Research on River Engineering

1. Introduction

River engineering is one of the most important subjects of hydraulic engineering. The main scientific fields necessary for understanding the basic principles of river engineering, include hydrology, hydraulics, and geomorphology.

Using hydrologic rainfall–runoff models, the river inflows originating from rainfall-induced overland flow can be calculated. River floods have to be routed during intense storms. Flood routing can be calculated using hydrologic or hydraulic models. Hydraulic models are based on water mass and momentum conservation equations, which are hyperbolic-type partial differential equations solved using numeric methods, e.g., finite difference schemes.

Soil erosion products, due to rainfall on the surrounding basins, are transported by the overland flow into the rivers and constitute the so-called wash load. The river bed can be eroded by the river flow, or suspended sediment can be deposited onto the river bed. Numerous computational models for the bed load and the total load have been developed in the past. To take into account sediment transport in rivers, the sediment continuity equation should be added to the water mass and momentum conservation equations. Sediment transport is mainly influenced by unsteady turbulent flows, which are normal physical condition in rivers. The vegetation on river banks also influences river flow and sediment transport.

The hydrologic and geomorphologic conditions in reservoirs and lakes are different from those in rivers. Generally, hydraulic structures, e.g., dams, modify the hydraulic and geomorphologic conditions in rivers.

2. Overview of the Topic “Research on River Engineering”

The above topic includes thirteen articles submitted to Water and one article submitted to Hydrology. The article authors are active at universities, research institutes, and water management authorities from eleven different countries: Bangladesh, Canada, China, Egypt, France, Iran, Italy, Japan, Mexico, Peru, and the USA. One article resulted from the cooperation of three universities in Iran, Italy, and the USA, respectively, while another article resulted from the cooperation of a university and a technical bureau in the USA and a research institute and a water management authority in Bangladesh. Generally, most authors are active at universities.

The fourteen articles can be divided into three categories: Category A: “Hydraulic geometry, bed roughness and sediment particle size of rivers”; Category B: “Hydraulic structures”; Category C: “Water quality recovery”. Articles [1,2,3,4,5,6] belong to Category A, articles [7,8,9,10,11,12] belong to Category B, and articles [13,14] belong to Category C.

The contents of the articles of Category A are summarized below:

In the article by Qin et al. [2], the well-known empirical relationships of regime theory, in the form of a power law in alluvial channels, that express river width, average flow depth and flow velocity as functions of discharge, were determined for six major exorheic rivers located in the Qinghai–Tibet Plateau (China). The included equations were also modified so that the discharge frequency was incorporated into these relationships.

The study by Takata et al. [5] attempted to back-calculate Manning’s roughness coefficients by repeating a two-dimensional flow simulation to fit the spatially and temporally dense river water level data observed in Japan’s Yamatsuki River, a typical mountainous river with an average riverbed gradient of 1/50 and an average river width of 17.9 m. The software Nays2DH was used for the river flow calculations. The flow field calculation model used a general curvilinear coordinate system, which allowed for complex boundaries and riverbed topography to be directly considered. The roughness coefficient during flooding was found to be correlated with the channel slope and step height (H)–step length (L)–channel slope (S) ratios (H/L/S), and a corresponding regression equation was proposed.

The study by Gabr et al. [3] assessed the maintenance condition of the main surface drains (Baloza and Elfarama) located in the Tina Plain (50,000 acres) and a portion of the Southeast Elkantara region (25,000 acres) in North Sinai, Egypt, based on the values of the Discharge Capacity Ratio (DCR) and Manning’s roughness. DCR is defined as the ratio of actual discharge to the projected or design discharge.

The study by Dehkordi et al. [6] developed an empirical relation capable of estimating the median sediment particle size in gravel river bends. Field data were collected from different cross-sections placed at the bend apex and crossovers in various rivers (Iran). The Buckingham π-theorem was applied to identify the effective dimensionless numbers, such as the Shields number, Reynolds particle number, Froude number, submerged specific gravity of sediment, and aspect and curvature ratios. Three regression techniques, containing the power function approach, the general additive model (GAM), and the multivariate adaptive regression spline (MARS), were chosen to achieve the best relation between the above-given dimensionless variables. It was found that two parameters, the curvature ratio and the Shields number, were the most important in affecting the median size of bed sediment at the bends of meandering rivers.

The study by Sarker et al. [1] provided detailed and quantitative insights into the properties of planform complexity and dynamics of channel patterns. This was achieved by investigating the applicability of anastomosing classification on the Brahmaputra River’s planform (Bangladesh) and computing the disorder/unpredictability and complexity of fluctuations using the notion of entropy and uniformity of energy conversion rate by the channels using a power spectral density approach.

In the article by Yi et al. [4], the effects of flooding characteristics, namely flooding depth and flooding duration, on riparian plants were studied in the case of Three Gorges Reservoir (China). The results of this study show that the riparian plant diversity and functional diversity varied by season. A significant negative relationship between plant diversity and flooding depth was observed, while the flooding duration was not a significant predictor in different seasons.

The contents of the Category B articles are reported in the following paragraphs:

According to the article by Cabarello et al. [7], the maximum scour that occurred at the Carrizal River hydraulic control structure (Tabasco, Mexico) was assessed experimentally and numerically. The physical model was built at a scale of 1:60 (without distortion) at the Engineering Institute of the National Autonomous University of Mexico. The maximum experimental scour was compared to the results of a 2D free surface numerical model and to the estimations of four empirical equations: Breusers, Farhoudi and Smith, Negm, and Dietz. The numerical model consisted of a mass conservation equation for water, Saint-Venant equations, a mass conservation equation for sediment (Exner equation) regarding bed load transport, and an advection–diffusion equation regarding suspended load transport. Only the Breusers method provided values close to the measured values.

