Soil physical characteristics related to failure of stormwater biofiltration devices

Abstract

The main theme of this dissertation research was to investigate and test solutions to overcome common failure mechanisms of bioinfiltration stormwater devices. Bioinfiltration can be an effective option for the management of stormwater runoff from urban areas, mainly through enhanced infiltration of the runoff to better balance the urban hydrologic cycle. There are increasing interests in the use of bioinfiltration practices for managing stormwater runoff, as infiltration practices promote groundwater recharge, reduce runoff peak flow rates and volumes, and can reduce pollutant discharges to surface water bodies. However, there are known problems causing failure of these devices (such as clogging, under-sizing, improper underdrain use, and inefficient treatment media). In addition, few quantitative guidelines are available for the design of biofilters and bioinfiltration devices for specific treatment goals while minimizing these operational problems. Some types of stormwater control practices are intended to include standing water for varying lengths of time for enhanced sedimentation and scour protection. In areas having restrictive soils, underdrains are used to minimize long periods of standing water (less than 3 days) to minimize nuisance conditions such as breeding of mosquitoes.. Alternatives that result in greater flexibility and efficiency in the design of biofiltration and bioretention devices were tested and developed during this research. The drainage rate in biofiltration devices (having an underdrain) is usually controlled using an underdrain that is restricted with a small orifice or other flow-moderating component. These orifices used for flow control frequently fail, because they are very small (<10 mm) and are prone to clogging over time. The performance of a foundation underdrain material (SmartDrainTM) that could be used in biofilter devices was evaluated. This material was found to have minimal clogging potential while also providing very low discharge rates. The flow capacity and clogging potential of the SmartDrainTM material was examined under severe service conditions. Laboratory and field-scale studies were conducted to provide insight into the existing soil characteristics of a poorly operating biofilter facility. Surface double-ring infiltration tests (comprised of three separate setups each) and bore hole infiltration measurements were conducted in the field to determine the surface infiltration and the subsurface infiltration characteristics of bioinfiltration sites in Tuscaloosa. The effects of different compaction levels on the infiltration rates through the soil (obtained from the surface and subsurface of the bioinfiltration sites) were examined during laboratory column tests for comparison to the field observations. A controlled laboratory column tests conducted using various media to identify changes in flow with changes in the mixture characteristics, focusing on media density associated with compaction, particle size distribution (and uniformity), and amount of organic material (due to added peat). The results of the predicted performance of these mixtures were also verified using column tests (for different compaction conditions) of surface and subsurface soil samples obtained from Tuscaloosa, AL, along with biofilter media obtained from Kansas City, North Carolina, and Wisconsin. The results of this research indicated that soil compaction has dramatic effects on the infiltration rates; the effects of compaction therefore need to be considered during construction of stormwater treatment facilities. Data from the infiltrometers also need to be cautiously evaluated as they show high rates that only occur during the initial portion of the event and are not representative of fully saturated conditions throughout the infiltration facility that would occur during actual storm conditions. It is important that stormwater practice designers determine the subsoil characteristics before designing stormwater treatment facilities and consider the use of added amendments (sand and peat) to the soils.