Effect of channel slope and roughness on hydraulic jump in open channel flow

An experimental study is conducted on hydraulic jump characteristics for understanding the impact of slope in an open channel flume. Hydraulic jump on different channel characteristics (horizontal smooth & rough and sloping smooth & rough) were analysed. The measured characteristics of hydraulic jump with different channel roughness and different slope were compared. The results showed that the sequent depth ratio (y2/y1 ) increases with the increase in Froude number (Fr1 ) for smooth horizontal bed and horizontal rough bed. It was also observed that with an increase in Fr1 , a decrease in y2/y1 in smooth sloping bed condition and decreases for rough sloping bed. New empirical relationships were also developed with the experimental data and results were found similar with the observed hydraulic characteristics data.

Morphologic signatures of autogenic waterfalls: A case study in the San Gabriel Mountains, California

Waterfalls can form due to external perturbation of river base level, lithologic heterogeneity, and internal feedbacks (i.e., autogenic dynamics). While waterfalls formed by lithologic heterogeneity and external perturbation are well documented, there is a lack of criteria with which to identify autogenic waterfalls, thereby limiting the ability to assess the influence of autogenic waterfalls on landscape evolution. We propose that autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and therefore form as a series of steps with spacing and height set primarily by channel slope. We identified 360 waterfalls split between a transient and steady-state portion of the San Gabriel Mountains in California, USA. Our results show that while waterfalls have different spatial distributions in the transient and steady-state landscapes, waterfalls in both landscapes tend to form at slopes >3%, coinciding with the onset of Froude supercritical flow, and the waterfall height to spacing ratio in both landscapes increases with slope, consistent with cyclic step theory and flume experiments. We suggest that in unglaciated mountain ranges with relatively uniform rock strength, individual waterfalls are predominately autogenic in origin, while the spatial distribution of waterfalls may be set by external perturbations.

Hydrotechnical arrangements influence on morphological and hydrological characteristics of Crişul Negru River and its tributaries - Western Apuseni Mountains, Romania

The anthropic interventions on riverbeds and banks have a direct and irreversible influence on the flow regime, primarily by changing the hydraulic characteristics of rivers, like channel slope and/or geometry, channel roughness, flow depth, consequently water velocity, propagation time of floods, etc. In the Upper Basin of the Crişul Negru River, the first hydro-technical arrangements were made in the 18th and 19th centuries, mainly for flood mitigation and swampy areas amelioration. But the most important and extensive works were designed following the exceptional floods of 1980-1981 (consolidations of banks, embankments, regularizations, bottom sills, etc.) and at the end of the 20th century. The purpose of the paper is to emphasize the effects of the hydro-technical works on both the morphology of the riverbed and the flow regime (average, minimum and maximum flow) of the most affected rivers in the study area. The data utilized are elevation profiles, stages and discharges recorded on more than 40 years at four gauging stations, namely including periods before and after the execution of the extensive works. The positive and negative influences of the arrangements are highlighted through statistical analysis and trend detection in data series, corroborated with the commissioning of the hydro-technical works and the evolution of rivers' morphology.

Laboratory Analysis of Debris Flow Characteristics and Berm Performance

In this study, laboratory tests were used to determine the deposition characteristics (runout distance, lateral width, and deposition area) of debris flow and their relationships with the flow characteristics (flow velocity and flow depth) according to the presence of a berm. An experimental flume 1.3 to 1.9 m long, 0.15 m wide, and 0.3 m high was employed to investigate the effects of channel slope and volumetric concentration of sediment with and without the berm. The runout distance (0.201–1.423 m), lateral width (0.045–0.519 m), and deposition area (0.008–0.519 m2) increased as the channel slope increased and as the volumetric concentration of sediment decreased. These quantities also increased with the flow velocity and flow depth. In addition, the maximum reductions in the runout distance, lateral width, and deposition area were 69.1%, 65.9%, and 93%, respectively, upon berm installation. The results of this study illustrate general debris flow characteristics according to berm installation; the reported relationship magnitudes are specific to the experimental conditions described herein. However, the results of this study contribute to the design of site-specific berms in the future by providing data describing the utility and function of berms in mitigating debris flow.

