| Environment Dam good benefits


Leaky barriers are being used to complement hard engineering flood defences in the UK, while a new Egyptian study underscores the importance of effective dam locations for preventing flash floods and minimising loss of life


ENGINEERED LEAKY BARRIERS ARE increasingly being used as natural flood management methods to complement hard engineering flood defences, new research has found. Whereas numerical modelling based on multiple assumptions had previously been used to measure their impact, a recent study led by scientists from Cardiff University and the University of Worcester in the UK has used field monitoring case studies to gather real data from leaky barriers. Human-made leaky barriers are made from trees, branches, logs, and leaves and designed to imitate beaver dams. They work in units of 50-100, deliberately raising water levels upstream to slow down river flow through storage and diversion, providing ecological benefits to the river corridor and on nearby farmland. For just over two years, the Cardiff and Worcester


team assessed 105 leaky barriers situated over a distance of 5km, at a natural flood management site on a Shropshire river, in order to understand how they operate when overtopped by flood waters. Published in the Journal of Hydrology, their study found that water levels increased by up to 0.8m at each barrier with an additional 100m3


of storage


behind each one. This meant that ten million litres of water were prevented from surging downstream, protecting communities from flooding as it was slowly released over a week or so.


Dr Catherine Wilson, a Reader at Cardiff University’s School of Engineering and one of the study’s authors, said: “In recent years, we’ve seen a significant increase in flood risk across the UK and internationally due to greater storm intensity and other climate change- related factors. Where flooding does occur, we often see extreme human and socio-economic cost. And so, it’s vital that we better understand how to combat these events in the most effective way possible.” For the first time, the study was able to provide quantifiable in-depth evidence of the effectiveness of nature-based solutions in tackling flood events. The leaky barriers were found to be most effective in narrow channels with steep banks and better at reducing flooding during smaller storm events than larger ones. The research team concluded that leaky barriers


are a low-cost and sustainable addition to existing flood management strategies, helping to increases biodiversity in rivers and on nearby land. They are continuing to monitor the effectiveness of leaky barriers at the Shropshire site, which is one of 60 identified by the Department for Environment, Food and Rural Affairs to evaluate natural flood management. Furthermore, the findings can be used by government and industry to develop flood defences for smaller, more frequent storms, and help


create an approach for modelling leaky barriers for larger storms as well.


Location, location Recent research has also offered insight into the


best dam locations for preventing flash floods and minimise loss of life. The study, published in the Journal of Hydrology: Regional Studies, was conducted by Dr. Ashraf Ahmed, a flood expert and chartered engineer from Brunel University London, in collaboration with researchers from Benha University and Zagazig University in Egypt. In the wake of the Libya dam failures during September 2023, that were triggered by the aftermath of Storm Daniel which led to widespread destruction, loss of livelihoods, thousands of fatalities, and pushed Libya into a state of extreme emergency, the research is shedding light on essential considerations for dam designers and decision-makers when selecting dam sites and constructing them. Focusing on Wadi Tayyiba in South Sinai, Egypt which is prone to flooding, the study explained how floods have impacted the region and emphasised the importance of developing dams with sub-basins to address such challenges. Rainfall data, soil type assessments, and computer simulations were used to model the rainfall-runoff relationship in Wadi Tayyibah over a 100-year period and the outcome was a comprehensive flood hazard map and the identification of cost-effective dam locations. The study also highlighted the significance of assessing the geological composition of basins and the presence of slopes which can intensify water outflows and flood risks, along with the need to evaluate the impact of highways that traverse an area as they are susceptible to surface runoff. With the recommendation of constructing eight dams in Wadi Tayyibah, dividing the area into four sub-basins alongside a storage lake, the study also highlighted the importance of choosing dam locations that align with the natural landscape, emphasising the need for dams to be situated between two high points and near a suitable lake or reservoir with adequate storage capacity.


It is hoped that these findings will offer valuable insights for decision-makers worldwide, aiding them in selecting optimal locations for storage dams. Beyond mitigating flash floods and safeguarding lives, the study underscores the economic benefits of strategically placed dams, which can store water for future use. Looking to the future, the researchers envision


the integration of artificial intelligence techniques to forecast dam locations for rainfall harvesting and flash flood mitigation, further advancing the field of dam construction and management.


Above: Human-made leaky barriers are designed to imitate beaver dams


Below: Aftermath of deadly flooding in Libya


References


www.cardiff.ac.uk/news/ view/2735456-beaver-like- dams-can-enhance-existing- flood-management-strategies- for-at-risk-communities,- study-finds


www.sciencedirect. com/science/article/pii/ S0022169423006868 ?via%3Dihub


Ibrahim, Amir & Al Zayed, Islam & Abdelhaleem, Fahmy & Afify, Mahmoud & Ahmed, Ashraf & Abd-Elaty, Ismail. (2023). Identifying cost-effective locations of storage dams for rainfall harvesting and flash flood mitigation in arid and semi-arid regions. 50. 101526. 10.1016/j. ejrh.2023.101526


www.waterpowermagazine.com | January 2024 | 15


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