United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/020 Feb. 1990
v/EPA Project Summary
Development and
Evaluation of a Rubber
"Duck Bill" Tide Gate
Peter A. Freeman, Angelika B. Forndran, and Richard Field
A unique 54 In. diameter "duckbill
rubber tide gate (RTG) was designed,
fabricated, and installed in a typical
New York City tide gate chamber. The
operation of the RTG was observed
over two years. The RTG was very
effective in preventing the Inflow of
tidal waters and generally showed
equal or improved performance com-
pared to a typical flap gate. Hydraul-
Ically, the RTG was supposed to open
to release storm flows at a positive
difference in upstream head of six in.
and to remain closed preventing
inflow at a downstream positive head
up to eight ft during high tide. Minor
inflow was observed when debris was
introduced into the RTG, and capa-
bility of self-cleaning was exhibited.
Inflow would be significantly greater
if similar size debris was lodged in
the conventional flap-type gate. The
maintenance crews observed no inci-
dent where the manual removal of
debris was required. The existing
chamber required minor modifica-
tions for the installation of the RTG.
The method of adapting the RTG to
an existing tide gate frame is critical
to ensuring the reliability of the
installation. The RTG was exposed on
occasions to gale force winds and
heavy rainfall during the two years of
operation in New York City.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
profect that is fully documented In a
separate report of the same title (see
Protect Report ordering Information at
back).
Introduction
Tide gates are a necessary component
of municipal combined sewer systems,
which discharge overflows into receiving
waters whose surface elevations vary due
to tidal or seasonal effects. In principle,
these perform a check valve function,
allowing excess flow mainly from storm
events to discharge into receiving waters,
while preventing back flow or leakage into
the combined sewer system. Leakage
can cause significant problems to the
treatment process and associated hard-
ware, due both to the presence of
dissolved salts or other substances, as
well as a waste of treatment plant
capacity.
The conventional flap tide gate
operates by swinging outward (toward the
receiving body of water) when the
upstream flow exceeds the capacity of
the regulator controlling flows to the
interceptor (normally during storm con-
ditions). The water level upstream of the
tide gate rises to whatever level is
necessary to offset the weight of the tide
gate and the water level downstream of
the gate. When there is no upstream flow,
the gate sits firmly against the frame and
does not permit backflow. Properly
operating tide gates do not permit tidal
inflow (backflow).
In New York City there are three types
of such tide gates. (1) Pontoon gates
which consist of hollow wrought iron flaps
mounted on cast iron frames; (2) Timber
gates predominantly made of three in.
thick Greenheart timbers, and (3) Cast
iron gates which are generally less than
48 in. high. A recently completed regu-
lator improvement program study re-
-------
vealed that these gates were functionally
adequate to prevent tidal inflow and
permit excessive storm outflow. The
design life is 20 years. Some existing
gates are as old as 30 years. Improperly
functioning tide gates permit inflow in
varying degrees. Malfunctioning gates
accumulate debris, have worn seats, have
corroded parts allowing entry of water,
have become misaligned, and/or are
warped. Inflow occurs as the tide rises
above the invert of the outfall sewer.
Inflow may be reduced when increasing
downstream static head tends to seal the
gate.
One investigation determined that
maximum inflow occurs at about two-
thirds high tide level when debris,
warpage, or mis-alignment causes incom-
plete closure of the gate. Another
problem which was identified is that the
hinge pins tend to become frozen.
Particularly in the dual hinge pin design,
the intended function is lost when the
lower pin is frozen. One recommended
solution is to replace the existing pins
with slightly undersized stainless steel
pins. Pontoon type gates tend to deteri-
orate due to graphitization of cast iron
components and corrosion/erosion of the
wrought iron flaps. As a result, timber tide
gates are recommended over pontoons.
The EPA has recognized the opera-
tional and economical problems of
conventional tide gates. Based on these,
improvements are required in tide gate
technology as follows:
1. The ability to both open and close
tightly in the presence of water borne
debris must be greatly improved,
both to prevent collection system sur-
charging and flooding, and also to
reduce the cost of existing treatment
efficiency by interfering with settling
and anaerobic digestion processes
and contributing to corrosion of plant
equipment.
