Introduction

Rivers stand as paramount water sources serving a multitude of purposes, underscoring the prime significance of comprehending river conditions for any river infrastructure undertaking [1]. The dynamic nature of rivers entails a continual state of change, making an understanding of sedimentation phenomena, including sediment transport, erosion, and consolidation, pivotal [2]. The Kabul River Basin (KRB) holds strategic importance for water, sustenance, and energy across Afghanistan and Pakistan, representing a global river basin. Central to this basin is the Kabul River, its primary and pivotal artery [3]. Human interventions within the Kabul River Basin significantly influence flood control, hydropower generation, and navigation, shaping the realm of water management challenges [4]. Notably, researchers have developed models to forecast bed material transport rates in alluvial rivers, contributing to a comprehensive understanding of sedimentation patterns [1].

Within Afghanistan’s context, the Ministry of Energy and Water is deeply concerned about sediment accumulation within the Kabul River, recognizing its far-reaching implications for the river’s utility and benefits [5]. Furthermore, this issue poses a considerable global threat to river and reservoir ecosystems [6]. Elevated erosion rates, escalated sediment loads from suspended materials, and encroachment of human activities into once undisturbed zones have emerged as primary focal points in the discourse surrounding sediment challenges [7]. Sedimentation, as a process, entails the erosion and subsequent conveyance of soil particles by running water, ultimately depositing these particles in bodies of water, including rivers and reservoirs [8]. As water demand grows, reservoir storage capacity is diminishing [2]. In Afghanistan, vital water resource projects like Nghlo, Srubi, and Darunta face significant constraints due to reservoir sedimentation.

Sediment loads within rivers are inherently shaped by the characteristics of their basins, including factors such as land use, soil types, vegetation cover, and topography [9]. However, human interventions along rivers and the influence of climate change add layers of complexity to sediment transport analyses [10]. Notably, increased flow rates and energy during the winter season elevate silt loads in rivers [11]. Thus, this study aims to propose solutions to address these challenges.

The principal objective of this research is to identify areas along the Kabul River prone to sediment exposure. Additionally, employing the HEC-RAS model, the study seeks to assess sediment transport and sedimentation capacity in the river. To inform this endeavor, a comprehensive literature review of sedimentation studies conducted globally was undertaken to discern prevalent methodologies [12]. Furthermore, additional literature reviews were conducted, specifically delving into sedimentation studies pertaining to the Kabul River [13]. Data collection was centered at the Tangi-Gharo hydrological station, situated downstream of Kabul city [14]. Utilizing the HEC-RAS model, this study examines sedimentation along the Kabul River, stretching from the Tangi-Gharo station within Kabul city to Maidan Wardak province.

Renowned as a water engineering software, the HEC-RAS model aptly assesses river sediment conditions, having been widely employed in analyzing rivers worldwide [15]. Given its significance, pivotal hydraulic engineering projects, and its influence on water politics, the Kabul River has been extensively studied in Afghanistan [3]. The outcomes of this research hold relevance for governmental and non-governmental entities as they consider multi-faceted river infrastructure projects along the Kabul River.

Materials and Methods

Study Area

The geographical extent of the Kabul River area, situated within the Kabul River Basin (KRB) and identified by Afghanistan’s coordinates, is of paramount importance. Geographically, it stretches from 69°08′ to 69°26′E longitude and 34°28′ to 34°36′N latitude, as depicted in Fig. 1. Notably, the KRB spans a total length of 700 km, with 560 km flowing within Afghanistan’s boundaries, encompassing a drainage area of 67,370 square kilometers [16].

Fig. 1
figure 1

Kabul River modeling for sediment transport

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Within the Kabul River Basin, six rivers are designated as the Kabul River’s branches, namely Lugar, Maidan, Shakar-Dara, Estalef, and Qargha Rivers. These rivers possess distinct catchment areas, with sizes of 12,850, 9,735, 1,625, 93, 83, and 70 km2, respectively, as illustrated in Fig. 1. These rivers consistently maintain a minimum flow discharge during dry months, except for Maidan and Qargha, which experience complete or partial dryness for four to seven months annually. It is pertinent to note that the Tangi-Gharo station witnesses a decline in silt input from the six rivers [17].

