Stormwater
Management
Program
City of Greenville
Greenville,
North Carolina
Draft
August 6,
2004
City of Greenville
1500 Beatty
Street
Greenville,
NC 27834
Phone: 252-329-4467
Table
of Contents
EXECUTIVE SUMMARY4
1.Introduction
1-A.Background
on the Tar-Pamlico Stormwater Rule5
1-B.Requirements
of the Tar-Pamlico Stormwater Rule6
1-C.Applicability
of the Tar-Pamlico Stormwater Rule7
2.New Development
2-A.Requirements
in the Rule9
2-B.Protecting Riparian
Areas on New Development10
2-C.Calculating Nutrient
Export from New Development10
2-D.BMPs for Reducing
Nitrogen and Phosphorus12
2-E.Calculating Peak
Runoff Volume15
2-F.Offsite Partial
Offset Option17
2-G.Regional or Jurisdiction-Wide
Approaches18
2-H.BMP Maintenance19
2-I.Land Use Planning
Provisions19
3.Illicit Discharges
3-A.Requirements
in the Rule21
3-B.What is an Illicit
Discharge?21
3-C.Establishing
Legal Authority22
3-D.Collecting Jurisdiction-Wide
Information22
3-E.Mapping and Field
Screening in High Priority Areas23
3-F.Identifying and
Removing Illicit Discharges25
3-G.Preventing Discharges
and Establishing a Hotline26
3-H.Implementation Schedule27
4.Retrofit Locations
4-A.Requirements
in the Rule 28
4-B.Approach
for Meeting Requirements28
4-C.Data
Collection and Notification 28
4-D.Mapping
Requirements29
5.Public Education
5-A.Requirements
in the Rule31
5-B.Public Education
Action Plan31
6.Reporting Requirements
6-A.New Development
Review/Approval32
6-B.Illicit Discharges32
6-C.Retrofit Locations33
6-D.Public Education33
Appendices
Appendix A15A NCAC 2B .0258
Tar-Pamlico River Basin - Nutrient Sensitive Waters Management
Strategy: Basinwide Stormwater Requirements
Appendix
BExport Calculation Worksheets
Appendix CLand Use Planning
and Design Techniques
Appendix DBMP Operation
and Maintenance Agreement
Appendix EChapter 9 “Stormwater
Management And Control” of Title 9 the City of Greenville Code of Ordinances
Appendix FScreening Report
Forms
Appendix
GPublic Education Action Plan and Report
Appendix HTimeline for City
of Greenville NPDES Phase II Comprehensive Stormwater Plan (Year 1)
EXECUTIVE
SUMMARY
In accordance
with the State’s rule, “15A NCAC 2B .0258 Tar-Pamlico River Basin
–Nutrient Sensitive Water Management Strategy: Basinwide Stormwater
Requirement,” the City of Greenville developed a Stormwater Management
Program. The purpose of this Program is to help improve water
quality in the Tar-Pamlico River Basin. The City’s jurisdictional
boundaries also extend into the Neuse River Basin. Figure 1 presented
in the City’s “Stormwater Management Program” is a general representation
of which areas of the City are located within the Tar-Pamlico River
Basin and which are within the Neuse River Basin. It also identifies
the city limits and the City's extra territorial jurisdiction (ETJ).
As part of
its program, the City amended Chapter 9 “Storm Drainage” of Title
9 “Building, Planning, and Development Regulations” located in the
“Code of Ordinances” for the City of Greenville. This chapter
is now referred to as “Stormwater Management And Control”.
This ordinance is presented in the program’s appendices.
Those requirements
as set forth by the Tar-Pamlico Rule will be applicable to that
portion of the City of Greenville located within the Tar-Pamlico River
Basin. For areas of the City’s ETJ located within the Tar-Pamlico
River Basin, the City will apply the requirements of its program to
the extent authorized under State statutes that govern municipality
operations in its ETJ. Under State statutes, the City is authorized
to perform inspections and report violations within its ETJ to the appropriate
County and State authorities. As areas within it's ETJ are annexed
into the city limits, the City will be able to enforce these requirements.
Those areas of the city and its ETJ located within the Neuse River Basin
will be subject to the requirements of this ordinance and program, with
the exception of having to meet the requirements for controlling phosphorus
releases.
The City’s
Stormwater Management Program is to be administered by the Engineering
Division of the Public Works Department and will be managed by the City
Engineer. The City’s Stormwater Utility will fund this program.
The Engineering Division is responsible for implementation of the program’s
various elements. The City of Greenville was identified as a NPDES
Phase II community. Many of the programs that the City is developing
to meet the Phase II requirements will be applied towards those of the
Tar-Pamlico Rule.
As part of
this program, the City will administer a public education program, addressing
residents within and outside Greenville’s city limits. The major
components of the City’s stormwater infrastructure located within
the city limits will be mapped as part of the City's NPDES Phase II
program, which is to begin in the second year of the City's NPDES Phase
II permit. As areas located within the ETJ are annexed into the
city limits, the stormwater infrastructure located within these areas
to be maintained by this City will be mapped following annexation.
The City has started evaluating retrofit opportunities. As presented
in the Stormwater Management Program, the City of Greenville will also
begin the development of its program to address illicit discharges.
The implementation schedule for this component is presented within this
program.
In Summary,
the overall program objective is to improve the water quality of stormwater
run-off that enters the natural waters located in and outside of the
City of Greenville.
1. Introduction
In accordance
with the State’s rule, “15A NCAC 2B .0258 Tar-Pamlico River Basin
–Nutrient Sensitive Water Management Strategy: Basinwide Stormwater
Requirement,” the City of Greenville has developed a stormwater management
program.
1-A.
Background on the Tar-Pamlico Stormwater Rule
The Tar-Pamlico River
Basin begins in Piedmont North Carolina and extends approximately 180
miles through the Coastal Plain to Pamlico Sound. Together, Pamlico
Sound and neighboring Albemarle Sound constitute one of the most productive
estuarine systems in the country. The 5,400 square mile Tar-Pamlico
basin is comprised primarily of agricultural and forest land and many
smaller municipalities. Despite the rural character of the basin,
in the mid-1970’s the Pamlico River estuary began to see increasing
frequencies of harmful algal blooms, fish kills, and other nutrient-related
problems.
