The next step in a 3,000m2 extension of a North Sea shoreline erosion-control effort on islands off the northwestern coast of Germany included the installation of more than 20 metric tons of a new polyurethane-mixture coating last summer.
The Elastocoast resin system, developed by BASF subsidiary Elastogran, is proceeding with shoreline protection projects in partnership with the Institute of Hydraulic Engineering (IHE), Hamburg-Harburg, Germany.
The most recent deployment was during July 2007 at Hallig Gröde, one of 10 German salt-marsh islands (see “What are Halligs?”), part of the Frisian Archipelago that stretches just off the North Sea coastlines of Denmark, Germany, and the Netherlands. The porous, transparent PU coating was previously applied on shores of Hamburger Hallig and the historic island of Sylt (see “The ‘disappearing’ island of Sylt”).
“Elastomer revetments utilize the property of polyurethanes by creating permanent and elastic bonds with stone surfaces,” explains Professor Erik Pasche of the IHE. “This creates sturdy, porous—but at the same time—very resistant revetments.”
Traditional coastal protection has usually used rigid “adhesive” materials such as concrete or asphalt to deflect wind-blown stormwater and erosive waves. But these have proven both costly to install and prone to rupture under severe conditions. This new elastomer system is a mixture of polyurethanes and a rock/stone aggregate laid on top of a stabilizing geomembrane.
The liquid, two-component, plastic polyurethane is stirred on-site, then mixed—for example, in a concrete mixer—with the crushed stone, which it envelops like a thin, transparent film. The finished mix of materials, which remains ready to use for about 20 minutes, can be applied in covering layers about 15-30cm (ca. 6-12in.) thick. The mixture even hardens underwater. Alternatively, it can also be sprayed onto a loose layer of stone ballast using a high-pressure technique.
The result is a highly stable, elastic, open-cell structure intended to absorb, rather than deflect, the energy of crashing waves and driving rain. The plastic bonds the broken stones to one another only at certain points, but elastically. In this way, it absorbs the energy of the insurgent volumes of water.
Nearly 500m2 of the new breakwater technology, containing almost 7 MT of the elastic covering, has been placed on Hallig Gröde since last year and has proved effective, according to Elastogran.
The PU resin system, which contains 60% renewable, plant-derived polyol, claims to be an ecologically sound solution that has no harmful impact on aquatic life. Further, the PU/rock-enveloped blanket can cut costs because of its relative ease of installation.
The installation process involves contouring the beach, placing a geomembrane to prevent undermining by sand erosion, then capping with a 15-30cm-thick layer of the PU resin-and-rock combo. The coating begins to harden within 20 minutes, reaching maximum strength after 2 days.
Currently, about 800km of dike systems protect the German North Sea shoreline against the forces of nature. The North Sea coastal states Schleswig-Holstein and Lower Saxony already invest almost €100 million (ca. $135 million U.S.) every year in coastal protection measures and for flood prevention.
The engineering principle of yielding slightly to the thundering volumes of water in order to contain them is the key. The same principle is applied in the construction of modern dike systems, which rise with a very gradual incline on the side facing the sea. This allows the breakers to gradually expend their force without causing damage, instead of explosively releasing their full forces upon immediate impact.
But the dikes also need a protective layer, considering the serious threats of erosion that are reaching dramatic proportions along exposed sections of coastline—as, for example, in Sylt (cited above). And in some cases, it is threatening the integrity of entire islands.
These efforts could increase even further in the future, as scientists predict that global warming will cause a rise in the sea level up to 70cm. Conjecture regarding a rise in sea levels in coming decades could make effective coastal protection even more important throughout the world.
The idea of bonding stone ballast together using polyurethane plastics was first realized with successes in railbed stabilization. The stability and durability of these stone ballast embankments is now transferring to a maritime context.
Dike systems, such as those on Sylt or the Hamburger Hallig, are only one of many possible application sites for the polyurethane/crushed stone revetments. Along coastlines, they could also protect harbor installations, flood barrages, riverbank promenades, and banks of inland waterways.