APWA has a detail for pipe anchors.  high velocity could be controlled using velocity control rings, also detailed by ACPA.  Or you could control the velocity by laying the drain flatter and using drop manholes.  This is a very small pipe and high velocity shouldn't be a large problem if it occurs only frequently - and if the run of pipe isn't too long.  If there is abrasive sediment (sands or gravels) in the stormwater, then you should expect erosion of the invert of the pipe.  Remove the sediment before it enters the storm drain system.

Don't even think about laying pipe with a42% grade. It will wear out in 1-2 years. Use the drop manholes as 'cvg' suggests. Add granite blocks to the bottom of the drop as a sacrificial element.

Depending on the fluid, scour will be a large concern.  Is the flow steady or intermittent?  Dissipation of the fluid energy will also help to reduce the longterm maintenance costs, but again, that will be dependant on the fluid characteristics.

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The pipe is carrying storm water and is only an inlet connection.  The local design manual gives a maximum velocity of 14 fps but only for pipes with dry weather flow.  I agree that a drop manhole is preferable.  What is a reasonable maximum velocity for RCP?  Maximum slope?

The reference that I have (DEBO) says 20 fps.