Infiltration: TTT

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Infiltration trenches and chambers are found within the LIDLow 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. toolbox
Surface
BermA 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. 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.
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 between 0.67 and 1.0 according to product geometry.
Seepage rate (mm/hr) Infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of native soilThe natural ground material characteristic of or existing by virtue of geographic origin.. See design infiltration rate
Clogging factor Typical value of 0.5 for aged structures.
Design drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). (hrs) Suggest that this be the time within which there's a 50 % chance of recurrent rainfall? drainage time
Drain (underdrainA perforated pipe used to assist the draining of soils.)
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. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.