Experiences with batch detention systems

[psmart]

Does anyone have experiences with batch detention systems they would like to share? I'm particularly interested in the design and modeling aspects of these systems. Since they involve active outlet controls (valves, gates, etc) they cannot be modeled with a standard head-dependent outlet device, such as a weir or orifice.

Here's a brief summary:

Batch detention basins capture and temporarily detain the water quality volume from a storm event using an automated controller and valve. They are intended to serve primarily as settling basins for the solids fraction, and as a means of limiting downstream erosion by controlling peak flow rates during erosive events. Batch detention basins may be constructed either online or offline, however, offline structures are preferred. Batch detention basins are designed to prevent clogging of the outflow structure and resuspension of captured sediment during a discharge. They also provide enhanced dissolved pollutant removal performance. The batch detention design typically incorporates a non-clogging outflow structure, such as an orifice protected by a trash rack, or a perforated riser pipe protected by riprap. Batch extended detention basins may be used in very small watersheds (no minimum size), since the discharge is regulated by a valve instead of an orifice. In addition, batch detention basins tend to accumulate debris deposits rapidly, making regular maintenance necessary to minimize aesthetic and performance problems. Batch detention basins can readily be combined with flood and erosion control detention basins by providing additional storage above the water quality volume.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

No experience, but there's some play in South Carolina and Georgia to move to floating skimmers for outlet controls on sediment basins. Idea is the orifice is just below the surface, maybe six inches, and is suspended by floats. Drains through a pipe that has a couple of adjusting bends so the float stays near the surface. The point is to pull water from the less sediment laden portion of the pond, letting the solids accumulate at the bottom.

I haven't had to model one of these yet, because they're temporary BMPs, but if I did I'd probably just back calculate the orifice flow based on constant head and use a constant discharge in the routing.

In these batch systems, do the valves adjust based on head or something? I would think unless there was some kind of widget that changed the valve opening based on head, the discharge relationship would still be based on head.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[gbam]

Peter - My guess these would be modelled similar to the water supply model with gravity tanks and operational valves. I have not analyzed these since college; but, I am sure Water-Gems, Water-CAD or other software packages could be used that use time enabled valves.

 

[cvg]

never used an automatic controller / valve on a basin. Designs I have been involved with have manually operated slide gates, generally intended for low level drainage. They can also be opened during flood stage if necessary for more operational control of the reservoir level. Assuming that the valve is either open or closed, than you would just need to set the date/time of opening and closing in your model. Once it is open, than discharge is just based on the stage-discharge relationship of the orifice.
Don't think a constant outflow is an accurate way to model an orifice.

 

[psmart]

These are active systems. Here's a typical sequence:

1 Wait till WSE rises to target level (treatment volume)

2 Wait a further 12-48 hours for settling

3 Immediately lower control weir to current WSE

4 Gradually lower weir to empty pond at constant rate over desired draw-down time.

5 Reset weir for next event.

All monitored and managed by a smartphone app.

Peter Smart
HydroCAD Software

http://www.hydrocad.net