# Shifting sands threaten flood-mitigation measures

Kristen Cook

► To cite this version:

Kristen Cook. Shifting sands threaten flood-mitigation measures. Nature, 2024, 636, pp.47-48.
10.1038/d41586-024-03773-9. hal-04927582

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<figure><img src="image_1.png" /></figure>
**News & views**

<figure><img src="image_2.png" /><figcaption>**Figure 1** | Flooding of the Nakkhu River in Lalitpur, Nepal. Record rainfall in late September caused extreme flooding of the Nakkhu River, a small river in the Kathmandu Valley. The water flowed over the embankments that are designed to mitigate the risk of flooding, seen here in the middle of the river.</figcaption></figure>

AMBIR TOLANG/NURPHOTO/GETTY

Environmental science

# Shifting sands threaten flood-mitigation measures

Kristen L. Cook

Climate change and urbanization are exacerbating the effects of flooding. A close look at a Nepalese river reveals that efforts to reduce the impact of floods must consider the geomorphic changes that occur during these extreme events.

In rapidly urbanizing areas, such as the Kathmandu Valley in Nepal, increasing population pressure is driving people to settle

on floodplains. In many of these regions, the pace of development is highest in areas that are most at risk of flooding¹. To make such

land available for urbanization, municipalities often turn to river engineering, building embankments that are designed to diminish this risk. But flooding is a multifaceted problem, and seemingly small factors can have dire consequences. Writing in *Water Resources Research*, Thapa et al.² modelled the effects of embankments on flooding of the Nakkhu River, a small river in the Kathmandu Valley, and found that sediment movement has a profound impact on the effectiveness of these protective embankments.

Floods are more than just water. The devastation in Spain that began last month was a grave reminder that floods can carry debris – both natural (trees and other plants) and human-made (vehicles, building materials and other detritus). Large pieces of debris can become jammed behind bridges, creating temporary dams that increase the water level, and can result in the sudden release of a wave
of water downstream³.

But floods can also alter the geometry of the channel by eroding the bed and banks of a river, damaging flood-control structures and potentially leading to embankment breaches. Upstream erosion leads to the downstream deposition of sediment, such as stones, sand, silt and mud, which raises the riverbed and decreases the channel's capacity⁴. This can occur gradually over time, or quickly during a single flood, causing the river to flow over its banks or walls at ever-lower thresholds as more sediment is deposited.

Members of the same team as Thapa *et al.* previously investigated the general impact of sediment on flood risk in the Kathmandu Valley⁵. They have now built on this work by modelling how sediment erosion and transport influence the effectiveness of the embankments that have been under construction along the river since 2017. Their results suggest that the embankments reduce flooding outside the channel, but that they are less impactful when sediment is included in the calculations.

For a high-magnitude flood—the kind that is expected to occur only once every 1,000 years—the authors predict that the inundated area increases fivefold when sediment is included in the modelling. In this scenario, embankments still provide some protection, in that they reduce the inundated area by 15%. However, a risk assessment must consider both flood hazard (the potential for flood) and flood exposure, which is the extent to which people and property would be affected by flooding. Overestimating the protection provided by flood-control measures can change the calculation of risk and encourage development in risky areas⁶.

The authors' findings highlight the importance of including sediment and river-channel change in calculations of flood risk⁴. Tragically, but perhaps not surprisingly, their predictions have already been put to the test. In late September, record rainfall hit central Nepal (Fig. 1). Stations in the Kathmandu basin recorded up to 323.5 millimetres of rain in 24 hours⁷, causing extreme flooding in many areas, including the Nakkhu River. The water flowed over the embankments, breaching them in at least one location, and inundating the floodplain. About 1,800 households in the district of Lalitpur were affected by the flooding of the Nakkhu and other rivers in the district⁸.