2. The reliability of tide gates must be
greatly improved to relieve the
requirement for frequent surveillance
and maintenance, and the corre-
sponding cost to the municipality.
3. Extended tide gate operating lifetimes
are required to reduce recurring
capital equipment costs.
Procedure
The subject program was set up to
explore a novel approach to the tide gate
problem. This approach offers consid-
erable promise in achieving the desired
performance discussed previously. The
proposed concept was based on a type
of check valve designed and currently
manufactured by the Red Valve Co.,
Inc.* of Carnegie, PA. (RV). This unit
consists of a flexible tube which tapers to
flattened sections with two or more sets
of sealing lobes. Forward hydraulic head
opens the lobes, to release flow. Reverse
hydraulic head collapses the lobes
together, to prevent reverse flow
(leakage). The duckbill part of these
valves is typically constructed of rubber-
impregnated fabric, in the manner of an
auto tire. This concept is shown in Figure
1. At the time of the program start, RV
manufactured these units in diameters up
to 12 in. It was the principal design task
of this program to extrapolate this
configuration to the 54 in. diameter
required to release storm flow at the
selected tide gate site.
Figure 1. Flanged end red check valve.
This approach is attractive for munic-
ipal tide gate use in a number of ways.
Mechanical moving parts, with their
attendant problems of corrosion, friction,
and wear, are replaced by flexible
structures of environmentally stable elas-
tomeric materials. The basic check valve
action is performed without abrupt
changes in flow direction, so that there is
a minimum tendency to entrap debris. If
debris is entrapped, the flexibility of the
unit permits it to conform closely to the
shape of the debris, minimizing the
leakage flow under reverse hydraulic
head conditions.
Manufacturing costs should remain
consistent with advancement in technol-
ogy in the tire industry.
Specifically, the program objectives
were:
1. To identify and select a site which
reasonably represented a typical tide
gate location and permitted a demon-
stration of an RTG.
•Mention of trade names or commercial products
does not constitute endorsement or recommen-
dation for use.
2. To install the RTG to the sele
dimensions of 54 in. in diameter
hydraulic head flow character!'
similar to those of the convento
flap gate.
3. To install the RTG in a typ
metropolitan combined sewer out
replacing a conventional flap g
with a minimum of site modificat
This would demonstrate the feasib
of retrofitting into existing outfalls.
4. To evaluate the performance of
RTG, as so installed, under typ
service conditions, for a period o
least 18 months. During this peri
comparison was to be made v
conventional flap gates as to ir
dences of malfunction (failure to Of
or close, leakage, etc.), necess,
surveillance, servicing, hydrat
characteristics, and capital cost rt
uired for replacement or new insi
lations.
The program was initiated in late 191
The project team selected a combin
sewer regulator site (Regulator #11)
89th St. and East End Ave in Manhatt.
at which a typical timber flap gate was
be replaced by the RTG. The si
configuration is shown in Figure 2. F
selected an initial configuration with fc
sealing lobes, in a "cross" arrangeme
A quarter-scale model was construct!
and successfully tested. The full-sea
prototype unfortunately was unsucces
ful, as the additional weight of the sealii
lobes caused them to sag and seat in
random manner, allowing large gaps ar
leakage flows with reverse hydraul
head. The four-lobed arrangement w<
abandoned in favor of a vertical
oriented, two-lobed configuration. A
experimental two-lobed unit, shown
Figure 3, was completed in Octobf
1983. Flow limitations at the RV te;
facility prevented full-flow hydraulic pe
formance calibrations, so a procedur
was generated to determine RTG flo
area vs differential hydraulic head unde
static (no flow) conditions. This procedur
showed that the RTG was marginally to
stiff (too much hydraulic differential hea
was required to achieve the desired flo>
area). The final unit was constructec
given limited testing, and delivered to thi
New York City Department of Environ
mental Protection in December 1983.
Site modifications undertaken b1.
