By Italo Gonzalez 

 This blog describes the three categories of drop structures used to convey flows to an underground storage facility or tunnel system.  In general, the intake structures of underground tunnel systems are divided in the following three categories: 

• Plunge drop structures
• Vortex drop structure
• Baffle drop structure   

The geometrical configuration, location and the type of drop structures to be implemented on a project have a unique set of criteria that has to be evaluated in order to provide the optimal drop structure solution to the project.  These criteria will be described in the next blog. 

Plunge drop structures 

A plunge drop structure coveys flow to the tunnel or storage facility by allowing the flow to enter a drop shaft in a radial direction.  Thereafter, the flow drops in free fall vertically in the shaft.  The energy generated by the flow free fall is dissipated throughout a plunge pool or an impact surface (made of high strength concrete) located at the base of the vertical shaft. 

The following figure shows a plunge type drop structure used on a project that connected a combined sewer overflow (CSO) to a tunnel in the area of Cleveland, Ohio. 

Plunge drop structure

Vortex drop structures 

A vortex type drop structure directs the flow to enter the shaft structure in a tangential direction.  Thus, the flow travels vertically attached to the drop shaft walls in a spiral type faction.  In general, a vortex drop structure consists of the following structural components: 

• Approach Channel
• Tangential Inlet
• Drop Shaft
• Deaeration Chamber
• Vent Pipe
• Adit 

The geometry of the tangential inlet makes possible the flow traveling downwards attached to shaft wall in a spiral motion.  The flow potential energy is dissipated by the friction between the flow and the wall of the drop shaft.  The approach channel projects the flow in a tangential direction to the drop shaft.  The deaeration chamber together with the vent pipe and the adit diminishes the amount of air entering to the tunnel. 

The following figure shows the design of a vortex structure

Vortex Drop Structure

Baffle drop structures 

A baffle-flow drop structures dissipates the potential energy of the flow by the use of baffle or slabs placed down the length of a drop shaft.  So, flow cascades downwards until it finds the end of the shaft.

 Following figure show the layout of a baffle drop structure. 

Baffle drop structure

Physical model of a baffle drop structure

Structures

This blog intends to provide insights into the design, technical advances and news of the use of deep underground hydraulic structures.

Deep underground hydraulic structures, transport, storage and treat flows.  In the most of the cases, hydraulic underground structures are used to mitigate contamination of lakes and rivers by preventing untreated combined sewer overflows (CSO) from entering the environment.  Structures, such as: storage tunnels, drop structures, gate structures, regulators, sewer lines and others, are used to control the contamination of the environment by restraining the discharges of combined sanitary and storm flows into the rivers and lakes. 

Physical model testing of a Baffle drop Structure - Italo Gonzalez