Environmental Engineering Reference
In-Depth Information
13
Embankment dam details
13.1 FREEBOARD
13.1.1
Definition and overall requirements
(a)
Freeboard
. The freeboard for a dam is the vertical distance between a specified reser-
voir water surface level and the crest of the dam, without allowance for camber of the
crest of the dam;
(b)
Normal freeboard
is the vertical distance between the crest of the dam without
allowance for camber, and the normal reservoir full supply level;
(c)
Minimum freeboard
is the vertical distance between the crest of the dam without
allowance for camber, and the maximum reservoir water surface that results from
routing the inflow design flood through the reservoir.
The objective of having freeboard is to provide assurance against overtopping resulting
from (USBR, 1981):
-Wind setup;
-Wave runup;
- Landslide and seismic effects;
- Settlement;
- Malfunction of structures;
-Other uncertainties in design, construction and operation.
Other factors which may influence the selection of freeboard include (ANCOLD,
1986):
- Reliability of design flood estimates;
- Assumptions made in flood routing;
-Type of dam and susceptibility to erosion by overflow;
- Potential changes in design flood estimates, either through changes in flood estimation
techniques or due to changed catchment conditions.
USBR (1992) have the following freeboard requirements for new embankment dams:
(1)
Freeboard at maximum reservoir water surface elevation
. The minimum freeboard
should be the greater of (a) 0.9 m or (b) the sum of the wind set up and wave runup
that would be generated by the average winds that would be expected to occur dur-
ing large floods, as determined after seeking advice from local authorities and meteo-
rologists. They point out that for large reservoirs and catchments the wind may be
independent of the storm event that created the flood, and suggest a wind with a 10%
exceedance probability (1 in 10 year) should be used.
(2)
Normal water surface freeboard.
The normal freeboard above full supply level should
be the wind set up and wave runup for the highest sustained wind velocity “that could
reasonably occur” e.g. 95-160 km/hr.
