storm sewer sizing & flood routing 1

[gjeppesen]

I live in south Florida where SWM systems are designed for specified 25-yr storm (varying durations depending on where the project is). However storm sewer systems (curb inlet/pipe/to detention area) are usually designed for the 10-yr storm using the Rational Method & IDF curve.

Why are these designed with completely different methodologies?

I think I know the answer but help me out...If the "project" must be designed to the 25-yr storm (a few inches of road ponding is ok), the curb inlet pipe systems will fail anyhow under the 25 yr storm, so they only have to be designed to the 10-yr?

I'm confused...

 

[RWF7437]

I think you're on the right track in your reasoning. Some Minor flooding can be tolerated as you note in your post so some parts of the system may have less capacity. All such regulations are, however, arbitrary. They may be reasonable but only a detailed cost benefit analysis can answer that question. Such anlyses are almost never done, in my experience. My own limited analyses suggest that local detention ponds are a waste of time and money and the community would be better served by a system of thoughtfully designed regional detention faclitities, publically owned and maintained.

You may safely ignore these comments since you will have difficulty finding a single public sector engineer who agrees with them.

 

[dicksewerrat]

Public Utilities cannot afford to design the transportation system for a 25 year storm. Or I should say the public, who has to pay for the storm water system, cannot afford it. 35 years ago the standard was a 2-3 year storm event. But they can make rules for private developers as strict as they want.

 

[RWF7437]

Are developers not also members of the "public" these agencies are created to serve ? Is private money different than "public" money ? Is waste desirable whether public or private ?

Just wondering.

 

[gjeppesen]

Not to get off the topic, but you're right. I live in Naples, FL where development is out of control (that's good for me because I'm a civil engineer). It seems like government agencies hold developers to different standards.

For example, I just receive an email from a local water and sewer utility (an email sent to all local engineers). They are holding a day-long class on a new software package that now must be used to "convert" as-built autocad data to GIS form. I've been told that it really isn't needed on small jobs, but big jobs yes. The cost to the engineer? $1,000 a seat. No other utilities are using this software. So basically 15-20 firms will have to shell out $1,000 a piece to keep this utility's as-built database state-of-the-art (and pay someone a day's salary to learn it).

Shouldn't the utility pay for it?

Also, the County makes developers whose project's require an open-cut of a local road for utility contruction install a 25' overlay of asphalt on either side of the cut.

However, when the County itself does it (repairs, drainage improvements, etc.) there's no overlay required. Is that right?

 

[RWF7437]

NO!

 

[KRSServices]

I just gotta weight into this one...even though I'm gonna get blasted, because I will argue from the County's perspective!

You all all correct, there is a double standard when it comes to what a developer has to provide as opposed to what the County provides. The bit about the GIS to Autocad brings back fond memories because I have made those requests (albeit from mylar sepia to the latest Autocad releases, and not GIS) to developers in several bylaw changes over the years as well. It all boils down to money and ultimate ownership.

When development occurs, the municipality only gets one chance to get it right, and alas, in some instances it is either too conservative or not good enough. When reviewing requests for new subdivisions, the municipal officials have to crystal ball to ensure that the municipality will not have to provide upgrades or rehabilitation for at least 25 years or so. Further, the developer, in terms of obligation is responsible for maybe three years at best, depending on the dvelopment agreement, and after that is essentially off the hook. After that, any problems, particularly subsidences or road failures, are now the resopnsibility of the municipality, whose funds are likely stretched to the limits by unhappy taxpayers and Councils.

On the other side of the street, comes the "substandard" repair or municipal upgrade. Those costs are borne directly by the municipality, and any screwups or deficient works are borne directly by the department. All too often, the subdivision servicing standards and the public works/engineering standards are not equal because of the direct cost. I know many near perfect engineers who cry foul every time a patch is half that of development or density tests for repair trenches are non-existant, but that is reality. I'm not saying it is right, but I know some of the departments I managed could never afford to repair trenches to the same standard as a developer, or the base material under the asphalt patch was never as thick as the private development. If a failure re-occured, or was cotinually problematic, the Cuncils sometimes allowed it to be properly done, but only under pressure from the public. In one municipality I managed, my annual budget was $32 million for operations/wages and equipment. Of that repairs were usually allocated at about $5 million or so in the provisional budget, but usually whitled down to about $2.5 million by the time the annual budget was adopted. Council ordered the cuts and understood that our repair work would not be equal to that of new development, and were prepared to live with it. I've had the pleasure of having several developers sit as Councillors and in all cases, they were the first to adopt more stringent development standards and also the first to waive those same standards when it came to municipal work because of cost and responsibility.

I could go on for hours, but would love to read more of your thoughts.



KRS Services

http://www.krs-services.com

 

[cvg]

I am a consultant and my company does it's fair share of development. But I will also argue the public works side.

Case in point:

I am writing standards and policies for a city which will then apply them to both public and private works. City wanted to apply a minimum standard height for box culverts constructed in an alluvial fan area. City noticed that the boxes tend to fill with sediment and limited headroom prevents easy maintenance. Developers whined and moaned for days when we suggested 6' high minimum boxes instead of 4' high minimum. Why was the city doing this to them, as it would cost millions of dollars? The answer is quite logical - the city spends a lot of time trying to maintain these boxes and that is a direct impact to the taxpayer. Plus, one of these substandard boxes (only 3 feet high) filled with sediment and subsequent storm was diverted into another watershed, causing much damage and threatened lawsuit. It seems developers rarely consider the big picture, as they are driven by owners that are striving for higher profitability to pay for their boat, car, house, etc. Even the way developers engineers "bid" jobs (so much $ per lot) shows the motivation for more profit, not protection for the public interest. I agree with KRSService, you only get one chance to get your pound of flesh out of the developer...

 

[johnhan76]

Most engineers here use the scs method for sizing the detention pond and the rational method for sizing the pipes. This can results in ponds which are oversized since the limiting factor is the interior piping.

I am contemplating a move to a hydrodynamic model such as SWMM. It allows one to model the interior piping and detention pond in tandem thereby eliminating problems with different storms and methods. I have purchased XP-SWMM and have been playing with it and I am trying to gain confidence in the system and and its use in my designs. Does anyone have any comments regarding the use of SWMM for subdivision modeling?

 

[RWF7437]

Does anyone remember gjeppeson's original question by now ?