Swanquarter Town Ditch Canal Study- 2021

"This report was prepared by the County of Hyde under grant award# NA17NOS4190066 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."
Greenman-Pedersen, Inc. 1308 HWY 258 N Kinston, NC 28504 p 910.663.4123
An Equal Opportunity Employer
December 30, 2021
Hyde County
Mr.Daniel Brinn
Flood Control Manager
30 Oyster Creek Road
Swan Quarter, NC 27885
Subject:Feasibility Study for Pump and Drainage Improvements for Swanquarter Town Ditch Canal
Dear Mr. Brinn,
Greenman-Pedersen,Inc.(GPI)is pleased to present a summary of findings for the Town Ditch Canal
Feasibility Study. GPI completed survey, H&H modeling, active water management design, and provided
these findings within the following report. GPI has deemed the drainage improvements feasible and
recommends the County proceed forward with additional stages of engineering to develop contract and
permit drawings to then proceed to the construction phase.
The staff at GPI and our project partners want to thank the County for allowing us to participate with this
project and look forward to future phases. Our project partners are from the County, while our project
manager has spent a significant portion of his life in the County and is passionate about improving the
drainage conditions and resiliency of its stakeholders. After your review if you have any questions please
feel free to contact our project manager, Mr. Jonathan Hinkle, PE.
Sincerely,
Jonathan D. Hinkle, PE
Lead Environmental Engineer / North Carolina Engineering Manager
Assistant Vice President
[e-mail]
910.663.4123
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BACKGROUND
The Town of Swanquarter has experienced significant
flooding from recent tropical events and other precipitation
events that has caused damage to the Town’s population
along Town Ditch Canal. Hyde County was awarded a grant
to study the feasibility of drainage improvements and active
water management for the Canal. GPI (formerly LDSI) was
selected as the engineering firm for this project. GPI
performed:
limited topographic survey on the existing drainage
infrastructure
first order H&H modeling
pump configuration analysis
assessment of future conditions
first order drainage network improvement modeling
Assumptions
The following assumptions were utilized in the development of this feasibility study:
The County will be responsible for all grant/contract administration and processing of documents with FEMA and
other applicable grant agencies.
The project includes the feasibility assessment of an active water management (pumped) drainage system and
recommendations for drainage improvement to the Town Ditch Canal; neither the complete design of the active
water management nor the design of recommended drainage improvements are part of this contract.
First order H&H modeling,
o Steady state
o Hydraulic Grade-line, modeling only
o Culvert analysis for the culverts along Town Ditch Canal
o No channel/attenuation modeling will be conducted
No major changes in land-use for future conditions analysis
Opinion of Cost will be based on best professional judgement, the current economic climate is extremely volatile,
therefore costs are difficult to accurately project
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SURVEY EFFORTS
GPI completed the surveying phase of the project by locating pipes and drainage infrastructure along Main Street. The
information gathered includes coordinates, elevations, ground cover, photos, and dimensions of structures, as well as
coordinates, dimensions, and material. Cross sections for the canal near the gazabo were collected and used to determine
adequacy of the pump operation and its feasibility.
HYDROLOGY
Historic rainfall data for this project was determined via
NOAA’s Atlas 14 tool. Watersheds were individually delineated
for each catchment leading into the stormwater system so
that a comprehensive analysis could be conducted of each
section of the network. As a part of the analysis, rainfall area,
percent impervious surface, time of concentration, and type
of surface flow experienced during a storm event were
determined for model inputs. These values were used when
calibrating the stormwater model to determine at which points
during a storm event any given section of the pipe network
would be under maximum load. Determining the maximum
load that the network might experience during a storm allows
deficiencies in the network to be predicted and analyzed. This
also allows for any tailwater effects that might be caused
upstream of that section to be determined.
When analyzing the hydrologic conditions of the Town Ditch Canal Watershed, it was necessary to account for the organic
and sandy, rapidly draining soils that are present near Swanquarter. These soils allow for very high stormwater infiltration
rates which prevent a significant portion of the stormwater from ever entering the drainage infrastructure. Organic and
sandy soils act as a sponge when the organic material is dry, the soil absorbs a majority of the precipitation events. On
the other hand, when they are saturated, they will contribute to runoff as they have reached the maximum absorption.
While runoff still occurs during large storm events, it is important to realize that surface flow does not mean that infiltration
is not occurring. This means that any stormwater that passes over local soils prior to entering the drainage network will
have a portion infiltrated into the soils, thus reducing the amount of water that must be routed through the drainage
network. The runoff from the catchments during a storm event was simulated for each individual catchment using the
rational method. This would simulate the peak flow experienced by the drainage network for the design events.
