Meandering Channel Research Articles

Hydrologic connectivity and morphologic variation of oxbow lakes in a pristine alpine fluvial system

The lifespan of oxbow lakes in a meandering system typically exhibits considerable disparity due to the diverse evolutionary processes that lead to their terrestrialization. Although oxbow lakes generally undergo a long-term gradual shrinking trend, their hydrologic connectivity and morphologic variations under variable hydrologic conditions of the river are not well understood. Here we focus on a group of oxbow lakes in a pristine alpine fluvial system in the Qinghai-Tibet Plateau to examine the hydrogeomorphic response of oxbow lakes to a variable hydrologic regime of an active meandering channel. We find that the oxbow lakes are characterized by extensively developed compound forms and connecting “tie” channels and that the lateral migration of the main channel discernably impacts lake morphology and evolution. With increasing discharges in the main channel, the oxbow lakes generally show increases in length under relatively lower flows (RI < 10 years) and more dramatic increases in width under higher flows (RI > 10 years), which collectively lead to their expansion regardless of their hydrologic connectivity. This implies hydrogeomorphic processes other than surface water connection between oxbow lakes and the main channel in adjusting water levels of the lakes. To quantify hydrologic connectivity, we propose a probability-based index and find the overall low hydrologic connectivity of the studied oxbow lakes, owing to the unique paleogeographic setting of the study area. The infrequent but pulsed sedimentation within the lakes and the relatively slow lateral migration of the meandering channel catalyze an inactive floodplain of the study reach.

Seismic attribute assisted analysis of the interpretational variations in the time and depth migrated datasets: An example from Taranaki Basin, New Zealand

In the regions with complex geology together with the apparent stratigraphic and seismic sequence events, time migration algorithms do not provide high-quality imaging for seismic interpretation due to strong lateral velocity contrasts, usage of average velocities in the vertical and horizontal directions, and without defining the ray-bending at interfaces. Therefore, the depth migration method using interval velocities and bended ray-traces at interfaces, provides more accurate results. There are also interpretational differences between the time and depth-migrated seismic data. The objective of this research is to compare these differences using several seismic attributes. In this context, we used the Toro 3-D pre-stack time and pre-stack depth migrated datasets from the Taranaki Basin, offshore New Zealand, because the Giant Foresets Formation presents suitable examples for the different types of channels like distributary, sinuous and meandering channels. Comparisons indicate that dip angles of the faults increased, channel walls become steeper and indicate more curvature, and structures get narrower in the depth-migrated data. There are also amplitude and phase variations in and around the channels. Most of the seismic data in the world are in the time domain and there is no sufficient borehole and velocity data. This situation may affect the attribute calculations in both migration data sets. The main goal of this investigation is to provide an understanding about the possible seismic differences in the time and depth migrations, the reasons behind them and, relationship between the lithology and probable seismic responses by using geometrical and physical attributes used in the seismic industry. These steps allow an accurate interpretation and more reliable evaluation of the subsurface structures. This study will also provide a toolkit and guidance for the interpreters in the time and depth migrations.

Surface microtextures of quartz grains and minor sedimentary components across the Paleolithic Jeongok‐ri sediments, Yeoncheon, South Korea

