United States
Environmental Protection
Agency
Chesapeake Bay
Program
Annapolis MD 21401
Research and Development
EPA-600/S3-82-084 Oct. 1982
Project Summary
Sediment Suspension and
Resuspension from Small
Craft Induced Turbulence
Hermann Gucinski
The objective of this study was to
determine if small vessels operating in
shallow waters have any measurable
effects in producing increased turbidi-
ties by the resuspension of fine
sediments that may affect submerged
aquatic vegetation (SAV).
A two-phase approach was used
consisting of field tests in a suitable
sub-estuary of Chesapeake Bay and
laboratory measurements of propeller
effects. During field trials, two differ-
ent vessel types were used to make
passes at set speeds over known water
depths. Before and after measure-
ments of light extinction and transmis-
sion, and determinations of gravimetric
suspended sediment were used to
identify effects. Laboratory experi-
ments were conducted to delineate
the contribution of propellers to possi-
ble resuspension. These experiments
were conducted using laser-doppler
anemometry to map the turbulence
field produced by propeller action.
This report was submitted in fulfill-
ment of Grant Number EPA-78-D-
XO426 by Anne Arundel Community
College and the U.S. Naval Academy
under the sponsorship of the Chesa-
peake Bay Program, U.S. Environmen-
tal Protection Agency.
This Project Summary was devel-
oped by Judy Broersma of EPA's
Chesapeake Bay Program, Annapolis,
MD. to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Causative factors for the widespread
disappearance of SAV in the Chesapeake
Bay, beginning in the early seventies,
have been the subject of intense study
Many mechanisms have been hypothe-
sized to explain this disappearance,
including increased sediment loads
caused by hurricane-produced runoff,
the increasing reliance on herbicides in
farm operations, nonpomt source pollu-
tion resulting from extreme pressures to
develop the coastal zone, and increased
turbidities from increased use of Bay
waters and tributaries by recreational
watercraft. No single factor appears to
be responsible for the stresses causing
reduction in SAV growth, but each
mechanism should be investigated to
find the possible and most significant
synergistic effects of several interacting
processes.
Small vessels operating in shallow
waters or with wakes travelling outward
to reach shallow waters may have
measurable effects in producing in-
creased turbidities as a result of the
resuspension of sediments. The amount
of time required for the sediment to
settle out of the water column depends
on particle size, background turbulence,
and motion that may retard the settling
rate and produce significant lateral
transport of the resuspended sediment.
The disturbance may have an effect
on rooted SAV if the erosive forces are
of sufficient magnitude to displace
organic detritus and inorganic silts and
muds normally stabilized by the rooted
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plants. Direct damage to the root may
subsequently result. If the resuspended
particles are small and have long
settling times, they contribute to
increased light extinction in the water
column, possibly reducing the photo-
synthetic rate of SAV. Sediments may
also settle onto the leaf structures of the
plants and further reduce photosynthe-
sis and respiration, thereby limiting
productivity and placing further stress
on the bed.
Laboratory and empirical field studies
were used to assess variables such as
depth to which effects can be felt, the
relative magnitude of the suspension,
and possible impact (of a biological
nature) of propeller generated sediment
resuspension
Procedure/ Methodology
Field trials were made at three sites
that have reasonably uniform water
depths, minimum variation of bottom
sediments with a high percentage of
small-sediment particles (<60 u),
available prior data or natural changes
in suspended sediments, and available
support facilities
Using two vessels, a 6.7 meter
speedboat and a 9 meter tugboat,
passes at set speeds chosen for maxi-
mum wave-making were made between
series of buoys at each site. Before and
after light extinction and eight trans-
mission measurements of water sedi-
ment samples were obtained for gravi-
metric determination.
Laboratory experiments to measure
the changes in water particle motion
due to the effect of a boat propeller were
conducted to predict the distribution of
stress sufficient for sediment resuspen-
sion. The results of these observations
were graphed and analyzed statistically.
Results/Conclusions
Light extinction measurements give
the most statistically reliable and
consistent results. They also show a
correspondence to the laboratory results
allowing formulation of a tentative
hypothesis concerning the most signifi-
cant variables that affect sediment
resuspension.
Transmissionmeter readings, taken
concurrently with photometer measure-
ments, corroborate measurements of
light extinction coefficients. The rela-
tively lesser effects of the boat having
the least propeller immersion is appar-
ent, and is borne out by statistical
comparison.
2
The gravimetric determination of
suspended sediments returned little
statistically significant data but does
follow the pattern established by the
photometer results.
The resuspension of sediments in the
path of a small craft is influenced by
water depth, depth of immersion, size of
the propeller, the advance ratio, and the
wave-making tendency of the vessel.
The depth to which stirring is sufficient
appears to bequite limited In this study,
at depths of greater than two meters,
reduction in SAV productivity was
calculated at about one percent.
However, the depths to which boating
effects allow sediment resuspension
coincide with depths where SAV growth
is limited in Bay waters Comparison of
SAV maps suggests that areas of least
SAV distribution and slowest recovery
are also areas of greatest boating con-
gestion. No conclusive studies have
been done on this correlation. Sandy
sediments predominate close to shore
in waters less than 2.5 meters deep, and
fine clay-like silts and muds are ubiqui-
tous in deeper waters. Such distribution
may have a protective effect for SAV
beds in high wave/wake energy envi-
ronments.
Recommendations
It is tentatively recommended that
ecologically sensitive areas be investi-
gated for the presence of fine sediments
(<60 u) Such areas should be protected
from excessive traffic, particularly deep-
draft, high-powered craft.
Hermann Gucinski is with Anne Arundell Community College, Arnold. MD.
William A. Cook was the EPA Project Officer (for information, see below).
The complete report, entitled "Sediment Suspension and Resuspension from
Small-Craft Induced Turbulence," (Order No. PB 82-265 489; Cost: $9.00,
subject to change/ will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For information contact David Flsmsr at:
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street, Suite 5G
Annapolis, MD 21403
US. GOVERNMENT PRINTING OFFICE 1902-559-017/0852
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