Storm Drain Study Report Excerpts-2020

-40- WithersRavenel
NOOur People. Your Success.
PHASE 4
DRAINAGE INFRASTRUCTURE
IMPROVEMENTS
CONCEPTUAL DESIGN FINAL REPORT
Sites 4,5,6,7,8,9110,12and 13
Nags Head, NC
Prepared For:
Town of Nags Head
Town Hall, P.O. Box 99
Nags Head, NC 27959
252.441.5508
ATTN: David Ryan
Prepared By:
WithersRavenel
115 MacKenan Drive
Cary, NC 27511
919.469.3340
License No. C-0832
August 2019
WR No. 02160539.30
Received
MAR 13 2020
DLI EC
Hunter C. Freeman, PE
Brandon J. Miller, El

EXCERPTS
COVERING SITES 10, 12 AND 13

Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
Nags Head, North Carolina
EXECUTIVE SUMMARY
Withers-eavene Our People. Your ss.
The Town of Nags Head selected WithersRavenel as their consultant for further analysis and
implementation of the Town's 2006 Stormwater Management Plan - Capital Improvement Plan. This
memorandum summarizes the additional analyses and conceptual designs completed for project areas 4, 5,
6, 7, 8, 9, 10, 12 and 13.
Project Area 4 - Wrightsville Avenue
The Town requested additional hydrologic modeling of a groundwater pumping system to reduce flooding
along Wrightsville Avenue. This work was completed by Moffat & Nichol, a sub -consultant to
WithersRavenel on this project.
Project Area 5 - North Ridge
WithersRavenel completed cost -benefit analyses of potential design alternatives at the request of the Town
Board.
Project Area 6 - Old Nags Head Place
Project Area 7 - Southridge
Project Area 8 - Soundside Road
Project Area 9 - Carolinian Circle / Nags Head Pond
Project Area 10 - Kipper Court
Project Area 12 - Old Oregon Inlet Road (Juncos St)
Project Area 13 - Old Oregon Inlet Road (E. Hardgrove St. to Fire Station)
WithersRavenel developed design alternatives and cost benefit analyses for each of these sites.
After discussing each project area with Town staff, WithersRavenel presented our findings to the Board of
Commissioners. The Board used the results of this phase of work to determine next steps for implementation
and construction projects intended to commence in the winter of 2019/2020.
No construction work was completed as part of this phase of the project.
PROJECT APPROACH
For project area 4, Moffat & Nichol completed detailed hydrologic and groundwater modeling to evaluate
the benefits of various groundwater pumping and discharge scenarios. The overall intent was to more
accurately predict the flood reduction benefits of proposed pipe systems, and also to determine the
maximum rate and volume of pumped groundwater that the soils under the Bonnet Street beach access
parking lot could accept without exacerbating existing flooding concerns on S. Virginia Dare Trail (NC-12).
The project report from Moffat & Nichol includes more detailed information on their approach, methods,
and results. The report is included in the appendices of this report.
For Site 5, WithersRavenel expanded on previous preliminary design work in an effort to find alternatives
which might reduce flooding along Lookout Road and Buccaneer Drive. This work focused on additional
coordination with NCDOT and work within their right of way, including potential improvements NCDOT
drainage infrastructure.
For the additional sites, WithersRavenel's approach was to develop simple models to represent complex and
unique hydrologic challenges. Each conceptual design is intended to improve the quality of life of Nags Head
residents by reducing the detrimental impacts of persistent standing water present after moderate rainfall
events.
The study of all sites relies on the development of hydrologic models. The models were developed to help
understand the hydrologic components existing in the present conditions, and also understand how the
physical site conditions were affected by rainfall events. Once the existing conditions models were
Phase 4 Drainage Infrastructure Improvements WR No. 02160539.30
Conceptual Design Final Report August 2019
1

Phase 4 Drainage Infrastructure Improvements ;0 WithersRavenel
Conceptual Design Final Report Our People. Your Success.
Nags Head, North Carolina
calibrated to more closely match field observations from Town staff and citizens, we began developing
proposed conditions models and retrofit options.
