Stormwater regulations vs a rural property

[cpacker]

I'm a computer programmer trying to make sense of storm water management regulations that affect the rural property on which I'm going to build a house. My site-planner's initial design for the wooded lot called for the removal of a stand of mature trees to make space for a rain garden. What!!? Long story short, I need to be directed to a tutorial that will let me do my own analysis: that doesn't assume an urban environment, that doesn't assume that required water retention features will be located next to the source of runoff, that lets me determine my runoff management obligations at the down-slope property boundary, taking into account the runoff coefficients of the woods between my house site and the property line.

 

[peter246]

what are your local and state stormwater requirements? typically, construction/ground disturbances over a certain size (depending on location) are subject to requirements for:
[ol 1]
[li]water quality (can be accomplished by raingardens);[/li]
[li]water quantity (typically by detention); and[/li]
[li]Best Management Practice (erosion control and design/construction) standards.[/li]
[/ol]

here is some information on the federal requirements that govern if your state or local laws don't apply:

http://water.epa.gov/polwaste/npdes/stormwater/EPA-Construction-General-Permit.cfm

sometimes, local codes require stormwater treatment for smaller developments. or, if you're purchasing a property developed as part of a subdivision, the applicant may have committed to stormwater BMPs as part of the subdivision and/or site plan permitting process.

a "stormwater planter" may be an option for you if you're stuck with treating your roof runoff. they can be incorporated into your landscaping and are typically more compact, but require more design.

 

[Twinkie]

SWM is not really something you're going to learn from an online tutorial, and regardless the design and calculations will need to be signed/sealed by a licensed engineer.

For large rural lots, I typically let the pre and post Tc's and CNs do the work .... I'm assuming you have a very long Tc to the hydraulically most distant point ... both pre and post rates and volumes typically go 1:1 in this case.

 

[peter246]

that's correct - if subject to local/state/federal stormwater permitting, you'll need a PE for the final submission. i recommend finding out exactly what you're required to do first - you may not even have to perform any analyses.

p.s. rainbarrels or a cistern are also often acceptable alternatives for handling roof drainage.

 

[beej67]

It all depends on where you're at. (State/Municipality)

Even in urbanized areas, I typically see exemptions for single family residential parcels under 5000 sf of impervious coverage. Check Municode. How bit is the house?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[brandonbw]

Over here in Los Angeles, the City of LA has handouts for small scale residential. The size of the rain garden would be based on square footage of the roof/impervious areas. Raised planter boxes, drywell, and rain barrels are some alternatives. I would take a trip down to the local agency and ask what they want.

B+W Engineering and Design | Los Angeles Civil Engineer and Structural Engineer

http://civilengineer.la

 

[cpacker]

Yes, there is a 5000 sq ft cutoff where I am, in Charles Co.,
Maryland. But because I'll have such a long driveway, I'm
over the limit on that alone. This puts me in the same category
with developers of shopping centers and housing tracts.
Therefore urban design criteria are being applied to my property,
which is crazy. There aren't any storm drains out there!
I've asked for a variance, but it was denied. The PE firm doing
my site planning is not experienced with this type of project,
and I'm racking up thousands of dollars in T&M fees as they go back
and forth with the county about it.

 

[beej67]

If it's only the driveway that puts you over the limit, do the driveway in pervious pavers or porous concrete to duck the limit and skip the rain garden entirely.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Example:

http://www.pavestone.com/tag/permeable-pavers/

So briefly, the water leaks into the ground through cracks between the pavers, so they don't count against your impervious area ... provided the municipality buys the argument. Instead of paying your civil firm hourly to confirm this, you could probably call the regulator directly and ask whether they'll agree to the methodology given your situation. If they say 'yes,' pick a product, tell your civil firm to spec. it on the drawings, and you'll be under the limit, exempting yourself from the SWM code.

