Please respond if you have comments

[RDS97]

Please respond if you have comments: Site is less than 1/2 ac. on a corner lot which has been used for the past 28 years as a used car sales lot at the intersection of two public roads. The site before demolition consisted of two buildings, concrete driveways & sidewalks, an asphalt car display area, and the balance of being a compacted gravel parking. Slope averages 2% and runoff is sheet flow toward adjacent properties. New owner wants to keep the site as a used car lot, but improve with a new building. The parking lot must be fully paved with concrete curbing, and landscaping on all perimeters in accordance with City regulations. Using NRCS Curve Numbers we provided a layout that met the Post-Dev. runoff did not exceed the Pre-Dev. runoff. After submitting the plan & drainage data the City disputed the Pre-Dev. calculations. We revised the plan to reduce the amount of pavement and thus the drainage even more. The city has now said we must evaluate pre-development drainage and evaluate the site ignoring the compacted gravel areas as such and using residential lot with grass numbers instead. Has anyone else run into this and how it was addressed? The owner is talking legal action. I'm not too sure he's wrong. Thanks.

 

[Drew08]

I've seen this a lot. Pre-development means pre-development. Not pre-your-development. The owner should address it the same way as if it was in fact a new development, with on-site storage, bioretention areas, etc. Just because the previous owner got away with not meeting the standard for whatever reason, doesn't mean every future owner should too.

 

[msquared48]

Me too.

If the site is contributing to downstream flooding via the percentage of impervious, the jurisdiction will pursue more on-site storage to mitigate the downstream flow.

This means going back to the original topo, if known, plus the original native vegetation prior to development. Some of the items are usually negotiable, but not the onsite storage.

Mike McCann, PE, SE (WA)

 

[civilman72]

While I agree that most municipalities will require you to go back to pre-development site conditions to represent your historic flows, I have found that attorneys have a different take on "historic" drainage. Here in Colorado, a landowner's responsibility is to maintain the historic quantity, quality and manner of stormwater runoff that is discharged to downstream neighbors. But many attorneys will state that if a landowner has been discharging a certain quantity/quality/manner of stormwater runoff to a downstream property for a certain amount of time without any objections from the downstream landowner, eventually the upstream landowner has a prescriptive easement to continue this discharge. Here in Colorado that time is usually around 18-20 years. With that theory, if the existing land-use (i.e. used car lot) has been discharging a certain quantity/quality/manner of stormwater to downstream neighbors for at least 20 years, this is what now constitutes "historic" drainage and should be the baseline for your drainage calculations. Any increase in quantity or any decrease in quality from these "historic" flows would not be covered by a prescriptive easement and therefore would have to be detained/treated, etc.

In summary, while your Client may have a leg to stand on in court, as an engineer, you likely will be fighting an uphill battle with the review agency.

 

[TerryScan]

The key here is checking what the actual regulations for your locale. I work where stormwater is primarily regulated by the state. These regulations DO have provisions for "redevelopment" which give credit for previous developed impervious.

 

[Drew08]

Specifics of the original question, local requirement, and legal standing apart, it is an interesting philosophical question as to the point of the rule.

One reason being that the municipal stormwater system is usually the downstream property. The stormwater system is not only the manholes and culverts, but also the receiving open channels, creeks and rivers. The municipality probably has an MS4 permit which limits pollutant discharges in stormwater and mandates that municipalities take measures to reduce pollutants. Probably one of the largest sources of stormwater pollutant in urbanized areas is sediment loading from stream bank erosion, stream bank erosion caused by stormwater discharges in excess of the natural stream-forming discharge. 28 years ago when the lot was built, the discharge in the receiving stream increased, and stream bank erosion began. Must urban and suburban municipalities want to claim that they are actively working to reduce pollutant loading, not just keeping the status quo. One way to do that is to have redevelopers actually improve drainage and reduce discharge, not just keep it the same.

Rules might be different in more rural areas where stream bank erosion and sediment loading is not the primary pollutant of concern.

