River intrusion
Definition and classification
River intrusion describes the process by which a river enters a receiving water body, which could be a lake, a reservoir, or an ocean. As the temperature, salinity, and sediment concentration of the river flow and the receiving water body may not be the same, the density of the river flow (ρr) may differ from that of the receiving water body (ρa). According to the relative relationship between ρr and ρa, the river intrusion process can be classified into three conditions with different hydrodynamic behaviors: hyporpycnal (ρr < ρa), homopycnal (ρr = ρa), and hyperpycnal river inflow (ρr > ρa). Under different hydrodynamic conditions, the influence of the river intrusion on the nearshore environment, ecosystem, and geomorphology are also different.
Hyporpycnal River
A hyporpycnal river flow usually happens when a river enters the ocean, as seawater has a larger salinity than freshwater, resulting in a larger density. A typical example is the Columbia River plume when it enters the ocean. The river flow transports longitudinally into the ocean due to its initial momentum, while at the same time, it spreads laterally on the water surface under the influence of positive buoyancy. If the size of the river plume is large enough, the Coriolis force would make it turn anticyclonically[3]. Since the river flow floats on the water surface, the pollution and nutrients it carries mainly float on the water surface as well.
Homopycnal River
'Homopycnal' means that the density of the river flow and the receiving water body is almost the same, meaning that buoyancy does not play a role in such a configuration. This condition may happen when a river flows into a lake or reservoir. An example of homopycnal river intrusion happens near Baton Rouge, LA, USA, where a channel outlet discharges into an oxbow lake[4]. Without the influence of buoyancy, the homopycnal river flow may detach from the lake bottom and thus only influence the water close to the water surface[1] or it may extend vertically to the bottom[5] depending on the nearshore geometry and the initial velocity of the river flow. If the river flow detaches from the bottom, materials it carries (e.g., microplastic, contaminant, sediment, and nutrients) would only stay close to the water surface, similar to hyporpycnal river intrusion. Otherwise, the whole depth of the nearshore area would be influenced by the river flow[1].
Hyperpycnal River
The excess density of a hyperpycnal river compared with the receiving water body may come from a lower temperature or higher concentration of suspended load (sediment). Some rivers source from snowmelt, meaning that the temperature of the river flow is lower than the surrounding environment. Considering that water density changes with its temperature and reaches its maximum when it is 4°C, the river flow can be denser than the lake/reservoir water due to temperature difference. Examples of this kind of hyperpycnal river are commonly found in lakes, for example, Canale Italsider River entering Lake Iseo (Italy)[6] and Tokaanu Tailrace River entering Lake Taupo (New Zealand)[7]. Besides the temperature, a much larger suspended sediment concentration may also result in a larger density. Such a flow is also called a turbidity current. For example, the sediment concentration of the Yellow River, the second largest river in China, can be as large as 400 kg m-3, meaning that the river water density is close to 1200 kg m-3 compared with 998 kg m-3 of pure water and 1030 kg m-3 of seawater[8]. Compared with hyporpycnal and homopycnal rivers, hyperpycnal rivers are usually much more powerful and able to transport much longer due to their negative buoyancy. This is because both the hyporpycnal and homopycnal rivers lose their momentum due to friction when they transport and thus can only transport several kilometers from the river mouth. However, for a hyperpycnal river flow, it can be accelerated during its transportation, similar to a car sliding down a slope, and as a result, is able to transport hundreds of kilometers. The longest transport of a hyperpycnal river ever recorded is the Congo river entering the Atlantic Ocean, which was reported to be 1100 kilometers[9]. As the hyperpycnal river is very powerful, it flushes everything in front of it and picks up a large amount of rocks and sediment from the bed[10]. This mechanism causes severe erosion of the lake/ocean bottom and is one of the most important causes of submarine canyons formation near river mouths. Congo Canyon and Nazaré Canyon are two typical examples of submarine canyons shaped by hyperpycnal rivers.
Significance of River intrusion
River flows usually carry a large amount of sediment, contaminants, and nutrients, and thus have an important influence on the environment and morphology of the receiving water domain[11][12]. Although the exact global sediment discharge to the ocean through rivers remains unknown, by extrapolating available data for all drainage basins, the total sediment delivered by all rivers to oceans is estimated to be around 15x109 tons annually[13]. While rivers usually pass by urban areas before they enter lakes and oceans, all the pollution that was discharged into rivers is transported into lakes and oceans along with river flows. For example, those hyperpycnal rivers are believed to play an important role in bringing microplastics into the deep ocean. The river intrusion process is also an important component of global carbon cycling, with approximately 430×1012 g of terrestrial organic carbon transported to the ocean in modern times[14].
