On most streets with a curb, the curb should be continued along the bioretention facility edge. Accommodate water flow into the cell in one of four ways:
- a depression in the gutter and a break in the curb that directs the flow through a graded channel to the facility’s bottom;
- a catch basin with a pipe or rectangular channel daylighting into the cell; or
- a depression in the gutter that connects to the concrete channel with a trench grate that then outfalls into the cell.
A cell along a curbed street may need one or more inflow points, depending upon the length of the cell, in order to maximize the amount of flow entering the cell and use the full capacity of the facility. Inflows should allow the cell to fill up to its full design ponding depth.
The outflow point serves to direct water back to the street’s gutter or to another cell along the corridor. The inflow point may also act as the outflow point for the facility, depending upon the elevations of the street, inlet, and cell. Alternatively, a storm structure, usually a raised drain, may be placed within the cell to direct the overflow downstream when the cell reaches capacity, or a piped connection to the gray stormwater system for overflow volumes.
The overflow elevation is the maximum ponding elevation (see page 103); runoff must be directed through an outlet, overflow drain, or bypass and not enter the bioretention area to prevent street or sidewalk flooding.
The opening and flow path into and out of the cell must be clear of obstructions. Drop the inlet grade 2 inches below the street grade, and allow another 1–3 inch drop behind the inlet to allow debris and sediment to settle without blocking the flow of runoff.
Locate other street infrastructure, such as posts for street signs, water meter boxes, valve boxes, and irrigation heads outside of the flow path if they are near inflow and outflow points.
Locate trees and shrubs at least 5 feet clear from the edge of the inlet to allow for tree/shrub growth without blocking flow path and ease of maintenance.
Review existing roadway and sidewalk surface grades and regrade or repair any uneven surfaces to ensure that overflow stormwater will flow from the bioretention cell into the street/gutter (or into an overflow drainage structure, if applicable), and not onto an adjacent property or the sidewalk.
Evaluate street grades to confirm that irregularities in the pavement surface do not prevent stormwater runoff or gutter flow from entering or exiting the bioretention facility. Sheet flow off the pavement can easily be unintentionally diverted via a crack or joint in pavement. In retrofit projects, it may be necessary to replace some street pavement to improve water flows from the crown to the gutter, and into the bioretention cell via the curb cut.