VISITORS TO SEASIDE RESORTS like Ocean City, Maryland, and Virginia Beach come for sun, surf, and sand. But the sand would not be there if it weren't for multi-million-dollar, long-term beach replenishment projects that protect the physical and economic foundations of these beach communities from damaging storms.

Virginia Beach taxpayers invest between $2 and $3 million annually to preserve this beachfront that draws a much-larger amount of tourism dollars to this community, says Phillip Roehrs, Virginia Beach's water resources engineer. "There's one thing we can do quite easily to address sea level rise," Roehrs says, "and that is to build the beaches higher."

Terry McGean, city engineer for Ocean City, also thinks that for large oceanfront communities, beach replenishment provides flexibility in the face of the uncertainty of how high — and fast — sea level will rise. "The beauty of a beach replenishment project is that you can adjust it in the face of what sea level rise actually is."

The risk of increased coastal flooding means that beach replenishment projects may become more important to seaside communities. But as supplies of offshore sand used for these projects become more scarce, and competition increases for federal funding to rebuild after larger, more damaging storms, will there come a point when building beaches higher becomes untenable economically and politically?

Ocean-facing beaches gain and lose sand through the seasons and over time as a result of natural processes. Currents that run along the shore move sand along the coast, while other forces continually move sand and other sediments between the beach and offshore sandbars through cycles of erosion and accretion.

Undeveloped beaches, such as those on Virginia's barrier islands, migrate over time, eroding in some places while elsewhere building vast sandy peninsulas like the tip of Assateague Island. Left alone, inlets fill in, and heavy storms cut new channels across islands.

But increasingly since the early 1900s, people have built hardened structures like jetties and seawalls to interrupt the dynamic equilibrium between sand and sea and to protect against the relentless forces of ocean and wind. As high tides and damaging storms threatened nearby hotels, houses, and businesses, an array of shoreline protection measures have been employed to protect life and property. Before those measures became available, retreating was the only option for many homeowners. In the 1920s, the 300 residents of Wash Woods, a community south of Virginia Beach, started to leave, weary of repeated inundations from the sea washing over the dunes. All that remains today is the Methodist Church steeple.

Today there are also what engineers call the "softer" structural options — like beach replenishment, where stretches of beach are rebuilt to resist erosion. Tons of sand are sculpted into a profile designed to provide protection to the hotels and boardwalks that anchor a way of life for many beachfront communities.

These engineered beaches must be replenished every few years or after especially damaging storms through fresh infusions of sand obtained from some other location. The larger projects, like those at Virginia Beach and Ocean City, are built by the U.S. Army Corps of Engineers and have multi-decade project timelines. John Headland, a private engineering consultant involved with coastal projects for over 30 years, says, "You have to replace pavement on streets, and likewise you have to replace sand on beaches."

The first designed beach "renourishment" project was constructed in 1922 at Coney Island, followed in the next 80 years by 333 engineered projects — and 517 million cubic yards of sand — along the Atlantic and Gulf coasts.

The Army Corps of Engineers advised the early projects and since the 1950s has managed all federally funded beach renourishment projects, expanding its traditional role in improving flood protection, harbors, and waterways. The beach sand projects are selected and funded based on "federal interest." This means they must meet a threshold where the cost of preventing storm-related damage to public infrastructure and private homes is significantly lower than the costs likely to be incurred as a result of significant storm damage.

In Maryland, combined costs for the Ocean City beach project have topped $115 million since 1998. But McGean says the latest estimate is that $717 million of damage has been prevented by these beach improvements.

For the Ocean City project — a nine-mile stretch of beach partly buttressed by a sea wall that protects the boardwalk — the federal government provides 53 percent of the cost. The remaining 47 percent is split evenly in two parts, one paid by Maryland and the other paid by the city of Ocean City and the surrounding county.

All parties share in the costs because all derive some benefits. For the Corps, the spending reduces the federal government's costs for disaster relief, insurance programs, and subsidies for rebuilding damaged structures.

Local communities share in the costs because they depend upon the beaches to generate the tax revenues that flow from homeowners and tourism. In Virginia Beach, the cost of periodic replenishment represents about 10 percent of tax revenue from oceanfront tourism, Roehrs says. For communities like Virginia Beach and its waterfront strand to the south, Sandbridge, the sand is, quite simply, why these communities exist.

The projects in Virginia Beach and Ocean City were designed before Hurricane Sandy demolished homes, hotels, and holiday dreams worth billions of dollars in New Jersey and New York coastal communities — and before billions of "hurricane relief dollars" were spent for rebuilding beaches in many mid-Atlantic communities.

Some critics think that large federal subsidies for beach replenishment are unfair because they are funded by many taxpayers living away from coasts to help support the beach-side homes of a wealthy few. The federal funding formula and Corps rules for replenishment projects give more weight to communities with expensive homes than to locales with less valuable real estate.

In 2008, the Corps of Engineers policy stated that all its projects must "incorporate" sea level rise in designs.

Justin Callahan, coastal engineer with the Corps' Baltimore District, explained that beach replenishment projects are more at risk from increases in storm surges — a function of sea level rise and the intensity, direction, and duration of repeated storms — than from sea level rise alone.

