NC0033111_Report_19830413

DIVISION OF ENVIRONMENTAL MANAGEMENT
April 13, 1983
MEMORANDUM
0
TO: Forrest Westall, Head
Operations Branch
FROM: Randy Williams G/
Technical Services ranch
THRU: Steve Tedder
SUBJECT: Fairfield Harbour Analysis
An analysis of the Fairfield Harbor situation was conducted. I
have attached some handwritten notes concerning the methods that were
used. Also attached is a table showing predicted concentrations of
phosphate at the outfall, at the confluence with the Neuse River and
one mile upstream of Fairfield Harbour's present site. Note that these
predicted concentrations are very low, even at the outfall. I have, of
course, made many assumptions in estimating the parameters. For example,
I assumed only 1000 sq. ft. cross-sectional area would be available for
mixing. This is a good assumption one mile above the outfall where
the channel is only about 200 ft. wide, but in the vicinity of the
outfall and below, the channel is over 0.5 mile wide. Also assumed
was a value for E, the dispersion coefficient, although a sensitivity
run was performed for this parameter, using one "high" value (1.0 mi /day)
and one "low" value (0.1 mi /day) . The settling rate of 0.3/day was
calculated from Lauria and O'Melia (1980) , "nutrient Models for Engineer-
ing Management of Pamlico River, North Carolina."
. The model shows that an order of magnitude increase in wasteflow
will result in approximately an order of magnitude increase in
nutrient concentrations in the estuary (Upper Broad Creek) . This
magnitude of increase could be significant. That is, the potential
for algal blooms to occur may increase given the 10-fold increase in
available phosphorus.
I would recommend that the total wasteflow be kept to a maximum
discharge of 500,000 gpd at the present site. Studies show that some
algae can grow in bodies of water with very low P concentrations. A
Ten-fold concentration increase could cause a different assemblage of
algae to grow, less desirable than that which grows at very low
concentrations. If the assemblage is no longer P-limited, N-limitation

Forrest Westall
April 13, 1983
- page two -
could occur, in which blue-greens may thrive. It would be advantageous,
then, to keep the P concentrations as low as possible in Upper Broad
Creek. A five-fold increase in flow would keep concentrations due to
the discharge in the P-limiting range in Upper Broad Creek.
RW:cs
Attachments

Table I
*
Waste Cross- Dispersion Settling Advective Concn Concn Concn
Flow Sectioal 2E 11 Velocity at -1.0 mi 0.3 mi
(mgd) Areal) (mi /day) (day ) (ft/sec) Outfall Upstream down
(ft (mg/1) (mg/1) stream
(mg/1)
0.1 1000 1.0 0.3 .000155 .002 , .0007 .001
1.0 1000 1.0 0.3 .00155 .012 .007 .0102
0.1 1000 0.1 0.3 .000155 .0038 .0007 .0023
1.0 1000 0.1 0.3 .00155 .038 .006 .0234
*Calculated at 7Q10 flow (=0 cfs) average flow is 63 cfs.

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