Impacts of Development on Water Quantity
Stream hydrology changes immediately in response to site clearing and development of the natural landscape. The existing stormwater storage capacity 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 so runs off the land. These modifications, along with the installation of "efficient" drainage facilities, such as catch basins and pipes, greatly alter natural drainage patterns, with the following consequences...
Changes in Stream Hydrology
- Increased volume of runoff, which raises the magnitude and frequency of severe flood events
- Greater frequency of "bankfull" floods - those that fill the stream channel to the top of its banks, but do not spill over into the floodplain. Increased bankfull flooding subjects the stream channel to continual disturbance and scour
- More rapid flow velocities, due to the combined effect of greater discharge, rapid time of concentration, and smoother hydraulic surfaces
- A dramatic increase in stream flow fluctuations, as runoff is concentrated into peaks that are sharper, faster and higher followed by equally abrupt returns to pre-storm level discharges. Increased flow fluctuations disrupt habitats and reduce the diversity of aquatic species regardless of water quality
- Reduced infiltration into the underlying water table, which in turn lowers the level of surface waterbodies 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
- Streambank erosion is accelerated, as channels are severely disturbed by undercutting, tree-fall and slumping
- 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
- Together, increased sedimentation and channel widening modify aquatic habitats. The pools and ripples that characterize natural streams are eliminated as the gradient of the stream adjusts to accommodate frequent floods, while the voids between stones on the stream floor are filled with sediment, destroying the habitat of insects at the bottom of the aquatic food chain, and 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 urban 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 stormdrains into our rivers and lakes.
In short, the ecology of urban streams may be completely re-shaped by the extreme shifts in hydrology, morphology and water quality that can accompany the development process. The stresses that these changes place on the aquatic community, although gradual and often not immediately visible, are profound. To mitigate these 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.
The Rules of the Washtenaw County Water Resources Commissioner govern only the design of stormwater management systems within certain new development projects: the following discussion applies to all aspects of managing land and stormwater.
Framework for the Design of Stormwater Management Systems
Thoughtful site planning can reduce or eliminate the negative 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 the consideration 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. This can be achieved through a coordinated network of structural and non-structural methods, designed to provide both source and site control. In such a system, each BMP by itself may not provide major benefits, but when combined with others becomes very effective.
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 vegetation along streambanks
- Minimization of impervious surface area through site planning that makes efficient use of paved, developed areas and maximizes open space. Impervious surfaces are also reduced by encouraging flexible street and parking standards, and the use of permeable ground cover materials
- Direction of stormwater discharges to open, grassed areas such as swales and lawns rather than allowing stormwater to run off from impervious areas directly into the stormwater conveyance system
- Careful design and installation of erosion control mechanisms and rigorous maintenance throughout the construction period. Effective erosion control measures include minimizing the area and length of time that a site is cleared and graded, and the immediate vegetative stabilization of disturbed areas
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 permanent wet ponds that discharge heated surface waters. But while each site will be unique, some universal guidelines for controlling stormwater quality and quantity can be stated.
Preferred Hierarchy of Structural Site Controls
- In general, the most effective stormwater quality controls are infiltration practices, which reduce both the runoff peak and volume. But to date, structural infiltration devices such as basins and, to a lesser degree , trenches have suffered extremely high failure rates due to clogging. Therefore, an aggressive maintenance program and extensive upstream pre-treatment measures (such as oil/grit separators, sedimentation basins and grass filter strips) must be incorporated into any stormwater management system that employs these devices. In addition, these practices are only feasible on small sites, with suitable soils and no potential for groundwater contamination.
- The next most effective stormwater site controls reduce the runoff peak, and involve storage facilities such as retention and detention ponds. In the selection of an appropriate stormwater pond design, wet ponds are generally preferable to detention ponds, since they hold stormwater much longer, allowing more particulates 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. Since this biological process is dependent upon the presence of water, it does not occur in dry ponds.
- Where site conditions make the use of a wet pond infeasible, ponds should be designed to provide extended detention of stormwater, again to promote as much settling of particulates 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, shorter retention ponds are preferable when stormwater management cannot be managed through source controls.
Volume: Whereas detention basin design for flood control is concerned with relatively infrequent, severe runoff events, such as the 25-, 50- or 100-year storm, design for water quality benefit is concerned with controlling the more frequent storm events (e.g. 1.5-year storm or less). By capturing and detaining the 1.5 year storm, the negative impacts of erosive "bankfull" floods are effectively avoided.
Also of primary importance to water quality, is the capture and treatment of the "first flush", a term used to describe the initial washing action that stormwater has on impervious surfaces. Pollutants that have accumulated on these surfaces are flushed clean by the early stages of runoff, which then carries a shock loading of these pollutants into receiving waterways. By capturing and treating the first 1/2 inch of runoff, up to 90% of all pollutants that are washed off of the land can be removed from stormwater before it enters the drainage system.
Treatment of the "bankfull" flood and "first flush" may be accomplished via the design of "dual detention basins". These basins control stormwater discharge rates for both extreme events to prevent flooding and more frequent runoff events to mitigate water quality impacts and channel erosion.
Pre-treatment: It is strongly recommended that stormwater be pre-treated prior to entering a retention or detention pond, by passing first through a sediment forebay. Sediment forebays function to reduce incoming water velocities, and to trap and localize incoming sediments, thereby reducing pond maintenance. Sediment forebays also extend the flow path of stormwater, increasing its residence time.
- Once all possible methods of reducing and treating stormwater on-site have been implemented, excess runoff must be discharged into conveyance systems and carried off-site. For this purpose, vegetated swales with check dams are generally preferred to curb and gutter systems and enclosed storm drains.
Role of Washtenaw County Water Resources Commissioner
The preferred hierarchy discussed above (summarized in Table 1), provides a comprehensive framework for evaluating the place and function of individual BMPs within a stormwater management system. While by far the most important BMPs are source controls that preserve and protect the natural environment, these cannot be mandated by the Washtenaw County Water Resources Commissioner. 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 earlier.
The Office of the Water Resources Commissioner exercises authority over the design and construction of structural facilities that convey and treat stormwater runoff that will be generated from a site as a result of its design. The Water Resources Commissioner's Rules will govern the design of such management facilities 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 rules
- Place greater emphasis on the maintenance of facilities
- Make the safety of facilities a priority
- Strengthen the protection of natural features
- Encourage more effective soil erosion and sedimentation control measures
Table 1. Hierarchy of Preferred Best Management Practices
Non-Structural (Source) Controls
- Preservation of the natural environment
- Minimization of impervious surfaces
- Use of vegetated swales and natural storage
Structural (Site) Controls
- Stormwater retention structures
- Stormwater detention structures
- Stormwater conveyance
- Infiltration trenches, basins, etc.