Document Type : Original Article

Authors

• zahra nikbakht ¹
• Alireza Emadi ²
• Mohammad Mirnaseri ³

¹ MSc. of water engineering department, Faculty of agricultural engineering, Sari agricultural sciences and natural resources university, Sari, Iran.

² Associate professor of water engineering department, Faculty of agricultural engineering, Sari agricultural sciences and natural resources university, Sari, Iran.

³ Assistant professor of water engineering department, Faculty of agricultural engineering, Sari agricultural sciences and natural resources university, Sari, Iran.

Abstract

Introduction

In recent years, pollution of surface water resources, especially rivers, has posed an environmental challenge. Pollution from municipal or industrial wastewater and waste disposal into rivers are important problems for human societies to protect the environment. Knowing the level of river water pollution as one of the sources of human water needs is essential and therefore modeling the quality of river water is very important. Hydraulic structures in rivers are one of the ways to control pollution in open-channel flows. Check dams are one of the types of these structures that due to the porosity of their environment can play a controlling role in the transport of contamination by increasing hyporheic exchanges as well as transient storage of contamination in their porous media. Transient storage model (TSM) is one of the methods of pollution transport analysis, especially in rivers with high hyporheic exchanges. The efficiency of the (TSM) is in the correct estimation of the four parameters of the model (Dx, As, A and α). Previous studies have not investigated the effect of hyporheic exchanges due to gabion check dams on the four parameters of thel (TSM). In this study, the effect of gabion check dams on pollution transport and the four parameters of the (TSM) with OTIS numerical model were investigated.

Methodology

Experiments of tracer material (NaCl) were performed in a flume with a length of 12 m, a width of 0.5 m and a height of 0.7 m in four flow discharges (2.5, 5 and 7.5 lit/s). An ultrasonic flow meter was used to measure the flow discharge in all experiments. Materials with medium diameter (D50) of 11.85 mm and porosity (n) of 0.28 were used to create a sedimentary bed with a length of 12 m and a thickness of 12 cm at the bottom of the flume. In this study, two types of gabion check dams with medium diameter (dg) of 11 mm (fine-grained) and 19 mm (coarse-grained) were used. In each experiment (except for the control experiment), 1 to 3 check dams were used at intervals of 2.5, 5 and 7.5 meters from the beginning of the flume, respectively. In this study, check dams with lengths of 0.75 and 0.35 m, widths of 0.5 and heights of 0.4 m were used. The length of the flume was divided into four equal reaches (L1, L2, L3 and L4). Two sensors were placed to measure the electrical conductivity (EC) of water at the end of each reach to measure the amount of contamination. Micro-propeller and ultrasonic depth-gauge were used to measure the velocity (V) and depth (h) of water flow in each reach. The laboratory results in L4 reach were simulated by the OTIS-P numerical model and the four parameters of the (TSM) were estimated.

Results and Discussion

The results showed that gabion check dams increased the transient storage of solute in the porous media of such dams, thus reducing the peak contamination concentration (Cmax) in the main flow area. On the other hand, check dams in the flow path will act as a sedimentary bed-form, which increases the hyporheic exchanges between the main flow area and the porous media of such dams. Increasing hyporheic exchanges into the porous media of the dams will also reduce the (Cmax) in the main flow area. Increasing hyporheic exchanges into the porous media of the dams also reduces the contamination concentration (Cmax) in the main flow area. Reducing the (Cmax) in the main flow area will also increase the longitudinal dispersion coefficient (Dx).

Comparison between the storage zone exchange coefficients (α) estimated by the OTIS-P numerical model showed that these coefficients decreased with decreasing the length of check dams (a). Reducing the length of check dams (a) will reduce the space of the porous media in the flow path. Therefore, the solute will leave these storage zones with a shorter residence time, so the storage zone exchange coefficient (α) decreases with decreasing the length of check dams (a).

Gabion check dams made of fine-grained materials reduce the exchange discharge between the check dams and the main flow area. The use of fine-grained materials reduces the rate of contamination transfer to the downstream reaches, so the (Cmax) in the downstream reaches will decrease, so the(Dx) will increase in the fourth interval (L4).

Conclusions

- Increasing the number of gabion dams (N) from one dam to three dams caused an approximately 1.43 to 1.71 times the value the (Dx).

- Increasing the length of gabion dams (a) from 35 cm to 75 cm caused approximately 1.43 to 2.49 times the value of the(Dx).

- Increasing the length of gabion dams (a) from 35 cm to 75 cm caused an approximately 1.10 to 4.43 times the value of the (α).

- The use of fine-grained materials in gabion dams increased the (α).

Keywords

• Advection-Disperion
• longitudinal dispersion coefficient (Dx)
• storage zone exchange coefficients (&alpha
• )
• Storage Zone

Main Subjects

• Hydraulic