rip rap scour protection design

[Chewi00]

I am looking for a design guide for rip-rap placement downstream of a vertical drop water control structure. I am looking for a design including stone sizing and length and width of the apron to prevent erosion of the river bed. The site is a 70' wide river which will flow over this submerged weir (stoplogs)There is a 2-5' difference between headwater and tailwater depending on conditions. Everything I have found relates to pipe or culvert discharges where the flow is not re-entering a downstream channel. This is a temporary control structure, so I would like to avoid creating a stilling basin/concrete apron if possible. Flows over this 70' wide structure range from 1100 to 5500 cfs running at about 3' deep over the top of the stoplogs. Any guidance would be appreciated.

Thanks in Adavance

 

[francesca]

The Nashville storm water manual has an equation that lets you calculate your manning's n based on channel characteristics and riprap size. It also has velocities for which various sizes or riprap are recommended. I think the upper limit for riprap in their manual is around 8-10fps. The latter, if I remember correctly, is for ditches, but I believe that the velocity limit is for stability of the riprap and underlying soil.

If this is a stream, it'll be a water of the state, so whatever you propose to do to it will need to be approved, and the USACE may need to be involved. It's probably worth a call to your local corps office who will be able to advise you, and they may have a publication on temporary channel stabilization. Better yet, follow this link.

 

[francesca]

Oh, I forgot... you can do a scour analysis in HEC-RAS.

 

[Chewi00]

Thanks for the quick response.....this design is going to the agency for review. Velocity wise we are talking about anywhere from 6 fps (median flow) to 35 fps (10-yr event).

 

[blueoak]

This seems like a pretty intense stoplog structure with 5' of head at a flow of 35 fps. With these velocities is the the river downcutting? I am used to these velocities in mountain (bedrock) streams. Depending on your toe it might be useful to create some backwater at the toe and flip the water into it (step the water back to the river instead of a single drop).

I'll check my ref's, but with these velocities you might consider a concrete blanket and add cobbles and boulders to the surface. This may provide some habitat or natural value that you could leave it in.

 

[Chewi00]

my error....The velocities range from 5-9 fps.

 

[Prepakt1]

8"concrete fabriform mat is good for flows of 4ft/sec on a long term basis.For a temp structure this might be adequate or you can go to a thicker mat 10" or 12 "


Intrusion Prepakt

http://www.marineconcrete.com

 

[cvg]

try USBR engineering monograph No. 25 - "Hydraulic Design of Stilling Basins and Energy Dissipators". According to figure 165, you will need a minimum of 12" riprap or approximately 100 lb rock for 9 fps. Layer thickness should be 24" thick minimum. If you are placing it on the channel banks, go a bit larger. You may want to recheck your tailwater since you will be placing the riprap on natural ground below the drop structure, it may cause the tailwater to come up higher.

 

[bltseattle]

I agree with cvg's suggested reference. Another reference is FHWA HEC-14 that shares much info with USBR #25. Either reference would faciliate analyzing potential scour hole formations and the length of the hydraulic jump downstream; you should extend rock protection at least far enough to contain the jump.

Make sure you check buoyancy, logs with +3 ft of water may have large uplift force to restrain against, in addition to the issue of potential undermining from scour hole.

This drop + apron sounds well-suited for gabions & reno mattress, consider checking those out. (no flotation, excellent scour resistance)

 

[blueoak]

With these numbers I would be surprised if you have a significant jump, however it gives you a conservative number for design.
I like bltseattle's mention of a reno mat with a thick geotextile. This will allow for much smaller stone than 12". This way if you have some scour the mat will drop into it. Also you can tie in thicker gabions at the end to produce a limited roll. This can shorten the length. However, with straight stone you may not need to go back to remove it. Sorry I think I boxed my stream references.

 

[Chewi00]

thanks everybody.......all those references were very usefull. We have decided to go with 1.5' gabion mattresses underlain with geotextile using maccaferri's Macra2 design software and verifying using hydraulic equations provided in the above references. Looks like about a 25' long by 70' wide (channel width) basin with a 3' high gabion counterweir at the end will normalize flow for our most severe condition.