Wednesday, April 24, 2013

Building Dunes Part 1:  Where sand goes from the beach

Written April 21, 2013, published in late April, 2013, all photos by Kathleen Sayce.

Along the oceans of the world, wherever there is surf and sufficient sand, there are beaches. These are dynamic landforms; they shape and reshape themselves in the surf and wind, to grow and recede with the tides and seasons, always on the move.  Sand, waves and wind are central to beach formation. 

Sand in the surf zone and on our local beach has several potential fates. First, in the intertidal zone, it can be drawn back into the water and moved along the beach by currents or drawn out into the ocean by very high waves. We see this in most winters, when high surf removes the wide flat summer beach, and changes it to a narrow slope that starts in or near the vegetation line.  Most of that sand goes into the surf zone and moves north along the beach during the winter. Some of it comes back onshore the next summer in a new location. Some of the sand moves west into deeper water, and stays there. 

Second, some of the sand on the beach blows into the dunes, the vegetated area, where it builds up the form of the dune. There’s a geologic term for this, of course, which is saltation, the movement of fine mineral grains by wind in bounces over a surface.

Originally used as an illustration of black sand on a winter beach, this also shows sand blowing from left to right into the dunes, to build up the elevation and the width of the fore dune––the dune immediately adjacent to the beach. 

In summer, strong northwesterly winds blow dry sand along the open flats and up into the dunes. In winter, with strong winds from southerly directions, sand is also blown/washed into the dunes, some of it carried on salt spray or at the surf edge. I haven’t set out measuring sticks to see which season’s winds (summer or winter) move more sand, but one big storm can deposit six inches of sand on a dune top.  From the air it looks like a water cannon plastered the dunes with sand slurry. The net effect is to build a dune at an angle to both wind directions, one that parallels the beach. 

Third, some sand is used for construction, to sand cranberry bogs, and land filling. It’s usually scraped up near beach approaches and outfalls, and hauled off by truck. In a typical year, about a million cubic yards of sand is hauled off the local beach. This sand moves permanently out of beach circulation and nearby dune building processes, or as permanently as our sand spit stays above sea level. 

Winter storms blow build fresh layers of black sand into dunes over beachgrasses, at Benson Beach, Cape Disappointment State Park.  Next spring these grasses will send rhizomes into the sand and bind it, while their leaves sprout above the new level. 

There’s often a visual difference in summer-wind-deposited sand versus winter-storm-deposited sand.  Summer sands are light-colored, light-weight, and high in quartz and feldspar. Winter sands may include darker, heavier sands, which have iron and manganese grains. Winter sands layer up like dark frosting over vanilla cake, with black sands atop the lighter-colored summer sands. 

One of the striking impacts of Hurricane Sandy on East Coast shorelines in 2012 was the fate of hundreds of miles of beaches. These beaches went away, washed over their barrier spits inland, into the bays behind, or dragged back out to sea. Many were built from sands mined from deeper waters by dredges, and deposited onshore. When Sandy hit, there wasn’t enough sand in the nearshore to keep the beaches or dunes intact in storm surf or surges. Luckily for us, we don’t have this problem.  Despite dams on the Columbia River that trap sediments, and dredging, which usually deposits the sand in deeper water than surf can pick up, there’s still considerable sand in the surf zone along most of our beach.  The proof of this is in the ongoing build-out and –up of dunes along the beach. 

Building Dunes Part 2:  Dunes As Seawalls

We are lucky to have healthy outer dunes and ample sand on the beach and in the surfzone to build and maintain them.  Our dunes aren’t just pretty places to walk and live. These dunes also form a seawall, a protective barrier between our communities and the ocean. Recent tsunamis in Japan and other countries have an important lesson to teach those who live along the ocean:  the higher and more continuous the seawall, the better the protection is for that community from storms and tsunamis. 

There is a second lesson to be learned from the recent tsunamis in Japan:  no seawall can be high enough.  Whatever the planners and engineers estimate is high enough, is probably not. Go higher.  We do not have to do this ourselves with bulldozers or shovels. With ample sand, onshore winds, beachgrasses and sand trapping structures, nature will build the seawall higher. She just needs sand and time.  All the basic aids to dune building are already here. 

Remnants of a wood fence at Benson Beach, Cape Disappointment State Park, placed by the US Army Corps of Engineers for a beach enhancement trial, where dredged sands from the Columbia River Entrance were directly placed on the beach above the tideline to build up an eroding section north of the north jetty. The fencing helps slow and hold sand, but erosion during winter storms continues to eat away at the sand each year. The lower 3-5 miles of beach is one area that is moving eastward and eroding, while most beaches to the north continue to grow up and westward.

Our onshore winds are legendary, and being eyed by wind and wave energy firms as power generating sources. Two introduced beachgrasses grow on the dunes, American and European beachgrass (Ammophila breviligulata and A. arenaria, respectively); each species does a good job of binding sand that blows into the dunes from the ocean beach. 

To build dunes up, winds blow the sand up into the dunes, sand fencing or beachgrasses slow the winds at the top, and the sand grains drop out over the crest of the dune. Then beachgrasses grow into the loose sand and bind it.  Low narrow dunes become broad higher dunes over several years. 

Sand fencing has been in widespread use along East Coast beaches for many years. This summer, there will be thousands of feet of new fencing in place on many beaches, to start rebuilding those storm-eroded dunes, badly damaged in Hurricane Sandy last fall. Sand fencing needs some tending, resetting the posts and lifting the fencing every year or so to keep the fencing in the air. Over time, it’s a simple, inexpensive and effective way to build broad high dunes. 

There’s one more problem to attend to:  The gaps in the seawall.  Approach roads that cut through dunes are gaps in the seawall.  Sloughs that drain across the beach through dune cuts are gaps in the seawall.  Dunes lowered for the view are gaps. Collectively, these become access paths for storm surges and tsunamis. Any breach in the dune is a gap in the integrity of the seawall.  

Beachgrassses, sand and time can close all these breaks, if we let nature go to work.  Roads and access paths can go over the top of the dunes. Outfalls for storm drainage and sloughs can go through culverts under dunes instead of open cuts through them. Sand fencing can build up those areas with especially low dune crests.  

Pacific County allows, with the right permits, the maintenance of dunes at 24 ft for ocean views. Having watched videos of tsunamis overtopping a 35 ft high seawall in Japan, I’m inclined to think that our beachside dunes should be much higher.  Eight to ten feet of subsidence on a 24 ft dune produces an effective height of 14 to 16 feet after the next big subduction zone earthquake. Forty feet might be a better target height for our beachside foredunes. 

We can build up this seawall with nature’s help, so that when the next big Cascadia earthquakes and attendant tsunamis hit, we’ll have more protection in place than we have right now. This might be enough to help some of us survive to rebuild after the next big one.