used in the analysis.
As discussed above, comparisons of historical and contemporary hydrographic and terrestrial
datasets can be important sources of information for quantifying changes in landform volume
and net sediment movement. Where the land and sea interact along the shores of Cape Cod, such
volumetric comparisons can be used to estimate long-term, regional scale sediment flux and
sediment budgets. To effectively use historical geospatial data, such as those central to the
methodology discussed above, however, potential sources of uncertainty inherent in data
collection methods must be minimized to ensure that quantitative estimates provide reliable
information at the scale of the analysis (Byrnes et al., 2002). In addition to limitations in
technology and equipment that could affect data quality, a potential source of significant
uncertainty for historical datasets lies with the ability to accurately translate horizontal and
vertical reference systems to contemporary datums (Jakobsson, et al.,2005).
For this study, all contemporary data is referenced horizontally to the Massachusetts State Plane
Coordinate System (North American Datum of 1983 (NAD83)) and vertically to the North
American Vertical Datum of 1988 (NAVD88)). Historical data were referenced horizontally to
the North American Datum of 1927 (NAD27) and vertically to a local tidal datum (either mean
low water (MLW) for the hydrographic survey or mean sea level (MSL) for terrestrial data),
requiring translation to the project datums (NAD83/NAVD88).
While the mathematical process for translating horizontally from NAD27 to NAD83 is well
established (Giese and Adams, 2007), the process for developing an accurate vertical translation
from a local tidal datum to a geodetic datum requires retracing previous survey work. The ability
to reproduce elevation data referenced to local tidal datums accurately, whether historical or
contemporary, depends on an ability to find and reoccupy reference stations established for the
tidal readings. Lacking recoverable reference points (benchmarks), the short term nature of the
tidal observations, inter-annual variations in tidal cycles, rising sea levels, and changing
environmental conditions make development of reliable translations of local, historical vertical
7
reference systems to contemporary systems problematic and greatly increase the uncertainty
associated with quantitative comparisons (Jakobsson et al., 2005; Van der Wal and Pye, 2003).
This can be particularly true for volumetric change analyses where rising sea levels can introduce
a significant bias towards erosion in the absence of an accurate translation.
To minimize this potential source of uncertainty, all historical data points were translated
vertically based on research, recovery, and reoccupation of historical tidal benchmarks identified
in the 1930’s USC&GS Hydrographic Descriptive Reports. Where benchmarks could be
recovered, they were occupied with high accuracy GPS survey equipment to provide a direct
translation to NAVD88. When USC&GS tidal benchmarks were found to have been destroyed,
the historical record was further investigated to establish relationships to other extant
benchmarks that could be occupied. These relationships were used to relate the tidal benchmark
to NAVD88 (Mague, 2012).
Benchmarks for historical hydrographic data sets have been recovered and occupied as part of
our previous work and the resulting translations to NAVD88 described in Giese et al, 2014(b),
Giese at al 2013, Giese et al 2012, and Mague, 2012. The present study required additional field
work to recover and occupy a historical benchmark located on Sandy Neck in Barnstable to
translate the hydrographic survey covering the westerly edge of the study area from local MLW
to NAVD88. Based on research of available technical documents, field work was conducted in
May of 2014 and using previous methods (Mague, 2012), Tidal Benchmark 1 set by the
USC&GS in 1934 (TBM 1 of 1934) was recovered and occupied (Figure 3).
Figure 3: TIDAL BENCH MARK 1 (1934) is a standard disk, stamped “Barnstable – BM 1/1934”, set in the
top of a 12”x12”x 3 ½” ft. concrete post, 8” x 8” at top and extending about 4 inches above surface of ground,
located on Sandy Neck, Barnstable Harbor (left). GPS reoccupation of TBM 1 by CCS to establish
hydrographic survey relationship to NAVD88 (right).
8
As in previous work, the referenced benchmark was occupied with CCS’s Trimble® R8 GNSS
Receiver and Trimble® TSC2™ utilizing Real-Time-Kinematic (RTK) GPS techniques and the
Keystone Virtual Reference Station Network (VRS) for data collection. Based on the results of
an on-going CCS accuracy assessment program, horizontal and vertical root mean square errors
(RMSE) values of this system have been determined to be within 2.0-2.6 centimeters.
