THE EFFECT OF
CHANNEL DREDGING ON
WATER QUALITY IN THE
DELAWARE ESTUARY
DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL A D M I N I N I S T R A T I O N
NEW YORK, NEW YORK
MAY 1966

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I. BJTRODUCTION
Request
This study was requested by the District Engineer, U. S. Army-
Engineer District, Philadelphia, Pennsylvania, by letter dated
June 30, 1965. The letter requested that the Public Health Service*
study the effect of widening and deepening the navigation channel on
salinity in the Delaware Estuary.
Purpose and Scope
The purpose of this study is to determine the effect of several
dredging schemes for widening and deepening the navigation channel
from Philadelphia, Pa. to the sea on salinity in the estuary. In
addition, the effect of channel dredging on dissolved oxygen has been
investigated since this parameter is of utmost importance in present
and future pollution abatement studies. The area of detailed study
is limited to the main stem of the Delaware River between Liston
Point, Delaware and Trenton,. N. J., since the effect on the Delaware
Bay is considered to be negligible and the tidal influence ends at
the Trenton rapids.
This study was carried out concurrently with the Delaware Estuary
Comprehensive Study and the Water Quality Control Study, Tocks Island
*Water pollution control activities of the Public Health Service were
assigned to the Federal Water Pollution Control Administration by the
Water Quality Control Act of 1?6£ (PL 89-23U)

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Reservoir, Delaware River Basin. Much of the data used in this report
was developed in connection with these other studies, and the reports
on these projects diould be consulted for more detailed information.
Five schemes for improving the channel from Philadelphia to the
sea were studied. These schemes are as follows:
a)	increase channel depth to US feet, maintain present width.
b)	increase channel depth to hS feet, maintain present width,
add anchorage improvements.
c)	increase channel depth to £0 feet, maintain present width.
d)	increase channel depth to 5>0 feet, maintain present width,
add anchorage improvements.
e)	increase channel depth to $0 feet, widen channel, add
anchorage improvements.
II. FINDINGS AND-CONCLUSIONS
1.	The following three, dredging schemes would have little or no
effect on the chlorides concentration in the estuary: '&) the k$ foot
deep channel, b) the U5 foot deep channel including proposed anchorage
improvements, and c) the f>0 foot deep channel.
2.	The 50 foot channel including anchorage improvements would
result in an increase in the chloride concentration at the intake to
Philadelphia's Torresdale Water Treatment Plant (Section 7 in Figure 1)

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to greater than 50 ppm in late autumn with present assured flows.
Based oh the added flow necessary to maintain the average chloride
concentration at mile 80 (Section 18 in Figure 1) at or below 250 ppm
compared to present channel depths, the $0 foot channel and anchorages
would result in a damage to water quality of approximately $900,000/year.
3. A fifty foot deep channel with the proposed anchorage improve-
ments plus widening the channel to proposed dimensions would increase
the chloride concentration at the Torresdale intake to approximately
85 ppm in late fall with present assured flows. Based on the cost of
providing the added flow required to maintain the chloride concentra-
tion at mile 80 (Section 18) at or below 250 ppm, as compared to the
present channel, this channel would result in damages of approximately
$1,300,000/year.
ii. A L|5 foot deep channel would cause a small decrease in the dis-
solved oxygen concentration in the estuary equivalent to the addition of
a waste load of 21ii,000 population equivalents. As a measure of the
damages this would cause, secondary treatment to remove a load of this
magnitude would cost approximately $650,000/year based on a 20-year
economic life, an interest rate of 3-l/8$> and including operation and
maintenance costs.
5. A 50 foot deep channel would have a larger effect on the dis-
solved oxygen, increasing the deficit by about 13%. This effect is
equivalent to an additional U28,000 population equivalents. A load

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this size would cost approximately $l,l50,000/year to remove, based on
a 20 year economic life, an interest rate of 3-1/8$, and including
operation and maintenance costs.
6.	Because of a lack of information on available spoils deposition
areas, all of the calculations on which the above conclusions were
based contained the assumption that the dredging spoils would be removed
entirely from the river. It may be possible through careful choice of
deposition areas that the salinity intrusion might be somewhat reduced.
As indicated, greater channel deepening will cause larger increases on
the salinity intrusion and correspondingly greater damages; thus, efforts
to plan spoils disposal areas to minimize this intrusion could reduce
the damages. However, it is important that organic sludge which is to
be found in many areas of the river bottom not be redeposited in the
river. Because of the high oxygen demand that this material would
exhibit when dispersed in the water, care should be taken at the dis-
posal site so that there is no chance of drainage back to the river.
7.	The dredging operations should be carried out at times of low
temperatures so that the effects of increased turbidity and disruption
of organic deposits that inevitably occur will be minimized.
8.	Disposal areas in Delaware Bay should.be selected so that
oyster beds are not damaged by silting.
III. NAVIGATION IMPROVEMENTS PROPOSED
The Delaware River, presently has a navigation channel maintained
by the Corps of Engineers at a nominal depth of UO feet from the sea

