Infiltration trenches are shallow surface or subsurface excavations lined with filter fabric and filled with clear stone which provide temporary retention within the stone voids until the stormwater can percolate into the ground. Trenches are typically sized for more frequent storm events and provides treatment for first flush storms which are high in sediment loading and pollutants.
There are several advantages to employing an infiltration trench into your stormwater management design. Reduction in volume runoff, mitigation of downstream flooding, smaller storm sewer sizes, removal of sediment and groundwater recharge are all elements provided by a properly constructed infiltration trench.
The subsurface trench is the type of trench typically employed in commercial and industrial type developments. This type of trench allows for pre-treatment practices such as oil/grit separators, enhanced swales, filter strips or bio-swales which reduce the amount of sediments entering the trench thereby extending its service life.
With the emergence of LEED and Low Impact Development objectives, infiltration trenches are being employed as an economical solution to meet both water balance requirements and stormwater drainage retention. The alternative best management practices to meet these objectives are grey water systems which re-use the stormwater, large ponds which provide evaporation and bio-swales which re-use the water through the plants. The disadvantage with the grey water system is that it’s cost prohibitive. The implementation of ponds and/or bio-swales provides some benefit but they need to be employed with other measures to meet the overall water balance objectives.
The practicality of the infiltration trench makes it the most practical measure to meet water balance objectives but there are often site constraints that limit its effectiveness or applicability. Site specific factors such as soils, water table location, depth to bedrock, type of development, and the proximity to wells or foundations will dictate whether or not an infiltration pit is feasible for the site.
There are several design considerations that can be employed to overcome some of the above noted site constraints. For example, increasing the trench footprint will allow you to reduce the trench height without reducing the required volume but will provide the allowable drawdown time in less favorable soils.
Every site has unique characteristics and requires a thorough stormwater analysis to minimize servicing costs while achieving stormwater objectives.
MGM has successfully designed numerous infiltration trenches throughout Southern Ontario.
For additional information on this topic, contact MGM directly.