Based on the fieldwork, a reference to NAVD88 was obtained for the local mean low water
value used for the 1930s surveys covering the study area. The translation of hydrographical and
terrestrial data for the study area, referenced to a local 1933/34 MLW or MSL datums, to the
contemporary geodetic datum, NAVD88, is represented by the relationships in Figure 4:
Figure 4. Contemporary and Historical Datum Comparisons for Study Area Units: Feet (meters)
After historical terrestrial data points were digitized, all data points were translated horizontally
and vertically and the contributing data sets were combined into one comprehensive file
(NAD83/NAVD88) for use in creating a 1930s three-dimensional surface, or surface model. This
surface model formed the basis for quantitative comparisons with a similar surface derived from
9
U.S. Army Corps of Engineers 2010 bathymetric lidar data, 2011 USDA-NRCS Terrestrial lidar
data and CCS’s 2014 vessel-based acoustic surveys.
Historical 1930s/40s Surface Model
A 3-dimensional model of the historical surface was created using the digital database to create a
point shapefile within the ARCGIS v10.0 software suite. These points were then converted into a
Triangulated Irregular Network (TIN) using the 3-D analyst extension with ARCGIS. These
triangles are formed using 3D data from three points to create a plane that represents a real-world
surface. The TIN was then converted into a terrestrial or bathymetric raster with latitude (y),
longitude(x), and elevation (z) attributes. Since there is rarely 100% coverage of a mapped area,
a krigging method was chosen as the best interpolation method for this study and utilized to
represent changes in natural topography and/or bathymetry. Before finalizing the surface model,
CCS coastal geologists reviewed the surface to identify potential data issues as well as to remove
outliers from the final surface. This was found to be a critical step in previous studies to ensure
that a processes-based assessment is conducted prior to accepting or rejecting points within the
surface and proceeding with the analysis.
Contemporary Data and Surface Models
Contemporary surface models for the study area were compiled from two lidar data sets, one
containing the terrestrial data, and the other bathymetric data. The terrestrial lidar was flown in
the spring of 2011 by the U.S. Department of Agriculture’s Natural Resources Conservation
Services. The bathymetric survey was flown in May of 2010 by the U.S. Army Corps of
Engineers. As part of its QA/QC program, representative areas of terrestrial lidar data were
tested using data collected with the Center’s GPS equipment.
Acoustic data were acquired by CCS in areas without bathymetric lidar coverage (figure 1, B and
D) using a Tritech PA500/6-S altimeter side-mounted to the R/V Marindin in June and July
2014. The data were processed using Hypack 2014 software and appended to the transect data set
using Microsoft Excel 2010. The acoustic data were also collected in areas that overlapped the
bathymetric lidar. QA/QC included the comparison of the overlapping 2014 acoustic data with
the 2010 lidar data in order to test for offsets.
Transect Construction, Volumetric Analysis and Sediment Flow Calculation
While the historical and contemporary surface models were being developed, a shore-parallel
baseline and shore-perpendicular transects were constructed along the 18 km shoreline of the
study area and combined with transects of previous studies, as shown in Figure 1. Transects were
spaced at 150-meter intervals (approx. 120 transects) and extended out to a depth of 10 meters.
Using the historical surface model and the contemporary lidar data sets, elevations were
extracted at 2 meter intervals along each transect. Using MATLAB software, elevations and
10
cross-shore and longshore distances derived from the historical and contemporary data sets were
plotted together to determine the local change in sediment volume, ΔV, between adjacent pairs of
transects over the intervening time period (77 years). These, annualized, provided ΔV/Δt rates for
each segment. Subsequent analysis based on profile comparisons of 1933-1934 and 2010-2011
data, documented changes in sediment volume and form thus permitting estimates of cross-shore
gain and loss rates, q, for each segment. The differences between ΔV/Δt and q at each transect-
pair segment yielded estimates of the local rate of change in net longshore transport, i.e.,
ΔQ = ΔV/Δt – q .
