Kiyosi Kawanisi

Given the changing climate, understanding the recent variability in large-scale rainfall patterns is a crucial task in order to better understand the underlying hydrological processes that occur within a watershed. This study aims to investigate how rainfall events in western Japan have changed due to climate change and how these changes have affected runoff–turbidity dynamics during the rainy season. To address the research objectives, we analyzed two decades of precipitation records in the Gōno River watershed and examined the associated runoff–turbidity dynamics during floods using turbidity–discharge (T-Q) loops, quantified using an enhanced hysteresis index. Our findings revealed a kind of intense rainfall event occurring every 3 to 4 years. Additionally, spatial pentad analysis showed varying intensities of accumulated precipitation, indicating that extreme rainfall is not confined to a specific spatial zone. Regarding turbidity–discharge behavior, we found that clockwise hysteresis patterns were caused by sediment sources from near-channel areas, while anticlockwise patterns were caused by soil erosion from nearby areas. Another notable finding was that turbidity peaks during floods may represent the earlier (or later) arrival of turbid water from distant upstream sources due to intense precipitation. One of the key challenges in quantifying hysteresis patterns is that there is no agreed-upon definition for how to determine the start and end of a flood event. This can lead to bias in the quantification of these patterns.

A recent study revealed that the amount of rainfall on the Kapuas River has increased over the last 30 years. The increase in rainfall increases the possibility of high discharge events, which might lead to destructive flooding of the Kapuas River and its tributaries. Hence, the ability to characterise the pattern of high discharge events is compulsory for the development and management of the Kapuas River watershed. The main objective of this study was to assess and characterise flood patterns in the Kapuas River watershed. To achieve this objective, we utilised information and complexity measures that consisted of mean information gain ( MIG), effective measure complexity ( EMC) and fluctuation complexity ( FC) in daily water level records from 2002 to 2011 from a gauging station in Sanggau, West Kalimantan Province. The results revealed that flood events in the Kapuas River were mainly generated by the Indo-Australian monsoon, which occurred from December to March. The anomaly in 2010, when intense flood events were observed during the dry season, can be identified as the effect of a strong negative El Niño-Southern Oscillation (ENSO). Additionally, the analysis of the information and complexity measures indicates that: (i) EMC, which reflects the length of flood events, tends to increase along with greater discharge, and (ii) MIG and FC, which denote the degree of randomness and fluctuation of flood events, respectively, tended to have higher values when the number of months without high discharge was less.

Understanding the streamflow hydrodynamics is important for forecasting flooding phenomena in mountainous rivers. Accordingly, based on long‐term runoff observations obtained using diverse runoff techniques (i.e., rating curves and fluvial acoustic tomography (FAT)), the authors (1) evaluated and explored flood phenomena, (2) discussed the effects of dams on flood hydrograph, (3) and revealed backwater effects caused by river channel bends, sand bars, and floodplains. The duration and slope of the falling limb of the hydrograph were directly influenced by the catchment characteristics that affected runoff recession, whereas the behaviour of the rising limb of the hydrograph was directly influenced by rainfall. A weaker correlation between discharge and rainfall in higher flood events indicates that rainfall events were strongly altered by water released from dams in the basin. Additionally, because of the backwater effects, the rate at which the flow velocity and boundary shear stress changed with the increasing discharge was reduced, potentially slowing the pace of erosion. Our findings are relevant for managing mountain rivers, particularly in river bend reaches prone to backwater effects. The FAT method could help increase the ability to monitor short‐term fluctuations in floods and associated hazards.

Advanced and precise acquisition of river flow in remote regions such as mountain rivers is essential for developing, correcting, and improving the current hydrological and physical models. The primary objective of this study is to shed light on the applications of travel time principle using the fluvial acoustics tomography system (FAT) as an advanced tomographic technology to evaluate river temperature, velocity, and discharge. Accordingly, a pair of the FAT system was installed on both riverbanks, then mutual transmission of acoustic signals between the stations was conducted to estimate the sound speed and thus to measure both river velocity, and flowrate. In addition, stream mean temperature was estimated using sound speed. To validate the precision of our measurement, comparisons with other data were conducted. Generally, this study reveals a promising and advanced system able to deliver accurate records of streamflow dynamics.

