New Federal Guidelines require all sediment basins over 1 acre to drain from the top of the water column.  Below are Wake County's minimum requirements for sizing the sediment  basin and   the associated weir (if utilized).  


Sizing the Storage Area

Surface area requirements must be met for all sediment basins.
435 X Q10* = surface area required for ALL types of sediment basins.
*Use minimum C=0.5 for areas draining to basin.
Sediment Basin volume is determined by 1800 cu/ft per Drainage Acre.
Sediment Trap volume is determined by 3600 cu/ft per Drainage Area.
The maximum depth allowed is 4 feet. (from bottom of basin to weir)
The minimum depth required is 2 feet. (from bottom of basin to weir)
The floor of all basins should be roughly level.
Basin should dewater in 3 to 5 days.
Use required Sediment Basin volume (1,800 ft3 per Drainage Acre) to size skimmer and orifice diameter. 

Sizing the Weir

Weir Equation: (Minimum weir length of 10')
Q= CLH3/2. C=3.0 (max). H for Q is not to exceed 0.5'.
An additional 1.0' of weir capacity is required for the conveyance of larger storm events; therefore, the total weir depth will be 1.5'.
A minimum weir length of 10' is required.

Show calculations for skimmer sizing and drawdown.
Show calculations for the sizing of the anti-flotation block.