The section of the Kabul River modeled for the sediment study is visualized in Fig. 1. This portion spans approximately 40 km, commencing from the Pul-i-surkh station in Maidan Wardak province, situated at coordinates 34.36684166°N and 68.769652778°E, and concluding at the Tangi-Gharo station within Kabul city, located at 34.569880556°N and 69.402169445°E. Characterized by a variable slope, the river channel’s average slope along this stretch is calculated as 0.00125, indicative of scouring processes.

Data Collection

The dataset underpinning this study comprises suspended sediment transport measurements conducted from 2016 through 2019, extending until June 2020, as documented [2]. Encompassing a span exceeding four years, these data observations encompass distinct seasons – spring, summer, fall, and winter. This seasonal breakdown is employed to discern the impact of snow cover, vegetation dynamics, and sediment availability on mobilization processes. For sediment analysis, the study employs the United States Army Corps of Engineers (USACE) HEC-RAS 5.0.7 model [18]. This comprehensive software facilitates the assessment of diverse water dynamics, including steady flow, gradually varying flow, sedimentation, and water quality. The model boasts advanced graphic capabilities, facilitating the visualization of software outputs [14].

Within the scope of sediment analysis, the HEC-RAS model is employed to investigate one-dimensional sediment deposition and erosion in the rivers under study [19]. Three pivotal input data parameters are central to the calculations governing sediment movement, as illustrated in Fig. 2. Moreover, to enable accurate sediment movement computations, the model necessitates the specification of the Transportation Function (TF), informed by the basin’s soil characteristics [18]. In line with the TF provided by Acker-White, Yang, and Wilcock, critical factors like the Sorting Method (SM) and Fall Velocity (FV) are determined. In this research, a comparative assessment of these models reveals the superior suitability of the Acker-White model for the prevailing conditions.

Fig. 2
figure 2

Parameters for sediment analysis using HEC-RAS model

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Geometry Data

Geometry constitutes the foundational input when addressing flow-related challenges. To establish this geometry, a synergy between Geo-RAS and the Digital Elevation Model (DEM) within the ArcGIS framework is harnessed. Notably, the Hydrologic Engineering Center (HEC) devised Geo-RAS as an ArcGIS extension, a tool instrumental in collating critical insights pertaining to river alignment, bank positioning, flow trajectory, and cross-sectional profiles within the GIS environment [20].

Within this framework, geometric data are seamlessly transferred from ArcGIS to HEC-RAS, encompassing key components such as cross sections, reach distances, and bank placements. An essential consideration for each river cross section entails the introduction of Manning’s n value, distinct for banks and flood plains. As per USGS recommendations, this study adopts Manning’s n values of 0.031 for rivers and 0.02 for flood plains, ensuring the accurate representation of flow characteristics [2].

Flow Data

The sequencing of data introduction within the model entails the prior incorporation of geometric data followed by flow data. In this trajectory, the Kabul River station supplied vital data, enabling a meticulous analysis of water height and solid particle concentrations. The repository of flow hydrographs spans a comprehensive dataset of annual discharge records, coupled with monthly discharge information covering an extended duration of over four years [20].

Notably, within the realm of quasi-unsteady analyses, the HEC-RAS software mandates the expression of liquid flow data through a specific format. This is embodied by a flow hydrograph presented as a time series of flow discharges, illustrated as a histogram in Fig. 3.

Fig. 3
figure 3

Flow series for Kabul River over a span of 4 years

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Sediment Data

Sediment data encompass essential information concerning sediment loads and the gradation of the riverbed, represented through a sediment rating curve [21]. A pivotal consideration involves the individual establishment of gradation curves for each cross section, accounting for the diverse sizes of sediment particles that profoundly influence erosion and sedimentation dynamics, as depicted in Fig. 4.

