Stormwater

Federal Regulations

Clean Water Act
National Pollutant Discharge Elimination System (NPDES)
NPDES Stormwater
Stormwater Discharges from Municipal Sources

State Regulations

SC Department of Health and Environmental Control (DHEC) Bureau of Water Laws and Regulations
Coastal Zone Laws and Regulations
Stormwater Permitting
Pollution Control Act
Stormwater Management and Sediment Reduction Act (72-300)
Water Pollution Control Permits (61-9)

Relevant Chapters:
Chapter 17.7 - Stormwater Management
Chapter 18 - Land Development Regulations
Chapter 9 - Flood Damage Prevention and Control
Chapter 10, Article II - Solid Waste Handling and Disposal
Chapter 4, Article III - Migratory Wild Fowl
Land Development Regulations

Introduction

Horry County requires that all land disturbance activities of 10,000 square feet or more, obtain a stormwater permit unless otherwise exempt. This page details the process and procedures to be used in the implementation of these regulations.

Statutory Authority

Horry County Stormwater Management and Sediment Control Ordinance went into effect Oct. 1, 2000.

Basic Criteria

a) All land disturbance activities associated with land development and construction are required to obtain a stormwater permit if one-half (0.5) acre or more of area will be disturbed. Land disturbance is defined as any use of the land that results in a change in the physical characteristics or topography that may cause erosion that contributes to sediment and alters the quality or quantity of stormwater runoff.

b) Exemptions – A number of activities are exempt and therefore do not need to obtain a stormwater permit. Single family residential construction, Agricultural land uses, timbering activity, and minor residential subdivisions as defined by the County Land Development Regulations. If a project received OCRM approval prior to the effective date of the County ordinance and the project is under construction, then an exemption may be issued.

The Permitting Process

1. Applicant submits a plan package for all projects including a completed Notice Of Intent (NOI) form from SC DHEC. (The county uses this form as your MS4 application). Click here for SC DHEC's water quality mapping tool that generates an NOI with water quality information. [ Generally the applicant will also submit a plan package to DHEC.]
   
2. The project information is reviewed to determine if it is exempt.
   1. If the project is exempt, the plan package is stamped by the plan reviewer and uploaded to the E-Plats page for download through the applicant’s E-Plats account. A copy will also be sent to the Horry County Code Enforcement Office.
   2. If the project is not exempt then the submittal is reviewed for compliance with Horry County Ordinances and the Horry County Stormwater Management Design Manual.
3. If the plans are not approved the applicant is notified and informed of any revisions or problems that need to be addressed. A copy of the review comments is then uploaded to E-plats page for the applicant to review and resubmit the plan with corrections.
4. If the plans are approved a letter of approval is issued and uploaded to the E-plats page for the applicant to download. The Applicant then submits the approved NOI and the Plan Approval Letter to DHEC as part of DHEC’s review process.
5. Once all review comments have been adequately addressed and the plans are approved, DHEC has issued the applicant thier permit for coverage and the permit is received by the county through the applicant, a storm water MS4 permit will be signed by the Deputy Storm Water Manager and awarded to the applicant.

For a flow chart of the process click here

What else will I need?

The following local, state or federal agencies may have additional requirements related to Stormwater permitting, which may apply to your proposed development.

South Carolina Department of Health & Environmental Control – Office of Coastal Resource Management (OCRM)

(843) 626-7217

US Army Corps of Engineers

(843) 365-4239

Bioretention

Bioretention areas that store and treat runoff are filled with a soil mixture that supports water-tolerant plants. These devices take advantage of natural processes in soils and plants to remove pollutants from stormwater runoff through physical filtration, chemical adsorption, and biological decomposition. The end result is an attractive, landscaped area that cleans and retains stormwater runoff.

NCDENR Stormwater BMP Manual

For information about bioretention sizing, underdrain design, soil media, construction and maintenance, visit NCSU’s Bioretention website.

For information from SC DHEC’s BMP manual, click here.

Enhanced Grass Swale

Enhanced grassed swales are gently sloping conveyance channels with a designed underdrain system to filter or infiltrate stormwater runoff as it flows through the channel. Slopes of less than 2 percent are necessary to avoid swale erosion. These are best used along roadways and parking areas in moderate or low density residential or commercial areas.

