Storm Design: You are…

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Storm Design:

You are proposing a 1% minimum slope of all sized top end pipe lengths (minimum 250mm diameter).
This is quite a steep slope, even for 250mm pipe. We typically design top end 200mm sanitary pipe at 0.50%.

I expect a 1% minimum slope for storm top end pipe will potentially create velocity issues, especially if that top end pipe is on the larger size. This will effectively cause the downstream pipes to possibly be sized larger with shallower slope to decrease overall velocity and increase storage capacity.

This also may create pipe cover issues. On the flip side, the entire storm system may need to be deeper in order to maintain proper cover due to this 1% slope minimum, which causes more digging (lengthier construction time, higher carbon impact).

I think a graduated top-end storm slope system should be in place taking into account pipe sizing, for example:
250mm storm to be 0.50%
300mm storm to be 0.??%
375mm storm to be 0.??%
....and so on.

This 1% minimum slope should maybe be reviewed in further detail.