Investigation and simulation of flood inundation hazard in urban areas in Iran

Extensive impervious area and the man-made streams are the characteristics of urban areas. In recent years, rapid urbanization has led to change of rural areas into urban areas, and urban runoff will increase as the result of spread and growth of impervious areas. Land use changes, increasing urbanization, unauthorized construction, inefficiency of sewage system and increased impervious surface in urban areas have significant impacts on inundation hazard. Therefore, to manage urban areas and prioritize regions to inundation elimination problems, the area most affected by inundation should be determined. In this study, the Storm Water Management Model (SWMM) is used to simulate the rainfall-runoff in the study area. The simulated runoff in the SWMM model is used as input to the HEC-RAS model and determines inundation hazard zones in 5, 25 and 50 return periods. Then, six factors such as distance from the main channel, slope, land use, drainage density, the main channel slope and elevation were selected to determine inundation hazard map using Analytic Hierarchy Process (AHP). The results showed that the combined model (SWMM and HEC-RAS) was suitable to analyze urban inundation and determine inundation hazard zones on urban areas. Simulated results can be used to develop urban inundation hazard forecasts. In addition, the result of inundation hazard map indicates that 8.2% of the case study is determined as a high hazard zone.

Geomorphic effects of recurrent outburst superfloods in the Yigong River on the southeastern margin of Tibet

Landslide dam outburst floods have a significant impact on landform evolution in high mountainous areas. Historic landslide dams on the Yigong River, southeastern Tibet, generated two outburst superfloods > 105 m3/s in 1902 and 2000 AD. One of the slackwater deposits, which was newly found immediately downstream of the historic dams, has been dated to 7 ka BP. The one-dimensional backwater stepwise method gives an estimate of 225,000 m3/s for the peak flow related to the paleo-stage indicator of 7 ka BP. The recurrence of at least three large landslide dam impoundments and super-outburst floods at the exit of Yigong Lake during the Holocene greatly changed the morphology of the Yigong River. More than 0.26 billion m3 of sediment has been aggraded in the dammed lake while the landslide sediment doubles the channel slope behind the dam. Repeated landslide damming may be a persistent source of outburst floods and impede the upstream migration of river knickpoints in the southeastern margin of Tibet.

A study of the insulating and anti-frost heave effects of polystyrene boards under molded bag concrete conditions

In this study, the channel frost heaving actions in the Hetao irrigation area of Inner Mongolia were examined and a field in-situ test platform was established. Then, experimental investigations were conducted regarding the insulating and anti-freeze effects of polystyrene boards under the conditions of concrete bags with different thicknesses. In this study's experiments, concrete bags with different thicknesses were set, along with a test block of polystyrene boards with different thicknesses. The research results showed that by adding 2–5 cm molded bags, the total accumulated temperature increased in the range of 3.93–9.22% and the frost heave rate decreased by between 18.28 and 55.44% concrete, on the basis of 10 cm molded bag concrete. In addition, when 4–8 cm polystyrene boards were laid, the total accumulated temperature increased by 207.63–272.25%, and the frost heave rate decreased by between 71.43 and 96.6%. The absolute slope of the curve fitting of the frost heave rates and the soil temperatures decreased by 44.6–58.7%. HIGHLIGHT The results show that, the biggest bending moment of channel slope is in the 1/3 of canal slope, the biggest shear is in the toe of slope, the biggest bending moment of canal bottom is in the 1/2 of the Canal bottom. According to the judgment of Typical Channel, frost heaving damage will occur on the slope and bottom of the South Branch Canal without taking insulation measures.

Erosion Rate of Lateral Slope Deposit Under the Effects of Different Influencing Factors

Lateral slope deposits along a channel represent an important source of material for initiation and development of debris flows/floods that are typically observed in many headwater tributaries. This study found that the failure process of such a deposit reflects combined interaction between external hydrodynamic factors (inflow discharge and channel slope) and internal factors (compactness and fine particle content). The erosion process comprises two stages: runoff erosion toward the toe of the deposit body and soil failure owing to gravity. Spatially, the erosion rate is distributed unevenly across the deposit; the highest value occurs at the section close to the middle of the deposit, on the upstream face. Temporally, the erosion rate decreases exponentially. Overall, the average erosion rate decreases (increases) with bulk density (inflow discharge and channel slope). However, a slope of 7 is a threshold at which the tendency of the erosion rate in relation to the fine particle content differs. In comparison with the other three influencing factors, the effect of the fine particle content is much smaller. Although the bulk density of the deposit imposes the most significant effect, it is of the same order as that of both inflow discharge and channel slope. As the failure process can be summarized as repeated runoff scouring of the toe of the deposit, deposit failure, and entrainment of the failure body by runoff, we proposed a calculation method for the total time required for a complete lateral erosion process, and validation of the calculation suggested its reasonability. The findings of this study enhance the understanding of the mechanism of lateral soil deposit failure, which could help improve runoff-induced debris flood forecasting in headwater regions of mountainous catchments.