In the face of more sophisticated data analysis on sea level rise trends, the Corps is now reviewing design criteria for all its coastal protection projects. Accommodating changing conditions and new data are part of the ongoing review process for long-term beach renourishment projects, says Jen Armstrong, a Corps project manager in the Norfolk district office. "We have a mechanism to go back and re-evaluate our projects based on changing conditions."

This kind of adaptive management, Headland says, offers a conservative approach for coastal communities to meet the challenges of accelerating coastal erosion resulting from rising seas. A beach replenishment project designed to a certain width and elevation geometry can provide protection today and be adjusted periodically to increasingly higher elevations, as needed. "This gives you a lot of flexibility," Headland says, over the planned project period, which can span up to 50 years.

But beach replenishment alone isn't sufficient to protect important infrastructure located "off the beach" like roadways, electrical power, and stormwater collection systems. All are increasingly vulnerable to damage from floodwaters associated with longer and more intense storms. Ocean City's stormwater system has limited ability to drain streets during flooding. During Hurricane Sandy, Ocean City's engineered beach survived mostly intact. But, McGean says, "All our flooding — and property damage — was from the Bay side."

The nine-mile strand of clean sand along Virginia Beach's boardwalk and shorefront neighborhoods is neither natural nor cheap.

Spikes in sand prices, resulting from demand from communities ravaged by hurricanes and nor-Easters, make project costs unpredictable for the feds and their local partners. "The most we've paid is a little over eight dollars a cubic yard," Roehrs says, "but if we'd had to buy sand after Hurricane Sandy, we would have paid as much as 15 dollars a cubic yard."

Supplies of sand can also be uncertain. The sand comes primarily from offshore sandbars, most of which have formed over millennia from erosional processes that have transformed upland rocks and sediment to sandy ocean bottom.

As the demand for beach-rebuilding sand has escalated, more Mid-Atlantic states are reaching beyond the three-mile state limit into federal waters to obtain it. The federal Bureau of Ocean Energy and Mining and the U.S. Geological Survey are studying these offshore resources on behalf of the states.

But coastal sand mining is not without ecological impacts. "Coastal sand mining disrupts the near-shore ocean food web by removing bottom-dwelling organisms," says Gwynn Crichton, senior scientist for The Nature Conservancy, "and this can disturb shoal areas where marine life congregates."

Using sand from other sources to rebuild beaches is fraught with other problems. Although benthic organisms in the surf zone can rebound with time, the composition and abundance of species there change when incoming sand grain characteristics are not closely matched with those of the replacement sand, according to Crichton. Migrating shorebirds, crabs, and fish depend on these organisms, like the millions of coquina clams that ascend through the sand after every receding wave.

And while beach replenishment projects are seasonally restricted to limit impacts on nesting sea turtles, changes in beach-face geometry can prevent females' access to safe nesting sites — and the composition of the sand itself may negatively impact sea turtle incubation rates.

Ironically, the language used to describe the process suggests an improvement in beach health — we say we are "nourishing" the beach. But the sand that is "borrowed" from offshore sites generally does not return to these same sites — and the overall health of the ecosystem has been compromised.

It's easy to forget that beaches are dynamic geologic structures, built up over millennia, constantly changing with seasonal cycles of wind, weather, and tides. But this is exactly what attracts us to them, as expressed in a traditional nursery rhyme.

"Down at the seashore, isn't it grand?
Wiggling my toes in the soft warm sand
Building a tall sand castle, where the king and queen stay
When the tide comes rushing in they will have to move away"

The words capture the lure and the dangers of building on oceanfront property — and an alternative to beach replenishment that no one is willing yet to address seriously, says geologist Robert S. Young, director of the Program for the Study of Developed Shorelines at Western Carolina University. That alternative: moving away from the beach.

Young says that "refreshing" damaged beaches is simply not a long-term fix for a serious and rising problem. In an August 14, 2014, New York Times op-ed, he wrote that spending post-Hurricane Sandy federal dollars to rebuild costal communities — including dunes and beaches — is leading to the waste of millions of dollars in the absence of a national plan for coastal adaptation. For Young, these strategies need to include "managed retreat," a politically unpalatable option, but one that increasingly crosses the minds of policy makers and local managers.

Virginia Beach is poised to commission a study on the cumulative impacts of sea level rise on the city and other strategies for adaptation in addition to beach replenishment.

As hard as it is to contemplate how large, beach-dependent communities might "move away" like the king and queen in the nursery rhyme, engineers like Phillip Roehrs of Virginia Beach admit that "what keeps us up at night is knowing that there can always be a [flooding] event that is bigger than the one you've been able to prepare for."

Or, afford.

"If we evaluated sea level rise and did the straight-line math," says Roehrs, to estimate what it would take to adequately protect the shorelines and inland infrastructure, "we'd find out that it would take some ungodly amount, and we'd throw our hands up in despair."

In the meantime, communities like Virginia Beach and Ocean City will continue to maintain — and nourish — their beaches while planning for the future.