Limit of Shallow concentrated flow

[C Cook]

I have been asked to limit shallow concentrated flow to 400' in a time of concentration calculation. Not sheet flow...… When I asked they said there were studies done. And I asked about the use of time of concentration equations for the area between the 400' and channelized flow. They stated they feel that after 400' it is channelized flow. But to me that requires Wetted perimeter and hydraulic radius.... which is channelized flow.

Here is the requirement from the City of Lawrence Kansas.

A. Overland Flow Time: Determined from Figure 1. The maximum sheet flow distance for calculations shall not be greater than 300 feet for developed areas and 500 feet for undeveloped areas.

B. Shallow Concentrated Flow Time: Determined from Table C or D. The maximum shallow concentrated flow distance for calculations shall not be greater than 400 feet. Shallow concentrated flow includes flow along side and rear lot swales and overland surfaces with no defined channel beginning at the point that the flow length exceeds the applicable maximum overland flow length.


Am I totally missing something?????

 

[Cairo Datum]

Hello! Are you designing a new site or evaluating existing?

a) New site:
Does this mean that after 400', the flow has to be intercepted by a channel or inlet? In this case it falls back on you to design it I suppose, but that would be an unconventional restriction.

b) Evaluating existing:
I suppose they want to protect against large times of concentration (i.e. potentially undersized drainage) if mostly sheet flow is assumed. Maybe I am also totally missing something, but if that is the case it aint that bad for you as it probably just makes your design more conservative, even if it seems arbitrary.

In these shoes I would estimate the cross section of the most likely natural/incidental "channel" along the hydraulic length from a topo survey, city gis, map, etc., or if all else fails going to the site and measuring the most fitting possibility. If estimating such a thing is blatantly unrealistic, you could always negotiate and hopefully they listen.

 

[Twinkie]

To me this appears to be an unsubstantiated attempt to make engineers over design for SWM, nothing else.

Sheet flow becomes shallow concentrated, but you can't have channelized without a channel.

 

[C Cook]

Thank you for your responses. This is exactly how I feel. They are wanting conservative. But in this instance there is no channel it truly is an area of shallow concentrated flow. Only one municipality is doing this, but they provide no guidance.