NYCDEP were minimal. After removal o
the existing flap gate, hinge brackets, anc
sealing frame, a stainless steel adapte
plate was installed. The adapter make:
the transition from the existing rectan
gular opening on a 15 degree slopec
tidegate chamber wall to the 54-in
-------
Plan Manhattan Sewer Datum 2.75 ft.
Above Mean Sea Level at
Sandy Hook
Figure 2. Wards Island - WPCP Regulator No. 11 (Plan).
circular /vertical opening required by the
RTG. The adapter plate is shown in
Figure 4. A clamping ring holds the RTG
in position on the adapter plate stub. The
RTG installation required about 2 days to
complete, and was placed in service on
August 11, 1984.
Results and Discussions
Upon reaching service status, the RTG
was included in the normal inspection
routine by NYCDEP regulator inspection
Maintenance crews. A special inspection
sheet format was generated to assist
them in making observations of the
RTG's performance under various service
conditions. Initial inspections were per-
formed weekly starting October 25, 1984.
The interval between inspections was
increased to 2 weeks, and then 4 weeks
after 8 months. A total of 18 months were
observed as part of this project.
All inspection sheets indicated negli-
gible or no leakage or inflow, even though
there was in nearly all cases a reverse
differential hydraulic head on the RTG,
even at most low tide conditions. The
inspection sheets also indicated that the
RTG was normal (clean), and that no
trapped debris was observed. A con-
dition of an RTG with entrapped debris
was simulated by inserting a 12-in.
length of lumber (4 in. x 4 in.) into the
RTG discharge section. A leakage flow of
about 50 gpm occurred at a reverse
hydraulic head of 2 ft. This simulated
debris was later washed out of the RTG
with the next occurrence of forward
hydraulic head, indicating an excellent
capability for self cleaning.
The principal observed difficulties with
the RTG were occasional instances
where hydraulic forces occurring during
storm events moved the RTG on its
mounting. On July 26, 1985, the RTG
came loose from the adapter plate. It
was reinstalled by the regulator main-
tenance crew in 7 hours, during which
techniques were improvised for handling
the heavy (800 Ib) unit within the
cramped confines of the tide gate
chamber. This event prompted recom-
mendations for suspension and handling
facilities to be built into the tide gate
chamber overhead, and the requirement
for "pinning" the RTG to the adapter
plate stub, in addition to the clamping
ring.
These recommendations appear par-
ticularly desirable for future, larger RTG
installations.
A rough, in situ hydraulic flow calibra-
tion of the RTG was performed during
August and September, 1985. Continu-
ous depth-of-flow measurements were
made in the trunk sewer upstream of the
regulator, and downstream of the RTG.
These, plus the known hydraulic charac-
teristics of the trunk sewer and regulator,
were used to compute standard hy-
draulic relationships based on Manning's
equation. The resulting flow calibration
was relatively linear with increasing
hydraulic head, as attributed to the fact
that the RTG flow area is itself a function
of hydraulic head differential. The un-
availability of data from the storm events
occurring during this period, plus some
instrumentation failures, did not permit
the generation of a complete flow cali-
bration; however, a reasonable extrapo-
lation of the obtained results indicated
that the RTG's maximum flow capacity
comfortably exceeds the maximum runoff
flow rate from the selected drainage area
without surcharging the trunk sewer.
A comparison of the hydraulic perf-
ormance of the RTG and the flap-gate it
replaced showed that the RTG starts to
release flow at a lower hydraulic head
differential for all conditions of down-
stream submergence. This difference
-------
Figure 3. Two-lobed being prepared for plant testing.
-------
1-112" (3.8cm)
~(7.6 cm)
(16 mm)
5/8" Dia. Holes for
(13mm) 112" Hex Head S.S.
Bolts, Nuts, and Washers
2' -6"
(0.76 m)
Regulator No. 11
•/v k/-
m;Vfc
$$
£W
' I ' I
flubber Tide Gate. „!; -, •' -.\ .'/
Adapter Plate
/- Remove Exist.