Finally, estimates of the volume, rate and direction of sediment movement along each segment
of the shoreline, Q, were determined by summing ΔQ, both north and south of the central “null
point.” Methodology for determination of the “null point” location - delineation of the littoral cell
boundaries - is described in “Theoretical Model Framework” above.
RESULTS
Comparisons of two profiles, one historical and the other contemporary, at single transects are
provided in Figure 5 and 6. Figure 5 illustrates the extreme erosion occurring in the west of the
study area, while Figure 6 shows the reverse, rapid deposition in the east. The reader should
recall that “E “ (volumetric erosion rate) is defined as the negative of area change between
profiles, hence a positive value for transect 2418 and a negative value for transect 2178.
Figure 5: Comparison of historical and contemporary profiles of transect number 2418. Arrows indicate
limits of cross-sectional area considered for calculation of E value of this transect.
11
Figure 6: Comparison of historical and contemporary profiles of transect number 2178. Arrows indicate
limits of cross-sectional area considered for calculation of E value of this transect. The contemporary “2010-
2011” profile includes the 2014 bathymetric data described above.
The distribution of volumetric erosion and accretion for the entire study area - beginning at
Skaket Beach in Orleans and ending in East Dennis – is presented in Figure 7. The graph shows
the results for all transects; the significance of the individual green, blue and red lines is
discussed below in “Discussion.”
Finally, in Figure 8, we present the results of our calculation of “Q”, the rate of net alongshore
sediment transport throughout the study area. Geographic points-of-interest are superimposed to
assist interpretation and application to management issues. Negative “Q” values indicate
eastward transport; positive values, westward transport. Sections in red are primarily
characterized by increasing “Q” values. These are areas of erosion – source areas for the littoral
drift in the region. Similarly, the green sections (decreasing “Q”) are primarily areas of accretion,
while black denotes fairly constant net transport with little total erosion or accretion. Red dots
designate the “null points” determined as explained in “Methodology”; net transport rates vary
from zero at those locations to a maximum of almost 30,000 cubic meters per year near
Brewster’s western boundary at Quivett Creek.
12
Figure 7: Distribution of E for all transects.
Figure 8: Distribution of Q for all transects.
13
DISCUSSION
The results of this study provide insight into the sedimentary conditions and processes associated
with the Brewster coast of Cape Cod Bay. As shown in Figure 8, they indicate that this coast
occupies more than half of a littoral cell, the “Brewster Cell”, in which sediment flows eastward
from a source null point lying between Nobscusset Point and Chapin Beach in Dennis, and a sink
null point near Rock Harbor in Orleans. Since the prevailing winter wind direction in Cape Cod
Bay is northwesterly, very likely this eastward flow of sediment is primarily driven by
northwesterly wind waves.
Figure 7, presenting volumetric erosion and accretion rates for the entire study area, shows those
rates for the Brewster Cell in green. The red line, a linear best-fit to the Brewster data, reveals a
strikingly regular trend of erosion decrease and accretion increase from source to sink throughout
the cell. Also apparent from the figure is the dominance of erosion over accretion. This sediment
imbalance within the cell is primarily the result of cross-shore sediment transport, q. Some of the
“missing” sediment is carried onshore by wind to dune fields and by tides into estuaries
(Figure 9); while other sediment, primarily the finer constituents, is transported to offshore
deposits by turbulent winter seas.
Figure 9: Marsh deposits within the study store sediment transported landward from the active coast into the
regional estuaries.
14
The model employed in this study (a “1-D model”) averages long term net transport across the
entire width of the active coast. It indicates that the Town of Brewster, at its westerly boundary,
receives sediment eroded from the coast of the adjoining Town of Dennis. Eastward sediment
transport maintains a fairly constant rate of between 25,000 and 30,000 cubic meters per year
throughout the western half of the Town’s coastline, and then decreases to some 10,000 cubic
meters per year at the Town’s eastern boundary. This reduction in transport rate indicates that
some 15,000 to 20,000 cubic meters of sediment per year are added to eastern section of the
active Brewster coast. However, all available data indicate that the deposition occurs offshore of
the Brewster shoreline.