Investigating subtidal friction and mass transport is pivotal for examining subtidal dynamics in tidal rivers. Although the behavior of subtidal friction and transport has been discussed in recent years, most studies have been conducted on tidal rivers that are affected by high amounts of river runoff. The aim of this study is to offer an initial understanding of the spatial and temporal behaviors of subtidal friction and subtidal flux in a tidal river channel with limited river runoff. This study utilized the frequency domain and theoretical decomposition analyses to determine the dominant tidal and subtidal mechanisms. Frequency domain analysis indicated the dominance of semidiurnal and diurnal tides in the observed tidal river channel. The rate of energy transfer owing to shallow water interaction was found to be stronger for the current velocity than for the water elevation. Decomposition analysis showed that subtidal friction and flux in a low-discharge tidal river channel were largely influenced by subtidal flow-induced subtidal friction and Eulerian return flux, respectively. The key findings of this study are as follows: (i) the limited amount of river runoff (4–20 m³/s) leads to the vertical variability of subtidal friction contributions from subtidal flow and subtidal-tidal interaction, as well as Eulerian return flux, and (ii) the vertical variability of the aforementioned terms can be associated with the existence of influential longitudinal subtidal density gradients along the tidal river. We believe that these findings advance our understanding of subtidal dynamics in tidal river systems, particularly those with limited discharge.

A challenge in exploiting tidal energy resources is to determine the most suitable sites for development, and as the industry grows, to monitor the progressive impacts of tidal farms on the flow field and sediment transport regimes. A feasibility experiment was carried out with the fluvial acoustic tomography (FAT) system to assess the combined effects of irregular bathymetry, horizontal and vertical sound-speed gradients, and macroturbulence on the quality of FAT-estimated velocities. The experiment involved reciprocal transmissions at 30-s intervals for four days with two 7-kHz broadband transceivers. To determine the origin and potential interpretive value of the frequent multiple arrivals in the unstratified, highly-turbulent flow conditions, we performed a series of simulations with the Bellhop3D model. We demonstrate that the multiple arrivals mainly arise from out-of-plane sound propagation paths resulting from the effects of the local bathymetry. The maximum mean difference between FAT-estimated currents and the coincident moving-vessel acoustic Doppler current profiler (ADCP) transects is found to be less than 10%. Comparisons are also made between velocity spectra during the full flood and the ebb determined using the FAT velocities and the vertically-averaged velocities from a bottom-mounted ADCP deployed in a later experiment. The spectra exhibit similar shapes in the frequency and wave number overlap regions, indicating that FAT is able to detect macroturbulence. Good agreement is also obtained between the sound-speed estimates based on the travel time and those from in situ temperature and salinity data. Overall, the results demonstrate the potential of shallow acoustic tomography for monitoring environmental conditions in high-flow tidal channels.

Understanding inflow dynamics in a dam lake forms the basis for optimal dam operation and management practices. However, methods pertaining to adequately determining negative inflows and addressing them, as well as quantifying uncertainties in dam inflow, have been scarcely investigated. In this study, the inflow was observed using two pairs of fluvial acoustic tomography (FAT) systems placed diagonally in a dam lake, forming a crossed-shaped pattern. The “travel-time” principle is the primary approach for measuring the inflow by FAT. The novelty of this study is in discussing the inflow characteristics within a slow water-flow environment monitored by FAT. Based on the reciprocal sound transmission, we upgraded an equation to estimate the flow direction; this newly proposed generalized equation can be used in a fluctuating flow environment. We also discussed the sound propagation characteristics for slow flow velocities. Finally, we demonstrated that a small inaccuracy in the acoustic signal, even by a sub-millisecond, can cause significant errors in measurements. One of the novel findings of this study is the detection of internal waves using the improved flow direction equation and acoustic travel-time records. Overall, this study presents a promising approach for inflow measurements under extremely slow flow conditions.

Fluvial Acoustic Tomography (FAT) is a powerful hydroacoustic system used in the investigations of estuarine dynamics. This research was designed to explore the effectiveness of using a novel and promising method of monitoring the continuous salinity intrusion and velocity distribution in a tidal channel using the FAT scheme. Four FAT units were installed near the riverbanks using a new zigzag system in a rectangular tomographic domain of 700 m × 170 m. The water velocities and salinities measured by FAT in this study were comparable to the data captured by traditional methods, including moving-boat Stream–Pro ADCP, CTD, and CT sensors. It was found that the delays in time between the maximum salinity and high water level along the channel ranged from 12 min to 1.5 h, with these time lags increasing seaward, primarily due to changes in freshwater flows upstream. In addition, the longitudinal salinity gradient was found to decrease toward the mouth of the river, with tide-driven mechanisms most likely being dominant in the dispersion process. The estuary is ebb-dominant, with an asymmetry in the ebb-tide and flood-tide velocities and the highest velocities occurring during the ebb tide. Furthermore, the residual current was found to be affected primarily by the freshwater discharge from upstream.