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<br /> <br /> <br /> <br />2 <br />Predicting Wave Runup on Dam <br />Slopes <br />Introduction <br />When wind blows over an open water surface, such as <br />within a reservoir, wind-generated waves can strike <br />the upstream slope of the dam embankment. This can <br />cause erosion of the embankment material and if <br />severe enough, waves can overtop the embankment, <br />both of which are dam safety issues. Therefore, the <br />dam embankment design must consider the potential <br />effects of wave action and protect against erosion of <br />the embankment materials and overtopping due to <br />wave runup. This is done by extending the <br />embankment up from the still water flood pool level to <br />an elevation equal to the still water pool plus the <br />maximum calculated wave runup and wind setup <br />height. <br />This article describes a procedure for calculating the <br />wind-generated wave characteristics for inland <br />reservoirs and lakes and the resulting wave runup on a <br />sloping dam embankment for small dams. <br />Dominant Factors and Procedure <br />The major variables used to calculate wind-generated <br />wave height on open water surfaces, such as <br />reservoirs, and influence embankment design are: <br /> Effective Fetch and Wind Direction <br /> Wind Speed over Water <br /> Wind Setup, Wave Height and Runup <br />The procedure presented in this article is based on <br />information presented in TR-69 (USDA, 1983) and <br />Bureau of Reclamation ACER TM-No. 2. Additional <br />information related to US Army Corps of Engineers <br />(USACE) procedures is presented in the reference <br />documents included at the end of this article. <br />Effective Fetch and Design Wind Direction <br />The fetch is an overwater length blown on at a <br />constant wind speed and direction. The longer the <br />fetch and the faster the wind speed, the more wind <br />energy is imparted to the water surface and <br />proportionally higher waves will be produced. TR-69 <br />recommends two approaches to determine the design <br />fetch and wind direction: (1) U.S. Weather Service <br />climatological data or (2) site orientation. Because <br />most dams/reservoirs are ungauged, wind data does <br />not typically exist and the site orientation method is <br />preferred to define the effective fetch and design wind <br />direction. <br />The design wind direction is obtained by determining <br />the longest stretch of open water from a point on the <br />shoreline opposite to the dam embankment. It is <br />assumed that wind and waves are developed along the <br />longest fetch of open water from the dam. According <br />to Saville’s 1954 study, the width of the fetch on inland <br />reservoirs normally places a definite restriction on the <br />length of effective fetch, which is the effective distance <br />of the water over which the wind blows without <br />appreciable change in direction. Figure 1 <br />diagrammatically shows the central (longest) and radial <br />fetch lines for a hypothetical reservoir. Simplistically, <br />this method involves drawing the central radial line <br />and then drawing seven radial lines at 6-degree <br />intervals on each side of the central radial line. <br /> <br />Figure 1: Central and radial fetch lines <br />The effective fetch, Fe, can then be computed using <br />Equation 1. <br /> ∑( ) <br />∑( ) <br /> Eq.1 <br />xi = Length of Radial Line i <br />αi = Angle Degree between the Central Radial Line <br />and the Radial Line i <br /> The procedure in this article is limited to <br /> reservoirs where 1.) Effective fetch is less than <br />10 miles and 2.) Wave height is less than 5 feet.