Author(s): Erik Bollaert; Stephanie Andre; Gregory Morris
Linked Author(s): Erik Bollaert
Keywords: Bolivia; Stormwater culvert; Physical modeling; Manning roughness; Air entrainment
Abstract: A comprehensive stormwater drainage analysis and design study was undertaken for the City of La Paz, Bolivia, incorporating both numerical and physical modeling. Behavior of major culvert sections within the system having non-standard stair-step geometry were simulated by a 1: 10 scale physical modeling. Physical modeling methodology, results and recommended design modifications are reported here. The prototype system contains a series of longitudinal single steps and contains significant discharge of gravel and cobble size sediment. Physical modeling analyzed slopes of 10 % and 27 % using a Plexiglas model which allowed visual observation of flow patterns. Equivalent Manning roughness coefficient values of 0. 025-0. 030 for slopes around 10 % and 0. 015-0. 020 for slopes of 20-30 % were defined. These values were subsequently used in numerical modeling. It was found that system capacity was limited by air entrainment, flow pressurization, and water jets hitting the roof at steps. A smaller than real cross section was recommended for numerical modeling to account for these complex flow phenomena. An alternative design using multiple smaller steps was proposed which doubles flow capacity without significantly impacting the roof or causing pressurization, while maintaining the current rate of energy dissipation. Design steps for the proposed culvert configuration are presented.