Scour is only an issue where the creek undergoes an unnatural contraction, such as at a bridge crossing or culvert. In order to use the HEC-18 contraction scour equation, you need both a natural width and a contracted width.

If you are not directly upstream or downstream of a manmade contraction, scour is probably not an issue. I would recommend looking through HEC-18 to gain a more complete understanding of this phenomenon. It available for free from the FHWA bridge office at http://isddc.dot.gov/OLPFiles/FHWA/010590.pdf

Good Luck,

Robert Weber, EI
Maurer-Stutz, Inc
www.maurerstutzinc.com
"I am satisfied that your answer is correct. I am not, however, satisfied that your answer is right."

you may need to consider scour and for several reasons

1.  Long Term Degradation - is the stream bed profile agrading or degrading - that is, is the stream bed in equilibrium, or not?  Is there headcutting occuring downstream of the sewer crossing which may move upstream?

2.  Local Scour - as described above by raweber, this could be caused by a flow contraction such as that caused by a bridge, culvert, etc.

3.  Bend scour - caused where the channel has significant curves which cause transverse currents and waves

4.  Low flow incisement - Is the depth and location of the low flow channel fixed, or will it vary over time?

5.  Is the channel stable under 100 year flow rates?  Under very high flow rates, will flow become supercritical?  Will sediment transport rates increase greatly to cause degradation  of the channel?

As a defense against a sewage spill, I would also recommend that the sewerline be constructed either within a steel casing, or use iron pipe with mechanical joints.  You may also consider encasement in concrete.