Rivers are natural tributaries, mostly of fresh water, that flow from the elevated areas toward the ocean. The area that collects the water that ends up in a river is called a river basin; however, a river is also connected to underground aquifers belonging to the same river basin and that is known as hydro-geological catchment area.
Although rivers on our minds seem like static elements through time that have always been there. But in fact, rivers are dynamic elements but the rate at which they change is small enough not to be noticed in the day to day. The speed at which a river changes depends on a lot of factors such as the water quantity that flows through the river, the place within the basin, the type of rock and the soil where is flows, the time of the year and the amount of vegetation in the adjacent area.
In this case study, you will be able to see in aerial images and satellite images the natural movements of the Tiribí River in a determined section where there are conflicts with the adjacent properties. This is a fragment of the studies that must be done in order to determine the level of hazard that a river poses to a property, investment or before the purchase of a plot of land.
The Case of the Tiribí River, in La Unión, Cartago
Within the area of La Unión, the Tiribí river sits beside the Ayarco community. By analyzing the landscape you can see the community that sits on top of a flat surface that is interrupted by the river, which is seen from above to have a sinusoidal shape, it travels through the deep end of a shallow canyon. The type of behavior of this river is responsible for the existence of the canyon, in short, the river makes a dent in the ground that keeps getting bigger as time progresses carving out the shape that we can see now; this process does not stop and keeps existing even to this day. We can see the process in the maps throughout the years in the following figure.
Even if the movement is slow, in the progression through time, we can barely see the movement on the river. However, it's effect on the hillside' of the river canyon is strong. To observe the phenomenon you can create the reconstruction of the river bed migration in the following animated image.
Now that you know that the river has movement throughout time, how can this become a hazard to the properties and buildings that are adjacent to the river?
The movement of water in the river has an energy quantity determined by the amount of water and it's velocity and therefore holds the capacity to move different materials with different characteristics, this can reflect in the movement of sediments in the water, these sediments are responsible of the rivers having a turbulent and chocolaty look, specially during the rainy season. But, where do the sediments come from?, in this case the sediment comes from the erosion of the hillsides because of the river action.
Given it's shape, the river has the capacity of exerting an erosive effect in different sectors while in other sections the opposite happens, the river deposits the material that has carried from uphill. The next image shows the areas where these processes.
The active sedimentation area belongs to the river bend and it is where material accumulates, usually sand, mud and rounded pebbles. These areas usually increase in size as time goes on. The area of active erosion is where the river is constantly removing material. Under normal conditions, this area can be observed as a small "wall" along the river bend that loses material when the river has a higher influx of water as it happens during heavy rainfall. The area of unstable slopes is located along side the river bend, this area is generally meta-stable throughout time, vegetation usually grows on it and gives us a sense that it isn't moving; however, this is not the reality given that in time, the active erosion area will cause instability until a critical point where the material fails; this mechanism is usually triggered by heavy rainfall or an earthquake but it can also happen without those triggers.
Does that mean, given the previous information, that stopping the erosion of a river bend margin will allow slope stability? The answer is an inconclusive Yes and No. Albeit, stopping the area of active erosion can significantly helps the slope stabilization it is not always viable. The field of engineering can create solutions to these type of problems, such as barrier building, dykes that will redirect the flow of water; however, these type of construction efforts can be very expensive and are generally reserved for large projects. The more cost-effective solutions tend to be only temporal or not that effective. The erosive power of a river should never be underestimated. Gabion walls, walls and barriers can be built but won't stop the river for long.
Such is the case observed in the year 2016, we can see in the above image that during the period the river migrated moving closer to the properties causing an impact on the buildings that were at risk of sliding into the river. The owners built barriers to attempt to defend their properties but those measures proved to be insufficient that couldn't stop the advance of the river. In these period for 2017 there was dredging of the river bend in order to move material around so that the flow of the river ran toward the sedimentation area; however, achieving that goal by just moving material around is a temporary fix given that the river will go back to the natural condition to continue the process. On site, we can see the evidence of these processes more clearly:
The best solution for these type of scenarios will vary depending on the specific conditions of the site, the characteristics and properties of the land, the river and the expert criteria of the consultant that was hired. It is important to mention that these type of problems are largely avoidable if you investigate the land that you are interested in buying before you pay a dime and here is where geological consultation will help you diagnose those problems in time. It is very hard for a family or small community to fight against a problem of this kind that are not immediate single events but persistent conditions with a great potential for harm which is why our number one recommendation is to get knowledge before investing. Research is not an expense, it is assurance and investment that will ensure the future of your project and development and more importantly, of human life.