WithersRavenel modeled the six new project areas (6, 7, 8, 9, 10, 12, and 13) using PondPack software. Both
physical (i.e., upgrade infrastructure) and mechanical (i.e., lowering groundwater through pumping) options
were considered. Metrics presented for each project area include the depth of roadway flooding and time
for floodwaters to recede. Projected performance of the proposed design alternatives was compared to
existing conditions using these metrics to gauge their effectiveness in addressing the concerns noted by the
stakeholders.
For the proposed groundwater lowering scenarios, WithersRavenel looked at the net result of the pumping
to assess the effect on floodwater levels during small storm events. This approach removed the need to
design and model specific pumping options for the time being. This level of detail will be added in the final
design if the Board of Commissioners earmarks the project area for construction. The hydrologic models
simulate the change in soil properties due to elevated and lowered water tables within each drainage area.
Existing conditions (e.g., no groundwater pumping operation) were considered to be saturated, resulting in
the loss of infiltration capacity as a starting boundary condition. To simulate the effect of the groundwater
lowering operation, the soil type within the project area was revised to reflect the available absorptive
capacity of the dewatered soil to infiltrate a portion of rainfall. This reflects WithersRavenel's assumption
that prior to a rainfall event the soil would be functioning sandy, high infiltration -capacity soils that exist in
most portions of the Town.
This approach was intended to address the primary impacts to quality of life voiced during community
meetings. It should be noted that we are not attempting to quantify the results using traditional engineering
metrics (peak flow, inches of runoff, level of service based on XX-year storm), but rather evaluating these
scenarios within a framework that quantifies how each option will improve day-to-day quality of life. This
approach is similar to the method in which previous project areas were evaluated.
PRELIMINARY RESULTS & COST ESTIMATES
*Note: the cost estimates below do not include potential costs of
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Problem Statement
Insufficient drain ge infrastructure, along with low-lying propel
flooding along W ightsville Avenue from Bonnett Street to Bainl
Existing Constraints and Design Considerations
or land acquisition.
and elevated groundwater, are causing
ge Street.
The detailed mo ling is aimed at determining the ability for the liumped groundwater to infiltrate through
the dune system 4nd reach the Atlantic Ocean. There is no availablt open space where pumped groundwater
could be dischar ed and expected to infiltrate (other than envir nmentally sensitive areas), therefore the
area under the p rking lot at the Bonnett Street beach access wal the most viable location.
Project Area P Conceptual Design
Previous studies f the project area investigated the potential b
potentially impr ing the culvert upstream of the Curlew St
cost/benefit retu ns, therefore this pumping alternative was inv
Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
of installing a french drain as well as
. Both alternatives showed limited
WR No. 02160539.30
August 2019

Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
Nags Head, North Carolina
Project Area #10 — Kipper Court
Problem Statement
%0WithersRavenel NO our People. Your Success,
Kipper Court is located at the low point of a 6.8-acre drainage area which includes many homes on W.
Baracuda Rd. runoff from upland areas floods the cul de sac and the roadway, including portions of Baracuda
Road.
Existing Constraints and Design Considerations
There is no storm drainage system on Baracuda or Kipper other than small shallow swales. The nearest storm
drainage system is on Danube St, and the system eventually discharges into a manmade canal adjacent to
Old Cove Road. The receiving waters are classified as SA waters.
Project Area 10 Conceptual Design
Study of the GIS survey of the storm drainage network indicates that gravity connection to the existing
network may be feasible but would require acquisition of easements on private property. A gravity flow
option was studied that utilizes subsurface drainage. A second gravity flow design that included a trapezoidal
swale has been excluded per Town comments. A third option of pumping water west along Baracuda Dr. to
an outfall on Shiner Ct. was not studied in detail, but could be investigated if easements on private property
cannot be obtained.
Conceptual Design — Install storm drainage system to connect to the existing system on Danube St.
Goal: Provide a french drain and outfall from Kipper Court to allow the area to drain more easily.
Assumptions: Two tailwater conditions were assumed based on field observations of the existing outfall.
One option, with a tailwater of 0.0 represents normal dry weather low tide conditions, the other, with a
tailwater of 3.21 represents observed storm conditions where the outfall pipe is'% submerged.
Design: Catch Basin on 207 W. Kipper Ct and installation of a new storm drainage system and cross pipe
under Danube St, tying into the existing system at a catch basin on the south side of Danube St.
