This is the second edition of the Stormwater Management Rules of the Kalamazoo County Drain Commissioner, Mary B. Powers, Drain Commissioner. These Rules continue a stormwater management philosophy that considers stream channel protection and stormwater management in addition to flood control. These revisions are based upon the most current body of knowledge concerning stormwater management from across the state and country. These standards also recognize that the Phase II Stormwater Rules will impact many communities in Kalamazoo County including numerous areas of plat drainage. It must also be noted that the intensity and duration of wet weather events (rain & snow) has changed dramatically in the past decade and that these events combined with a dramatic increase in impervious surface are quickly changing drainage patterns countywide. These Rules outline the basic ideas and principals of stormwater management and provide a conceptual foundation for the design standards contained in this document.

IMPACTS OF DEVELOPMENT ON WATER QUANTITY

The hydrology of a watershed changes immediately in response to site clearing and development of the natural landscape. A site's existing stormwater storage capacity (recharge to the groundwater) is quickly lost as vegetation is removed, natural depressions are graded and both topsoil and wetlands are eliminated. As the soil is compacted and resurfaced with impervious materials, rainfall can no longer penetrate into the ground and thus runs off of the top of the land, roofs, driveways and streets, burn barrels, junk piles and all the numerous surfaces that come in contact with rain, snow and other less appealing substances. These surface modifications, along with the installation of "efficient" drainage infrastructure such as catch basins, pipes and basins, greatly alter natural drainage patterns. In a groundwater drinking community such as Kalamazoo County, what is in drainage and where drainage goes should be of grave concern to everyone who drinks or uses public water supplies or private wells.

Changes in Watershed Hydrology

• Volume of runoff increases. This raises the magnitude and frequency of severe flood events.
  
  
• Frequency of bankfull floods increases. These floods fill the stream channel to the top of its banks, but do not spill over into the floodplain. Increased bankfull flooding subjects stream channels to continual disturbance and scour causing soil erosion and sedimentation resulting in increased drain maintenance costs to residents and damage to natural resources.
  
  
• Flow velocities increase. This is due to the combined effect of greater discharge (increased impervious surface), rapid time of concentration, and smoother hydraulic surfaces.
  
  
• Stream flow fluctuations increase dramatically. As runoff is concentrated into sharper, faster and higher peaks, equally abrupt returns to pre-storm level discharges will follow. Increased flow fluctuations have numerous impacts including the need for even larger drainage basins due to lack of infiltration prior to discharge to "the drainage system". Other large wet weather event impacts are the damage to and disruption of plant and animal habitats and reduction of the diversity of aquatic species regardless of water quality.
  
  
• Infiltration to Surface and Groundwater is reduced/eliminated. This in turn negatively impacts the conductivity between surfacewaters and groundwater supplies, lowering the level of surface water bodies dependent on groundwater to maintain base flows during dry periods.

Changes in Stream Morphology

• Channel widening and downcutting are the primary consequences of increased runoff and flow fluctuations resulting in water quality reduction, property damage and soil erosion.
  
  
• Streambank erosion is accelerated as channels are severely disturbed by undercutting, tree-falls and bank slumping. Erosion and sedimentation results in increased maintenance costs and damage to natural resources and habitat that often takes years to recover.
  
  
• Sediment loads increase sharply due to streambank erosion and construction site runoff. These sediments settle out and form shifting bars that often accelerate the erosion process by deflecting runoff into sensitive bank areas causing property damage and loss of land.
  
  
• Increased sedimentation and channel widening modify aquatic habitats. Pools and riffles are eliminated as the gradient of the stream adjusts to accommodate frequent floods. Sediment deposition destroys insect and benthic organism habitat as well as fish spawning areas.

IMPACTS OF DEVELOPMENT ON WATER QUALITY

As development occurs, changes in land use contribute new or additional pollutants to stormwater runoff. In addition, the accompanying impervious surfaces provide efficient delivery of these pollutants into receiving waterways. Leaves, litter, animal droppings, exposed soil from construction sites, fertilizer and pesticides are all washed off of the land. Vehicles and deteriorating surfaces deposit trace metals, oil, and grease onto streets and parking lots. These and other toxic substances are carried by stormwater and conveyed through creeks, ditches and storm drains into our lakes, streams, and wetlands and potentially to drinking water supplies.

In short, the ecology of lakes, streams, and wetlands and drains may be completely re-shaped by the extreme shifts in hydrology, morphology and water quality and quantity that accompanies the development process. The stressors that these changes place on aquatic communities, although gradual and often not immediately visible, are profound.

To mitigate groundwater and surface water impacts, it is necessary to reevaluate the way that stormwater and land development are managed. The following discussion provides a framework for this reevaluation, which must encompass the entire development process from land use planning and zoning to site design and construction and future maintenance of "drainage systems".

FRAMEWORK FOR THE DESIGN OF STORMWATER MANAGEMENT SYSTEMS

Thoughtful site planning can substantially reduce environmental impacts associated with development. Towards this end, communities, regulatory agencies, and designers must begin to evaluate the impact of each individual development project over the long term, and on a watershed scale. Such an approach requires consideration and use of Best Management Practices (BMPs) that function together as a system to ensure that the volume, rate, timing and pollutant load of runoff remains similar to that which occurred under natural conditions (pre-development). This can be achieved through a coordinated network of structural and nonstructural methods, designed to provide both source and site control. In such a system, each BMP by itself may not provide major benefits, but becomes very effective when combined with other actions.

