Time of Concentration / Peak Runoff 1

[Rookie2]

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

 

[Rookie2]

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

 

[RWF7437]

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.

 

[Rookie2]

Thanks again

 

[Rookie2]

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.

 

[RWF7437]

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

 

[Rookie2]

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

 

[RWF7437]

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

Keep asking questions, I do.

Russ

 

[TerryScan]

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.

 

[bltseattle]

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

 

[Rookie2]

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&quot; 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.

 

[johnhan76]

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.

 

[TerryScan]

I may not be visualizing the bigger picture here but:
&quot;into an existing channel which drains across the backyard of a residential property&quot; 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.

 

[Rookie2]

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

 

[TerryScan]

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 &quot;branches&quot; 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.

 

[Rookie2]

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.

 

[johnhan76]

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?

 

[TerryScan]

I was actually refering to retaining the individual site if appropriate. But, offline &quot;spill over&quot; 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.

 

[johnhan76]

Considering &quot;splill over&quot; 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?

 

[TerryScan]

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.

 

[sretlaw]

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