In the area in which Thapa *et al.* conducted their study, satellite imagery shows that around 95% of the affected structures were constructed in the past 15 years, suggesting that — although the probability of flooding might have been raised by climate change — the disaster was closely linked to a surge in building on the risky floodplains. I walked through the affected area shortly after the event and saw first hand that sediment had indeed had a

crucial role in exacerbating the impact of the flood. The deposition of sediment was evident, as was the reduced capacity in portions of the embanked channel, including the area just upstream from the catastrophic breach.

In the case of the Nakkhu River, it is clear that September's flood vastly exceeded the capacity of the embankments, which were designed to contain only moderate floods (the sort that have happened roughly once every 25 years). This highlights another challenge associated with designing flood-mitigation measures in the face of climate change: the floods of the past are not adequate models for the floods of the present and future. This applies not only to the hydrology, but also to the geomorphic component of a flood, because extreme events can bring substantial levels of sediment erosion and deposition⁹. The geomorphic changes that happen during an extreme event are often triggered by certain thresholds being crossed — for example, in the erosion of channel banks, the mobilization of coarse sediment or the occurrence of landslides. Such thresholds could well be difficult to anticipate if they have never previously been crossed.

Although Thapa *et al.* examined just one small river in a single city, their study serves as an example of the complexities involved in managing urban flood risk. As both the flood hazard and the exposure to floods increase

around the world¹⁰, a full analysis of both the hydrologic and the geomorphic processes of flooding is key to effective mitigation measures.

Kristen L. Cook is at the French National Research Institute for Sustainable Development (IRD), Marseille, 13000, France, and at the Institute of Earth Sciences (ISTerre), Grenoble Alpes University, 38058 Gieres, France.

e-mail: kristen.cook@univ-grenoble-alpes.fr

1. Rentschler, J. et al. *Nature* **622**, 87–92 (2023).

2. Thapa, S. et al. *Water Resour. Res.* **60**, e2024WR037742 (2024).

3. Martín-Vide, J. P. et al. *J. Hydrol. Reg. Stud.* **47**, 101348 (2023).

4. Vázquez-Tarrio, D. et al. *Geomorphol.* **446**, 108976 (2024).

5. Thapa, S. et al. *Earth Surf. Process. Landf.* **49**, 706–727 (2024).

6. Georgic, W. & Klaiber, H. A. *Land Econ.* **98**, 78–97 (2022).

7. Dept of Hydrology and Meteorology, Ministry of Energy, Water Resources and Irrigation. *Situational Report on Extreme Precipitation and Flooding Event of 27–29 September 2024* (Govt of Nepal, 2024).

8. https://www.bipad.gov.np/np/1354

9. DeLisle, C., Yanites, B. J., Chen, C.-Y., Shyu, J. B. H. & Rittenour, T. M. *Geology* **50**, 16–20 (2022).

10. Devitt, L., Neal, J., Coxon, G., Savage, J. & Wagener, T. *Nature Commun.* **14**, 2801 (2023).

The author declares no competing interests.
This article was published online on 26 November 2024.

Engineering

# Bird-inspired leg enables robots to jump into flight

Aimy A. Wissa

A bird's ability to transition swiftly from walking to flying has been emulated in the design of a simple robotic leg. The feat is a win for drone design, and one that could reveal why some birds' legs make up so large a fraction of their weight. See p.86

Many organisms can transition smoothly from one mode of locomotion to another, often by using the same body parts. For example, flying fish use their fins to steer underwater, and these same structures turn into wings when the fish begin to glide above the water's surface¹. Such versatility is an endless source of inspiration for engineers, but using the same locomotor modules to perform different movements can create trade-offs, because these manoeuvres often have opposing requirements².

On page 86, Shin *et al.*³ report the design of a bird-inspired robotic leg that has many locomotion modes – and can transition between

them with remarkable speed and efficiency.

Little is known about the evolutionary pressures and mechanistic trade-offs that govern transitions between different modes of locomotion from a biological perspective. Furthermore, some organisms that show multimodal locomotion are among the hardest to study, especially when they are transitioning from one mode to another – from walking to burrowing, for example, or from flying to swimming.

This gap in understanding has hampered efforts to emulate multimodal locomotion in engineered systems – a feature that would benefit robots in several settings, including