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Time of Concentration / Peak Runoff

Time of Concentration / Peak Runoff

Time of Concentration / Peak Runoff

(OP)
I'm confused, please help. Why is it that as time of concentration for a watershed goes up, peak flow goes down?
And does it make sense to split a relatively small watershed(<100ac)into subwatersheds with different time of concentrations and combine hydrographs? Does this assume that a storm with different rainfall intensities is occurring at the same time? TIA

RE: Time of Concentration / Peak Runoff

1. As time of concentration goes up, the Rational method assumes the storm duration also goes up.  Because of the shape of Intensity-Duration curves, the intensity goes down as duration increases.  I-D-F curves are derived statistically from precipitation records.

2. It seldom, if ever, makes sense to divide a drainage basin into pieces which have a time of concentration of less than 5 or ten minutes. My own opinion is that ten minutes is a reasonable minimum.

3.  It never makes sense to use the Rational method for large drainage basins.  FEMA limits its use to basins smaller than 1 square mile ( 640 acres).  Many local reviewing agencies limit it even more.  Where I live, Salem, Oregon, the City limits it to 100 acres or about 0.16 sq. mi.

4. The Rational method was developed in 1873 in Rochester NY and is based on only a few, small urban drainge basins studied then. It hasn't been shown to be accurate much beyond those original conditions.  For larger basins you are probably better off to use stream gage records or programs such as NFF the National Flood Frequency program available free on the net.  Do a web search for it.  You'll find lots of other useful information along the way.

Good luck

RE: Time of Concentration / Peak Runoff

(OP)
Thanks so much for responding to my post. I am using the SCS TR55 method, but every method I know of (I generally work  in small watersheds(<500ac), as time of concentration goes up, peak flow goes down. I understand that it is because that's what the statistics say, but if you have a five minute storm intensity over a watershed with a time of concentration of 45 minutes, does that peak from the five minute storm not travel to the point of interest?  I know my question may not be clear; my specific problem may help.

We are developing a site at the downsteam end of a 100 acre watershed. The watershed consists of three well definable sub watersheds. 3/ac residential, 8/ac residential and pasture. All with a time of concentration of 30 to 45 min.
The site we are developing is at the end of this watershed with a tc of 5 min. I can show by combining these hydrographs that we are not increasing the peak flow at all because by the time the peak of the larger watershed has reached our site, our site peak flow has already left. Is this reasonable? And in the process of thinking through this I stopped to think, why does peak flow go down as time of concentration goes up. I know that's what the formula says, but i need more.

Thanks again

RE: Time of Concentration / Peak Runoff

BE CAREFULL !

Your post suggests that you're making a number of unsubstantiated assuptions.  Whether or not your development will increase the peak flow depends on the the size, shape, pattern and direction of travel of the "storm cell" which is causing the runoff.  We don't know any of those things, nor can we predict them with any accuracy.  The prudent thing to do is assume the worst combination of these things which will almost certainly increase the runoff downstream from your development.

Routing only one assumed combination of hydrographs does not answer the question either since they are based on a single type of storm, not all possible storms.

There is more art than science to hydrology so try to find an artful solution.  Remember too, some reviewing agency will look at your design and they are very unlikely to accept an assertion that development will not increase runoff.

Good luck

RE: Time of Concentration / Peak Runoff

(OP)
That's why I'm asking for advice. I don't know what the conservative approach is. If we route the flow from our site through a pond then the tc goes up and actually increases the peak flow for the larger watershed. Our options are decrease the runoff for the 5 min 10 year storm and increase the runoff for the 45 min 10 year storm; Or increase the runoff for the 5 min 10 year storm and decrease the runoff for the 45 min 10 year storm. We can provide detention to do either. help? any answers concerning the tc / runoff relationship would still be appreciated.

RE: Time of Concentration / Peak Runoff

If you're building a pond and outlet control structure using TR-55 you can't possibly be increasing the runoff either upstream or downstream !  Remember, the pond has no effect on the flows coming in to it and can only protect lands downstream from it. Juggling numbers cannot change the physical and climatological facts.