Figure 10, based on a section of NOAA chart 13250, shows several major areas of deposition
offshore along the front, and most significantly along the outer edge, of the large, easterly
trending, tongue-shaped shoal (Area #1) associated with the Brewster intertidal flats. Onshore,
the chart indicates exposed boulders in the nearshore zone (Area #2), suggesting erosion rather
than deposition there. Shoreline and nearshore erosion along the eastern section of the Brewster
coast is confirmed by our comparison of historical and contemporary profiles along transects in
that section (e.g., Figure 6).
Figure 10: Portion of NOAA chart 13250 showing offshore areas of deposition identified in this study, and
exposed boulders (indicating coastal erosion) in the nearshore.
15
Namskaket Shoal
The tongue-shaped shoal is shown in cross-section in Figure 6. To better define this feature,
which we tentatively refer to as Namskaket Shoal, Figure 11 provides a series of cross-sectional
plots of the sea floor covering the area of converging transects lying in the vicinity of Namskaket
Creek (Figure 1). This shoal has the basic characteristics of fluvial bed forms known as linguoid
bars, found in rivers transporting large volumes of sediment at locations of increasing river
width. In Figure 11, color codes for the profile dates are reversed from those used earlier (e.g.,
Figure 6); the 2010-2011 profile is shown in black; the 1933-1934 profile in red.
The eight plots in Figure 11 cover the area extending eastward from transect 2232 to transect
2166, located geographically in Figure 8. The contemporary (black) profiles indicate that at the
present time the distinct tongue-shaped deposit first appears after - east of - transect 2232 and
ends west of transect 2166, i.e., between transects 2214 and transect 2172. Within that region the
shoal is clearly bounded offshore by a steep slope extending to depths greater than 7 m., and
inshore by a shallow declivity separating it from the landward inter-tidal flats.
In contrast, the red profiles indicate that at 1933-1934 distinctive tongue-shaped form was
present farther eastward, at transect 2232, than at present; also that it terminated farther eastward
than a present - after transects 2190. Summarizing, we find that in 1933-1934 the shoal’s form
was well defined between transects 2232 and 2190, while at present it is well defined between
transects 2214 and 2172. In addition to this longitudinal, alongshore extension of the shoal, a
comparison of the profiles reveals a lateral, primarily offshore, extension that increases eastward.
Taken together, these observations suggest that Namskaket Shoal is a growing and eastward
migrating depositional feature. Since it lies southeastward of the gradually submerging
Billingsgate Shoal (Uchupi, et al., 1996), it seems likely that the development of Namskaket
Shoal is closely tied to an increase in energy of wind waves produced by the prevailing winter
northwesterlies.
2214 2232
16
Figure 11: Eight cross-shore transects lying in the vicinity of Namskaket Creek beginning, in the east, with
transect 2232 and ending in the west with transect 2166. Transect profiles representing 1933-1934 conditions
are shown in red; contemporary (2010-2011) profiles are shown in black. The significance of these profiles
are discussed in the text.
2196 2190
2184 2178
2172 2166
17
ACKNOWLEDGEMENTS
The authors would like to thank Chris Miller, Director of the Town of Brewster’s Natural
Resources Department; and Steve Spear and Sharon Randall, of the Cape Cod Office of the
Natural Resource Conservation Service, United States Department of Agriculture in Hyannis,
MA for electronic copies of the post hurricane1938 aerial photographs.
GLOSSARY OF TERMS
Term Symbol Units Description
Alongshore gradient of
annual net longshore
transport
dQ/dy
meters2/year or
meters3/meter/year
The slope of Q when it is plotted against alongshore
distance “y”. It describes the gains or losses in area
at a shore-perpendicular transect due to longshore
sediment transport.
If q = 0, erosion results when dQ/dy increases
alongshore (i.e., positive dQ/dy); deposition results
when dQ/dy decreases alongshore (i.e., negative
dQ/dy).