Now, if you've got over 5k of impervious in your driveway, AND over 5k of impervious in your house and associated hardscape, then it's not really that crazy to apply SWM criteria to your parcel. Particularly considering you're under the umbrella of the Bay Act and such. It's fingers reach all the way down into my father's agricultural practices over on the Northern Neck, despite him being a much longer drive from any urbanized area than you are.

Folks are pretty sensitive about water quality where you're at.

Presuming you're stuck at over 5k impervious coverage, then the answer to your original question is to begin by reading this cover to cover:

http://www.mde.state.md.us/programs...entandStormwater/stormwater_design/index.aspx

By the time you're done, you'll probably realize why you hired a civil firm. :/

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cpacker]

The pervious pavement idea was raised and then rejected.
I forget why the county wouldn't accept it for my project,
but I didn't like it because, aesthetically, it's just another
kind of pavement. I want to preserve the rustic character of
the site with a driveway of bank run gravel, which, after a
couple of seasons, will revert to a pair of wheel tracks
with grass in between. Ironically, even if you assume a runoff
coefficient of 1.00 for those two tracks, it would probably
be the best actual outcome for SWM.

 

[Twinkie]

What kind of soils (HSG) and how long are the pre and post TC paths??

I've done 100s of jobs like this, and the TCs usually take care of everything, meaning no pre or post runoff.

Put a crown in the driveway so the water doesn't sheet flow down, but goes into the lawn areas.

This isn't that hard, don't keep racking up T&M fees if your engineer can't get this approved.

 

[beej67]

Twinkie:
I'm under the assumption that it's not a pre/post q match issue. It's a "water quality treatment" issue.

packer:
A regulator who's throwing you over the 5000 sf threshold because they're counting your gravel driveway as impervious coverage is what we like to call "stuck on stupid." But you probably already realize that. You're completely correct that your preference for a driveway is going to be totally fine for the environment, but you've run into bureaucratic brainlock.

Try to convince them that you'll use "washed 57 stone" for the driveway, with "no fines," and that the rainfall will infiltrate the ground through the voids in the stone. The driveway is not impervious. Keep using the phrase "not impervious" with every communication you have with them. "How do I best show you that my not impervious driveway, in so much that it's verifiably not impervious, should not be erroneously counted as impervious for the purposes of the SWM ordinance?" Document everything. Go over their heads if they're being checklist nazis. Find out the name of the county commissioner. Have your engineer draw up his post development drainage basin map with a separate take-off for the driveway. Label the house and hardscape, "House and Hardscape, XXX sf impervious," and the driveway, "XX sf driveway, not impervious."

As a practicing engineer, I would not take this route purely because I have to maintain a good relationship with the regulators I work with on a regular basis, even when their decisions are silly. But you only need 1 permit and you're done with them. You really have two options. 1) Bow up on their definition of imperviousness to get yourself under the regulatory threshold, or 2) put in a rain garden or similar. They might let you get away with a cistern, or lots of rain barrels. Maryland might have an "overland flow filtration" credit or similar you can apply for to meet the SWM ordinance. Your engineer should know about those options.

Good luck with it. I hear horror stories of the regulatory environment surrounding DC. Just be glad you're not in Miami-Dade or Durham NC.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Twinkie]

Even so, for a WQ issue. If the TC is as long as the picture I'm conjuring up in my head, the PE should be able to argue a case of treatment just from overland flow. Especially with a majority of this being roof runoff.

What is WQ anyway? We achieve it by promoting the infiltration of the runoff. If you have no runoff, how do you treat for WQ?

 

[civilman72]

There should be some reduction in potential pollutants when comparing a gravel driveway to an asphalt driveway, but you are going to struggle to convince the reviewing agency that a gravel driveway has no potential impact. In almost all circumstances, a well-compacted gravel driveway does create more runoff than undisturbed ground, and due to potential pollutants from vehicles, fines in the gravel and potential for concentrated flow in the roadside ditches there is a higher likelihood that storm runoff from the gravel driveway will contain pollutants.

 

[LincolnPE]

Gravel(packed) surfaces get a 40% imperviousness value for runoff calculations. They are not pervious...

