Modeling a dry swale?

[briandestefano]

I am working on a commercial project in New York State, not the city, and my boss has asked me to see if a dry swale will be able to treat a portion of the runoff. The stormwater design manual says that the swale should be sized to carry a ten-year storm.

So, how could I model this in the hydrographs program?

I am confused about the modeling because if the swale is at some slope, say 1% or 2%, how can you run this through the hydrographs program with a stage and storage? Wouldn't the water just run through to the bottom without any detention.

Also, what happens to the 100-year storm runoff from this area? I still have to do a pre- and post-development comparison for the site, so I would think that this would need to be accounted for.

Thanks for all the help so far guys.

 

[cvg]

you may have to consider infiltration and channel storage. I'm not sure how this works in NY, but around here a basin is required, not a swale. At 1 or 2% slope, your swale will not be very effective. Maybe you should flatten to 0.1% slope to slow the water down.

 

[briandestefano]

There will be some infiltration, as it is requried that there be permeable soil for 30" below the bottom of the swale with an underdrain at the bottom which discharges to a wetland area.

There are two other stormwater basins on the site, but we are trying to pick up a small portion of the site that will need very deep pipes to get to the other ponds.

As for the slope, the design manual says that it may be as steep as 4%.

I hope this will help to clear up my post

 

[hydroponder]

A flatter slope will allow for more detention time and thus more infiltration. You may want to look at putting in small berms in the swale to create small infiltration basins. This will increase detention, provide storage and make the modeling easier.

You will need to have an emergency overflow to direct excess water to the wetland and yes, it should be accounted for in the modeling.