Scour Dynamics and Cost–Benefit Analysis of Series-Arranged I-Head and T-Head Groynes

Abstract This study investigates the hydraulic behavior and economic performance of a hybrid groyne system composed of an upstream I-head groyne (IHG) set at 45° and a downstream series of three T-head groynes (THG), under a clear water, subcritical flow over fine sediment beds. A total of 76 flume experiments were conducted to evaluate scour dynamics as a function of groyne geometry, Froude number, and interspur spacing. Temporal scour evolution curves confirmed that equilibrium conditions were achieved within 48 h. Results show that THGs without protection experience significant upstream scour, reaching depths up to 0.43 times the depth of flow. Introducing a protective IHG reduced maximum scour at THGs by up to 75%, particularly when oriented between 45° and 135° and spaced at 3–3.2 times the groyne web length of a THG, dimensional analysis and nonlinear regression yielded a four-parameter empirical equation predicting normalized scour depth with R 2 = 0.983 , RMSE = 0.0047 , and MAPE = 1.50 % . K-fold cross-validation, residual analysis, and ANOVA confirmed the robustness, statistical significance, and generalization capability of the model without evidence of overfitting. Comparative validation against existing scour prediction equations showed that the proposed model significantly outperformed the already proposed equations. Sensitivity analysis indicated that primary spacing ratio and flow intensity were the dominant parameters governing scour development. A life-cycle-based cost–benefit analysis incorporated Indian market rates, uncertainty quantification, and sensitivity analysis. The study also integrates a detailed cost–benefit analysis using geometric design variables and Indian market rates. The IHG consistently delivered the lowest cost per unit of protected bank length, while the hybrid layout (1 IHG + 3 THGs) offered a balanced trade-off between performance and cost, outperforming a series of four THGs in economic efficiency. The findings provide a statistically validated and economically informed framework for the design of sustainable river-training structures in alluvial channels.