Bioretention Design (Pre-treatment): Sediment Forebays vs GPTs

Abstract

Bioretention basins have emerged as an effective solution in urban developments, addressing challenges associated with stormwater runoff quality and peak flow regulation. With expanding urban landscapes and increased impervious surfaces, the need to manage and treat stormwater runoff has never been greater. Among the various treatment train solutions available, the industry often leans towards incorporating either sediment forebays or Gross Pollutant Traps (GPTs) as pre-treatment solution for bioretention basins. This white paper compares these two primary pre-treatment techniques, evaluating their performance, cost implications, and physical footprints. The performance of a sediment basin is dependent on a variety of factors such as forebay size, inlet flow velocity, catchment size, level of maintenance, etc. Sediment forebays are typically built inline, and so the primary concern is the scouring or resuspension of sediment that could be flushed into the bioretention basin. This can severely compromise the filter media functionality, leading to prolonged ponding and degradation of vegetation. Conversely, GPTs have a more consistent performance which is typically verified through rigorous testing protocols (e.g. SQIDEP). As they can be built offline, sediment is much less prone to resuspension. Three main cost factors are considered: Capital Expenditure (Capex), Operational Expenditure (Opex), and footprint costs. Capex: Sediment forebays come with a relatively lower upfront cost. For a standard 5ha catchment, the estimated investment lies between 10-20k. In contrast, GPTs, typically demand a higher initial outlay, ranging from 40-60k for a comparable catchment. Opex: The ongoing maintenance costs for sediment forebays is dependent on a variety of factors such as its sediment capture capacity, catchment sediment loading and accessibility. Generally, sediment forebays require maintenance at least twice a year. However, if the performance is compromised as discussed, captured sediment will flush into the downstream basin. The cost of resetting the basin if clogged can be substantial, up to the original construction capital expenditure. Underground GPTs typically require suction removal for maintenance. The annual maintenance expenditure of a GPT is expected to be more than that of a sediment forebay due to higher sediment removal performance, resulting in increased disposal tonnage. However, the lifecycle maintenance cost is likely to be lower due to the difficulty and cost in resetting a failed bioretention basin. Footprint Cost: Beyond capex and opex considerations, the spatial footprint of these systems plays a pivotal role. A sediment forebay catering to a 5ha catchment, as prescribed by the Bioretention Technical Design Guidelines (2014), occupies a minimum of 95m^2. In contrast, the footprint of a GPT is generally two to four 600mm diameter lids. This reduction in valuable area provides additional urban land area which can be utilised - a frequently underappreciated advantage. The choice between GPTs and sediment forebays in bio-filtration design requires an understanding of not just initial costs, but also long-term maintenance implications and land use considerations. While sediment forebays offer a cost-effective upfront solution, their performance vulnerabilities and substantial footprint make GPTs an attractive alternative for modern urban stormwater management.