ABSTRACTThe Jeongok‐ri Paleolithic site in the central Korean peninsula is known as the first findspot of Acheulean‐type hand axes in East Asia. Although many previous studies have investigated the depositional environments of Jeongok‐ri sediments across the Paleolithic occupation, they remain controversial. In this study, we collected an 11‐m‐long core to obtain evidence from systematic surface microtextures of quartz grains (SMQs) concerning depositional environments in the Jeongok‐ri archeological site. SMQs in the Jeongok‐ri sediments are characterized by dominant angular to subangular outlines and small to large conchoidal fractures with arcuate steps, as well as sparse V‐shaped percussion cracks; these features suggest short distances and subaqueous transportation with grain‐to‐grain collisions and violent impacts of quartz grains. On the basis of simultaneous sedimentation of the hosting silt deposits, our SMQ results suggest that Jeongok‐ri sediments were originally fluvial, regardless of their intermediate sedimentary history. This evidence from quartz grain microtextures is consistent with minor sedimentary components found in the Jeongok‐ri core sediments, such as diatom grains as microfossil constituents, along with macroscopic charcoals and titanium‐bearing detrital magnetite grains. The geological and geomorphological conditions of the Jeongok‐ri site are also favorable for floodplain environments given the position adjacent to river terraces formed by the previous lateral erosion of a meandering channel in the nearby Hantan River. Our findings will help to understand environmental conditions during the development of Paleolithic cultures in East Asia.

Hydrogeomorphology of asymmetric meandering channels: experiments and field evidence

Hydrogeomorphology of asymmetric meandering channels: experiments and field evidence

A curved boundary treatment for discrete Boltzmann model of shallow water flows based on a partially saturated method

In the present study, a boundary treatment scheme named the partially saturated method (PSM) is incorporated into the discrete Boltzmann model (DBM) for shallow water flows to deal with complex curved boundaries. In this PSM-DBM, the two-dimensional 16 velocity levels scheme is adopted and the finite difference method is used to solve the governing equation. Then, the PSM-DBM has been applied to simulate four cases, i.e. such as steady flow in a bending channel, the flow past a stationary cylinder, a jet-forced flow in a circular basin and flow in a meandering channel with 90° consecutive bends. The simulated results have been compared with the experiments and the simulation by traditional numerical simulation. The study shows that the agreement between simulation and experiments is good. It is demonstrated that the PSM-DBM is stable and accurate to deal with the curved boundaries. Moreover, the implementation of the PSM is relatively straightforward in treating stationary curved geometries and is easy to be incorporated into the DBM for shallow water flows. In conclusion, the proposed PSM-DBM can be used widely for flows with curved boundaries.

Fluvial Responses to Late Quaternary Climate Change in a Humid and Semi-Humid Transitional Area: Insights from the Upper Huaihe River, Eastern China

Research into river processes in different climatic and geomorphic areas is vital for a clearer understanding of the non-linear responses of rivers to climate change. The Huaihe River (HHR) Basin, located in China’s North–South Transition Zone (NSTZ), provides an ideal environment in which to explore river responses to climate change within a humid/semi-humid transitional area. Based on optically stimulated luminescence (OSL) and 14C dating, combined with sedimentary stratigraphic analyses, we reconstructed the river processes of three sedimentary sequences in the upper HHR since the Late Quaternary. Our results showed that the upper HHR was characterized by aggrading meandering channels from the Last Glacial Maximum (LGM) to 0.5 ka, and an aggrading wandering channel from 0.2 ka to the present. Two periods of downcutting occurred during 5.8–3.0 and 0.5–0.2 ka, respectively. The river incision is potentially linked to changes in the climate during the Mid–Late Holocene transition and the Little Ice Age (LIA). However, there have been no marked changes in channel patterns in the upper HHR since the LGM. This phenomenon reflects the influence of vegetation on channel patterns during climate change. Our results showed that the fluvial processes in different climatic and geomorphic areas are controlled by the local hydroclimatic regime.

Riparian vegetation's effects on the evolution of an experimental meandering channel driven by an upper vortex flow

