Stormwater Management in Maryland
The Stormwater Management Act of 2007 (Act) set forth new statewide standards for implementing Stormwater Management (SWM) on both Residential & Commercial projects. The Act requires that Environmental Site Design (ESD), through the use of nonstructural best management practices and other better site design techniques, be implemented to the maximum extent practicable. To aid Land Surveyors & Civil Engineers in designing these practices, the Maryland Department of the Environment (MDE) published an update to the Maryland Stormwater Design Manual, Volumes I and II (October 2000, Revised May 2009) (Manual).
Three Step Approach to ESD
1. Smart Design: Minimizing Impervious Areas & Implementing Alternative Surfaces. The less impervious areas you have, i.e. sidewalks, driveways, and buildings, the less SWM you have to provide. So minimizing impervious area, if possible, is a great way to reduce the magnitude & cost of SWM. An alternative is to incorporate SWM into your impervious surface utilizing a green roof or permeable pavement.
2. Nonstructural Practices: If the site has relatively flat slopes (<5%) and soils that infiltrate well, a portion of the required SWM can often be satisfied using simple and relatively inexpensive methods. These methods will allow for rain water runoff to drain over a flat grassed area and naturally infiltrate into the ground. Two great opportunities to use this credit, as the Manual refers to it, are with Rooftop Disconnect (water runoff from downspouts), and Non-rooftop Disconnect (water runoff from roads, driveways, and sidewalks).
3. Micro-Scale Practices: Often, even after utilizing Smart Design & Nonstructural Practices the SWM requirements for the project have not been satisfied. At this point the Manual requires the implementation of Structural Practices. Also referred to as Micro-Scale Practices, these devices all implement a method for holding rain water runoff, and allowing it to infiltrate directly into the ground or releasing it slowly to incerase the opportunity for downstream infiltration. These devices can include: Rain Barrels, Submerged Gravel Wetlands, Landscape Infiltration, Infiltration Berms, Dry Wells, Micro-Bioretention, Rain Gardens, Bio-swales, Wet Swales, and Enhanced Filters.
How Much SWM / ESD Do I Need?
The Manual establishes a formula which factors in the projects total disturbed area, total impervious area, and existing soil infiltration rates to calculate a volume of rain water runoff that must be stored. Furthermore, each one of the devices described above, beyond having certain rules for it's application, also has a formula to calculate how much of this volume it satisfies. Projects take one of two possible routes, either the entire required volume is stored, thus satisfying the SWM / ESD requirements for the site, or after exhausting all available methods, it is proven that ESD has been implemented to the Maximum Extent Practicable (MEP). The later implying that no other SWM / ESD devices are possible due to site conditions, such as; steep slopes, insufficient space, or soils that do not infiltrate (clay soils). Notably, aesthetics & costs are not valid justifications to support implementing ESD to the MEP, so it is very important to factor SWM into a projects budget.