Outlet erosion protection 1

[Trackfiend]

I am designing a outlet (24" culvert) to discharge into an existing roadside ditch. The discharge pipe will be entering the ditch at roughly a 45 degree angle to the ditch bank. The ditch has a 3' bottom and directly across from my dicharge pipe is an existing 24" concrete box culvert with wingwalls. The velocity coming out of my discharge pipe is 8 ft/s.

I am wondering what type of device would you use to prevent scouring out the ditch bottom between the two pipes? My first reaction was to use rip-rap in the affected area, but my experience is limited in the effectiveness of rock-lined channels (i.e., I've only generally used it around splash blocks exiting parking lots). I realize that rip-rap is not an effective energy dissipator, but installing a concrete stilling basin with baffles is kinda overkill. Any thoughts would be helpful.

 

[dicksewerrat]

Take some pitch off the pipe and slow down the discharge. then put a stilling basin on the end of your pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[tsgrue]

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See information on Compact Weir Outfalls in the threads below:

thread194-199558

thread160-190926

Such an outlet structure could address your problem.

.



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

http://hhwq.blogspot.com

 

[gbam]

Have you considered concrete lining between the two culverts; maybe grouted riprap? Also, if dumped riprap is sized properly it should not fail. Is the outlet of the second culvert protected? If not does the box velocity warrant outlet protection, ie plunge basin.

 

[cvg]

it is hard to recommend something because you really have not provided much information. you don't give any information on the flow in the ditch or in either of the pipes. How wide is the box culvert? Is the ditch flat and flowing slow and deep or steep and flowing swiftly? Is the ditch lined with anything? What is the capacity of the ditch? How deep is it and how deep are the pipes? what is the slope of the banks of the ditch?

You will need to protect from erosion and riprap can do that. You can also grout the riprap, use concrete lining or other erosion protection. All will work, some with higher cost and some with more maintenance required. It seems that your larger problem is that you will need to have sufficient freeboard so the flow can turn and you could have significant turbulance due to the flow from the two culverts intersecting in the middle of the ditch.

 

[Trackfiend]

I apologize if my initial description was vague. As far as the additional info, here's what I have to work with:

1. Due to restrictions coming from the client, I am not able to contact any state or county official in regards to method, or regulations. I am however following that state's guidelines per their DOT regulations.
2. The only flow in the ditch is the flow that I have redirected via the new discharge pipe. All other flows have been rerouted thru the new underground system. This essentially renders the "old" ditch useless.
3. The existing ditch containing the 24" concrete box culvert is overgrown and filled in with sediment. I will be calling for the ditch to be cleaned and regraded to accomodate my new drainage. This will only be done in the immediate area around the inverts of the two pipes and some portion of the ditch both north and south so as not to have a pit. Installing a catch basin is not an option. The existing 24" box culvert has wingwalls.
4. The slopes on the ditch are 2:1. If the ditch were fully dug to its original design, then a trapezoidal section with a 3' bottom roughly 8' deep would due it justice.
5. No the ditch is not lined with anything.
6. Both the existing pipe (roadside and projecting perpendicular) and my new discharge pipe (property side and projecting 45 degrees to ditch bank) inverts are located at the ditch bottom. (If the top of bank of ditch is at elevation 100.00', then the inverts of both pipes would be at elevation 92.00'.
7. The flow is 22.84 cfs (discharge pipe is operating like an orifice). The pipes are located directly across from each other so any discharge from my pipe will immediately hit the ditch bottom and go directly (at least I hope) into the existing 24" box culvert.

I appreciate all of the comments and it seems that rip-rap would be a decent (and cheaper), route to take.

 

[dicksewerrat]

The ditch owner/operator may have abandoned the ditch and your client will own it. Sounds like your client may be trying something not quite kosher. Once the ditch owner is contacted, you will have to do whatever they say. Doesn't make sense to do the job twice.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Trackfiend]

"The ditch owner/operator may have abandoned the ditch and your client will own it."

This is not the case since the ditch seems to be in a right-of-way owned by the state. I say "seems" because I was able to glean some information about the regulations governing said ditch before I was reprimanded for "muddying up the waters" by talking to county/state officials. The client specifically stated that they have "permit people" that deal with that sort of thing. Luckily I was fairly persistent in the early stages of the project (before being told to not do my job) to fairly confirm that the ditch is governed by the state (it is alongside a rural road designated by a state highway number). I was in the process of fortifying this confirmation when told to back off. Since then, I have made sure that my design conforms to state regulations concerning drainage/detention (there are no county restrictions for this site) so as to cover my back, so to say.

 

[dicksewerrat]

I am not sure I would work for a client that wants to keep each part of the project blind from the other. Does he have this permit? Will you be liable ifhe doesn't? Who is going to do the actual work?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com