RE: [PipingDesign] wheres the math/ Drainage and Sewers

From: <SARE>
Date: Thu Nov 08 2001 - 00:31:00 EST

> --- kloskie@hotmail.com wrote:
> a 1000 ft lenght of smooth wall corragated pipe.<BR>
> the manufacturer states a&nbsp; n value of the pipe
> <BR>
> of 0.010-0.012 manning and the pipe is 15&quot; ID<BR>
> <BR>
>

the value you stated for the roughness coef. of 0.012 ( or manning's n ) is for cement lined pipe. For corrogated pipe, the mannings n number is about 0.024.

> than that of <BR>
> metal corrugated and that the reason such a small pipe
> is being used <BR>
> combined with the depth of instalation is to reduce
> cost and keep the <BR>
> velocity up to keep the drain self-cleaning.<BR>
>

         i would agree with this reasoning. self cleansing can be achieved by going for the max. flow velocity (not exceeding 7fps). but you stated you're in a flat areas (unpaved?), therefore considereation must be made to clean out the settled solid in during dry season.

> whereas I would think that a deeper depth of a
> 20&quot; pipe should be <BR>
> used.<BR>
>

for underground or buried pipe, i found that its a good rule to maintained no less than 450 mm (which is a little short of 20 inch.measured from top of pipe) factoring the backfill and the sand cushion.

> what can I do?<BR>
> I need to find the max possible flow through this
> pipe...<BR>
> I am not sure of the pipes ability to carry the storm
> waters.<BR>
> <BR>
> if you can supply a formula with data I would greatly
> appreciate it.<BR>
> <BR>

          i havent been involve with designning drainage and sewers for buildings and houses other than industrial storm sewers and drainage system that is pitched (with slope). Here's the equation.

	 Continuity eq. is flowrate Q = AV
	 A is cross sectional area and V is the flow velocity

	 Basic Chezy-Manning eq : V is also equal to CxSquare root ( RxS)
	 where R= hydraulic radius and equal to Pipe Dia/4;  S is the bottom
slope
	 Relationship between C and R is that C= (R raised to the power of
1/6) divided by n (this is where the roughness coef. comes in)
	 Combining,  Rewriting and solving for Q
	 Q = (A/n)x (R raised to the power of 2/3) x square root of S.

	 May i suggest that you determine the vel, calc the A then solve for
Q. May i further suggest that you allow for silt accumulation anf flow surges ( meaning run at 70% depth of low).

         Hope this helps and good luck.As ive said before, method is for industrial application. Received on Thu Nov 08 00:31:00 2001

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