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| OF02-5 Executive Summary | contact: D.V. Wells (dwells@mgs.md.gov) |
Shoreline Erosion
as a Source of Sediments and Nutrients Northern Coastal Bays, Maryland
by
Darlene V. Wells, E. Lamere Hennessee and James M. Hill
(2002)
The Maryland Geological Survey (MGS)
began a multi-year study to determine the flux of sediments and nutrients eroding
from unprotected
shorelines
bordering
Maryland’s coastal bays. The first year of study focused on the northern-most
bays: Assawoman and Isle of Wight Bays and the St. Martin River.
Sampling locations
were selected on the basis of linear rates of shoreline change, as well as
geology and geomorphology (marsh, bluff, or beach). At each
of the 16 sites, bank heights were measured. Sediment samples were collected
from marshes and beaches and from distinct geologic horizons within banks.
Samples were analyzed for grain size composition, bulk density, total organics,
total carbon (TC), nitrogen (TN), phosphorus (TP), and a suite of trace metals.
The analytical results were then used in conjunction with coastal land loss
estimates to determine sediment and nutrient loadings to the bays. Annual land
lost was based on a digital comparison of two historical shorelines dating
from 1942 and 1989.
Based on geomorphologic variability and differing rates of
shoreline erosion, the study area shoreline was divided into 23 reaches, ranging
in length from
about 600 m to 45,000 m; most were less than 9,000 m long. A template of irregular
polygons was constructed to demarcate the reaches, and total land loss (m2)
during the 47-year period was determined for each polygon. These “land
loss” polygons provided a structure for organizing the results of the
physical and chemical analyses. Each sampling site was associated with one
or more of the land loss polygon. Mean bank heights and concentrations of the
measured constituents (i.e., TN, TP, TSS, etc. in kg/m3), averaged for each
of the sampling sites, were used to calculate annual loadings (kg/yr) for each
polygon.
During the 47-year period, shoreline erosion contributed an estimated
11.6 x 106 kg/yr of total sediments into the three-bay system (Table ES-1).
Of the
total sediment load, approximately 42%, or 4.9 x 106 kg/yr, were total suspendable
solids (TSS). That amounts to about one-third of the TSS load from upland (surface)
run-off. Annual total sediment loadings were greatest in the St. Martin River
(6.9 x 106 kg/yr), due in part to high bank elevations and relatively dense
bluff material. Bulk densities of sediments collected from bluffs averaged
1.4 g/cm3. Total sediment loading from shore erosion in Assawoman Bay was about
half that of the St. Martin River (3.2 x 106 kg/yr). Sediment loadings from
Isle of Wight Bay shorelines were even lower (1.5 x 106 kg/yr). Much of the
shoreline bordering Assawoman and Isle of Wight Bays is low-lying marsh, composed
of sediments with average bulk densities of 0.4 g/cm3.
Sand-sized sediments account
for approximately 57% of the total sediments contributed from shoreline erosion.
The sand contributed from erosion about
half of the sand coming into the bays. More than one-third of the sand is eroded
from the St. Martin River shoreline.
Shoreline erosion is a significant source
of nutrients, contributing up to 8.5% of the total nitrogen and total phosphorus
delivered to the system. Nutrient
contributions from shoreline erosion slightly exceed input from point sources.
In addition to nutrients, erosion also contributes significant amounts of Pb
and Zn, accounting for 4% and 9.5%, respectively, of the total loadings of
those metals to the bays.
| Table ES-1. Annual loadings (kg/yr) of nutrients and sediments,
northern coastal bays. The 1989 shoreline length applies to the shoreline included in the land loss polygons. |
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| Component | Assawoman Bay | Isle of Wight Bay | St. Martin River | Total |
| 1989 shoreline length (m) | 81,164 | 25,296 | 59,378 | 165,839 |
| Total Solids | 3,206,065 | 1,471,477 | 6,888,572 | 11,566,114 |
| Suspendable Solids | 2,151,542 | 955,737 | 1,771,102 | 4,878,381 |
| Carbon | 214,743 | 87,424 | 122,398 | 424,565 |
| Nitrogen | 11,649 | 4,477 | 7,247 | 23,373 |
| Phosphorus | 1,070 | 497 | 777 | 2,344 |
| Pb | 80 | 29 | 99 | 208 |
| Zn | 182 | 77 | 260 | 519 |
(Updated 9/24/03 )