From Maryland Stormwater Design Manual (2009)

Rooftop Runoff Management

Rooftop runoff management includes the use of cisterns or rain barrels and roof gardens to detain stormwater runoff. Rain barrels and cisterns collect runoff from roofs and store it for household use, such as garden watering. A green roof uses vegetation to soak up and evapotranspire rainfall, much like a bioretention area, before it is discharged. Green roofs require extra engineering to support the weight of the gardens, but can be highly effective in removing pollutants.

From NCDENR Stormwater BMP Manual

NCSU Rainwater Harvesting Modeling Tool

Stormwater Wetlands

Stormwater wetlands are constructed to treat stormwater runoff by mimicking the physical, chemical, and biological functions of natural wetlands. Constructed wetlands consist of shallow pools that support wetland vegetation and wildlife. The natural processes remove pollutants from runoff before discharging to local streams and waterways.

From NCDENR Stormwater BMP Manual

Infiltration Trench

Infiltration trenches are devices that are designed to fill with stormwater runoff and let it soak into the surrounding soil. These devices allow the water to be filtered and allow biogeochemical processes in the soil to treat the water and remove pollutants. They are most effective for small drainage areas.

From NCDENR Stormwater BMP Manual

Click here for entry in SC DHEC's Stormwater BMP Manual.

Pervious Pavement

Pervious pavement differs from conventional concrete or asphalt by using air spaces to allow infiltration and storage of stormwater runoff. Conventional pavement generates significant stormwater runoff. Pervious pavement can often reduce the size needed for stormwater detention elsewhere on a developed parcel. Pervious pavement is typically used in low-traffic areas such as parking stalls, emergency lanes, bike paths and sidewalks.

Horry County Pervious Parking Stalls in Garden City Beach

Technical Resources
For background and research on pervious pavement, visit NCSU's Biological and Agricultural Engineering pages .


Click here for the entry in SC DHEC's Stormwater BMP Manual.

Filter Strip / Level Spreader

Filter strips are vegetated buffers through which diffused stormwater runoff is filtered. The system includes a forebay to dissipate concentrated stormwater runoff, a channel with a level spreader to diffuse the flow, and vegetated buffer or filter strip through which the diffused runoff flows before it enters a waterway or ditch. The buffer is intended to remove sediment and pollutants before it reaches the waterway.

Filter strips are vegetated buffers through which diffused stormwater runoff is filtered. The system includes a forebay to dissipate concentrated stormwater runoff, a channel with a level spreader to diffuse the flow, and vegetated buffer or filter strip through which the diffused runoff flows before it enters a waterway or ditch. The buffer is intended to remove sediment and pollutants before it reaches the waterway.

From Low Impact Development: A Guidebook for North Carolina (2009)

LID Construction

Construction Planning

Construction Sequence

LID Maintenance

Maintenance for LID devices, just like any infrastructure, should be anticipated. Ideally, a maintenance plan for each LID device should be developed and followed.

The following tips may be useful:

• For the first year after installation, inspections should be conducted monthly and after major storms. Check for sinkholes, erosion, washouts, or excessive ponding.
• Bioretention areas should be kept free of weeds.
• Stormwater wetlands may need to be treated to remove unwanted invasive species such as cattails.
• An adequate layer of mulch (2-3 inches) should be maintained.
• Clogged soils must be replaced to restore function to the device.
• Pruning of plants should be done as needed to maintain proper infiltration.
• Do not add additional fertilizer. Stormwater runoff and compost in the soils should provide adequate nutrients for plantings.

LID Manuals

Design Manuals are helpful in site planning and implementation of specific devices. Below is a list of design guidance manuals from this region:

• South Carolina Coastal Low Impact Development Manual
• South Carolina Stormwater Best Management Practices Handbook
• North Carolina Low Impact Development Guidebook
• Georgia Stormwater Management Manual and Coastal Stormwater Supplement
• Low Impact Development Strategies - Prince Georges County, Maryland
• LID for Big Box Retailers

LID Cost Analysis vs. Conventional Development

A common question about low impact development is, "What does it cost?"
Compared to conventional development techniques, a low impact development design can often cost equal to or even significantly less than conventional development due to savings on infrastructure costs. Also, because LID designs frequently incorporate open and green space, sale prices in LID residential developments are often higher due to demand for the neighborhood features offered by this type of residential setting.
Below are some resources analyzing the costs of low impact development design relative to conventional development:

• Cost-Benefit Resources for LID
• Reducing Stormwater Costs through LID - EPA report

LID Design Tools and Calculators

There are many design tools and calculators that can assist in the design of low impact development devices and project planning. Below are a few:

• Urban Design Tools
• Green Values Stormwater Toolbox - National Stormwater Management Calculator
• North Carolina State University - Rainwater Harvesting Model
• North Carolina LID BMP Spreadsheet Tool

LID Design

LID Design Principles

Treatment Train

• Low Impact Development in South Carolina : A Planning and Design Guide
• LID Practices for Stormwater Management - National Assoc. of Homebuilders Research Center
• The Practice of Low Impact Development - U.S. Dept. of Housing and Urban Development
• Chapters 2-3 of Low Impact Development Design Strategies - Prince Georges County, Maryland

For SC DHEC's stormwater permit coverage applications and forms for construction activities, click here.