By the mid-1980’s,
the state began to consider actions to control nutrient inputs to the
estuary. Those actions have included the following:
Phase I:
In 1989, the North Carolina Environmental Management Commission (EMC)
designated the entire basin “Nutrient Sensitive Waters.” The
first phase of management through 1994 focused primarily on point sources,
establishing an annually decreasing nutrient loading cap for an association
of dischargers, and an innovative “trading” program that allowed
dischargers to achieve reductions in nutrient loading more cost-effectively.
PCS Recycling:
In 1992, a phosphate mining company then known as Texas Gulf, which
is located on the Pamlico River estuary, instituted a wastewater recycling
system that reduced its phosphorus discharges to the estuary by 93 percent.
Phase II:
Modeling of estuary conditions showed that despite the gains made to
that point, significant reductions in nitrogen and phosphorus loading
were still needed to restore water quality standards and minimize the
recurrence of harmful algal blooms. The second phase of the nutrient
strategy, which runs through 2004, established a biologically based
goal of 30 percent reduction in nitrogen loading from 1991 levels and
holding phosphorus loading at 1991 levels. Load reductions were
apportioned among point sources and the major nonpoint sources.
The point sources were given steady annual nitrogen and phosphorus loading
caps. A program was designed with the nonpoint sources to achieve
the goals through voluntary measures. After two years of voluntary
implementation, the EMC found insufficient progress and called for rules
for nonpoint sources.
Rules:
Beginning in 1998, Division of Water Quality (DWQ) staff conducted a
lengthy public input process to evaluate source categories and develop
rules where needed. Over the course of 2000, the EMC adopted rules
for agriculture, fertilizer application across all land uses, urban
stormwater, and rules to protect the nutrient removal functions of existing
riparian buffers. These rules were modeled after a similar set
of rules recently adopted in the adjacent Neuse River Basin. The
Neuse rules were given extensive public review and modification, and
the Tar-Pamlico rules similarly received extensive scrutiny. The
resulting rules provide increased flexibility for the regulated community
while maintaining the focus of the nutrient reduction goals.
1-B.
Requirements of the Tar-Pamlico Stormwater Rule
The Tar-Pamlico
Stormwater Rule identifies the City of Greenville as one of the local
governments with the greatest likelihood of contributing significant
nutrient loads to the Pamlico estuary. The EMC may designate additional
local governments in the future through rule amendment based on criteria
given in the rule.
The affected
local governments are:
Municipalities
Greenville
Henderson
Oxford
Rocky Mount
Tarboro
Washington
Counties
Beaufort
Edgecombe
Franklin
Nash
Pitt
For these local governments,
only their geographic areas that fall within the Tar-Pamlico River Basin
are subject to the rule. Part of The City of Greenville’s jurisdiction
is located with this Basin. Figure 1 in Section 1-C identifies
which portion of the City and its extraterritorial jurisdiction is located
within the Tar-Pamlico River Basin. in subject counties, applicable
areas are those under the direct jurisdiction of the counties, which
would not include incorporated cities, towns, or villages within county
jurisdictional limits. Cities and counties are encouraged to coordinate
to establish implementation responsibilities within municipal extraterritorial
jurisdictions. Counties administering development regulations
by interlocal agreement on behalf of municipalities would implement
the rule within only those municipalities that are subject to the rule.
The activities of state entities within subject local governments would
be subject to the rule.
The rule establishes
a broad set of objectives for limiting nutrient runoff from urban areas
and then lays out a set of specific elements that the City of Greenville
has included in its program. Timeframes for implementation of the rule
are as follows:
April 1, 2001:Effective
date of the rule.
February
13, 2003:Target date for approval of the Model Stormwater Program by
the Environmental Management Commission (modified through EMC approval
from the date of April 1, 2002, established in the rule).
February
13, 2004:Deadline for submittal of local Stormwater Programs (including
ordinances) to the EMC (modified as above).
August
13, 2004:Deadline for local governments to begin implementing local
Stormwater Programs (modified as above).
Following implementation
in August 2004, the City of Greenville is required to make annual progress
reports to the EMC that will include nitrogen and phosphorus loading
reduction estimates.
The elements
that must be included in the City of Greenville’s management program
are:
1.
New Development Review/Approval
New development
is required to meet the 30 percent reduction goal through site planning
and best management practices. The rule imposes a 4.0 pounds per
acre per year (lb/ac/yr) nitrogen loading limit and a 0.4 lb/ac/yr phosphorus
loading limit on new development. Proposals that exceed these performance
standards may partially offset their load increases by treating existing
developed areas offsite that drain to the same stream.
New development
shall avoid causing erosion of surface water conveyances. At minimum,
post-development peak flows leaving the site may not exceed pre-development
for the 1-year, 24-hour storm event. The rule provides the City
of Greenville with the option of using regional stormwater facilities
to help meet nutrient loading and attenuation requirements under certain
circumstances.
2.
Illicit Discharges
Illicit discharges
are substances deposited in storm sewers (that lead to streams) that
should instead be handled as wastewater discharges. Illicit discharges
may contain nitrogen. The City of Greenville will develop and
implement a program to identify, remove, and prevent illicit discharges.
3.
Retrofit Locations
There are a
number of funding sources available for water quality retrofit projects
such as the Clean Water Management Trust Fund and the Wetland Restoration
Program that the NC General Assembly has recently established.
To assist technical experts, the City of Greenville is required to identify
sites and opportunities for retrofitting existing development to reduce
total nitrogen and phosphorus loads.
4.
Public Education
Citizens can
reduce the nitrogen pollution coming from their lawns and septic systems
if they understand the impacts of their actions and respond with appropriate
management measures. The City of Greenville shall develop and
implement public and developer education programs for the Tar-Pamlico
basin.
1-C.
Applicability of Tar-Pamlico Stormwater Rule
Figure 1 identifies which
areas of the City are within the Tar-Pamlico River Basin and which are
within the Neuse River Basin. It also identifies the current city
limits and the City's extraterritorial jurisdiction (ETJ). Those
requirements as set forth by the Tar-Pamlico Rule will be applicable
to the portion of Greenville’s city limits located within the Tar-Pamlico
River Basin under its “Stormwater Manangment and Control” ordinance.