Additionally, we have modeled the watersheds through the
Cypress Creek Drainage equations which are discussed in the
NRCS Hydrology Handbook. “
Work that
we have previously done in Hyde County and with NRCS has
shown that these curves are good estimates for drainage
removal rates.
The following table is a summary of flow rates from both
Rational Method and Cypress Creek Equations.
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Model and Recurrence Interval Flow Rates
Rational 005-year 13.3
Rational 010-year 16.6
Rational 025-year 20.1
Rational 050-year 24
Rational 100-year 27.7
Cypress Creek 11.6
HYDRAULICS
The rainfall runoff data generated by the simulated storms and watershed parameters was input to the model so the
existing network’s hydraulic properties could be analyzed. These properties included Hydraulic Grade Line (HGL), Water
Surface Elevation (WSE), and maximum flow. It was found that there are multiple choke points or restrictions within the
existing drainage network. This means that there are multiple locations where downstream pipes are smaller than their
upstream pipes, tailwater limited pipes, undersized pipes, or other network restrictions. Transitioning from a pipe of a
larger diameter to one of a smaller diameter severely restricts the quantity of water that can pass through and results in
increased tailwater upstream of that pipe. Additionally, there are junction choke points where multiple pipes all lead into
a single pipe that is not capable of adequately handling the volume of water that is provided. These have the same effect
of reducing the quantity of water that can pass through and increasing the upstream tailwater. Several sedimentation
restrictions have also been identified within the existing network including multiple clogs and blockages. There were
multiple severe blockages due to sedimentation, including several surface exposed pipes that became partially or
completely filled with soil. Sedimentation reduces the flow capacity within the pipe network by limiting the available area
for water to flow through. Additionally, sedimentation will also reduce storage capacity and stormwater attenuation
capability of the ponds within the drainage network.
FINDINGS
After analyzing the H&H model, GPI found that multiple pipes operate under surcharged conditions during storm events.
The more intense, higher recurrence interval storms caused more pipes to operate under surcharged conditions. A
surcharged pipe condition is a sign that the pipes in place are likely undersized or are inhibited in some way. This could
include pipes being installed at a reverse grade, choke/throttle points within the pipe network, or an excess of pipes being
drained through a single outlet. The following table summarizes the pipe network as modeled in the existing conditions.
Existing Conditions
For existing conditions, we performed a steady state hydraulic grade line analysis, neglecting the storage of canals or
spread. This is a conservative and first order method for determining drainage network capacity. Given the feasibility
study level and limited survey information this allowed for determination of potential drainage network restrictions.
Existing Conditions Summary
Pipe ID Flow HW TW Notes
Pipe -01 9.5 5.56 5.56
Pipe -02 9.5 5.86 5.56 BRANCH AT START OF SYSTEM
Pipe -03 9.5 5.56 5.53 overtopping driveway/freeboard elev.
Pipe -04 9.5 5.53 5.50 overtopping driveway/freeboard elev.
Pipe -05 9.5 5.5 5.47 overtopping driveway/freeboard elev.
Pipe -06 9.5 5.47 5.47 overtopping driveway/freeboard elev.
Pipe -07 9.5 5.47 5.32 overtopping driveway/freeboard elev.
Pipe -08 9.5 5.32 5.31 overtopping driveway/freeboard elev.
Pipe -09 9.5 5.31 5.27 overtopping driveway/freeboard elev.
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Pipe -10 9.5 5.27 5.23 overtopping driveway/freeboard elev.
Pipe -11 15.4 5.23 5.04 overtopping driveway/freeboard elev.
Pipe -12 15.4 5.04 4.93 overtopping driveway/freeboard elev.
Pipe -13 15.4 4.93 4.84 overtopping driveway/freeboard elev.
Pipe -14 15.4 4.84 4.65 overtopping driveway/freeboard elev.
Pipe -15 15.4 4.65 4.46 overtopping driveway/freeboard elev.
Pipe -16 15.4 4.46 4.27 overtopping driveway/freeboard elev.
Pipe -17 20 4.27 3.98 overtopping driveway/freeboard elev.
Pipe -18 20 3.98 3.84 overtopping driveway/freeboard elev.
Pipe -19 20 3.84 3.71 overtopping driveway/freeboard elev.
Pipe -20 20 3.71 3.60 overtopping driveway/freeboard elev.
Pipe -21 20 3.60 3.47 overtopping driveway/freeboard elev.
Pipe -22 20 3.47 3.33
Pipe -23 20 3.33 3.11 overtopping driveway/freeboard elev.
Pipe -24 20 3.11 3.04 overtopping driveway/freeboard elev.
