(a)

The purpose of this article is to establish minimum stormwater management regulations to protect the health safety and welfare of the general public residing in the town and to protect the natural marine eco-systems. This ordinance seeks to meet that purpose to the maximum extent possible through the following objectives:

(1)

Minimize increase in stormwater runoff peak discharges, volumes and velocities from new land development and re-development;

(2)

Minimize increase in non-point source pollutant loading caused by stormwater runoff from new development;

(3)

Protect and promote healthy habitats, nurseries and other natural ecosystems such as coastal wetlands, riparian areas and groundwater recharge zones;

(4)

Maintain or replicate predevelopment natural speed and volume of runoff for new land development;

(5)

Ensure adequate design, construction and maintenance of stormwater management facilities to meet this purpose while still protecting the character and style unique to the Town of Manteo;

(6)

Promote public education, a multi-objective purpose and stakeholder participation to meet these objectives; and

(7)

Protect marine recreation areas from pollutants associated with stormwater runoff or discharges from floating structures.

(b)

These regulations are set forth so as to:

(1)

Maintain or enhance the public health, safety, and welfare;

(2)

Maintain or enhance the value of adjacent property or be a public necessity;

(3)

Comply with the general intent of the respective zoning district for the physical development of the area; and

(4)

Contribute to the traditional village-like atmosphere of the historic town.

(Ord. of 9-14-2005, § 17.1)

This ordinance shall apply to all new development requiring a site plan review within the town's planning jurisdiction, except where such new development is exempt pursuant to one or more of the following criteria:

(1)

A development or redevelopment proposes to disturb less than 5,000 square feet of land area and will not include any future phased development or any other adjacent or phased development;

(2)

The development is a single-family residence with customary accessory structures including accessory dwelling units, and is not a subdivision of record; and

(3)

B-1 historic village district.

(Ord. of 9-14-2005, § 17.2)

(a)

All non-stormwater connections to the stormwater system are prohibited.

(b)

Non-stormwater connections generally include but are not limited to sanitary sewer connections, floor drains, commercial car washes, or sump pump connections that discharge non-stormwater waste and materials.

(c)

The following activities may discharge into the stormwater system:

(1)

Street washing;

(2)

Irrigation;

(3)

Non-commercial car washing;

(4)

Firefighting discharges;

(5)

Water line flushes.

(Ord. of 9-14-2005, § 17.3)

(a)

Criteria, design information and/or standard details for proper implementation of this ordinance, specifying acceptable stormwater management facilities (best management practices), design criteria, materials used and maintenance to be preformed are described herein.

(b)

The town may, but is not required to, accept proposed stormwater management facility design and standard details on a cases by case basis that are based on generally accepted engineering practices in the State of North Carolina Department of Environment and Natural Resources, Division of Water Quality, Best Management Practices Design Manual.

(c)

The Town of Manteo Stormwater Management Design and/or standard details may be updated periodically at the discretion of the town, based on experience, new science and technology, monitoring and maintenance considerations.

(d)

All facilities will be designed to meet the minimum performance and maintenance standards set forth in the ordinance and as required by all other applicable local, state and federal regulations.

(Ord. of 9-14-2005, § 17.4)

(a)

Appropriate stormwater management facilities shall be used for all new developments and redevelopments in order that stormwater runoff on one parcel of land must enter adjacent property with the same characteristics of the predevelopment hydrology.

(b)

Stormwater management plans for all developments must meet the following criteria:

(1)

The vegetation in all stormwater areas must conform to the design guide and/or standard details, and compliment adjacent property landscaping.

(2)

A contiguous 100-foot buffer must be maintained with a maximum slope of no more than two percent. The buffer will extend from the property boundary at minimum distance of 100 feet measured horizontally on a line perpendicular to the property boundary.

(3)

Development may occur with in this buffer as long as stormwater is managed on site.

(4)

Use drainage and hydrology as a design element: The basic concept for stormwater management in utilizing LID is to design, implement and manage so that the volume and rate of stormwater movement from a proposed site will be the same both before and after project implementation.

(c)

For the town, LID stormwater management is an opportunity to create public spaces and add gardens that fit in with the Manteo style while managing stormwater generated on site.

(d)

The use of LID techniques provides the opportunity to use materials such as native plants, soil and gravel which allows these systems to be more easily integrated into the landscape. Small, distributed, at the source controls also offer a major technical advantage: one or more of the systems can fail without undermining the overall integrity of the site control strategy.

(e)

Land in a predeveloped scenario contains a variety of vegetation. Some of the stormwater runoff can return to the underground aquifers through infiltration, some may travel slowly through vegetation to a nearby intermittent stream, and the trees on the site take up some of that stormwater and disperse it through evapotranspiration.

