STRUCTURAL MEASURES: CONSTRUCTION, OPERATION, AND MAINTENANCE - Low Impact Development and Sustainable Stormwater Management - page 166

4. Pervious pavement and infiltration beds should not be placed on areas of
recent fill or compacted fill. Any grade adjustments requiring fill are made using
stone subbase material. Areas of historic fill ( > 5 years) may be considered for
pervious pavement, but permeability testing is essential.
5. For the bed to infiltrate, the bed bottom must not be compacted. The stone
subbase should be placed in lifts and lightly rolled according to the specifications.
6. During construction, the excavated bed may serve as a temporary sediment
basin or trap. This will reduce overall site disturbance. If used as a temporary
sediment basin or trap, the bed is excavated to an elevation of at least 12 in.
higher than the designed bed bottom elevation. Following construction and site
stabilization, sediment is removed and the final grades established.
7. Bed bottoms must be level or nearly level. Sloping bed bottoms will lead
to areas of ponding and reduced stormwater distribution within the bed.
8. All systems are designed with an overflow system. Water within the sub-
surface stone bed should never rise to the level of the pavement surface. Inlet
boxes can be used for cost-effective overflow structures. All beds should empty
within 72 hours.
9. While infiltration beds are typically sized to handle the increased volume
from the more frequent small storms, they must also be able to convey and
mitigate the peak rates of the less frequent, more intense storms (such as the
100-year storm). Water-level control in the beds is usually provided in the form
of an outlet control structure. A modified inlet box with an internal weir and
low-flow orifice is a common type of control structure (see Figure 7-6). The
specific design of these structures may vary, depending on factors such as rate
and storage requirements, but it must always include positive overflow from the
system. Standing water must never be allowed in the porous pavement.
10. The subsurface bed and overflow should be designed and evaluated in
the same manner as a detention basin to demonstrate the mitigation of peak flow
rates. In this manner, the need for a detention basin will be eliminated or reduced
significantly in size.
11. A weir plate or weir within an inlet or overflow control structure may be
used to maximize the water level in the stone bed while allowing sufficient cover
for overflow pipes.
12. Perforated pipes along the bottom of the bed may be used to distribute
runoff evenly over the entire bed bottom. Continuously perforated pipes are
connected to structures (such as cleanouts and inlet boxes). Pipes must lay flat
along the bed bottom and provide for uniform distribution of water. Depending
on size, these pipes may provide additional storage volume.
13. Roof leaders and area inlets may be connected to convey runoff water from
adjacent areas to the bed. Water quality inserts or sump inlets are used to prevent
the conveyance of sediment and debris into the bed.
14. Infiltration areas should be located within the immediate project area to
control runoff at its source. Expected use and traffic demands are also considered