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Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
River deltas are structures formed by the deposition of sediment at their mouth when the streamflow enters an ocean, sea, estuary, lake, or reservoir. The rate of deposition should be higher than the rate of removal by waves and currents, and it depends on flow velocity and load and the salinity of the stream water and receiving water body. The stream flows into the sea with a turbulent flow into the relatively still body of another fluid, interacting with waves and currents. As river water mixes with saline water, the fine clay parts flocculate into aggregates that are too large to remain long in suspension. Therefore, sedimentation is rapid, but suspended sediments may flow offshore even for several kilometers forming turbidity or density currents.
River Delta Processes and Shapes
Published in Yeqiao Wang, Wetlands and Habitats, 2020
Douglas A. Edmonds, Rebecca L. Caldwell
A river delta is an accumulation of sediment on a coastline adjacent to a river. Deltas can form on any coastline, and recognizing them on satellite images is not always straightforward, especially if the “sediment accumulation” of the delta is small or not easily visible. We define two geomorphic criteria for recognizing deltas. These criteria are chosen because they imply substantial sediment accumulation. First, deltas may have a noticeable depositional protrusion (or bump) that causes deviation from the mean shoreline position and is unambiguously linked to the river (see Figure 8.1e for example). The minimum scale of the protrusion is not easy to define, but it should be wider (in a shore-perpendicular direction) than a few multiples of the river width to avoid confusing deltas with prograding river mouths or tie channels that have no delta.[5] The second criterion is clear evidence for a distributary channel network (Figure 8.1a–d). Distributary channels are river channels that branch off from a parent channel and do not rejoin before intersecting the coast (Figure 8.1b). By their nature, these channels facilitate sediment accumulation as they distribute sediment over a broad area. Only one of these criteria is necessary for classifying a geomorphic feature as a delta, and often deltas exhibit both a protrusion and a distributary network (Figure 8.1c).
Climate Change Impacts on Groundwater
Published in Mohammad Karamouz, Azadeh Ahmadi, Masih Akhbari, Groundwater Hydrology, 2020
Mohammad Karamouz, Azadeh Ahmadi, Masih Akhbari
As the temperature rises globally, the melting of ice sheets and glaciers will increase and will lead to sea-level rise. When sea-level rising happens, the saltwater will penetrate farther inland and upstream in the low-lying river deltas (IPCC, 1998). Salinization endangers the surface water, urban water, and groundwater supplies, damaging the ecosystems, and the coastal farmland (IPCC, 1998). Furthermore, the lower rainfall reduces groundwater head and exacerbates the impacts of sea-level rise. The saltwater intrusion into the limestone aquifers is higher than the alluvial aquifers. The saltwater intrusion in the coastal aquifers is another key groundwater quality concern. As the groundwater pumping increases for municipal demand along the coast, the freshwater recharge in the coastal areas declines, and the sea level rises. Thus, the groundwater aquifers are affected by the seawater infiltration. In areas with the low land surface elevations, the relative sea level has been greater. Further increases in the sea level may accelerate the salinity intrusion into the aquifers and affect the coastal ecosystems.
Community-of-interests across source-to-sea systems: an international law perspective
Published in Water International, 2021
Flavia Tavares da Rocha Loures
At the same time, river deltas ‘are increasingly vulnerable to flooding and submergence through the combined effects of sediment-trapping by dams and sea-level rise due to climate change and, in some cases, over-abstraction of groundwater’ (Mathews et al., 2018, p. 3). Many estuaries and coastal ecosystems starve for water, sediments and nutrients because of resource diversions far above sustainable levels, so that downstream flows are severely diminished and impacted rivers often fail to reach the sea. Already, many coastal communities find themselves trapped between the impacts of climate change and upstream activities that deprive deltas, estuaries and mangroves from the sediments they need for replenishment.
An improved method for mapping tidal waterways based on remotely sensed waterlines: A case study in the Yellow River Delta, China
Published in Marine Georesources & Geotechnology, 2020
Xiang Yu, Chao Zhan, Mengquan Wu, Xueli Niu, Xueyu Zhang, Qing Wang, Buli Cui
The Yellow River Delta is the youngest and second largest river delta in China. It is an alluvial plain where much sediment is carried by the Yellow River deposit in the Bohai depression (see Figure 1). It is well known throughout the world as a sediment-laden river. In recent years, Yellow River had carried approximately 1.68 × 108 tons of sediment to the estuary (Xu et al. 2002). The sediment deposited in the slow-flowing delta formed an extensive intertidal zone and special marsh landscape. In 1855, the Yellow River migrated to the Daqing River and was injected into the Bohai Sea by Lijin. Taking Ninghai as the apex, a fan-shaped delta from the north of the Saier River and south to the Zimaogou was formed. The Yellow River Tail River channel frequently oscillated within the delta alluvial sector. Due to the dual effects of sedimentation and ocean dynamics from the Yellow River into the sea, the erosion and siltation were fierce. The coastal siltation and erosion were alternated, and the silt tidal flat had been widely developed. On the tidal flat above the average high tide line, the divergence of the tidal flat increases, forming a plume tidal system. River routes in the Yellow River Delta changed due to global warming, sea-level rise, runoff decrease and water-sediment level, as did the waterlines and the tidal waterways. Seawater intrusion caused by storm surge led to a series of ecological risks and socioeconomic losses (Zekri et al. 2008). Coastal ecosystems tended to be more vulnerable to seawater intrusion due to the intensified soil salinization, which was caused by extensive development and use of coastal zones (Gamito 1997). To survey the risk of seawater intrusion and promote sustainable development in coastal ecosystems, it was essential to understand the distribution status of tidal waterways in the Yellow River Delta. We carried out an experiment in mapping the tidal waterways based on remotely sensed waterlines following the process below (Figure 2).