Hydraulic Conditions of Flood Wave Propagation in the Valley of the Narew River after the Siemianówka Dam Overtopping Failure

Abstract

In this paper, the results of the dam break wave forecast in the valley of the Narew River, after the failure of the Siemianowka Reservoir earth dam, are presented. The Siemianowka Reservoir was built in 1991 due to the planned agricultural management of the Narew Valley. It is located in the Upper Narew Catchment as a typical lowland reservoir of a low mean depth, equal to 2.5 m and the principal spillway of 145.00 m above sea level. The reservoir is located about 82 km upstream of the border of the Narew National Park and considerably influences the hydrological regime of the Narew River within that park. The total capacity of the reservoir is 79.5 million m3. The presented example of calculations for the Siemianowka Reservoir underlines the most important issues related with the forecast of the dam break, that is: the determination of maximum water levels and discharges in the valley downstream of the failure dam, the calculation of wave-front occurrence time at particular points of the valley, the inundation range along with the longitudinal profiles of water levels and velocities of water on inundated areas. The knowledge of the water depths on inundated areas, the duration of inundation and maximum water velocities enable as well the estimation of the damage degree on inundated areas. As it results from the calculations, the washout process of the Siemianowka Reservoir earth dam, which is 6 m high, happens very quickly, and after about 2 h the top of the triangular breach reaches the dam bed. The discharge from the reservoir after dam failure drops from 465 to 304 m3/s and the water level in reservoir decreases 1.36 m as fast as within 300 h. The occurrence of wave in the valley causes the increase of water levels by 0.1–4 m in comparison to the water level before the dam break, and a 4-time increase of discharge rate in relation to the design flow at the cross-section of the dam of the Siemianowka Reservoir. The velocity of the wave-front propagation in the Narew Valley does not exceed 5 km/h.