9-2 | Aquabella Subdivision - Stormwater Engineering Responses

ATTACHMENT 2

M E M O R A N D U M

TO : George Small, P.E., Director of Engineering

FROM : Sue Burke, P.E., CFM, Stormwater Management Engineer

SUBJECT : Aquabella Subdivision – Application for Preliminary Plat Approval

Responses to Stormwater Issues from September 18, 2006 Public Hearing

DATE : September 28, 2006

The purpose of this staff report is to provide responses to questions and issues posed by Council members at the September 18^th Public Hearing for the Aquabella Subdivision’s Application for Preliminary Plat Approval.

The three issues associated with stormwater management are:

The location of the proposed bio-retention basins in the public right-of-way
The application of Low Impact Design (LID) principles to stormwater management
The efficacy of bio-retention basins in general and, specifically, of the structures proposed by the applicant.

Right-of-Way and Resource Conservation District location

The primary reasons that staff cannot support installation of private or non-essential facilities into/onto public rights-of-way are:

The Town regulates and maintains public right-of-way property. Rights-of-way are established and reserved to (1) allow placement of facilities that benefit and support public health, safety and welfare (such as streets and utility service lines) and (2) to restrict/regulate placement of facilities that can be placed elsewhere, that are non-essential, and/or that primarily benefit private property owners. Facilities that are allowed in public rights-of-way are maintained by the Town or by facility owners via formal franchise or encroachment agreements that include specific stipulations protecting the Town.
Public rights-of-way must remain open and available for future public improvements that may become necessary. Non-essential encroachments into/onto public rights-of-way could impede future use of the right-of-way for public benefit.
The Town is responsible for maintaining and managing public rights-of-way in a manner that reasonably protects public safety. The installation of non-essential facilities into/onto public rights-of-way would unnecessarily increase the Town’s liability exposure.
The installation of non-essential facilities on public rights-of-way would set a precedent for future encroachments, potentially exacerbating problems associated with the previously noted right-of-way considerations.

The proposed installation of a bio-retention area intended to serve private development of adjacent land into/onto public rights-of-way would create problems for the Town for all of the reasons noted above.

The bio-retention basins are proposed in the Pinehurst Drive public rights-of-way. As such, the application fails to meet the requirements for stormwater rate and volume control. Paragraphs (b) and (c) of Section 5.4.6 of the Land Use Management Ordinance require the “stormwater runoff volume leaving the site post-development shall not exceed the stormwater runoff volume leaving the site pre-development…” and the “stormwater runoff rate leaving the site post-development shall not exceed the stormwater runoff rate leaving the site pre-development…” (emphasis added). The bio-retention basins are proposed to be located offsite in the rights-of-way and therefore the plan does not meet the Town’s stormwater management criteria.

Additionally, the proposed sites would fall within the Resource Conservation District streamside zone. While the RCD is an overlay zoning district and as such is not applicable on public rights-of-way, it must still be shown on development plans.

The streamside zone functions as a buffer for the intermittent stream that crosses the property. This stream buffer appears to be in good condition and stable (please see photo below).

As can be seen in the photo below, the northern bio-retention basin cannot be constructed without grading and associated land disturbance necessary to remove a rise that runs parallel to the sidewalk. The full extent of land disturbing activity directly related to construction of the proposed bio-retention basins is not clear based on the information provided by the applicant.

The streamside zone also functions as a natural floodplain for an existing intermittent stream crossing this property. During rain events that exceed the one-inch capacity of the bio-retention basins and overflow the basins, the possibility exists that sediment could be resuspended and transported downstream, along with the grass clippings and other vegetative debris, resulting in blockages of the downstream conveyance system.

Site of north bio-retention basin. Photo taken from north corner of property along Pinehurst Dr., looking south.

Application of Low Impact Design (LID) principles to stormwater management

A Council member asked if the Town’s Land Use Management Ordinance incorporated Low Impact Design principles for stormwater management.

The stormwater management requirements are contained in Section 5.4 of the Town’s Land Use Management Ordinance. Section 5.4.7, ‘Integrated Management Practices’, states that “Applicants shall utilize Integrated Management Practices/Best Management Practices to meet the standards established in Section 5.4.6, using one or more approved design options. Low Impact Design options are encouraged” (emphasis added). Similar language is contained in Section 2.4 of the Town’s Design Manual.