(f)

In a LID development site, the stormwater will be managed using tools that mimic the natural hydrologic functions on the site which involves both a physical and time expanded dispersal of stormwater when compared to standard stormwater management of disposing of stormwater as quickly as possible and collecting it and disposing of it in a centralized method. In the low impact development scenario (below), the function of the LID tools is to manage the stormwater on the developed site where it was generated while at the same time adding useful features to the human landscape. Pervious paving allows infiltration of stormwater. The swale helps control the direction of the flow of stormwater along with infiltration. The cistern can be located near a building to capture stormwater runoff from its rooftop and hold that water for later use. A bioretention area can be a garden space designed to temporarily store stormwater. The temporary storage is a designated space to hold stormwater until it can evaporate and infiltrate gradually. The trees on the site will take up some of the water and some will be released during the evapotranspiration process. All the LID elements work together on the site to manage the stormwater on site.

(Ord. of 9-14-2005, § 17.5)

(a)

Rain barrels and cisterns are low-cost retention devices that can be used for residential and commercial/industrial LID sites. Rain barrels retain a predetermined volume of rooftop runoff and provide permanent storage for water reuse on lawns and gardens. Any type of roofing material can be directed to rain barrels. Gutters and downspouts are used to convey water from rooftops to rain barrels. Filtration screens should be used on gutters to prevent clogging of debris. They should also be equipped with an overflow outlet to bypass runoff from large storm events and a drain spigot that has garden hose threading, suitable for connection to a drip irrigation system. They must be designed with removable, child-resistant covers and mosquito screening on water entry holes.

(1)

Sizing: The size of the rain barrel is a function of the rooftop surface area that will drain into the barrel and the inches of rainfall to be stored.

(2)

Uses: On-lot storage with later reuse of stormwater also provides an opportunity for water conservation and the possibility of reducing water utility costs. Can be used on residential, commercial, and industrial LID sites.

(3)

Design considerations: Locate beneath each downspout, or storage volume can be provided in one large container. Cisterns should be located for easy maintenance or replacement.

(b)

Porous pavement systems: Note: Minimum/maximum dimensions and other specifications are product-specific and shall comply with manufacturer's recommendations.

(1)

General description: There are many types of porous pavement on the market today. Numerous products and design approaches are available, including special asphalt paving; manufactured products of concrete, plastic, and gravel; paving stones; and brick. Porous pavement accepts only precipitation, not stormwater runoff. It may be used for sidewalks, driveways, and parking lots.

(2)

Checklist of minimal information to be shown on the permit drawings:

• Additional information may be required on the drawings during permit review, depending on individual site conditions

• Facility dimensions and setbacks from property lines and structures

• Profile view of facility, including typical cross-sections with dimensions

• Porous pavement materials specification

• Subgrade and base course specifications

• Filter fabric specification (if applicable)

• All stormwater piping associated with the facility, including pipe materials, sizes, slopes, and invert elevations at every bend or connection

(c)

Vegetated swales: These systems are often used on highway designs. Designers can design these channels to optimize their performance with respect to the various hydrologic factors. Two types of vegetated swales are used for this purpose:

(1)

Dry swale-which provides both quantity (volume) and quality control by facilitating stormwater infiltration.

(2)

Wet swales-use maximum infiltration time and natural growth to reduce peak discharge and provide water quality treatment before discharge to a downstream location. The wet swale typically has water tolerant vegetation permanently growing in the retained body of water.

(d)

Level spreader: An outlet designed to convert concentrated runoff to sheet flow and dispense it uniformly across a slope to prevent erosion. One type of level spreader is a shallow trench filled with crushed stone. The lower edge of the level spreader must be exactly level if the spreader is to work properly.

(1)

Runoff in a thin layer (usually less than one inch in depth) over a wide surface, which begins when water ponded on the surface of the land becomes deep enough to overcome surface retention forces known as sheet flow or overland flow.

(2)

The receiving area of the outlet must be uniformly sloped and not susceptible to erosion. Particular care must be taken to construct the outlet lip completely level in a stable, undisturbed soil to avoid formation of rilling and channeling. Erosion-resistant matting might be necessary across the outlet lip, depending on expected flows. Alternative designs to minimize erosion potential include hardened structures, stiff grass hedges, and segmenting of discharge flows into a number of smaller, adjacent spreaders.

(3)

Uses.

a.

Convey sheet flow runoff from lawn areas within graded areas to bioretention facilities and transition areas.

b.

Deliver runoff from parking lots and other impervious areas to infiltration areas.

c.

Over well-vegetated areas, particularly lawns, to achieve additional retention and increase the time of concentration.

(e)

Filter strips: Low-grade vegetated areas that permit sediment deposition during sheet flow.

(1)

Usually used as one component of a stormwater management system, filter strips are considered pre-treatment devices; meaning water is routed through them before entering systems such as bioretention areas.

(2)

For the systems to be fully effective, slopes should be minimal (zero to two percent), with channelized flows eliminated.

