Storm Sewer for Industrial Park
Storm Sewer for Industrial Park
(OP)
I'm designing a 90 acre industrial park with a maximum impervious for each lot at 83.3%, obviously very high. The project will discharge to one large wet pond and outfall to an adjacent off-site ditch.
The roads will be private rural section with 2.25 deep swales to accomodate both road runoff and lot runoff. These shallow swales will then discharge into ditch bottom inlets, into the storm sewer system and into pond.
Now, the question. In Florida we typically design the storm sewer/roadside swales for local roads to the 10 year storm. I'm used to designing curb inlets such that the grates need to be at or above the HGL to elimate spread. But with these inlets being 2.25' below the edge of pavement, this doesn't apply.
How should I determine the minimum bottom elevation of the roadside swale, and as a result, the road elevation?
FYI the 25 yr, 24 hr storm stage is 35.22 and the 10 yr, 24 hr storm stage is 34.90.
The roads will be private rural section with 2.25 deep swales to accomodate both road runoff and lot runoff. These shallow swales will then discharge into ditch bottom inlets, into the storm sewer system and into pond.
Now, the question. In Florida we typically design the storm sewer/roadside swales for local roads to the 10 year storm. I'm used to designing curb inlets such that the grates need to be at or above the HGL to elimate spread. But with these inlets being 2.25' below the edge of pavement, this doesn't apply.
How should I determine the minimum bottom elevation of the roadside swale, and as a result, the road elevation?
FYI the 25 yr, 24 hr storm stage is 35.22 and the 10 yr, 24 hr storm stage is 34.90.





RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
Did you mean to say elevation 32.97 OR BELOW ?
RE: Storm Sewer for Industrial Park
I thought that too, but my tailwater is halfway up the ditch, thereby reducing the actual conveyance capacity of the ditch (I think....).
If I raise it so the bottom of the ditch is at the 10 year stage, I'm bringing in a ton of fill. And I can't increase the pond size any more than it is.
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
A diagram would make this all a lot clearer.
good luck
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
You're exactly right. The way I see it, I have 2 choices:
1. Raise the site so the ditch bottom is higher than the design tailwater so I can claim all the conveyance capacity of the ditch. This way I can use Manning's very easily. Big problem is fill quantity needed.
2. Leave as is and use ICPR computer model to model the whole ditch/pipe system and account for storage in the ditch. Is this a good idea though? I'm not used to designing rural roads. Is using roadside ditches for storage (not just conveyence) a normal thing?
RE: Storm Sewer for Industrial Park
raise the site with fill
lower the pond, if possible
widen or deepen the ditches and include the storage volume in the ditch as CVG suggested
flatten the ditch grades
line the ditches to improve (lower) their Manning's "n"
increase the pond volume if possible
even, consider lowering or improving the receiving "stream"
good luck
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
Another question. Given the fact that it looks like I'm going to use the ditches for storage, should I just make the ditches flat (the site is flat so it's not a big deal) and model them as "ponds" instead of "channels"?
Pros/cons?
RE: Storm Sewer for Industrial Park
Suggest you set the grade of the ditches at minimum slope; the slope which will produce a minimum velocity of 2.5 ft/sec at the design flow ( 10 year ?). This will probably provide "self cleaning" velocities at least once a year, on average. It should also prevent standing water for long periods of time which are not just a nuisance but allow mosquitoes to lay their eggs in still water ( remember West Nile Virus ?). Setting grades flatter than this will not gain much storage volume anyway.
You may also want to consider underground pipe storage and infiltration trenches below the detention pond. It is expensive but if you have well drained soils it can be very effective. It is widely used in Southern California for "retention storage" and there they use large (36") perforated pipe embedded in open graded round rock. I don't know if Florida has design standards for such things but it might be a good idea to inquire.
good luck
RE: Storm Sewer for Industrial Park
htt
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
If the elevation is set below what I specified, then the roadways will be under water for the 25 year event. I did mean ABOVE.
RE: Storm Sewer for Industrial Park
You may also want to consider some underground storage in perforated pipes embedded in open graded round rock. It is expensive but often used in Desert areas where soils are well drained. This also has some water quality advantages and gives you much greater flexibility in setting grades. The pipes may be buried below your detention pond or under other open areas on the site, such as parking lots.
good luck
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
where is "around here" ?
RE: Storm Sewer for Industrial Park
RE: Storm Sewer for Industrial Park
Where is your industrial park ?
RE: Storm Sewer for Industrial Park