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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. This article describes a procedure for calculating the 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 The procedure presented in this article is based on 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 documents included at the end of this article. 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 and the Radial Line i The procedure in this article is limited to reservoirs where 1.) Effective fetch is less than 10 miles and 2.) Wave height is less than 5 feet.