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Ten threat zones of the Cauca River in face of the scenario of a Hidroituango disaster

A Hidroituango dam failure is a scenario which has been assessed by the Medellín Public Utilities Company (EPM, for its Spanish acronym), the Colombian Engineering Society, national and international experts and the Colombian government. They all include the possibility of a breakage of the dam due to different reasons, which are not part of this analysis. Here is an estimation of the morphodynamic processes (study of landscape changes due to erosion and sedimentation) which could take place on the streambed of the Cauca and Magdalena downstream.

Follow-up of the reservoir through daily and high-resolution monthly satellite imagery (using PlanetScope) shows an alarming daily growth (1-2 m/day). The imagery of April and May show how the water has changed notably, reaching a length of 35 km upstream by the end of May with a width between 261 m and 1.02 km.

The design parameters established a maximum height of 220 m, for a 70 km long and de 2,720,000 m3 reservoir. The current situation shows that the height of the dam is very close to that established by the project. A length of 35 km and 1,000,000 m3 in case of failure could produce a wave between 20 and 100 m high.

The following are ten morphodynamic sectors which are determined and described in case of a breakage scenario based on geomorphologic analysis, satellite imagery, digital modeling and field surveys.

Sector 1. Hidroituango Dam - El Cinco (avalanche passage)

The first sector is known as “avalanche passage” is the area of the Cauca River from the dam to the area known as “El Cinco”, at 60 km downstream.

This mass could have a very strong wavefront and would rip through the slope leaving a polished rock surface and residual soils.

Municipalities such as Puerto Valdivia (30 km) and El Quince (47 km) could be totally destroyed. The avalanche would be favored by the straight passage linked to its control for geological faults associated with the Cauca Romeral system (Espíritu Santo fault).

Sector 2. El Cinco Cáceres – Tarazá (avalanche deposit in a fluvial-torrential fan) 

In this stretch, the river passes from being in a canyon to foothills between the municipalities of El Cinco (60 km), Puerto Antioquia (65 km), Cáceres (70 km) and Tarazá, the latter over the river of the same name, 5 km of Cáceres.

From El Cinco the Cauca River exits the canyon towards a slightly sloped area forming a small valley where the water could be freed of its energy, depositing the material carried and forming a 3,808-hectare fluvial-torrential fan.

The effect of the discharge could impact great part of the municipalities of the sector pressuring the Tarazá River, which could be restrained upstream and probably the Cauca River would lose its course in this sector and form a temporary dam with the accumulation of sediments. 

Sector 3. Cáceres - Guarumo (torrential avenue) 

This is a transition area before the plains and characterized by the presence of alluvial material historically carried towards the high basin and deposited here in form of alluvial terraces; therefore the Cauca riverbed is limited by terraces composed of conglomerates with a general direction of N15E to SN.

The active river course is slightly wandering, tabular with two 50 and 300 m wide channels. The greater river floodplain is restrained by terraces with a width between 100 m and 1.8 km. Here, the Cauca river flow could produce a torrential avenue type event with high river flow and level. Slopes could have continued landslides taking into account the low resistance to the terraces to erosion. The municipalities which could be impacted in this sector are Corrales El Playón, Puerto Bélgica, El Jardín, Piamonte, and Guarumo. 

Sector 4. Guarumo Caucasia – La Ilusión (material deposit of the torrential avenue) 

This morphodynamic sector is characterized by an enlargement of the largest riverbed of the Cauca River in two areas, one from Guarumo to the Man River (4.3 km) and the second from the Man River to La Ilusión, with a maximum width of 9 km. The area has 10,817 hectares.

The extension of the larger riverbed is a consequence of a right angle sudden change course direction from south-north to west-east. There would be a torrential deposit of materials coming from the narrower valley of sector 3. Part of the municipality of Caucasia could suffer from flooding and the lower terraces bordering the river and could be impacted by torrential flows.

Sector 5. La Ilusión - Nechí (flooding) 

This stretch is characterized for its west-east heading limited by alluvial terraces. The river has a meandering and sinuous course with variable widths between 2.4 and 6.6 km.

In this sector, the river would behave as a water surge in case of dam failure impacting the human settlements of Palanca, Palomar, Colorado y Nechí, which could be totally flooded. The area has 31.549 hectares.

Sector 6. Colorado - Nechí (water streams) 

The limits of the alluvial terraces end on the river course and the course is higher on the left margin of the plains, making the area susceptible to water streams forming large water flows towards the plains of Ayapel – San Jorge, as occurred in 2010 in the area known as Nuevo Mundo.

Sector 7. Río Nechí (onrush and flooding) 

The Nechí River joins the Cauca River near Nechí. The onrush linked to a failure of the dam would increase the level of the Cauca River forming a flood of 28,972 hectares. This flooding could be restrained by metamorphic crystalline rocks and upstream of the river.

Sector 8. San Jacinto del Cauca – Pinillos (onrush and overflowing) 

This sector has the lower floodplain of the Cauca River between the municipalities of Nechí, San Jacinto del Cauca, Guaranda, Achí, and Pinillos. The greater floodplain of the river is predominantly south-north and with a width between 1 and 3.5 km.

The right margin of the Cauca River between Nechí and Guaranda is limited by Paleozoic metamorphic rocks. Later the course and greater floodplains are restrained by slim alluvial dikes which border with floodplains and marshes.

Sector 9. Delta of the Cauca River (meander closure, reservoir, erosion) 

The Cauca to Magdalena River delta is a meander or an area where the river forms a closed curve in closure process, only remaining 20 m for this closure.

With an onrush, the river would exert power over the right margin of the Pinillos River forcing the closure of the meander and producing an avalanche of the delta to the north.

This would cause a small dam in the arms of the meander of the Cauca River and cause erosive events on the slopes of the Pinillos riverbed with overflowing.

Sector 10. Río Magdalena. Pinillos – Zambrano (flooding) 

After the confluence of the Cauca and Magdalena Rivers through the Pinillos meander, there could be moderate flooding in the floodplains of the Magdalena River up to the municipality of Zambrano.

Due to the sensitivity of the alluvial dikes which form the floodplain in this sector, there could be erosion processes on the dikes and overflowing towards La Mojana.

La Mojana region (fluvial flooding buffer) 

La Mojana is a flooding plain of approximately 13,483 km2, and a type of buffer zone for the flooding of the Cauca and Magdalena Rivers in a dam failure situation it would collect the water overflowing from fluvial flooding.

Consejo Editorial