Dry and wet river intakes; unstable bank

Wet river intake

A wet intake is a tower (or well) built in the river, with its interior filled with water up to river level. Ports at different levels admit water, and the intake pipe leaves from the bottom of the well. The valves are operated from the top, since they are underwater.

  Foot bridge =====#=====
        Stem (control rod) |
  HFL ~~~~~~~~~~~~~~~~~~~~ |
  NWL ~~~~~~[port + screen]|  <- water inside
  LWL ~~~~~~[port + screen]|     well = river level
  bed ///////[port]--------|
              |   intake pipe
              +---------------> to pump well / plant

Operation and maintenance: The tower is filled with water and the pipe is always flooded. Gates at each port are operated from the bridge by a stem. Screens must be raked and cleaned from above, and divers or stop-logs are used when the tower has to be examined. It is cheaper than a dry intake, but harder to maintain.

Dry river intake

A dry intake is a tower built on the river bank (or just inside it) whose interior is kept dry. Water enters through ports in the wall, flows into a separate sump by a connecting pipe, and only the valves and pipes are in the dry chamber. The operator can work inside it at any time.

 Foot bridge ===+
                | operating floor
 HFL ~~~~~~~~~ ||   gate valves
              ||[=]---+
 NWL ~~~~~~~~ ||[=]---+--> pipe to
              ||[=]---+    sump/pump well
 LWL ~~~~~~~ ||  dry chamber
   ///////// ||____________ screens at ports
   river bed  |   (water-tight walls)

Operation

• Water flows from the river through the screened ports, then through the valves in the dry chamber, to the sump or directly to the pump.
• The operator selects the port at the best level and closes the others.
• Valves and screens can be inspected, cleaned or repaired from inside, in dry conditions.

Merits and demerits: Operation and maintenance are easy, but it needs stronger water-tight walls and a heavier, deeper foundation (to resist floatation when it is empty), so it costs more than a wet intake.

Modification if the river bank is unstable

• Move the intake into the river away from the bank, so it stands as an isolated tower (a wet or dry tower) connected to the bank by a foot bridge, with the intake pipe laid in a protected trench under the bed.
• Provide bank protection: stone pitching or gabion, groynes, spurs or retaining walls upstream and downstream of the intake.
• Use deep foundations (well, pile or caisson) for the tower so that scour cannot undermine it.
• Fix the river course near the intake with river-training works (guide banks, training walls, spurs), or choose a straight reach where the bank is firmer; avoid the eroding face of an outer bend unless it is protected.
• Use a canal or channel intake (a cut leading water to the intake) if the river migrates, and build the structure well away from the moving bank.