SMALL DAMS PETITS BARRAGES

Fig. 5.2 – Design Freeboard
5.4.3 Freeboard Design for Wave Run-up and Wind Setup
In general, freeboard design should consider reasonable combinations of appropriate
components defined on Figure 5.2. However for most small dams, wind setup and wave runup
are the principal design criteria that are most often used to determine freeboard
requirements. Wave run-up and wind setup are highly sensitive to site-specific conditions. In
the U.S., for example, hurricane wind conditions may control freeboard requirements for
reservoirs located along the Atlantic and Gulf coasts, whereas controlling wind conditions in
mountainous regions are more strongly influenced by “orographic” effects of topography and
seasonal meteorological conditions that can produce sustained high winds (USACE, 1997).
Widely used design procedures in the U.S. for calculating freeboard based on wave runup
and wind setup are outlined in USBR (1992) and EM 1110-2-1100, Part II (USACE, 2003).
These procedures generally involve the following steps:
Evaluate Effective Fetch (Fe)
Fetch is the straight-line distance across a body of water subject to wind forces, and is
limited for inland reservoirs by the surrounding topography. Effective fetch (Fe) is defined as
the average horizontal distance in the general direction of the wind over water, corrected for
reservoir plan geometry, over which a wind acts to generate waves.
Evaluate Design Wind
Use of the actual wind records from a site is the preferred method for establishing wind
speed-duration curves (USACE, 2003). Alternatively, generalized maps have been developed
for the continental U.S. that show contours of overland wind velocities for summer, fall, winter,
and spring (USBR, 1992). Maps for the fastest mile (approximately 1-minute duration) and
sustained 1-hour winds are published. USBR (1992) suggests developing 2-hour wind velocity
55