Preliminary Engineer's Cost Opinion: $45,000 - $60,000 (including french drain)
"The reports for project areas #10, 12, and 13 were prepared by Withers Ravenel
under the direction of the Town of Nags Head under grant award #
NA15NOS4190066 to the Department of Environmental Quality, Division of
Coastal Management from the Office for Coastal Management, National Oceanic
and Atmospheric Administration. The statements, findings, conclusions, and
recommendations are those of the author(s) and do not necessarily reflect the
views of DEQ, OCM or NOAA."
Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
WR No. 02160539.30
August 2019
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Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
Nags Head, North Carolina
Project Area 10 Preliminary Results
A% WithersRavenel our Peopte. Your Success.
We estimate that Kipper Ct floods approximately 3 times every five year, but localized flooding on private
property is more frequent (multiple times per year). The pipe option improves the flooding conditions during
larger storms event, and Kipper Ct would reduce to flooding once in five years. Road flooding duration is
reduced by 89% across the road in the 5-year 24-hour storm.
Table 10. Projected depth of roadway flooding and time to recede for Project Area 10
Existing
Staging
Event
WSEL
Start
Finish
Time (hrs)
2-YEAR
5.52
12.50
13.50
1.00
5-YEAR
5.57
12.25
14.50
2.25
1.5-YEAR
5.50
12.75
13.25
0.50
Proposed
Design
(Tailwater
EL.
0.0)
Event
WSEL
Start
Finish
Time (hrs)
WSEL 0%
Time A%
2-YEAR
5.40
-
-
-
-
-
5-YEAR
5.53
12.5
12.75
0.25
-1%
-89%
1.5-YEAR
5.35
-
-
-
-
Proposed Design (Tailwater EL. 3.21)
Event
WSEL
Start
Finish
Time (hrs)
WSEL A%
Time A%
2-YEAR
5.40
-
-
-
-
-
5-YEAR
5.53
12.5
12.75
0.25
-1%
-89%
1.5-YEAR
5.35
-
-
-
*Road flooding benchmark assumed to be EL. 5.5
Discussion and Next Steps
The Town preferred the design option which included a french drain system near Kipper Court. The
underdrain from the french drain would flow into a new storm drainage pipe which would flow east before
turning south and connecting to the existing system on the south side of Danube St. This keeps the new
alignment in Town owned property, minimizing the need for easements or property acquisition.
The Town requested that WithersRavenel obtain field survey information along the project corridor, and the
Town would lead the design and construction effort.
Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
WR No. 02160539.30
August 2019
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Phase 4 Drainage Infrastructure Improvements ;% WithersRavenel
Conceptual Design Final Report Our People. Your Success.
Nags Head, North Carolina
Proiect Area #12 —Old Oregon Inlet Road @ Juncos St
Problem Statement
The area between Olympic St and Juncos St remains flooded for prolonged periods of time after rainfall
events. S Old Oregon Road is often restricted to one lane of travel due to standing water. Saturated soils
and damage to driveways was noted during the site visit.
Existing Constraints and Design Considerations
This southern portion of the Town has little relief and high groundwater elevation are evident by the water
elevation in the existing Swale west of Old Oregon Road. The volume of groundwater, ocean influence, and
the proximity of this flooding to the existing ditch results in the assumption that localized groundwater
lowering is not a feasible option.
Project Area 12 Conceptual Designs
Conceptual Design Alternative #1 - Install french drains and perforated pipe system.
Goal: Improve effective storage below the existing ground level and provide a high infiltration area to relieve
standing water concerns as quickly as possible.
Assumptions: Effective depth of the french drain is limited to 1' - 2' below the existing edge of pavement,
any depth below that elevation is expected to be continually saturated with groundwater.
Design: Add 2,100 If of shallow french drain (5' wide, 1'-2' deep) and perforated pipe along east side of Old
Oregon Inlet Rd. Repair and elevate driveways where possible, adding driveway culverts.
Preliminary Engineer's Cost Opinion: $100,000 - $125,000
Conceptual Design Alternative #2-Add pump system to Alt 1.
Goal: Pump water from the french drain to a new infiltration area to be incorporated into the design of new
beach access parking north of Juncos St.
Assumptions: Higher elevations in the parking area, especially closer to the ocean, provide separation from
the groundwater table and suitable soils for infiltration.
