Preserving large trees as runoff interceptors is an integral part of low impact development as well as incorporating trees and vegetation in filtration beds, rain gardens, and bioretention systems. These systems are extremely beneficial and provide essential functions of the natural landscape: infiltration, evaporation, transpiration, interception, and shading.
Depending on the type of tree or vegetation retained on-site or planted within the facility there is a different type of load released throughout the year….organic matter. Organic matter for a green field is extremely beneficial as the load can easily be assimilated, allows for decomposition, and provides essential nutrients for soil and nourishment for future vegetation. Organic matter for a stormwater facility can be beneficial, but also can increase excessive nutrient loads that cannot be assimilated, block inlets, blind surface beds, alter flow paths, and change percolation rates through the soil.
In general, stormwater policy has not yet been able to integrate specific design guidelines associated with mass or material load, as it is site specific and variable. The preference has been to deal with mass and material load as an inspection and maintenance activity, rather than a design element. While it is easier to digest a surface blinding load as an inspection and maintenance activity, it’s better to incorporate additional design features and safety factors that account for organic load rather than to rely on modifications to the original design through inspections or through increased maintenance activities.
Whether it is a new development, redevelopment, or retrofit project it is likely that the surrounding area contains additional sources of organic load. Depending on the frequency and type of organic load encountered, additional modifications to the system can be incorporated at the design stage to account for on-site additions of organic load. A few additional design modifications to consider are:
Minimizing organic content in the soil and mulch
Add soil amendments to the soil mix that prevent phosphorus leaching
Increasing storage capacity within the structure to prevent early bypass
Additional safety factors to increase the facility size and surface loading capacity
Additional inlets and/or wider inlets to increase avenues into the facility
Select vegetation that release minimal organic material
Designing a facility with maintenance in mind to allow for the accumulation of organic matter within the facility should improve the long-term function, and with that peace of mind.