Feel like starting a war... 4

[SMIAH]

A city reviewer here ask me to do retention calculation by using "fixed parameters" in the rational method formula (...I know...)

As an example, if someone is planning to extend a parking lot, they ask to use a C of 0.5, a Tc of 15 min and an Intensity related to 100-year frequency as "actual conditions".

Then you have to calculate the "real" parameters for the parking lot for the "proposed conditions" (e.g. C = 0.95, Tc = 5 min and I still related to 100-year).

And Voilà! This dumb procedure leads us to a high retention volume - City reviewer is happy.

Am I the only one thinking this is a pretty stupid procedure?!

I feel like using my own common sense and calculate the retention volume required based on a procedure that makes sense - then arguing with the City reviewer.

 

[gbam]

Have you reviewed the city/county/state code in regard to retention? If all are in line with the city's request, I don't think you have any other choice.

 

[SMIAH]

Believe it or not, They don't have any...

100k population city.

That's the parameters to use - that's it that's all.

 

[SMIAH]

[Edit] Even worse ... They ask for a I = 10-years as actual conditions and ask to do retention for a I = 100-years for the proposed conditions...

 

[cvg]

how are you getting volume out of the rational equation? the rational equation only gives you an estimate of peak discharge, not runoff volume.

the city can require retention based on your shoe size, it's their drainage ordinance.

 

[SMIAH]

Modified Rational Method...

Bogus

 

[gbam]

SMIAH - Does your state or county have regs regarding retention that you can hang your hat on?

 

[Ryb01]

SMIAH,

I think anyone that has completed and/or used the rational method for a project of similar calibre would sympathize with your frustrations.

I have yet to see a municipality that only accepts the rational method for hydrology. Is it possible to use a more accurate approach? (i.e. modelling with HydroCAD, SWMM, etc)

 

[SMIAH]

I will use HMS and route an hydrograph (3-hour 100-year storm) through the basin. Based on what the outlet is capable of releasing we'll see what if the basin's shape is ok.

Hopefully not too far from the Modified Rational Method results...

 

[3feethighandrising]

If you are going to start a war, I would first:

1. predesign to the reviewers "standards",
2. size a detention facility,
3. show the costs to the client.

Then he/she will be onboard for the fight and will anticipate and appreciate your time and expenses.

If you are going to fight, track who has the general NPDES discharge permit from the EPA for your area. They really have the final say on stormwater regulations, since they hold the permit from the EPA. Look on the EPA website to find what agency holds your areas NPDES Stormwater Discharge permit. Then find their approved stormwater manuals. Although towns can still write ordinance that allows them to adopt something more stringent than the EPA, I don't think this is the case here. The Rational method for sizing detention is not accepted by any NPDES permit that I have seen.

Use a hydrologic model/method that is accepted by the regulatory Stormwater Manual under the NPDES permit for flow control. Use it size the detention and submit it to the reviewer along with all the data, and the Section of the NPDES approved Stormwater Manual that says you can use it. If the review refuses it, forward his correspondance and comments to the agency holding the NPDES permit.

 

[francesca]

If they don't have their own regs, use the DOT's.

 

[SMIAH]

Sorry I didn't mention that this project is in Canada...

I will do a cost analysis based on both "procedures" (City Reviewer onboard).

Thanks

 

[ccampttu]

Unless this is a postage stamp size site and the client is trying to maximize every square inch because of ineffective site plan layout (ie. a favorite pastime of architects - convincing the client that they can develop the entire site without any allotment for stormwater BMPs), then I believe you will find it is not worth the fight. I'm sure every stormwater engineer here has been in a similar situation. 3feet is right get the cost, talk wit the client and see if this is a fight they want to be a part of. Sadly unless this is a large developer who will be working in this municipality over and over again it will not be worth it. Build it big, curse them under your breath, and let it go. Just be glad that they are not making you use a C = 0.98. One of the best quotes i've heard in a long time = "the city can require retention based on your shoe size". Thanks cvg LOL

 

[SMIAH]

True that it is not worth the fight!

The guy reviewing isn't an hydrologist (no wonder) and I can't see myself arguing about the fact that the Modified Rational Method shouldn't be applied to retention volume calculation.

Lost the fight

 

[beej67]

I pink-starred 3 feet high. The first thing to do when starting a war is make sure someone else is banking your time on it.


For what it's worth, a lot of the municipalities in my area have moved from regulations that ask you to "match existing conditions" to regs that make you "match an undeveloped condition," where they specify the CN you've got to pretend was on your site before you got there. Sounds a bit like what you ran into, outside the Rational headaches.

One option you may have, if you're trying to keep your volumes down, is the Dekalb Rational Method. I don't use it anymore, but City of Atlanta used to accept it, and we would sometimes go with a Dekalb analysis on urbanized projects at my last company.



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

http://www.campbellcivil.com

 

[SMIAH]

Oh didn't know about that Dekalb method. Might check that.
I know that we have to follow what the reviewer ask but there's a limit... Mine is when someone tells me to "calculate the required volume with Q=CIA".

This is leading to over-design retetion facilities.
Over-designed + based on a 100-year flood. I mean it's a parking lot...

 

[elev8848]

As professionals, if we do not voice these concerns, of misuse of procedures and mathematics, who will? Many over zealous and under-educated regulators are out there. They must be challenged in a professional setting.