Design: Install Alternative 1, and include submersible pumps and a force main to the discharge location. The
system could be designed as a continuous low flow pump operation or an emergency pump connection.
Preliminary Engineer's Cost Opinion: Groundwater lowering option - $250,000 - $300,000
Emergency pump option - $200,000 - $250,000
Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
WR No. 02160539.30
August 2019
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Phase 4 Drainage Infrastructure Improvements ;A WithersRavenel
Conceptual Design Final Report Our People. Your Success.
Nags Head, North Carolina
Project Area 12 Preliminary Results
Under existing conditions, roadway flooding through the project area is expected multiple times per year.
The proposed alternatives eliminate flooding for small storm events expected to occur 3 times per year (+/-
2" of rainfall). There is insufficient storage area and volume to prevent flooding from any larger storms. Flood
pending times are expected to be improved for very small storms.
In Alternative 2, the frequency of roadway flooding is expected to be slightly better than Alternative 1,
however it is difficult to quantify this improvement as the effect of low flow pumping (2 cfs average flow
rate) is insignificant compared to the rate of inflow from rain events. Both low flow and emergency pumping
could reduce the long-term flood durations to less than 1 day after larger rainfall events.
Table 11. Projected depth of roadway flooding and time to recede for Project Area 12
Event
Existing WSEL
Alt-1 WSEL
Alt-2 WSEL
4-MONTH
1 1.53
1.35
0.89
Discussion and Next Steps
The Town directed WithersRavenel to proceed with field survey and design documents for installation of
Alternate #1 (french drain). All design drawings will need to be reviewed by NCDOT prior to bidding.
Phase 4 Drainage Infrastructure Improvements
Conceptual Design Final Report
WR No. 02160539.30
August 2019
16

Phase 4 Drainage Infrastructure Improvements %0 WithersRavenel
Conceptual Design Final Report NO our People. Your success.
Nags Head, North Carolina
Project Area #13 — Old Oregon Inlet Road — East Hard9rove St to Fire Station
Problem Statement
Flooding along the east side of Old Oregon Inlet Road for prolonged periods after small and large rainfall
events. Saturated soils and high groundwater were noted during the site visit.
Existing Constraints and Design Considerations
High groundwater elevation and sensitive natural areas near the project site limit the design options and the
ability to release concentrated stormwater or construct new stormwater outfalls west of the roadway. There
are no significant swales or ditches in the project area.
Project Area 13 Conceptual Designs
Conceptual Design Alternative #1 — Install french drains in the project area and pump groundwater to an area near the
fire station.
Goal: Improve local infiltration and dewater area using low flow groundwater pumps
Assumptions: Existing utilities would not pose significant conflicts and permits could be obtained to allow
for the discharge of the pump system.
Design: 2,050If of shallow french drain (5' wide, l'-2' deep) and perforated pipe, new catch basins and wells,
3,100 If of force main
Preliminary Engineer's Cost Opinion: $225,000 - 275,000
Conceptual Design Alternative #2 - Expand scope of Alternative 1 to include additional french drain
Preliminary Engineer's Cost Opinion: TBD based on extent of additional french drain
Project Area 13 Preliminary Results
Under existing conditions localized standing water is to be anticipated after all measurable rainfall events.
Installation of the system would improve dewatering times (often reduced to less than 1 day following
rainfall) and sufficient storage could be installed to eliminate roadway flooding for storms less than 1.5" of
total rainfall.
Table 12. Projected depth of roadway flooding and time to recede for Project Area 13
Event
Existing WSEL
Alt-1 WSEL
4-MONTH
1.55
0.67
Discussion and Next Steps
The Town directed WithersRavenel to proceed with field survey and design documents for installation of
Alternate #1. The design will also include construction of an infiltration area north of the fire station on the
west side of the road. All design drawings will need to be reviewed by NCDOT prior to bidding.
This report concludes Phase 4 of this project. Additional work described above will be conducted in
Phase 5.
Phase 4 Drainage Infrastructure Improvements WR No. 02160539.30
Conceptual Design Final Report August 2019
fVA

APPENDIX A:
EXHIBITS OF SELECTED DESIGNS FOR
PROJECT AREAS 10, 12 AND 13
"This exhibit was prepared by Wither Ravenel under direction of The Town of Nags
Head under grant award # NA15NOS4190066 to the Department of Environmental
Quality, Division of Coastal Management from the Office for Coastal Management,
National Oceanic and Atmospheric Administration. The statements, findings,
conclusions, and recommendations are those of the author(s) and do not necessarily
reflect the views of DEQ, OCM or NOAA."