A pond designed for only one small set of conditions is not very useful.  You use TR-55 to get an initial pond size (volume) and outlet control size.  Then, you check it against a range of possible storm conditions.  Even if the design standards you are required to use don't require this level of checking, the prudent engineer will want to do this.  Without such a check, you don't know how good, or bad, your design is.  When will it fail? How will it fail ? What is the overflow control ?  Are you satisfied that the pond will meet not only the mandated design criteria but will it function safely over the range of possible storm events ?

"Local" detention ponds are not very useful, efficient or rationally designed in most cases and may represent a liability for the people who have to maintain them.  Regional ponds and conveyance systems make much better economic and engineering sense, in my view.

End of lecture.

Good luck anyway,

Russ

RE: Time of Concentration / Peak Runoff

(OP)
Thanks again, but you're getting the wrong idea. I'm not trying to manipulate the numbers to give us the most "economical" solution. I just really don't know the answer. We have a channel coming through our site (not a stream) on which we could provide detention for the larger watershed (maybe a 48" culvert) or we could pass the channel through our site with a culvert and provide a pond to retain the site only (8" orifice). You know what, nevermind, we're gonna do the culvert and retain the 2 and 10 from our site. I think that's conservative. Thanks, and have great weekend.

PS - still looking for answers on the tc / runoff relationship

RE: Time of Concentration / Peak Runoff

The simple answer is just that you have the same relative volume of water.

short tc -- the rate at which the water leaves the basin is very high.

long tc --  there is a longer period of time that the water is leaving the basin, hence lower peak.

As the tc is the time for the most hydraulically distant part of the watershed to reach the basin outlet point then with a longer tc the lower parts of the basin are all contributing water as that most hydraulically distant point winds its way down.

Try four or five different tc's, say 5, 10, 30 minutes, etc., and while your peak rate will vary substantially your volume will probably stay within 5%.

Detention can have high unintended consequences if the entire receiving water (stream) is not carefully considered.  Can you use small infiltration/bioretention areas throughout the development instead of one detention area?

RE: Time of Concentration / Peak Runoff

(OP)
Thanks ptmoss for the tc/runoff explanation. That's the direction I was headed in (no, really). I actually was doing that calculation. Not much of an option on the detention pond. As I said before we are developing about 10 acres at the end of the watershed on which about 300' of the channel draining 100 acres passes. We either let the channel pass through and retain the small site to pre-developed conditions or pass the small site through and retain the larger watershed to predeviloped conditions. Or am I missing something completely?

RE: Time of Concentration / Peak Runoff

To answer your predicament you really need to evaluate what's downstream from you.  What does this channel empty into?  What are you actually needing to design for?  I think one of the things I read into Russ's comments, and I agree with, is that you may have a regulatory requirement to detain to a pre-development peak (or some other parameter), and you can fairly easily do that, but by doing so you might be creating a whole other set of problems.  And these would be problems that you're not mandated to check by the reviewing authority you have to satisfy.

If you have a lake right below you it might not make much difference (detention-wise).  But if there is another development right below you then you need to evaluate a larger range of events than the reviewer might require to see if your detention is having some other adverse effect.

There's a school of thought that says staged detention with some form of water quality treatment included is the way to go.  Another puts higher stock in the water quality.  Then there's the client (developer).  They just want you to meet whatever the minimum requirements are that the regulating authority is looking for.  And there are plenty of engineers that will do that and feel they are meeting their obligations just fine.

Am I starting to stray off track here?


RE: Time of Concentration / Peak Runoff

(OP)
One more question about tc / runoff relationship.

Sorry, i've been known to ask stupid questions and make things more difficult than they should be but.. tcmoss  spoke about the runoff volume being relatively the same
for different tc's and I understand this....here is the way I'm thinking, tell me where I'm wrong (I'm thinking in terms of SCS method)..

In terms of a 10 yr Storm Hydrograph:
1) A ten year 24hr storm distribution (rainfall intensiy) looks the same no matter what the time of concentration is. (rainfall not runoff)
2) The 5 minute peak runoff is based on the average rainfall that occurs in the 5 minutes centered on the peak ordinate of the 10 year hydograph.
3)  The 45 minute peak runoff is based on the average rainfall that occurs in the 45 minutes centered on the peak ordinate of the 10 year hydograph.

