- Beach prisms

. Appendix 3 - Potomac River Site 1
Chapel Point Rd. (Site 1)
Bel Alton, MD 20611
General Location Description: The site is located on the eastern side of Port Tobacco
River a tributary to Potomac River approximately 6 miles north of the 301 bridge (see
Maps 1 and 2 for vicinity and project site locations). The general area is rural in nature
consisting primarily of agriculture land use. The project site location has a pool and pool
house in the riparian zone. As depicted in Exhibits 1, and 1a the riparian landuse both
upstream and downstream is forested and commercial agriculture, respectively.
Erosion Potential: As reflected in Exhibit 5 (Historical Shorelines) the project site has
historically experienced erosion as do both upstream and downstream adjacent properties.
The property as exposure to fetch lengths of 1 mile to northwest, over 7 miles to
southwest, and 3.8 miles to south, southwest. Prevailing winter winds are strongest in
March (see Exhibit 9) reflecting dominance from the northeast, northwest, and south.
This suggests moderate to strong wave action from south and southwest. Additional
factors affecting the site is its exposure to elevated river flows in the Potomac River
during high rainfall events and the soil composition of the bank. The latter being a sandy
loam susceptible to moderate erosion (see Exhibits 8, 8a)
Description of Erosion Control Project: This prototype beach prism breakwater was
constructed and completed in 1985. The Beach Prisms span 160 feet consisting of 16
units measuring 4 foot high x 4 foot wide x 10 foot long. On September 2, 2009 an onsite survey (see Appendix 1) was conducted to supplement and validate archival
information sources (see Appendix 2) The alignment of the structure is north northwest to
south southeast at an angle of 20 degrees to shoreline. At the time of the survey, the
southern and northern ends of the Beach Prism measured 15 feet and 50 feet, respectively
from the shoreline. Picture 1 depicts the alignment of the Beach Prisms as viewed from
the north. There are no other structures upstream or downstream of the project site (see
Exhibit 3 and 3a)
Over the last 20 years the Beach Prisms have maintained both alignment and elevation.
The latter as reflected in the top elevation Beach Prism retaining its 1.31ft above MHW
as based on rectified field measurements taken at the time of the survey. A soil profile
taken at the base of the Beach Prism was homogenous to depth of 12 inches and consisted
of a sandy clay as based upon sand, silt and clay partitioning.
Assessment: Performance of the Beach Prisms is based on the bank and buffer
conditions behind the structure and adjacent properties both upstream and downstream of
the structure. The bank and buffer conditions as observed (see Picture 3) and as reflected
on Exhibits 2 and 2a indicate an eroding, partially vegetated bank and buffer landward of
the beach prisms. The failure of Beach Prisms in this case is in large part due to the lack
of protection from waves as generated from the northwest. Although of a low to moderate
energy (i.e. fetch length of 1 mile) the gap of 50 feet between the shoreline and structure
provides no protective barrier. In contrast to the failing shoreline which characterizes
much of this site is the southern quarter of the shoreline which exhibits an accretion and
small wetland formation (see Picture 2). This portion of the adjacent Beach Prism tapers
closer to the shoreline providing greater protection. As based upon these observations
and archival information, the Beach Prisms as used in the context of this site have not
demonstrated an ability to provide an effective barrier against erosion due to improper
alignment not providing protection to winds from the northwest. A correction to provide
such protection would be landward extension of Beach Prisms at the northern end of the
structure perpendicular to shoreline.
Onsite Survey
1) Date and Conditions The site was surveyed on September 2, 2009 at approximately
12:30pm. Winds were light and variable with slight water chop. Skies were mostly
sunny. Predicted high tide at 2:38pm.
2) Project Description The Beach Prisms span a distance of 160 feet. The alignment of
the structure is north to south. At the time of the survey distances of 50 feet and 15
feet were measured respectively at the north and south ends of the structures to the
shoreline and the top 1.5 feet of the beach prism was exposed and at an elevation of
3.1 feet above MLLW or 1.31 feet above MHW. Picture 1 depicts the alignment of
the structure as viewed from the north.
3) Site Description
a) A pool, associated structure and agricultural land are landward of the bank (see
M2) .
b) A partially vegetated and eroding bank in the near shore area ranged in height
between 10 to 30 feet consistent with Exhibit 2b.
c) Channelward of the bank and landward of the Beach Prism and predominately at
the southern end of the structure and is emergent vegetation . ( see Picture 2)
d) Depths ranged from .6 feet MLLW at the Beach Prism to 2.2 feet MLLW at a
distance of 200 feet channelward of the Beach Prism consistent with Exhibit 7.
e) A soil profile taken at the base of the Beach Prism was homogenous to depth of
12 inches and consisted of a clayey sand.
Archival Information
1) Riparian Land Use
a) At Site: Residential
b) Upstream: Forested
c) Downstream: Agriculture
2) Bank and Buffer Conditions
a) At Site:
i) 10-30 ft bank / high erosion
ii) Partial vegetation
b) Upstream:
i) 10-30 ft bank / low erosion
ii) Total vegetation
c) Downstream:
i) 10-30 ft bank / low erosion
ii) Total vegetation
3) Structures
a) At Site: Breakwater
b) Upstream: None
c) Downstream: None
4) Submerged Aquatic Vegetation
a) At Site: 0% coverage
b) Upstream: 0% coverage
c) Downstream: 0% coverage
5) Historical Shorelines
a) At Site: erosion
b) Upstream: erosion
c) Downstream: erosion
6) Fetch
7) Bathymetry
8) Soils
1, 1a
2, 2a
3, 3a
8, 8a