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ADDENDUM
1) Stakeholder Input
2) Conceptual Design Alternatives — 3/15/2019

Appendix A — Stakeholder Input
Project Area #10
January 28, 2019 — Meeting with Town Staff, site visits, discussion of flooding problems, discussion of
flooding frequency, and discussion and observation of site constraints -
Hunter Freeman & Brandon Miller (WR) toured project area 10 with David Ryan. David described the
general problem and pointed out areas where flooding occurred. They discussed the existing
infrastructure and local constraints (property boundaries, vegetation, utilities, and other utility or
infrastructure improvement projects planned for the vicinity of the project area). They also discussed
performance goals and possible solutions/strategies worth investigating.
April 23, 2019, Board of Commissioners Budget Workshop— Presentation of Preliminary Concepts -
Hunter Freeman (WR) presented the preliminary concept for project area 10, the addition of storm
drainage alignment on Danube Street. No comments were received from the Public or Board of
Commissioners.
June 5, 2019, Town Board Meeting— Project area 10 was approved for Engineering contract to proceed.
Project Area #12
January 28, 2019 — Meeting with Town Staff, site visits, discussion of flooding problems, discussion of
flooding frequency, and discussion and observation of site constraints - Hunter Freeman & Brandon
Miller (WR) toured project area 12 with David Ryan. At the site, David described the general problem
and pointed out areas where flooding occurred. They discussed the existing infrastructure and local
constraints (property boundaries, vegetation, utilities, and other utility or infrastructure improvement
projects planned for the vicinity of the project area). They also discussed performance goals and
possible solutions/strategies worth investigating.
April 16, 2019 - Town Board Meeting & Committee Meeting - The Town and Withers Ravenel met with
NCDOT Division Staff to review the initial concept designs for Project Area 12. The preliminary design
includes a french drain on the east side of S. Old Oregon Inlet Road connected to stormwater pumping
systems. They acknowledged that there was frequent localized flooding in the area which restricted
traffic flow. Maintenance work had been done on the existing swale within the past few years, this
maintenance had helped, but not fully resolved the issues. NCDOT was unsure about the availability of
funds, but indicated that the Division would need to review and approve the construction documents
and bid documents prior to greenlighting the project for possible fund assistance.
April 23, 2019— Board of Commissioners Budget Workshop -
Hunter Freeman (WR) presented the preliminary concepts for project area 12, in addition to other
concept areas. The board had questions, but none related to project area 12.
June 5, 2019, Town Board Meeting— Project area 12 was approved for Engineering contract to proceed.

Project Area #13
January 28, 2019 — Meeting with Town Staff, site visits, discussion of flooding problems, discussion of
flooding frequency, and discussion and observation of site constraints - Hunter Freeman & Brandon
Miller (WR) toured project area 13 with David Ryan. At each site David described the general problem
and pointed out areas where flooding occurred. They discussed the existing infrastructure and local
constraints (property boundaries, vegetation, utilities, and other utility or infrastructure improvement
projects planned for the vicinity of the project area). They also discussed performance goals and
possible solutions/strategies worth investigating.
April 16, 2019 - Town Board Meeting & Committee Meeting - The Town and WithersRavenel met with
NCDOT Division Staff to review the initial concept designs for Project Area 13. The preliminary design
included french drains on the east side of S. Old Oregon Inlet Road connected to stormwater pumping
systems. Project area 13 includes an infiltration area north of the Fire Station on the west side of S. Old
Oregon Inlet Road. NCDOT had few initial concerns regarding the french drain systems. They
acknowledged that there was frequent localized flooding in the area which restricted traffic flow.
Maintenance work had been done on the existing swale within the past few years, this maintenance had
helped, but not fully resolved the issues. NCDOT was open to allowing the wet well on project area 13
within the right of way and agreed that impact to private property should be minimized. NCDOT was
unsure about the availability of funds, but indicated that the Division would need to review and approve
the construction documents and bid documents prior to greenlighting the project for possible fund
assistance.
