A presettling zone or structure serves to capture debris and sediment from the surrounding catchment area. A presettling zone is especially recommended in areas with concentrated point pollution discharges or significant sources of upstream sediment (e.g. exposed soil, automobile service and repair uses or industrial uses, or debris from tires). Designating a presettling zone or structure allows targeted maintenance in that area to remove sediment build-up.
The presettling zone should be designated within the first (upstream) portion of a bioretention cell. For multiple cells in a series on a block, the presettling zone will typically be in the first upstream cell at the inlet that receives larger flow volumes before it is distributed to the remaining cells down the block. Alternatively, use a presettling structure, such as a catch basin with a sump where the outlet pipe daylights into the bioretention cell.
The design of the presettling zone will vary depending on the tributary area characteristics such as amount of sediment load, street maintenance, traffic volumes and adjacent use in the right-of-way. Streets with higher traffic or with adjacent land use that is susceptible to erosion will generate more sediment than neighborhood streets with lower traffic volumes.
If the presettling zone includes a wider curb cut, review the potential for people to park a car with its tire in the curb cut. Some city agencies have used a continuous metal lid across the top of the curb in the curb cut to prevent vehicle incursions. With this approach, consider the maintenance equipment needs for clearing debris from the narrower opening.
If vehicular parking or loading is adjacent to the facility, provide a level step-out zone. If it is a critical inflow point, use no-parking designations to prevent cars from parking in a position that blocks the curb cut.
Consider the city’s operations and maintenance equipment when designing the presettling zone, as well as the amount of impervious area footprint taken up by the presettling zone. For example, city agencies that use vactor trucks may prefer presettling zones with concrete pads rather than a rock or cobble pad. Other cities may prefer cobbles, which minimize the amount of impervious surface created in the cell compared to a concrete pad. Some city agencies may use a screen on the vactor hose to capture finer sediments without capturing cobbles. Some agencies prefer catch basins with sumps, given adequate space and depth to daylight pipes from catch basins into the bioretention cell.
Plant materials can be highly beneficial in the presettling zone to withstand inundation, soil accumulation, or erosive conditions. Plant materials provide a soft edge treatment, compared with cobbles or a concrete pad.
There is no straightforward method to size presettling zones. Streets with higher traffic or with adjacent land use that is susceptible to erosion will generate more sediment compared to neighborhood streets with lower traffic volumes. Design judgment with input from maintenance staff is important so the presettling zone is within the context of the neighborhood character, street scale, and maintenance approach.
Provide a scaled presettling zone at point discharges with concentrated flows. In most cases, a presettling zone should be located at the upstream end of the first cell in a series if the first cell is intended to receive at least half a street length of gutter flow.
Provide a presettling zone at inflow points on streets with higher traffic volumes, especially truck or bus traffic.
Where cobblestones are the desired material, use a mortar treatment to reduce the maintenance demands and replacement requirements of loose cobble.
Fall maintenance leaf collection prior to anticipated storms is recommended if this is the primary inflow into the facility. See Operations & Maintenance for more information.