So, for a watershed with a time of concentration of 45 minutes the 5 minute peak intensity occurs. Is this not the peak runoff? The runoff from the 5 minute peak will travel 45 minutes and impact the point of interest, right? But the way I think the peak is calculated for the 45 minute storm is by using the average rainfall that occurs in the 45 minutes centered on the peak ordinate of the storm hydrograph.Does this make any sense? I think I've confused myself; Be gentle....The only way to learn is to ask questions.

RE: Time of Concentration / Peak Runoff

(OP)
Nevermind, I've sorted the whole thing out. The results I was getting made me question, and want to be sure of my understanding of hydrology. I think this illustrates why detention ponds might not be all they're cracked up to be.
By detaining the peak from our small site, we actually increase the peak from the larger watershed. By increasing the time of concentration from our site (routing through pond) our site peaks at a time closer to the peak of the larger watershed, therefore increasing the total peak. We are going to retain runoff from our site to a point where neither the site runoff or the runoff from the larger watershed is increased. Who knows what this does to the peak of the recieving stream. I know that this one site will not effect the peak greatly, but what about when the entire watershed is developed with detention ponds?

RE: Time of Concentration / Peak Runoff

Please BE CAREFUL!

From what you've told us so far,you have demonstrated that for one type of storm, and for one storm duration ( 24 hours ) and for one case of antecedent moisture conditions, that it is possible for the peak flow to be increased.  But you have NOT shown that this is true for every possible storm.  You haven't even analyzed other storm intensities, durations, frequencies, or sets of assumptions about development patterns.

Better luck,

Russ

RE: Time of Concentration / Peak Runoff

(OP)
Russ, I have analyzed for the 2, 10, and 50 year 24 hr type II storm distribution. I think regardless of amc and duration of storm the relationship of the time to peak for the subwatersheds will remain the same, Am I wrong in saying that the 24 hr storm duration is the conservative approach in determining the peak flow? Yes there may be a storm distribution that produces different results, but I can't analyze for the unknown. All I can do is analyze for the worse possible case for the typical storm for this area, and I think I have done that. If you disagree please let me know, and be specific. The general criticisms and wishing me better luck is not very helpful.
Thanks

RE: Time of Concentration / Peak Runoff

Partly in answer to your question, we don't know what the critical storm duration is. It may be 24 hours or it may not.  TR-55 uses 24 hours only because there was lots of 24 hour rainfall data available in 1972 073 when the method was developed.  So I cannot say whether in your case 24 hours is either conservative or not.

Also, TR-55 is not the only method available. There are others including the Snyder Unit Hydrograph, Clark Unit Hydrograph, Santa Barbara Unit Hydrograph and simulation programs such as HEC-HMS, SMADA, SWMM and others.  All of these help us to look at the situation from many points of view and use estimates of peak flow calculated by other means such as Regional Regression Equations as does NFF   ( mentioned in an earlier post).

Finally we should ask what level of protection we are really providing.  The "100 year storm" has a 1% chance of ocurring in any one year.  Over the life of a 30 year mortgage, the "100 year storm" has a 24% chance of occuring; 1 in 4.  Over 100 years the "100 year storm" has a 67% chance of ocurring.  In other words, it more likely than not that it will occur one or more times.

Since you find luck un helpful, I'll send you no more.

RE: Time of Concentration / Peak Runoff

(OP)
Russ, I didn't mean to offend. Thanks for the input, it has been very helpful. I'll take another look at it.

RE: Time of Concentration / Peak Runoff

Rookie and Russ,

I am going to attempt to jump in.  Keep in mind that some storms move from the downstream up and some storms move from the upstream down.  In your case the entire watershed would probably receive the storm at the same time since it is only 100 acres.  I would just keep this in mind for future jobs because sometimes detention will not increase the peak downstream if the storm moves in one direction but will increase the downstream peak if it moves in the other.

Russ, i was under the impression that the 24 hour storm has all of the other storms (6 hour, 12 hour, etc.) nested in it.  I may be wrong.

