Reconstructing a Traditional Sandbar Polder Landscape Based on Historical Imagery: A Case Study of the Yangzhong Area in the Lower Yangtze River

Abstract

In regional traditional landscape studies where continuous literature and physical relics are scarce, image-based materials serve as a crucial medium for reconstructing historical spatial structures. This study focuses on the sandbar polder landscapes in the Yangzhong area, located in the lower Yangtze River. By integrating historical maps, military cartographic surveys, CORONA satellite imagery, and modern remote sensing data, this study developed a multi-source image interpretation framework to reconstruct the traditional dike–water–field–settlement spatial structure. Employing image recognition and morphological analysis, the study extracted features such as dikes, water systems, and settlements, revealing their adaptation mechanisms to microtopography and associated ecological functions, including multi-level irrigation and drainage, hydrological buffering, and flood prevention. The results demonstrate that traditional sandbar polder landscapes exhibit a high degree of experiential adaptation, and their spatial organization offers valuable insights for future green infrastructure planning. The study confirms the applicability of image-based interpretation methods for historical landscape reconstruction and provides a practical path for the activation and translation of traditional landscape units in contemporary urban–rural governance.

1. Introduction

As the product of long-term interactions between nature and human activity, traditional agricultural landscapes embody regional ecological wisdom and cultural memory [1,2,3]. In recent years, they have attracted increasing attention in the fields of human geography, landscape studies, and urban–rural planning [4,5,6]. However, under the combined pressures of rapid urbanization, land consolidation, and agricultural modernization, these landscapes face accelerated degradation and structural fragmentation [7,8,9,10]. The identification of their traditional spatial forms and the re-evaluation of their ecological functions have thus become key issues in achieving cultural continuity and sustainable development.

Polders represent a widely distributed type of historical agricultural landscape in low-lying riparian areas. Their typical characteristics include constructing dikes to enclose water, establishing drainage and irrigation canals, and adapting land use to topography and hydrological conditions such as tides and floods. These spatial construction patterns can be studied using a typological approach [11,12,13]. In the Yangtze–Huai region of China, polder landscapes are particularly prominent, forming a complex agricultural structure highly coupled with regional geomorphology, water systems, and settlement patterns [14,15]. In the lower Yangtze River’s riverside region, sandbar polders exhibit unique spatial textures [16,17], demonstrating a precise and effective adaptation to sandbar microtopography and flood level differentials [18]. However, since these polders are mainly located in the economically developed but low-lying riverside region of the Yangtze River Delta, they have been severely impacted by rapid urbanization and agricultural modernization, often resulting in the destruction or obscuration of their morphological structures [19,20]. Understanding how to scientifically reconstruct the historical configuration of riverside sandbar polder landscapes and reveal their formation mechanisms and spatial logic has become a key issue in both landscape history research and practical transformation.

In recent years, the widespread availability of spatial data such as historical cartographic materials and remote sensing imagery has provided critical support for the morphological reconstruction and evolutionary analysis of traditional landscapes [21,22]. In particular, the release of early satellite data such as CORONA has effectively compensated for the limitations of traditional sources in terms of spatial coverage and information dimensions. These data have enabled researchers to infer landscape structures based on imagery interpretation in regions lacking detailed historical documentation, making it an essential technical approach for identifying and reconstructing traditional agricultural landscapes [23,24,25]. However, existing studies often remain at the level of image comparison and morphological restoration, lacking in-depth interpretation of the reclamation processes and spatial structures of traditional agricultural landscapes and failing to extract the underlying ecological intelligence and construction logic.

Located in the lower reaches of the Yangtze River, the Yangzhong area offers distinct advantages for research on sandbar polder landscapes, including clearly documented land formation history, abundant imagery sources, and well-preserved agricultural patterns. These characteristics make it an ideal testing ground for image interpretation and morphological reconstruction of riverside sandbar polder landscapes in the Yangtze River Delta. This study integrates historical maps, military cartographic surveys, CORONA satellite imagery, and modern remote sensing data to develop a systematic image-based identification framework. It reconstructs the fundamental landscape structure of this polder type, analyzes its spatial organization logic and ecological adaptation mechanisms, and further explores its potential relevance for contemporary urban–rural spatial restructuring.

The objectives of this study are threefold: (1) to explore image-based identification methods for sandbar polder landscape structures and to clarify the potential of image interpretation in historical agricultural landscape research; (2) to systematically reconstruct the structure of dikes, water systems, field plots, and settlements in the Yangzhong area, and to analyze their overall spatial organization logic and ecological functions; (3) to propose insights from traditional sandbar polder landscapes for green infrastructure development and spatial planning in response to the current spatial fragmentation challenges in urban–rural development.

Through an in-depth identification and structural analysis of the sandbar polder landscapes in the Yangzhong area, this study not only provides a representative case for exploring the morphological evolution of reclaimed lowlands in traditional periods but also demonstrates methodological and theoretical innovations in image interpretation and vernacular landscape cognition. On the methodological level, it integrates historical maps, remote-sensing imagery, and military survey charts to develop an image interpretation workflow specifically adapted to traditional agricultural landscapes. On the theoretical level, by focusing on the spatial texture and settlement patterns of sandbar polders in Yangzhong, the study identifies key characteristics of vernacular landscapes, thereby offering practical support and academic reference for the understanding, heritage conservation, and spatial regeneration of traditional agricultural environments.

