Hydrological Impact of Typhoon on Rivers

Abstract

Rivers link terrestrial and marine ecosystems, not only transporting numerous substances downstream but also shaping landscapes and fostering aquatic ecosystems through physical interactions and biogeochemical processes with numerous agents. On the other hand, hydraulic facilities, such as reservoirs, hydropower plants, and banks are deployed to utilize water resources for sustaining human society. In the river network systems, rainstorms, as episodic/periodic strong triggers, can induce mass wasting from hillslopes, accelerating nutrient transport, which causes sequential effects. In recent decades, global warming has been accelerating water cycling via thermodynamics, and thus, the frequency and intensity of extreme rainstorms are increasing in intensity. In the West Pacific, typhoons (alias tropical cyclones in Asia) characterized by strong wind and torrential rainfall are evidenced to be getting stronger. The intensified typhoons inevitably stimulate the response of river systems through sediment and nutrient transport and threaten the safe operation of the hydraulic facilities and even coastal communities through storm surge flooding. These strong impacts on river systems should be comprehensively explored. This issue aims to improve the understanding of typhoon effects in river systems. Inter- and cross-disciplinary studies on different watershed scales, linking ecosystem services and watershed management, are particularly addressed.

1. Introduction

Rivers are not only important for wildlife but also vitally important for the human race. Humans still rely on rivers to support their livelihoods and economies. To serve as a water resource for humans, two-thirds of the longest rivers no longer flow freely owing to human-made constructions, such as dams and reservoirs, leading to the rapid decline of some of the most diverse, complex, and dynamic ecosystems on the planet [1]. Meanwhile, rivers act as pipes receiving terrestrial matter, particularly excess nitrogen and phosphorus, and discharging them into the oceans, surpassing the planetary boundary and threatening the sustainability of the Earth [2]. It has been found that human influence on climate has affected rivers in terms of the magnitude of low, mean, and high flows on a global scale [3]. In a drainage system, the associated hydrological processes not only transport numerous substances downstream but also shape landscapes through physical interactions and biogeochemical processes. Humans deploy hydraulic facilities, such as reservoirs, hydropower plants, and banks, to utilize water resources to support societal development. However, the fluctuation of extreme precipitation is a challenging issue for water resources management as well as hydraulic facilities. Meanwhile, extreme precipitation also shifts the flow regimes in watersheds and drainage systems, which regulate the dynamics of sediment and nutrient transport [4]. For example, typhoons, which bring strong wind and torrential rainfall within a short duration, are episodic, periodic, and strong triggers to induce mass wasting from hillslopes, accelerate riverine transport, and threaten the hydraulic facilities. This issue is becoming more challenging due to global warming, which is likely accelerating the water cycle and intensifying the typhoon magnitude. In this regard, understanding the typhoon-induced impacts on watersheds and drainage systems will enable the conservation of soil and water resources, ecosystem services, and avoid damage to hydraulic facilities. These strong impacts on river systems should be comprehensively explored. This issue aims to improve the understanding of typhoon effects in river systems. Inter- and cross-disciplinary studies on different watershed scales, linking ecosystem services and watershed management are particularly addressed.

2. Summary of This Special Issue

In this Special Issue, we attempted to discuss and address the typhoon impacts on watersheds and drainage systems. This issue is becoming more challenging not only due to the intensification of typhoons but also the increasing population and societal infrastructure. The more population and societal infrastructure, the more vulnerable hydraulic facilities are. Obviously, there are many impacts from different aspects as typhoon events. We tried to collect the studies which can raise the typhoon-induced impacts on rivers from different dimensions. After collecting manuscripts written during 2020, we published nine papers in this special issue. In this issue, the studies are from Taiwan, China, Korea, and the Philippines, where typhoons are the main force to trigger catastrophic impacts. We classified the studies into two main dimensions: one is sediment-associated, and the other is nutrient-associated impacts. Those studies elucidate how running water regulates aquatic ecosystems and hydraulic facilities and how typhoons impact the rivers from the two aspects. The simulation of the impacts, design of mitigation strategies, and responses of aquatic ecosystems associated with nutrient transport comprehensively demonstrate typhoon impacts in this issue.

For sediment-associated impact, sediment transport from rivers to the oceans is a big issue in soil water resources management and the biogeochemical processes [5]. Basically, the sediments are generated from hillslope via surficial soil erosion, landslides, and resuspension in water bodies (e.g., rivers, lakes, and reservoirs). Surficial soil erosion degrades the hillslope agro-ecosystems severely, and landslides result in massive mass wasting, which is a risk to mountainous villages and reservoir siltation. In this special issue, Teng et al. 2020 [6] demonstrated that landslides prefer reactivating on old scars, and the landslide-triggered sediment is a function of rainfall intensity and landslide reoccurrence. On the other hand, sediment transport is a function of stream power (e.g., water discharge) and sediment amount. Their coupled model, which simulated the sediment transport during typhoon periods, provides a basis for estimating sediment export under climate change scenarios. Liu et al. 2019 [7] investigated the long-term sediment load in the Jinsha River Basin, which is upstream of Three Gorges Reservoir. They found that even under the same climates, the sediment loads are quite different due to human–landscape interactions. The generated sediment deposited in the local reservoirs effectively hinders the sediment delivery downstream. Chiu et al. 2019 [8] applied a hydrodynamic and sediment transport module to estimate sediment yields and identify the unstable stream reaches in a large-scale watershed. They found that the considerable sediment volume, which remains at the foot of the slope near streams, becomes a potential source for sediment hazards in the future. For the exposure of coastal areas to storm surge flooding in the Philippines, Dasalla et al. 2019 [9] used topographical analysis of the inundated regions to show the effects of coastal shape, elevation, and position relative to the typhoon’s approach angle on storm surge flow depth and velocity.

For nutrient-associated impacts, the nutrient export is strongly affected by the source locality and pathway condition (reaction condition in rivers). Chang et al. 2020 and Lee et al. 2020 [10,11] attempted to use the nutrient budget and statistical analysis to identify the source locality of different nutrients. Their results showed that agricultural land and atmospheric input are the main sources of nutrients. The agricultural land cover is the major factor, while the input via precipitation plays a relatively minor role. This nutrient budget approach provides a preliminary assessment to evaluate the impacts of agriculture and atmospheric deposition on nutrient export. Except for source locality, the pathway condition relevant to water residence time and local environmental setting, has a great influence on the interaction of the bacterial community and nutrients. Kim et al. 2019 [12] presented a great case on this topic. The locality of the weirs, even along the same river continuum, evolve various habitats of bacterial communities. Coincidentally, Ryu et al. 2020 [13] demonstrated that the heterogeneity of water quality (or habitats) in reservoirs is governed by water circulations. However, the density flow regimes can alter the spatiotemporal distribution of dissolved and particulate nutrients and organics. They found stagnant areas with long residence times corresponding with areas of observed algal blooms and hypoxia. The understanding of water quality or biologic processes should be based on source locality and pathway conditions. Baek et al. 2019 [14] used an effective approach to find sources of DOM (dissolved organic matter) in rivers by coupling the SWAT (Soil and Water Assessment Tool) model with organic matter characterization and concluded that lateral flow transported hydrophobic and large-molecule organic matter after rainfall.

This issue demonstrates that extreme rainstorms impact the rivers via many aspects, and those periodic and episodic strong pulses, in fact, occur in watersheds and river systems where humans and landscape interact. There is still much work to be done in this field. Only through multi- and trans-disciplinary discussion can we improve the understanding of river resilience and strive towards sustainability. We hope this issue can stimulate more comprehensive and incisive discussions and promote more collaborations across disciplines and countries.