SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=421627
The above URL contains the entire discussion. Below is a snippet.
QUESTION
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 that 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. The city has now said we must evaluate pre-development drainage and evaluate the site ignoring the compacted gravel areas, and use residential lot/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.
REPLIES
beej67
Different government agencies treat this different ways.
Just in my region, I’ve seen the following approaches used:
- pre-dev = forested no matter the current ground cover
- pre-dev = grassed no matter the current ground cover
- pre-dev = current ground cover
- pre-dev = current ground cover but provide a 30% reduction of post-dev flow rate
- pre-dev = forested and provide a 10% reduction of post-dev flow rate
- 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)
- pre-dev doesn’t matter, match a fixed unit flow rate per area of watershed for the design storm
- 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.
- 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 pre-development runoff attenuation?
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, 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.