“Integrated Management Practices” is a term that generally refers to the Best Management Practices used for stormwater management in a Low Impact Design project. Low Impact Design techniques are conservation-oriented and help to minimize and negate the impacts of increased impervious area on the environment. The stormwater goals of Low Impact Design are to maintain groundwater recharge and quality; reduce stormwater pollutant loadings; protect stream channels; and prevent increased flooding. This can be accomplished by providing multi-functional stormwater controls that are decentralized and disconnected to the extent practicable.

Some examples of structural Low Impact Design Best Management Practices (or Integrated Management Practices) are bio-retention basins, infiltration structures, filter strips, permeable pavement, and green roofs.

Bio-retention basins

There were several questions about the proposed bio-retention basins. One Council member asked staff to comment on the efficacy of the proposed stormwater treatment. Other Council members asked for more information about bio-retention basins in general.

Bio-retention basins, also referred to as rain gardens, are shallow depressed areas constructed with a specific soil mixtures and plant materials suitable for receiving stormwater runoff. As noted in the North Carolina Department of Environment and Natural Resources’ Best Management Practices Manual, bio-retention basins are effective in removing pollutants such as suspended solids, heavy metals, and nutrients.

Design criteria for bio-retention basins are contained in both the Town’s Design Manual and the North Carolina Department of Environment and Natural Resources’ Best Management Practices Manual. A summary of the design criteria is included below.

Size	5 to 7% of the contributing drainage area, depending on the amount of impervious area
Facility depth	2 to 4 feet
Ponding depth	9 inches
Minimum soil permeability at bottom	Greater than 0.5 in/hour (generally soils that are categorized as Hydrologic Soil Group A or B). An underdrain must be installed for soils in Hydrologic Soil Group C or D.
Planting mixture	2 to 4 feet of sandy loam material
Pre-treatment	Forebay or sheet flow. Avoid concentrated flow.
Plantings	Diversity of planting materials to include trees, shrubs, grasses, and other herbaceous plants
Top layer	3 to 4 inches of coarse hardwood mulch. Grass clippings and pine chips are not suitable.

Bio-retention basins may be sized to provide volume control and rate control if soils are suitable for infiltration.

Photos of several bio-retention basins at University Mall in Chapel Hill and in other North Carolina jurisdictions are included as Exhibit A. More detailed information about bio-retention basins and photos are included as Exhibit B (North Carolina Department of Environment and Natural Resources’ Best Management Practices Manual) and Exhibit C (Puget Sound Action Team’s Low Impact Development Technical Guidance Manual for Puget Sound).

Bio-retention Basin Proposed by the Applicant

Two bio-retention basins, using modified design criteria, have been proposed for this site. The applicant was unable to provide specific design and performance data for the proposed basins because the design is currently being tested by North Carolina State University staff at a site in Graham, NC. Because performance data is not available at this time, we are unable to determine whether the average annual 85% Total Suspended Solids water quality requirement could or would be met by the proposed facilities. Performance data is expected to be published by the NCSU researchers later this year.

Key differences between the proposed design and established designs include the lack of planting materials other than turf and the lack of a mulched top layer. Other concerns from our perspective include:

Flow will be concentrated via two flumes from the new street.
The soils at this location are in Hydrologic Soil Group D and are not suitable for infiltration.
The applicant has not explained how the stormwater rate and volume requirements will be met. In his presentation, the applicant stated that runoff from the first one-inch of rainfall would pond in the bio-retention basin to a maximum depth of nine inches; anything more would overflow the berm.

In conclusion, a bio-retention basin designed using established criteria is an excellent stormwater management practice. It is not known if the modified design proposed by the applicant will meet the Town’s stormwater management requirements.

The success or failure of any stormwater management structure is dependent on proper selection of structures that are appropriate for the site. The selection process and design must consider and evaluate a host of factors such as soil types, slopes, land cover, degree of imperviousness, hydrology, and hydraulics. As noted at the Hearing, we have to be smart enough to recognize that one size does not fit all.

Exhibits: Exhibit A – Bio-retention photos

Exhibit B – Bio-retention section from the North Carolina Department of Environment and Natural Resources’ Best Management Practices Manual

Exhibit C – Bioretention section excerpt from the First Puget Sound Action Team’s Low Impact Development Technical Guidance Manual for Puget Sound