simple question - design for 30 year storm

A so-called 30 year event is one in which the probability of it being exceeded in any one year is 1/30. This is what the statisticians call a poission distribution. From any basic statistical text you can find out how to calculate the probabilities of this event occurring n times in any x year period.

From the way that you worded your post, you really do not have a clue what you are talking about. It appears that you are attempting to design a storm water system. Ground water is water that is under the surface of the ground and as such cannot flood which is too much water on the surface of the ground.

In storm water system design in addition to having to know what the intensity, duration and frequency of the rainfall event that you are designing for, you also must know something about hydrology, runoff coefficients, contributory areas, pipe and open channel flow etc.

Since you obviously do not even know about these things, I would suggest that either you go back to school and learn about them (we will not teach the basics here) or get a qualified engineer to do the design for you.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

thanks for being patronising rick, good to know that there's support out there for young engineers starting out in the profession, yes ground water was an abvious faux-pas on my part, as regards appearing a little green on the topic I prepared you for that by stating at the outset that I was a structural engineer

i'll try to expalin a little better where I was coming from:

the phrase "such that flooding does not occur more frequently than once in 30 years" was perhaps misleading as to what i was looking for

the "surface" water from the proposed development is to be collected in a drainage system which will discharge to a new culvert running through the site, this culvert, upon reaching the edge of the site discharges to a stream which discharges to an attenuation pond on a neighbouring site before discharging to a major watercourse.

I am looking for directions as to where I can find information on the rainfall intensity and duration that i should be using for a reccurence interval of 30 years in order to calculate the potential rise in water level that we will generate in the attenuation pond.

i've searched the net and can't find any charts, maybe i'm using the wrong phrase as to what i'm searching for

if i'm still being unclear then perhaps i do need to 'go back to school' or 'get a qualified engineer to do the design for me', as rick (politely) suggested.

rick, i'm looking for assistance not a lecture, if it's still a case of 'you really do not have a clue what you are talking about' then please don't respond

You're going to have to tell us what state you're in...generally each state or region has its own, industry accepted source for that information. Once we know, perhaps someone in this forum from that area can help you find the information.

In Canada you buy the rainfall duration curves from Environment Canada through their web site. Any standard textbook will also give some generalized information on this and there are some standard formulas that will approximate the rainfall for different locations.

How hard did you search for this before you asked us to do your work for you? Try searching "IDF rainfall curves" This will give you several hundred hits.

I am assuming that a PE or P.Eng will have to sign off on the design for you. You could try asking him or her for some guidance. If there is no qualified professional willing to sign off on this then you are practicing without a license. That’s sort of frowned on here and by the authorities.

I note, sparkylynch, that you are new here. You made this post the day you joined eng-tips and never made any attempt to see how this site operates or who the regular posters are here. It’s even in the wrong forum. You asked a basic question far out of your depth, without apparently doing any real research on the issue.

You expected us to educate you on a complete area and to willingly give up our time to provide answers.

You got snippy when I pointed out the obvious that you were out of your depth so far that you did not know the basic terminology. If you are going to survive in this profession you will need to learn some respect for those of us who have been there, done that and got the scars to prove it.

If you don’t like my response, too bad. This is not grade school where we are concerned with your self-image and personal development. This is the real world where you have to take some responsibility for your actions.


Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

thank you EricC and Rick......and rick, i'm sorry that we got off on the wrong foot...............while it may seem that I hadn't tried very hard before asking the forum, i had..........i can't do anything to convince you of that

as soon as I searched for 'IDF rainfall curves' i was presented with the information i had been looking for, thanks

as regards my forum manners, i am new to this and will endeavour to learn from my mistakes this time round......
and if i can be of help to anyone, i will

as regards what state i'm in.............it's a small green state, a few thousand miles to the east of new york.......

go raibh maith agat

No problem

Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

Sounds like you're in Vermont. You should be able to obtain a copy of the Vermont Highway Department's Hydraulics Manual. Nearly every State has one as far as I know. There you should find IDF curves and much other useful information. Often the manuals are free but always they are reasonably priced since they were paid for with your tax dollars.

