The state stormwater management law was enacted in 1982, requiring local governments to enact ordinances supporting stormwater management and approve stormwater management plans for new development projects by July 1, 1984. Since the law went into effect, experience and research have demonstrated the benefits to water quality of effective stormwater management planning, implementation and maintenance. Planned densities and open space must now include the accommodation of stormwater management needs. Maryland's stormwater management approach for new development projects is a unified sizing criteria for stormwater best management practices (BMPs) to meet pollutant removal goals, maintain ground water recharge, reduce channel erosion, prevent overbank flooding and mitigate extreme flooding.
Performance standards have been established for the design criteria of the five groups of structural and nonstructural BMPs. Innovative site planning is an integral part of this approach, relying on nonstructural site design techniques such as roof top disconnection, natural area conservation and impervious surface area reductions that reduce the generation of stormwater runoff and the reliance on structural BMPs.
Maryland encourages wise, environmentally sensitive site design techniques, such as Low Impact Development (LID), that reduce the overall volume of runoff and the generation of runoff borne pollution. LID promotes infiltration using ground water recharge criteria from Natural Resources Conservation Service (NRCS) soil type data. To further upgrade the state's stormwater management program, the Maryland General Assembly enacted the Stormwater Management Act of 2007, which codified the requirements for implementation of Environmental Site Design (ESD) techniques. Maryland's approach also requires that appropriate volumes be controlled to protect stream stability (channel protection volume) and large rainfall events (overbank and extreme flood protection).
Limiting the amount of impervious surfaces by implementing the most effective stormwater management practices, such as ESD, must be incorporated in every new development and redevelopment project. Stormwater utilities are effective and should be incorporated in local ordinances to ensure that dedicated financial resources are available to support the maintenance of stormwater controls. They should also provide the resources needed to address existing developed lands that do not yet have stormwater management. This stormwater management approach provides flexibility to localities and developers/designers by ensuring that innovative site design techniques are blended into local grading, building and development codes, while mandating a specific pollution reduction performance standard.
Maryland has adopted smart growth policies that are geared toward concentrating development where it currently exists, thereby reducing suburban sprawl and impervious surface. Therefore, redevelopment of existing areas is strongly encouraged. A stormwater management policy for redevelopment has been established that specifies a 20% reduction in impervious surface area below existing conditions. Because this may be impractical due to site constraints, water quality treatment of the volume of runoff from 20% of a site's impervious surfaces is allowed. Locally approved practical alternatives such as fees, off-site implementation, watershed or stream restoration and retrofitting are allowed as well.