How the Sunken Bridge Works

By Tyler Silvers

When charged with the task of bridging the gap to Fort de Roovere, now a recreational space located in Halsteren, The Netherlands, RO&AD Architects pursued the idea of invisibility. The result was the picturesque Moses Bridge – a promenade that is sunken into the West Brabant Water Line that, from a distance, achieves virtual invisiblity. The beautifully designed structure, however, raises a few questions that I would like to solve; namely:

:Why a sunken bridge?
:How does the design account for rising water levels and rainfall?
:How does the structure fare during the harsh winters of the Netherlands?

 
 

Through my research, I have found that the history of the site played a large role in the development of the “sunken bridge.” The structure crosses the West Brabant Water Line, a moat-like area that was used by Fort de Roovere as a defensive line during the 17^th century while the fort was in use. This fort, and several others like it, suffered damage during the 19^th century, but has recently been zoned for recreational usage. When charged with the task of bridging the gap to the fort, RO&AD Architects found that constructing a bridge across a defensive moat to be “highly improper…especially on the side of the fortress the enemy was expected to appear on. That’s why we designed an invisible bridge.” The design of the bridge features a wooden pathway that is sunken into the existing landscape. From a distance, the bridge becomes virtually invisible as only a small sliver of the accoya wood material (a rot resistant wood that is waterproofed with EPDM foil below the water) peeks through the surface.

With the question of aesthetics solved, I turn to functionality. Initially, the idea of a sunken bridge seemed absurd. If water levels rise, the bridge would be of no use. RO&AD Architects, however, seamlessly incorporated functionalities such as this into their design of this remarkable bridge. The design of the bridge is complimented by a set of dams (one on each side of the bridge) that help to control the water levels around the sunken structure. The heights of the dams are specifically calculated to direct water away from the bridge element in the event that water levels rise, as pictured in the diagram.
 

We now know that water cannot flow over the top of the bridge element, but what about water from above? The designers have taken into account the inevitable rainfall in the area with a practical design solution. Each of the floor planks has an intentional gap between them which allow rain water to permeate the walking surface (and also helps to account for the shrinking and swelling of the material). Once the rainwater finds its way to the space below the wooden surface, a sump pump expels the water into the moat and the dam system takes hold to ensure that the bridge element remains passable.

The harsh winters of the Netherlands take quite a toll on the bridge. Heavy snowfall and temperatures well below freezing often push the bridge past its limits and cause the bridge to flood and become impassable during these months. The recreational area of the fort, however, has little to no use during this time, so the bridge need not be crossed. Come spring, the land thaws and the bridge once again becomes operational thanks to accoya wood’s resistance to rot and the EPDM waterproofing enabling the structure to handle the mass exposure to water.

 






Bibliography:

Jett , Megan . "Moses Bridge / RO&AD Architecten" 17 Nov 2011. ArchDaily. Accessed 26 Sep 2012. <http://www.archdaily.com/184921>

Joo Kim, Sun, ed. "RO&AD Architects: How the Moses Bridge Works." Smart Planet. SmartPlanet, 29, Jan 2012. Web. 26 Sep 2012. <http://www.smartplanet.com/blog/design-architecture/ro-ad-architects-how-the-moses-bridge-works/3838>

:Pictures courtesy of RO&AD Architects and http://www.naturalhomes.com