Ditch Hydraulic Grade Line 1

[shorty]

I have an open ditch with many culverts that dumps into a small creek. How do you calculate the hydraulic grade line of an open ditch system? I was told by someone here in the office to take the water surface elevation at the creek and carry it upstream at the same slope as the ditch, making some small adjustments for headloss through the culverts. Is this correct? I know that the HGL in an open system is just the water surface elevation, but nobody seems to be able to tell me exactly how to calculate the water surface elevation in a ditch. Doesn't the ditch cross section play a part? What happens if I decrease the amount of water coming to the ditch? You would think that the HGL would be lowered.

Can someone explain how to calculate the HGL in an open ditch system?

 

[waterguy]

I thought I'd just add some thougths regarding your problem.

1) Aside from considering headloss at the culverts, you also need to consider headloss due to friction, particularly if the ditch is long.

2) Your culverts could be a "bottle neck" in your system creating a backwater situation upstream of the culverts, particularly if they are undersized, or do not have the same capacity as your ditch section (which is probably the case).

3)If the determination of HGL is critical, you might consider modeling it it HEC-RAS. This will allow you to properly account for the cross-section properties and characteristics, as well as the affect of the culverts.

4) If I recall, the assumption that the HGL is the same slope as the channel would only be valid if the flow and magnitude and flow depth are constant.

6) Get a hold of a book on open channel flow, it will probably be of better help than I was. Find a copy of the HEC-RAS manual (I think you can get a free download in PDF format from the Army Corp. of Engineers), as it should explain some of head loss considerations, as well as discuss the "standard-step method" that can be used for manual calculations.

John

 

[cvg]

Waterguy is correct, and let me just add that explaining how to calculate the hgl of a stream simply can't be done in this forum. It is much too complicated. I would suggest some basic study of river and open channel hydraulics before continuing.

Some good references are:

Open Channel Hydraulics, Chow, 1959

Engineering Hydraulics, Rouse, 1959

Handbook of Hydraulics, Brater & King, 1963

Open Channel Hydraulics, French, 1985

The Army Corps also has some excellent reference material online (free) look for the following:

EM 1110-2-1416 River Hydraulics

EM 1110-2-1601 Hydraulic Design of Flood Control Channels

The FHWA has a program to analyze flow through culverts called "HY8" which is free online
Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[muzair]

It is true that Hydraulic Grade Line in Open Channel Flow is the Water Surface Line. The reason being that Hydraulic grade line is the line joining the pressure heads at different points. Since in pipe flow, the flow is under pressure, that is why the watersurface lines and the HGL are different, but in open channel flow, the pressure is atmospheric pressure. If you take the bed of the channel as datum and do a simple calculation

If p is the density
g acceleration due to gravity
h Depth of water at any section

Then pressure exerted by water at bed = pg h
And Pressure head of Water = pg h/(pg) = h

Since h is the depth of water, hence the point on the HGL will coincide with the point on the Water Surface line.

As far as calculation of backwater effect is concerned, here is what you have to do,

1. Determine the Normal flow depth of the channel by using the Manning's Formula.
2. Now determine the specific energy of the channel corresponding to the discharge and channel cross-section.
3. Using these parameters determine the critical depth and hence determine if the flow is subcritical or Supercritical. Also determine the critical width at which flow will be critical.
4. If flow is subcritical (which would normally be the case) and width of culvert is more than that of the channel section, then water level will rise. If it is supercritical then the waterlevel will fall. This can be best understood with the help of discharge diagram (specific energy remaining constant).

If you can go this far, then it is very easy to determine the height of water surface. Keeping E as constant, determine the alternate depths corresponding to this E. Then we can plot the water-surface profile by using the subreach method.

I hope all of your querries have been answered.

 

[sta7240]

First, my advice is to study up on open channel hydraulics. Chow's "Open Channel Hydraulics" is a good source. Second, if you want to do this correctly, I suggest using the HECRAS computer program to model the ditch and the culverts. There can be a lot of headloss at certain culverts which will have a major effect on the HGL. To run HECRAS you will need to get surveyed cross sections throughout the channnel reach. Don't forget to extend the cross section well beyond the ditch banks to capture out of bank flow. Also, you will need detailed survey data at each culvert. One other important item is your starting water surface elevation at the downstream end. Depending on the storm frequency, you will need to know what the starting elevation is at the stream (if the ditch outlets into a stream) because the backwater effect of the stream can influence the ditch.