Exh 2BAGENDA ITI;NI #2B
AUGUST 8, 2005
A STUDY OF
DRAINAGE IMPROVEMENTS AT
OCEAN BOULEVARD
FROM 13TH STREET TO 16TH STREET
PREPARED BY
BOWEN CIVIL ENGINEERING, INC.
PREPARED FOR
CITY OF ATLANTIC BEACH
PUBLIC WORKS DEPARTMENT
(P.O. # 037155)
JULY 2005
AGENDA ITIrN1 #2B
AUGUST 8, ?005
EXECUTIVE SUMMARY
The purpose of this report is to investigate the possibility of adding
additional stormwater collection systems to the system proposed to serve the
Ocean Blvd. and 13`" Street intersection. The Ocean and 13`" intersection
(Phase 1) is currently under design and is intended to relieve ponded water at
the intersection. The Phase 1 portion promises to offer a point of connection
for a new collection system to serve problem areas at Ocean and 14cn~ lscn~
and 16`". This additional system would be Phase 2. The intersections at
Ocean Boulevard and 14`" and 15`h Streets as well as that section of Ocean
Boulevard between 15`h and 16`h Street are not served by a stormwater
collection system at this time and they experience nuisance flooding in
response to even mild precipitation events. Runoff from these intersections
would be added into the Phase 1 system and possibly also the system in
place at Ocean and 16`h Street. The various options for stormwater
management considered include slotted pipe in a gravel envelope,
exfiltration trenches, and pipe bursting for the purpose of providing relief to
the ponding areas. After meeting with the St. Johns River Water
Management District (SJRWMD} to determine which options were
permittable, and after reviewing the cost effectiveness of these options, the
recommended solution is to construct an inverted crown roadway section
with inlets in the center and a collection system composed of slotted pipe in
a gravel envelope. The results of this study indicate that the proposed
improvements for Ocean & 13th will not be adversely impacted by adding
the Phase 2 system upstream if certain measures are put in place such as a
constriction plate at the point of connection.
AGENDA ITEM #2B
AUGUST S, 2005
EXISTING CONDITIONS
During a site visit to the project area on the afternoon of June 30, 2005, it
was observed that after a storm with moderate rainfall, significant ponding
occurred at the intersection of Ocean Blvd. and 13`" St. even though a
stormwater collection system serves this area. In addition, areas of minor
ponding at Ocean and 14`" St., 15`" St. just east of Ocean and on Ocean just
south of 16`" St. were also noted. Upon returning to the site the following
morning on July 1, 2005, the areas of ponding had dissipated, which
suggests that the branch of the existing collection system which begins at
Ocean Blvd. and 13`" St. is operational but is undersized. Ponding at Ocean
and 14`" and Ocean and 15`" also had subsided due to percolation.
OBJECTIVE FOR REMEDY
Since the infrastructure downstream of the intersection of Ocean Blvd. and
13`" St. is restricted to 6-inch pipe in places (6" on one survey, 10" on
another), the inlets at this intersection back up from the excess flow. Phase
1 is currently intended to upsize all pipes smaller than 15 inches. The
objective for remedy for this study (Phase 2) is to provide a collection
system along Ocean Blvd. north of 13`" St. where there is none. This would
be connected to the Phase 1 system for an outfall. Because even the new
15"-18" pipes will be too small to handle the flows coming to them, it is
anticipated that there will still be some minor flooding on Ocean Blvd. The
benefit is that the flooding will dissipate faster since it will have larger pipe
to carry it away.
SLOTTED PIPE COLLECTION SYSTEM
The proposed Phase 2 collection system would consist of an inverted crown
roadway section for Ocean Blvd. from 13`" St. to 16`" St. with inlets at the
centerline connected by 15-inch slotted pipe in a 8 ft. wide by 2 ft. high
gravel envelope as shown on Figure 1 -Plan, and Figure 2 -Cross Section.
