Some questions...

[c2h2u2]

Dr Quek,

Thanks for answering my questions. I still got some doubts regarding the culvert design.

How if the information obtained from site investigation or through the surveyor is not enough? The following assumptions can be accepted?

(a) Acceptable upstream flood level = Finished road level - Min. height of pavement over water (free board level)

(b) Estimated downstream tail water = Acceptable upstream flood level - 1

Example 27.B.2 Box culvert Step 2 stated the ways to determine the suitable size of the culvert. How about the Manning's Equation? The value of velocity is fixed to be equal or less than 3m/s (to reduce erosion) for Example 27.B.2. but the value of velocity obtained from Manning's Equation is different. Therefore, Manning's Equation is not suitable in determining the culvert size?

Thanks.

Best regards,
c2h2u2

[drquek]

How if the information obtained from site investigation or through the surveyor is not enough? The following assumptions can be accepted?
Nothing can substitute site investigation. What you proposed below can only be used as rough assumptions for preliminary design.
(a) Acceptable upstream flood level = Finished road level - Min. height of pavement over water (free board level)
Keeping the flood level below the finished road level is no guarantee that flooding will not affect other properties.
(b) Estimated downstream tail water = Acceptable upstream flood level - 1
Not necessarily so.
Example 27.B.2 Box culvert Step 2 stated the ways to determine the suitable size of the culvert. How about the Manning's Equation? The value of velocity is fixed to be equal or less than 3m/s (to reduce erosion) for Example 27.B.2. but the value of velocity obtained from Manning's Equation is different. Therefore, Manning's Equation is not suitable in determining the culvert size?
In Step 2, it is assumed that the flow velocity is 2 m/s, and when the flow is divided by the velocity it gives an estimate of the flow area and hence the height and width of the box culvert. This is required to start the design calculation. There is no need to use Manning for this purpose.