rip rap design for detention basin? 2

[Cherie]

We're designing a detention basin with a 25' wide, 2' deep concrete inflow spillway, that will take a maximum inflow of 336 cfs during the 50 yr storm. The spillway flows into a concrete low flow swale at the floor of the basin. On either side of the spillway at the bottom of the slope in the basin, we have riprap.

Does anyone know what the standard sizing of that riprap should be? If I look at the "Greenbook" SSPWC standards section 200-1.6.2, I would take a wild guess and say the 375 lb class would be appropriate, but I don't know the design criteria. Let me know if you have experience with this, and what your advice would be. Thanks!

 

[bltseattle]

Some references are:

FHWA HEC-15 Design of Roadside Channels with Flexible LInings
THis ref presents a shear stress design approach. Once you know the shear stress, the riprap d50 (ft) is (rule-of-thumb) about 1/4 of the shear stress (psf).

http://www.fhwa.dot.gov////////bridge/hydpub.htm#hec

I think there is also free govt software to do the calcs.


Another good reference is USACE EM 1110-2-1601 Hydraulic Design of Flood Control Channels

http://www.usace.army.mil/usace-docs

Since you have high flow dropping into the pond you should also consider an energy dissipation pool sized to contain the hydraulic jump. Check out FHWA HEC-14 for guidance

http://www.fhwa.dot.gov////////bridge/hydpub.htm#hec

BLT

 

[LHA]

Why riprap? At 336cfs on the side of a basin (slope over 10%?), you are going to need the largest stone available.

There are now very tough, very affordable Turf Reinforcment Mats available, and grass will grow through them without degrading them. Material cost might exceed stone, but placement is labourers using hand tools with a loader moving the rolls. North American Green and ACF Landlok are two which provide free design software to determine which material you need, with printable reports. I would take a look at them first.