Maximum allowable velocity in sewer for stormwater drainage

Question

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

Ajit Seshadri Seshadri · Answer

Agreeing with Arturo Constante -- For developing countries viz India, even Pressurised system needs to have extra safety margin, as most times, 
Systems are functioning with no power, hence be safe on flows.
At best it would be 0.5 m per sec, with 5% slope of flow pipe.. or have sunken trenches with extra HRT in pipes and system..
Well wishes.
Prof Ajit Seshadri. India .

Yongseok Choi · Answer

@ YIANNIS Tzeranis , @ Alejandro Gomez 
Could you send the design standard you mentioned to my e-mail?
My e-mail address is yscatfish96@gmail.com or yscatfish@hanmail.net  
Many thanks to you in advance.

T Walski · Answer

If the velocity becomes unacceptably high, you can install drop structure to dissipate the energy.  Here is a link that discusses how to model it. https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/13308/what-is-a-drop-manhole-and-how-do-i-model-one

Alejandro Gomez · Answer

Hi, in Colombia, South América the National Code recommends a maximum velocity of 5 m/s unless you are using plastic pipes then you can use 10 m/s. Anycase, structures of the system must be protected for velocities over 4 m/s.

Arturo Constante · Answer

I ever considered 3 m/s as pressure system; you can refer to pipe engineering or calculate directly from pipe specs sheet about permitted ft/s for the whole area
for non pressure sewage system is considered since 0.5 m/s with 3% to 5% length slop; the effective sewage area segment is the standard minimum chord; hope it helps

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