For areas of the City’s ETJ located within the Tar-Pamlico River Basin,
the City will apply the requirements of its program to the extent authorized
under State statutes that govern municipality operations in its ETJ.
Under State statutes, the City is authorized to perform inspections
and report violations within its ETJ to the appropriate County and State
authorities. Those areas of the City and its ETJ located within
the Neuse River Basin will be subject to the requirements of this ordinance
and program, with the exception of having to meet the requirements for
controlling phosphorus releases. As
areas within it's ETJ are annexed into the city limits, the City will
be able to enforce these requirements.
2. New Development Review/Approval
2-A.
Requirements in the Rule
The Tar-Pamlico Stormwater
Rule has the following requirements for new development located within
that portion of the City of Greenville and its ETJ within the Tar-Pamlico
River Basin (Please refer to Appendix A for complete language):
The nitrogen load contributed
by new development activities is held at 4.0 pounds per acre per year.
This is equivalent to 70 percent of the estimated average nitrogen load
contributed by non-urban areas in the Tar-Pamlico River Basin (as defined
using 1995 LANDSAT data). Similarly, the phosphorus load contributed
by new development activities is held at 0.4 pounds per acre per year,
which is equivalent to the estimated average phosphorus load contributed
by non-urban areas in the basin. The Environmental Management Commission
may periodically update these performance standards based on the availability
of new scientific information.
Property owners shall have
the option of partially offsetting projected nitrogen loads by providing
treatment of existing developed areas off-site that drain to the same
stream. However, the total nitrogen loading rate cannot exceed
6.0 pounds per acre per year for residential development or 10 pounds
per acre per year for non-residential development.
There is no net increase in
peak flow leaving the developed site from the predevelopment conditions
for the 1-year, 24-hour storm.
The City of Greenville will
review new development plans to assure compliance with requirements
for protecting and maintaining riparian areas as specified in 15A NCAC
2B .0259.
The City of Greenville
may include regional stormwater facilities in their programs to provide
for partial nutrient and flow control. Such facilities may not
degrade surface waters. This is further discussed under Section 2-G.
To comply with
the aforementioned requirements, the City renamed and amended Chapter
9 “Storm Drainage” of Title 9 “Building, Planning, and Development
Regulations” located in the “Charter and Code of Ordinances” for
the City of Greenville, and this chapter is now referred to as “Stormwater
Management And Control”, which is presented in Appendix E).
The Engineering
Division of the Public Works Department is responsible for reviewing
plats and plans for private developments for compliance with the “Building
,Planning, and Development Regulations” Ordinance. Once the Engineering
Division has completed its review and determines them to be in compliance,
the documents are forwarded to the Planning Division for development
permit issuance.
During this
review process, review comments for each plat or plan is entered into
a Land Development Tracking System. On a quarterly basis, data
on approved projects will be retrieved from the Land development Tracking
System. This data will be used to generate annual reports to DWQ
on Nitrogen and Phosphorus loading from new development projects.
2-B.
Protecting Riparian Areas on New Development
The Tar-Pamlico Riparian
Buffer Protection Rule, 15A NCAC 2B .0259, requires the City of Greenville
to ensure that riparian areas on new developments are protected in accordance
with the buffer rule’s provisions. The buffer rule requires
that 50-foot riparian buffers be maintained on all sides of intermittent
and perennial streams, ponds, lakes and estuarine waters in the basin.
The buffer rule provides for certain “allowable” uses within the
buffer with DWQ approval such as road and utility crossings.
The City of Greenville
shall disapprove any new development activity proposed within the first
50 feet adjacent to a waterbody that is shown on either the USGS 7.5
minute topographic map or the NRCS Soil Survey map unless the owner
can show that the activity has been approved by DWQ. DWQ approval
may consist of the following:
An on-site determination that
surface waters are not present.
An Authorization Certificate
from DWQ for an “allowable” use such as a road crossing or utility
line, or for a use that is “allowable with mitigation” along with
a Division-approved mitigation plan. A table delineating such
uses is included in the buffer rule.
An opinion from DWQ that vested
rights have been established for the proposed development activity.
A letter from DWQ documenting
that a variance has been approved for the proposed development activity.
2-C.
Calculating N and P Export from New Development
New Development
Described: For the purposes of the City of Greenville’s
Stormwater Management Program, new development shall be described to
include the following:
Any activity that disturbs
greater than one acre of land to establish, expand, or replace a single
family or duplex residential development or recreational facility.
For individual single family residential lots of record that are not
part of a larger common plan of development or sale, the activity must
also result in greater than ten percent built-upon area.
Any activity that disturbs
greater than one-half an acre of land to establish, expand, or replace
a multifamily residential development or a commercial, industrial or
institutional facility.
Projects meeting the above
criteria that replace or expand existing structures or improvements
and that do not result in a net increase in built-upon area shall
not be required to meet the basinwide average non-urban loading levels.
Projects meeting the above
criteria that replace or expand existing structures or improvements
and that result in a net increase in built-upon area shall achieve a
30 percent reduction in nitrogen loading and no increase in phosphorus
loading relative to the previous development. Such projects may
achieve these loads through onsite or offsite measures or some combination
thereof.
Multi-family residential,
commercial, industrial, and institutional projects may choose to achieve
all of this reduction by providing treatment of off-site developed areas,
or by permanently conserving land from future development in conformance
with the the City of Greenville’s approved land conservation plan
as described in Section 2-G.
Alternatively, any project
that is subject to the above loading requirements and that is located
within an area that the City of Greenville has established for redevelopment,
as characterized here, in a pattern conducive to the goals of the Tar-Pamlico
nutrient strategy, may not be required to achieve those nutrient reductions
if the project meets certain conditions that are established for that
area as follows:
A “fix it first” policy
that reserves public funds for repair of existing infrastructure in
these areas before investing in new infrastructure of the same
type in new growth areas.
Mixed use/mixed density zoning
provisions.
Retrofits are consistent with
NCDOT definitions for pedestrian scale in traditional neighborhood developments
(e.g., 80% of users are within a ¼ mile walk from schools, libraries,
and recreational/athletic facilities, 60% of students and 50% of teachers
are within ½ mile walk from schools, and 40% of congregants are within
¼ mile of churches).
