stormwater energy dissipator for steep slope

[1plan2design3constr]

Need suggestions on energy dissipator for an urban trunk line w/ limited space for downstream structures before connecting to existing 10' x 8.5' RCB at 0.08 slope w/ already 150 cfs upstream of the connection : urban trunk line Data -31% slope ( 13.6 ' drop for 44 ' length) w/ 145 cfs to 175 cfs of flow ( 25 year storm) -
One suggestion is internal energy dissipator with 3 velocity control rings. HECRAS shows both the headwater and tailwater as submerged with hydraulic jumps between the rings.
Questions:
1- is HECRAS the right tool for this analysis ?
2- is there a limit to the effectiveness of the rings?
3- Need considderation for more frequent, smaller flows ( 10 year, 5 year storms) where the headwater and tailwater may not be submerged ?
4- Need suggestion for constructability on steep slope .
5- How does velocity control ring compare to the impact type energy dissipators?

 

[tsgrue]

.

I suggest you consider a compact weir outfall and check out the following posts:

Length of culvert's apron
thread160-190926

Hydrology Software
thread162-201630

Hope that helps!

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[sknight701]

You might want to take a look at the Federal Highway Administration HEC-14 document.

 

[saruman26]

Is this an open channel or closed conduit (i.e. pipe) flow condition? A stepped approach with pipes/channel at slopes less than 5% or so and a few vertical drops in the system may help to reduce velocity while still providing adequate capacity.

 

[msquared48]

How about a drop Catch basin?

Mike McCann
MMC Engineering

 

[1plan2design3constr]

Hi All,

already seen HEC 14 but not enough info for design.

drop structure seems good, but need details on the height of drop, length of base and protection for the base slab. Apprecaite a detail if available. If multiple drops are needed, need design guide on such a structure. Total length required for the steps of drops will be a concern as 44' is all we can have.

the upstream structure is proposed to be a 60" pipe connecting to the existing RCB.

Drop catch basin is in line with the drop concept, but the total fall is 13', maybe too much for a CB!

 

[saruman26]

I've worked on a similar situation before, though with smaller pipes and lesser flows. In this case, it sounds like to have adequate space to do two drops in the new run of 60" RCP. This will likely require a

 

[saruman26]

Accidentally hit 'submit'

I've worked on a similar situation before, though with smaller pipes and lesser flows. In this case, it sounds like to have adequate space to do two drops in the new run of 60" RCP. This will likely require at least an 84" inside diameter manhole to accommodate the outside diameter of the pipe. Each drop can be 4-5 feet or so, with each individual pipe run set at 25% or so. The end run can core directly into the existing RCB, and a manhole riser can be cut into the top of the existing RCB if necessary.

As for erosion of the bottom slab, thickening the slab a few extra inches to allow a 'wearing surface' so to speak, should be reasonable.

 

[1plan2design3constr]

Thanks !

The drop MH concept sounds reasonable and will be an improvement over the original 31% sloping pipe of 44' length.

However, if the total drop is only 13.6' & we are dropping 4 to 5' each MH, the pipe slope between MH's can be very mild, why "25%" pipe run?


If we connect the drop MH with pipes, they will be acting like orifice at high flow, the concern may not be erosion at bottom, but flow restriction at orifice. Any comments or suggestions?

Thanks!

 

[saruman26]

Oops, typo on my part. That should read 2.5%.

As for the potential for orifice flow, that's a good question. Without running a hydraulic model, I'd guess that orifice flow would be a greater concern if the pipe is surcharging a bit. Perhaps I'll try to model such a system in StormCAD if I get a little extra time.

Good luck!

 

[1plan2design3constr]

Thanks!

I assume we are in agreement that a drop MH system works better than velocity control rings .

If the orifice situation also works for the high flow, then this maybe the way to go!!

Many thanks !

 

[dicksewerrat]

When we did drops with high flow or long drops, we paved the bottom of the drop with 8 inch thick granite blocks. They last alot longer than concrete.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[1plan2design3constr]

Thanks Richard!

Do you have a sample spec. for the granite blocks?

Do you have any history and descriptions of how they perform ?

Appreciate your reply!

 

[dicksewerrat]

I hate to tell you this but we used old granite paving blocks that we dug out on other projects. We stored them for this purpose. I suppose you could look back in time and find a paving block spec from about 1880. that iswhen these went in. My guess is they used whatever was available in Minnesota at the time.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[1plan2design3constr]

OK Richard, we'll find something equivalent , thanks!