Concrete
If Necessary
1 l&;fr
f^v.i!/.^.:'--
, jlte^M^
Figure 4. Adapter plate design details (fitting to frame).
occurs since the flap-gate is ballasted
with lead to ensure closing under high
tide conditions. The flap-gate has a
higher maximum flow capability than the
RTG (see Figures 5 and 6). Both units
release more at less differential hydraulic
head with increasing downstream sub-
mergence. The lower maximum flow
capability of the RTG indicates a
requirement for careful estimation of peak
storm flows or oversizing, in selecting an
RTG for a particular tide gate location.
Comparative costs for RTG and con-
ventional flap-gates are given in Figure 7.
These costs are manufacturer's costs
only. Installation costs are.dependent on
location and ranged from $5,000 to
$15,000 for retrofit with a RTG, while the
more predictible flap gate replacement
cost is approximately $9,000.
Factors to consider in estimating costs
ire the related savings due to:
(a) Operation and maintenance of tide
gate system.
(b) Preventing inflows and treatment
upsets caused by settling, digestion,
and hydraulic overloading.
(c) Corrosion protection from industrial
wastewaters. Structural limitation for
each gate location, e.g., chamber
modification, adapter plates must also
be considered.
Conclusions
The basic conclusion from this pro-
gram is that the rubber tide gate (RTG) is
a practical, cost-effective alternative to
the typical flap-type tide gate.
The RTG showed significant improve-
ment over the flap-type tide gate in terms
of leakage inflow, entrapment of debris,
capability to self-clean, and susceptibility
to marine fouling during 18 months of
observed operation.
The RTG required virtually no labor-
intensive surveillance or maintenance
during routine inspection. Maintenance
was required to reattach the rubber
sleeve onto the adapter plate.
The design used in this prototype
installation for attaching the RTG onto a
smooth adapter plate using clamping
rings was not sufficient to hold the RTG
in place during the heavy storm and tidal
action.
Non-stainless steel metals or stainless
steel hardware not of type 316 will
corrode in the brackish environment and
cause failure of the installation by
permitting the RTG to slip during storm
and tidal action.
The RTG material consisting of neo-
prene over vulcanized rubber has shown
no signs of any surface deterioration due
-------
J.5.-
0.5--
-4-
Zero Submergence -
% Submergence =100 (Tide
4.5 (ft)
2 ft Submergence
4 ft Submergence
-i-
-4-
-4 J-
-i-
10 20 30 40 50 60 70 80 90 100
Q-CFS
Figure 5. Estimated hydraulic performance of conventional tide gate.
70
*
60-
50-
30--
20--
10--
20% - Submergence
Figures Indicate
& Submergence of
Each Data Point
0% - Submergence
Reference Height of RTG
at Discharge End = 54 in.
% Submergence = (100-Tide Height)/4.92
(ft)
Figure 6. Estimated RTG flow characteristic.
-------
') either tidal saltwater, wastewater con-
tituents, or temperature fluctuation over
the 18 months of observed operation.
The RTG is expected to have a lifespan
of 20 years or more, which is comparable
to conventional tide gates. Smaller indus-
trial installations of this type check valve
are currently in operation up to 15 years.
There was no record of any backup
flooding during storms or measurable
tidal inflow when submerged at high tide
during the observed operation. The
maximum flow capacity through the RTG
for any size tide gate is less than that for
a flap-type gate. However, the maximum
available RTG flow for this specific
installation is estimated at 120 cfs, which,
based on historical rainfall data, is
adequate for the particular drainage area.
Genetically, a slight lessening of maxi-
mum outflow capacity does not cause
any measurable decrease in the way of
flood protection because the return storm
frequency design concept is based on a
stochastic phenomenon. Hydraulic com-
parisons between the RTG and conven-
tional gates are developed in the final
report. The release flow of the RTG starts
at a lower differential hydraulic head
when compared to a flap gate.
Debris caught in the RTG will cause
»idal inflow to occur, however, no debris
ras discovered in the RTG during in-
spections. Inserted debris washed out
without intervention by the maintenance
crew and was measured to cause a
relatively small inflow of 50 gpm during
high tide.