 

RE: Time of Concentration / Peak Runoff

(OP)
Hadn't thought of that, thanks

RE: Time of Concentration / Peak Runoff

johan76,

I don't believe the 24 hour storm has all the other, shorter duration, storms "nested" in it. Nor can it possibly represent longer duration events.  It is an artificial storm derived from statistical analysis of rainfall records back  in the 1970s. Many states have recently updated those record adding 40 more years of data.

Also, it is very unlikely that weather accross the US can adequately be represented by the four SCS storm types.  There are many microclimates, especially in the mountainous northwest, which experience different weather patterns.

Whenever possible, I try to route actual, historical storms through detention ponds and look at the results. (Rainfall data is usually available to do this these days ) Did the pond fill ? Was it overtopped ? Might it have failed in some way during such an actual storm ?

There is a lot of software now on the market which will take much of the work out of this so there is little reason NOT to do it.

Even so, flow and volume estimates are rarely more accurate than plus or minus 30% and we can only say t6hat these are the probable flows and volume.

RE: Time of Concentration / Peak Runoff

And some useful information on the "shape" factor for the SCS hydrograph:

Basin Characteristics            Attenuation Factor

Rural, flat                        150
Rural, gently sloping                    200
Rural, rolling hills                    300
Mixed urban and rural                    400
Mixed urban-rural, gently sloping            484
Urban, steeply sloping                        575
Rational Formula                    645

RE: Time of Concentration / Peak Runoff

(OP)
Russ, could you briefly explain the Rational Formula attenuation factor? TIA

RE: Time of Concentration / Peak Runoff

Sorry Rookie2,

I'm not familiar with the Rational Formula Attenuation factor.  I believe it was developed by Dr. Maidment.  He has a home page which is:

http://www.ce.utexas.edu/prof/maidment/home.html

The attenuation factors in my previous post are for use in the SCS hydrograph (TR-55) method, not the Rational Formula.  Q=CIA is simple but the limitations it has make it useless in many situations.  FEMA for example would not accept a flood study based on Q=CIA and many local agencies discourage its use for all but the smallest basins.

Regional Regression equations are, poor as they may be, probably the best tools we have to estimate peak flows.

Hope this helps.

RE: Time of Concentration / Peak Runoff

(OP)
Thanks again

RE: Time of Concentration / Peak Runoff

(OP)
OK, I've switched to TR-20, which is giving me higher peak flows. Thanks to Russ, I have come to the understanding that the 24 hr storm may not be the critical storm. From what I've read the volume required to retain other durations may or may not be higher. I'm a little foggy about why, But I am trying to route these storms to gain some understanding. I have 2 questions:
1) I have found a 6 hour balanced distribution. The software I am using (based on TR-20) allows me to enter
a custom distribution. Is it correct to use this distibution in TR-20, or are you limited to the SCS 24 hr distributions?
2) Where can I find or how do I create a distribution for other storm durations?

Your time and patience are greatly appreciated.

RE: Time of Concentration / Peak Runoff

I'm not familiar with TR-20 but an excellent program which allows you to create your own or input historical rainfall distributions is SMADA.

The SMADA programs were written to accompany the textbook
Hydrology: Water Quantity and Quality Control 2nd Edition by
M.P. Wanielista, R. Kersten, and R. Eaglin. The text is
available from John Wiley and Sons publishers.
 
http://www.wiley.com/
 
A manual for the computer programs is available
by sending a check or money for $95.00  
(made out to R. Eaglin) to (Non-US orders should
add $20 foreign shipping cost):
 
SMADA Manual c/o Ron Eaglin
1155 Elm Street
Oviedo, FL   32765
 
Support for this software is through e-mail only,
questions should be sent to:
 
eaglin@magicnet.net
 
The documentation contains information on;
 
Using SMADA
SMADA Theory
Using TCALC
Using REGRESS
Using DISTRIB
Using EZMAT
 
Consulting services are available and inquiries can be made to
the e-mail address shown above.
 