Source Controls

Source Controls reduce the volume of runoff generated on-site, and eliminate initial opportunities for pollutants to enter the drainage system. By working to prevent problems, source controls are the best option for controlling stormwater, and include the following key practices:

• Preservation of existing natural features that perform stormwater management functions, such as depressions, wetlands, and woodland and vegetative buffers along streambanks.
  
  
• The minimization of impervious surface area through site planning that makes efficient use of paved, developed areas and maximizes open space. Encouraging flexible street and parking standards, and the use of permeable ground cover materials can also reduce impervious surfaces.
  
  
• Direction of stormwater discharges to open grassed areas, swales and lawns rather than allowing stormwater to run off of impervious areas directly into the stormwater conveyance systems.
  
  
• Careful design and installation of erosion control mechanisms and rigorous maintenance throughout the construction period and after. Effective erosion control measures include minimizing the area and length of time that a site is cleared and graded, retention of existing vegetation as buffers, and the immediate vegetative stabilization of disturbed areas.

Site Controls

Site Controls are the subject of this document.

After the implementation of source controls, site controls are then required to convey, pre-treat, and treat (e.g., detain, retain or infiltrate) the stormwater runoff generated by development. The range of engineering and design techniques available to achieve these objectives is to some degree dictated by site configuration, soil type, and the receiving waterway. For example, flat or extremely steep topography may preclude the use of grassed swales which are otherwise preferable to curb and gutter systems. Likewise, sites upstream of cold-water fisheries may not be suitable for shallow basins that discharge heated surface waters. While each site will be unique, some universal guidelines for controlling stormwater quality and quantity can be stated.

Preferred Hierarchy of Structural Site Controls

1. In general, the most effective stormwater quality controls are infiltration practices, which reduce both the runoff peak and volume. To date, structural infiltration devices such as retention basins and, to a lesser degree, trenches have suffered high failure rates due to clogging from poor erosion control during construction. Therefore, an aggressive maintenance program and extensive upstream pre-treatment measures, such as oil/grit separators, sedimentation basins and grass filter strips, should be incorporated into any stormwater management system that employs these devices. In addition, these practices are only feasible for smaller drainage areas with suitable soils and no potential for groundwater contamination.

2. The next most effective stormwater site controls reduce the runoff peak, and involve storage facilities such as detention basins. In the selection of an appropriate stormwater basin design, wet ponds and extended detention ponds are generally preferable to dry detention ponds, since they hold stormwater much longer, allowing more particulate matter to settle out. In addition, the aquatic plants and algae within wet ponds take up soluble pollutants (nutrients) from the water column. These nutrients are then transformed into plant materials that settle to the pond floor, decay, and are consumed by bacteria. This biological process is dependent upon the presence of water, it does not occur in dry ponds.

3. Where site conditions make the use of a wet pond infeasible, dry ponds should be designed to provide extended retention of stormwater, again to promote as much settling of particulate matter as possible. A notable exception to this preference exists within areas where thermal impacts are a concern. Since they hold stormwater longer, wet and extended detention ponds tend to increase the exposure of runoff to solar warming before releasing it. Where thermal impacts are of primary concern, a balance must be struck between the goals of pollutant removal and the reduction of thermal impacts. Source controls and infiltration of stormwater, where feasible, are preferable approaches.

4. Once all feasible methods of reducing and treating stormwater on-site have been implemented, excess runoff may be discharged into conveyance systems and carried off-site. Discharges must be at rates, velocities and volumes that will not cause adverse downstream impacts to land or waterways. For this purpose, vegetated swales with check dams or terraced slopes are generally preferred to curb and gutter systems and typical enclosed storm drains.

5. Regardless of the design, any stormwater system will lose effectiveness without regular maintenance. Depending on the specific BMP, maintenance must be performed at regular intervals. This may include inspection, sediment removal, maintenance of vegetation and structures, replacement of filters, et cetera. Maintenance plans should be developed concurrent with the system designs. The design must include adequate maintenance access.

THE ROLE OF THE KALAMAZOO COUNTY DRAIN COMMISSIONER

The preferred hierarchy discussed previously and summarized in Table 1 provides a comprehensive framework for evaluating the place and function of individual BMPs within a stormwater management system. While the most important BMPs are source controls that preserve and protect the waterbodies of the state and natural environment, the Kalamazoo County Drain Commissioner cannot mandate all of these. We must look to the staff and officials of local governments, as well as to developers and their design engineers and planners, to implement the source reduction approaches described.

The Office of the Drain Commissioner exercises authority over the design and construction of structural facilities that convey and treat stormwater runoff that will be generated as a result of a sites design. The Drain Commissioner's Rules will govern the design of such management facilities expected to come under the Drain Commissioner's final maintenance authority with the following objectives:

• Incorporate design standards that control both water quantity and quality.
  
  
• Encourage innovative stormwater management practices that meet the criteria contained within these Stormwater Management Rules.
  
  
• Ensure future maintenance of facilities by planning for it as a part of system design.
  
  
• Strengthen the protection of Natural Features.
  
  
• Encourage more effective soil erosion and sedimentation control measures.

Table 1