BMP Monitoring Sites - Urban Drainage and Flood Control District

UDFCD BMP MONITORING SITES
Open Bed Sand Filter
Impermeable
Membrane and
Geotextile Liners
When expansive or NRCS Type D soils are present, or
when standard percolation tests show percolation
drawdown rates exceeding 60 minutes per inch, or
potential for groundwater contamination exists, install a
16 mil thick impermeable liner on the bottom and
sides of the basin. If vertical walls are permeable or of
stacked blocks, extend the impermeable liner behind the
walls.
If soils are not expansive (i.e., NRCS Type A, B or C),
use a woven geotextile material that meets the
following ASTM requirements:
o D-4751 – AOS U.S. Std. Sieve #50 to #70
o D-4633 – Trapezoidal tear strength >= 100x60 lbs
o COE specified open area >= 4%.
Products meeting requirements: US Fabric US 2070 and
US 670, Mirafi Filterweave 500 and 700, Carthage Mills
Carthage 6%.
Wrap all liners to top of the sand filter basin and attach
firmly with staples to the soil vertical wall using staples or
concrete anchors. Provide sufficient slack so that the
liners are not stretched when rock and sand/peat mix is
placed. If tears are seen or discovered, repair them as
recommended by manufacturer with no less than 18
inches of overlap on all sides of the tear.
Outlet Works
When underdrains are needed, the outlet
works consists of 4” perforated HDPE
pipe to convey water to the overflow outlet
structure. Space perforated pipe on 20 foot
centers or less. At the outlet of the HDPE
pipe into the box, install an orifice sized to
empty the WQCV above the sand in no less
than 24 hours.
Provided an overflow outlet pipe out of the
overflow structure to convey flows away from
the filter basin when the runoff volume exceeds
the WQCV at rates required by local
jurisdiction to control the flood detention,
typically the 10- and the 100-year storm.