Function of dams and reservoirs

As for control, it is to reduce peak flows, which enter an area prone to flooding. The operation of a reservoir to control floods is very different from that of hydroelectric operation, or for irrigation. Instead of keeping water levels high in order to have a greater hydrostatic head or source of water for irrigation, its flood control operation requires that water levels be kept low, before and during the flood season, to have the capacity to store rising waters. However, the eventual release of the water can cause problems. Instead of being flooded to a greater depth, for a shorter period of time, some lands are flooded to a much shallower depth, but for a much longer period of time. This may not be compatible with existing agricultural systems. The environmental effects of dams and reservoirs, which are discussed in detail in a separate article, will not be analyzed here.

Structural measures

To control floods, in riverbanks and channel improvements, they increase the capacity of the river, increase its flow speed, or achieve both effects, simultaneously. Modifications to the channel that can be made are: dredging it to make it wider or deeper, clearing vegetation or other debris, leveling the bed or walls, or straightening it; All of this helps increase the speed of water passing through the system, and prevent flooding. By straightening the channel, eliminating meanders, the risk of water breaking the river bank on the outside of bends, where the current is faster and the level is higher, is reduced.

It may cause some negative environmental impacts. Any measure that increases the speed of the current will increase the water's ability to cause erosion. Erosion and sedimentation problems can occur on site, or downstream. By lining channels, all factors impeding flow are reduced or eliminated, but this practice produces aesthetic and environmental problems, including reduced groundwater turnover, and disrupts populations with the disposal of dredging waste. Canal improvement can alleviate problems caused by flooding in the treated area; However, the peaks will be higher downstream, and the problem has simply been transferred elsewhere.

Artificial banks, improved natural banks and dikes

They increase the capacity of the canal and keep all the water inside it, with the exception of floods that are extraordinarily high. However, like canal improvement measures, these structures tend to transfer the problem to downstream areas, which in turn suffer, or public funds need to be invested to implement urgent flood control measures in those areas. Levees that are built on alluvial terrain, with the purpose of excluding water from certain areas, intervene in the hydrology of the area, and can have impacts on the habitat and movement of fauna and livestock.

Overflow depressions are usually swamps between the river banks and the sides of the valley. Likewise, it is possible to build artificial depressions for the waters. Detention depressions or small reservoirs are often used near urban areas to intercept and collect flow before it reaches the river; They are also effective in reducing peak flows. The depressions have a positive effect on replenishing the aquifers, and allow suspended sediments that would otherwise flow into the channel to settle. However, they can become habitats for disease vectors.

Relief channels

Also called high-flow diversions or spillways, they are natural or artificial diversion channels, or conduits that send water away from urban centers, or high-density populated areas. Further downstream, the water can be returned to the river where it originated.

Flood control structures are expensive. Additionally, they give a false sense of security, because the agent thinks that the risk of flooding has been eliminated, rather than just reduced. This can encourage them to increase floodplain development, causing disastrous results in the event of unusually high flooding, or failure of control structures. Each protection structure is calculated for a certain flow rate, which corresponds to an average return time (T).

In addition to the impacts of structures on the environment, environmental factors that influence flood control must be considered. Infrastructure or other works in the floodplain not only mean risks for the people there (depending on their vulnerability), but also increase the risk for neighbors and other people who live in downstream communities. Buildings, for example, by obstructing the flow of water, can increase the height and speed of flooding, reduce the storage capacity of the floodplain, and increase flow.

Natural events, such as fires, windstorms, or changes in river course, influence flooding. Human activities in the watershed, such as tree cutting or clearing for agriculture, generally increase the flow, as does the plowing of sloping land without implementing adequate terraces, or contour furrowing. By waterproofing the land of the hydrographic basin and the alluvial zone, the flow and volume of the floods will increase; installing storm sewer systems will increase flow; Installing storm sewer systems will increase the amount and speed of stormwater entering the river system.