AbstractFew investigations of river evolution have incorporated the series of upper vortex flow disturbances induced by quasi‐knickpoints, and it is unclear whether the regulating ability of riparian vegetation in maintaining meandering channel patterns is valid. In this study, we therefore conducted meander evolution experiments to confirm the effects of vegetation species and vegetation strength on this complex system. Two nonleguminous plants with salinity and alkalinity resistance had non‐uniform degrees of normal seedling growth and different root network lengths. Our findings show that braided swales formed between the upstream developing zone of the vortex flow and the nearest bend and the riparian vegetation generally consolidated the single‐thread channel planform without branching outward under the conditions of flooding flow. Transverse point bars on inner banks were replaced by downstream scroll bars in the overall channel for long time, unless riparian vegetation and flood scour were coupled. Shallow‐rooted plants were inadequate to withstand the inner‐banks being cut by the upper vortex flow. Deep‐rooted plants can significantly stabilize bank lines and thalwegs but are vulnerable to locally low vegetation coverage. Using evolutionary spectral analysis based on thalwegs, we found that the streamwise high‐frequency distribution of bed topography was primarily concentrated downstream of the bifurcation interface when a flood was present in the unvegetated scenario, shrank to the isolated turning interface of the upper reach and the large‐scale spiral swale of the lower reach in the shallow‐rooted scenario and stood out along the area disturbed by bare roots in the deep‐rooted scenario. This experimental study broadens our understanding of vegetation effects in hydro‐bio‐geomorphological system engineering.

Effect of sediment supply on the flow characteristics in a meandering channel

Meandering channels are common in natural rivers, especially in lowland alluvial plains. In this research, a meandering channel was designed in order to explore the effect of sediment supply on the flow characteristics. The water level, three-dimensional (3D) instantaneous velocity and riverbed elevation were measured using a digital wave altimeter, a 3D acoustic Doppler velocimeter and a 3D laser scanner, respectively, and the flow characteristics with and without sediment supply were compared. Under the same discharge, the transverse slope of the water surface in the case with sediment was larger than that without sediment and the change was steep; the change in the longitudinal slope of the water surface was small in the upper half bend and large in the lower half bend. By increasing the water discharge, the number of secondary flow vortices at the apex of the meandering channel increased. The trends of the mainstream lines with and without sediment supply were roughly opposite and the difference between them decreased with increasing discharge. The lateral and vertical turbulence intensities of the flow near the outer bank were larger in both cases (with and without sediment). In the case with sediment, the turbulent kinetic energy of the cross-section was larger near the bed and the water surface.

Turbulent flow mechanisms in meandering channels with sediment transport

Abstract This research aims to investigate the near-bed turbulent flow characteristics in a meandering channel with both mobile bed and immobile bed conditions. Experiments were performed in a prismatic rectangular meandering channel with a non-uniform sand bed of size d50 = 0.523mm. The three-dimensional instantaneous fluid velocity was collected using the Acoustic Doppler velocimeter which will provide important results related to the flow turbulence such as mean flow velocity, turbulence intensity, Reynolds shear stress, turbulent kinetic energy, skewness, kurtosis and turbulent anisotropy. The secondary current flow and the exchange of momentum in the form of turbulence kinetic energy, Reynolds shear stress and turbulent intensity at the inner layer of the flow are identified more in a mobile bed condition as compared to an immobile condition, which causes sediment transport. For the inner layer of the flow, turbulence intensity and turbulent kinetic energy are decreased in magnitude and gradually increase in the outer layer of flow for both the bed conditions. Higher turbulence anisotropy is noticed in the mobile bed condition than in the immobile bed condition, which shows more nonuniformities near the bed level for the mobile bed condition. This study may help in understanding the effect of sediment transport due to a turbulent flow structure in a sinuous alluvial channel.

Using Simple LSTM Models to Evaluate Effects of a River Restoration on Groundwater in Kushiro Wetland, Hokkaido, Japan

