Stormwater Manual - Pima County Flood Control District

Embankment heights shall not exceed 20 feet (measured from the downstream toe) and
storage volumes shall not exceed 25 acre-feet. The minimum storage volume shall be 0.3
acre-feet. Regional facilities may exceed these limits, but they must comply with the
applicable requirements of the Kentucky Division of Water.
Design earthen embankments with side slopes not steeper than 3:1 (horizontal to vertical).
Provide anti-seep collars where the spillway barrel passes through the embankment.
Stabilize earthen embankments immediately with temporary or permanent vegetation in
accordance with requirements of Chapter 11.
Reserve adequate access from public or private right-of-way by establishing a maintenance
easement. Design the access to be at least 10 feet wide and not steeper than 5:1 (h:v) or less.
Design the access way to connect to the embankment so that equipment can access the top of
the embankment on a slope not greater than 5:1 (h:v).
Provide a minimum 25-foot wide buffer strip between the pond and the nearest property line.
Landscape the buffer strip with low-maintenance native grasses, shrubs, and trees. Provide a
landscaping plan for the pond and the buffer. Objectives of landscaping include improving
the appearance for adjacent residents and providing wildlife habitat.
10.8.3 Design Procedures
Design procedures for stormwater quantity and peak discharge control are the same for
extended detention ponds and traditional dry detention ponds, except that the design WQV
will be retained longer in the extended detention pond. To design the storm detention
volume and peak control structure for an extended detention pond, follow the procedures
given in section 10.7.3 and assume for design purposes that the elevation of the dry pond
bottom corresponds to the elevation of the surface of the design WQV (i.e., the top of the
extended detention pool).
Sand Filter Outlet
Figure 10-13 illustrates an outlet configuration that may be used to regulate discharge of the
extended detention pool. A perforated riser may also be used.
To size this device pick a preliminary configuration and check it using the falling head
permeability equation. Set t equal to 24 hours and calculate k. If the calculated k varies
significantly from 3.54 ft/hr, adjust the filter dimensions and recalculate.
The falling head equation is:
k = 2.303 * (aL/At) * log (H/h)
where:
k = coefficient of permeability (ft/hr),
a = average surface area of extended detention pool (ft 2 ),
L = depth of sand (ft),
STORMWATER MANUAL 10-29 January 1, 2009
LEXINGTON-FAYETTE CO., KY.