UNCERTAINTIES IN ANALYSIS and DESIGN OF FLOOD CONTROL/SEDIMENT RETENTION DAMS IN NW TEHRAN

Alborz Mountains in northern boundary of metropolitan Tehran are a major potential of floods and sediments threatening urban areas. The developing District 22 bounded in north by the mountains is one of the high-risk areas. Checkdams and the so-called flood control/sediment retention earth dams are designed for protecting the district. The flood control/sediment retention earth dams are having different characteristics from regular earth dams, detention dams, and tailing dams. Therefore, special criteria are required to be established for their analysis and design. Lack of water in reservoir during service life except after floods for a short period of time, sediment accumulation in reservoir with time, probability of finegrained sediments infiltration into dam and hence permeability reduction of a narrow band in upstream, chance of coarse-grained impact during floods, and relatively high natural ground slope are among the main characteristics of such dams. Each of these characteristics may endanger the dam safety, therefore it is necessary to evaluate their quantified impact on the dam behavior for analysis and design purposes. The main objective of this paper is to study the effect of aforementioned uncertainties on flood control/sediment retention dams of District 22, and performing parametric numerical analysis to evaluate their quantified impact. Two-dimensional plane strain numerical analysis is performed for various loading combinations including initial construction, steady state seepage, rapid draw down, and seismic loading in dry and steady state seepage conditions. Some guidelines are then presented for analysis and design of sediment retention dams and the requirements of further studies on the subject are pointed out