Wetland ecosystems with proper functioning provide various ecosystem services. Therefore, their conservation and restoration are of fundamental importance for sustainable development. This study used a deep learning model for groundwater level prediction to evaluate a wetland restoration project implemented in the Kushiro Wetland in Japan. The Kushiro Wetland had been degraded due to river improvement work. However, in 2010, a wetland restoration project was carried out to restore the meandering river channel, and a decade has passed since its completion. In this study, the wetland restoration project was evaluated by comparing the response of the groundwater level using a model that reproduced physical conditions with different characteristics before and after the restoration. At first, a deep learning model was created to predict groundwater levels pre- and post-restoration of a meandering river channel using observation data. Long short-term memory (LSTM) was used as the deep learning model. The most important aspect of this study was that LSTM was trained for each of the pre- and post-restoration periods when the hydrological and geological characteristics changed due to the river channel’s restoration. The trained LSTM model achieved high performance with a prediction error of the groundwater levels within 0.162 m at all observation points. Next, the LSTM models trained with the observation data of the post-restoration period were applied to evaluate the effectiveness of the meandering channel restoration. The results indicated that the meandering channel restoration improved hydrological processes in groundwater levels, i.e., their rainfall response and average groundwater water levels. Furthermore, the variable importance analysis of the explanatory variables in the LSTM model showed that river discharge and precipitation significantly contributed to groundwater level recovery in the Kushiro Wetland. These results indicated that the LSTM model could learn the differences in hydrological and geological characteristics’ changes due to channel restoration to groundwater levels. Furthermore, LSTM is a data-driven deep learning model, and by learning hydrological and geological conditions to identify factors that may affect groundwater levels, LSTM has the potential to become a powerful analysis method that can be used for environmental management and conservation issues.

Spatio-temporal variations of a meandering channel morphodynamics: A study along the Kaljani River in Bhuchungmari-Moamari Section of North Bengal Dooars, West Bengal (India)

Rivers being the most dynamic and natural engines receive the maximum outcome of process-form activities over space and time. Displacement and shifting of course of the alluvial channels over the lower gradients result into severe bed-bank scouring activities temporally. River Kaljani within the neo tectonically active zone of North Bengal plains is an example of temporal variations of high sinuosity and braiding over the softer lithology. The present work is focusing on a temporal study from 1970 to 2019. The SOI Topographical map of 1:50,000 scales has been the base layer along with temporal satellite imageries (1990, 2000, 2010 and 2019). The schematization has followed digitization, rectification and overlaying of conformal layers extracted from the USGS website under UTM Projection system and WGS of 1984 datum for north 45 zones under GIS platform. Multi-temporal satellite data identified serious course changes of the selected reach of lower segment of Kaljani River. The rectified multi-spectral bands are used to make the NDWI for identification of the water content on earth surface followed by selective morphometric analysis e.g. sinuosity index, braided index and roundness of curvature to access the spatio-temporal changes accomplishing some derivative mathematical equations. Descriptive statistics viz. mean, standard deviation (SD), coefficient of variation (CV), skewness and kurtosis were used and explained maximum course oscillations. The authors tried to examine the fact for six temporal frames i.e. 1970, 1990, 2000, 2006, 2010 and 2019 and received high sinuosity of all categories. Conclusively the work claims to set a documentary approach to examine the morphodynamics of an alluvial channel of Sub-Himalayan foothills.

Geomorphic Landscape Features Affect Plant Invasion in Salt Marshes: The Role of Tidal Channel Meanders

