Aguaje de la Tuna
[typing] The sediment retention dam at Aguaje de la Tuna has resulted in the degradation of the streambed, to a depth of approximately 8 m, reaching the underlying bedrock. [VMP] Good morning. We are here in the Aguaje de la Tuna, one of the watersheds that drains into the Tijuana river, in Tijuana, Baja California, Mexico. We are accompanied by Alberto Castro Garcia, a hydrologic civil engineer, practicing in Tijuana. [ACG] Good morning to all. I wish to thank Dr. Ponce and his team of San Diego State University students for the opportunity to present to you a few aspects of Tijuana's hydrology, particularly about this creek, which is referred to as the Aguaje de la Tuna. In 1991, there was a heavy storm in the city, which produced a flood in this basin, transporting a large quantity of sediment to the mouth of the basin. In 1993, an extraordinary storm event pounded the city, especially the Aguaje de la Tuna. About 400,000 cubic meters of sediment were dumped into the delta. As a consequence of this situation, the need was felt to construct flood protection works, consisting of channelizations and sediment retention dams, like the one shown here.
It is practically at the level of where I am now standing. This is the base level of the first retention dam. The objective of this dam was basically to retain sediment.
Originally, the dam was designed to retain a certain amount of solids. During the useful life of the structure, degradation occurred, which led to the construction of a first step. Ten years later, another step was built because degradation continued. The steady erosion by water continued, and the erosion depth eventually reached the underlying rocks, classified as shales, as observed here.
[VMP] Here we are located at a distance of approximately 50 m downstream of the sediment retention dam. As you can see, here the amount of degradation of the creek bottom has reached the underlying rock, which appears to be a disturbed shale. Here we have an example of a modern-day geologic, anthropogenic process in which a small canyon is being formed in the rocky streambed of the Aguaje de la Tuna watreshed. [VMP] Here we can observe a curiosity of Nature --- how the water flowing in this reach of the Aguaje de la Tuna, above the rocky shale bottom, has been able, over the past 20 years, to carve out this feature, basically a natural dam of irregular shape, apparently a spillway, but with intermediate channels.
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