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Why is there flooding in the lower Cauca area?

Close to 3,000 families of the municipalities of San Jacinto del Cauca, Bolívar, Ayapel, and Córdoba were affected by the waters. What were the natural and anthropic conditions that played out in these floods?

Among the factors or natural conditions that intervened are:

Hydrographical context. The great rivers in the low basins, as in this area of the Cauca River, have ample plains with conditions proper for natural flooding. The greater margin of the river (channel where a river can move or cover), the floodplains, and the marshland basins are natural buffer zones of the river in times of highwaters.

Geological condition. The riverbanks (active flow and greater margin) of plain rivers such as the Cauca River are made of lithological material such as low erosion-resistant sand and silt. Near the floodplains and marshlands, the flow naturally develops over alluvial dikes that allow it to preserve its flow without overflowing. These structures are predominately clayey and just a few meters wide.

Geomorphological condition. A geomorphological condition is present when the river flow (active flow and greater flow) in plains areas are higher in one or two of its margins. The Cauca River has a particular geomorphological condition between the Nuevo Mundo sector near the municipalities of Nechí and San Jacinto del Cauca, Province of Bolívar. Here the active riverbank is at a greater height of 39-31 meters above sea level (masl) than its western margin that pours to the San Jorge River and Ayapel marshlands in heights of 18 to 20 masl. This condition of the active flow makes it highly susceptible to form breakers (rupture of alluvial dikes) with great water flow towards these lower areas.

Morphodynamical condition. Rivers of low plains, due to their low gradient have movement of the greater bank from frontal or lateral of the flow in form of meanders. The river flow can change in a determined time determined by its right to left margin producing erosion processes or differential sedimentation. The Cara de Gato sector of the Cauca River migrated since 1985 from the right to the left margin with a closing meander trend, which also moves straight toward the northeast.

Erosion. Plain rivers move in their greater bank in their hydrological and sedimentological balance forming different patterns or straight or meander configurations. When the active flow impacts the shore of the greater bank of the river it exerts greater force to erosive forces to the dikes or margins as occurs in the sectors of Mundo Nuevo, Santa Anita, Pedro Ignacio, Cara de Gato, and San Jacinto del Cauca.

Climate conditions. Some hydroclimatic factors thattrigger flash floods in the flows of plain rivers are associated with climate variability phenomena such as La Niña, which by the effect of cooling of the water of the Pacific Ocean produces in the continent an increase of the rains. These rains impact rivers, highly increasing the levels, flows, and sediment loads producing overflowing, flash flooding, and flooding.

The effect of these phenomena was very incidental for the end of 2010 and beginning of 2011, in 2017, and recently on 2021. Between 1976 y 2021, the Cauca River between Caucasia and it's pouring into the Magdalena River –near the town of Pinillos– had close to 50 flooding points. A great part of these events occurred during the La Niña of 2010 and 2011.

The human influence

Among the anthropic conditions where the hand of man has directly intervened are:

Civil work. Building roads inside the greater banks or over natural alluvial dikes causing obstacles to the fluvial dynamic and producing breakers. If the frontal movement of a meander crashes with the dike of a road it suffers a slight retreat and looks for weaker areas breaking the alluvial dikes of the road.  A contributing anthropic factor in this sector of the Cauca River is the presence of the road Caucasia – Nechí, San Jacinto del Cauca, which in some sectors its route is inside the greater bank. Effects of this condition are evidenced by the occurrence of breakers such as the ones of Mundo Nuevo, Santa Anita, and Cara de Gato.

Urban areas in the greater riverbed. The ancestral settlement of communities on the riverbanks without considering the fluvial dynamic is highly vulnerable to flooding due to flash floods or changes linked to its dynamic. Towns such as Caucasia, Nechí, San Jacinto del Cauca, Guaranda, among others have sectors in the bank or flood plain.

Intervention of natural alluvial dikes. Natural river dikes for being safe areas against floods generally are used to build houses or roads impacting their natural condition and making them susceptible to erosive processes. Several sectors of the existing road on the left margin of the Cauca River are on natural dikes.

Flooding on the lower Cauca linked to the La Niña phenomenon

When this happened towards the end of 2010 and beginning of 2011, there were 5 breakers, three in 2011 on the right margin of the Cauca River near the towns of Nechí called Nuevo Mundo, Santa Anita, and Pedro Ignacio.

The water flow on the Nuevo Mundo and Santa Anita breakers join on the 11th km from Nuevo Mundo and 4th km from Santa Anita to form a sole flow that flows for 60 km. This flooding reached the basin of Ayapel marshlands. This flow covered a total surface area of 278.7 km2, as seen in Figure 3.

This shows the recurrence of breakers and flooding of the NechíSan Jacinto del Cauca sector linked to the La Niña in sectors with highly susceptible geological and geomorphological conditions. As explained in Figure 3.

The case of the Cara de Gato-San Jacinto breakwater

The La Niña event of 2020-2021 has been categorized by world and national meteorological organisms as the Hydrology, Meteorology and Environmental Studies Institute (IDEAM, for its Spanish acronym) as light to moderate intensity, producing flash floods and flooding in several sectors of the Mojana region from the Magdalena and Cauca Rivers.  The main event is that of Cara de Gato, San Jacinto del Cauca.

The conditions explained apply totally to the breakwater event and flooding in Cara de Gato-San Jacinto del Cauca on August 27 of 2021.

The Cauca River in this Cara de Gato sector is higher (31 masl), than in the banks and plains of the left margin reaching 25 masl on the basin of the Ayapel marshlands, provoking water flow and flooding of these areas.

The records of multi-temporal flows in the Cara de Gato sector and other meander show a fluvial dynamic in this sector with the migration of an active flow to the right margin of the greater bank toward the left margin. Also, between 1985 and 2021 a meander formed that closes in the breakwater sector and shows a frontal migration towards the northeast in its closing process pressuring the shores of this margin. This natural dynamic and the presence of a road to the north has hindered the mobility of this meander causing the breaker and flooding. As explained in Figure 4.

The breakwater known as Cara de Gato broke in approximately 500 m and formed two flows in the northeast direction reaching close to 31 km to the southern sector, flooding the plains of Ayapel, and the northern corridor reached close to 33 km impacting part of the municipalities of Majagual and San Benito de Abal. On August 29, the flooding had impacted a surface of around 112,151 km2 and on September 2 it covered a surface area of 320,114 km2. As explained by Figure 5.

The Colombian Government announced an investment of Col $1 billion to control the flooding in the area of la Mojana –by building a longitudinal levee of about  57 km on the left margin of the Cauca River and 5 floodgates– therefore it is important to consider good knowledge of the natural behavior of the fluvial zones to intervene and the buffer and connection systems with the plains and marshlands to not impact the balance of the fluvial and lacustrine system of the area. It is also important to correct the stretches of road over the inside of the greater bank of the Cauca River that hinders the mobility of the river.

Consejo Editorial