The Plan Package

The Storm Water Report

A Stormwater Report must be submitted with the Notice of Intent permit application to document compliance with the Stormwater Management Standards. The following checklist is NOT a substitute for the Stormwater Report (which should provide more substantive and detailed information) but is offered here as a tool to help the applicant organize their Stormwater Management documentation for their Report and for the reviewer to assess this information in a consistent format. As noted in the Checklist, the Stormwater Report must contain the engineering computations and supporting information set forth in Horry County Storm Water Manual. The Stormwater Report must be prepared and certified by a Registered Professional Engineer (RPE) licensed in the state of South Carolina. The Stormwater Report must include:

Summary table which includes:

• Peak inflows, peak outflows, and max pond water surface elevations for the 2, 10, 25 and 100yr storms.

• A brief narrative describing stormwater management practices.
• All elevations must be based on NAVD 88.
• NRCS soil identifications and characteristics
• Describe pre- and post-conditions and any flooding issues
• Describe topographic and soil conditions
• Upstream and down stream conditions (known flooding issues)
• Existing tail water elevations and any assumptions
• An analysis and details for any streams that may be affected or within ½ mile radius of a water body listed on the 303D DHEC list or if a TMDL is established. If the project does not fall into these criteria then this shall be clearly stated in the narrative.
• A general description of adjacent properties and existing structures located on surrounding properties.
• Site acreage also pond area in (Ac Ft)
• Narrative of 10% downstream analysis. Include pinch points and outfall conditions determining the effects from the project and any potential flooding.
• Anticipated start and completion dates.
• If using a feature which is sensitive to water table elevations state the water table elevation and the method of determination.

Maps included are:

• FEMA and Flood Zones
• Project location
• USGS quadrangle sheet (show project location, route runoff from site to nearest water body)
• Wetland permit or certification letter if required
• Map of 10% down stream analysis map with points of interest.
• Map to show off site drainage through or onto the site

Supporting documents included are:

• Lake maintenance agreement For a blank copy click Here
• Off site temporary easement agreements for outfalls (may not be required)
• Adjacent property owner’s agreement for new point discharge onto adjacent property that was not previously natural (may not be required)
• If an infiltration system used: Geotechnical report with infiltration rates

Site Hydrology:

• Size of Areas (Acreage)
• Post-development is less than pre-development by 20%  (for projects over 5 acres) Note: Post- vs pre-discharge rates are based on point discharges and not accumulative site discharge.
• CN designations calculated avg. or designation
• Soils information (type, special characteristics, perk rate if a perk system is used, water table if dry pond is used)
• Proper rainfall intensities used for 2, 10, 25, 100 yr storms – Use NOAA 14 charts (Click Here)
• Correct Peaking factors used (256 pre, and 323 post)
• Volume Hydrographs for SCS 24 hr
• 10, 25, 100 yr peak inflow/outflow and max water elevation of ponds (note on drawing as well)
• Drainage nodes
• Provide Basin and node map
• Design calculations for culverts and storm sewers
• Design calculations, cross sections and method of stabilization of existing and planned channels (include temporary linings) 10 yr storm velocities of 5fps
• Design calculations and construction details of energy dissipaters below culverts and storm sewer outlets (for rip-rap aprons, include stone sizes (diameters) and apron dimensions)
• Tail water conditions (elevations of outfall ditch match and tail water condition should reflect an existing 25 yr event in the ditch if using a static condition)
• Water Quality Calculations for sediment basins and/or other construction site BMPs
• Water Quality Calculations for post-construction BMPs
• 10% downstream analysis

The Stormwater Management Report shall document compliance with each of the Stormwater Management Standards as provided in the Horry County Stormwater Manual and Horry County ordinances.

NOI Information

This form can be filled out online at the DHEC website by following this link. A mapping tool for assisting with NOI completion may be found here. Please submit a copy of this form along with the other documents as part of the Plans Package.