Russ

RE: Time of Concentration / Peak Runoff

(OP)
That looks interesting, I could obviously use the text book, I'm just a little concerned that this method is not on the approved list for the jurisdiction I am dealing with. From what I understand, TR-55 is just a simplified and less accurate  version of TR-20. I will definitely check into the SMADA software though. Thanks

RE: Time of Concentration / Peak Runoff

Don't worry about "approved lists".  The book is excellent and the program is affordable and, most of the time, works great.

Keep asking questions, I do.

Russ

RE: Time of Concentration / Peak Runoff

Rookie:

Could you tell us what is downstream from your site?

Several people have mentioned this and it is highly relevent.  Are you truly at the bottom of the total watershed?  Is your site on a large river or tidal waters?

If you are at the bottom of the watershed it is highly likely that no detention will yield the best overall peak flow results.

As far as your original Tc questions, here is my example of why peak discharge goes down when Tc goes up:
There are two rectangular drainage areas (A and B) with the same area.
Area A is 200 ft. wide across the bottom and 5000 ft. to the top (up the slope).
Area B is 5000 ft. wide across the bottom and 200 ft. to the top.
We will assume both have the same slope.

Both watersheds are approximately 23 Acres, so the same amount of rain falls on them and the same volume of runoff is generated for each watershed.

Because the Tc for Area A will be substatially longer than the Tc for Area B, this volume is spread out over a longer period of time.  

It is similar to your site being at the bottom of a larger watershed. The bottom 200 ft. of Area A has already drained off site before the next 200 ft. etc (for the remaining 4800 ft.).  The bottom 200 ft. of Area B is the entire watershed!  It should be obvious that this area with a shorter Tc will produce a much larger peak.

RE: Time of Concentration / Peak Runoff


You mention that the method must be on an "approved list" for the jurisdiction.  If they have a list of methods they are likely to have prescribed which design storms to use.  Just a thought....

RE: Time of Concentration / Peak Runoff

(OP)
Terryscan, we are discharging into an existing channel which drains across the backyard of a residential property. We are the bottom 10 acres of a 100 acre watershed. Your comment about no detention being possibly the best alternative, according to my calculations is true; but how would I convince a reviewer of this? But this leads me back to my earlier question. Assuming that there's no way around detention, is it better to route the total watershed through
through a pond and attenuate this runoff with a 48" culvert(This essentially provides no detention for our site and the downstream property would experience higher flows for a lower frequency storm; right?. The 48 in culvert does nothing for the runoff coming from our site.) Or should we pass the offsite watershed through the site with a culvert, and then route the site flow through a pond and slightly increase the peak of the total watershed. Am I making any sense? I just want to be sure to provide the best level of protection. Being that the downstream property is a home owner I'm sure they will be very sensitive to the increased runoff. Thanks to everyone for the input.

RE: Time of Concentration / Peak Runoff

Rookie2,
You are facing a dilemma that I have faced before.  I was unable to convince the reviewer that the detention pond would increase the total peak flow.  He may have believed it but probably knew if the downstream areas were flooded the new development would get blamed because of the lack of a detention pond.  That is the reality of the situation.

I dont know if I did the right thing but I went ahead and designed it anyway - I did oversize it though.

RE: Time of Concentration / Peak Runoff

I may not be visualizing the bigger picture here but:
"into an existing channel which drains across the backyard of a residential property" does not sound like the bottom of the watershed.  It sounds like a point where a subwatershed joins with other subwatersheds.

When a channel is crossing  my site, I usually find it much simpler to do off-line management.  That is: not to put the existing channel through my pond (on-line).  Aside from being simpler, I imagine one is assuming a liablility when placing obstructions (control) in an existing channel.

RE: Time of Concentration / Peak Runoff

(OP)
You're right Terryscan, it is a subwatershed for a larger watershed. I was thinking only in terms of the area draining to my site. That's the way I have the site designed (off-line), but I haven't thought of a liability aspect upstream, I've mainly been concerned with the property immediately downstream. Thanks

RE: Time of Concentration / Peak Runoff

Ok, since your site is not truly at the bottom of the overall watershed, in order to really evaluate the effects of detention vs. no detention, one would have to evaluate the other "branches" that meet with yours downstream.

