Empirical Evidence of Catastrophic Geology: Case 4

The Ricobayo dam is a 320-ft tall concrete arch dam constructed in 1929-1933 in northwest Spain.  The original spillway was originally a simple 1300-ft long channel cut in the rock (unlined channel) that discharged over a rock cliff.  Floods (high water levels) occurred in 1934-1936 and scour (loss of rock mass) was observed within the spillway channel and plunge pool.   100 meters of granite rock along the spillway was scoured away during this time (80 ft in 1935 and 100 feet in 1936 to start), creating an entirely new downstream valley profile.

  

(Photo was reproduced from an article by George Annandale, P.E.).  The Ricobayo spillway consisted of granite with vertically dipping joints and 10 to 20 degree off horizontal joints.  The plunge pool of the spillway was relatively stable after a certain depth of scour had occurred (80 feet), due to the presence of harder rock starting at a certain elevation, although the concrete lining was destroyed by forces from a flood in 1939 and again in 1962.  The fractured/jointed condition of the rock permitted water infiltration and a reduction of effective stresses within the joints that facilitated loss of rock mass.  This case history reminds us of the power of water and the ability to alter topography and geologic profile very quickly.  A more recent photo of the dam is provided below.  Note the huge vacant rock space.