A great view of how a H flume installation can be built-up to ensure free-spilling discharge.  Free discharge is a requirement for the H series of flumes due to their low resistance to the effects of downstream submergence.

Remember that this elevation does make the installation unpassable for aquatic life.  If this is a concern, a Cutthroat, Parshall, or Trapezoidal flume should be used instead.