Channel Restoration 4

[hacrock]

I am currently working on a channel restoration design. The problem is a woefully undersized channel for the amount of flow running through it which causes flooding on adjacent properties.

Is it better to follow the existing path of the channel or can I realign the channel in certain areas in an effort to "give back" yard area to the property owners adjacent to the channel.

Another issue is the size of the channel. To contain the runoff from the entire drainage basin would require a very wide channel. I have broken the drainage basin into 5 subbasins being separated in the channel by grade structures in an effort to keep the width of the channel down. It is my understanding (and I may be incorrect) that once flow drops over a grade structure, the velocity of that flow is zero and does not transfer to the next subbasin. Since the velocity is zero, the flow is zero and it is like there is no flow coming into the next subbasin. Is this thinking correct or will I have to consider the cumulative flow and ultimately end up with a channel getting wider as it goes downstream?

Thank you for any help/clarification/guidance!

 

[cvg]

are you talking about a river or a man made channel? If it is man made, you might consider re-alignment. If it is a river, probably not.

I am not tracking you on the whole "drainage basin / grade structure" question. A sketch might help to define what you are asking. In general, when water flows over a channel grade control structure, it does not stop - it keeps flowing down the channel. Constructing grade controls in a channel would probably require a wider channel.

 

[CPENG78]

hacrock,
Please provide more detail. If it is a man made channel, you may consider making it deeper to increase the capacity of the channel from the point where capacity is an issue all the way to the outfall (to avoind increasing the footprint). Of course, I don't know if this falls under your scope of work. If it doesn't, other alternatives would have to be explored. This "grade structure" is it like a retaining structure or will the excess flow be diverted somewhere else?. Velocity may be zero but at some point that water will discharge unto the next subbasin. Possibly a sketch may help.

 

[Ryb01]

Before starting your natural channel design take the time to obtain some background information of the watercourse in your area. Data to collect should include the following to start;

1)Historical Aerial photography
2)Flows (stream gage or calculated)
3)Floodline Mapping (if available)
4)Information pertaining to aquatic life in your area (if applicable)
5) Soil types
6) Meander Belt
7) Slope of channel
8) Upstream and downstream channel conditions. What is their current states? Erosion problems? Flooding? etc
9) Existing vegetation within the watercourse, near your project site
10)Buget constraints
11) Client expectations

These are a few main areas of interest that will have a significant impact of the direction and overall outcome of the design.

Channels generally have an alignment for a reason. This could be due to a man made struture or natural causes.

Realigning your channel may be possible to create additional backyards for the local residents. Before proceeding you'll need to consider impacts of the realingment. Channel realignment projects, if not designed properly tend to cause erosion downstream within a couple of years. Erosion thresholds and flow velocities would have to be considered in your new design, if your considering the realignment. Depending on overall grade, and length of intended realignment, try and maintain a reasonable grade through the meandering section if possible to minimize deposition.

The hydraulics for a vertical drop structure can become very complicated but the general concept is.....
A jet of water over a vertical drop structure approaches supercritcal flow (Froude # approx 1) and plunges to the bottom of the drop structure. The jet hits the bottom of the basin and is directed horizontally(downstream or toward the channel banks ). If there is a sufficient tailwater available, a hydraulic jump is iniitated, reducing the engergy and velocities of the flow. This tailwater effect can be created or intiated by a shallow plunge pool at the bottom of the vertical drop. Without the tailwater condition at the bottom of the drop, flows will continue to the next drop witout a reduction in velocity. For more details on the hydraulics of the vertical drops, Open Channel Flow by Ven Te Chow is an excellent resource.

Creating a channel to convey more flows may have impacts downstream, consider the above in your design.

Hope this helps.

 

[Ryb01]

My appologies the the long winded previous response..

If you have a manmade channel you may be able to realign, pending property constraints, and standard agency guidlines.

As per my previous message, condsider your downstream impacts. Designing a channel to convey flows through your site doesn't necessarily mean that channel sections downstream could convey and or withstand the added energy to the system.

Hope this helps.

 

[beej67]

1)
Is this an existing stream? Are you going for a 404 on it? Army Corps generally claims jurisdiction over just about anything with base flow in it, in my experience, which puts you into a channel restoration scenario.

Channel restoration is a very complicated engineering process, and even very experienced engineers get it wrong. Not only is there flow capacity to think of, such as with a ditch design, there's also quite a bit more to take into account, such as bed forms, sediment transport, habitat, etc, and engineers who simply throw a trapezoidal channel at a design flow invariably fail at stream restoration.

There are two schools of thought, which generally fall into "Rosgen Method" and "Non-Rosgen Method." Doing either requires analyzing all of the issues Ryb01 enumerates above, and that's just to get started. Learning Rosgen design takes about a month's worth of solid training (8 hrs / day) and about $10,000 worth of training costs. Learning other methods may cost less, and your design tools may be more easily available, such as HEC-RAS, but they're almost invariably based on graduate level academic principles. The Chow text referenced above is a good one. So is Sturm's "Open Channel Flow," which discusses sediment transport, bed forms, and flow regimes quite a bit.

If you're trying to do a true stream restoration project, and don't have any experience at it, I highly suggest hiring an expert.


2)
If you're merely rerouting dry ditches, then the issue is much more simple, but if I understand your post you're making a grave error. Flow is cumulative in your reach, and you can't discount flow just because you've stuck a structure somewhere in your ditch. The continuity equation dictates that what goes in must come out less what remains, so your ditch MUST get bigger as you move down the watershed. If capacity is a problem, but it's not a creek under any sort of environmental regulation, then consider paving the ditch to decrease your manning's n. That'll help some. Otherwise you're basically stuck, unless you want to put a detention pond in there someplace to reduce the peak design flow.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[engrTman]

Listen to beej67,

If this is under the corp 404 jurisdiction, it's time to step WAY back and evaluate your project. The new thinking for restoration/channelization is very different than Q=AV. You won't meet many engineers who have successfully completed a channelization with recent regulatory guidelines (other than the specialized firms). If at all possible stay out of the Corps POTENTIAL jurisdiction which is usually top bank/ordinary high water. Overbank grading may be an option depending on your location but I would stay out of the 404 quagmire. They can wait you out. Permit review times could be in years. Another jurisdictional issue may be wetland potential, check the NWI maps.

 

[eea]

Ryb01 gives a great narrative. I'd add for you to make a preliminary submittal to the Corps. Let them determine if a permit is needed or not. Better you have a letter back from them stating you do not need one than them showing up on the job site and issuing a stop work order for lack of one.

A nationwide 404 permit is not that bad, but if you are doing more than 300' of bank protection you'll probably be subject to an individual permit. Of course I say that working within the boundaries of a pretty decent Corps District. I understand that some are not pleasant to deal with. Make sure to discuss the permitting with the client beforehand.