Parking maximums or shared
parking ratios.
Residential density bonuses
where parking maximums, pedestrian scale, or “fix it first” are
proposed.
Built-upon area means that
portion of a development project that is covered by impervious or partially
impervious cover including buildings, pavement, and gravel area.
Slatted wooden decks and the water surface area of pools shall be considered
pervious.
Land disturbance is defined
as grubbing, stump removal, grading, or removal of structures.
New development shall
not include agriculture (including intensive livestock operations),
mining, or forestry activities.
Vested Rights:
All site plans and preliminary plats for new development projects that
have received approval from the City of Greenville prior to the effective
date of this program (September 10, 2004) and that have implemented
that development in accordance with the City of Greenville’s vesting
provisions shall be exempt from the requirements of the Tar-Pamlico
Stormwater Rule. Vesting provsions and timelines shall be incordance
with Section 9-4-34 of the City of Greenvile Code of Ordinances for
site plans and Section 9-5-41 for preliminary plats.
Projects that require
a state permit, such as landfills, NPDES wastewater discharges, land
application of residuals and road construction activities shall be considered
exempt if a state permit was issued prior to the effective date (September
10, 2004) of the City of Greenville’s Stormwater Management Program.
Calculating N and
P Export: The nitrogen and phosphorus export from each new development
must be calculated. This export will be calculated in pounds per
acre per year (lbs/ac/yr). Worksheets to carry out this method
are provided in Appendix B.
It is expected that some
values provided in the methodology will be refined over time.
The Division of Water Quality plans to provide those refinements to
the City of Greenville on a periodic basis as they are established.
For example, additional research may lead to refined export values for
the various urban land covers, particularly rooftop and transportation
impervious surface. Also, stormwater management practices are
typically in various stages of refinement around the country.
Several nutrient reducing BMPs are being applied and studied around
North Carolina toward better designs and more accurate knowledge of
long-term nutrient removal efficiencies. The City of Greenville
will incorporate these refinements into its program from time to time
as they are substantiated.
For a given project,
the methodology calculates a weighted annual load export for both nitrogen
and phosphorus based on event mean concentrations of runoff from different
urban land covers and user-supplied acreages for those land covers.
The user chooses BMPs that reduce the export to rule-mandated levels.
Two versions of the spreadsheet were developed based on rainfall differences;
one (the “Piedmont” version) for the jurisdictions of Oxford, Henderson,
Rocky Mount, and Tarboro, and the counties of Franklin, Nash, and Edgecombe,
and the other (the “Coastal Plain” version) for the remaining communities.
A residential worksheet
is also provided in Appendix B to calculate acreages dedicated to different
land covers in residential developments where impervious footprints
are not shown. One situation not addressed by the methodology
is a non-residential subdivision where the impervious surfaces are not
shown on the plans at the time of submittal. In this case, the
applicant shall determine a worst-case scenario for the areas of impervious
surface and managed open space for the type of development specified
and then apply the methodology. This determination shall be presented
on the preliminary plat as part of its approval.
2-D.
BMPs for Reducing Nitrogen and Phosphorus
The Tar-Pamlico Stormwater
Rule requires that all new developments achieve a nitrogen export of
less than or equal to 4.0 (and a phosphorus export of less than or equal
to 0.4 pounds per acre per year. If the development contributes
greater than 4.0 pounds nitrogen (or 0.4 pounds phosphorus), then the
following options exist.
For residential (or commercial
or industrial) development:
If the computed nitrogen export
is greater than 6.0 (or 10.0) lbs/ac/yr, then the owner must either
use on-site BMPs or take part in an approved regional or jurisdiction-wide
stormwater strategy or some combination of these to lower the nitrogen
export to at least 6.0 (or 10.0) lbs/ac/yr. The owner may then
use one of the following two options to reduce nitrogen from 6.0 (or
10.0) to 4.0 lbs/ac/yr.
If the computed nitrogen export
is greater than 4.0 lbs/ac/yr but less than 6.0 (or 10.0) lbs/ac/yr,
then the owner may either:
Install BMPs onsite or take
part in an approved regional or jurisdiction-wide stormwater strategy
or some combination of these to remove nitrogen down to 4.0 lbs/ac/yr.
Provide treatment of an offsite
developed area that drains to the same stream to achieve the same nitrogen
mass loading reduction that would have occurred onsite.
The owner must install BMPs
that also achieve a phosphorus export of less than or equal to 0.4 lbs/ac/yr
but may do so through on-site or offsite measures or some combination
thereof.
As with most resource
impacts, an ounce of stormwater prevention is worth a pound of cure.
A sound site planning process first considers the ability to achieve
the needed reductions using site design measures that avoid or minimize
runoff to begin with. These planning measures include reducing,
disconnecting, and rerouting impervious surfaces, maximizing time of
concentration for stormwater, and protecting open spaces for infiltration
and evapotranspiration. More detail on planning measures that
reduce hydrologic and nutrient loading is given in Appendix C.
Often, structural management
practices cannot be avoided. BMP selection is an important and
challenging craft. Available data indicate that most BMPs remove
only 20 to 40 percent of total nitrogen or phosphorus on a consistent
basis. There are a number of issues to consider to ensure this
sustained performance. It is crucial to consider the issues of
aesthetics, long-term maintenance, safety, and reliability in BMP design.
All BMPs require regular maintenance and some have varying performance
depending on soil type and season. The efficiencies provided below
and in the load calculation worksheets in Appendix B assume correct
sizing and other design per the referenced manuals and optimum performance
based on regular, effective maintenance as well as proper siting of
the practices.
The BMPs available for
nutrient reduction and their removal rates based on current literature
studies are provided in Table 2 below. These median values
are based on a literature review conducted by a contractor that updated
Neuse nitrogen efficiencies and established phosphorus values.
Also provided in the table are the design standards to be adhered to
in permitting BMP design.
The design of Best Management
Practices that remove nitrogen and phosphorus from stormwater is a developing
field. Researchers throughout the country, particularly in the
Southeast, are conducting studies to identify and refine effective means
of controlling nitrogen and phosphorus. As stated in Section 2-C,
the Division of Water Quality plans to provide refinements in the stated
BMP removal efficiencies to the City of Greenville on a periodic basis
as they are substantiated.