Negative of annual rate
of change in cross-shore
area
E
meters2/year, or
meters3/meter/year
Total loss (+) or gain (-) per year in cross-sectional
area of the “active” zone (wave transport zone) of
beach at any specific location along the shore. Equals
dQ/dy + q. (+) E = erosion; (-) E = deposition or
accretion.
Annual rate of change in
cross-shore area along a
transect
dA/dt
meters2/year or
meter3/meter/year
Time (“t”) rate of change in cross-sectional area
(“A”) between two cross-shore transects at a single
location or the volume rate of coastal change per unit
shoreline distance. (Note: dA/dt = - dQ/dy – q).
Littoral cell
A coastal compartment that contains a complete
cycle of sedimentation including sources, transport
paths, and sinks. Cell boundaries delineate the
geographical area within which the sediment budget
is balanced, providing the framework for the
quantitative analysis of coastal erosion and accretion.
(See Berman, 2011, for full discussion)
Littoral drift or
(net) longshore
sediment transport
Q
meters3/year
The annual net flow of sediment along the coast
expressed as the volume rate of sediment crossing a
shore-perpendicular transect that extends across the
active coast from the landward limit of wave-
produced sediment transport seaward to the
approximate limit of sediment movement. (The result
of the integration of dQ/dy along the shore).
The model assumes that net longshore sediment
transport results from waves striking the coast at an
angle, thereby producing a flow of sediment along
the shore in the direction of wave travel.
18
Local rate of rhange in
net longshore transport -
estimate
ΔQ =
ΔV/Δt–q
meters3/year
Where ΔV/Δt represents the local change in sediment
volume, ΔV, between adjacent pairs of transects over
the intervening time period, Δt (77 years).
Long-term sediment flow
At any particular location along the shore, the result
of the composite of all waves (i.e., the actual waves)
that acted on the shore over the time period of the
study
Model wave
A theoretical single wave representing the composite
of all “actual” waves which, acting continually on
the shore over the time period of the study, would
have produced the same net sediment flow as the
actual waves.
Net cross-shore transport
per unit shoreline
distance
q
meters2/year or
meters3/meter/year
Gain or losses in area at a shore-perpendicular
transect due to cross-shore sediment transport , e.g.,
wind-transported sand exchange between a beach
and coastal dunes, tidal inlet losses, or offshore
transport of very fine sediment by storm seas.
Null point
A point along the shore that defines the updrift or
downdrift boundary of a littoral cell. Where Q = 0,
or dQ/dy is a maximum (in the case of a source).
Located where model waves approach shoreline at
right angles, i.e., the angle, “θ”, between wave
approach and a line drawn perpendicular to the shore
is zero. This point is sometimes referred to as a nodal
point.
Wave angle θ The angle between wave approach and a line drawn
perpendicular to the shore
19
REFERENCES
Berman, G.A., 2011, Longshore Sediment Transport, Cape Cod, Massachusetts. Marine
Extension Bulletin, Woods Hole Sea Grant & Cape Cod Cooperative Extension. 48 p.
Byrnes, M.R., J.L. Baker and F. Li, 2002, Quantifying Potential Measurement Errors and
Uncertainties Associated with Bathymetric Change Analysis. ERDC/CHL CHETN-IV-
50. Coastal and Hydraulics Engineering Technical Note (CHETN). U.S. Army Corps of
Engineers. September 2002.
Dean, R.G., and Dalrymple, 2002, Coastal Processes with Engineering Applications. Cambridge
University Press, Cambridge, UK, 475 p.
Giese, G.S., M. Borrelli, S.T. Mague, T. Smith and P. Barger, 2014, Assessment of Multi-
Decadal Coastal Change: Provincetown Harbor to Jeremy Point, Wellfleet. A Report
Submitted to the Massachusetts Bays Program. January 2014. 23 p.
Giese, G.S., M. Borrelli, S.T. Mague, and P. Hughes, 2013, Evaluating century-scale coastal
change: Provincetown/Truro line to Provincetown Harbor. Marine Geology Report
No.13-1, Center for Coastal Studies, Provincetown, MA, 11 p.
