Problem

What causes transient waves to dampen? According to the Help and the Advanced Water Distribution Modeling and Management book, transient waves will persist indefinitely in the absence of friction.

For example you may notice in a model with frictionless pipes that the waves from a transient event still dampen over time (reduce in intensity, toward a new steady state).

Solution

In general, waves generated by a transient event are dissipated by friction from the movement of water.

In a frictionless pipe network the pressure waves will persist indefinitely as demonstrated in Example1.wtg.  However one may ask “What is meant by a ‘frictionless’ hydraulic system?”  A key element is that all constituent pipes must be perfectly smooth (i.e., DW f is exactly zero).  Furthermore, there must be no entrance/exit losses from the network.  In the case of other models such as Lesson2.wtg, there may be nodes at which the water demands exist at positive pressures.  In other words, the net inflows to such consumption nodes involve pressure losses as the hydraulic grade line drops to pipe elevation.  In effect, each demand point is satisfied by dissipating the pressure by means of a partially open valve or equivalent. 
 
At lossless nodes such as dead ends, reservoirs, and junctions (with no demand), waves will be reflected and/or transmitted, albeit there will be no loss of energy.  The waves will be redistributed and can interact with other waves, but from a macro perspective there will be no dampening since no energy is lost.

Note that the above still holds true when considering cavitation (vapor pocket formation and collapse) - this phenomenon does not introduce energy loss. So, in a frictionless system in HAMMER, cavitation would not result in dampening over time.

See Also

Transient pressure wave not dampening or unexpected lack of headloss

How can I model a frictionless pipe?