The Bonnet Carré Spillway, which he now put in his rearview mirror, was the upstream bulwark of The Wall, aka HSDRRS (Hurricane and Storm Damage Risk Reduction System): a system of flood defenses, largely invisible to the non-expert observer, that encircled greater New Orleans. Dutch experts had been brought in to consult, and this was how Willem had come into occasional contact with people like Hugh and Margaret.