Storm Sewer for Industrial Park 2

[gjeppesen]

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.

 

[civilperson]

Sounds like the the elevation of the swale must be 32.97 or above.

 

[RWF7437]

civilperson,

Did you mean to say elevation 32.97 OR BELOW ?

 

[gjeppesen]

civilperson,

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.

 

[francesca]

I'm not sure I'm understanding properly, but with tailwater in the swale, it really becomes a different animal. Does the time of concentration for the swale equal the peak volume time in the pond? Have you factored the swale storage into your pond storage area?

 

[RWF7437]

Also, how can you calculate the HGL elevation of you haven't set the swale invert elevation ?

A diagram would make this all a lot clearer.

good luck

 

[cvg]

does your pond hold the entire runoff volume? If so, then shouldn't you assume that the pond water elevation is lower when the storm runoff peak enters the basin? If not and you are going to assume that tailwater enters your swale, then the swale becomes an extension of your retention pond. The volume should be included in your stage-storage relation. This water surface should be the controlling elevation and back water analysis should be done to estimate the hgl in the swale.

 

[gjeppesen]

CVG,

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?

 

[RWF7437]

You can also model this in HEC-RAS, I believe. You can also try any of the following ideas:

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

 

[cvg]

using ditches for stormwater retention is commonly done, especially where the existing topography is flat. Culverts can even be sized to maximize the detention within the ditch. But you will need to use ICPR, HECRAS or any other software to do the analysis which can't be done with just normal depth computation and mannings.

 

[gjeppesen]

CVG,

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?

 

[RWF7437]

I would NOT make the ditches "flat" i.e slope = 0.000 !

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

 

[RWF7437]

See also:

http://hdsc.nws.noaa.gov/hdsc/pfds/other/fl_pfds.html

 

[cvg]

agree that the ditches should be designed to drain, otherwise you will have more standing water which is a nuisance and safety hazard.

 

[civilperson]

Flat diches with no slope are common and not a nuisance. Any height of water produces flow and thus will drain. (+/- the 0.1' variability of normal grading accuracy).
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.

 

[RWF7437]

I don't know where Civilperson lives and works but ditches with 0.00 slope would not be allowed where I've worked (Oregon, Washington, California, Mississippi and few others). Apparently cvg also thinks this is true ?

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

 

[cvg]

flat "ditches" around here are called "retention basins" and are usually designed without an outfall. They are required to hold the entire 100-year, 2-hour storm in most cases. They are required to fully drain in 36 hours or less because of concerns with safety, including drowning and mosquitoes. West Nile is a huge problem in this area and any more than 48 hours for standing water is enough for the eggs to hatch. Retention basins are required to have either dry wells installed and tested to confirm rate of water infiltration, or a bleed off line to an outfall.

 

[RWF7437]

cvg,

where is "around here" ?

 

[cvg]

arizona

 

[RWF7437]

gjeppesen,

Where is your industrial park ?