Table 2: BMP
Types, TN and TP Removal Rates, and Design Standards
BMP
Type
TN Removal Rate per Literature
Review
TP Removal Rate per Literature
Review
Appropriate
Design Standards
Wet
detention ponds
25%
40%
NC Design Manual (*)
Constructed wetlands
40%
35%
NC Design Manual (*)
Restored riparian buffers
30%
30%
Tar-Pamlico Riparian Buffer Rule
(15A NCAC 2B .0259)
Grass Swales
20%
20%
NC Design Manual (*)
Vegetated filter strips
with level spreader
30%
30%
NC Design Manual (*) and other literature
information
Bioretention (rain
gardens)
40%
35%
NC Design Manual (*)
Sand Filters
35%
45%
NC Design Manual (*)
Proprietary BMPs
Varies
Varies
Per manufacturer subject to DWQ approval
Other BMPs
Varies
Varies
Subject to DWQ approval
(*) The
North Carolina Department of Environment and Natural Resources, Division
of Water Quality, Water Quality Section, Stormwater Best Management
Practices Manual, 1999, and all amendments
Multiple BMPs:
The worksheet provides calculation space for the case where more than
one BMP is installed in series on a development. It determines
the removal rate through serial rather than additive calculations.
This is important to understand in projects where the automated worksheet
is not used to estimate the effect of multiple BMPs.
As an example, if a wet
detention pond discharges through a restored riparian buffer, then the
removal rate shall be estimated to be 47.5 percent, determined as follows:
The pond removes 25 percent of the influent nitrogen mass and discharges
75 percent to the buffer. The buffer then removes 30 percent of
the remaining 75 percent of the original nitrogen amount that discharged
from the pond, or 22.5 percent of the original influent amount.
The sum of 25 and 22.5 is 47.5. The removal rate is NOT 25 percent
plus 30 percent.
Assigning Values
to Pervious Cover: Large-lot residential development may involve
substantial open space that, at least initially, may remain in an undisturbed
wooded or reforesting condition. While it may seem logical to
enter this acreage as wooded pervious, without conservation easements
or some other mechanism for ensuring protection of these areas, the
City of Greenville has no control over their eventual condition.
Thus, unless specific protection instruments, such as conservation easements,
are established and provided in the development application or by the
City of Greenville, lot areas shall be assigned the lawn/landscape managed
pervious export rate. The worksheet will do this automatically.
Riparian buffers protected
under the Tar-Pamlico Riparian Buffer Protection rule, 15A NCAC 2B .0259,
are divided into two zones, moving landward from the surface water,
that are afforded different levels of protection. Zone 1, the
first 30 feet, is to remain essentially undisturbed, while zone 2, the
outer 20 feet, must be vegetated but may be managed in certain ways.
The user shall enter the acreage in zone 1 into the worksheet as wooded
pervious, while zone 2 acrage shall be entered as managed pervious (lawn/landscape).
2-E.
Calculating Peak Runoff Volume
The Tar-Pamlico Stormwater
Rule requires that new development not cause erosion of surface water
conveyances. At a minimum, new development shall not result in
a net increase in peak flow leaving the site from pre-development conditions
for the 1-year, 24-hour storm event. A number of Neuse local governments
sought to use the 2-year rather than the 1-year storm as the design
storm for peak flow control given that the 2-year storm is more consistent
with current hydrologic modeling methodologies.
The main reason that
the rule requires a 1-year design storm for peak flow control is to
protect stream channels from erosion. Development on land causes
many changes in stormwater hydrology. One of the major causes
of streambank erosion in urban streams is the increase in the frequency
of the bankfull-flooding event. The bankfull-flooding event generally
occurs at approximately a 1.5-year frequency. The Tar-Pamlico
Stormwater Rule requires control of the 1-year storm to predevelopment
levels to insure that the rate of release will be below bankfull and
therefore less erosive to the stream channel. Releasing the 2-year
storm at predevelopment levels would likely have the effect of increasing
the frequency of a storm that is just a bit larger than the most erosive
storm.
Protecting streambanks
from erosion is a crucial part of the overall Tar-Pamlico Nutrient Sensitive
Waters Management Strategy. Riparian buffers are protected under
this program because in most situations they are effective at removing
nitrogen resulting from nonpoint source pollution. The use of
nitrogen reducing BMPs on new development does not obviate the need
to maintain valuable riparian buffers.
In the Neuse process,
DWQ staff devised a strategy, which is incorporated here, to allow use
of the 2-year design storm while also providing a similar level of protection
for streambanks as the use of the 1-year design storm. The strategy
is to give the City of Greenville the option of using the 2-year storm
as the design storm for peak flow control; however, requiring that it
be controlled to the pre-development levels of the 1-year storm.
This can be done by computing the peak flow associated with the 2-year
storm for pre-development conditions and then reducing it by an appropriate
percentage to reflect the difference between the 1-year and 2-year storm
peak flows. The City of Greenville will allow either of the following
two options:
Option
1: Use the 1-year Design Storm
The US Weather Bureau
(Technical Paper 40) published maps of rainfall depths for the 1-year
storm of duration 30 minutes to 24 hours. The 1-year, 24-hour
precipitation, varies along the Tar-Pamlico River Basin.
For the City of Greenville, the amount of precipitation for a 1-year,
24-hr storm is 3.4 inches.
The Rational Method is
an acceptable method for estimating peak discharge in the design of
stormwater facilities for relatively small watersheds (up to 50 acres).
The basic equation is:
Q
= CIA
Where:Q is the peak flow
for the design storm in cubic feet per second
C is the coefficient of runoff based on land
cover (dimensionless)
I is the storm intensity in inches per hour
A is the drainage area in acres
The rational equation
is based upon the assumption that rainfall is uniformly distributed
over the entire drainage area at a steady rate, causing the flow to
reach a maximum at the outlet of the watershed at a time to peak, Tp.
The Rational Method typically gives a conservative estimate of runoff.
In order to use the Rational
Method to determine peak flows, it is necessary to compute the storm
intensity in inches per hour for the 1-year storm. The intensity
is computed by the formula:
I
= g/(h+T)
Where: I is the storm intensity in inches
per hour
g and h are empirically derived constants
T is the duration in minutes (or (L3/H)0.385)/128)
The values
for constants g and h for the 1-year storm are not presently available.
