Frequently Asked Questions

Q. What is a drop shaft?

A. Drop shafts are used in water conveyance tunnels to transfer flows from a higher elevation to a lower elevation.  Such drop shafts are typically used for flood control, CSO systems and deep tunnel systems.

Q. Why are drop shafts needed?

A. Drop shafts are needed to dissipate the increase in energy of falling water.  A drop shaft is also needed to remove any air that mixes with or is entrained in the water as it descends to eliminate cavitation and hydraulic instability that falling water would cause and to minimize hydraulic head losses when drainage tunnels are surcharged with storm flows.

Q. What kinds of drop shafts exist?

A. Present drop technologies include conventional technologies such as weirs, cascades, flumes, backdrops and vortex drops.  Innovative technologies include advanced vortex drops. 

Q. Aren’t drop shafts unsafe and dangerous?

A. It is true that conventional drops can cause flowing water to have a high velocity and turbulence.  Improperly designed drop shafts can cause hydraulic shock and glugging (transient and surging flows causing rapid pressure releases of air and water in the opposite direction of the flow, causing dynamic instability and possible tunnel collapse). Additionally, improperly designed drop shafts may accelerate erosion of the pipes if entrained air is not properly removed.  Entrained air can cause cavitation, the phenomenon where micro-bubbles form and explosively collapse on the surface of a pipe due to rapid changes in water elevation.

Q. How does a properly designed drop shaft prevent hydraulic shock and cavitation?

A. In order to prevent the formation of damaging high-pressure air buildups, a properly designed drop shaft will ensure a smooth transition between air-entrained and full-pipe modes of operation. 

Q. What are the limitations of conventional drop technologies?

A. Weirs, cascades, flumes and backdrops are simple to design.  However, weirs and backdrops have practical height limits sometimes as low as 8 ft.  Cascades are expensive and cause water to gain energy on each “step” down, risking the loss of control and overshooting.  Flumes are simple but risky because it is difficult to match energy dissipation with energy gain.

Q. Are vortex drops better than conventional drop technologies?

A. Vortex drops can offer a more acceptable drop solution because they can be used for very tall drops.  However, conventional vortex drop technologies require very large diameter pipe to accommodate the large air core that forms within the vortex.  The systems also require elaborate entry structures and a large stilling basin to dissipate energy at the bottom. 

Q. Does the Hydro Vortex Drop Shaft require very large stilling basins and pipes?

A. No.  The Hydro Vortex Drop Shaft has innovative components that allow for a smaller, simpler inlet structure and utilizes a compact energy dissipation unit at the base.  The Hydro Vortex Drop Shaft also offers a more acceptable drop solution because it enables the use of small diameter pipes as they can run full.  The small pipes can be fixed to the tunnel wall allowing for safe man access to the base of the drop.  This eliminates the need for auxiliary shafts for maintenance access.