References
- ↑ 1.0 1.1 1.2 Broekema, Y. B.; Labeur, R. J.; Uijttewaal, W. S. J. (2019-12-18). "Suppression of vertical flow separation over steep slopes in open channels by horizontal flow contraction". Journal of Fluid Mechanics. 885. doi:10.1017/jfm.2019.972. ISSN 0022-1120.
- ↑ Horner-Devine, Alexander R. (January 2009). "The bulge circulation in the Columbia River plume". Continental Shelf Research. 29 (1): 234–251. doi:10.1016/j.csr.2007.12.012. ISSN 0278-4343.
- ↑ Schiller, Rafael V.; Kourafalou, Vassiliki H. (January 2010). "Modeling river plume dynamics with the HYbrid Coordinate Ocean Model". Ocean Modelling. 33 (1–2): 101–117. doi:10.1016/j.ocemod.2009.12.005. ISSN 1463-5003.
- ↑ 4.0 4.1 ROWLAND, JOEL C.; STACEY, MARK T.; DIETRICH, WILLIAM E. (2009-05-25). "Turbulent characteristics of a shallow wall-bounded plane jet: experimental implications for river mouth hydrodynamics". Journal of Fluid Mechanics. 627: 423–449. doi:10.1017/s0022112009006107. ISSN 0022-1120.
- ↑ Ortega-Sánchez, Miguel; Losada, Miguel A.; Baquerizo, Asunción (January 2008). "A global model of a tidal jet including the effects of friction and bottom slope". Journal of Hydraulic Research. 46 (1): 80–86. doi:10.1080/00221686.2008.9521845. ISSN 0022-1686.
- ↑ Hogg, Charlie A. R.; Marti, Clelia L.; Huppert, Herbert E.; Imberger, Jörg (2013-02-09). "Mixing of an interflow into the ambient water of Lake Iseo". Limnology and Oceanography. 58 (2): 579–592. doi:10.4319/lo.2013.58.2.0579. ISSN 0024-3590.
- ↑ Spigel, Robert H.; Howard‐Williams, Clive; Gibbs, Max; Stephens, Scott; Waugh, Barry (June 2005). "Field calibration of a formula for entrance mixing of river inflows to lakes: Lake Taupo, North Island, New Zealand". New Zealand Journal of Marine and Freshwater Research. 39 (4): 785–802. doi:10.1080/00288330.2005.9517353. ISSN 0028-8330.
- ↑ Xia, Junqiang; Li, Tao; Wang, Zenghui; Zhang, Junhua; Deng, Shanshan (June 2017). "Improved criterion for plunge of reservoir turbidity currents". Proceedings of the Institution of Civil Engineers - Water Management. 170 (3): 139–149. doi:10.1680/jwama.15.00046. ISSN 1741-7589.
- ↑ "Underwater avalanche continued for two days". BBC News. 2021-06-07. Retrieved 2023-01-14.
- ↑ Parker, Gary; Fukushima, Yusuke; Pantin, Henry M. (October 1986). "Self-accelerating turbidity currents". Journal of Fluid Mechanics. 171 (-1): 145. doi:10.1017/s0022112086001404. ISSN 0022-1120.
- ↑ Branch, R. A.; Horner‐Devine, A. R.; Kumar, N.; Poggioli, A. R. (July 2020). "River Plume Liftoff Dynamics and Surface Expressions". Water Resources Research. 56 (7). doi:10.1029/2019WR026475. ISSN 0043-1397. PMC 7507782 Check
|pmc=value (help). PMID 32999509 Check|pmid=value (help). - ↑ Lamb, M. P.; McElroy, B.; Kopriva, B.; Shaw, J.; Mohrig, D. (2010-05-10). "Linking river-flood dynamics to hyperpycnal-plume deposits: Experiments, theory, and geological implications". Geological Society of America Bulletin. 122 (9–10): 1389–1400. doi:10.1130/b30125.1. ISSN 0016-7606.
- ↑ Milliman, John D.; Meade, Robert H. (January 1983). "World-Wide Delivery of River Sediment to the Oceans". The Journal of Geology. 91 (1): 1–21. doi:10.1086/628741. ISSN 0022-1376.
- ↑ Schlünz, B.; Schneider, R. R. (2000-03-22). "Transport of terrestrial organic carbon to the oceans by rivers: re-estimating flux- and burial rates". International Journal of Earth Sciences. 88 (4): 599–606. doi:10.1007/s005310050290. ISSN 1437-3254.
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