How to assume bank full elevation to find the travel time in channel?

[BillsSis]

I'm fairly new to this much in-depth study of hydrology and hydraulics; so forgive me if I sound ignorant. I used to use mainly the Rational Method; now I'm using TR-55 and HEC-HMS. Most of the drainage areas I come across are less than one square mile or 640 acres.
My question is this: when using TR-55 and trying to figure the channel travel time, how do I go about assuming what the bank full elevation is of the channel? Once i figure what that is, I can figure out what the velocity is, then the travel time.

Thanks everyone!

 

[beej67]

Bankfull means filled up to the top of the bank.

Perhaps I misunderstand your question?

What you're going to discover for watersheds your size, is that your Tc is going to be heavily influenced by your sheet flow assumptions, even though the sheet flow is a relatively small portion of the flow path. That means you really have to use good engineering judgment on selecting a flow path that you feel is representative of the site. I also recommend vetting the Tc you calculate against several different methods that aren't so sheet flow dependent.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[BillsSis]

Hi Beej67,
I'm sorry; what I meant by 'how do I go about assuming what the bank full elevation is of the channel' is how do I figure out what that elevation is, in feet? There's no survey, so I can't get it that way. With me being a 'newbie' at this, I'm sure I'm missing something to help me figure out what the elevation is. Then, I guess with that, I can deduce that is the depth of the water? Also, how to figure out what the cross sectional area is or wetted perimeter? I would have to have at least one of these items and depth to figure out the rest of the items for deducing the travel time in the channel.

The site I'm currently figuring the travel time in the channel is a tributary feeding into a reach, which then has another tributary feeding into it further downstream. This reach takes the water to the cross drain structure and under the roadway. The tributary area has a sheet flow area, which then turns into shallow flow. After shallow flow, then the channel starts as there is a 'blue line' to denote this channel in this drainage area. The attached pdf file shows the arrow which points to the beginning of the channel in the drainage area.

 

[psmart]

You can also use an iterative process: Take a guess at the flow depth so you can determine the approximate Tc and get an initial peak runoff. Use this peak to determine the actual flow depth and adjust the Tc accordingly.

As beej67 stated, your channel flow time is probably a small fraction of the overall Tc. You'll also find that the channel velocity doesn't change much with modest changes in depth. So in the end, knowing the exact flow depth has a minimal effect on the results. If in doubt, do a quick sensitivity analysis.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

well, if you dont have a surveyed channel slope or cross section, you have options. a) make an assumption based on the map b) field trip and take along a tape measure and a level c) send out the survey crew to get cross sections. If there is water in the channel than you will have to estimate anyway. dont forget the camera. n value might be just as important as the slope or cross section. realize that most natural channels are highly variable, so you will need to simplify and use conservative, engineering judgement. channels may or may not handle the design flood, so they might or might not be bank full.

 

[TerryScan]

Obviously, it is best to have the most accurate data possible as mentioned (survey, field check etc.) However, I thought I would offer some other lower accuracy aides as well: USGS, or other topo data. Some LIDAR data is pretty accurate, however it often does not capture the definition of smaller channels.

In your case, if I could not acquire the preferred data, I fell back to Google Maps. There is a Street View of the channel where it reaches the highway. The estimate I could make on channel dimensions referencing other visible features (fence posts) would certainly be better than nothing. And as CVG mentions, one can use the street view photo to get an idea of what mannings n would be applicable.