Fig. 4
figure 4

Sediment rating curve for Kabul River

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The Ackers-White equation serves as the cornerstone of this investigation [20]. Employing a robust approach, cross sections equipped with statistical gradation data spanning from upstream to downstream are strategically employed. To achieve this, data generation for cross sections is facilitated through the application of HEC-GeoRAS. This study specifically employs both the rating curve and bed gradation data [23]. Fundamentally, the rating curve, contingent on river discharge, computes sediment discharge into the river, a pivotal parameter in sediment transport assessments.

Methodology

The efficacy of the sediment data analysis conducted on the Kabul River reach, using the HEC-RAS 5.0.7 model, hinges upon a well-defined set of precise input data requisites [18]. The compilation of this model’s inputs encompasses diverse elements, including the Digital Elevation Model (DEM), exported HEC-GeoRAS data, flow data, and sediment data. Through this input, the HEC-RAS model yields crucial insights encompassing channel bed and water surface profiles, velocity profiles, shear stress distribution, and variations in mass distribution, pivotal for the study’s objectives [22].

A plethora of data constitutes prerequisites for the sediment model’s execution, including the DEM of the study area integrated within the GIS framework, discharge records, gage height, sediment/sediment load series, geometric specifications, quasi-unsteady flow dynamics, and the sediment analysis plan [19]. The calculation of water surface profiles, a crucial aspect of flow analysis, is facilitated through the utilization of the HEC-RAS 5.0.7 model. For sediment analysis, hydraulic properties play a pivotal role within the HEC-RAS 5.0.7 environment, enabling calculations encompassing sediment deposition, bed change analysis, and erosion through sediment continuity assessments [14].

The methodology unfolds with a generic approach to numerical modeling, encompassing data preprocessing and the establishment of the computational domain [18]. Subsequently, Fig. 5 delineates the post-processing phase, where acquired data are subjected to model execution and visualization, culminating in comprehensive conclusions drawn from the analysis outcomes.

Fig. 5
figure 5

Model calibration using sediment load

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Results and Discussion

The exploration of sedimentation dynamics within the HEC-RAS model has been conducted through the strategic incorporation of quasi-unsteady flow considerations, standing in lieu of the traditional geometric and flow data of the Kabul River. Within this framework, the Ackers-White equations play a pivotal role, facilitating the comprehensive computation of the overall sedimentation load. Graphical representation vividly portrays various outcomes, including the maximum elevation variations, projected bed level changes, and other consequential aspects.

The HEC-RAS model emerges as a powerful tool, offering extensive insights into diverse facets of sedimentation. Through its analytical capabilities, this model furnishes intricate data encompassing cross-sectional erosion and deposition patterns, alterations in river invert and water surface elevation, flow dynamics, velocity profiles, shear stress distribution, and mass distribution variations.

Figure 6 vividly illustrates the longitudinal bed profile of the Kabul River over the course of the study period. Notably, the river exhibits marked variations in slope along its upstream, downstream, and middle sections. These distinct slope characteristics are manifested through significant erosional processes in both upstream and downstream reaches, juxtaposed against variable sedimentation phenomena in the intermediate stretch, spanning from 4.5 to 33.8 km, as well as the downstream zone, ranging from 0 to 1 km. Figure 7 comprehensively captures the fluctuations in velocity spanning the entire length of the Kabul River. The channel’s considered reach is characterized by substantial velocity variations, wherein the 22.08 km section experiences notable velocity dynamics. Within this span, the observed velocities oscillate, with a maximum of 1.48 m/sec and localized instances of comparatively lower velocities, primarily associated with sections featuring increased water depth and larger surface areas.

Fig. 6
figure 6

Longitudinal surface water profile along the Kabul River

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Fig. 7
figure 7

Distribution of velocity along the Kabul River

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Figure 8 provides a comprehensive depiction of the variation in shear stress along the course of the Kabul River. Shear stresses, representative of frictional forces counteracting gravity, exhibit a direct correlation with specific water weight, hydraulic radius, and channel slope. Notably, the shear stress distribution showcases discernible patterns within the river’s stream.

Fig. 8
figure 8

Variation in shear stress along the Kabul River

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Intriguingly, the upstream reach of the river, spanning from 33.8 to 40 km, emerges as an area characterized by elevated shear stress levels, in contrast to the downstream segment, encompassing 0 to 33.8 km. These variations in shear stress distribution could be attributed to changes in bed slope and bed material properties along the Kabul River.