The appropriate values for g and h were estimated by graphing the 2,
5, 10, 25, 50 and 100-year values of g and h for Wake and Wilson Counties
as a function of return period on a log-normal scale and determining
the y-intercept of the best-fit line. For the City of Greenville,
the resulting values of g = 112 and h = 20 are applicable in the
Tar-Pamlico River Basin.
Option
2: Use the 2-year Design Storm, but Control it to 1-year Predevelopment
Levels
This option involves
the following three steps:
First, compute the peak flows
(both pre- and post-development) from the drainage area based on the
2-year design storm using one of the methodologies listed below.
Second, estimate the 1-year
pre-development peak flow by multiplying the 2-year predevelopment peak
flow by 80%.
Third, design a BMP that will
control the 2-year post-development peak flow to 1-year pre-development
peak flow levels (estimated by the second step).
Exceptions
to the Peak Flow Requirement
Peak flow control is
not required for developments that meet one or more of the following
requirements:
The increase in peak flow
between pre- and post-development conditions does not exceed 10
percent (note that this exemption makes it easier to conduct redevelopment
activities).
The development occurs in
a part of a drainage basin where stormwater detention can aggravate
local flooding problems.
Acceptable
Methodologies for Computing Peak Flow
Acceptable methodologies
for computing the pre- and post-development conditions for the design
storm include:
The Rational Method
Dr. Rooney Malcom, P.E., Small
Watershed Method
NRCS Methodologies applied
through the Corps of Engineers HEC-1 Program
The Peak Discharge Method
as described in USDA Soil Conservation Service’s. Technical Release
Number 55 (TR-55)
The Putnam Method
Other methods approved by
the Environmental Management Commission
The same method must
be used for both the pre- and post-development conditions.
2-F.
Offsite Partial Offset Option
The Tar-Pamlico Stormwater
Rule provides the option to partially offset nitrogen load increases
from new development by providing treatment of offsite developed areas.
The developer must provide legal assurance of the dedicated use of the
off-site area for the purposes described here, including achievement
of specified nutrient load reductions and provision for regular operation
and maintenance activities, in perpetuity. The legal assurance
shall include an instrument, such as a conservation easement, that maintains
this restriction upon change of ownership or modification of the off-site
property. Before using off-site treatment, the new development
must attain a maximum nitrogen export of six (6) pounds/acre/year for
residential development and ten (10) pounds/acre/year for commercial
or industrial development.
Typical features of such
an offsite offset project that distinguish it from regional systems
(described in section 2-G) include the following:
The new development site does
not typically drain into the offsite treatment facility.
The offsite facility is retrofitted
to treat an existing developed property.
The offsite facility may address
only the nutrient requirements unless a development proposal demonstrates
that meeting some or all attenuation requirements offsite will not result
in degradation of surface waters to which the new development site discharges.
In consideration of this
option, the City of Greenville will require a developer submit the appropriate
documentation and calculations with their plat and plans that demonstrate
the following:
Projects reduce nitrogen load
onsite to 6 lb/ac/yr for residential, 10 lb/ac/yr for commercial, industrial.
Offsite location achieves
remaining nitrogen reduction requirement.
Projects reduce phosphorus
loading to 0.4 lb/ac/yr between onsite and offsite BMPs.
Projects meet the flow attenuation
requirements of the Rule.
The offsite property drains
to the same receiving body of water as the new development project.
Current owners agree in a
documented, enforceable manner that offsite facilities are dedicated
to achieving the specified nutrient and flow reductions for the life
of the new development.
All future owners of both
properties will understand and accept these restrictions at the time
of purchase.
Current and future owners
of the new development will maintain stormwater facilities on both the
new development and the offsite properties.
Plats and plans for projects
are required to show easements, buffers, and other applicable restrictions.
The Engineering Division maintains records of plats within its planning
jurisdiction. Consideration of these records is part of the Engineering
Division’s review process for projects. As per Section 9-9-8
of the Code of Ordinancnes for the City of Greenville, a developer is
required to submit a maintenance plan and complete annual inspection
reports for BMPs on their properties. In addition to its annual
inspection program, the City will utilize this information as a mechanism
for tracking offsite partial offsets and to assure that these areas
will be maintained.
2-G.
Regional or Jurisdiction-Wide Approaches
The Tar-Pamlico Stormwater
Rule provides local governments the option to develop regional or jurisdiction-wide
stormwater facilities in its program as an alternative means for developers
to address nutrient or flow control requirements. Currently, the
City of Greenville does not have plans for a regional facility.
However, if the City determines that it would be beneficial to develop
such facilities, they would require the review and approval of the North
Carolina Department of Environment and Natural Resources, Division of
Water Quality. At such time, the City of Greenville will develop
appropriate ordinances, guidelines, and requirements for these types
of facilities and also establish appropriate tracking processes, mechanisms,
legal instruments, etc. to ensure that regional or jurisdiction-wide
approaches continually meet attenuation and loading requirements of
the Tar-Pamlico Stormwater Rule.
Regional Facilities:
Within the context of the rule, the concept of a regional facility means
generally a stormwater facility that serves more than one development
project, each of which drains to the facility for treatment or attenuation.
Inflows to regional facilities may already be partially treated or attenuated.
Many individual developments
include stormwater designs that could be interpreted as “regional”
under the broadest of definitions but which are not intended for the
type of review and approval process described here. Projects such
as phased developments or commercial projects with outparcels may use
common stormwater facilities that receive runoff from more than one
development under different ownership. However, common facilities
that are permitted under single projects are intended for permitting
by the City of Greenville.
Regional facilities provided
for in the rule would serve more than one development project.
They could be publicly or privately owned but would be proposed to DWQ
by the City of Greenville. Basic elements of regional system proposals,
to be permitted by DWQ, and other “common-facility” individual projects
permitted by the City of Greenville would be the same.
Jurisdiction-Wide
Approach: Within the context of the rule, the concept of a jurisdiction-wide
approach means generally a nutrient-reducing management measure implemented
under the authority of a local government to offset one or more increases
that may take place in the same or a separate watershed within the jurisdiction.
