The width and number of inlets determines the stormwater inflow and outflow capacity. Inlets should be wide enough to accommodate the expected stormwater volume, but their minimum size is usually related to the type of maintenance equipment that will be used to clean the curb cut (for example, the minimum width of a shovel). Inlets are typically 8–12 inches wide, but inlets up to 24 inches wide are not uncommon.
If there are multiple cells in a series or multiple inlets in a cell, some water can bypass the first inlet and then flow into the next inlet downstream.
Building an inlet often involves minor modifications to the existing flow path in the gutter or pavement at the face of the curb. These changes can include a sloped surface through the curb cut to the cell to facilitate conveyance of stormwater into the cell, as well as a stabilized surface inside the cell at the inflow point (e.g. cobbles or a concrete pad) to minimize erosion as flow enters the cell.
Carefully evaluate the slope of the inlet to ensure flows can enter the bioretention cell. Small differences in slope may mean that low-flow runoff bypasses the inlet.
Vertical notches can be installed along the sidewalk edge of the facility. A width of at least 4 inches will provide convenient maintenance access while minimizing the risk that people will get their feet caught in the openings.
Design inlets to minimize the opportunity for entry by vehicles. Even narrow curb cuts can allow incursion, especially if perpendicular to the parking lane. Metal or concrete lids can reduce incursion risk.
Design inlets to resist blockage. The accumulation of garbage, debris, or sediment at the inlet will prevent runoff from entering the bioretention cell, causing stormwater flows to bypass the cell and eliminating its value for flood prevention and runoff capture. Consider how ice and snow may block inlets and mitigate for this issue if necessary. Include a designated presettling zone—at the inlet for the first cell in a series, or at the primary inlet for a single cell on a block—for collecting debris and sediment, and to allow cleaning efforts to focus on a small space within each project.