Stabilizing Sand Dunes

A. D. Stoesz and Robert L. Brown.

To many of the millions of Americans who see them each year, the large sand dunes of our country are mounds and ridges of great beauty. To many others the dunes mean terror, when the shifting sands threaten to cover and destroy homes; a menace, when they make highways unsafe; an economic danger, when they make farms and ranches worthless.

Sand dunes in the United States occupied an area one-tenth as large as that of the agricultural land in 1957.

Inland dunes occur in geologic lake-beds, old beach lines and deltas, glacial outwash, along rivers, and in deserts and midgrass areas, where they are formed by wind action.

The second major group of sand dunes includes the ones on the Atlantic and Pacific coasts and the shores of the Great Lakes. They are developed naturally as coastwise currents take sand from the rivers where they enter the ocean or as coastal headlands are eroded. Winds take the sand as it is deposited on the beach and dunes form as the sand is blown inland.

Sand dunes may become unstable at any time because the vegetative cover is destroyed by fire, by overgrazing with livestock, by jetties at the mouths of rivers, and by the construction of buildings and roads, wearing of trails, and cultivation.

Dunes lack a soil profile because they are unstable and undeveloped. Water percolates through the sand freely. Because the sand has no organic matter, it does not retain very much moisture for plants. The fertility level is low. The sand is fine, loose, and easily moved by wind. It is abrasive and destructive to plants when it moves.

The basic problem in stabilizing a dune is to still the sand as near as possible to its source of origin or the point of disturbance and to establish and maintain permanent vegetation on it. A renewed supply of fresh sand is continually being washed up on coastal dunes by the waves. On inland dunes new material is not added, and the movement of sand is caused by the disturbance or destruction of the existing vegetative cover.

Once the sand is stilled, a plant cover can be established.

Sand dunes can be stilled temporarily by vegetation or by artificial barriers.

VEGETATION used on the sand must thrive in shifting sand and survive inundation or keep pace with the sand.

The plants generally have coarse, stiff stems that resist sand blasting. They should be unpalatable to livestock, because any grazing will be detrimental to dune control. They will have to grow fast on practically a starvation diet, or their nutrients will have to be supplied in fertilizers. Plants seldom provide a complete cover.

Grasses of primary importance for stilling sand in the Pacific Northwest are European beachgrass (Ammophila arenaria), a naturalized grass used to control coastal sand dunes since 1896; American beachgrass (Ammophila breviligulata), a native of the Great Lakes dunes and of the Atlantic coast as far south as North Carolina; and Volga wildrye (Elymus giganteus), a strain developed by Soil Conservation Service nurseries from a grass from Siberia.

The primary sand-stilling grass on the Great Lakes dunes and on those of the Atlantic coast north of North Carolina is American beachgrass.

The Carolina dunes are stilled most effectively by plantings of sea-oats (Uniola paniculata) and sea panicgrass (Panicum amarum). Inland dunes of the Great Plains are anchored effectively by seeding the dunes to such fast-growing plants as broomcorn (Sorghum vulgare, var. technicum), Sudangrass (S. vulgare var. Sudanese), and Black amber cane (S. vulgare var. coffrorum).

The seed heads are clipped before they mature to prevent reseeding the following year. The high stubble is left standing. It makes a protective mulch in which to seed a permanent cover.

Seeding these rapid-growing species as sand-stillers has not been successful on the inland dunes in the Western States because of the Mediterranean (rather than continental) climate. Seeding for initial sand-stilling is not successful on any of the coastal dunes because of the continual wind, whose velocities exceed 15 miles an hour.

Sand-stilling grasses such as the American and European beachgrass and Volga wildrye are planted as clones or culms rather than by seeding, because the winds cause the sand to shift too much to permit the establishment of seedlings. (A clone is a single stalk taken from a clump; it can produce a new clump.)

Plantings on coastal or lake dunes will usually start if a foredune (or "barrier" dune) that will stop the sand is established as near the water line as possible rather than let it continue to blow inland.

Planting on these dunes is in spaced hills rather than as a broadcast planting. Spacing of hills and number of culms to a hill vary. When the sand slopes to be planted are gentle, spacing of hills should be 18 by 18 inches, with 5 culms to a hill. On steep slopes, road cuts, and on the foredune, spacing should be 12 by 12 inches, with 3 culms to a hill. On protected, flat areas, a spacing of 24 by 24 inches, with 5 culms to a hill, will be close enough. Wider spacing will cause the planting to scour out, and loss will occur. Culms must be planted 12 to 14 inches deep for good survival and effectiveness against wind.

