Mohamad Basel Al Sawaf

Recent decades have seen a dramatic increase in destructive rainfall events, leading to concerns regarding the uniqueness and severity of these events. The literature does not provide an accepted definition for “patterns” for assessing rainfall severity. Hence, the main challenge lies in utilizing flexible patterns that can effectively reflect variations within annual rainfall datasets and suitable for diverse seasons and climate contexts. This study aimed to address the severity of annual rainfall records and investigate how they vary between lenient and intense years in Wajima City, Japan. For this, a clustering-based framework was proposed. The framework comprises two key approaches: (i) pentad precipitation integrated with hierarchical clustering, and (ii) information-complexity analysis integrated with partitional clustering. Pentad analysis could provide a comprehensive image of key patterns at sub-monthly scales. As a result, it was found that heavy-rainfall incidents were intense mainly during the autumn rain front. Importantly, this study offered an innovative application of information and complexity analysis, which may provide a new perspective for the assessment of hydrological systems and diverse environmental systems. The findings demonstrated that information and complexity metrics can be used as deductive indicators to capture hidden patterns in annual rainfall incidents, such as the frequency and persistence of major rainfall events. As the mean information gain increased, the number of days that exceeded the daily rainfall threshold increased, and vice versa. In addition, a higher effective complexity was associated with more prolonged rain incidents. Overall, the findings provide an important reference for rainfall assessments.

The recent improvements in streamflow measurement approaches have boosted the reliability and accuracy of river flow measurement. In this study, long-term measurements of river discharge in the Tokoro River, Japan, were conducted. The key objective of this work is to investigate the extent of river flow measurement in a very shallow and narrow silt stream using the fluvial acoustic tomography system (FAT). Despite the preliminary nature of the measurement results, the recorded data were subject to analysis from three different outlooks. First, examinations were performed under very shallow and high-water conditions. Second, examinations were performed using double acoustic frequency. Third, examinations were performed using multiple independent flow datasets. As a new achievement in terms of advanced monitoring capabilities, it was documented that the measurement by the FAT was possible even in extremely shallow conditions. However, the minimum water depth along the measured cross-section must be ≥9 cm. Moreover, the FAT system demonstrated its capability to monitor streamflow in high water levels. In addition, it was found that using high transmission frequency can provide shorter wavelengths, permitting better spatial resolution and higher velocity resolutions and hence desirable measurement accuracy. Nevertheless, measurements in the presence of high suspended sediment particles were lacking. Alternatively, a lower transmission frequency offers a longer wavelength, which might be less sensitive to small-scale variations and results in an imprecise degree of measurements. Nonetheless, measurements can be accomplished even during the mobilization of a high concentration of suspended sediment matters. Finally, using multiple independent streamflow measurement records, the results proved that the flow measured by the FAT system was in very good agreement with the records acquired using sophisticated measurement approaches such as HADCP and STIV with a very low range of uncertainty.

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 recent variability in massive rainfall patterns is essential to provide a clear image about underlying hydrological processes that occur over a watershed scale. The main objective of this study is to investigate the variability of rainfall events in Western Japan as a result of climate change and examine the related runoff-turbidity dynamics during rainy season. To fulfill the research objectives, we investigated the changeability of the precipitation records in the Gōno River watershed for the last two decades and the related runoff-turbidity behavior during floods using the turbidity-discharge (T-Q) loops and quantified using an improved hysteresis index. The findings revealed a kind of intense rainfall periodicity of 3 ~ 4 years. In addition, spatial pentads analysis exhibited various intensities of accumulated precipitation suggesting that there is no specific spatial zone of extreme rainfall. Regarding the turbidity-discharge behavior, it was found that clockwise hysteresis patterns were induced by sediment sources from near channel areas, alternatively, anticlockwise pattens were produced due to soil erodibility from the nearby areas. Another interesting finding was a notable behavior of turbidity during floods such as “bursts” or “local peaks” may represent an earlier (or later) arrival of turbid waters from nearby distant sources at an upstream section as a result of an intense precipitation. One of the fundamental challenges in the quantification of hysteresis patterns is that there is no accepted definition on how to determine the start and the end of a flood event which may led to a bias in the quantification of these patterns.

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.

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.