April 23, 2019 — Board of Commissioners Budget Workshop —
Hunter Freeman (WR) presented the preliminary concept for project area 13, in addition to the other
areas. Responses to board questions were presented to the board by David Ryan at the June 5, 2019
board meeting. There were no questions related to project area 13.
June 5, 2019, Town Board Meeting— Project area 13 was approved for Engineering contract to proceed.

%0 WithersRavenel
NoOur People. Your Success.
MEMORANDUM
To: David Ryan, PE, Town of Nags Head
From: Hunter Freeman, PE, WithersRavenel
Date: March 15, 2019
Project: Nags Head Stormwater Master Planning
Subject: Phase 4 Conceptual Design Alternatives Summary - Sites 10, 12, and 13
Executive Summary and Overall Approach
This memorandum summarizes the preliminary conceptual design and results for an additional 3 drainage
projects located in the Town of Nags Head.
In this fourth phase of our contract, 7 additional sites that were previously identified by staff will be studied.
Grant funding was obtained by the Town to study sites 10, 12, and 13, and therefore these designs are
separated from sites 6, 7, 8, and 9 for reporting purposes.
The study of all sites relies on the development of hydrologic models. The models were developed to help
understand the hydrologic components existing in the present conditions, and also understand how the
physical site conditions were affected by rainfall events. Once the existing conditions models were
calibrated to more closely match field observations from Town staff and citizens, we began developing
proposed conditions models and retrofit options.
WithersRavenel modeled the three project areas using PondPack software. Both physical (i.e., upgrade
infrastructure) and mechanical (i.e., lowering groundwater through pumping) options were considered.
Metrics presented for each project area include the depth of roadway flooding and time for floodwaters to
recede. Projected performance of the proposed design alternatives was compared to existing conditions
using these metrics to gauge their effectiveness in addressing the concerns noted by the stakeholders.
For the proposed groundwater lowering scenarios, WithersRavenel looked at the net result of the pumping
to assess the effect on floodwater levels during small storm events. This approach removed the need to
design and model specific pumping options for the time being. This level of detail will be added in the final
design if the scenario reaches that stage. The hydrologic models simulate the change in soil properties due
to elevated and lowered water tables within each drainage area. Existing conditions (e.g., no groundwater
pumping operation) were considered to be saturated, resulting in the loss of infiltration capacity as a starting
boundary condition. To simulate the effect of the groundwater lowering operation, the soil type within the
project area was revised to reflect the available absorptive capacity of the dewatered soil to infiltrate a
portion of rainfall. This reflects WithersRavenel's assumption that prior to a rainfall event the soil would be
functioning sandy, high infiltration -capacity soils that exist in most areas of the Town.
This approach was intended to address the primary impacts to quality of life voiced during community
meetings. It should be noted that we are not attempting to quantify the results using traditional engineering
metrics (peak flow, inches of runoff, level of service based on XX-year storm), but rather evaluating these
scenarios within a framework that quantifies how each option will improve day-to-day quality of life. If a
more traditional engineering reporting is requested, we can add this type of analysis to the report.
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Preliminary Results & Cost Estimates
'Note: the cost estimates below do not include potential costs of easements or land acquisition.
Project Area #10 —Kipper Court
Problem Statement
Kipper Court is located at the low point of a 6.8 acre drainage area which includes many homes on W.
Baracuda Rd. The cul de sac floods the roadway, including portions of Baracuda Road.
Existing Constraints and Design Considerations
There is no storm drainage system on Baracuda or Kipper other than small shallow swales. The nearest storm
drainage system is on Danube St, and the system eventually discharges into a manmade canal adjacent to
Old Cove Road. The receiving waters are classified as SA waters.
Project Area 10 Conceptual Design
Study of the GIS survey of the storm drainage network indicates that gravity connection to the existing
network may be feasible but would require acquisition of easements on private property. Two gravity flow
options were studied. A third option of pumping water west along Baracuda Dr. to an outfall on Shiner Ct.
was not studied in detail, but could be investigated if easements on private property cannot be obtained.
Conceptual Design Alternative #1 — Install a swale and storm drainage system to conned to the
existing system on Danube St.
Goal: Provide an outfall from Kipper Court to allow the area to drain more easily.
Assumptions: Two tailwater conditions were assumed based on field observations of the existing outfall.
