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Why does the maximum discharge for major does not change after changing diameter & number of orifice

Some may encounter this issue when the Post-development Major Design Storm Outlet Maximum Discharge will always remain the same despite proposing different sizes and numbers of orifice.

You may be wondering if there is a bug somewhere. Therefore, before we dive deeper, let’s do some quick revision on the basis formula use to calculate the maximum discharge (orifice flow rate);

Reference: MSMA 2nd Edition

Image 1: Equation 2.6 use to calculate orifice flow
Image 2: Figure 2.8; Show on how to measure Ho

As shown in the above screenshot, the H is calculated with reference to the center point of the orifice. So if the maximum water level is not over the half of the proposed size orifice yet,  it will cause the ‘Post-development Major Design Storm Outlet Maximum Discharge’ to remain the same with no changes.

Example using project file #

Project file for reference here

*note that the original project file use 200mm diameter and 1 number of orifice for major outlet design, hence you may need to tune in the diameter and orifice number as per example below

Example 1: D= 200mm  #

Note: H = 0.75m

Image 3: water level reaches the half of the orifice
Image 4: Discharge at stage 3.85m is 0.4202m3/s
Hence, the Post-development Major Design Storm Outlet Maximum Discharge = 0.419m3/s

Example 2: D=400mm #

Note: H = 0.75m

Image 5: when proposed a 400mm diameter for the orifice, the water level does not reach the half of the orifice
Image 6: hence the Post-development Major Design Storm Outlet Maximum Discharge remains at 0.419m3/s

Note that when the water level exceeds/reaches half of the orifice, then it is expected that the ‘discharge’ at stage 3.85m will be different since the Ho is now changed. Which then will definitely affect the Post-development Major Design Storm Outlet Maximum Discharge.

Example 3: D=150mm  #

Note: H = 0.75m

Image 7: water level exceeds/reached the half of the orifice when proposed 150mm diameter
Image 8: Discharge at stage 3.85m is now 0.436m3/s (previously was 0.4202m3/s)
Hence, the Post-development Major Design Storm Outlet Maximum Discharge now equals to 0.431m3/s

Conclusion #

The fact that the maximum water discharge doesn’t change with changing orifice number and diameter is not necessarily a bug, check the maximum water level first.

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