Plunge Pool Design 2

[Ryb01]

Hello All,

I'm currently working on a project requiring erosion protection. We have a 1.2m dia culvert discharging 3.7m3/s to a channel, 120m long @ 5.0% grade, 2.0m bottom width, 2:1 side slopes, approx 1.8m deep. This channel has an abrupt transition to approx 0% near the outlet to a environmentally sensitive creek. I'm trying to determine ways to 1) armour the channel with rap for erosion protection and 2) determine the best possible solution for erosion protection at the transition from 5% to 0%.

I was initially thinking a plunge pool may be appropriate for erosion protection and also reducing the velovity. I've been referencing HEC-11,14 and 15. The plunge pool design in HEC-14 is for a culvert outlet only. Could anyone recommend some additional information or provide any advice on desiging plunge pools or another method of erosion protection at the transition? Thank you in advance for your advice.

RYB

 

[gbam]

It appears that if you do not protect the channel it will erode, v=14fps +/-. Have you considered flattening the slope and using drops? Then you will need to evaluate the cut-off walls/sills for scour and protect them.

 

[msquared48]

How about orienting the end of the pipe to a vertical position about 10 degrees off vertical, casting the outlet end in concrete, and allowing for the vertical drop to the plunge pool. This would minimize the horizontal vector for any scouring, and dissipate a lot of the system energy. Obviously, a large thrust block would have to be designed.

Mike McCann
MMC Engineering

 

[cvg]

it seems that your channel will experience the erosion with sediment deposition being the problem at the abrupt slope change at the outlet end. drop structures in the channel to reduce the flow to subcritical might be the way to go along with channel lining. Also, check your channel flow depth - think 1.8 meter looks a bit high

 

[Ryb01]

Thank you to all who has replied. This stream/channel is located in a hilly type region with an interesting scenario. The soil types along the channel banks are a composite of A and B soil groups, while the bottom of the channel consists of shale rock. Sediment deposition is definielty an issue. Currently, sediment has been deposited over time filling in the channel at the transition point and causing runoff to chosse alternate routes through residential areas before entering the creek. We do have a little flexibility to create some drop structures.

Could anyone recommend a specific reference for this type of drop structure design?

cvg...i agree 1.8m looks high, this is the total channel depth(inv to TOB), flow depth is approx 1m.

Thanks again in advance for your help.

 

[cvg]

see attached reference for UDFCD (Denver) which has a very good analysis of several types of drop structures.

 

[civilperson]

A naturally occurring hydraulic jump will use up the velocity energy and bring the water level near the hydraulic grade line at the transition to the flat portion. No need for a pool, just bank lining for the first 100 feet of the flat channel.