24 m diameter) the rotation of which raise or lower the gates, which in the
open position are level with the river bed and in the flood control mode
are vertical (Lewin, 2001). The barrage has four 61 m wide rising sector
gates (protecting the navigation passages), a further two 31.5 m wide rising
sector gates and four 31.5 m falling radial gates.
The protection of Veniceagainst flooding is a great engineering chal-
lenge. To avoid piers in the navigation channels connecting the Venice
Lagoon and the Adriatic it is proposed to use in the three channels four
barriers each 400 m wide, with bottom hinged buoyant 20 m wide, 18–28 m
long and 3.6–5 m deep flap gates recessed in closed position in reinforced
concrete caissons with a hydraulic sediment ejector system (i.e. altogether
79 gates). In their operating position the gates are at an angle approxi-
mately 45°; they are not linked and move independently under wave
action with leakage between the gates (and the hinges). An experimental
full-scale module was built and operated 1988–92 (Lewin and Scotti, 1990,
Bandarin, 1994); in 2003 agreement was reached to proceed with the
project. For further details see Lewin and Eprim (2004).
6.5 Hydrodynamic forces acting on gates
The main force acting on gates is usually due to hydrodynamic pressures
caused by nonuniform turbulent flow with subsidiary forces caused by
waves, ice, impact of floating bodies etc. The hydrodynamic forces are
usually considered in two parts: time averaged mean component (or
steady flow part) and fluctuating components induced by various excita-
tion mechanisms (see Section 6.6).
6.5.1 Forces on low–head gatesIn their closed position gates and valves are subjected to hydrostatic
forces determined by standard procedures for forces acting on plane
and curved surfaces. For radial or sector gates the vector of the resultant
force must pass through the gate pivot and thus its moment will be zero;
equally it can be converted into one tending to open or close the gate
by placing the pivot above or below the centre of curvature of the skin
plate.
The computation of hydrodynamic forces acting on partially opened
gates is far more complicated as it is closely related to flow conditions.
Although it is tempting to express these using various coefficients of dis-
charge and contraction, this approach is acceptable only in the simplest of
cases (see Worked example 6.1), but hardly sufficient in the final design,