Facilities planning and design
Historically, emphasis was given to the rapid removal of storm water from areas, carrying pollutants to waterways, and eroding pond and stream banks. In response, storm water management has become more comprehensive. Objectives now include controlling bank erosion, water quality, and flooding. To achieve these objectives, the volume, velocity, and pollutant load of runoff leaving a site after development must be similar to that which occurred under natural or pre-development conditions. This can be accomplished by establishing a coordinated network of both natural and engineered "best management practices" (BMPs) that work together to reduce, convey, and treat storm water runoff. In such a system, each BMP by itself may not provide major benefits, but when combined with others it becomes very effective.

Reduce runoff and pollutants at their source
Source controls are considered the best option for controlling storm water because they can reduce the volume of runoff generated on-site and eliminate opportunities for pollutants to enter the drainage system. Source controls include:

• Preserve wetlands, vegetation and other existing features that naturally manage storm water.
• Minimize impervious surface area to maximize storm water infiltration.
• Direct storm water to open grassy areas such as swales and lawns rather than to pavement or storm drains.
• Control soil erosion through a variety of means such as creating diversions, using fabric fences, etc.

Reducing effects of parking lots, sidewalks and roads
Paved parking lots, roads, and sidewalks can introduce large amounts of pollutants into the storm system with every rain. Slightly reducing the amount of impermeable surface makes a great difference in the amount of pollutants traveling to waterways through runoff. Gravel parking lots with paved traveling lanes (as in part of the Glazier Way Commuter Lot) increase permeable surfaces while keeping the heavily used lanes smooth. To prevent the gravel from washing away, a minimal slope is required. Paving blocks that allow grass to grow through an open cell of concrete or plastic are among the most permeable parking surfaces, but are intended for use in lightly traveled areas because grass will not survive consistent daily traffic. Reducing the size of parking spaces and aisle width can also reduce the amount of paved areas. Keeping trees alive in a parking lot is difficult if not enough permeable surface has been provided to allow water to reach the roots. Studies have shown that long planting strips that are at least 8 feet wide are sufficient for trees to survive. Innovative designs are most useful in lightly traveled areas. The amount of impervious surface can be reduced by placing a sidewalk on only one side of the street unless the design code forbids that. A sidewalk that is a few inches more narrow also adds up to substantial savings. Lightly traveled roads can also be narrowed to decrease impervious surface areas.

Removing Pollutants from runoff
Even after source controls have been used, runoff still needs to be managed. By capturing and treating the first 1/2 inch of runoff (also referred to as the "first flush"), up to 90% of all pollutants that are washed off the land can be removed from storm water before it enters the drainage system. The order of preference of these control designs are:

1. Infiltration devices: Basins and trenches are the most effective at reducing the runoff peak and volume, but also the most susceptible to clogging. With an aggressive maintenance program these systems are possible but only on small sites, with suitable soils, and no potential for ground water contamination.
2. Wet ponds: Designed to reduce the runoff peak, wet ponds hold storm water much longer than dry ponds, allowing more pollutants to settle out. The aquatic plants in wet ponds also take up soluble pollutants (such as nutrients) from storm water.
3. Extended detention ponds: Where wet ponds are not feasible, design detention ponds to hold storm water as long as practical to allow more pollutants to settle out and to reduce the speed of flowing water, which in turn helps to control erosion.