Erosion-deposition Patterns Research Articles

Spatio-temporal variation of sediment transport in the Selenga River Basin, Mongolia and Russia

Many Asian rivers have been intensively used to boost economic growth, resulting in sudden and drastic changes in sediment transport patterns. However, a few rivers are still undisturbed. The present paper considers the unregulated Selenga River and its basin, located in Russia and Mongolia. The river contributes to 50 % of the total inflow to Lake Baikal. Pending scientific challenges include the quantification of sediment loads and erosion–deposition patterns along the Selenga River system, the understanding of suspended particulate matter composition and the importance of peak flow events for total sediment discharge and heavy metal transport. Field data and hydraulic modeling converge on showing that peak flow events during spring and summer contribute to the main part (70–80 %) of the annual sediment and pollution loads in upstream parts of the basin. The Selenga River carries mostly silt and sand. The average particle size differs by a factor of four between summer floods and base flow periods. The low amount of particulate organic matter (ranging between 1 and 16 % in the studied rivers) is consistent with the significant role of sediments originating from mining areas and in-channel sources. The bed load transport in the downstream part of the river basin is high (up to 50 % of the total transport), and channel storage plays an important role in the total sediment transport to Lake Baikal. Reported statistically significant multi-decadal declines in sediment fluxes in the downstream Selenga River can be attributed to the abandonment of cultivated lands and (most likely) to changing hydroclimatic factors.

A Three‐Dimensional Structure and Process Model for Integrated Hydro‐Geo‐Pedologic Analysis of a Constructed Hydrological Catchment

A structure model describes the boundary geometries and the three‐dimensional solid phase distributions of the catchment. In contrast to an established ecosystem, feedback relations can be assumed for an initial geosystem between the development of spatial structures and the water and element fluxes. The objective was to develop a hydrological catchment model as a three‐dimensional spatial database of the solid phase that allows an integrative analysis, a periodical mass balance, and the generation of distributed model parameters for the developing system. Data were from the constructed catchment “Chicken Creek.” The initial texture and bulk density distributions were generated by imitating sediment dumping, segregation, and compaction. Boundary geometries and changes in surface topography due to erosion and sedimentation processes were quantified on the basis of digital elevation models (DEMs) derived from aerial photographs. The catchment was visualized with the three‐dimensional software GoCad; the “emergence” of the water table and other structures (e.g., soil horizons, root zone) could be spatially assigned and quantified to identify regions with specific processes. A combination of three‐dimensional catchment with time‐dependent two‐dimensional surface models allowed generating the development of spatially distributed erosion–deposition patterns that formed new initial surfaces. The three‐dimensional distributed solid phase structure of the catchment allowed for a more direct comparison with observations using minimal invasive methods. The mass balance of the solids could be related with pedologic development and transfer to other geopedologic systems achieved by adapting sediment transport and deposition in the descriptions of the model. This model may help improve the integrative analysis of hydrological catchments.

TIDAL WAVE REFLECTION FROM THE CLOSURE DAM IN THE GUADALQUIVIR ESTUARY (SW SPAIN)

Closure dams in estuaries and tidal areas have a strong impact on their hydrodynamics and morphology. Among other reasons, this impact can be due to tidal reflection. In the Guadalquivir estuary (SW Spain), data measured allowed to identify a partially standing motion along the upper third of the estuary. We estimate the frequency-dependent complex reflection coefficients at the head dam, and analyze the effect of the combined action of the incident and reflected tidal waves on the residual currents. Erosion-deposition patterns induced by the tidal reflection were identified. The most energetic tidal constituent, the semidiurnal M2, favors sedimentation near the entrance of the Port of Seville, where the most intense dredging works take place. Finally, we briefly discuss the potential consequences of reflection on the turbidity spatial distribution.

Changes in planview geometry and erosion–deposition patterns along a channel–levee complex in the southern corner of the Kurile Basin, Okhotsk Sea

High-resolution seismic profiles reveal the changes in planform and erosion/deposition pattern in the Abashiri-oki Trough (AOT), the southwestern corner of the Kurile Basin. The AOT originated from a syncline in the Late Miocene–Early Pliocene, and is characterised by Plio-Pleistocene channel–levee complexes and debris flows, mainly composed of hummocky, chaotic and convergent reflectors. The channel planform was changed by eustacy, tectonics, climate, and hinterland volcanism. Regressive surfaces probably occur as the two reflectors of erosional surface and debris flow deposits. The erosional/depositional pattern suggests that the channel profile tends to evolve towards an exponential-like form. In the upper trough, channels are incised. Probably inland volcanism induced to increase sedimentary supply, consequently the slope progradation and steepened gradient. Volcanism also caused an additional channel. In the middle trough, channels show braided pattern, related to subsidence and mildened gradient of the trough floor, which lead to accommodation. In the lower trough, channels anastomose, and in the lowermost trough finally, the merged channel incises and cut-offs the anticline dividing the AOT from the Kurile Basin.

ESTIMATION OF SEDIMENT TRANSPORT IN THE COASTAL ZONE OFF THE HOOGHLY ESTUARY MOUTH

ABSTRACT The coastal zone lying within the confluence of River Bhrahabalang to Hooghly estuary mouth is very sensitive to wave and current related transport due to strong tidal and wave action. The quantum of sediment movement involved in the region has been estimated using formulae proposed by Rijn, Bijker, Engelund-Hansen and CERC after evaluating appropriate parameters related to wave, current and sediment. The wave field for the study zone has been determined by solving the differential equation of Munk et al. by an explicit finite difference numerical scheme. Results of computation indicate that estimate by Rijn, Engelund-Hansen and CERC formulae are of the same order of magnitude and that proposed by Bijker gives less transport rate. Assessment of erosion deposition pattern in the surf zone has been made based on formulae proposed by Swart which shows the study zone comes under erosion except near the river mouth where deposition has been found to occur.

River dynamics and the diversity of Amazon lowland forest

We suggest here that large-scale natural forest disturbance and primary succession in the lowland rainforests of the Peruvian Amazon is caused by lateral erosion and channel changes of meandering rivers. Our results indicate that in the upper Amazon region, primary succession on newly deposited riverine soils is a major mode of forest regeneration. Landsat imagery analyses show that 26.6% of the modern lowland forest has characteristics of recent erosional and depositional activity; 12.0% of the Peruvian lowland forest is in successional stages along rivers. This successional development is used to classify the western Amazon rainforests according to their geomorphological erosion–deposition pattern. We also propose that by causing high site turnover, disturbance and variation in forest structure, the river dynamics may be a major factor creating and maintaining the high between-habitat (β-type) species diversity characterizing the upper Amazon1.

Antidune Movement

A new mechanism is introduced to explain the intriguing movement of antidunes. It is suggested that this mechanism may exist concurrently with sediment erosion-deposition patterns used heretofore to explain the apparent movement of antidunes. The mechanism introduced herein may be responsible for bulk movement of antidunes.