Hydraulic calculation using the Rational method

[lKobRal]

When performing a hydraulic calculation using a rational method, the following question arises: How much can Q[sub]full[/sub] and Q[sub]des[/sub] differ?
For correct hydraulic calculation, it is necessary that the V[sub]full[/sub]≥V[sub]min[/sub]. if this condition is not met, how to adjust the slope. Is there any table of normative slopes, or I substitute any numbers?
Does V[sub]full[/sub] have to constantly increase?

 

[Ekenny_G]

if I get your question, you are asking how close should your design be to the full capacity of the channel?
So if your Qfull is 100cfs, how close should your design be?
Well, that depends on your local or design requirement. I usually keep my design 80% of Qfull.
For velocities, that depends on the type of channel it is (grass channel, stone, concrete channel...etc). Each type of channel have maximum allowable velocity. For grass channel, its around 3.5 feet per second (fps), for stone/rock channel it can be be upwards of 15-fps.
IF the channel cross section is set, you can adjust the slope of the channel to get the velocities you want. There is no "normative slope" per say, but there is "normative velocity" . Whatever slope gives you the velocity (or sheer stress) you need for the type of channel you are designing, is what you go with.
To find what these "normative velocity" or what i called "allowable velocity" just google it.

Here is another link:

https://www.roads.maryland.gov/OHD2/Highway_Drainage_Manual-part-1.pdf

see table on page 86.

I hope this answered some of your questions and didn't confuse you more.

 

[gbam]

To build on what was already mentioned above, key items for design of an open channel are typically based on the governing agency's design criteria. This can include:
- Freeboard - Dictates the % full. Freeboard varies with velocity, flow regime etc...
- Velocity / Stable Channel Design
- Channel Material Types, what is acceptable versus not. If you are designing an earthen or lined channel etc... Also, will the end of the channel require energy dissipation?
The design element is driven by design criteria and policy. Make sure that you are familiar with these and meet the minimum criteria and your design should be approved.