Giese, G.S., M. Borrelli, S.T. Mague, and P. Hughes, 2012, Evaluating century-scale coastal
change: a pilot project for the Beach Point area in Truro and Provincetown,
Massachusetts. Marine Geology Report No.12-2, Center for Coastal Studies,
Provincetown, MA, 18 p.
Giese, G.S., M.B. Adams, S.S. Rogers, S.L. Dingman, M.Borrelli and T.L. Smith. 2011, Coastal
sediment transport on outer Cape Cod, Massachusetts. In P. Wang, J.D. Rosati and T. M.
Roberts (eds.) Coastal Sediments ’11, American Society of Civil Engineers, v. 3, p. 2353-
2356.
Giese, G.S. and M.B. Adams. 2007, Changing orientation of ocean-facing bluffs on a
transgressive coast, Cape Cod, Massachusetts. In: Kraus, N.C., and J.D. Rosati (eds.),
Coastal Sediments ’07, American Society of Civil Engineers, v. 2, p. 1142-1152.
Jakobsson, M., A. Armstrong, B. Calder, L. Huff, L. Mayer, and L. Ward, 2005, On the Use of
Historical Bathymetric Data to Determine Changes in Bathymetry: An Analysis of Errors
and Application to Great Bay Estuary, NH. International Hydrographic Review, Vol. 6,
No. 3, pps. 25-41. November 2005.
Komar, P.D., 1998, Beach Processes and Sedimentation, Second Edition. Prentice Hall, New
Jersey. 544 p.
20
Mague, S.T. (2012) Retracing the Past: Recovering 19th Century Benchmarks to Measure
Shoreline Change Along the Outer Shore of Cape Cod, Massachusetts. Cartography and
Geographic Information Science, Vol. 39, No. 1, pp. 30-47.
Uchupi, E., G.S. Giese, D.G. Aubrey and D.J. Kim, 1996, The Late Quaternary Construction of
Cape Cod, Massachusetts: A Reconsideration of the W.M. Davis Model. Geological
Society of America Special Paper, 309, 69 pp.
Van der Wal, D. and K. Pye. (2003). The use of historical bathymetric charts in a GIS to assess
morphological change in estuaries. The Geographic Journal, Vol. 169, No. 1, pps. 21-31.
March 2003.
T and H-Sheet Descriptive Reports
U.S. Coast and Geodetic Survey. 1909. Descriptive Report, Topographic Sheets [revisions to]
1062, 901, 795, 1088, 1078, 1077, 1704, 368, 579, 259, 260, 616 a & b and 1982.
Massachusetts, Cape Cod Bay. 11 pages.
U.S. Coast & Geodetic Survey. Tidal Bench Marks, State of Massachusetts, Department of
Commerce, Washington, D.C. Re-issued by: Mass. Geodetic Survey, 100 Nashua St., Boston.
1938.
U.S. Coast and Geodetic Survey. 1933. Descriptive Report, Topographic Sheets 6033& 6034.
Massachusetts, Cape Cod, Provincetown & Vicinity, Wellfleet & Vicinity. 35 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Hydrographic Sheet 5588. Cape
Cod, Vicinity of Barnstable Harbor. 18 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Hydrographic Sheet 5589. Cape
Cod, Barnstable Harbor. 25 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Hydrographic Sheet No. 5534. Cape
Cod, Billingsgate Shoal. 22 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Topographic Sheet 6112.
Massachusetts, Cape Cod, Brewster to Wellfleet Harbor. 13 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Topographic Sheet 6113.
Massachusetts, Cape Cod, East Dennis to Brewster. 11 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Topographic Sheet 6114.
Massachusetts, Cape Cod, Yarmouthport to Nobscusset Pt. 10 pages.
U.S. Coast and Geodetic Survey. 1934. Descriptive Report, Topographic Sheet 6122 & 6123.
Massachusetts, Cape Cod, Barnstable Harbor & Scorton Neck. 16 pages.
U.S. Coast and Geodetic Survey. 1943. Descriptive Report, Topographic Sheets 5734.
