Effects of Tc and Tt on SCS hydrographs?

[proletariat]

From what I understand, the Tc is the distance from the beginning of runoff to the inflection point of the hydrograph. So, Tc has an effect on the peak runoff rate, but not the overall runoff volume in the SCS TR55 and TR20 models.

For example, if you have 3" of runoff for a given type II design storm, that runoff volume is constant regardless of the Tc. However the runoff hydrograph time span will lengthen or shorten, and the peak will rise and fall depending on Tc, but the overall area under the hydrograph will remain the same.

On the other side of the coin, the mass rainfall distribution for a type II storm always stays the same (speaking in terms of percent). I guess that would make the Tc the main factor that dictates the runoff RATE, aka the relationship of runoff from a given site to a standard type II rainfall mass distribution. (This is assuming CNs stay the same).

Tt would have no effect on the hydrograph shape and is only used to cause a lag in the runoff hydrograph reaching the study point.

I'm trying to understand SCS better. I can employ it in practice using standard cookie cutter methods, but I want to know the theory behind the numbers. This would help me model the siutation in my earlier thread about Offsite DAs.

 

[proletariat]

By the way, these are "Quatements," questions phrased as statements.

 

[psmart]

The SCS PEAK runoff is determined by:

1) Peak rainfall intensity, as contained in the rainfall distribution (Type II, etc).

2) Time of Concentration: Often defined as the time from a given burst of precipitation excess to the point of inflection of the UNIT hydrograph (as opposed to the total runoff hydrograph.) Travel time may be included in the Tc, or handled as a separate routing operation, depending on the methodology and layout of the watershed.

3) Unit Hydrograph (often described in terms of the peak factor, such as 484)

4) Curve number.

The total runoff VOLUME is strictly a function of the CN. Items 1-3 have no effect on the volume.

 

[proletariat]

"Travel time may be included in the Tc, or handled as a separate routing operation, depending on the methodology and layout of the watershed."

Can I take this to mean that if your watershed is only divided by a property line, it would be conducive to including travel time within the Tc? If the watersheds are divided by actual drainage divides and then conveyed through a stream or pipe, then they would warrant separate Tc and Tt.

Is there a reference or textbook that goes into detail about this? I was a construction engineering major and never took a hydrology or stormwater course. OJT is great, but lacks theoretical background.

 

[psmart]

It looks like you have two threads going on the same basic issue...

I would leave the property line out of the picture (at least at the start) and do a proper job of modeling the watershed. Then look for a way to apportion the runoff between the two sites.

There are lots of texts and training resources, but no shortcuts to understanding hydrology. I think you will find it is one of the most challenging fields of civil engineering. Many of the folks on this forum have been at it for decades, and we're still learning. If you have any doubts, I would bring in an experienced H&H engineer.

 

[proletariat]

I read part of the HydoCAD manual today that deals with unit hydrographs and TR20. It was a good overview.