Accidents Related to the 2014 Rains and Their Socio-Economic Consequences in the City of Abidjan: The Case of the Municipalities of Abobo and Attécoubé(Côte D’Ivoire)

The urban environment is confronted with recurring calamities including floods, landslides and forced escapes of populations during periods of rain. In Côte d’Ivoire, the municipalities of Abobo and Attécoubé suffered enormous material and human damage following the torrential rains of 2014. These urbanized areas are built on large plateaus bordered by thalwegs where 61% of these areas have no drainage network, thus facing a lack of sanitation. Moreover, topographical, rainfall, anthropogenic and technical conditions are factors explaining the occurrence of natural disasters and their consequences in the said communes. The objective of this article is to assess the socio-economic impact of the occurrence of hazards. It focuses on the identification of risk sectors in the municipalities of Abobo and Attécoubé and the material, human and economic consequences that result from rain accidents.

Bed Forms and Sediment Characteristics along the Thalweg on the Tanana River near Nenana, Alaska, USA

Sediment sampling and longitudinal river-bottom surveys were conducted along the thalweg on the Tanana River near the city of Nenana, Alaska, USA, to provide basic information for the engineering design requirements of hydrokinetic devices to be deployed in the area. The study reach was located at approximately 64°33'50'N and 149°04'W. The Tanana is a large glacier-fed river, with open-water flow conditions from May to October. The river presents a single channel in the study area. Granulometric analyses of sediment moving near the riverbed reveals the coexistence of three distinctive types of sediment along the study reach: 1) nearly uniform fine sand;2) bimodal distributions containing fine sand and medium gravel;and 3) medium gravel. Preliminary relationships between sediment loads and discharge were developed. Dunes with small superimposed dunes were found along the reach. The basic geometric parameters (i.e., wavelength and height) of dunes were measured, and steepness was calculated. In general, dune wavelength increased with increasing discharge. Dune wavelengths ranged from 41 to 67 m, while small-dune wavelengths ranged from 13 to 16 m. Steepness increased slightly with increasing discharge.

Experimental Study on the Flow Characteristics around the Refraction Groyne

The refraction groyne is a complex structure consisted with groyne and groyne’s arm. This study conducted a experiment on the flow influences around the refraction groyne due to changes in the arm angle (θ) and length (AL). Results of experiment were analyzed on the impacts of the refraction groyne according to the projection length (L’). Velocity increase in main channel occurred greater the upward groynes than downward groynes. The vortices occurring at recirculation area of the upward and downward refraction groynes were formed in different shapes. The thalweg height did not have great impact vis-a-vis the extended arm length ratio and refraction angle change. The length of the recirculation area showed a gradual uptrend as the arm length of the groyne increased. Such area was formed at the range of 29% - 47%. For the length of the recirculation area, it was observed to be 10.2 - 14.7 times (URG), 8.4 - 12.7 times (DRG), and 10.6 - 13.8 times (right angle groyne) the projection length (L’) incensement.

Experimental Study on the Flow Characteristics of ￢-Type Groyne

A hydraulic model test for ￢-type groynes (with “￢” shape) was conducted to analyze the flow characteristics around these groynes. The results of the model tests are expected to be used as fundamental information in designing the ￢-type groyne constructed in the field. Main hydraulic factors such as velocity and thalweg line changes in the main channel and separation area were analyzed in this study. The thalweg line is a stream line where the maximum velocity occurs, whereas the separation area is the boundary between the main flow and the recirculation zone. Model tests with 5 different arm lengths of the ￢-type groynes were conducted by changing the velocity. The LSPIV (Large-Scale Particle Image Velocimetry) technique was used to measure and analyze flow variation around the ￢-type groynes. The velocity in the main channel measured to be increased by 1.5 times. The velocity variation on groyne arm length is little. The width of the thalweg lines (TCL) was changed to 55 - 57% of the channel width. The Froude number did not affect the thalweg line (TCL) and separation line (Sh) changes, however.

Sedimentary erosive processes and sediment dispersal in Kaoping submarine canyon

The Kaoping submarine canyon, connected to the Kaoping River in the coastal plain in SW Taiwan, continues the dispersal path of modern Kaoping River sediments, from an active small mountainous drain basin to the receiving basin of the South China Sea. Using seismic reflection sections, Chirp sonar profiles, and bathymetric mapping, we reveal characteristic erosive processes responsible for multiple cut-and-fill features, deeply entrenched thalweg, and sediment dispersal that are closely related to turbidity currents in the canyon. The river-canyon connection setting, along with extreme climatic conditions and active tectonism, is favorable for generation of turbidity currents at the canyon head. The upper reach of the Kaoping Canyon is distinguished into three distinct morpho/sedimentary features. The canyon head is characterized by V-shaped axial thalweg erosion. The sinuous segment of the upper reach is dominated by a deeply incised canyon pathway with trough-like morphol- ogy. Relatively small-scaled features of cut-and-fill associated with the dominant incision process are commonly along the canyon floor, resulting in a flat-floored pathway. Sliding and slumping dominated the steep canyon walls, producing and transporting sediments to canyon floor and partially filling up canyon thalweg. The meandering segment is characterized by erosive features where deeply down-cutting occurs in the outer bend of the major sea valley, forming V-shaped entrenched thalweg. The recurrences of turbidity currents have allowed continuous incision of the canyon head and have kept the connec- tion between the canyon head and the river mouth during Holocene highstand of sea level. The upper reach of the Kaoping Canyon is linked to drainage area and maintains as a conduit and/or sink for terrigenous and shallow marine material. Sediment-laden river plume operates in the Kaoping River-Canyon system, with turbidity currents flushing fiver sediments into the canyon head where the canyon thalweg is the most erosive. Presently, the upper reach of the Kaoping Canyon can be considered as a temporal sediment sink.

The barrier river reach identification and classification in the Middle Yangtze River

Adjustments of upstream river regimes are one of the main factors affecting downstream fluvial processes. However, not all adjustments of river regimes will propagate downstream. There are some distinctive river reaches where upstream and downstream adjustments have no relevance. However, the irrelevance is neither caused by different river types nor by the different conditions of water and sediment;but rather, the channel boundaries and riverbed morphologies block the propagation effect. These are referred to here as the barrier river reach phenomena. The migration of the thalweg line is the essential reason for causing the propagation effect. Numerous influencing factors for thalweg migration exist, including 1) the average flow rate above the critical bankfull discharge, the average flow rate below the critical bankfull discharge, and their ratio, 2) the ratio of the duration of the aforementioned two periods, 3) the thalweg displacement at the entrance of the river reach, 4) the deflecting flow intensity of the node, 5) the ratio of the river width to water depth, 6) the relative width of the floodplain, and 7) the Shields number. In this study, the correlativity between the measured distances and the restricting indicators of thalweg migration in the Middle Yangtze River over the years was established. The barrier degree of 27 singlethread river reaches was subsequently assessed. These reaches included 4 barrier river reaches;5 transitional reaches transforming from barrier to non-barrier;10 transitional reaches transforming from non-barrier to barrier;and 8 non-barrier river reaches. Barrier river reaches were found to be important for maintaining the stability of the river regime and the transverse equilibrium of sediment transport in the downstream reaches. To some extent, the barrier river reaches may protect the natural dynamical properties from being destroyed by artificial river regulation works. Thus, they are of great significance for river management.