Massachusetts, Wellfleet Harbor-Atlantic Ocean. 22 pages.
21
U.S. Coast and Geodetic Survey. 1944. Descriptive Report, Topographic Sheets 5735.
Massachusetts, Atlantic Ocean –Cape Cod Bay, South Orleans-North Eastham. 31 pages.
U.S. Coast and Geodetic Survey. 1952-58. Descriptive Report, Topographic Sheet T-11193,
11193a, & 11194. Cape Cod, Barnstable Harbor. 42 pages.
U.S. Coast and Geodetic Survey. 1952-55. Descriptive Report, Topographic Sheet T-11187 &
11188. Cape Cod Bay, Nobscusset Point to Boatmeadow River. 26 pages.
U.S. Coast and Geodetic Survey. 1952-55. Descriptive Report, Topographic Sheet T-11183.
Cape Cod Bay, Eastham 20 pages.
Cartographic and Bathymetric Data Used in this Project
1933 - U.S. Coast & Geodetic Survey Topographic Survey, T-6034, Wellfleet & Vicinity, Cape
Cod, Massachusetts. Scale 1:20,000.
1934 - U.S. Coast & Geodetic Survey Hydrographic Survey, H-5534, Billingsgate Shoal, Cape
Cod, Massachusetts. Scale 1:20,000.
1934 - U.S. Coast & Geodetic Survey Hydrographic Survey, H-5588, Vicinity of Barnstable
Harbor, Cape Cod, Massachusetts. Scale 1:20,000.
1934 - U.S. Coast & Geodetic Survey Hydrographic Survey, H-5589, Barnstable Harbor, Cape
Cod, Massachusetts. Scale 1:10,000.
1934 - U.S. Coast & Geodetic Survey Topographic Survey No. 6112, Brewster to Wellfleet
Harbor, Cape Cod, Massachusetts. Scale 1:20,000.
1934 - U.S. Coast & Geodetic Survey Topographic Survey No. 6113, East Dennis to Brewster,
Cape Cod, Massachusetts. Scale 1:20,000.
1934 - U.S. Coast & Geodetic Survey Topographic Survey No. 6114, Yarmouthport to
Nobscusset Pt., Cape Cod, Massachusetts. Scale 1:10,000.
1934 - U.S. Coast & Geodetic Survey Topographic Survey No. 6122, Barnstable Harbor, Cape
Cod, Massachusetts. Scale 1:10,000.
1938 – U.S. Coast & Geodetic Topographic Map, T-5734, Massachusetts, Cape Cod, Wellfleet
Harbor and Vicinity. Scale – 1:10,000.
1938 – U.S. Coast & Geodetic Topographic Map, T-5735, Massachusetts, Cape Cod, Orleans
and Vicinity. Scale – 1:10,000.
1938 – United States Department of Agriculture, Natural Resource Conservation Service, Aerial
Photographs. Date of Photographs: November 21, 1938. Ground Scale 1:24,000. Time of
photos: ~ 1000 HRS. Time of High Water: ~ 1000 HRS. Weather: Clear, west winds 10-15 mph.
22
1939-1941 - U.S. Geological Survey, Wellfleet, Orleans, Harwich, Dennis, & Hyannis
Quadrangles. Scale: 1:31,680.
1952, ‘53, & ‘55 – U.S. Coast & Geodetic Shoreline Manuscript, T-11187, Massachusetts, Cape
Cod Bay, Nobscusset Point to Quivett Creek. Scale – 1:10,000.
1952, ‘53, & ‘55 – U.S. Coast & Geodetic Shoreline Manuscript, T-11188, Massachusetts, Cape
Cod Bay, Quivett Creek to Boatmeadow River. Scale – 1:10,000.
1952, ’53, & ‘55 – U.S. Coast & Geodetic Shoreline Manuscript, T-11194, Massachusetts, Cape
Cod Bay, Dennis. Scale – 1:10,000.
1952, ’53, & ‘55 – U.S. Coast & Geodetic Shoreline Manuscript, T-11193, Massachusetts, Cape
Cod Bay, Barnstable Harbor. Scale – 1:10,000.