In the study by Zhang et al. [8], a quasi-stump group structure was proposed and placed upstream of the bridge piers to mitigate the scour of water flow on the riverbed. Both experimental and numerical simulations using FLOW-3D were employed to study the protective effect of this structure. The experiments were carried out in the hydraulic laboratory at Zhengzhou University’s School of Water Conservancy and Civil Engineering (China). The numerical model consists of the continuity equation for turbulent flow and the Reynolds-averaged Navier–Stokes equations. FLOW-3D uses the RNG (Renormalization Group) k-ε turbulence model to solve the latter equations. A separate equation, including the critical shear stress, was employed to calculate the volumetric bed load transport rate. The numerical results were in good agreement with the experimental findings. It was found that the quasi-stumps group could effectively reduce the flow velocities around the bridge piers, thereby promoting the deposition of suspended sediment.

The study by Tu et al. [10] adopted a flume model experiment to investigate the scouring and deposition around geotube groins and mixed clay–geotube groins. The motivation of the study was the fact that during the cofferdam construction of the toe reinforcement project at the Qiantang River estuary (China), the scouring of the riverbed at the groin head often led to the collapse of geotube groins due to strong tidal currents. Results indicated that the influence of tidal surges on geomorphic changes surrounding the groins was more pronounced during spring tides than neap tides. Under the same flow conditions, the scour depth at the head of the geotube groin was notably deeper than that of the mixed clay–geotube groin. Additionally, sediment silting behind the mixed clay–geotube groin was significantly greater than that behind the geotube groin.

The study by Girolami et al. [9] focused on the mechanisms that trigger erosion of the pervious foundation of flood protection dikes. The origin of these permeable areas is generally attributed to the presence of a paleo-valley and paleo-channels filled with gravelly sandy sediments beneath the riverbed and dikes. These layers may extend into the protected area. The possibility of internal erosion must also be considered as the cause of leaks, sand boils, and sinkholes. Two classical geophysical methods (Electromagnetic Induction and Electric Tomography) were applied to Agly dikes (France) for the geological observation of the foundation soils. A three-dimensional finite element numerical model for internal flows was used for quantifying the soil permeability.

The study of Chenari et al. [11] investigated the problem of low efficiency and lack of water supply at the Hemmat Water Intake, Iran, where severe sediment accumulation was observed at the intake mouth. The FLOW-3D software was used to simulate the flow patterns under various scenarios of hydraulic regimentation works. The numerical model uses the Volume-of-Fluid (VOF) method on a gridded domain to solve the Navier–Stokes–Reynolds equations for three-dimensional analysis of incompressible flows. The considered parameters include the following: (i) three alternative locations of the spur dike; (ii) four spur dike lengths; and (iii) five spur dike deviation angles. Overall, the main flow of the river with the highest velocity and depth and best directed towards the water intake occurs for the placement of the longest spur dike in front of the inlet and for a spur dike deviation angle of 135 degrees.

In the study by Zhang et al. [12], flume experiments were conducted to study the upstream water level rise of submerged rock weirs. The flume was installed in the Key Laboratory of Hydraulic and Waterway Engineering of the Ministry of Education at Chongqing Jiaotong University (China). The main findings were as follows: The recorded water level along the flume showed that for submerged conditions, the rock weirs primarily rose the upstream water level while exerting minimal influence on the changing tail-water level. For a given tail-water depth and void ratio of rock weirs, the upstream water level rise increased with increased discharge. However, this response became insignificant as tail-water depth increased. Furthermore, as weir void ratio increased, the water level rise upstream of the weir was expected to decrease for a given discharge and tail-water depth. Based on the experimental data and observations, a predictor including the effects of Froude number before building the weir, weir submergence, and weir void ratio was proposed for estimating the water level rise upstream of I-shaped rock weirs for submerged conditions.

The contents of the Category C articles are described below:

In the article by Mori-Sánchez et al. [13], a two-dimensional numerical model (Iber model) was applied to the Lurin River to improve the water quality. Lurin is one of the main sources of water for the city of Lima (Peru). The main water quality parameters were Dissolved Oxygen (DO), Biochemical Oxygen Demand at 5 days (BOD₅), and Escherichia Coli (E. Coli). The Iber model consisted of two submodels: a hydrodynamic submodel (Saint-Venant equations) and a convection–diffusion submodel for each polluting substance. Using this model, the authors found where the greatest contamination occurred. It was proposed to improve the river by optimizing the San Bartolo WWTP (Waste Water Treatment Plant) and building a new WWTP in Pachacámac to avoid diffuse contamination.

The study by MacKenzie et al. [14] presented a novel methodology to evaluate the relative contributions of the stream corridor and upland watershed contributions to total sediment and phosphorus loads in receiving watercourses. The new method could be used to develop a cost-optimized mitigation plan for the impacted watersheds, including the implementation of Low-Impact Development (LID) for urban areas and Best Management Practices (BMPs) for rural areas of the watershed, as well as stream restoration projects for degraded stream sections. The estimation of stream banks and bed erosion was based on the concept of stream power. The new method was applied to the Tannery Creek watershed (Canada), which has experienced significant urbanization in recent decades.