Function of dams and reservoirs

As for control, it is to reduce peak flows, which enter an area prone to flooding. The operation of a reservoir to control floods is very different from that of hydroelectric operation, or for irrigation. Instead of keeping water levels high in order to have a greater hydrostatic head or source of water for irrigation, its flood control operation requires that water levels be kept low, before and during the flood season, to have the capacity to store rising waters. However, the eventual release of the water can cause problems. Instead of being flooded to a greater depth, for a shorter period of time, some lands are flooded to a much shallower depth, but for a much longer period of time. This may not be compatible with existing agricultural systems. The environmental effects of dams and reservoirs, which are discussed in detail in a separate article, will not be analyzed here.

Structural measures

To control floods, in riverbanks and channel improvements, they increase the capacity of the river, increase its flow speed, or achieve both effects, simultaneously. Modifications to the channel that can be made are: dredging it to make it wider or deeper, clearing vegetation or other debris, leveling the bed or walls, or straightening it; All of this helps increase the speed of water passing through the system, and prevent flooding. By straightening the channel, eliminating meanders, the risk of water breaking the river bank on the outside of bends, where the current is faster and the level is higher, is reduced.

It may cause some negative environmental impacts. Any measure that increases the speed of the current will increase the water's ability to cause erosion. Erosion and sedimentation problems can occur on site, or downstream. By lining channels, all factors impeding flow are reduced or eliminated, but this practice produces aesthetic and environmental problems, including reduced groundwater turnover, and disrupts populations with the disposal of dredging waste. Canal improvement can alleviate problems caused by flooding in the treated area; However, the peaks will be higher downstream, and the problem has simply been transferred elsewhere.

Artificial banks, improved natural banks and dikes

They increase the capacity of the canal and keep all the water inside it, with the exception of floods that are extraordinarily high. However, like canal improvement measures, these structures tend to transfer the problem to downstream areas, which in turn suffer, or public funds need to be invested to implement urgent flood control measures in those areas. Levees that are built on alluvial terrain, with the purpose of excluding water from certain areas, intervene in the hydrology of the area, and can have impacts on the habitat and movement of fauna and livestock.

Overflow depressions are usually swamps between the river banks and the sides of the valley. Likewise, it is possible to build artificial depressions for the waters. Detention depressions or small reservoirs are often used near urban areas to intercept and collect flow before it reaches the river; They are also effective in reducing peak flows. The depressions have a positive effect on replenishing the aquifers, and allow suspended sediments that would otherwise flow into the channel to settle. However, they can become habitats for disease vectors.

Relief channels

Also called high-flow diversions or spillways, they are natural or artificial diversion channels, or conduits that send water away from urban centers, or high-density populated areas. Further downstream, the water can be returned to the river where it originated.

Flood control structures are expensive. Additionally, they give a false sense of security, because the agent thinks that the risk of flooding has been eliminated, rather than just reduced. This can encourage them to increase floodplain development, causing disastrous results in the event of unusually high flooding, or failure of control structures. Each protection structure is calculated for a certain flow rate, which corresponds to an average return time (T).

In addition to the impacts of structures on the environment, environmental factors that influence flood control must be considered. Infrastructure or other works in the floodplain not only mean risks for the people there (depending on their vulnerability), but also increase the risk for neighbors and other people who live in downstream communities. Buildings, for example, by obstructing the flow of water, can increase the height and speed of flooding, reduce the storage capacity of the floodplain, and increase flow.

Natural events, such as fires, windstorms, or changes in river course, influence flooding. Human activities in the watershed, such as tree cutting or clearing for agriculture, generally increase the flow, as does the plowing of sloping land without implementing adequate terraces, or contour furrowing. By waterproofing the land of the hydrographic basin and the alluvial zone, the flow and volume of the floods will increase; installing storm sewer systems will increase flow; Installing storm sewer systems will increase the amount and speed of stormwater entering the river system.