PREDICTING DOROODZAN DAM HYDRAULIC BEHAVIOR DURING RAPID DRAWDOWN

Doroodzan dam is a 57m high earth dam with rip-rap cover constructed on Kor River in Bakhtegan Basin. The dam is located at 30° 12'' 2'' north latitude and 52°25'' 5'' east longitude ~70km north of Shiraz, Iran. The dam reservoir supports a large amount of agricultural, industrial, and urban demands in the region. The reservoir volume is 993 M.C.M at the normal pool level and the dam crest length is ~700m. In this paper, a 3-D finite element model of the dam was constructed and analyzed for steady and transient conditions. Transient pore water pressure fluctuations were predicted at different piezometer locations for a 21-day rapid drawdown of 23.9m. It was found that seepage through the dam is not sensitive to hydraulic conductivity of downstream dam body, apparently due to the effective hydraulic behavior of the chimney drainage there. Under rapid drown down conditions, a maximum of 11.8m excess pore water pressure on upstream part of the dam was observed (compared to the steady state conditions) while no significant excess pressure was seen at the downstream part of the dam. Dynamics of the phreatic line location during the 21-day rapid drawdown was monitored in four 5.25-day time steps. A gradual phreatic line change at time steps ending at the 21-day period was predicted. Phreatic line at the upstream face of the dam closely followed the reservoir level rapid drawdown. However, phreatic line at the interior sections of the dam did not drop as fast. As a result, a gradient towards upstream face of the dam was developed after ~10 days which might jeopardize slope stability there. It is recommended that the excess pore water pressure be carefully considered in dam analysis researches, especially during the transient periods. In general, rapid drawdown should be cautiously analyzed in dams, especially those with short emptying times, as it may reverse the seepage direction, endanger the slope stability, and not allow excess pore water pressure to dissipate in an acceptable manner.