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
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
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
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
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
RE: Time of Concentration / Peak Runoff
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
PS - still looking for answers on the tc / runoff relationship
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
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
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
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
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
Thanks
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
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
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
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
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
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
RE: Time of Concentration / Peak Runoff
Keep asking questions, I do.
Russ
RE: Time of Concentration / Peak Runoff
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
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
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
"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
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
RE: Time of Concentration / Peak Runoff
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
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
RE: Time of Concentration / Peak Runoff
Try this website:
http://www.cee.ucf.edu
RE: Time of Concentration / Peak Runoff
Thought it might interest you.
RE: Time of Concentration / Peak Runoff
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
RE: Time of Concentration / Peak Runoff
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