An example of the costs of planting stock and the planting operation is the rates paid in the Siuslaw district of Oregon in 1956. Planting stock of European beachgrass cost 1.75 dollars for 1 thousand culms (95 thousand culms are used to an acre for an 18 by 18 inch planting of 5 culms to a hill). Planting by hand was contracted at 80 dollars an acre for 18-inch planting. Planting by machine has required approximately 18 man-hours and 3 tractor-hours an acre, compared to 80 man-hours for hand planting.

Good, vigorous, clean, and carefully graded stock is essential. All dead trash should be cleaned from the culms. Underground stems are broken back to one or two nodes. The long tops should be cut off, for they serve no useful purpose and cause excessive evaporation. The culms should be about 20 inches long when ready for planting.

In trials and field plantings at Warrenton, Oreg., maximum temperature and not precipitation determined the survival of European beachgrass. Survival of plants in 50 percent or more of the hills was always obtained when maximum temperatures for the 72 hours following planting did not exceed 55 F. Survival was always less than 50 percent when maximum temperatures exceeded 60 . Between the two temperatures survival was erratic. A survival of 50 percent or more is necessary for stilling eroding dune sand.

AMERICAN BEACHGRASS is much less sensitive to environmental factors than the European beachgrass. Successful dune plantings on the Pacific coast can be made with American beach-grass any time between early fall and late spring. Volga wildrye should be planted when maximum temperatures are under 55 .

The best survival of these plants on the Great Lakes and Atlantic coast dunes was when they were planted between the time the grass matures in the fall and the ground freezes. The period of planting may be extended until growth starts in the spring in places where the ground does not freeze.

Plantings for sand stilling should be fertilized as soon as growth starts in the spring. Nitrogen at the rate of 40 pounds an acre must be applied to insure success. Inorganic fertilizers, such as ammonium sulfate, have been more successful than organic fertilizers.

Seedings for sand stilling on the inland dunes in the Great Plains are made with regular farm equipment at the same time as regular crop seedings of the same species are made.

ARTIFICIAL BARRIERS can be used to stabilize temporarily the dune sands that cannot be planted effectively. The barriers include oil, clay, gravel, picket fence, brush, and hay.

Emulsified asphalt has been used in emergencies by several State highway departments. The North Carolina State Highway and Public Works Commission used AE-5 emulsified asphalt cut to 25 percent asphalt emulsion and 75 percent water and applied at the rate of 0.4 gallon the square yard. The application penetrated the sand 1 inch and did not crust. Heavier concentrations of asphalt crusted. Seedings made under the lighter application germinated and grew, but they failed under crusts.

The concentration of the emulsified asphalt depends on climatic conditions. The higher temperatures require lighter concentrations for penetration without crusting. A danger exists of burning seedlings in sands treated with asphalt because sand and asphalt become extremely hot from the rays of the sun. Hot crude oil sprayed on the unstilled sand has been used as a temporary stilling agent in several of the arid or semiarid dune areas.

Clay is effective, but it is expensive. Blankets of clay 4 to 6 inches thick are spread over the unstable dune sand. Permanent seedings or plantings can then be made.

Gravel or crusher waste was used by the Corps of Engineers on a railroad relocation near McNary Dam in Oregon and Washington. Sand dunes with slopes flatter than 1 to 3 can be stabilized with relatively small amounts of material. The important thing is to make complete coverage with materials not subject to movement by the wind. A 2-inch cover was used on 1:3 slopes. A 4-inch cover should be used on steeper slopes. This work cost 400 to 645 dollars an acre.

Picket fences have two primary uses. Located approximately at right angles to the prevailing winds, they cause temporary deposits of sand. Located at a tangent to the prevailing winds, they direct the sand-laden wind and may cause sand scouring.

Picket fences were used on the Clatsop Plains dune area at the mouth of the Columbia River in Oregon to build up a foredune. Two parallel lines of fence 30 feet apart were needed to get proper and stable shape in the dune. (This distance between fences may not be the same in other localities; the size of the sand particles and the velocity of the winds determine the spacing.) The pickets were 4 feet long and made from 1 x 4-inch lumber. The openings between pickets was equal to the width of the picket. A smaller space caused scour along the fence. A greater space allowed the wind to move sand through the fence unstilled. The top of the picket fence should be even, because unevenness at the top causes the dune to be uneven.