Infiltration: TTT

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Infiltration trenches and chambers are found within the LID toolbox
Berm height (mm) This is the height of the curb which constrains the overland sheet flow of water.
Where curb cuts or other inlets exists at the lowest point of an infiltration surface, the suggested value is 0.
Surface roughness (Manning’s n) Lower numbers indicate less surface obstruction and result in faster flow.
Suggested range for a surface of decorative stones 0.03 – 0.05 [1]
Surface slope (%) If the slope > 3%, consider using Check dams or weirs to prevent erosion of the surface under high flow velocities.
Thickness (mm) Entire trench
Void ratio Suggest value 0.4 (unless otherwise tested) for designs using just clear stone reservoir aggregate for storage. For any design using preformed infiltration chambers, the overall ‘effective porosity’ will vary between 0.50 and 0.95 according to product-specific geometry.
Seepage rate (mm/hr) Infiltration rate of native soil. See design infiltration rate
Clogging factor Typical value of 0.5 for aged structures.
Design drawdown time (hrs) Suggest that this be the time within which there's a 50 % chance of recurrent rainfall? drainage time
Drain (underdrain)
Flow coefficient Suggested value 1
Flow exponent Suggested value 1
Offset height This is the height from the base of the cell to the height at which the drain discharges. In some designs this may be the height of the perforated pipe within the storage layer. In other designs this height is adjusted by creating an upturn in the discharge pipe.
  1. Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233. Accessed August 23, 2017.