Pipe -25 20 3.04 2.96 overtopping driveway/freeboard elev.
Pipe -26 20 2.96 2.89
Pipe -27 20 2.89 2.78 overtopping driveway/freeboard elev.
Pipe -28 24 2.78 2.53 overtopping driveway/freeboard elev.
Pipe -29 24 2.53 2.31 overtopping driveway/freeboard elev.
Pipe -30 24 2.31 1.86 overtopping driveway/freeboard elev.
Pipe -31 24 1.86 1.47 overtopping driveway/freeboard elev.
Pipe -32 24 1.47 0.49
Pipe -33 24 0.49 0.02
Pipe -34 24 0.02 0.00
Proposed Improvements
Keeping the same assumptions for development of drainage network improvement, the proposed improvements of the
drainage network are summarized in the table below. The HGL/steady state analysis shows a significant improvement of
the computed water-surface-elevations.
Proposed Improvement Summary
Pipe ID Flow HW TW Notes
Pipe -01 9.5 2.41 2.41
Pipe -02 9.5 5.86 2.41
Pipe -03 9.5 2.41 2.38
Pipe -04 9.5 2.38 2.35
Pipe -05 9.5 2.35 2.32
Pipe -06 9.5 2.32 2.32
Pipe -07 9.5 2.32 2.28 Replace with (2) 36
Pipe -08 9.5 2.28 2.27
Pipe -09 9.5 2.27 2.26 Replace with (2) 36
Pipe -10 9.5 2.26 2.25 Replace with (2) 36
Pipe -11 15.4 2.25 2.20 Replace with (2) 36
Pipe -12 15.4 2.20 2.15 Replace with (2) 36
Pipe -13 15.4 2.15 2.11 Replace with (2) 36
Pipe -14 15.4 2.11 2.06 Replace with (2) 36
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Pipe -15 15.4 2.06 2.01 Replace with (2) 36
Pipe -16 15.4 2.01 1.96 Replace with (2) 36
Pipe -17 20 1.96 1.89 Replace with (2) 36
Pipe -18 20 1.89 1.85 Replace with (2) 42
Pipe -19 20 1.85 1.82 Replace with (2) 42
Pipe -20 20 1.82 1.79 Replace with (2) 42
Pipe -21 20 1.79 1.76 Replace with (2) 42
Pipe -22 20 1.76 1.72 Replace with (2) 42
Pipe -23 20 1.72 1.70 Replace with (2) 42
Pipe -24 20 1.70 1.68 Replace with (2) 48
Pipe -25 20 1.68 1.66 Replace with (2) 48
Pipe -26 20 1.66 1.64 Replace with (2) 48
Pipe -27 20 1.64 1.61 Replace with (2) 48
Pipe -28 24 1.61 1.59 Replace with (2) 48
Pipe -29 24 1.59 1.53 Replace with (2) 48
Pipe -30 24 1.53 1.50 Replace with (2) 48
Pipe -31 24 1.50 1.47 Replace with (2) 48
Pipe -32 24 1.47 0.49
Pipe -33 24 0.49 0.02
Pipe -34 24 0.02 0.00
Culvert summary sheets are attached to this report in Appendix A. Again, it should be noted that this is a first order
model and has significant assumptions in order to estimate the need of drainage improvements and suitability of the
installation of a pump near Landing Road.
Pump Recommendations
Based upon the H&H investigation and modeling, it appears we have three probable scenarios we should plan for in the
selection and design of the proposed pump.
1. The conditions that exist currently with the identified choke points and canal sediment presence which gives us
the lowest expected flow.
2. An improved condition scenario that assumes undersized culverts are replaced with recommended sizes, and
flow restricting sediment is removed to connect a series of adequately sized culverts to the pump basin.
3. Lastly, under either of the previous noted flows, tropical events or events which yield, “out of bank flooding, (i.e.
flooding in the streets event),” requiring the need for higher pumping rates in order to manage the out of bank
flooding.
With these three different flowrates required, the pump needs to have a variable flowrate, after review a 24” axial flow
pump with an electric drive motor, would be the recommended pump. This pump design would allow a flow rate of
approximately 10,000 gpm to 17,000 gpm depending on operational speed. Additionally, this pump could be controlled
using a Variable Frequency Drive (VFD) controller allowing the reduction of the drive motor speed in times of average to
below average flows, while providing the ability to increase the drive motor speeds in a heavy flow or flood event to gain
the maximum pump output. This operational flexibility is like powering the pump with a diesel engine where throttle
control would achieve the same results but without the noise and environmental concerns associated with diesel powered
equipment. For times of power outage, a switchgear would be provided for functional use as a backup.