The invasion of smooth cordgrass (Spartina alterniflora Loisel.) has greatly threatened China's coastal salt marshes, yet how geomorphic landscape features, such as tidal channel meanders, influence cordgrass invasion processes remains unclear. We found that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated the seedling establishment and subsequent growth, whereas the concave side considerably inhibited the cordgrass invasion. Our findings suggest that the meandering geomorphology of tidal channels could act as stepping stones to greatly facilitate cordgrass landward spread along tidal channels by creating invasion windows. Photo 1. Invasion range of S. alterniflora is promptly extending landward via tidal channels in salt marshes, as shown in this photograph (taken in the Yellow River Delta, China). As a consequence, the drainage density of tidal channels, the lateral hydrological connectivity between tidal channels and their served marsh flats, and bio-geomorphic landscape formation could be considerably altered. Photo credit: Zhonghua Ning. Photo 2. UAV aerial image presents the process and pattern of S. alterniflora (green and green-yellow color in tidal channel margins) landward expansion attached to the meandering geomorphic structure of tidal channels, particularly the convex sides of channel meanders, which could provide consecutive spatial “windows of opportunity” for S. alterniflora establishment. Photo credit: Zhonghua Ning. Photo 3. As shown in this photograph, S. alterniflora seedlings preferentially colonize on the convex side of tidal channel meanders. Compared with the concave side, the eddies and reduced flows generally exist on the convex side, thereby providing suitable physical environments (lower hydrodynamic disturbance, bed shear stress, and salinity-moisture stress) for seed stranding, seedling establishment, and subsequent growth of S. alterniflora. Photo credit: Zhonghua Ning. These photographs illustrate the article “Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows” by Zhonghua Ning, Baoshan Cui, Cong Chen, Tian Xie, Weilun Gao, Youzheng Zhang, Zhenchang Zhu, Dongdong Shao, Dongxue Li, and Junhong Bai, published in Ecological Applications. https://doi.org/10.1002/eap.2813.

Anthropogenic amplification of geomorphic processes on fluvial channel morphology, case study in Gilgel Abay river mouth; lake Tana Sub Basin, Ethiopia

As a result of ongoing human induced or natural factors acting on river channels, banks, and within a catchment, alluvial river systems change their course and morphology over time. The base level changes and the backwater effects affect rivers entering a static water body. For coastal rivers, the planform changes are highly pronounced at the fluvial deltas and flood plains. Aggradation, degradation, progradation, meandering, and formation of islands and distributary channels are common processes on coastal rivers. This study investigates planform changes and landscape responses of Gilgel Abay river about 36 km stretch starting from the bridge, near Chimba, to its entrance to Lake Tana by using historical images for the last 60 years (1957–2020) and field observation. The study reach was divided in to three parts based on feature characteristics. For data preparation and analysis, image analysis software's; ENVI, ERDAS IMAGINE, and ArcGIS were used. From land use land cover classification, the land use pattern near the river flood plain and delta area changed significantly. The overall planform results (sinuosity, width, and island) show that, Gilgel Abay River within the study reach has shown little change in its planform for the last 60 years. However, the landscape of alluvial delta formed at the mouth of the river has shown huge change. Accretion-erosion result map shows, a maximum of 18.73 ± 1.97 m/y accretion and −12.48 ± 1.44 m/y erosion at the right side (eastward) and a maximum of 50.06 ± 1.44 m/y accretion and lost only 3.95 ± 1.97 m/y westward. The delta area analysis shows in the last five decades, 17.13 ha/yr of land has formed disproportionally out of which over 56% is added at the right shore of the river. In general, for the planform changes that occurred in Gilgel Abay river channel and fluvial delta, human-induced factors have a great contribution. Increasing interest for new settlements inside delta flood plain, productive agricultural land and artificial lake level rise changes the river planform and the delta appearance. Quantitative and qualitative mapping of river and delta coupling with feeding basins and floodplains is important to enhance and understand the socioeconomic influences on river morphology and needs an integrated management framework.

Numerical Investigation on Flow Characteristics in a Mildly Meandering Channel with a Series of Groynes

In single-bend channels or meandering channels, groynes are widely used as a river-training structure. Geometric factor is an important principle of groyne design. In this paper, the numerical method based on the Renormalization Group (RNG) k-ε turbulence model is used to study the effects of groynes with different lengths and orientations on the mean flow pattern and turbulence characteristics in a mildly meandering channel. The analysis shows that compared with equal-length groynes, the groynes arranged in descending order perform well in improving the flow velocity of the main channel, and the maximum longitudinal velocity at the channel center can be increased by 1.57 times the average velocity. However, at the same time, they bring higher Normalized Turbulent Kinetic Energy (NTKE) and Normalized Bed Shear Stress (NBSS) values. The attracting groynes arranged in descending order help to reduce scouring. The groynes arranged in ascending order reduce the velocity of the riverbank in some downstream areas and are suitable for bank protection. It effectively decreases the NTKE and NBSS around the groyne fields and can reduce the inner bank scouring of the mildly meandering channel. The findings are helpful for the management of meandering rivers.

Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows.

Understanding the mechanisms by which the geomorphic structures affect habitat invasibility by mediating various abiotic and biotic factors is essential for predicting whether these geomorphic structures may provide spatial windows of opportunity to facilitate range-expansion of invasive species in salt marshes. Many studies have linked geomorphic landscape features such as tidal channels to invasion by exotic plants, but the role of tidal channel meanders (i.e., convex and concave sides) in regulating the Spartina alterniflora invasion remains unclear. Here, we examined the combined effects of tidal channel meander-mediated hydrodynamic variables, soil abiotic stresses, and propagule pressure on the colonization of Spartina in the Yellow River Delta, China, by conducting field observations and experiments. The results showed that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated Spartina seedling establishment and growth, whereas the concave side considerably inhibited the Spartina invasion. Lower soil abiotic stresses also significantly promoted the invasibility of the channel meanders by Spartina. Based on these findings, we propose a conceptual framework to illustrate the effects of the meandering geomorphology of tidal channels on the mechanisms that might allow the landward spread of Spartina and related processes. Our results demonstrate that the meandering geomorphic structures of tidal channels could act as stepping-stones to significantly facilitate the landward invasion of Spartina along tidal channels. This implies that geomorphic characteristics of tidal channels should be integrated into invasive species control and salt marsh management strategies.

Extracting a Connected River Network from DEM by Incorporating Surface River Occurrence Data and Sentinel-2 Imagery in the Danjiangkou Reservoir Area

Accurate extraction of river network from the Digital Elevation Model (DEM) is a significant content in the application of a distributed hydrological model. However, the study of river network extraction based on DEM has some limitations, such as location offset, inaccurate parallel channel and short circuit of meandering channels. In this study, we proposed a new enhancement method for NASADEM V001 in the Danjiangkou Reservoir area. We used Surface Water Occurrence (SWO) and Sentinel-2 data to describe vertical limit differences between morphological units to complement actual flow path information from NASADEM data by a stream burning method. The differences between the extracted river network and the actual river network were evaluated in three different geographical regions. Compared with the actual river centerline, the location error of the river network extraction was significantly reduced. The average offset distances between river network extraction and the actual river network were 68.38, 36.99, and 21.59 m in the three test areas. Compared with NASADEM V001, the average offset distances in the three test areas were reduced by 7.26, 40.29, and 42.35%, respectively. To better estimate accuracy, we also calculated and compared the accuracy of the river network based on MERIT Hrdro and HydroSHEDS. The experimental results demonstrated that the method can effectively improve the accuracy of river network extraction and meet the needs of hydrological simulation.

