Proper placement of Emergency Spillway for Basin 3

[MuddyCrypts]

I am having trouble deciding where the best place to locate an emergency spillway on a retention basin would be.

Would it be directly over the main outlet barrel, cutting through the middle face of the basin's embankment, and meeting with the outlet's flow at the toe of the basin?

Or, would it be better to offset the spillway away from the main outlet's centerline, and placing it off of the main face of the embankment?

The basin life will most likely be over a year. Also, which one would be the most practical to build, but the safest in longer term retention. I am based in Southern California, if that helps. Thanks.

 

[jugglerbri]

MuddyCrypts,

I don't think it matters too terribly much where the spillway goes, especially since you say the life of the basin is around a year. Some things to consider, is the spillway going to inundate anything that should stay above water? Will you be able to protect the slope (concrete, riprap, etc.)? Are there any areas that may scour? The main consideration that I've seen in emergancy spillway design is the depth of flow over the spillway (around here, it's usually around 6&quot. Since your pond is temporary (I'm assuming), what flow are you designing your spillway for?

Brian

 

[MuddyCrypts]

Brian-

Thanks for the tip. Would it be safe to say that a more permanent basin would be a more appropriate place to have the emergency spillway offset from the main outlet?

The emergency spillway is actually draining into a dissipated common channel, that eventually leads to another basin down stream. I'm designing the spillway for a maximum peak runoff discharge of a 25-year 24-hour storm event. This is not including the 10 year flow for the principal outlet.

Thanks for the tip on the "Spillway" depth of flow. Would that be 6" from the top of the rip rap to the the top of the spillway crest? Right now, I have the spillway over-excavated to allow energy dissipaters, rip rap, to be flush to the designed slope.

Also, is a pond that's going to be in place for five years considered a temp basin? Thanks again.

 

[jugglerbri]

MuddyCrypts,

I don't think spillway location has a whole lot to do with permanency. Some permanent spillways I've designed have gone right over the outlet structure. What's more important is the level of protection that is needed. The level of protection needed depends on the volume of water stored and the possibility of downstream damage in case of failure.

The 6" flow depth is a measure of the distance from the crest of the spillway to the water surface elevation when flowing at the design flow. I believe this is kept small so the velocities will be lower and there would be less chance of failure.

As far as temporary vs. permanent structure, I'm not sure what a design for 5 years would be. I usually think of temporary as during construction (and a five year project is pretty long). Are you POSITIVE the pond will be removed/replaced in five years? What if future projects get put on hold because of budget constraints or interest backout?

I would strongly suggest getting in touch with whatever entity is in charge of approving/accepting/ok'ing the design. If it's a large detention pond, perhaps the Army Corps would be in charge. If not, check with the city or county (I'm assuming you are in the United States).

I hope this helps,

Brian

 

[intrigueing]

As a general rule of thumb, try and locate the spillway over native material and not over the principal spillway. The embankement will likely have differences in compaction that could cause weak points, and will tend to erode more quickly. The embankment can also have differential settlement that could cause flow to occur in a different place than you anticipate. You better add at least a foot of freeboard above the water level anticipated in the spillway flow. Structures often exceed the intended design life, and are often not removed within the intended timeframe, you may want to consider bumping up the principal spillway size to take a larger event. What drainage area, basin area, basin volume, and embankement height are we talking about here?

 

[treg]

I agree that, if possible, the emergency should be located over native material. If the spillway is located over the embankment, any erosion of the spillway will erode your emankement. Failing that, it should be well armored.

It has also been my experience that basins do not get removed when the reason for them being built ends. The owener invariably does not want to spend the money to restore them, or they are simply forgotten.