{"id":5329,"date":"2015-07-23T04:33:18","date_gmt":"2015-07-23T04:33:18","guid":{"rendered":"http:\/\/revoscience.com\/en\/?p=5329"},"modified":"2015-07-23T04:33:18","modified_gmt":"2015-07-23T04:33:18","slug":"predicting-the-shape-of-river-deltas","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/predicting-the-shape-of-river-deltas\/","title":{"rendered":"Predicting the shape of river deltas"},"content":{"rendered":"

New method may help engineers determine coastal impact of dams and levees.<\/strong><\/em><\/p>\n

\"A<\/a>
A variety of deltas: the Mississippi birdfoot delta (left) and Mexico’s Grijalva cuspate delta (right).
Image: NASA Landsat<\/figcaption><\/figure>\n

CAMBRIDGE, Mass. —\u00a0The Mississippi River delta is a rich ecosystem of barrier islands, estuaries, and wetlands that\u2019s home to a diverse mix of wildlife \u2014 as well as more than 2 million people. Over the past few decades, the shape of the delta has changed significantly, as ocean waves have carved away at the coastline, submerging and shrinking habitats.<\/p>\n

To keep flooding at bay, engineers have erected dams and levees along the river. However, it\u2019s unclear how such protective measures will affect the shape of the river delta, and its communities, over time.<\/p>\n

Now researchers from MIT and the Woods Hole Oceanographic Institution (WHOI) have devised a simple way to predict a river delta\u2019s shape, given two competing factors: its river\u2019s force in depositing sediment into the ocean, and ocean waves\u2019 strength in pushing that sediment back along the coast. Depending on the balance of the two, the coastline of a river delta may take on a smooth \u201ccuspate\u201d shape, or a more pointed \u201ccrenulated\u201d outline, resembling a bird\u2019s foot.<\/p>\n

The new metric may help engineers determine how the shape of a delta, such as the Mississippi\u2019s, may shift in response to engineered structures such as dams and levees, and environmental changes, such as hurricane activity and sea-level rise.<\/p>\n

Jaap Nienhuis, a graduate student in the MIT-WHOI Joint Program in Marine Geology and Geophysics, says the effects of climate change, and the human efforts to combat these effects, are already making an impact on river deltas around the world.<\/p>\n

\u201cBecause there are so many people living on a river delta, you want to know what its morphology or shape will look like in the future,\u201d Nienhuis says. \u201cFor the Mississippi, the river supplies a lot of sediment. But because there are a lot of dams on the Mississippi nowadays, there is not as much sand coming down the river, so people are very worried about how this delta will evolve, especially with sea-level rise, over the coming centuries.\u201d<\/p>\n

Nienhuis, and Andrew Ashton and Liviu Glosan of WHOI, report their results in the journal\u00a0Geology.<\/em><\/p>\n

Shaping a shoreline<\/strong><\/p>\n

Over hundreds of thousands of years, a river\u2019s sand and silt flow toward the coast, ultimately piling up at a river\u2019s mouth in the form of a low-lying delta. A delta\u2019s coastline can be relatively smooth, with most sand depositing from the main river, or it can fan out in the shape of a bird\u2019s foot, as the river bifurcates into tributaries and channels, each of which deposits sand in finger-like projections.<\/p>\n

Scientists often characterize a delta as either river-dominated or wave-dominated.<\/p>\n

In a wave-dominated delta, such as the Nile River delta in Egypt, incoming ocean waves are stronger than the river\u2019s flow. As a result, waves push outflowing sediment back along the coast, effectively smoothing the coastline. By contrast, a river-dominated delta, such as the Mississippi\u2019s, is shaped by a stronger river, which deposits sand faster than ocean waves can push back, creating a crenulated coastline.<\/p>\n

While this relationship between rivers and ocean waves is generally understood, Nienhuis says there is no formal way to determine when a delta will tip toward a smooth or pointy shape.<\/p>\n

The researchers came up with a simple ratio to predict a delta\u2019s shape, based on a river\u2019s sediment flux, or the flow rate of sediment through a river, and the strength of ocean waves, determined by a wave\u2019s height, frequency, and angle of approach.<\/p>\n

Based on the various factors that determine the overall ratio, the team determined the point at which a delta would no longer be a smooth outline, shaped by ocean waves, but instead, a pointy coastline, influenced more by the river.<\/p>\n

\u201cAt some point there\u2019s so much sediment that you exceed the maximum of what waves can do,\u201d Nienhuis says, \u201cand then you become a \u2018bird foot,\u2019 or river-dominated delta, because the river is so much stronger.\u201d<\/p>\n

A delta\u2019s tipping point<\/strong><\/p>\n

Nienhuis and his colleagues applied the new method to 25 river deltas on the north shore of the Indonesian island of Java, a region where sediments have deposited on a shallow continental shelf, creating a wide variety of delta shapes.<\/p>\n

For each delta, the team used a global wave model developed by the National Oceanic and Atmospheric Administration to determine the height, frequency, and direction of each incoming wave. The researchers also used a model to determine the corresponding river\u2019s sediment flux.<\/p>\n

Using data from both models, Nienhuis determined the ratio of river-to-ocean wave strength for each delta, and found that those deltas with a ratio greater than or equal to 1 were more likely to have multiple river channels, with deltas that project out from the shoreline. The main factor determining this transition turned out to be the angle at which ocean waves generally approach the coast: If the angle of approach is 45 degrees or greater, then ocean waves are no longer able to smooth out the amount of sediment coming from a river, tipping a delta\u2019s shape toward a river-dominated morphology.<\/p>\n

Nienhuis says the group\u2019s method may help engineers predict the shape a delta may take if erected dams or levees change a river\u2019s sediment flow. Similarly, the method may estimate the evolution of deltas with climate change, as rising sea levels and increased hurricane activity will likely alter the behavior and magnitude of ocean waves.<\/p>\n

This research was funded, in part, by the National Science Foundation.<\/p>\n

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New method may help engineers determine coastal impact of dams and levees. CAMBRIDGE, Mass. —\u00a0The Mississippi River delta is a rich ecosystem of barrier islands, estuaries, and wetlands that\u2019s home to a diverse mix of wildlife \u2014 as well as more than 2 million people. Over the past few decades, the shape of the delta […]<\/p>\n","protected":false},"author":2,"featured_media":5330,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-5329","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0-300x200.jpg",300,200,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",600,400,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",600,400,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",540,360,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",95,63,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",639,426,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/07\/MIT-Delta-Shape-1_0.jpg",150,100,false]},"author_info":{"info":["RevoScience"]},"category_info":"Research<\/a>","tag_info":"Research","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/5329","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=5329"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/5329\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/5330"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=5329"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=5329"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=5329"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}