Improved Understanding of Minor Reservoirs Optimizes Full-Field Productivity

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 21225, “Defining Heterogeneity and Compartmentalization Predictions of Minor Reservoirs in Fluvial Environments: Geological and Dynamic Context,” by Nur Azah Zulkifli, Lisa C. Chisholm, and Amy M.M. Yusoff, Petronas, et al. The paper has not been peer reviewed. Copyright 2021 International Petroleum Technology Conference. Reproduced by permission. _ Reservoirs in the MN field offshore Malaysia consist of predominantly fluvial delta deposits. Several modeling studies have been performed for major reservoirs. After more than 20 years of production, a project for minor reservoirs has been created. The primary objective of this study was to improve the understanding of the uncertainties affecting well performance and reservoir connectivity and to identify potential infill opportunities. Introduction From the initial analysis of a 2017 full-field review (FFR), the team decided that a detailed subsurface evaluation was necessary. This study was completed with the integration of subsurface data to ensure that hydrocarbon volume and geological knowledge have been calibrated with production performance. The field is 160 km north-northeast offshore peninsular Malaysia in a water depth of 69 m. This field is divided into five areas: Main, West, North, South, and Southwest. The depositional environment of this field is fluvial coastal plain and consists of multistacked oil and gas reservoirs with an average net thickness of 2–20 m, interbedded with shale and coal in a coastal plain setting. Geological Concept and Overview Reservoir X is one of 26 major and minor hydrocarbon-bearing units in the field. Twenty-two minor reservoirs were analyzed for the FFR with an integrated study work flow. Geological analysis concluded that these reservoirs were deposited in a fluvial environment consisting of meandering channels, point bars, overbanks, and abandoned channel-sand mouth bars with deltaic influence (Fig. 1). Seismic data have been used extensively to tackle key uncertainties with either quantitative or qualitative approaches, depending on the seismic detectability limit of the reservoir. The use of seismic in this case was challenging because seismic attributes are not optimal and mostly are used as soft data guided by primary well data. Additionally, several reservoirs required stratigraphic compartmentalization to be introduced. Heterogeneities in the Fluvial Environment Capturing and preserving the different level of heterogeneities was complex because of varied data resolution, grid definitions, and hierarchies of impact. This field has relatively large net-to-gross (NTG) ranges from 20 to 98% with individual channel width averaging from 20 to 100 m. Because the existing dynamic models struggled to replicate field performance, the model created in this study had limited predictive capabilities. The four levels of fluvial heterogeneities of megascopic, mesoscopic, macroscopic, and microscopic scales were used to better understand the heterogeneities of these minor reservoirs. This has helped to compartmentalize the reservoir based on interpretation of the dynamic data without compromising geological understanding.

Unravelling the impact of geological heterogeneities in the reservoir management. An integrated study of the deep-water Akpo field (Niger delta)

Geological and seismic data have been integrated to describe the impact of structural and sedimentological heterogeneities on fluid dynamics for one of the reservoirs (Reservoir X) of the Akpo field (deep offshore of Niger Delta). Akpo is a sinuous 4-way anticline formed because of strong deformation occurring within the over pressurized shales of the Akata Formation. Weak deformation may have started since 9 Ma; however, most of the strain associated with shortening was accumulated between ca 7 Ma and ca 3 Ma. The cumulative shortening has been estimated in the order of 4% (ca 400 m). From 3 Ma (during the latest stages of growth of the Akpo anticline) up to Present-day, the residual strain was taken up by oblique (right lateral strike-slip component) extensional faults, between 2 km and ca 15 km in length, organized into two main trends: N-dipping, ENE-WSW to NE-SW striking faults, and NNW-SSE striking faults mainly dipping to the east. These faults are segmented and have a maximum throw in the order of 70 m. Based on the architectural elements identified in this work, the Reservoir X of the Akpo field has been subdivided into three main intervals: a basal lobe (Interval 1a) and a related channel (Interval 1 b); two intermediate generations of meandering channels (Interval 2a and 2 b); and an upper and strongly erosional constructive channel (Interval 3). The 4D signal has been integrated to better understand the complex fluid flow within Reservoir X in time and space. The extensional faults play a role in baffling mainly in the central part of the field, where across-fault flow is likely to occur in correspondence with the spill points. Facies distribution and the nature of the contact between various reservoir intervals are also important because vertical and lateral communication between sand bodies can be complex. Levees or mud barriers can inhibit flow, impacting both efficiency of support between injectors and producers. On the other hand, sand-to-sand communication due to the strong erosional character of the contacts enhances dynamic connectivity between sands belonging to different reservoir intervals. As an example, even if sands belonging to Interval 3 totally erode (i.e. dynamically isolate) those of Interval 2 in the southern part of the field, in the northern sector the two reservoir intervals are in (vertical) communication as suggested by the 4D signal. A key result of this integrated work is that of documenting in detail the modes in which faults and sedimentary features impact fluid flow within and across reservoir intervals in a deep-water environment.

