Stormwater trenches that soak, not choke

Posted on March 4, 2011

Infiltration trenches are below-ground constructions designed to intercept stormwater runoff from impervious surfaces in urbanised catchments.  By diverting runoff into the subsoil they help to reduce runoff volume, recharge groundwater supplies and capture suspended solids, nutrients and chemical pollutants.  Although some infitration trenches are still working well after 100 years, a large proportion fail due to soil pore clogging, lack of maintenance or improper location.  Few studies have tracked their performance over time with the aim of improving trench design.   An infiltration trench receiving runoff from a parking area nearPhiladelphiawas monitored for three years following construction.  The trench, dug in an area of loamy sand, was 1.8 m deep, 3.9 m long and 3 m wide and lined with geotextile and crushed stone.  The trench was deliberately undersized relative to its drainage area in order to increase loading rates and accelerate the ageing process.  There was an exponential reduction in infiltration efficiency over the first eighteen months of operation; after this time water movement through the walls remained active but movement through the clogged bed was negligible.  As the study progressed the time taken for the trench to drain increased from less than a day to over a week.  Infiltration efficiency improved in summer because increased temperatures reduced the viscosity of water and boosted conductivity through the soil.  Although the temperature-corrected infiltration rate dropped by about 50% over the first six months and 70% over the first year, performance was rated as satisfactory, since in twelve months the trench received runoff and sediment equal to that likely to be intercepted by a normally-loaded trench over 30 years.  The authors recommend the following design features for infiltration trenches:  a low drainage area to trench footprint ratio (c. 5:1), the use of pretreatment systems (e.g. planted vegetation) to remove suspended solids where possible, a relatively shallow storage bed to minimise soil compression by ponded water, and soil amendments to improve permeability in the surrounding area. 

Reference: Emerson, C.H., Wadzuk, B.M. & Traver, R.G.  2010.   Hydraulic evolution and total suspended solids capture of an infiltration trench.  Hydrological Processes 24, 1008–1014.

Posted in: hydrology, pollution