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Western Dam Engineering Technical Note May 2017 2 Thinking Fast - Emergency Response to Seepage and Internal Erosion Introduction One of the most challenging threats to the safety of an earthen dam is concentrated, uncontrolled seepage that has initiated internal erosion. The progression of such an event may be avoided by inspection, monitoring, maintenance, and, if warranted, rehabilitation. But what if those measures have not been implemented, or in spite of them there is a failure to recognize a progressing problem, and it culminates in an emergency situation? This article focuses on answering that question. The previous Western Dam Tech Note article titled Is Your Embankment Dam under Pressure – Underseepage Impacts describes the mechanics, monitoring, and investigation of seepage through the soil foundations of earth dams in some detail, and provides a good background to the following discussion. Why we care about concentrated seepage As shown in Table 1 more than half of all dam failures worldwide in the modern dam era have been due to seepage-induced piping (i.e., internal erosion). Failures and incidents continue to happen today, incurring significant consequences to the owner and the public. From January 1, 2005 through June 2013, state dam safety programs reported 173 dam failures and 587 "incidents" - episodes that, without intervention, would likely have resulted in dam failure [1]. Examples of incidents involving concentrated seepage that could have led to dam failure without timely, aggressive, and ultimately successful intervention include:  AV Watkins Dam, UT [2,3,4,5]  Lake Needwood Dam, MD [4,6,7]  Washakie Dam, WY [4,8,9]  Powell Dam, MT [4] – see inset this page  Salt Fork Dam, OH [4] – see inset next page Brief case histories of concentrated seepage events at AV Watkins, Lake Needwood and Washakie Dams are presented at the end of this article. Table 1. Seepage-Induced Internal Erosion Dam Failures Worldwide [2]. Powell Dam Foundation Seepage Blowout Construction of a seepage berm likely concentrated the seepage exit from a gravel layer to the edge of the berm, causing a blowout (see photo to left). Emergency response included the placement of on-hand woven-geotextile fabric covered with stockpiled drain rock (see photo below). A permanent toe drain was later constructed to intercept and drain the gravel layer believed to be the source of the high pressures. Mode of Failure % Total Failures (where mode of failure was known) % Failures pre 1950 % Failures post 1950 Overtopping 34.2%36.2%32.2% Spillway/gate (appurtenant works)12.8%17.2%8.5% Piping through embankment 32.5%29.3%35.5% Piping from embankment into foundation 1.7%0.0%3.4% Piping through foundation 15.4%15.5%15.3% Downstream slide 3.4%6.9%0.0% Upstream slide 0.9%0.0%1.7% Earthquake 1.7%0.0%3.4% Totals (1)102.6%105.1%100.0% Total overtopping and apurtenant works 47.0%53.4%40.7% Total piping 49.6%44.8%54.2% Total slides 4.3%6.9%1.7% Total no. embankment dam failures (exc. During construction(124 61 63 Total embankment dam years operation (up to 1986)300,400 71,000 229,400 Annual probability of failure 4.1 x 10-4 8.6 x 10-4 2.7 x 10-4 (1) Subtotals and totals do not necessarily sum to 100% as some failures were classified as multiple modes of failure