One option, with a tailwater of 0.0 represents normal dry weather low tide conditions, the other, with a
tailwater of 3.21 represents observed storm conditions where the outfall pipe is y: submerged.
Design: Vegetated swale across 207 W. Kipper Ct and 0 W. Danube St to a cross pipe under Danube St,
tying into the existing system at a catch basin on the south side of Danube St.
Preliminary Engineer's Cost Opinion: $30,000 - $40,000
Conceptual Design Alternative #2 — Install storm drainage system to connect to the existing system
on Danube St.
Goal: Provide an outfall from Kipper Court to allow the area to drain more easily.
Assumptions: Two tailwater conditions were assumed based on field observations of the existing outfall.
One option, with a tailwater of 0.0 represents normal dry weather low tide conditions, the other, with a
tailwater of 3.21 represents observed storm conditions where the outfall pipe is'% submerged.
Design: Catch Basin on 207 W. Kipper Ct and installation of a new storm drainage system and cross pipe
under Danube St, tying into the existing system at a catch basin on the south side of Danube St.
Preliminary Engineer's Cost Opinion: $45,000 - $60,000 (including french drain)
Project Area 10 Preliminary Results
We estimate that Kipper Ct floods approximately 3 times every five year, but localized flooding on private
property is more frequent (multiple times per year). The swale option allows for additional flood storage on
207 W Kipper Ct, reducing the frequency of road flooding to once per five years, and also reducing property
flooding to once per year, with the exception of the swale itself, which would be saturated frequently and
may be suitable for wetland planting. The duration of flooding is reduced from 50% to 67% depending on
downstream tailwater conditions and the time it takes for the canal to recede to normal low water levels.
The pipe option does not improve the flooding conditions during larger storms event, and Kipper Ct would
still be expected to flood 3 times per five years, however flooding duration is reduced by 50% (depending
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on the water level in the canal) and property flooding from small events would be expected to be less
frequent if some portions of private property were also graded to drain to the swale. A french drain and
perforated pipe design alteration would further reduce localized flooding on private property and should be
considered.
Table 10. Projected depth of roadway flooding and time to recede for Project Area 30
Existing
Staging
Event
WSEL
Start
Finish
Time (hrs)
2-YEAR
5.52
12.50
13.50
1.00
5-YEAR
5.57
12.25
14.50
2.25
1.5-YEAR
5.50
12.75
13.25
0.50
Proposed
Alternative
#1
(Tailwater EL.
0.0)
Event
WSEL
Start
Finish
Time (hrs)
WSEL A%
Time 0%
2-YEAR
5.49
-
-
-
-1%
5-YEAR
5.58
22.00
22.75
0.75
0%
-67%
1.5-YEAR
5.42
-
-
-
-1%
-
Proposed Alternative #1 (Tailwater EL. 3.21)
Event
WSEL
Start
Finish
Time (hrs)
WSEL A%
Time 0%
2-YEAR
5.49
-
-
-
-1%
-
5-YEAR
5.58
12.00
13.00
1.00
0%
-56%
1.5-YEAR
5.42
-
-
-
-1%
-
Proposed Alternative #2 (Tailwater EL. 0.0)
Event
WSEL
Start
Finish
Time (hrs)
WSEL A%
Time A%
2-YEAR
5.53
12.25
12.75
0.50
0%
-so%
5-YEAR
5.57
12.25
12.75
0.50
0%
-78%
1.5-YEAR
5.50
12.50
12.75
0.25
0%
-50%
Proposed Alternative #2 (Tailwater EL. 3.21)
Event
WSEL
Start
Finish
Time (hrs)
WSEL A%
Time A%
2-YEAR
5.53
12.50
12.75
0.25
0%
-75%
5-YEAR
5.57
12.25
12.75
0.50
0%
-78%
1.5-YEAR
5.50
12.50
12.75
0.25
0%
-50%
Project Area #12 - Old Oregon Inlet Road @ Juncos St
Problem Statement
The area between Olympic St and Juncos St remains flooded for prolonged periods of time after rainfall
events. S Old Oregon Road is often restricted to one lane of travel due to standing water. Saturated soils
and damage to driveways was noted during the site visit.
