21.3 A300 - Flow Spreading Devices
Flow spreaders function to uniformly spread flows There are five flow spreader options presented in this section:
Option A – Anchored plate (Figure 4 - 56)
Option B – Concrete sump box (Figure 4 - 57)
Option C – Notched curb spreader (Figure 4 - 58)
Option D – Through-curb ports (Figure 4 - 59)
Options A through C can be used for spreading flows that are concentrated. Any one of these options can be used when spreading is required by the facility design criteria. Options A through C can also be used for unconcentrated flows, and in some cases must be used, such as to correct for moderate grade changes along a vegetated filter strip (BMP T1040).
Options D and E are only for flows that are already unconcentrated (such as entering a Vegetated Filter Strip (BMP T1040) or a continuous inflow biofiltration swale (BMP T1030). Other flow spreader options may be allowed with written approval from Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001.
21.3.1 General Design Criteria
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
Where flow enters the flow spreader through a pipe, it is recommended that the pipe be submerged to the extent practical to dissipate as much energy as possible.
Flow spreaders are difficult to maintain in a way that allows for continual evenly distributed flow. Flow spreaders should not be used on slopes greater than 5% or areas easily accessible to the public.
For higher inflows (greater than 5 cfs for the 100-yr storm), a Type 1 catch basin shall be positioned in the spreader and the inflow pipe shall enter the catch basin with flows exiting through the top grate. The top of the grate shall be lower than the level spreader plate, or if a notched spreader is used, lower than the bottom of the v-notches.
An anchored plate flow spreader must be preceded by a sump having a minimum depth of 8 inches and minimum width of 24 inches. If not otherwise stabilized, the sump area must be lined to reduce erosion and to provide energy dissipation.
The top surface of the flow spreader plate must be level, projecting a minimum of 2 inches above the ground surface of the stormwater treatment facility, or V-notched with notches 6 to 10 inches on center and 1 to 6 inches deep (use shallower notches with closer spacing). Alternative designs may also be used if approved by Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001.
A flow spreader plate must extend horizontally beyond the bottom width of the facility to prevent water from eroding the side slope. The horizontal extent shall be such that the bank is protected for all flows up to the 100-year flow or the maximum flow that will enter the stormwater treatment facility.
Flow spreader plates must be securely fixed in place.
Flow spreader plates may be made of either wood, metal, fiberglass reinforced plastic, or other durable material. If wood, pressure treated 4 by 10-inch lumber or landscape timbers are acceptable.
Anchor posts must be 4-inch square concrete, tubular stainless steel, or other material resistant to decay.
Figure 4 - 56: Flow Spreader Option A – Anchored Plate
The wall of the downstream side of a rectangular concrete sump box must extend a minimum of 2 inches above the flow spreading area. This serves as a weir to spread the flows uniformly across the area.
The downstream wall of a sump box must have “wing walls” at both ends. Side walls and returns must be slightly higher than the weir so that erosion of the side slope is minimized.
Concrete for a sump box can be either cast-in-place or precast, but the bottom of the sump must be reinforced with wire mesh for cast-in-place sumps.
Sump boxes must be placed over bases that consist of 4 inches of crushed rock, 5/8-inch minus to help assure the sump remains level.
Figure 4 - 57: Flow Spreader Option B – Concrete Sump Box
Option C -- Notched Curb Spreader
Notched curb spreader sections must be made of extruded concrete laid side-by-side and level. Typically five “teeth” per four-foot section provide good spacing. The space between adjacent “teeth” forms a v-notch.
Figure 4 - 58: Flow Spreader Option C – Notched Curb Spreader
Option D -- Through-Curb Ports
Unconcentrated flows from paved areas can use curb ports or interrupted curbs (Option E). Curb ports use fabricated openings that allow concrete curbing to be poured or extruded while still providing an opening through the curb to admit water to the stormwater treatment facility.
Openings in the curb must be at regular intervals but at least every 6 feet (minimum). The width of each curb port opening must be a minimum of 11 inches. Approximately 15 percent or more of the curb section length shall be in open ports, and no port shall discharge more than about 10 percent of the flow.
Figure 4 - 59: Flow Spreader Option D – Through-Curb Ports
Interrupted curbs are sections of curb placed to have gaps spaced at regular intervals along the total width (or length, depending on facility) of the area where flows are spread. At a minimum, gaps must be every 6 feet to allow distribution of flows into the treatment facility before they become too concentrated. The opening must be a minimum of 11 inches. As a general rule, no opening shall discharge more than 10 percent of the overall flow entering the facility. See City of Tacoma Standard Details GSI-09.