The effect of building arrangement on the flow characteristics in meandering compound channels

Due to the fact that most of the cities and villages are built on the banks of the rivers, the hydraulic characteristics of water flow always undergo changes during floods, so it is necessary to study the interaction of the flood flows of meandering compound channel in these areas. In this paper, the effect of building arrangement on the flow characteristics in meandering compound channels was investigated by using laboratory models. For this purpose, a smooth floodplain (without building) and three different types of building arrangement on the floodplain including structural obstacles perpendicular and parallel to the floodplain flow and checkered structural obstacles have been used. The tests have been conducted at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. All experiments were performed under a quasi-uniform and fully developed flow condition. The measurement data were recorded at different sections along the meandering main channel. Stage-discharge curves, Manning's n, velocity distribution, boundary shear stress and Darcy-Weisbach friction factor are determined over 12 test runs. The results of this research show that the arrangement of buildings has a great effect on the longitudinal velocity of the main channel. At a relative depth of 0.49, in a meandering compound channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the dimensionless longitudinal velocity of the main channel compared to the smooth floodplain increases by 113%, but with the arrangement of structural obstacles parallel to the floodplain flow, the dimensionless longitudinal velocity increases by 27%. Also, the building arrangement is effective in reducing the discharge capacity of the meandering compound channel, so that in a building density of 18.5%, structural obstacles perpendicular to the floodplain flow reduce the discharge capacity of the channel by 50% compared to the smooth floodplain. In the channel with structural obstacles arranged perpendicular to the floodplain flow, Manning's roughness coefficient increases sharply with increasing the relative depth, but in the channel with the structural obstacles parallel to the floodplain flow, Manning's roughness coefficient decreases with increasing the relative depth. In the channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the bed shear stress of the main channel is higher than the other building arrangements, so that at the relative depth of 0.49, the average maximum bed shear stress in all sections increases by 42% compared to the smooth floodplain.

Experimental assessment of velocity and bed shear stress in the main channel of a meandering compound channel with one-sided blocks in floodplain

Meandering rivers have an important role in transporting runoff from rainfall, especially during floods. With many cities and villages along rivers and the construction of water-management structures in floodplains, the flow pattern of the meandering rivers changes in time of floods. Here the flow behaviors in meandering compound channels with smooth floodplains and one-sided building floodplains are investigated using laboratory experiments. The experiments have been performed at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the block density in a floodplain has a significant effect on flow behavior in the main channel. So that, with the increase of the relative depth, the ratios of main channel discharge compared to the total discharge (Qmc/Q) decreases in the bends with minimal extension of blocks floodplain, the crossover areas and the bends with maximal extension of blocks floodplain. For a given Dr, the significant increase in the Manning's n values as the density of the blocks increases (approximately 30%). With increasing the blocks density from zero to 23.7%, the maximum Darcy–Weisbach friction factor in sections CS1 and CS7 increases by 110% and 2%, respectively, which shows that increasing the density of blocks does not have a great effect on f in the section CS7. Under one-sided blocked floodplain conditions, the maximum longitudinal velocity in the main channel in the section with maximal extension of the blocks in the floodplain (CS7) is greater than the section with minimal extension of blocks in the floodplain (CS1). At the relative depth of 0.49, the maximum value of depth-averaged velocity in sections CS1-4 decreased by an average of 52% compared to the smooth floodplain. The maximum shear stress on the bed at the apex section for all channels occurs near the convex side and the bed shear stress at all sections decreases with decreasing relative depth and in the section with minimal extension of blocks floodplain (CS1) and cross-over sections. The maximum shear stress of the bed decreases with increasing the block density and these changes are more near the convex side than the concave side.