An offsite offset project (see Section 2-F) that is implemented under
the authority of a local government would be a specific type of jurisdiction-wide
approach.
2-H.
BMP Maintenance
BMPs implemented to achieve
the nitrogen and phosphorus loading reductionand flow attenuation requirements
for a development must be maintained as established in Sec.9-9-8 in
the City Code of Ordinances. (Presented in Appendix E.) An example
of an operation and maintenance agreement for BMPs is presented in Appendix
D. BMPs shall be maintained in accordance with the methods presented
the North Carolina Department of Environment and Natural Resources,
Division of Water Quality, Water Quality Section, Stormwater Best
Management Practices Manual.
The City of Greenville
shall inspect all BMPs on an annual basis and will maintain records
for BMPs to include types of BMPs, their locations, approved maintenance
plans, and required inspection process. The City of Greenville
shall notify the owner upon finding that maintenance is needed on a
BMP in accordance with Sec. 9-9-8 of the City Code. If the owner does not
complete the maintenance in a timely manner, then the City of Greenville
shall contract out the maintenance itself and recover its costs in the
manner as permitted by this section of the City Code.
2-I.
Land Use Planning Provisions
An objective of the Tar-Pamlico
Stormwater Rule is to provide the flexibility and incentives for the
City of Greenville to improve its growth management practices and for
developers to considerusing impact-reducing site design techniques that
will reduce nitrogen and phosphorus loading from their developments.
One such measure, reducing impervious surfaces, reduces the need for
BMPs to control nitrogen and peak stormwater flows and also reduces
associated BMP maintenance concerns.
The City of Greenville
encourages developers to consider the following planning techniques
and the general advantages and disadvantages of incorporating the following:
Pervious Paver
Pervious concrete/asphalt
mixes
Minimizing use of curb and
gutter
Cluster or open-space developments
Traditional neighborhood developments
Mixed-use developments
Low Impact Development principles
Other impact-reducing approaches
Descriptions of these
techniques are provided in Appendix C.
3. Illicit Discharges
3-A.
Requirements in the Rule
The Tar-Pamlico
Stormwater Rule requires the City of Greenville establish a program
to prevent, identify, and remove illicit discharges. Illicit discharges
are flows in the stormwater collection system that are not associated
with stormwater runoff or an allowable discharge.
3-B.
What is an Illicit Discharge?
Stormwater
collection systems are vulnerable to receiving illicit discharges (even
though the person responsible for the discharge may be unaware that
it is illicit). Depending on their source, illicit discharges
may convey pollutants such as nutrients, phenols, and metals to receiving
waters. Table 3a identifies some potential flows to the stormwater collection
system that may be allowable. Table 3b identifies some discharges
that are not allowed.
Table 3a: Discharges
that may be allowable to the stormwater collection system
Waterline
Flushing
Landscape Irrigation
Diverted Stream Flows
Uncontaminated Rising
Ground Water
Uncontaminated Ground Water Infiltration
to Stormwater Collection System
Uncontaminated Pumped Ground Water
Discharges From Potable
Water Sources
Foundation Drains
Uncontaminated Air Conditioning Condensation
Irrigation Water
Springs
Water From Crawl Space Pumps
Footing Drains
Lawn Watering
Non-Commercial Car Washing
Flows From Riparian
Habitats and Wetlands
NPDES Permitted Discharges
Street Wash Water
Fire Fighting Emergency
Activities
Wash Water From the Cleaning of Buildings
Dechlorinated Backwash and Draining
Associated With Swimming Pools
Table 3b: Types of
Discharges that are not allowed to stormwater collection system
Dumping
of Oil, Anti-Freeze, Paint, Cleaning Fluids
Commercial Car Wash
Industrial Discharges
Contaminated Foundation
Drains
Cooling Water Unless no Chemicals Added
and has NPDES Permit
Washwaters From Commercial/ Industrial
Activities
Sanitary Sewer Discharges
Septic Tank Discharges
Washing Machine Discharges
Chlorinated Backwash
and Draining Associated With Swimming Pools
Note: Some
of these items may be disposed of through the sanitary sewer system.
Please contact the GUC WWTP at 551-1542 for further information.
3-C.
Establishing Legal Authority
In accordance
with Chapter 9 “Stormwater Management and Control” of Title 9 in
the City Code of Ordinances (Appendix E), the City of Greenville has
established the legal authority to control and prohibit illict discharges
under Section 9-9-17. The purpose and objectives of establishing
this authority by ordinace is as follows:
Control
the contribution of illegal pollutants identified in Table 3b to the
stormwater collection system.
Prohibit
illicit discharges to the stormwater collection system.
Prohibit
discharge of spills and disposal of materials other than stormwater
to the stormwater collection system.
Determine
compliance and non-compliance.
Require
compliance and undertake enforcement measures in cases of non-compliance.
3-D.
Collecting Jurisdiction-Wide Information
As part of
its program, the City of Greenville will collect geographic information
within its legal boundary, as defiened under State statute, at three
increasing levels of detail:
The
first, most cursory level is information that shall be collected for
the City’s legal jurisdiction. The associated requirements are
discussed in this section.
The
second level is a more detailed screening for high priority areas within
the City’s legal jurisdiction. The associated requirements are
discussed in Section 3-E.
The
third level is a very detailed investigation that shall be done upon
the discovery of an illicit discharge. The associated requirements
are discussed in Section 3-F.
The purpose
of collecting jurisdiction-wide information is to assist with identifying
potential illicit discharge sources and characterizing illicit discharges
after they are discovered.
The City of
Greenville will compile maps that may include, but not necessarily limited
to, the following:
Location of sanitary sewers in areas
of the major stormwater collection systems and the
location of areas that are not served by sanitary sewers.
Waters
that appear on the USDA – Natural Resources Conservation Service Soil
Survey Maps and the U.S. Geological Survey 1:24,000 scale topographic
maps.
Land
uses, such as residential, commercial, agriculture, industrial, institutional,
publicly owned open space, and others.
Currently
operating and known closed municipal landfills and other treatment,
storage, and disposal facilities, including for hazardous materials.
Major
stormwater structural controls.
Known
NPDES permitted discharges to the stormwater collection system .
