Siltation

About: Siltation is a research topic. Over the lifetime, 1420 publications have been published within this topic receiving 20983 citations.

Climatically induced levee break and flood risk management of the Gorakhpur region, Rapti River basin, Ganga Plain, India

Abstract: A densely populated city, Gorakhpur, located on the bank of Rapti river in the Ganga plain, is frequently affected by flooding. The Rapti river exhibits narrow channel within wide valley, channel bars, natural levee and river terraces. Artificial levees are constructed in the valley during low discharge period to mitigate the flood and also to provide the additional land to the society. These levees break during prolonged heavy rain and induce the catastrophic flood because it is not constructed by analyzing the capacity of the bracketed channel to accommodate the high discharge of the river. The precipitation, discharge, sediment load, and river water levels are correlated by making graphs between these parameters to analyze and identify the threshold limits and main reasons for flood. It explains that discharge and sediment load increases with precipitation during monsoon season, whereas the water storage capacity of the river decreases due to siltation and artificial levee. Hence, during heavy rain in this region, water rises in the channel, which either overtops the bank or breaks the levee and creates the flood. Flood inundation map was prepared using GIS techniques from 70 m base level to 81.5 m high level, which indicate the inundated area with every 1–2.5 m rise in the Rapti river water level. Rating curves and flood frequency curves have been prepared to identify the recurrence interval for major floods. It is concluded that prolong heavy precipitation, discharge variability of the river, siltation in the river bed, artificial levee, and anthropogenic impacts on younger river terraces and river valley leads to flood. Its affect is devastating when water level, discharge and sediment loads are above 77 m, 5000 m3/sec, and 5 metric tones respectively. The flood inundation map and recurrence interval are useful parameters for flood risk management, whereas the upland terrace is flood-free surface suitable for settlement. This study can be used as a model for other flood prone regions.

Combining field monitoring and aerial imagery to evaluate the role of gully erosion in a Mediterranean catchment (Tunisia)

Abstract: The objective of this study was to address the role of gully erosion on sediment delivery at the catchment scale by assessing the contribution of gullies to the total sediment yield at the outlet of the Kamech (Cape Bon, Tunisia), which is a small cultivated catchment in the Mediterranean environment. The methodology was based on the long-term analysis of the evolution of gully volume, and the total volume of sediment that reached the catchment outlet. The evolution of gully volume was assessed by field surveys and image analysis. The total volume of sediment was calculated using intensive field monitoring of the runoff and erosion in the reservoir at the catchment outlet. The contribution of gullies to the total flux of sediment that reached the catchment outlet was estimated to be less than 30%. This means that siltation of the reservoir was mainly due to erosion processes other than gully erosion, i.e., topsoil erosion processes such as rill and interrill erosion in cropland areas. This result was consistent with a previous independent estimation of the apportionment of gully and topsoil erosion sources conducted at the same site using the fingerprinting approach. Therefore, the study confirms that, even in a heavily eroded agricultural catchment area with a dense network of gullies, gully erosion nevertheless accounted for less than 30% of the total erosion balance. The results also showed that the cumulated gully length has decreased in the last fifty years, thus revealing the declining role of gullies. However, the decrease in gully activity could not be linked to an isolated factor. Complementary observations, such as the appearance of vegetation in the permanent gullies and the decrease in the percentage of active gullies, showed that the degree of hydro-sedimentary connectivity in the Kamech catchment has probably entered a decreasing phase. We also analysed the potential use of historical aerial images for studying gully erosion. We found that these images were useful for calculating accurate ortho-photography but failed to provide a DEM with the precision required to measure the historical volume of gullies.

Hydropedology and soil evolution in explaining the hydrological properties of recharge dams in arid zone environments

Abstract: The effects of anthropogenic activities on soil evolution due to siltation and the impact on hydraulic properties of the vadose zone on the augmentation of the aquifer recharge are investigated for the Al-Khoud dam in Oman. Inside the dam reservoir and in areas adjacent to the embankment, downstream, 33 pedons (of depths between 1.5 and 2 m) were excavated and studied during 2011 and 2012. Soil analysis revealed that the subsoil’s physiochemical properties of the study area are continuously changing due to damming, i.e., alteration of the natural runoff, intensified sedimentation, and infiltration. Variation of hydropedological properties caused by the geotechnical construction is evident in a distinct vertical stratification of texture of accrued sediments and almost an order of magnitude drop in saturated hydraulic conductivity (K s ) of the dam bed. Correspondingly, spilling of ponded water over the dam crest occurs more frequent and therefore increases the potential hazards of flooding of the downstream recharge area. Some fine particles of the suspended load carried to the reservoir by the feeding wadi migrate vertically downward, driven by seepage, into the originally coarse matrix of the parent soil and cause clogging of large pores (with time, hard pans in the subsurface are developing) even without visible cake formation on the soil surface. Development of hard pans was also discovered in pedons at depths close to 1 m. This is attributed to presence of a pedogenic carbonate derived from the parent rock and formed by precipitation of dissolved salts due to a vertical upward moisture evaporation to a hot and dry bed surface during prevailing dry bed periods of dam operation. K s measured downstream of the dam was relatively high (6 m/day) and was three times higher than the average value inside the reservoir (2.1 m/day), ranging there between 0.01 and 3.96 m/day, but less than at the upstream site outside the reservoir.

Channel geometry and flow estimates for two small mountain streams in the middle hills, nepal

Abstract: As part of a hazard mapping and evaluation study of the Kathmandu-Kakani area in the Middle Hills, Nepal, conducted as a United Nations/Unesco project, channel conditions at 43 stations on Thulo Khola and Ghatte Khola have been measured and peak discharges for the 1979 monsoon (a season of approximately normal precipitation) have been estimated. The results show that variations in the channels and the flows they carry in this tropical, high-relief area conform to the patterns established by other studies. This should allow hydrologic concepts and models that are derived from the long records and widespread studies of mid-latitude regions to be used in tropical mountains, where there are few direct observations. Flood hazards in the Kathmandu-Kakani area appear to be limited to those riparian zones which consist of constructed agricultural terraces. Damage to these terraces by channel expansion could provide a large influx of sediment to the fluvial system with consequent downstream siltation.

Comprehensive analysis on the sediment siltation in the upper reach of the deepwater navigation channel in the Yangtze Estuary

Abstract: The results from both the field measurements and numerical simulation were reported to comprehensively analyze the sediment siltation in the upper reach of the Deepwater Navigation Channel Project in the Yangtze Estuary after the project has been implemented. In this research, firstly some basic information about the river evolution in the Yangtze Estuary is analyzed, including the variations of water depths in the Hengsha Passage and the inlet cross-sections of the North Passage and the South Passage, and changes of diversion ratios of ebb flow and sediment flux in the North Passage and the South Passage. Then the Delft3D-FLOW model is applied to simulate the hydrodynamics and sediment transport in the Yangtze Estuary. This model has been calibrated and the simulated results agree well with the measured data of the tidal levels, flow velocities and suspended sediment concentration (SSC), indicating that the model can well simulate the hydrodynamics and sediment transport of the Yangtze Estuary caused by the Deepwater Navigation Channel Project. The research results show that the development of the Hengsha Passage and decrease of diversion ratio of ebb flow and sediment flux in the North Passage are the main reasons of sediment siltation in the upper reach of the Deepwater Navigation Channel in the Yangtze Estuary.