July 2010 - Parks & Recreation - 19

dredging of the lake by a crew of contractors with digging equipment and
trucks once the lake was drained. Cost
for that was estimated at $180,000.
In addition, this cost for conventional
dredging did not include erosion and
sedimentation (E&S) that was expected to drive the cost upwards to between $100,000 and $300,000. Finally,
the conventional method would create
ponds as deep as seven feet from the
lake dredgings. It could take months or
even years to dewater and as such would
be a liability to a facility that services
many thousands of youths each year.
Also, conventional method would take
months to clean woods, build a retention
area and roads and then weeks to for
and loaders trucks to remove sediment
and transport it to loading area for dewatering by evaporation. Hence, further
evaluation of the conventional dredging
alternatives was not pursued and the hydraulic dredging method was selected
for sediment removal.
Discussions with the Pennsylvania's
Department of Environmental Protection (DEP) indicated that due to the size
of lake and drainage basin, dredging in
Lake Henrietta qualified for Section 105
permit exclusion and, therefore, no permit was required. A report was still required to be submitted to DEP for this
work, which was approved for a period of four to five weeks using hydraulic dredging. A permit for conventional
dredging option would have been for
four weeks. Additionally, cost for the
permit application was quite less compared to the conventional dredging.
Brinjack's plan called for hydraulically dredging the lake bottom and return it
as near as possible to original contours.
This included removal of island that had
formed at the influent end of the lake.
Hydraulically dredging the lake did
not require drawing down the lake and
this technology had minimal impacts on
aquatic life in the lake including fish. The
E&S control plan indicated the location of
the bags and filter fence. A county conservation district officer visited the site

and approved plan, since the bags were
simply large filter that trap sediment
while protecting the cold water fishery.
Hydraulic dredging process involves
removal and transportation of waterways
material via sediment/water slurry. The
process does not require water drawdown in the lake; rather the dredging
equipment floats on the water surface
and " sucks up " sediment/water slurry
from lake with the help of centrifugal
pumps and conveys it to offsite location.
The process mainly involves a barge,
dredge unit, pumps, and discharge line.
Since, water is used as a media for transportation of sediments during dredging
process; it needs to be separated from
sediments at the offsite location prior to
final disposal of sediment material.
The various methods for separation of
sediments from water include use of a settling basin, filter bags, geo-textile tubes,
mechanical dewatering, and other devices. The geo-textile, or filter bags, option
is suitable for applications with limitations of area available for separation and
sediment placement. For Lake Henrietta,
polypropylene filter bags were used for
the separation. With this method, slurry
is typically pumped into the filter bags
where sediment gets trapped and water filters out through the polypropylene
material of the bags. After separation, the
dredged sediment material needs to be
disposed of. Various potential alternatives
available for disposal of dredged material includes open-water disposal sites, disposal pits, and upland confined disposal

facilities. Other options for disposing of
the dredged material include habitat development, agricultural uses, construction material, and capping open-water
disposal sites. For the Lake Henrietta
Project, the filter bags served as a final
disposal method for containing and retaining the trapped sediments on site.
Prior to dredging, sediment deposition
depths were measured in the lake for approximating sediment area and volume
needed for excavation. Material volume
was estimated using conventional engineering estimating practices for volumetric calculations and the depths measured.
Dredging area was estimated from topographic survey of the lake area and existing survey data for the property. Based
on the measurements, Brinjack calculated that more than 10,000 cubic yards of
material would have to be removed from
the lake to return it to its original contours.
Conversations with experienced dredging contractors indicated that material less
than 1.5 feet in depth would not warrant
the cost of removal. Therefore, revised estimates indicated that depths greater than
this amounted to approximately 7,000 cubic yards of material to be removed from
the lake. This volume included the island
that had formed at the head of the lake as
a result of sedimentation.

ARCHIS AMBULKAR is an environmental

engineer with Brinjac Engineering Inc. in
Harrisburg, Pennsylvania.
STEPHEN N. ZELLER is a project manager
with Brinjac Engineering.

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