That is to say, your assumption that letting the water go with no detention is better, because it avoids the peak from upstream areas in your subwatershed may not hold true if it ends up coinciding with peaks from other branches of subwatersheds downstream.

Example:
   Your subwatershed = A 100ac.
   Other subwatershed= B 200ac.

   Peak time of sub A = 14hrs
   Peak time of sub B = 12hrs
   Peak from subject site= 12hrs
(we'll assume larger peak on B even though it could be Tc dependent.  And assume these watersheds meet a short distance downstream)

Although it would lower the peak from your subwatershed, letting your site runoff go with no detention would coincide with peak from sub B and aggravate downstream flooding.

RE: Time of Concentration / Peak Runoff

(OP)
Terryscan you're right, thanks. I guess I am thinking in terms of what's immediately downstream from me. I guess If it were my house downstream, I would rather be protected from higher flows from the more frequent storms. The extra 4-5 cfs is not gonna make alot of difference on the less frequent storms. The questions I've asked are based more on understanding the problem in a general sense, not specific to this project. I'm sure I'll run into it again. I think thanks to everyone's input I've got a good grasp on the best solution. Thanks for all the input.

RE: Time of Concentration / Peak Runoff

Would it be possible to do an offline storage of the main channel.  For instance, instead of placing controls in the main channel, provide a way for the channel to spill over into a storage area that you provide adjacent to the channel.  Terryscan, is this what you were referring to?  OR were you referring to retaining the individual (10 ac.) site?

RE: Time of Concentration / Peak Runoff

I was actually refering to retaining the individual site if appropriate.  But, offline "spill over" storage could be a great idea.  I'd be much more inclined to to that than put structures in the existing channel.

I believe one still needs to study the overall picture before exerting great expense constructing this storage.  It seems like it could improve matters, but it also has potential to aggravate conditions downstream if a timing effect is involved.

RE: Time of Concentration / Peak Runoff


Considering "splill over" storage:

I know that it is complex, but what are the basics in designing one of these in a situation like rookie's. (ie 10 acre disturbance at end of 100 ac. subwatershed?

RE: Time of Concentration / Peak Runoff

Basics? I have not done one of these but I would approach it like this:
1. Determine if shaving the peak and releasing it later is beneficial. (whole watershed study).
2. Determine desired amount of peak to be shaved.
3. Determine elevation in channel corresponding to flows exceeding the desired peak.
4. Calculate the volume of the shaved peak.
5. Size storage area and based on this volume along with the configuration of its link to the channel.
6. Route a model of this design and tweak the configuration until desired results are obtained.

Perhaps someone with more experience with this configuration can correct or add to this approach.

RE: Time of Concentration / Peak Runoff

Mr Eaglin, My email to you has been rejected by my mailserver, Tiscali, Please contact me, Sretlaw at : dwalters@icon.co.za

RE: Time of Concentration / Peak Runoff

(OP)
Russ, You may be interested to know, or you may already be aware of it, but NRCS now has a windows based version of TR-55, which has expanded it's rainfall distributions. It now has a NM rainfall distribution and also allows you to enter historical storm events. Haven't used it yet, but it looks like a nice program. You can download it on their website.
Thought it might interest you.

RE: Time of Concentration / Peak Runoff

Thanks Rookie2,

I'll check out the NRCS website.  Wyoming has also developed their own rainfall distributions.  I live and work in Oregon and have for 34 years.  Still, we're backward here and have no such developments  to report.  Some of us only heard about rain last week!

Thanks again,

Russ

RE: Time of Concentration / Peak Runoff

(OP)
Sure thing, but I better clarify. I don't think the NM stands for New Mexico (I may be wrong). What it stands for I have no idea, I can't find it in the attached documentation. But it's worth a look.

RE: Time of Concentration / Peak Runoff

Thanks Rookie2,

This thread has become way too long for me.  I've learned a lot.   But, at the end, I can only quote Henry David Thoreau ( which he pronouced like thourough )

" The mass of men lead lives of quiet desperation."

Good luck in your career.  Keep asking questions.  They don't always lead to answers but to ask them is NOT wrong.

later and luck,

Russ

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