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5.
to Philadelphia, Pa., (Section 10) and a nominal depth of 30 feet from
Philadelphia, pa. to Trenton, New Jersey.
Five different dredging schemes for improving the navigation
channel to Philadelphia have been proposed. All five configurations
were used for the evaluation of the effects on salinity, as follows:
a.	foot channel - This configuration assumes an increase
nominal depth of channel from Philadelphia to the sea of
$ feet, with no other changes.
b.	kB foot channel with anchorages - This configuration
assumes an increase in nominal depth of £ feet in the
channel as well as in the present anchorages. In
addition, anchorages at Reedy Pt., Pea Patch Island,
Cherry Island, Marcus Hook, and Hog Island would be
added or enlarged to proposed dimensions.
c.	£0 foot channel - This configuration assumes an increase
in the nominal depth of the channel of 10 feet, with no
other changes.
d.	$0 foot channel with anchorages - This configuration
assumes an increase in nominal depth of 10 feet in the
channel and present anchorages. In addition, the new
or enlarged anchorages cited in configuration b) would
be incorporated.
e.	Optimum channel - This configuration assumes that all
modifications made in configuration d) would be incorporated

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as well as widening the channel to the proposed new
dimensions.
IV. METHOD OF ANALYSIS
A time varying model of the estuary was used for evaluating the
changes in salinity in the estuary. The flow regime for the last half
of the 1961* record at Trenton, which has a recurrence interval of about
fifty years, was used as a basis for analysis. The conclusions on the
dissolved oxygen effects are based on investigation of configurations,
a) and c), above. These investigations used a steady state flow
approximation of 3300 cfs which is approximately equal to a 90 day
average summer flow with a recurrence interval of 3 years, and is in
the order of the assured flow expected with presently proposed develop-
ment.
V. RESULTS
The effect on D.O. is primarily due to a decrease in reaeration
rate which is considered to be inversely proportional to the average
depth. Therefore, the configurations which have larger average depths
associated with them, result in increased deficits.
The steady state solution for the effect of Configuration a)
showed that in the area between Section 7 and Section 12, the dissolved
oxygen deficit was increased by an average of .1 ppm. An increase in
deficit of .2 ppm was seen from Section 12 to Section 18 with the
effect tapering off to zero between there and Section 29. This

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represents an overall increase in D.O. depletion of about 6%. The
actual magnitude of change is not great but it is equivalent to an
increase in BOD load of about 2lU,000 population equivalents which
would cost approximately $650,000/year to remove by treatment in a
secondary plant based on a 20 year economic life, an interest rate of
3—1/856, and including operation and maintenance costs.
Configuration c) gave effects that were almost exactly double
those under configuration a) at each point along the estuary. The
dissolved oxygen deficit was increased by .2 ppm from Section 7 to
Section 12, and by .h ppm from there to Section 18, an overall increase
of 13%* This increase is equivalent to the effect of an additional
1*28,000 population equivalents which would cost approximately $l,l50,Q00/year
to remove by treatment.
The effect on salinity is primarily due to an increase in the eddy
exchange coefficient and a decrease in the net velocity brought about
an increase in the cross-sectional area of the river.
The salinity study, accomplished on a time varying, digital com-
puter model, yielded the following results with the percentage change
at any one point on the estuary remaining essentially constant through-
out the year:
Configuration a) showed an increase in chlorides of about lk%
from Section 1 to Section 10. The actual magnitude of chloride con-
centration in this region is relatively small so the effect is not as

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large as might be presumed from the percentage change. As the magnitude
of the chlorides rose from Section 10 to Section 19, the percentage
change dropped from 12% to about 3%- Below Section 1?, the change in
chlorides amounted to about 2%.
Configuration b) and c) were very similar with an increase of
about 2$% in chloride concentration from Section 1 to Section 10,
decreasing from there to about $% at Section 19. The river below
Section 19 showed a 1$ increase in chlorides.
In Configuration d) the chlorides between Section 1 and Section
10 were increased by almost $0%, dropped to 6% at Section 19. This
increase was sufficient to yield ah average chloride concentration in
excess of 50 ppm. at the Philadelphia water intake (Section 7) in late
Autumn. In order to maintain the average chloride level at mile 80
(Section 18), at 2$0 ppm, an increase of about 1;00 cfs at Trenton over
the present requirement of 6200 cfs would be necessary. In order to
supply this additional flow, it was calculated that a single purpose
dam would require an added draft on storage of 133,000 acre feet.
Using an economic life of $0 years and 3-1/8$ interest, the annual
added cost for this additional storage would be $900,000/year including
operation and maintenance. The damages which would result from allowing
intrusion past Chester based on some recent benefit surveys carried out
by the DECS in connection with the Tocks Island report would be $l;71,500/year
to the industries and municipalities between Chester and Philadelphia.

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9.
The added cost to maintain the 2£0 ppm average chloride level
at the mouth of the Schuylkill River would be approximately the same.
Configuration e) yielded an increase of approximately 90% in the
estuary above Section 10. The increase in chlorides below Section 10
drops to 25$ at Section 19 and zero at Section 29. At the Torresdale
intake, the average chloride concentration in lage Autumn in increased
to approximately 85 ppm. An additional flow of 600 cfs above the present
requirement of 6200 cfs at Trenton would be required to maintain the
average chlorides at mile 80 (Section 18) at 250 ppm. The additional
cost to provide the necessary 2^0,000 acre feet added annual draft on
storage for a single purpose reservoir would be approximately $l,300,000/year.
The quantifiable portion of the damages due to the loss of water use
benefits to the industries and municipalities between Philadelphia and
Chester would be $671,5O0/year.

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As$u npink
" ..Cre ok
DELAWARE ESTUARY
COMPREHENSIVE STUDY
SECTIONS FOR
MATHEMATICAL MODE),
Smyrna
River
U.S DEPARTMENT OF HEALTH.EDUCATION.8 WELFARE
MDERAL WATER P.OIfiUTION
CONTROL AD.klHISf RAT ION
REGION II	NE W YORK, NEW YORK

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