(3)

Pollutant removal efficiency depends largely on the quantity of water treated, flow path and length, type of vegetation, and the soil infiltration rate.

(4)

Depending on the amount and type of vegetation planted and the need for replacement or amendment of soils, filter strips can be inexpensive to construct and maintain. (Source: The Practice of Low Impact Development, PDR/PATH, July 2003; p. 39)

(5)

Typically bands of close-growing vegetation, usually grass, planted between pollutant source areas and a downstream receiving water body.

(6)

Can also be used as outlet of pretreatment devices for other stormwater control practices. For LID sites, a filter strip should be viewed as only one component in a stormwater management system.

(f)

Bioretention: Management and treatment of stormwater runoff with the use of a conditioned planting soil bed and planting materials to filter runoff stored within a shallow depression.

(1)

Siting problems can be overcome if the issue of stormwater quality treatment is addressed at the very first stages of development planning. They are intended to address the spatial constraints that can be found in intensely developed urban areas where the drainage areas are highly impervious. They provide some nutrient uptake and ground water recharge in addition to physical filtration.

(2)

The runoff is infiltrated though a surface organic layer of mulch and/or a ground cover to the planting soil. The runoff is stored in the planting soil where it is discharged over a period of days to the in-situ material underlying the bioretention area or through an under drain.

(3)

In bioretention. Pollutant removal is performed through physical and biological treatment processes occurring in the plant and soil complex, including transpiration, evaporation, storage, and nutrient uptake. Excess runoff that cannot be infiltrated or ponded or lost through evapotranspiration is diverted away from the bioretention area. The diversion of excess runoff is accomplished by grading the bioretention area so that the ponded surface elevation is equal to the elevation that is discharging the water into the bioretention area.

(4)

The bioretention area components have complementary roles or functions to improve water quality. The major components are:

a.

Ponding area-over the root zone provides for some surface storage of the stormwater runoff, and provides for the evaporation of a portion of the runoff. Settling of the particulates that have not been filtered by the grass buffer or the sand bed occurs in the ponding area.

b.

Planting soil-provides the source of water and nutrients for the plants to sustain growth. The voids in the soil also provide for stormwater storage. Clay particles that compose a portion of the soil absorb heavy metals, nutrients, hydrocarbons, and other pollutants.

c.

Plant material-use nutrients and other pollutants and to remove water through evapotranspiration.

(g)

Stormwater wetlands: Stormwater wetlands primary goal is to maximize pollutant removal and create generic wetland habitat. They do not function or replace natural wetlands and are not located within delineated natural wetland areas, which are protected under state and federal statute.

(1)

Pocket wetlands are well adapted to serve smaller sites. Pocket wetlands must be used in combination with other BMPs.

(2)

Pocket wetlands differ in design from extended detention basins in that they have a smaller permanent pool of water, and a fore bay will not be required if the pocket wetland is being fed stormwater from a grassed swale or vegetated filter strip.

(3)

Collectively, sediment, plants, water and detritus remove multiple pollutants through a series of complementary physical, chemical and biological pathways. Sheetflow conditions across a wetland, slow velocities and the hydraulic resistance afforded by the wetland vegetation work together to provide very good conditions for particle settling and removing pollutants (i.e. phosphorus, trace metals, and hydrocarbons) that are adsorbed to the surfaces of suspended particles. The emergent plants that greatly characterize stormwater wetlands help to stabilize the sediments that settle and thereby reduce the amount of resuspension.

(4)

Pocket wetlands can be thought of as a small wet detention pond with special emphasis placed on the wetland plantings. The sizing process should follow that of the wet detention basin.

(h)

Planting plans for BMPs.

(1)

Species diversity-plant species should be selected based on a complex community concept with combinations of ground covers, herbaceous upright perennials, shrubs, vines and small trees.

(2)

Use of native plant species-typically native plants demonstrate a greater ability of adapting and tolerating physical, climatic, and biological stresses.

(3)

Plant species for BMPs should be selected based on their adaptations to the soils climate, and moisture regimes in which they are to be used. Plant species should not be invasive and should be compatible with the appearance of the streetscape and surrounds of Manteo.

(Ord. of 9-14-2005, § 17.6)

The following requires minimum runoff volume retention for stormwater management performance. To achieve this criterion, the use of best management practices (BMPs) such as Cisterns, Level Spreaders, Infiltration Trenches, Vegetated Swales, Wet Swales or Infiltration Swales are recommended. This criterion does not include any additional regulations as may be required by the North Carolina Environmental Management Commission or other State and Federal regulations.

(1)

Stormwater runoff volume generated by the new site development or redevelopment shall not exceed the predevelopment site volume for the first one and one-half inches of total rainfall depth and the net increase in the volume shall be infiltrated into the existing soils, detained in approved underground tanks or wells or otherwise reused on-site or at an approved location.

(Ord. of 9-14-2005, § 17.7)