Dredging is not the best long-term and economically-viable solution for Karun River in southwest Khuzestan Province against floods, head of the Faculty of Water Science Engineering at the Shahid Chamran University of Ahvaz said.
“Dredging helps improve the flow of floodwater into rivers. When it comes to Karun, this operation will cost too much and not produce the desired results,” ILNA quoted Mehdi Qomeshi as saying.
“For a big river like Karun, dredging will cost close to $53.5 million, but will have very little effect on reducing the volume of water (during flooding). Moreover, the riverbed will return to its previous status within three years,” he added.
Dredging refers to increasing the depth and width of a river channel by removing silt and sediment from the bottom and on the sides of rivers accumulated over years.
It may be effective in some watercourses by enabling them to hold more water and in turn reduce the risk of flooding. “But the dredged channels need regular and long-term maintenance as new silt piles up within the riverbed over time.
Karun is Iran's largest and only navigable river stretching over 950 kilometers. It begins in the Zard Kuh Mountains in the Zagros range, has many tributaries, such as Dez and Kuhrang, before passing through Ahvaz, capital of Khuzestan and finally emptying into Arvand Rud.
Rains unseen over the past six decades hit across the country from March 19 and caused torrential floods in 24 provinces.
Thousands of homes were washed away along with millions of acres of farmlands as rivers, including Karun, overflowed and burst their banks.
The primary aim of most dredging activities is to reduce flooding and act as a flood management tool. However, having a precise understanding of the extent to which dredging can mitigate the flood risk and flooding is rather intricate because of differences between location and river system hydrology and ecology.
Flood Management
To better understand the effect of dredging in flood management, the advantages and disadvantages of dredging have been assessed by the UK Environment Agency using six pilot study sites.
In some cases, dredging did reduce the water levels of rivers, but this did not lead to reductions in flooding risk during extreme flows because other features, such as infrastructure, upon the floodplain increasingly affect water levels rather than the capacity or shape of the river channel itself. In some cases, dredging actually increased the flood risk further downstream.
Qomeshi has a similar opinion about Karun. “Even if we increase the depth of Karun by 5 to 6 meters (by dredging) , it will not reduce the volume of water passing through Ahvaz,” he was quoted as saying.
Those in favor of dredging presume that a bigger river will allow water to flow faster and more efficiently, but it would not carry enough water when compared to the size of the floodplain.
Even if the capacity of a river is increased by 50%, it is relatively a small amount compared to the scale of flooding in the area.
Therefore, using natural flood management techniques should be a focus of flood mitigation measures, and dredging should not be considered as the best solution to contain the floodwaters.
Creating upstream holding areas for water, restoring wetlands and putting up more flood barriers are ways to slow the flow of water that can prevent floods from racing downstream.
