Infiltration: TTT

Infiltration trenches and chambers are found within the LID toolbox

Surface
Berm height (mm)	This is the height of the curb which constrains the overland sheet flow of water. Where curb cuts or other outlet 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 rocks 0.03 – 0.05
Surface slope (%)	If the slope > 3%, consider using Check dams or weirs to prevent erosion of the surface under high flow velocities.
Storage
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 void ratio’ will vary according to product geometry. Contact manufacturer(s) for recommended value.
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)	Suggested value 72 - 96 hours, see 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.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The natural ground material characteristic of or existing by virtue of geographic origin.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

A perforated pipe used to assist the draining of soils.