Maximum allowable velocity in sewer for stormwater drainage

When we design sewer for storm water drainage, there will be a design standard as for the allowable maximum velocity in sewer in each country. For example, the one is 3.0 m/sec in South Korea.  However, when installing ​sewers for ​storm drainage ​in steep slope ​terrain, it ​often causes ​the increase of ​excavation ​volume due to ​gentle slope of ​sewer to meet 3 ​m/sec (​allowable ​maximum ​velocity, ​design standard)​. And temporary ​soil retaining ​wall will be ​used according ​to the site ​situation.  Someone ​sent me "Urban ​Drainage ​2nd  ​Edition" and ​there is a ​statement for ​the following and Hong Kong design standard allows the maximum velocity up to 4.5 to 6.0m/sec if using abrasion resistant pipe or interanal lining is chosen.  Historically, ​many sewerage ​systems were ​designed so ​that velocity ​would ​not exceed a ​specified ​maximum. This ​was no doubt a ​sensible ​criterion for ​early brick ​sewers with ​relatively weak ​lime-mortar ​joints. However,​ research ​has shown ​that abrasion ​is not normally ​a problem with ​modern pipe ​mater- ​ials. Perkins ​(1977) has ​suggested that ​no fixed maximum ​limit is ​required, ​but where ​velocities are ​high (>3 m/s) ​careful ​attention needs ​to be given to: ​  ​So I'd ​like to gather ​some information ​about the ​exceptional ​cases in other ​countries. It ​will be very ​appreciated you ​share your ​experiences or ​let me know ​exceptional ​design ​standards in ​your countries. ​  Thank you.  Best regards,  Yongseok CHOI