Good luck,

Russ

Russ

Vermont is a little closer than a few thousand miles east of NY. Try Ireland




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

It seems that Sparky could have prevented much of this misunderstanding by stating that he was in Ireland from the outset. Perhaps a "groundwater system" in Ireland is equivalent to a "stormwater drainage system" as defined in the United States. Similarly, Environmental Engineering in the States doesn't usually include drainage design, however, Civil Engineering does. A "30 year storm" should have been the tip-off. In the United States this would be a 25 year storm.

But, I do agree somewhat with Rick and have stated so in the forum before, that engineers need to be careful about not practicing outside of their area of expertise. I would hope that Sparky has plans to have an experienced "drainage engineer" review his work and provide the necessary assistance so Sparky can learn drainage design and keep himself out of civil court.

thanks for your advice cvg

no, groundwater here is the same as groundwater over there, my slip-up........

just so you can sleep, this will (eventually) be signed off by an experienced drainage engineer (when we find one) but i am attempting to cover as much of the ground as i possibly can before i hand it over (i'd like to be able to give him/her a reasonably complete/competent package in order to minimise their workload and consequently their fee + theres no better way to master a new subject than starting from scratch, trying to resolve issues from first principles, and pulling your hair out at regular intervals)

thanks again everyone who responded

Hey, I’ll come and look at it for a reasonable fee (+travel expenses)

Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

thanks for the offer rick, i don't know if i'll have the time to make it to canada and review your portfolio, anyway, as they say in showbiz: "don't call us, we'll call you"

Hi sparklynch

It seems that the forum has given you a hard time - I trust you will help us out when we have a structural problem.

You need to solve your problem by the "rational method". It is a long time since I worked in UK but used to be well described in Road Note 35. (HMSO). You should be able to find rainfall inntensity duration frequency curves for your area. In UK they used to be available from the met office. You could also make a rough estimate from Billam's formula.

R = 267.7 X N^.2817/t^.7183-152.4/t

R = mm rain per hour
N = return period
t= storm duration mins

The storm duration is equal to the time of concentration.

And, the time of concentration is the time of travel from the furthest point to the outfall of your drainage sytem.

The run off flow is then calculated from the the run off coifficientthe rainfall intensity and the catchment area .

Q= CRA (you need to make sure the units are consistent)

That is the method in a nutshell but if you just follow what i have said above you will get it wrong. You need to read and understand the method. I suggest you read Road Note 35

Brian C Eng

Sparky

I have just noted that you want to calculate the change in storage in the attentuation pond. This is a much more complicated problem and is not something you will be able to tackle. You will need a comptetent set of calculations to support your planning applications. The attentaution pond will have been provided in the first place to satisfy planning restrictions.

From your description it appears that your drainage combines with drainage from other catchments and then discharges through the attenuation pond into a main water course.

You will need an overall catchment model which will need to include routing calcualtions through the attentuation pond pond. (water will escape from the pond when it exceeds the outfall level and the rate of escape will depend on the hydraulics of the outlet structure).

If you want to learn a good guide is the CIRIA publication B014 "Design of flood storage reservoirs" 1993 ISBN 0 7506 9057 3

Brian

Met Eireann will have the details you're looking for. Don't forget to be county specific, and a lot of CO.CO.s are doing catchment plans at the moment, which should help. What area of the country are you in?
Michael

About the year 1860, Emil Kuichling, City Engineer of Rochester, New York proposed the so called "Rational Formula"; Q = CIA, for calculating the peak runoff from an urban drainage basin. He presented his ideas in a paper published in the ASCE Journals along with the data from five test basins he had monitored. His simple equation tries to model the complex series of events which take place during a storm. The method is easy to use, requires only a small amount of data and, for small watersheds, appears to give reasonable answers. It is thought by many to be conservative by overestimating the flow.

Despite it’s obvious drawbacks, the method continues to be widely used today, although sometimes it is misapplied to larger watersheds for which it has never been demonstrated to be applicable.

In the U.S the method is not accepted by Federal agencies for basins larger than 100 acres.

Good luck,

Russ

thanks for all the help guys, i've now got enough bed-time reading to take me through to retirement...........

Ouch, a lttle harsh there RDK....