 

[cvg]

calling stream bank erosion "pollution" is a bit of stretch in my opinion

 

[Spartan5]

"A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal species, or by interfering with human amenities, comfort, health, or property values."

Seems to meet that criteria.

 

[cvg]

seriously? is wikipedia the source of all knowledge?

water, wind, sunlight and everything else might also meet that criteria
also, who determines what is an "undesired effect"?

 

[Spartan5]

Is there something wrong with the information from Wikipedia? Water, wind, and sunlight do not meet that criteria because they are not "introduced" into the environment. They are an inherent part of it.

Here, I grabbed the Civil Engineering Reference Manual (10th ed.) as it is a basic engineering text. Page 32-2, A pollutant is a material or substance that is accidentally or intentionally introduced to the environment in a quantity that exceeds what occurs naturally.

Does that definition better suit you?

 

[miningman]

Strong parallels here..... definition of pollutant notwithstanding... with new mine permitting. EVery new proposed minesite is located in an area of known elevated "pollution" be it Zn , As, Cu, Pb, Ca , or whatever is naturally provided into the local watercourses by the local geology and its interaction with local water. If common sense prevailed at the permitting stage , you'd think that the new mine should have to prove that any contamimination from the mine was at or below natural levels. But oh no. The new mine has to be willing to clean up its effluent to almost drinking water standards.....and be prosecuted if every now and then the systems dont get it perfect.

 

[cvg]

depending on who wrote it, wikipedia can be a good or a bad source. so yes, I would not believe about half of what wikipedia says and the other half is suspect.

actually the second definition is no better than the first. but lets stay on topic here. I'm not saying that stream bank erosion is necessarily good or bad, but calling it pollution is still a stretch

 

[Twinkie]

Every municipality around me requires you to model the pre as "meadow in good condition" regardless if it's completely paved over or not. The idea is to represent the natural hydrologic cycle that existed in the stone age. I don't disagree with this approach, but I feel this could be a hinderance to redevelopment on some parcels.

I've seen some municipalities give you credit for some of the existing impervious if it is a redevelopment job.

 

[beej67]

Different government agencies treat this different ways.

Just in my region, I've seen the following approaches used:

1) pre-dev = forested no matter the current ground cover
2) pre-dev = grassed no matter the current ground cover
3) pre-dev = current ground cover
4) pre-dev = current ground cover but provide a 30% reduction of post-dev flow rate
5) pre-dev = forested and provide a 10% reduction of post-dev flow rate
6) pre-dev = current ground cover but provide a percentage reduction of post-dev flow rate equal to half of the impervious ratio of the current site (no kidding, this is a real reg)
7) pre-dev doesn't matter, match a fixed unit flow rate per area of watershed for the design storm
8) pre-dev = current ground cover, but run your 1,2,5,10,25,50 year storms, and then your post-dev 2,5,10,25,50,100 year storms have to match those allowable discharges. Basically slide the events by one notch and match those.
9) I even saw one that required us to develop our predev allowable discharges with rational method and our proposed discharges with SCS method. That was a head scratcher.

Then you run into TC issues. Ten acre parking lot today, treat it as wooded, do you give it a wooded TC or its current TC? And how would you know what the wooded TC was given that the site has been graded?

Then you run into historic storage issues on top of that. If the site has a pond today, do you match the releases of the pond? Or do you have to pretend the site is wooded going into the pond? What if there are depression zones on the site, do you route them as zero discharge ponds to model predevelopment runoff attenuation?

Etc.

There's no one way to do this. That's what makes civil engineering fun.

If I were tasked with writing a regulation for a local municipality, I'd pick a soil type, pick a land cover, pick a TC, run an analysis for one acre of watershed, and then develop an allowable discharge per acre of site. Strip the modeling requirement out entirely and just hold engineers to an allowable discharge per existing basin area. Make it simple and inarguable. As sad as it is to say though, the weirder the regulations get, the more job security there is for engineers who know how to navigate them. So I guess there's a strange kind of silver lining to the whole deal.

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

http://www.campbellcivil.com