STP

Determine the most appropriate and accurate STP type based on definitions provided. Enter the STP type in the calculator. Calculate the STP storage volume based on the “method for calculating design storage volume” column, below.

STP Type	Description	Method for calculating Storage Volume (SV)	Storage Volume Formula
Extended Dry Detention Basin	Provides temporary detention storage for the design storage volume to drain in 24 hours through multiple out let controls.	Ponding volume prior to high flow bypass	SV = Apond × Dpond (does not include pretreatment volume)
Grass Conveyance Swale	Conveys runoff through an open channel vegetated with grass. Primary removal mechanism is infiltration.	Volume of swale at full design flow	SV = Lswale × A crossect. swale
Gravel Wetland	Provides surface storage of runoff in a wetland cell that is routed to an underlying saturated gravel internal storage reservoir (ISR). Outflow is controlled by an orifice that has its invert elevation equal to the top of the ISR layer and provides retention of at least 24 hours.	Pretreatment volume + ponding volume + void space volume of gravel ISR.	SV = (Apretreatment × Dpretreatment) + (Awetland × Dponding) + (AISR × Dgravel × ngravel)
Hydrodynamic (swirl) Separator Structure	Devices designed to improve quality of stormwater runoff by physically removing sediment and nutrients. Must be a stand-alone practice to receive P reduction credit, if included as pretreatment for another practice, no additional credit is given.	Hydrodynamic Structure	N/A (Must be properly sized and installed according to manufacturer)
Infiltration Trench	Provides storage of runoff using the void spaces within the soil/sand/gravel mixture within the trench for infiltration into the surrounding soils.	Void space volumes of stone and sand layers	SV = (Atrench × Dstone × nstone ) + (Atrench × Dsand × nsand)
Porous pavement w/ impermeable underlining or underdrain	Provides filtering of runoff through a filter course and temporary storage of runoff within the void spaces prior to discharge by way of an underdrain.	Depth of Filter Course	DFC
Porous Pavement with infiltration	Provides filtering of runoff through a filter course and temporary storage of runoff within the void spaces of a subsurface gravel reservoir prior to infiltration into subsoils.	Void space volumes of gravel layer	SV = (Apavement × Dstone × nstone)
Rain Garden / Bioretention (w/ underdrain)	Provides storage of runoff by filtering through an engineered soil media. The storage capacity includes void spaces in the filter media and temporary ponding at the surface. After runoff passes through the filter media it discharges through an under-drain pipe.	Ponding water storage volume and void space volume of soil filter media.	SV = (Abed × Dponding)+ (Abed × Dsoil × nsoil)
Rain Garden / Bioretention (no underdrains)	Provides storage of runoff through surface ponding and possibly void spaces within the soil/sand/washed stone mixture that is used to filter runoff prior to infiltration into underlying soils.	Ponding water storage volume and void space volumes of soil filter media.	Example for raingarden: SV = (Apond × Dpond) + (Asoil × Dsoil × nsoil mix)
Sand Filter w/ underdrain	Provides filtering of runoff through a sand filter course and temporary storage of runoff through surface ponding and within void spaces of the sand and washed stone layers prior to discharge by way of an underdrain.	Pretreatment volume + ponding volume + void space volume of sand and washed stone layers.	SV = (Apretreatment × DpreTreatment) + (Abed × Dponding) + (Abed × Dsand × nsand) + (Abed × Dstone × nstone)
Subsurface Infiltration	Provides storage of runoff using the combination of storage structures and void spaces within the washed stone within the system for infiltration into the surrounding soils.	Storage volume of storage units and void space of backfill materials.	Example for subsurface galleys backfilled with washed stone. SV = (L × W × D)galley + (Abackfill × Dstone × nstone)
Surface Infiltration	Provides storage of runoff using the combination of storage structures and void spaces within the washed stone within the system for infiltration into the surrounding soils.	Volume of storage structure before bypass.	Example for linear trapezoidal vegetated swale. SV = (L × ((Wbottom + Wtop@Dmax) / 2) x D)
Wet Pond	Provides treatment of runoff through routing through permanent pool.	Permanent pool volume prior to high flow bypass	SV = Apond × Dpond (does not include pretreatment volume)

Key
SV	Storage Volume = physical storage cpaacity to hold water
L	Length
W	Width
D	Depth
n	Porosity fill material
A	Average surface area for calculating volume