Suspended Solids 2

[dik]

Does the added weight of silt carried in a river increase the density of the water in a significant fashion?

I'm looking at a coffer dam design and the engineer from another office has used 62.43 for the density. I usually use 62.4 (62.42 is the greatest density for 4 degC), and I was wondering if this extra .01 pcf would account for silt in the flow. The amount of silt load is fairly heavy (Red River (North)).

The precision isn't an issue, I just want to get a bit of a handle on the increased density for better understanding.

Thanks, Dik

 

[beej67]

Most sediment transport equations I've seen neglect the density changes of water due to suspended solids. Unless it's honest to god mud, I doubt it would be significant enough to change the density of the fluid.

I'd bet you're just looking at a typeo, unless the water's brackish, which does change the density.

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[dik]

Thanks, beej... it's not a typo, because it's repeated. I don't think it's significant, but just want to confirm this. I can get a container fill it and weigh it... and compare this with tapwater... but, I didn't want to do this...

Dik

 

[cvg]

I have a chart that shows a maximum of 62.43 at 4C. Reference CRC handbook or the following also:

http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

assuming a dry unit weight of 100 pcf for the silt, you would need just 0.08% silt per cubic foot of water to increase the weight to 62.43. However, I have not seen the density of water ever changed from 62.4 for a hydraulic or structural analysis. I would be more inclined to assume some deposition of silt at the base of the cofferdam rather than assuming any change to the density of water.

 

[dik]

Thanks... my reference had 62.42... happy for the correction and explains his use of the number.

Dik

 

[beej67]

The more important thing in my mind, is if your coffer dam holds up under 62.42 water and fails under 62.43 water, you need to redesign your coffer dam.

/grin

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[hokie66]

62.4 has always been good enough for me.

 

[dik]

beej:
I agree; I try not to design anything that tight...

Hokie:
same here...

thanks, gentlemen

Dik