Existing Constraints and Design Considerations
This southern portion of the Town has little relief and high groundwater elevation are evident by the water
elevation in the existing swale west of Old Oregon Road. The volume of groundwater, ocean influence, and
the proximity of this flooding to the existing ditch results in the assumption that localized groundwater
lowering is not a feasible option.
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Project Area 12 Conceptual Designs
Conceptual Design Alternative #1— Install french drains and perforated pipe system.
Goal: Improve effective storage below the existing ground level and provide a high infiltration area to relieve
standing water concerns as quickly as possible.
Assumptions: Effective depth of the french drain is limited to 1' - 2' below the existing edge of pavement,
any depth below that elevation is expected to be continually saturated with groundwater.
Design: Add 2,100 If of shallow french drain (5' wide, 1'-2' deep) and perforated pipe along east side of Old
Oregon Inlet Rd. Repair and elevate driveways where possible, adding driveway culverts.
Preliminary Engineer's Cost Opinion: $100,000 - 125,000
Conceptual Design Alternative #2 —Add pump system to Alt 1.
Goal: Pump water from the french drain to a new infiltration area to be incorporated into the design of new
beach access parking north of Juncos St.
Assumptions: Higher elevations in the parking area, especially closer to the ocean, provide separation from
the groundwater table and suitable soils for infiltration.
Design: Install Alternative 1, and include submersible pumps and a force main to the discharge location. The
system could be designed as a continuous low flow pump operation or an emergency pump connection.
Preliminary Engineer's Cost Opinion: Groundwater lowering option - $250,000 - 300,000
Emergency pump option - $200,000 - $250,000
Project Area 12 Preliminary Results
Under existing conditions, roadway flooding through the project area is expected multiple times per year.
The proposed alternatives eliminate flooding for small storm events expected to occur 3 times per year (+/-
2" of rainfall). There is insufficient storage area and volume to prevent flooding from any larger storms. Flood
ponding times are expected to be improved for very small storms.
In Alternative 2, the frequency of roadway flooding is expected to be slightly better than Alternative 1,
however it is difficult to quantify this improvement as the effect of low flow pumping (2 cfs average flow
rate) is insignificant compared to the rate of inflow from rain events. Both low flow and emergency pumping
could reduce the long-term flood durations to less than 1 day after larger rainfall events.
Table 11. Projected depth of roadway flooding and time to recede for Project Area 12
Event
Existing WSEL
Alt-1 WSEL
Alt-2 WSEL
4-MONTH
1 1.53
1.35
0.89
Project Area #13 — Old Oregon Inlet Road — East Hardgrove St to Fire Station
Problem Statement
Flooding along the east side of Old Oregon Inlet Road for prolonged periods after small and large rainfall
events. Saturated soils and high groundwater was noted during the site visit.
Existing Constraints and Design Considerations
High groundwater elevation and sensitive natural areas near the project site limit the design options and the
ability to release concentrated stormwater or construct new stormwater outfalls west of the roadway. There
are no significant swales or ditches in the project area.
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WithersRavenel
Project Area 13 Conceptual Designs
Conceptual Design Alternative #1— Install french drains in the project area and pump groundwater
to an area near the fire station.
Goal: Improve local infiltration and dewater area using low flow groundwater pumps
Assumptions: Existing utilities would not pose significant conflicts and permits could be obtained to allow
for the discharge of the pump system.
Design: 2,050If of shallow french drain (5' wide, l'-2' deep) and perforated pipe, new catch basins and wells,
3,100 If of forcemain
Preliminary Engineer's Cost Opinion: $225,000 - 275,000
Conceptual Design Alternative #2 — Expand scope of Alternative 1 to include additional french drain
Preliminary Engineer's Cost Opinion: TBD based on extent of additional french drain
Project Area 13 Preliminary Results
Under existing conditions localized standing water is to be anticipated after all measurable rainfall events.
Installation of the system would improve dewatering times (often reduced to less than 1 day following
rainfall) and sufficient storage could be installed to eliminate roadway flooding for storms less than 1.5" of
total rainfall.
Table 12. Projected depth of roadway flooding and time to recede for Project Area 13
Event
Existing WSEL
Alt-1 WSEL
4-MONTH
1.55
0.67
Upcoming Work
WithersRavenel intends to compile conceptual design alternatives into 1-page exhibits for each project
area to graphically represent design goals and results.
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