Also, complete and submit the DHEC Plan Review Checklist when submitting the plan package.

For a list of SC DHEC Stormwater Design Aids and Technical Documents, click here.

Storm Water Pollution Prevention Plan (SWPPP)

A Storm Water Pollution Prevention Plan (SWPPP) identifies structural and non-structural controls that will be put in place to minimize negative impacts, caused by offsite storm water discharges, to the environment. The purpose of these controls is to minimize erosion and run-off of pollutants and sediment. A SWPPP establishes procedures for minimizing the potential for pollutants to be carried away in storm water discharges. These procedures emphasize the use of Best Management Practices (BMPs) to provide the flexibility to address varying sources of pollutants. A SWPPP must describe the site characteristics and list the pollutants that could impact storm water quality. The plan must also identify pollution prevention measures designed to minimize the discharge of pollutant laden storm water. For a SWPPP template from SC DHEC, click here. The SWPPP is required to be onsite or readily accessible upon request. The Inspector will request this document during inspections to verify it is current. The SWPPP must include:

• Narrative
• Stormwater Management and Sediment Control
• Reporting forms and logs of inspections
• Sequence of construction
• Site features and sensitive areas (Buffer zones)
• Sources of pollution
• BMPs
• Maps
• Engineering report
• Construction site plans
• Non-Numeric effluent limits
• Management of non-stormwater discharges
• (TMDL) Documentation of permit eligibility related to Total Maximum Daily Loads

Permanent Maintenance Covenants for Stormwater Practices

Permanent maintenance covenants for stormwater practices are to be recorded among the land records at the Horry County Register of Deeds. The document outlines the responsibilities of the property owner to properly maintain the stormwater practices. The document also allows the county permission to inspect the stormwater facilities and to communicate those findings to the property owner. The document also grants the county the opportunity to take necessary actions to correct any deficiencies in the event that the property owner fails to take such actions.

Preliminary / Final Plat

Preliminary / Final Plat is a plat with easements and legal boundaries. These plans include the following:

• All Easements labeled Public or Private
• Proper easement widths (check for both pipe and ditches)
• Pond maintenance easement
• Note: “All drainage easements shall be cleared and remain free of structures and obstructions.”
• Engineer's or Surveyor's Seal
• Wetland limits
• Ponds shall be located in Common areas rather than on individual lots in residential areas

Additional information may be required by the Stormwater Department as deemed necessary.

Grading And Drainage Plan

Site Plans (General observations)

• Engineers Seal
• Can each phase stand on its own hydraulically if no other phase is built?
• Size of Areas (Acreage)
• Adjoining lakes, streams, or other major drainage ways
• Existing and planned drainage patterns (include off-site areas that drain through project)
• Include Landscape Plan and check if it is encroaching on an easement.
• Note: "The land disturbing activity will be accomplished pursuant to the concept plan and the county has the right to conduct on-site inspections."
• Note: Final inspections will not be conducted until a final as-built has been submitted to the county.
• Show average bottom elevation, normal water level, 25-yr and 100-yr water level, top of bank elevation, area of storage in Acre-Feet on all ponds
• Average depth of pond
• Utility lines
• Directional arrows should be shown on the plan for each planned lot in the subdivision to show the drainage direction
• Trash racks and safety grates on weir structures
• Existing and proposed contours 25 feet outside of all property lines and further if county deems necessary for clarity.
• Show drainage flow patterns for sheet flow
• Show spot elevations for swales every 100 feet
• First floor elevation of proposed buildings
• Show all drainage lines with size, length, type, slope and invert elevations
• Show all catch basins and junction boxes with rim and invert elevations

• North arrow
• Scale (not to exceed 1:100)
• Property lines
• Legend
• Planned and existing buildings location and elevations
• Planned and existing roads location and elevations
• Lot and/or building numbers
• Limits and acreage of disturbed area
• Streams, lakes, ponds, drainage ways, dams

• Show Pipe detail
• Show catch basin detail (frames fully supported on box wall or structural top.
• Provide detail of pond
• Outfall structure details with dimensions and design elevations
• Swale detail
• Ditch detail
• Note: All Structures are to meet SCDOT standards
• Emergency spillway detail

• Profiles provided for the 25-yr HGL (starting tail water at 25-yr elevation and grades stay below the inlet grate)
• Provide pipe data (Slope, Type, Size, Length)
• Catch basin data (grate elevation, sump elevation, size of box, type of box, pipe inverts)
• Existing and proposed surface grade
• Length of pipes not to exceed 400 ft
• Provide profiles to show tops and toes for all ditches over 1 ft in depth

Additional information may be required by the Stormwater Department as deemed necessary.

