Water and Soil

Numerical Investigation of the Effect of Increasing the Geometric Dimensions of the Horizontal Drain on the Performance and Stability of the Nahand Earth Dam

Document Type : Research Article

Authors

T. Omidpour Alavian 1
E. Asadi 2
M. Soltani Sotobadi 2
E. Hematvand 3

1 Department of Civil Engineering, Faculty of Engineering, Water and Hydraulic Structures, University of Maragheh, Maragheh, Iran

2 Department of Civil Engineering, Faculty of Technical and Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran

3 Civil Engineering, Soil and Foundation Engineering, Islamic Azad University, Tabriz, Iran.

https://doi.org/10.22067/jsw.2026.97826.1533
Abstract
Introduction
Clay-core earth dams are indispensable hydraulic structures for water supply, irrigation, and flood control, yet more than 35% of their historical failures stem from excessive seepage and internal erosion. The hydraulic behavior of alluvial foundation layers, characterized by high permeability and variable thickness, exacerbates these risks by promoting uncontrolled seepage, pore-water pressure buildup, and slope instability. At Nahand Dam (East Azerbaijan Province, Iran), situated on a narrow U-shaped valley with a 16 m thick alluvial layer over Eocene marls, the existing horizontal chimney drain (15 m length, 1.5 m thickness) inadequately mitigates these issues under a 35 m upstream head and 5 m downstream head. Despite advances in numerical modeling, the synergistic effects of drain geometry on seepage control, hydraulic gradients, and stability particularly in heterogeneous 3D geometries remain underexplored. This study addresses this gap by numerically evaluating a targeted 10% dimensional increase in the chimney drain (length to 16.5 m, thickness to 1.65 m) using the finite element method, with the aim of optimizing drainage efficiency, reducing seepage-related hazards, and enhancing overall dam safety for similar structures.

Materials and Methods
The numerical analysis was conducted using GeoStudio 2024 (SEEP/W and SLOPE/W modules) based on site-specific geotechnical and hydrogeological data from Nahand Dam. The 2D and 3D models incorporated realistic geometry (35.5 m dam height, 730 m crest length, central vertical clay core), material properties (saturated hydraulic conductivities ranging from 1×10⁻⁹ m/s for the clay core to 5×10⁻² m/s for filters and drains, along with mechanical parameters calibrated via 124 triaxial tests and inverse analysis), and boundary conditions (fixed upstream head of 35 m at normal reservoir level 1607 m asl; downstream head of 5 m). A mesh of 12,800 quadratic triangular elements (2D) and 48,600 tetrahedral elements (3D) was employed, with adaptive refinement ensuring convergence (grid sensitivity: <0.3% change in seepage and pressure). Five scenarios were simulated: baseline (S0: 15 m × 1.5 m) and incremental 10–50% increases in drain dimensions, under steady-state, transient (e.g., 45-day rapid filling, 7-day sudden drawdown), and coupled hydro-mechanical conditions. Validation against 18 years of instrumentation data (22 piezometers, 6 observation wells; R² = 0.982, RMSE = 0.68 m) confirmed model fidelity (98.2% agreement). Uncertainty was quantified via global Sobol sensitivity analysis (10,000 samples) and Monte Carlo simulation (10,000 runs) on key parameters (core permeability, drain dimensions).

Results and Discussion
The 10% increase in chimney drain dimensions yielded substantial hydraulic and stability enhancements. Controlled seepage discharge rose 22.3% (0.008 to 0.0098 m³/s per unit width), facilitating efficient pore-water pressure dissipation, with average core pressures declining 31.7% (2.180 to 1.123 kPa) and total hydraulic head loss reduced by 9.3%. Exit gradients remained below the safe threshold of 0.8 in 94.2% of cases, minimizing internal erosion risk. Downstream slope factor of safety improved 25.5% (1.45 to 1.82) under steady-state conditions, with greater gains (up to 31.5%) in transient scenarios (e.g., rapid filling post-earthquake, PGA = 0.45 g). Three-dimensional arching effects reduced overall discharge by 18.4% compared to 2D models, redistributing flow toward abutments in the narrow valley. Permeability dominated over thickness (745.92% vs. 16.85% seepage impact), as confirmed by Sobol indices (S₁ core k = 0.88–0.91). Monte Carlo results indicated failure probability <0.3%, underscoring the cost-effective nature of this optimization. These outcomes align with and extend prior studies (e.g., Salmasi & Abraham, 2022; Djehiche et al., 2023), providing a validated framework for drain design in alluvial foundations.

Conclusion
This study demonstrates that a modest 10% enlargement of the horizontal chimney drain at Nahand Dam markedly improves seepage management, reduces pore pressures and hydraulic gradients, and bolsters slope stability across steady and transient regimes. The resulting enhancements 22.3% higher controlled discharge, 31.7% lower core pressures, and 25.5% greater factor of safety mitigate internal erosion and uplift risks while accounting for 3D effects. Monte Carlo analysis affirms high reliability (<0.3% failure probability), offering a practical, generalizable (94.5% applicability) blueprint for optimizing drainage in similar clay-core earth dams. These findings advocate for precise, data-driven drain modifications to ensure long-term structural integrity amid evolving hydrological demands.

Acknowledgement
The authors express sincere gratitude to the Nahand Dam Operation and Maintenance Authority for providing long-term instrumentation data and geotechnical records. Special thanks are extended to GEO-SLOPE International for software support and to the University of Maragheh for computational resources.

Keywords

Alluvial layer
Dam stability
Hydraulic conductivity
Numerical modeling
Seepage flow
Subjects

Authors retain the copyright. This is an open access article distributed under Creative Commons Attribution 4.0 International License (CC BY 4.0)

 

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Water and Soil
Volume 40, Issue 2
Summer 2026
Pages 179-153

  • Receive Date 06 March 2026
  • Revise Date 07 August 2026
  • Accept Date 18 August 2026
  • First Publish Date 18 August 2026

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