AGENDA #5d
MEMORANDUM
TO: Mayor and Town Council
FROM: W. Calvin Horton, Town Manager
SUBJECT: Response to Petition Regarding Stormwater Management and Mosquitoes
DATE: November 24 , 2003
On October 8, 2003, a Council member petitioned the Council to ask the Town Manager for comments about stormwater management facilities and mosquito habitats.
PETITION
A Council member suggested that the staff look at Town regulations for impounding water as new development is occurring, in response to growing concerns about mosquito habitats. One of the concerns focused on provisions in Chapel Hill’s Land Use Management Ordinance that the rate and volume of stormwater runoff from new development be controlled, often requiring developers to install stormwater detention or stormwater retention facilities on a development site. The petition was referred to the Town Manager for comment.
ORDINANCE REQUIREMENTS
The Council enacted a new Land Use Management Ordinance in January, 2003, that contains new stormwater management requirements. Prior to this new ordinance Town regulations required that, for new development other than single-family and two-family residences, stormwater detention devices be installed such that the rate of runoff post-development would not exceed the rate of runoff in pre-development conditions. The new ordinance tightened those regulations in two major ways. First, the requirement was expanded to state that volume of water leaving a site post-development could not exceed pre-development conditions. This means that water must be held on a site for infiltration into the ground, for re-use in irrigation or other purposes, or for evaporation. The second major shift was to apply these requirements to most new development, including new single-family residences.
MOSQUITO CONTROL
Chapel Hill’s approach to mosquito control in general is as follows:
We have a modest outreach public education program to solicit public involvement in reducing mosquito breeding habitats and a modest program to treat and/or address breeding sites on property owned by the Town. Our existing web page explains our current program and services and provides information and links to other sites that inform the general public about action they may take to reduce breeding habitats. The address of the website is:
http://townhall.townofchapelhill.org/publicworks/mosquito_control.htm
The majority of mosquito breeding habitats on property owned by the Town is associated with open space/wetlands (i.e., along Bolin Creek/Little Creek between Glen Lennox and Ridgefield). Such habitats are very flat and slow to drain. In the past, a common approach to address such problems in low lands was to pipe and channel the drainage system in order to dry up the swampy areas, thus eliminating the wetlands. We now have a better understanding of the deleterious effects of such an approach. We generally limit our work to removing obstructions created by deadfalls or storm-borne debris; breaching beaver dams; and using an environmentally friendly biological larvicide to destroy the mosquito larvae. Under the law, we must limit this service to public lands and rights-of-way. We also provide periodic maintenance of drainage ditches that are a part of the street system, and respond to service requests for problem correction. Because of the community benefit, staff members are available to offer advice to private property owners about how to address standing water problems on private property.
STORMWATER MANAGEMENT
Detention basins are commonly used to control the rate at which stormwater leaves a property. These facilities are designed to hold water for slow release, and generally drain out completely within 72 hours. We believe that there should be no mosquito issues associated with detention basins that are properly designed and properly maintained.
Retention basins are designed to hold water for long periods of time. We note that State regulations require retention in water supply watershed areas, whenever impervious surface on a newly developing site would exceed 24% of the area of the site. In addition, new provisions in the Land Use Management Ordinance require most new development to control for volume of runoff, and the solution for controlling volume often involves holding water on a site. There are several design approaches to retention:
- Deep pool, to allow for water column mixing and higher oxygen content, creating a favorable habitat for fish (natural predator of mosquitoes).
- Installation of aquatic plants (submerged, emergent and basin fringe plants) that proved cover and nesting opportunities for dragonflies and birds (natural predators of mosquitoes).
- Water circulating/aerating systems that prevent stagnant water. (Mosquito larvae require the presence of stagnant, shallow water for a minimum of 72 hours to become adults.)
These standards are being included in the new Design Manual that is currently being prepared for the Council’s consideration early in 2004.
Prior to construction of any detention basin, retention basin, or bio-retention basin, an Operation and Maintenance Plan must be prepared and approved by the Town Manager.
SUMMARY
We believe that Chapel Hill’s stormwater management regulations are an important part of the Town’s growth management systems, and help assure that downstream properties will not be negatively impacted by stormwater runoff from upstream properties. We believe that, in the case of retention facilities, proper design can minimize the incidence of creating new mosquito habitats.