At Rhyl, a tourist resort on the North Wales coast, the East Promenade is susceptible to damage by marine scouring processes. Concealed scour cavities were suspected following the sudden collapse of a segment of the promenade and the recognition of numerous depressions and zones of cracking elsewhere in the concrete pavement. The full threat posed to the integrity of the promenade and to the safety of pedestrians remained uncertain and costly reconstruction was anticipated. Ground-penetrating radar was used along a 260 m stretch of the promenade to determine the full extent of voiding beneath the surface. The geophysical survey delineated the known cavity and, contrary to expectations, demonstrated that the sub-slab voiding beneath the remainder of the promenade was relatively minor. The more subdued radar features encountered were interpreted as minor separation beneath slabs due to consolidation. The results provided a basis for design and construction of effective and inexpensive repairs to the promenade foundations that ensured the public safety.