Detention Time vs Discharge Ratio chart query 1

[stioffan]

In using the universal figure that is the "Detention Time vs Discharge" chart (where you find the ratio of outflow to inflow by inputting the unit peak discharge), I am wondering how the 12-hour and 24-hour curves were formulated. I have searched for the answer to no avail. It seems the Maryland Department of the Environment may have created this chart, but I don't see how they came up with the the curves on it.

 

[psmart]

It would help if you could cite the document this appears in.

Perhaps the MD DOE can be of assistance?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[stioffan]

The chart I speak of is in the New York State Stormwater Design Manual Chapter 8 and the Maryland Stormwater design manual and many others. It seems it is universally used by everybody. I tried in vain to attach it to this post.

 

[cvg]

well, "universally used by everybody" if you don't count the rest of the world. never heard of this.

 

[beej67]

I've seen reference to something like this in a training manual put out with a course I took on the Georgia Blue Book, and I've used it as a first guess ballpark for CPV calcs down here. It works pretty good. (+/- 20%) The problem with relationships like this is they're based on assumed detention pond characteristics, but they're not terrible depending on your application. Don't forget, you're not building a watch. Usually anyway.

(to everyone, I'll try and scan the reference and stick it up here for general gawking if I get the time tomorrow)

As a professional engineer turned full time hydrologist, who's spent the last seven years doing site hydrology for land development in eight radically different states in the south eastern US, I recommend you find out what the local guys do, and do that. Stochastic hydrology is only as easy (or as hard) as you (or they) make it.

My hydrology reports for Gwinnett County Georgia are 180 pages long, but some of my best and most creative work was in Mississippi where they've barely even heard of stormwater management, and the 18 page report I gave them blew them away. They thought they'd hired NASA to build a hardware store.

You have two goals as a civil engineer dealing with stormwater management for new development:

1) Make sure the neighbors aren't going to have reason to sue your client, the best way you know how.

2) Then, separately, give the regulatory agency whatever they ask for in whatever format they're most used to seeing.

The larger lesson is this - If the locals want you to use a qo/qi relationship, then that probably means it's worked for them for years, and if so, keep using it. Hydrology is so different regionally, that everyone's come up with different ways to do it, and stuck with them if they work. Use what works.


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

http://www.campbellcivil.com

 

[stioffan]

Thanks Beej67. I wouldn't mind gawking at the reference you mentioned.

 

[beej67]

As promised, see attachment.

Some history..

In Georgia, the Blue Book (Georgia Stormwater Management Manual) has a channel protection criteria that dictates you draw down the volume of runoff from the 1 year storm over a duration of 24 hours. The problem is that the Blue Book is NOT a regulatory guideline, it's supposed to be a big book of suggestions for local municipalities to draw from when they're forming their own regulations. Georgia does not have and has never had overall watershed management districts like Florida, so each municipality wrote their own stormwater regulations. Since the ARC released the Blue Book, a bunch of municipalities have decided to simply say "we follow the blue book." But what they really *mean* when they say that, is they follow bits and pieces of it, and still want you to do the same stuff they used to require under their old regs, and if you use something out of the Blue Book they're unfamiliar with, they're likely to refuse your permit even though they claim to follow it. Then there's the issue of how the details in the Blue Book don't really work for coastal engineering, and blah blah. Big headache. But I digress.

The channel protection requirement is one of the most frustrating, because every municipality treats it differently. Some want you to detain the 1 year storm. Some want you to calculate the volume of the 1 year storm and provide that as a drawdown volume in your pond. Some want you to figure out what orifice size it would take to discharge that volume in 24 hours, then use that orifice size and your pond geometry, route the 1 year 24 hour hydrograph through your pond, then take a note of how much that fills your pond up, and then call that peak routed volume the "channel protection volume." That convoluted version of the rule is increasingly becoming the "normal" way to interpret it, but it's impossible to calculate if your pond is only for CPV, because your pond will be too small to calculate the imaginary 'head' necessary to size the orifice.

Which is why the ARC is now recommending simply "estimating" the CPV with some nomagraphs and calling it done. That's what the scans are of, from that training course. The funny thing is the guys who wrote the manual, and the guys who give the courses, generally don't have any idea how wacky the different interpretations of their criteria are. There's this never ending drama of different reviewers misinterpreting the Blue Book and then claiming their interpretation is right, and site engineers basically just have to ask in advance which 'version' of CPV they're dealing with on a job.

Anyway, there's some qo/qi graphs from a power point presentation, so you can see what stioffan is talking about. That same figure shows up in the Blue Book itself on page 2.2-10, and it says:

(Source: MDE, 1998)

..but nothing in the references section at the end of 2.2 mentions MDE or was dated 1998, so I have no idea where the thing came from.

The Blue Book can be downloaded here:

http://www.georgiastormwater.com/

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

http://www.campbellcivil.com