Environmental Engineering Reference
In-Depth Information
Table 13.1.
Freeboard requirements for preliminary studies (USBR, 1977).
Largest fetch (km)
Normal freeboard (m)
Minimum freeboard (m)
Less than 1.6
1.2
0.9
1.6
1.5
1.2
4
1.8
1.5
8
2.4
1.8
16
3.0
2.1
(3)
Intermediate water surface freeboard.
The intermediate freeboard requirement should
be determined so that it has a remote probability (e.g. a 10
4
/annum probability) of
being exceeded by any combination of wind generated waves, wind set up and reser-
voir surfaces occurring simultaneously. (Note that USBR, 1992, do not mention wind
setup but logically it should apply.)
The adopted freeboard should be the largest of these three.
For existing embankment dams USBR (1992) require that, if the maximum water sur-
face elevation is so close to the dam crest that wind generated waves and setup would
wash over, or if the maximum water surface elevation is higher than the existing crest, the
potential of this overtopping to cause failure of the embankment should be assessed.
They point out that freeboard requirements for a new dam may be different from that
which may be accepted for an existing dam, because the cost of adding freeboard to a new
dam is small, but to add to an existing dam may be very costly. They advocate the use of
risk analysis to assess this. The authors would also advocate this analysis for existing dams.
ANCOLD (2000) requirements are generally consistent with these and suggest that the
freeboard at maximum water surface elevation should be below the top of the dam core.
(In reality this means the dam core should be taken to the dam crest level.)
When considering water levels in the reservoir, consideration should be given to spill-
way gate reliability, since the water level may be very different if the gates fail to operate.
In some cases there may be a need to consider waves which could be generated by landslides
failing into the reservoir and waves or seiches caused by earthquake faults in the reservoir.
It should be noted that camber is not part of the freeboard, since it is provided to allow
for the long term settlement of the dam.
13.1.2
Estimation of wave runup freeboard for feasibility and preliminary design
For feasibility and preliminary design studies and for small dams, the USBR (1981) indi-
cate that the method outlined in USBR (1977) may be adopted. This is based on a wind
velocity of 160 km/hr (100 miles per hour) for determination of normal freeboard and
80 km/hr (50 miles per hour) for minimum freeboard. The effect of wind setup is ignored.
For rip-rapped slopes the freeboard requirements are as tabulated in Table 13.1. For
dams with a smooth pavement or soil cement upstream slope, depending on the smooth-
ness of the surface, freeboard of up to 1.5 times those shown in Table 13.1 should be used.
13.1.3
Estimation of wind setup and wave runup for detailed design
The following outlines the method described in USBR (1981). This is based on US Corps
of Engineers (1976) and Saville et al. (1962). USBR (1992) up-dates this procedure, taking
account of US Corps of Engineers (1984a), and favours a more probabilistic approach. The
updated procedures are too lengthy to include here and for detailed design of important struc-
tures readers should refer to the original texts (USBR, 1992; US Corps of Engineers, 1984a).