Figure 8 vividly captures the spectrum of shear stress values, ranging from the minimum of zero Pascal to a peak of 0.59 Pascal. This distribution pattern accentuates the notable contrast in shear stress levels across the river’s extent. Particularly in the upstream reach, where the Kabul River courses through gritty and narrow valleys, heightened shear stresses manifest, leading to increased friction losses and, subsequently, exacerbating erosion concerns.

Figure 9 unveils the intricate interplay of erosion and deposition processes unfolding sporadically along the course of the Kabul River. This dynamic phenomenon is encapsulated by fluctuations occurring in both positive and negative directions, symbolizing sedimentation and erosion events. Notably, the river segment spanning from 10 to 12.2 km emerges as a focal point for the highest levels of erosion and deposition. Conversely, from 22.6 to 32.4 km, a river stretch characterized by significant width, remains remarkably stable without evident bed changes.

Fig. 9
figure 9

Mass alterations along the river in Kabul

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Figure 10 presents a comprehensive illustration of bed level variations across distinct segments of the Kabul River. Throughout the river’s expanse, intermediate levels of both erosion and sedimentation are observed. The maximum erosion measures approximately 1.27 m, juxtaposed against the peak deposition of around 0.35 m, localized downstream at a reach of 33.7 km. The Kabul River emerges as a dynamic entity, perpetuating a sequence of erosion followed by deposition. Similarly, the river stretch from 22.6 to 32.4 km, distinguished by increased width, registers minimal bed level variations.

Fig. 10
figure 10

The Kabul River’s modeled length features varying bed levels

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By virtue of the river’s flow, sediments are conveyed through diverse modes encompassing dissolved, suspended, saltation, washed, and bed loads. Suspended sediments are integral constituents of the clastic load, traversing the channel within the water column. In a specific 10 km span of the Kabul River, the calculated maximum sediment load reaches 151,511.09 tons. Figure 11 encapsulates the dynamics of suspended load fluctuations across the Kabul River’s course.

Fig. 11
figure 11

Sediment load along Kabul River

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Figure 12 presents a dynamic and geographically grounded visualization of the study river’s entire length, thoughtfully embedded within the familiar terrain of Google Maps. This immersive portrayal not only places the study’s focus within a real-world context but also provides a tangible geographical understanding of the Kabul River’s trajectory.

Fig. 12
figure 12

Google map of Kabul River length model

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Conclusions

  1. 1.

    In the initial phase, a comprehensive sediment transport analysis was executed within the HEC-RAS framework. The outcomes underscored notable erosion occurrences within the upstream and downstream stretches of the Kabul River, attributed to the greater slope characterizing these sections, in contrast to the relatively stable middle length. This equilibrium in sediment accumulation across the middle length is attributed to the smaller slope therein.

  2. 2.

    Erosion dynamics, influenced by factors like elevated flow velocities and bed material characteristics, emerged as a dominant contributor to sediment transport. The pronounced steepness of the river’s upstream slope plays a pivotal role in stimulating erosional processes.

  3. 3.

    The confluence points of the tributary rivers with the Kabul River, situated upstream, prominently exhibit deposition phenomena. A discernible shift in the river’s longitudinal slope at these confluence points potentially contributes to the sedimentation trends observed in Kabul city.

  4. 4.

    Urban congestion, unregulated construction along the riverbanks, and the imperative for improved sanitation infrastructure in Kabul city are identified as prime drivers of sediment deposition. Furthermore, the presence of a low and variable slope within the middle length passing through Kabul city accentuates sediment-related challenges.

  5. 5.

    The establishment of sediment-induced bars, coupled with infrequent river dredging, has yielded adverse consequences. These encompass reduced river capacity for accommodating flood discharges and a notable decline in Kabul River’s water quality.

  6. 6.

    The significance of this research transcends its immediate scope, extending to the realm of infrastructural planning along the Kabul River. Given its central positioning within Afghanistan, the findings hold profound implications for the region’s development endeavors.