Early News media reporting are showing to be incomplete in information details on the Dam. An Emergency Safety feature is engaging to control the overtopping failure. This area is on the South end of the Dam, not the North as reported.
Note: Without design specifications to this dam, information was constructed from satellite elevation data, imagery data, technical report information, and dam design background knowledge.
Fig. 1 reveals that the Dam was designed for an Emergency Overtopping Area in the southern end of the dam. This is the lowest elevation in the curved section. The embankment in this area is sloped to guide water into a circular concrete collection zone at the top of a concrete spillway. Fig. 2 Shows the overtopping flow at this low elevation point. The flow is entering into the collection zone of the circular concrete spillway. At the time of the video, the overtopping has not eroded to the left into the main dam. However, this water flow volume is testing the limits of the spillway capacity.
Fig 3. shows that the lower spillway is damaged from these high flows. The riprap rock has been washed away. This is a primary indicator that the riprap was handling flows exceeding the dissipation energy it was designed for. Another possibility is that the spillway concrete was not maintained and experienced a failure in a section of the spillway concrete. If so, this failure would have allowed the flows to undermine the base foundation that the riprap rock was emplaced upon, making it easier to wash the rock away.
The overtopping failure flow has been somewhat mitigated IF the dam does not develop a deeper erosion "V" breach. A "V" breach could expose a much higher surge in flows as the lowest section of the "V" determines the height and volume of the reservoir release in a shorter period of time.
Fig. 4 shows the failure of the spillway and the erosion that is cutting into the base of the dam surrounding the spillway. This erosion is likely "back cutting" or back stepping the loss of the soil like foundation supporting the spillway. If high runoff conditions continue to enter into the reservoir, there could be a point where a collapse of this overtopping & spillway section could result. Thus, if so, a larger surge of flood water would cascade down the river channel.
Fig. 1 Guajataca Dam was designed having a lower elevation "Controlled" failure "Overtopping point" in the southern end of the dam. A "Controlled failure spillway", concrete lined, was designed to guide this water to energy dissipation riprap of large rocks.
Fig. 2 Image shows a wide water flow into the "Controlled Failure Overtopping" area and the water entering the "Controlled Failure Spillway".
Fig. 3 Image reveals the broken spillway (see depth of water drop) from high flows and "back cutting" erosion. The Energy Dissipator RipRap rocks have been washed away. The high flows are causing more flooding downstream.
Fig. 4 Image shows the failure of the spillway and the erosion that is cutting into the base of the dam surrounding the spillway. This erosion is likely "back cutting" or back stepping the loss of the soil like foundation supporting the spillway. If high runoff conditions continue to enter into the reservoir, there could be a point where a collapse of this overtopping & spillway section could result. Thus, if so, a larger surge of flood water would cascade down the river channel.
