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Credit: David Wallace, Severn River Association

Build an infiltration system to collect stormwater on your property and return it to the ground. This will keep nutrients and sediments from entering the Creek and replenish groundwater systems.

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Infiltrator Sizing*


10' x 60' driveway = 600 sq. ft.

Assume trench 4' deep, 2' wide

Solution: 600 sq. ft. = 120 cu. ft. gravel (6.7 tons)

Trench length = 120 cu. ft. / 4' x 2' = 15 ft. long

Impervious Area
(sq. ft.)
Gravel Vol.
(sq. ft.)
200 40 2.2
400 80 4.4
600 120 6.8
800 160 8.9
1000 200 11.0
1200 240 13.3
1400 280 15.5
1600 320 17.8
1800 360 20.0
2000 400 22.0
* 1" rain-water quality volume washed gravel 3/4" or larger; 40% voids


Cobbles are random diameter round/flat stones. They are often referred-to as washed gravel or tailings and are used for drainage structures because of their void ratio. A bucket or trench full of cobbles is 40% air. That air space is filled with water when it rains. The table above, Infiltrator Sizing, will help you compute quantities. Note that the larger stones are especially for the exposed surface because they are heavier and will absorb the moving waters energy better than smaller stone. However, it is easier to handle or shovel 3/4" washed gravel so one may choose to use smaller gravel below grade and the larger cobbles on the surface. Cobbles of 1-3" diameter are called "2 inch" and sell by the ton. Delivery may cost extra.

There are approximately 1.5 tons per 1 cubic yard.

Drains and Grates

T'here are many choices for surface drains that can be installed in new or existing concrete and gravel driveways. Since they are sold as components, it's easy to fit any situation. If a vehicle will be driving over it, use steel or cast iron systems. For custom applications steel and cast iron grate is available by the foot.