Zero increase to Post-Construction Runoff - help 3

[Bhotar3]

A friend of mine asked me to complete the pre and post construction runoff calculations for this proposed 0.53 acre site. They are constructing a 45x70 metal building. The parking lot is existing, and other than that the existing pre-construction conditions are empty grass lot.

The county in question wants to see net zero increase to runoff, post construction. Any ideas? A detention pond for a development this size seems to be overkill.

There is a drainage swale on the north end of the property that runs the entire length to the main ditch by the road.

 

[dik]

Can you add a small weir, or holding tank and pump? What sort of volume are you looking at?
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bimr]

You don't mention the condition of the parking lot nor the design rainfall intensity. The parking lot appears to be a good location for a stormwater detention vault or permeable pavers. Your drawing needs elevations as well.

 

[dik]

How are you calculating the run-off? I have a small spreadsheet that does small ones, I'll see if I can dig it up... haven't used it for 20 years. Uses Rational Airport method for small watersheds.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Bhotar3]

The parking lot is concrete and already existing. The client would like to not make modifications to it. The design storm is 25 year 24 hr. Pre construction intensity is 6.1 in/hr. So far, my post-construction intensity for the portion of the site flowing over the parking lot toward the ditch is 8.1 in/hr.

Dik, yes, the rational method is being used.

 

[Ron]

Since you have a required zero difference pre vs post construction, your storage will be difficult, but workable. I have designed many of these systems under the same requirements in Central Florida (25y24h storm) with no additional runoff allowed. As bimr noted, vaults may be used; however, you're in Louisiana so your water table might be too high for that. That leaves surface swales for the most part. You might consider a small retention pond to take the first inch of runoff and then pop off to swales that detain the flow to allow the final runoff to be as low as pre-construction runoff. This will require iterations of your stage-storage computations.

 

[dik]

Does your pre condition include the parking lot? or is this a 'fix up' of an earlier condition?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Bhotar3]

Yes my pre-existing condition includes the parking lot.


I’ve thought about modifying the existing swale to act as a linear retention pond to accommodate the additional runoff.

 

[Bhotar3]

So my next question is this:

If I design the swale or modify the existing swale.

Lets just say I have 1.5 CFS from the entire site being drained to the front, main, drainage ditch.
So, Once They improve it, they have 3.0 cfs now.
If I divert 2.0 cfs to the drainage swale, how do I calculate the swale such that it handles the excess or reduces that flow to Net Zero upon discharging to the main ditch?

How do I design the swale such that the site is still net zero, even tho 2.0 cfs is flowing to the swale??
Manning’s equation I’m sure, but I’m still unsure of the total process.

 

[Ron]

Design your outlet drainage structure to limit the offsite flow to 1.5 CFS by setting the weir height to allow your retention area and detention swale to store the difference for the temporary condition of the storm

 

[Bhotar3]

So I calculated my excess flow to be 0.70 CFS. The design storm is 25yr 24 hr storm. Would my retention Volume be 0.70 cfs * 24 hr * 3600 sec = 60,480 cu.ft? Seems really excessive, or am I just doing the calc wrong?

 

[cvg]

no, that is incorrect. you cant use peak discharge to calculate volume. you need to use the rainfall depth, not the intensity

 

[Bhotar3]

Sorry for my ignorance.

I've got a peak flow rate, and I've got an intensity. How can I arrive at my rainfall depth? Is it simply the intensity * 24hr?

And then from there, how would I get my required retention volume?

 

[cvg]

no, you need the total precipitation during the 24-hour storm to calculate the runoff volume. it has nothing to do with intensity

 

[Bhotar3]

Ok. Just determined that the 25yr 24hr rainfall depth for my location is 10.5 inches.

Now I would take the area (acreage) that is flowing to my rentention area, and calculate my volume?

 

[cvg]

precip depth x area x rational coefficient

 

[bimr]

Here are some examples:

Link

It looks like you need 11,000 to 12,000 cu.ft. of storage.

 

[cvg]

my interpretation is that net zero increase would indicate that there was some runoff previously and that amount cannot increase. so you need to compare pre- and post-development runoff and store the difference. that might only be a small volume needed

 

[dik]

Do you have to accommodate the whole site? or just the part that's been changed? Seems like a lot of water for just the change.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bimr]

Your print says the project is located in Long Beach, Mississippi which is within Harrison County, Mississippi. Correct?

"A Drainage Management Site Plan (DMSP) shall be required for any new single family residential developments and for any new commercial, multi-family residential,
institutional, industrial, or utility developments, and shall be required for land disturbing activities greater than one acre."

[URL unfurl="true"]http://co.harrison.ms.us/departments/engineering/stormwater.asp[/url]

Based on the wording in the link, it appears that you don't have to do anything since your site is less than one acre.

Rather than ask questions on this forum, why don't you sit down with the County Engineer and confirm what your actual requirements are?

"The Harrison County Engineering Department is obligated by State and Federal law to monitor the rate and quality of stormwater runoff on nearly all parcels of land in the unincorporated portion of the County. All properties between one and five acres where onsite activity disturbs the existing vegetation are specifically targeted. Parcel of less than one acre may be included if these are part of a larger development or if more than 20,000 square feet of impermeable surface is created on a site."

Harrison County Stormwater