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A sample of refuse was tak en from the l ef t abut ment area at th e time the inspection team made it s vis it . Specific gravit y , gr ain s i ze , and direc t shea r tests were condu c ted on t he sample. Vis ually, the refuse was classified as a black gran ul ar mat erial co ntai nin g frag - ments of sandstone, shale, and low grade coal. Lar ge r fragment s va ry from blocky to platy in shape . According to the Unified Soil Classifi - cation System, th e refuse was classified as an SP material, i.e . , poorly graded san d, gravelly sand, little or no fines. In the field, slabs of refuse were observed that were seve r al feet wide and several inches thick, The grain size distribution curve for the laboratory sample i s prese nt ed in Figure 7. Middl e Fork had a drainage area of app roxi mately 800 ac re s , most of which contributed run - off to the impoundment behind Darn No. 3 . Prior to the failure it was estimated that the impounded water had reached a height in excess of 40 ft, and ext ended upstr eam a distance of abo ut 2500 ft. Based on these estimates, the total volllllle of water impounded was estimated to be 21 million cubic fe e t, IV . DESCRIP TI ON OF FAILURE At 8:00 a.m. on February 26, 1972, Dam No. 3 fail ed , The released water over-topp ed the two small dams, No. 1 and No. 2, be low Dam No. 3 and rushed down the r ight side of a l arge waste dlll!lp of c oa l r efuse . An ae rial view of the ar ea taken seven days after the failure is shown in Figu r e 8. 9
Only the ab utme nt s of Dam No. 3 re main ed . The left ab utment shown in Fig ure 9 was estimated to be app ro ximately 60 ft. hi gh . At the time o f fa ilur e the es timated cres t width was 100-200 f t. and the base wid th was 400-500 ft. Trees, stwnps, and natur a l soil present in the f ound a tion can be seen in Figure 10. Figure 11 shows some of t he coal s ilt an d s l udge which rema ined in t he reservoir bottom . The two smal l dams were completely e roded away . Fi gure 12 is an upstream view of the loc at ion of Dam No. 2 . The left abutment of Dam No. 3 is in the center background . The water followed a haul road down the right side of the coal waste pile and eroded the 50-ft.-deep gulley shown in Figures 13 and 14. Steam explosions occurred when portions of the waste pile which were burn in g were flooded by the advancing water. The flood waters empt ied out of the gulley into Buffalo Creek (Figure 15) and started th e ir devastating way downstream. It was estima te d by the U.S . Depart - ment of Interior Task Force that the entire quantity of impounded wa ter was released within a 15-minute pe rio d. Based on data gathered by the Task Force, it was estima t ed that water flow velocities ranged from 15 - 20 fps ne ar Saunders to 4-5 fps at the mouth of Buffalo Creek at Man. A peak flow of 49 ,000 cfs was es tim ated at a location 4,500 ft . downstream from Saunde rs which cor r es ponded to a water depth of 15- 20 ft. and a flo w velocity of app roxim a tely 20 fps. Farther downs tream at Lor ado , flood depths 10