Due to the close proximity of salt water and the salt air environment, we recommend the pump be outfitted with a full
anti-corrosion package including stainless steel impeller, wear band, and bolt fasteners in all flange connections. The
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pump, discharge pipe, and flapgate should be coated with 2-part coal tar epoxy coating and have zinc anodes attached
in below water line locations. Additionally, all electrical equipment beyond the meter base should have stainless outdoor
enclosures to ensure long life in the elements.
OPINION OF COSTS
The following table is a summary of potential costs to implement all the recommendations. Given high market fluctuations
following the COVID pandemic and this is currently at a planning level study, significant changes to costs can happen as
the design moves forward or with product cost/availability.
Opinion of Probable Costs
Item Estimate of Probable Cost
840 Linear feet of AASHTO M 294 Type ‘S’ 36 DIA Pipe $114,000
460 Linear feet of AASHTO M 294 Type ‘S’ 42 DIA Pipe $ 83,000
660 Linear feet of AASHTO M 294 Type ‘S’ 48 DIA Pipe $138,000
Pump $ 140,000
Design Fee for Construction Implementation & Permitting $ 95,000
Contingency (35% Given COVID supply-chain disruptions)$ 165,000
Total Planning Level Opinion of Costs $735,000
RECOMMENDATIONS
The feasibility analysis of the Town Ditch Canal shows significant improvement with major modifications to the existing
infrastructure. Again, it should be noted that this is a planning level study and additional engineering needs to be
performed to analyze key portions of the drainage network and coordination with project stake holders on implementation.
Our team recommends proceeding with an additional engineering project to develop construction documents for
implementing the entire proposed conditions as we have modeled. A refined engineering study will be needed to
determine if cost-savings can be gleaned from using existing pipes with a higher order drainage network analysis, as well
as if project stakeholders want to include/exclude portions of the proposed recommendations.
FUNDING OPPORTUNITIES
There are several potential funding options for implementing the Town’s stormwater outfall infiltration projects. Some of
these are included in the table below.
Name Funding Cycle Application Deadline(s)
FEMA BRIC 1 – per year Late Fall (November LOIs)
Water Resource Development Grant 2 –per year Late June, Late December
Clean Water State Revolving Fund (CWSRF) Loan NA NA
Stormwater Utility Fee NA NA
The following sources were utilized for this list of funding opportunities: The Environmental Finance Center at the
University of North Carolina, Chapel Hill Methods and Strategies for Financing Green Infrastructure, and Individual web
sites from funding sources.
FEMA – Building Resilient Communities and Infrastructure (BRIC)
Overview: Building Resilient Infrastructure and Communities (BRIC) will support states, local communities, tribes and
territories as they undertake hazard mitigation projects, reducing the risks they face from disasters and natural hazards.
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The BRIC program guiding principles are supporting communities through capability- and capacity-building; encouraging
and enabling innovation; promoting partnerships; enabling large projects; maintaining flexibility; and providing
consistency. (FEMA website)
Information: https://www.fema.gov/grants/mitigation/building-resilient-infrastructure-communities
Water Resource Development Grant
Overview: This grant program provides cost-share grants and technical assistance to local governments. Applications
for grants are accepted for seven eligible project types: general navigation, recreational navigation, water management,
stream restoration, water-based recreation, Natural Resources Conservation Service Environmental Quality Incentives
Program (EQIP) stream restoration projects and feasibility/engineering studies. The non-navigation projects are
collectively referred to as state and local projects.
Award Decision: Range $10,000 ~ $200,000
Cycles: There are two grant application cycles per fiscal year for state and local projects. The current spring 2019 grant
cycle began Jan.1 and applications are due by June 30. The second cycle is from July 1 – December 31.
Contact: Amin Davis [e-mail]
Information:
https://files.nc.gov/ncdeq/Water%20Resources/documents/WRDG%20WSN%20New%20Bern%20102317_A%20Davis.pd
f
Clean Water State Revolving Fund (CWSRF) Loan
The North Carolina State Water Infrastructure Authority (SWIA) overseas a number of water and wastewater loan and
grant programs including the joint state/federal (EPA) funded Clean Water State Revolving Fund (CWSRF). According to
the UNC Environmental Finance Center report entitled Methods and Strategies for Financing Green Infrastructure, local
governments can obtain loans at rates as low as 0% for 20 years to fund eligible projects including stormwater projects.
Stormwater Utility Fee
Under North Carolina law, stormwater fees can be used to cover a wide range of stormwater quality and quantity
programs.
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Existing Conditions
Model
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Proposed Conditions
Model
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Study Area Map
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Pump Schematic