2
Predicting Wave Runup on Dam
Slopes
Introduction
When wind blows over an open water surface, such as
within a reservoir, wind-generated waves can strike
the upstream slope of the dam embankment. This can
cause erosion of the embankment material and if
severe enough, waves can overtop the embankment,
both of which are dam safety issues. Therefore, the
dam embankment design must consider the potential
effects of wave action and protect against erosion of
the embankment materials and overtopping due to
wave runup. This is done by extending the
embankment up from the still water flood pool level to
an elevation equal to the still water pool plus the
maximum calculated wave runup and wind setup
height.
wind-generated wave characteristics for inland
reservoirs and lakes and the resulting wave runup on a
sloping dam embankment for small dams.
Dominant Factors and Procedure
The major variables used to calculate wind-generated
wave height on open water surfaces, such as
reservoirs, and influence embankment design are:
 Effective Fetch and Wind Direction
 Wind Speed over Water
 Wind Setup, Wave Height and Runup
information presented in TR-69 (USDA, 1983) and
Bureau of Reclamation ACER TM-No. 2. Additional
information related to US Army Corps of Engineers
(USACE) procedures is presented in the reference
Effective Fetch and Design Wind Direction
The fetch is an overwater length blown on at a
constant wind speed and direction. The longer the
fetch and the faster the wind speed, the more wind
energy is imparted to the water surface and
proportionally higher waves will be produced. TR-69
recommends two approaches to determine the design
fetch and wind direction: (1) U.S. Weather Service
climatological data or (2) site orientation. Because
most dams/reservoirs are ungauged, wind data does
not typically exist and the site orientation method is
preferred to define the effective fetch and design wind
direction.
The design wind direction is obtained by determining
the longest stretch of open water from a point on the
shoreline opposite to the dam embankment. It is
assumed that wind and waves are developed along the
longest fetch of open water from the dam. According
to Saville’s 1954 study, the width of the fetch on inland
reservoirs normally places a definite restriction on the
length of effective fetch, which is the effective distance
of the water over which the wind blows without
appreciable change in direction. Figure 1
diagrammatically shows the central (longest) and radial
fetch lines for a hypothetical reservoir. Simplistically,
this method involves drawing the central radial line
and then drawing seven radial lines at 6-degree
intervals on each side of the central radial line.

Figure 1: Central and radial fetch lines
The effective fetch, Fe, can then be computed using
Equation 1.
∑( )
∑( )
Eq.1
xi = Length of Radial Line i
αi = Angle Degree between the Central Radial Line