This envelope will be situated above the groundwater elevation such that it
will have both vertical and horizontal discharges. Refer to Appendix C for
the soils report information. This, in combination with sumps at the
proposed inlets, will offer a measure of pollution abatement. This system
will connect to an inlet at the intersection of Ocean Blvd. and 13`" St. (Phase
1). The initial calculations, based on a pair of Double Ring Infiltrometer
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AGENDA ITEM #2B
AUGUST 8, 2005
tests, predict that stormwater will flow from the system into the ground at a
rate of 1.3 cfs. This is similar to the carrying capacity of a 10" pipe, and
should enable ponded waters to dissipate from the intersections at a much
higher rate. Even if flows into the ground are not considered, the
preliminary calculations predict that floodwaters could dissipate in 2-3 hours
following the end of rainfall.
The performance of this system was then evaluated using the MODRET
• ~ software for infiltration modeling. MODRET is considered to be much
more reliable because it considers the impact of the soil filling up with water
. , on the performance of the system. The results confirm that discharges to the
soil will be relatively rapid during the early stages of the storm event when
the soils are unsaturated. But it is clear that once the soils become saturated,
the infiltration rate declines to a rate which is in equilibrium with the
groundwater table. In other words, the ponded water surface elevation will
. recede at the same rate that the groundwater recedes, which is essentially
• what is happening today without the slotted pipe system in place. What this
suggests is that there would be noticeable improvements for storm events of
• ~ lesser volume, i.e. less that two inches. Rainfall events of greater volume
• will more nearly saturate the soil leaving no space for additional waters from
the pipe.
In order for Phase 2 to operate well with Phase 1 it is important that all the
• storm grates have similar elevations. A potential disadvantage is that if the
Phase 2 system has significantly higher grate elevations, this might offer
.. sufficient hydraulic potential to send stormwater into the Phase 1 system at a
• much higher rate and cause worse problems there than were originally being
experienced. This has been confirmed by field elevations. The grates at 14`"
• acid 15`" Streets are on the order of a foot higher than those at 13`"Street. A
foot differential over 300 feet of pipe would drive about 3-1/2 cfs through a
15 inch pipe. This would be a faster rate than we expect water to be leaving
• Ocean and 13`"'because the rest of the system would ordinarily require a 36"
pipe instead of the 15" pipe currently in 12`" Street. Therefore it can be
` ~ concluded that ponding at Ocean and 13`" would be worse during a rainfall
• • event if Phase 2 is simply connected by a 15 inch pipe. Furthermore, the
.. construction of an inverted crown road section would lower the grate
elevations and reduce the theoretical hydraulic potential thereby reducing
flows to Ocean and 13`". This would make a larger area to pond, but it
• ~ would subside in significantly less time. One solution to this problem might
be to bolt a plate over the connection pipe and put an orifice through it to
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AGENDA ITEM #2B
AUGUST 8, 2005
throttle the flow. This might take some "tweaking" to get the desired result,
but changing the size of an orifice is neither difficult nor expensive. The
orifice would offer additional treatment advantages to the system since it
would force a longer residence time in the slotted pipe system. If certain
measures are taken, such as a constriction plate over the Phase 2 outfall, the
connection of Phase 2 to the Phase 1 system can be controlled and no
adverse impact will result by adding this future system upstream. Hence this
project can be designed, permitted and constructed as Phase 1 and Phase 2 of
a single project.
EXFILTRATION TRENCH
This study analyzed the use of an exfiltration trench under Ocean Blvd.
Using an approximate drainage area of 11 acres and a runoff coefficient of
0.6, an exfiltration trench of 4 feet deep, 22 ft wide and 900 feet long was
modeled for four storm events. A compatible outfall was assumed. The
following table shows the results, with the spreadsheet models included as
Appendix A. The model indicates that exfiltration trenches could be used to
meet both treatment and attenuation criteria. Significant design and
construction activity would be required to bring the concept to fruition. This
was determined not to be a practical alternative since it is relatively
expensive and the SJRWMD does not require meeting treatment criteria for
this project as long as certain other criteria are met (see SJRWMD Criteria
section).