A survey of municipal installations
since 1984 indicates costs for RTGs are
comparable to timber tide gates. Factors
to consider in pricing an RTG versus a
timber flap gate are equipment, installa-
tion and operation and maintenance
needs for the specific location. For an
equivalent area of about 25 square feet,
hardware cost for flap gates averages
$19,000 in New York City and $24.000 for
RTGs in other municipal installations.
Installation costs vary greatly, averaging
about $9,000 for timber tide gates in NYC
and ranging from $5,000 to $15,000 for
RTGs.
Recommendations
Operational experience with the pro-
totype rubber tide gate (RTG) indicated
that some design modifications for the
installation of the RTG are recommended
as follows:
The RTG attachment to the adapter
plate should be modified to provide a
positive restraint against axial movement.
The prototype installation in this project
had a friction arrangement only which
proved to be inadequate under heavy
storm hydraulic loading and tidal action.
The adapter plate and all related
hardware, should be made of stainless
steel type 316 for corrosion resistance in
the brackish water environment.
The RTG design should be modified
for suspension near its discharge end to
relieve cantilever loading on the mounting
flange and adapte plate. Two larger units
84 in. and 72 in., currently being
fabricated by RV, will have holes through
the top end of the lip to facilitate
attachment to the tide gate chamber
ceiling.
The liquid level upstream of the RTG
decreases and flow capacity increases as
the cross-sectional area of the RTG
increases. Therefore to alleviate flooding
(from an elevated upstream flow profile)
during intense storms, it is important to
maximize discharge area. A probable
modification would be to make an over-
sized adaptor plate to accommodate the
largest practicable and workable RTG.
The modifications to existing tide gate
chambers should include provisions for
overhead suspension of the RTG to facil-
itate installation and/or servicing since the
weight of large sized units exceeds
manual lifting capability when working in
the confines of typical tide gate cham-
bers.
It is recommended that the 54 in-
diameter RTG at the current site remain
in operation subject to routine O&M pro-
cedures. Observations should continue to
monitor durability of material, reliability of
performance, and consistency of low
maintenance requirements over time.
Interested municipalities should con-
tinue to monitor NYCDEP's continuous
26
24-
22-
20-
18-
16-
14-
12-
8-
6-
4-
2-
0
RTG
Conventional Flap
Gate
I 1 I I ) I
4 8 12
16 20
Equivalent Area (sq ft)
24
28
figure 7. Comparison between costs of RTG and conventional flap gate.
-------
experience with this unit during which the
RTG costs, maintenance requirements,
and hydraulic performance wilt be more
precisely established over time. Further
developments of this technology should
include:
• Establishing design criteria for new
installations. New chambers would
have cost-saving benefits such as (a)
design for attachment which does not
require an adaptor plate, (b) access
manhole over discharge end which
permits direct observation from street
surface, (c) appropriately dimensioned
access chimney and overhead sus-
pension or trolley system as required
for installation and removal of RTG.
Establishing comparative costs be-
tween RTG retrofitting and repairing
existing traditional flap gates. These
costs would include savings from
reduced surveillance and maintenance
and savings in wastewater processing
from reduced tidal inflow.
Establishing protocol for repairs
maintenance. This would identify
type of damage the RTG m
sustain, methods of patching
repair that are suitable, and typi
training and tools required
maintenance crews servicing mul
installations.
Establishing life expectancy of
rubber/neoprene in a sewer/ou
environment. This would involve s<
outfall materials testing investigatio
Peter A. Freeman is with Peter A. Freeman Associates Inc., Berlin, MD 21811;
Angelika B. Forndran is with the New York City Department of Environmental
Protection, Wards Island. NY 10035; and the EPA author Richard I. Field (also
the EPA Project Officer, see below) is with the Risk Reduction Engineering
Laboratory, Edison, NJ 08837.
The complete report, entitled "Development and Evaluation of a Rubber 'Duck Bill'
Tide Gate," (Order No. PB 89-188 379/AS; Cost: $15.95, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory—Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAH
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-89/020
000085918 «EHL
JOH8 COLLETTI
EP1 REGIOM 5
230 5. DEAHBORH
CHICAGO
STREET
IL 60604
0
5
------- |