 

[cpacker]

Maryland law is explicit about gravel: it's impervious.
If I have to sue the county to get my building permit, it would
seem not a good idea to base my case on a contrary interpretation,
not least because I have no idea how to locate an favorable expert
witness. Instead, I think it would be better to focus on the
fact that my project is clearly residential, so I shouldn't have
to follow shopping center rules just because I fall above the
5000 sq ft cutoff that seems to have been designed specifically
to exempt residential properties.

So...was it? I noticed a comment from somewhere besides Maryland
that mentioned a 5000 sq ft rule there. Is it some kind of national
standard? How was it arrived at? Curious minds want to know,
especially if they'll have to coach a technically naive lawyer
on strategy.

 

[Twinkie]

5,000 sf is a pretty common threshold to exempt you from land development process and regs. It is in place to allow you to build a shed, or expand a driveway, etc. There is no difference in interpretation of regs based on land use. You either fall under the threshold or you don't.

I'll say it for the last time, if your property is as big as you are making it out to be, I can't fathom how your engineer can't make the math work in regards to WQ.

Unfortunately, I can't add anything more to this discussion.

 

[beej67]

I noticed a comment from somewhere besides Maryland
that mentioned a 5000 sq ft rule there. Is it some kind of national
standard? How was it arrived at?

There are no national standards, because stormwater is different everywhere you go. (different soils, different rainfall characteristics, different topography, different land use stressors, different pollutants of concern) Everything is either local, or at best regional.

5000 sf was pulled out of a hat by somebody, and many other places stole it because they considered it to be intuitively reasonable. Some places use different thresholds. I've seen 500 sf. I've seen straight exemptions for single family residential properties, not for any scientific reason, but mostly just to keep residents from raising a stink. It all just depends on where you are. If there's not a residential exemption specifically spelled out in the rules, then the land use argument is a dead end and not worth pursuing. The rain doesn't care whether it lands on a shopping center or a house.

Gravel(packed) surfaces get a 40% imperviousness value for runoff calculations. They are not pervious...

They're not impervious either, by that very same rationale. And the 5000 sf threshold isn't calculated by weighted runoff coefficients, it's calculated by adding up the total amount of (completely) impervious area on your site. They don't count grass as "slightly impervious" for reg applicability even though it has a higher runoff coefficient than trees.

If his regulator is counting the gravel driveway as 100% impervious for the purposes of determining applicability of the code, and then turning around and counting it as "40% impervious" for the purposes of hydrology, then he's talking out of both sides of his mouth. If he's counting it as "100% impervious" for both reg applicability and hydrology, then he's ignoring science.

..which, sometimes regulators do, because regulators aren't just worried about what makes sense, they're worried about what makes precedent. They may be worried that if they let you count your driveway as pervious for SWM reg applicability, that some developer will come along with a 4900 sf retail store and 15,000 sf worth of gravel parking lot, and claim to be exempt from the regs. That's probably what's on their mind.

What if you did your driveway in pea gravel? Oyster shells? Sand? Ask the regulators if there is a driveway material they would permit to be considered "pervious."

Another idea is to use something like this:

https://www.pinterest.com/pin/488640628289190519/

Image came up on a search for "grasscrete" but I think the product shown in the image is actually Strataweb. They can't possibly tell you that's impervious.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[TerryScan]

For what it is worth the exemption from providing stormwater management in Maryland is that the project must have lees than 5000 sf of disturbance (NOT impervious). Besides Conveyance and Flood control, Maryland stormwater regs have water quality AND recharge requirements.

For a drive, we would typically use filter strips, grass swales, or bioswales for the BMP rather than a rain garden. It would be great if the home could use the same BMP.

However, I have run into the same directive of taking down woods to put in a BMP which rubs me the wrong way too.

 

[beej67]

So if you build soccer field in the floodplain in Maryland, with no impervious coverage at all, you still have to put in a stormwater pond?

If that's the case, the OP is definitely up the proverbial creek.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com