Table 1 - Stormwater Discharge
Storm Event Allowable Discharge
(cfs) Peak Discharge (cfs)
Mean Annual 34.53 19.85
5- ear 45.75 38.86
25- ear 62.15 59.49
100- ear 79.12 96.16
PIPE BURSTING
Pipe bursting of a pipe in the Phase 1 portion of this project (existing
collection system from Ocean Blvd. and 13`" St. to East Coast Drive and 12`"
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AUGUST 8, 2005
St.) was studied. The 10" orangeburg pipe runs from an inlet in a yard on
the west side of East Coast Drive just north of 13`h Street to an inlet at the
north end of Selva Marina Circle. Pipe bursting, which can be either
pneumatic, hydraulic expansion or static pull, fractures a pipe and displaces
the fragments outwards while a new pipe is drawn in to replace the old pipe.
The front end of the bursting head is connected to a cable or pulling rod
while the rear of the bursting head is connected to the new pipe. The size of
pipe currently being replaced by pipe bursting typically ranges from 2 inches
to 36 inches, so the 10-inch pipe under consideration falls within this range.
Pipes suitable for pipe bursting are typically made of brittle materials, such
as vitrified clay, cast iron, plain concrete, asbestos or some plastics. The
brittle orangeburg pipe would be suitable for pipe bursting. Pipe bursting
has limitations. Difficulty can arise in close proximity of other service lines
or a collapsed pipe at a certain point along the pipe. Pipe bursting operation
creates outward ground displacements adjacent to the pipe alignment,
especially when the pipe to be burst is shallow. If the existing pipe runs
below residences' foundations, this could be a problem. Pipe bursting may
not be successful if the bursting head gets stuck at an unexpected obstruction
in or around the pipe, such as root balls of trees that encircle pipes. If pipe
bursting is determined to be feasible, the cost for this alternative is
approximately $50 per linear foot for 10-inch pipe according to a national
study. For the approximate 350-foot length of pipe, this would amount to
$17,500. This option could be used in conjunction with the slotted pipe in
gravel envelope to alleviated some of the stormwater backup in the Phase 1
system from Ocean Blvd. at 13`" St. to East Coast Drive at 13`" St.
SJRWMD CRITERIA
A meeting was held with the SJRWMD on June 29, 2005 to discuss
treatment and attenuation requirements for proposed improvements in Ocean
Blvd. Minutes of this meeting are included in this report as Appendix B. To
summarize, the project must meet three criteria to meet a Category C
Environmental Resource Permit (40C-42). (1) There must be no increase in
pollutant loading. (2) There must be no increase in peak discharge. (3)
There must be no change in the point of discharge. Upon review of the
Aikenhead and & Odom Preliminary Drainage Report dated October 2,
2000, it appears that the areas which outfall to the Selva Lagoon at 11`h St.
add up to 155 acres (basin numbers 32320, 32330, 32340 and 32350). By
comparison, the area being modified (11 acres) is relatively small. Since the
AGENDA ITEA~I #2B
AUGUST 8, 20x5
slotted pipe with gravel envelope option offers a measure of treatment and
meets these three criteria, it is the preferred alternative.
CONCLUSION
Through a meeting with the SJRWMD, it was determined that this project
meets the requirements of a Category C Environmental Resource Permit
without excessive measures for stormwater management such as wet
detention ponds or exfiltration trenches. Therefore, more affordable project
scenarios were possible. It has been proposed that the City construct an
inverted crown roadway section with inlets in the center with a collection
system composed of 15" slotted pipe in an 8-foot by 4-foot rectangular (16
sq.ft.) gravel envelope. This resembles an exfiltration .trench, but it is not
intended to meet design criteria for an exfiltration trench. Rather it is
intended to relieve the existing constricted system of some of the excess
flows by discharging them to the soils. For this reason, separations from the
groundwater table are not required, nor is the aggregate voids volume as
important. This collection system will be connected to proposed Phase 1
improvements at Ocean Blvd. and 13`~ St. Appropriate constriction devices
may be necessary.
A preliminary cost estimate was made using composite unit prices from
numerous Better Jacksonville Plan jobs. The estimate predicts that
construction of the recommended solution will cost in excess of $250,000.
Because this cost may be higher than the budget will allow, the elimination
of certain lengths of the slotted pipe and gravel envelope may have to be
. considered. These lengths would be replaced by solid pipe alone.
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