Example Sizing For Infiltration Trenches:

STEP 1 Determine Total Impervious Surfaces

STEP 2 Determine Required Infiltration Volume using Equation

(2.04 in. x 824 sq. ft.)/ 12 = 140 cu. ft.

(140 cu. ft.) / 0.4* = 350 Cu. ft. (*assumes 40% void ratio in gravel bed)

STEP 3 Sizing of Selected Infiltration Method

Volume of facility = Depth x Width x Length

Set D = 3 ft; determine required surface area of trench

350 cu. ft. / 3 ft. = 117 sq. ft.

The width of the trench should be greater than 2 times it depth (2 x D); therefore, in this example a trench width of 6 feet is selected;

Determine trench length: L = 117 sq. ft. / 6 ft. = 20 ft.

Final trench dimensions: 3 ft. (D) x 6 ft. (W) x 20 ft. (L)