Written descriptions
for the map components will be as follows:
A
summary table of municipal waste facilities that includes the names
of the facilities, the status (open/closed), the types, and addresses.
A
summary table of the NPDES permitted dischargers that includes the name
of the permit holder, the address of the facility, and permit number.
A
summary table of the major structural stormwater control structures
that shows the type of structure, area served, party responsible for
maintaining, and age of structure.
A
summary table of publicly-owned open space that identifies size, location,
and primary function of each open area.
The City of
Greenville shall complete this collection of jurisdiction-wide information
by the time the second annual report is due.
3-E.
Mapping and Field Screening in High Priority Areas
Beginning in
its third year after implementation , the City of Greenville shall identify
a high priority area of its jurisdiction for more detailed mapping and
field screening. This high priority area shall comprise at least
ten percent of the jurisdiction’s area. Each subsequent year,
the City of Greenville is responsible for selecting and screening another
high priority area that comprises at least 10 percent of its jurisdiction.
“High priority”
means the areas where it is most likely to locate illict discharges.
The most likely locations for identifying illicit discharges are areas
within older developments. Each year, the City of Greenville shall
explain its basis for selection of its high priority areas.
The first part
of the screening process for the selected high priority areas is mapping
the stormwater system. At a minimum, the map that is produced
should include the following:
Locations
of the outfalls, or the points of discharge, of any pipes from non-industrial
areas that are greater than or equal to 36 inches.
Locations
of the outfalls of any pipes from industrial areas that are greater
than or equal to 12 inches.
Locations
of the outfalls of drainage ditches that drain more than 50 acres of
non-industrial lands.
Locations
of the outfalls of drainage ditches that drain more than 2 acres of
industrial land.
An
accompanying summary table listing the outfalls that meet the above
criteria that includes outfall ID numbers, location, primary and supplemental
classification of receiving water, and use-support of receiving water.
The second
part of the screening process for the selected high priority area is
conducting a dry weather field screening
of all outfalls that meet the above criteria to detect illict discharges.
The dry weather field screening shall not be conducted during or within
72 hours following a rain event of 0.1 inches or greater.
Figure 3 illustrates
a process for conducting field screening sampling activities and following
up with any findings of dry weather flow. As shown in this figure,
if the field screening shows that an outfall is dry, then the outfall
will be checked for intermittent flow at a later date.
If a field
screening shows that an outfall has a dry weather flow, the City of
Greenville shall complete a screening report for the outfall.
The information that will be contained in the screening report is outlined
in Table 3c. Screening reports shall be kept on file for a minimum
of five years. Example screening report forms are provided in
Appendix F.
Table 3c: Field Screening
Report Information
General
Information
Sheet Number
Outfall ID Number
Date
Time
Date, Time and Quantity
of Last Rainfall Event
Field Site
Description
Location
Type of Outfall
Dominant Watershed Land
Use(s)
Visual
Observations
Photograph
Odor
Color
Clarity
Floatables
Deposits/Stains
Vegetation Condition
Structural Condition
Biological
Flow Estimation
Sampling
Analysis *
Temperature
pH
Nitrogen-Ammonia
Nitrogen-Nitrate/Nitrite
Fluoride or Chlorine
Total Phosphorus
Ortho-Phosphate
* Analytical
monitoring is required only if an obvious source of the dry weather
flow cannot be determined through an investigation of the upstream stormwater
collection system.
Outfalls with
flow will be screened again within 24 hours for the above parameters.
Figure 3: Field
Screening Process
Screen outfall in high
priority area
Check for signs of intermittent
flow *
Inspect and sample flow
Remove illegal discharge
Investigate source
of flow, considering
the following:
Jurisdiction-wide information
collected
Field investigation of drainage
area of
outfall
Sampling data
Qualitative observations --
sheen, odor,
turbidity, etc.
Flow found
No flow
Flow
found
Outfall OK
No flow
* Checking
for intermittent flow includes rechecking outfall at a later date as
well as visual observations for evidence of intermittent flow.
Note:
Analytical monitoring may be conducted only if an obvious source of
the dry weather flow cannot be determined through an investigation of
the upstream stormwater collection system.
3-F.
Identifying and Removing Illicit Discharges
After the field
screening is complete, the City of Greenville shall take measures to
identify and remove illicit discharges. Identifying illicit discharges
may require a combination of office and field work. After the
field screening, the City of Greenville shall consult the jurisdiction-wide
information they have compiled (see Section 3-D) to obtain information
about the land uses, infrastructure, industries, potential sources,
and types of pollution that exist in the drainage area of the outfall.
After potential
sources have been identified in the office, a systematic field investigation
will be completed minimizing the amount of resources required to identify
the source. Several field methods may be used to identify illegal
discharges. The City of Greenville will use a simple approach
if that will suffice. Listed below are several approaches, but
not necessarily limited to, that may be used as part of the inspection
process:
Site
Investigation
Additional
Chemical Analysis (recommend testing for fecal coliform if the ammonia
concentration was found to exceed 1.0 mg/L)
Flow
Monitoring (recommended to use multiple site visits rather than a depth
indicator)
Dye
Testing (fluorescent dye is recommended)
Smoke
Testing
Television
Inspection
Documentation
of the results of the office and field investigations shall be kept
on file for a minimum of five years with the screening report.
After the City
of Greenville identifies the source of an illicit discharge, it shall
take enforcement action to have the source removed. The legal
authority that was established for the illicit discharge program shall
provide the means to accomplish this requirement. Enforcement
shall include requiring the person responsible for the discharge to
remove or redirect it to the sanitary sewer. If redirected to
the sanitary sewer, prior approval from Greenville Utilities Commission
will be required by the person responsible for the discharge.
There shall be remedies to deal with cases of non-compliance.
Records of all compliance actions shall be kept for a minimum of five
years with the screening report.
In addition
to keeping all screening reports on file, the City of Greenville shall
maintain a map that includes the following:
Points
of identified illicit discharges.
Watershed
boundaries of the outfalls where illicit discharges have been identified.
An
accompanying table that summarizes the illicit discharges that have
been identified that includes location, a description of pollutant(s)
identified, and removal status.
3-G.
Preventing Discharges and Establishing a Hotline
The City of
Greenville shall contact persons who are responsible for establishments
that are likely sources of illicit dis