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Scripps Scientist Explains Beach Nourishment And Sand Movement Along The Coast

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Posted: Friday, June 21, 2019 12:13 pm

Bonnie Ludka, PhD, of Scripps Institute of Oceanography (SIO), spoke to about 40 people at the Tijuana River National Estuarine Research Reserve June 15 as part of the their bi-monthly speaker series.  

Ludka presented the results of her research into the movement of beach sand along the coast and the impact of beach nourishment on the closure of the Tijuana River in 2016.

Southern California beaches are experiencing a sand deficit. She explained that beach sand is normally created by sediment flowing from rivers and cliff erosion. Unfortunately, river sand that would normally flow into the ocean is now trapped behind dams that are present in every southern California river except the Santa Margarita river near Oceanside. 

Many southern California ocean cliffs have been armored to prevent erosion which further reduces the amount of natural sand available for the beaches.

Other sources of sand used to nourish beaches in the past were harbor dredging and excavation from development but both of these sources have slowed over the years.

Beach nourishment is one way to address the sand deficit and Ludka’s research used beach nourishment events as natural laboratories to study how sand moves from the beach to the ocean and back, called cross shore transport, and how sand moves up and down the coast which is called long shore transport.

SIO has been measuring the amount of sand on Imperial Beach monthly for the past 11 years using all terrain vehicles, push dollies and jet skis. Since sand movement is primarily caused by wave action, they also measure waves using buoys that are part of the Coastal Data Information Program (CDIP). 

As many people have observed over the years, Ludka showed a chart which displayed the cyclical variation of beach width as waves from the north during the winter tend to remove beach sand while waves from the south during the summer tend to restore beach sand. Over time, more sand is removed in the winter than is replaced in the summer which causes the gradual reduction of beach sand over time. 

Imperial Beach’s most recent nourishment was in 2012 when the equivalent of 120 Olympic sized swimming pools of sand were placed on the beach. The sand grains used in 2012 were much larger than previously used which resulted in the sand lasting longer than previous nourishments. When other cities used sand that was the same size as native sand, the beach nourishment benefit only lasted a year or two. However, this coarser sand acted as a “sponge” and while the wider and higher beach protected infrastructure from direct wave action, it may have contributed to the flooding of homes along south Seacoast Drive.

Over time, much of the sand from this nourishment continued moving south which eventually filled the mouth of the Tijuana River requiring emergency dredging in April of 2016. While the additional sand from beach nourishment contributed to the 2016 closing of the Tijuana River, the 2016 El Niño event also played a major role. The Tijuana River previously closed in 1983 due to an El Niño even though there wasn’t any beach nourishment involved. 

Beach sand movement is an active area of research that is very complex and scientists expect that better modeling will help develop improved methods to maintain healthy California beaches.

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