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# Economical Section Design for Rect Open Channel Stormdrain

## Economical Section Design for Rect Open Channel Stormdrain

(OP)
For Stormwater Drain Design there are two approaches:

1. Design drain as Non-Uniform Steady Flow (Y1 > Y2)
Step for Design:
- Calculate Design Discharge for Segment of Drain.
- Calculate Capacity of Rect. Drain at Downstream Node.
- Using Manning Formulae Calculate Velocity of Drain(QDES,B, d(Wetted)=D-F.B(0.15M),A,P,R)
- Considering Top of Water Surface throughout the Drain = Top - F.B

CONs:
1. Even though the Method is quick the report Velocity is for Fictious Depth of Flow.
2. Practically the Flow through Drain is Uniform Steady Flow.
3. Whereas in above method we are considered same Free board for all senctions.
4. Where as Free Board increases downstream.

2. Design Drain for Economic Section D = B/2---refer attached excel.
Steps of Design:
- Calculated Ynormal such that LHS(Q*n/S^0.5) = RHS ((B.y)*((b*y)/(b+(2*y))
- Calculate B = 2 * Yn, Round off Base width
- Calculate Manning's Actual V for Adopted B (width) ------ should this be for actual width?
- Calculate Actual Discharge = Actual V * Area.
Cons of Above Actual Method:
1. Slope to achieve Actual velocity of 0.9m/s is very steep
2. Goal seek function used for calculating Y n DOES NOT GIVE convergence.
3. For calculating Actual Velocity I am using Adopted Width multiplied by Yn , WHICH is not correct as per me.

Question:
I have been traditional using Method 1 : Uniform Freeboard
Cleint as asked to present the calculation such that Velocity is calculated for Actual Flow depth./

refer attached snap, if possible please correct me if I am wrong.

https://files.engineering.com/getfile.aspx?folder=...

### RE: Economical Section Design for Rect Open Channel Stormdrain

I am not sure what your spreadsheet is doing, I did not attempt to open that. However, it appears you are using normal depth methods. suggest that if flow is subcritical, than use a standard step backwater method to determine the water surface profile. Yes, the velocity should be based on the actual flow depth, not on a normal depth.

http://onlinemanuals.txdot.gov/txdotmanuals/hyd/ch...

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