Plan review ensures that sufficient post-construction structural controls will be implemented to protect water quality and mitigate the impacts of development on stream flows after construction is complete. Storm water Management regulations are found in several documents in the Unified Development Ordinance (UDO).
A. Development - Storm Water Management Strategy
Both water quantity and water quality must be addressed to protect the water quality of streams. Gwinnett County's storm water management strategy has four essential components.
- Flood Protection - Public Safety (100-year storm)
- Flood Protection - Property Protection (25-year storm)
- Channel Protection (1-year storm)
- Stream Quality Protection (TSS Criterion)
I. Flood Protection - Public Safety (100-year storm)
To address safety concerns caused by flooding, Gwinnett has restricted development in the 100-year flood plain since 1957. Floodplain protection was a part of our Zoning Resolution until a Floodplain Management Ordinance was passed in January 1988. The major change we made in 1999 was to regulate the future developed 100-year flood plain determined based on the county's future land use plan. We believed that the flood plain is a natural detention basin that will do a better job of attenuating 100-year peak flows than on-site detention ponds. Flood plain also does not have the maintenance, permitting or liability issues of a regional detention facility. The second part of this component is to size the bridges and culverts for our road system based on this future 100-year flow. We do not want the roads to block the flow and cause flooding of homes upstream of the road crossing. We also do not want the flow going over the road and either washing out the road or causing potential road safety hazards.
Communities that do more than FEMA's minimum flood protection requirements can participate in FEMA's Community Rating System (CRS) and obtain premium reductions for flood insurance policy holders. Gwinnett County's efforts have earned a rating of 9 instead of 10.
II. Flood Protection - Property Protection (25-year storm)
The second component of the storm water strategy has been part of Gwinnett County regulations since the early seventies. Gwinnett has required peak detention for the 2-year through the 25-year storm to protect property since the early 1970's and detention was a part of the County's Development Regulations passed in 1988.
III. Channel Protection (1-year storm)
The third essential component is the protection of the channel by detaining the 1-year storm volume for 24 hours. The 1-year storm flow is often considered the bank full flow and has the velocity and the volume of flow to define the channel shape. Keeping this flow to pre-developed conditions is critical to keeping the channel from eroding to accommodate the larger flow and saving the stream habitat.
IV. Stream Quality Protection (TSS Criterion)
The fourth storm water design component is the Total Suspended Solids (TSS) performance criteria. This criterion was a one of the major results of Gwinnett County's Watershed Assessment. The benthic score was determined to be one of the best overall indicators of the health of the stream. The quality of the benthic community was the best indicator of habitat quality, water quality and the quality of the fish population. The stream monitoring also showed that a relationship existed between benthic score and TSS. A lower TSS loading in the stream indicated a higher benthic score or better stream health. Conversely, a higher TSS loading indicated a lower benthic score and poorer stream health. After lengthy discussions in the Citizen's Advisory Group, they decided to set an in-stream TSS goal of 1600 pounds/acre/year at the bottom of the watershed.
The next step was to determine the allowable load off a new development up in the watershed. After analyzing the different loading rates and trying to balance the cost to development and cost to existing property owners to meet the goal, a loading rate from new development sites of 850 pounds/acre/year was selected. These discussions also recognized that it is much more costly to fix a problem than prevent the problem. It was decided that it is much less costly and less disruptive to residents for treatment BMPs to be constructed as part of the development. To implement the loading rate performance criteria, a spreadsheet was developed that required the determination of site land use and the water quality BMPs to be used. The land use was broken into four categories; undisturbed buffers, undisturbed pervious, disturbed pervious and impervious. The different land uses are assigned loading rates in the spreadsheet of 125, 500, 1200 and 4000 pounds/acre/year respectively. The designer then fills out the spreadsheet indicating how much area drains to different BMPs that are to be constructed on site. If the result of the analysis is less than 850 pounds/acre/year then the site meets the requirement.
B. Maintenance of BMPs
Maintenance requirements are important because if the BMPs are not maintained, then they will become ineffective over time. Best Management Practice facilities must be on individual lots with recorded access easements. We require mandatory property owner associations (POAs) with recorded maintenance agreements to insure that the POAs know that they are responsible for the maintenance of the facility and have instructions on how to maintain it. We have also started an inspection program of existing facilities. We know that without maintenance agreements and an inspection program, the facilities will probably not be properly maintained.
Addressing water quality and storm water quantity in areas that are already developed well be especially difficult and expensive. In addition to the master planning and capital improvement projects that are discussed above, individual projects will address these issues as projects come in for redevelopment. If more than 50% of the project site is redeveloped, then water quality and water quantify for the entire project area must be addressed. To determine how much detention is required, existing pavement areas are treated as forested areas when the pre-developed flows are calculated. When between 5% and 50% of the site is redeveloped, then detention and water quality requirements are applied only to the re-developed area. If less than 5% of the site is redeveloped, the water quality impact is considered de minimus. A hydrology study is required to determine if detention is required.
Another passive water quality BMP that Gwinnett strongly encourages is natural stream buffers. We encourage stream buffers by giving the loading rate of 125 pounds/acre/year to these areas. Since the target, loading rate for a site is 850 pounds/area/year and impervious areas have a loading rate of 4000 pounds/area/year, one acre of stream buffer "treats" 0.18 acres of impervious surface. The second method Gwinnett County uses to encourage keeping stream buffers is by requiring a stream buffer variance to disturb the buffer and by having very stringent criteria to justify a variance. The state has required a variance to disturb a stream buffer, but the weakness has been in defining what is a stream and then what justifies disturbing the buffer as discussed above. In March of 1999, Gwinnett County established a Buffer Variance Procedure. The essential part was a rebuttable presumption based on field data that a stream starts when the drainage area is 20 acres. If the site designer felt that the stream does not start until further downstream, then he/she could request that Gwinnett County make a field inspection. Likewise, if a field inspection shows that the stream started above the 20-acre assumption, then that field location would be used. To justify encroaching into a buffer, we required a three-step process similar to the Corps of Engineers wetland permitting process. First, the site designer had to prove that the buffer had been avoided where possible, second the encroachment was minimized and third that any impact was mitigated. The avoidance phase required much more than a statement that it cost too much to stay out of the buffer. A hardship had to be shown that the site could not be developed unless a variance was granted. We asked the developer to consider options such as walls, two story buildings, reduced building size and parking decks. We had to approve the variance before we would send the variance request to the State EPD to get their approval. In January of this year, this process was made part of our Erosion Control ordinance.
Gwinnett County also has stricter stream buffer requirements in our two water supply watersheds. In the Big Haynes Creek Watershed, within a 7 mile radius of the water supply reservoir, there is a requirement for a 100' undisturbed buffer and 150' impervious setback from the stream bank. Outside the 7- mile radius, the undisturbed buffer is 50' and the impervious surface setback is 75' from the stream bank. In the Alcovy watershed the buffer requirements are for a 50' undisturbed buffer with a 75' impervious setback since the entire watershed within Gwinnett County's borders is outside the 7-mile radius.
The protection of wetlands has also been enhanced in our regulations since they can be included in stream buffer areas and receive the low loading rate of 125 pounds/acre/year. In addition, Gwinnett County's Erosion Control Ordinance was revised in October 2000 to require verification from the COE that a 404 permit has been obtained when wetlands are destroyed by development.