Sediment and Erosion Control Plan

Erosion Control Notes

• Standard Notes (OCRM) For a copy click here. 
• Construction sequence related to sedimentation and erosion control
• Maintenance requirements during construction
• Person responsible for maintenance during construction and where records are kept
• Maintenance schedule, description and procedures for permanent structures
• As-built to be supplied to the county prior to final inspection (and certified by a either a PE or PLS in South Carolina)
• Must have the statement:” The land disturbing activity will be accomplished pursuant to the Stormwater Management Plan and the County has the right to conduct on-site inspections.”

Erosion Control Details

• Location of temporary and permanent measures
• Concrete wash out detail (locate the washout on the plans)
• Construction drawings and details for temporary and permanent measures
• Silt fence detail
• Construction entrance
• Wetland crossing details if used
• Provide details for dewatering onsite (ponds, inlets, and ditches)
• Provide Individual lot erosion control plan/detail
• Show spillway detail
• Stockpiled topsoil or subsoil location 

Vegetative stabilization details

• Areas and acreage to be stabilized
• Requirements for temporary ground cover
• Planned vegetation with details of plants, seed, mulch, fertilizer
• Rates for seed, fertilizer and mulch
• Specifications for permanent and temporary vegetation
• Method of soil preparation

Additional information may be required by the Stormwater Department as deemed necessary.

Landscape Plan

• No landscaping or berms to be located in drainage easements
• No landscaping in swales
• Berms are to have a slope of 3:1 and be stabilized with vegetation

Additional information may be required by the Stormwater Department as deemed necessary.

Preliminary As-Built Plan Requirements

Prior to completing the installation of the asphalt and curbing and prior to a proof roll inspection scheduling a preliminary "as-built" plan signed and sealed by a professional registered in South Carolina shall be submitted to the Horry County Engineering Department for review and approval. The purpose of this plan is to ensure the pipe and catch basin structures are located in the proper place as well as installed at the proper elevations to avoid disturbance of a final finished product in the asphalt and curbing. In the as-built plan the registered professional shall state: 

• The facilities have been constructed as shown on the Preliminary As-built Plan, and 
• The facilities meet the approved Stormwater Management and Sediment Control Plan and specifications or achieve the function for which they were designed. 

Also, the minimum information to be provided on the Preliminary As-built Plans shall include the following:

• Boundary, phase and lot lines. 
• Lot numbers and street names.
• Drainage structures with elevations.
• Drainage pipes with size, material, length, slope and invert elevations.
• Ponds or lakes with average bottom, top of bank and water surface elevations.
• Acre-feet of storage provided

Additional information may be required by the Stormwater Department as deemed necessary.

As-Built Plan Requirements

Upon completing the installation of the stormwater management facilities included in the Stormwater Management and Sediment Control Plan, an "as-built" plan signed and sealed by a professional registered in South Carolina shall be submitted to the Horry County Engineering Department for review and approval. The as-built plan must be submitted prior to any final inspection scheduling. In the as-built plan the registered professional shall state: 

• The facilities have been constructed as shown on the "as-built" plan, and 
• The facilities meet the approved Stormwater Management and Sediment Control Plan and specifications or achieve the function for which they were designed. 

Also, the minimum information to be provided on the "as-built" plans shall include the following:

• Boundary, phase and lot lines. 
• Lot numbers and street names.
• Easements.
• Road locations with centerline stationing and curve data.
• Road centerline elevations at 100-foot intervals.
• Drainage structures with elevations.
• Drainage pipes with size, material, length, slope and invert elevations.
• Ponds or lakes with average bottom, top of bank and water surface elevations.
• 25-year and 100-year storm elevation of pond
• The Acre-feet of storage in a 25-year rain event
• Any control structures shall be shown in detail with key-way dimensions and stage elevations. If structure is off site include a note of its location. 
• Drainage ditches and swales, with elevations at 100-foot intervals and critical locations.
• Recommended maintenance plan and schedule for drainage system components.

Additional information may be required by the County Engineer as deemed necessary to adequately document the condition of the road and drainage systems.

Stormwater Master Plan

• Master subdivision or site layout showing buildings, roads, lots, parking areas, utilities, and grassed or landscaped areas
• Vicinity map
• Master Stormwater Report

Stormwater Facility Maintenance