Scale Modeling of the Bouzina Dam Flood Mitigation Structures

Danica Starinac1, Predrag Vojt1, Marijana Damnjanović1, Dragiša Žugić1, Ljubodrag Savić2, Radomir Kapor2, Budo Zindović2 and Radmilo Glišić3

 

 

 

 

1 Jaroslav Černi Institute for the Development of Water Resources, Jaroslava Černog 80, 11226 Pinosava, Belgrade, Serbia, E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

2 University of Belgrade - Faculty of Civil Engineering, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbia

3 Energoprojekt Hidroinženjering PLC, Bulevar Mihaila Pupina 12, 11070 Belgrade, Serbia

 

 

 

Abstract

The design of the RCC (roller-compacted concrete) Bouzina Dam (Batna Province, Algeria) was verified using a scale model. The length scale of the physical model was 1:40 and the hydraulic parameters were scaled applying the Froude similarity. The tests focused on a standard Creager profiled spillway crest (with five bays), a converging stepped chute and a stilling basin. The main design problem was selecting the optimal shape of the stepped chute side walls. The converging chute generated standing waves, resulting in a significant flow depth increase in the vicinity of the walls. The optimal shape of the side walls was selected through a trial-and-error procedure, ensuring the most favorable flow conditions. Improving the flow conditions in the chute resulted in better flow conditions inside the stilling basin as well, so that the initially proposed energy-dissipation baffle blocks could be removed. Measurements of hydraulic parameters (water levels, depths, velocities, pressures) on the scale model verified the proposed improvements (compared to the original design).

Keywords: scale model, stepped chute, standing wave

 

Introduction

 

Stepped spillways present one of the most efficient energy dissipators and can significantly reduce the size and the cost of the stilling basins. The development of new construction techniques, primarily roller-compacted concrete (RCC), have increased the interest in stepped spillways. Flow over the steps can be classified as nappe, transitional and skimming (Chanson, 1994). Although, the substantial energy dissipation occurs during nappe flow, spillways usually operate at large unit flow-rates and exibit skimming regime. As the velocities in skimming flow are significant, flow exibits strong air entrainment. Inception point is the location where boundary layer intersects the free surface. It is located downstream of the weir and marks the onset of air-entrainment (Castro-Orgaz, 2009).

Air-entrainment is important as the presence of bubbles strongly influence the energy dissipation (Toombes, 2008). In spite of significant research activities in the past two decades regarding air-water mixing process, the phenomenon is not yet fully understood. For a constant width spillways, Boes and Hager proposed number of empirical equations to assess energy dissipation along the spillway (Boes, 2003a and 2003b). Energy dissipation along the spillway increases with the increase of its width. As the available width is constrained by the width of the river-section, the spillway should be as narrow as possible to minimize the construction costs (due to excavation). These two requirements can be satisfied with the construction of the converging side-walls. Hunt et al. (Hunt, 2008 and Hunt, 2012) conducted experiments for converging spillways but their findings are somewhat limited due to the lack of the air-entrainment. As such, recommended equations cannot be used to assess the height of the side walls and scale-modelling is required to verify the optimal design.

 

Bouzina Dam Overview

The Bouzina Dam is under construction on the Bouzina River in Batna Province, Algeria. The approved design concept calls for a roller-compacted concrete (RCC) gravity dam, 76.5 m high, with a dam crest elevation of 1050 m above sea level. The dam will create a reservoir with a full reservoir level (FRL) at 1045 m.a.s.l. The main function of the dam is to manage the flow of the Bouzina River and support drinking and irrigation water supply. Additionally, the dam and its appurtenances will ensure safe conveyance of the design flow rate of Q0.1%=1320 m3/s.

Given that this is a highly specific and costly capital project in an environment of complex hydraulic conditions, the design (Figure 1) was tested on a scale model.

 

Fig1
Figure 1: Proposed configuration of the Bouzina Dam.

 

The model tests (Starinac, 2013) focused on the flood mitigation structures of the dam, including a spillway, stepped chute and stilling basin.

The spillway is of the Creager profiled type, comprised of five bays, with a total crest length of 75 m. The elevation of the crest is 1045 m.a.s.l., while the design head at the design discharge rate is 4 m.

The function of the stepped chute on the downstream face of the dam is to partially dissipate the energy of the rushing water. The chute converges downstream, from a width of 81 m at the entry cross-section to 55 m at the exit. The longitudinal gradient of the chute is 9:8 and the step height is 0.9 m.

The chute extends into a concrete stilling basin, 50 m long and 55 m wide, whose function is to settle the flow to the downstream natural river channel as much as possible. The stilling basin had been designed as an USBR III type structure, with 4 m high baffle blocks.

 

Methods

Physical Model and Effect of Scale

The physical model of the Bouzina Dam (whose length scale was 1:40, Fig. 2), was built in February 2013 at the Hydraulics Laboratory of the Jaroslav Černi Institute for the Development of Water Resources, Belgrade, Serbia.

 

Fig2
Figure 2: Physical model of the Bouzina Dam at the Hydraulics Laboratory of the Jaroslav Černi Institute for the Development of Water Resources.

 

The model reproduced a 60 m long section of the reservoir, the dam and its main flood mitigation components (spillway, stepped chute and stilling basin), and a 190 m long downstream river channel.

In the central part of the model, the dam, spillway and stepped chute were made from fine concrete, while the walls and stilling basin were fabricated using transparent plexiglas plates mounted on a special metal structure. All the components of the model reflected the design dimensions and the roughness of the materials met all similarity requirements to prototype conditions.

The selected scale of 1:40 allowed for effective modeling of the spillway capacity, the generation of standing waves in the stepped chute, and the wave propagation in the stilling basin. Given that the Froude similarity requirements were fulfilled, a model of this scale could not effectively reproduce the flow of the air-water mixture (Boes et al., 2000), such that the chute depths were smaller and the residual energy at the end of the chute higher. This meant that the model could not reliably assess the height of the side walls of the chute, while the dimensions of the stilling basin were on the safe side (Kapor et al., 2013).