2. Data and Methods

2.1. Study Area Overview

The Yangzhong area is located between the river systems of the mid-channel shoals in the Jiangsu section of the lower Yangtze River, situated between the Zhenjiang and Jiangyin areas. Surrounded by the Yangtze River on all sides, it covers an area of approximately 211 km² and represents a typical shoal geomorphology (Figure 1). The temporal scope of this study spans from the late 19th century to the 1960s. By the late 19th century, the shoal landforms in the Yangzhong area had largely stabilized and undergone extensive reclamation, with relatively detailed historical image materials preserved from this period, providing a crucial basis for analyzing the spatial morphology of sandbar polder landscapes. Before the 1960s, China had not yet entered the phase of rapid urbanization, and traditional agricultural landscapes were largely intact. Moreover, as the Yangzhong area was not a core area of economic development, its overall landscape structure has remained well preserved to this day. The agricultural texture in the Yangzhong area is clearly distinguishable, with well-preserved spatial configurations and a wealth of detailed, consecutive historical imagery. These conditions offer a robust foundation for multi-temporal and multi-scalar image comparison and morphological reconstruction, making this area particularly suitable for landscape typology identification and evolution analysis.

2.2. Data Sources and Processing

This study takes image data as the core source, supplemented and validated by historical documentary materials. The image data used mainly includes the following four categories (Figure 2):

(1) Military Cartographic Maps: These are 1:50,000-scale topographic maps produced by the Land Survey Bureau of the Military Committee, dating from approximately the 1910s to the 1940s. The maps are preserved by the Institute of Modern History, Academia Sinica (https://www.mh.sinica.edu.tw/). They feature a moderate scale and detailed graphical expression and contain rich information such as water systems, dikes, and place names in the Yangzhong area. These maps offer high spatial accuracy and considerable historical reference value.

(2) CORONA Remote Sensing Imagery: Provided by the United States Geological Survey (USGS) (https://www.usgs.gov/), the CORONA satellite imagery from the 1960s was captured by early reconnaissance satellites. With a spatial resolution of approximately 1.8–2.1 m, the imagery is clear enough to identify landforms, water systems, farmland, and settlements. It effectively reflects the landscape pattern of the Yangzhong area prior to the onset of urbanization. This type of remote sensing data offers a unique combination of temporal specificity and high interpretation accuracy, making it a valuable base map for the reconstruction and comparative analysis of traditional agricultural landscapes.

(3) Historical Maps: This refers primarily to the Map of Newly Purchased Polders on the Jiangyin Sandbars, drawn in the 1880s and archived at the Taipei Public Library (https://tpml.gov.taipei/). It was created using the traditional "bow-step" surveying method and preserves detailed information such as the outlines of dikes, river ports, drainage ditches, and naming systems.

(4) Modern Remote Sensing Imagery: Satellite images with a spatial resolution of approximately 0.5 m, obtained via Google Earth Pro, were used as the reference base for assessing the current state of sandbar polder landscapes, settlement distribution, and development patterns in the Yangzhong area. These images help evaluate the spatial continuity of traditional agricultural landscapes and support field verification efforts.

2.3. Methods for Landscape Structure Identification and Reconstruction

This study establishes an operational framework centered on image comparison, morphological reconstruction, and structural identification to reconstruct landscape patterns from localized image analysis to holistic spatial interpretation. By overlaying multi-source imagery across different time periods, the fundamental dike–water–field–settlement spatial structure of the sandbar polder landscape is progressively identified, enabling a systematic reconstruction of the overall landscape configuration in the Yangzhong area (Figure 3).

2.3.1. Preliminary Identification Based on Remote Sensing Imagery

Within the ArcGIS environment, this study utilized modern satellite imagery from Google Earth Pro as the base layer, covering approximately 31,665 km² in the Jiangsu section of the lower Yangtze River. All data were uniformly georeferenced to the WGS_1984_UTM_Zone_32N coordinate system. Overlaid on this base were CORONA satellite images from the 1960s, matching the same spatial extent. Although some CORONA imagery had been pre-georeferenced via the Jiangsu Tianditu platform (https://jiangsu.tianditu.gov.cn), technical constraints related to the interface and data formats prevented its direct application. As a result, a high-precision georeferencing procedure was conducted manually. Despite the 60-year time span between the two datasets and the occurrence of land consolidation and urban expansion, the boundaries of most settlements and the morphological texture of traditional agricultural landscapes have remained largely intact. Stable geographic features—including major riverbanks, hills, intersections of waterways and ports, settlement centers, and shoreline locations—were selected as control points to support spatial alignment and error correction of the CORONA imagery.

Military topographic maps from the 1910s to 1940s differ significantly in cartographic style from modern remote-sensing datasets, rendering direct feature matching highly challenging. However, key geographic reference points in the Yangzhong area—such as settlements, dikes, and river–port intersections—remain clearly identifiable in the CORONA imagery. While the CHMAP platform (https://chmap.mpiwg-berlin.mpg.de/lgtu-new/map.html, accessed on 28 August 2025) has performed large-scale mosaicking and preliminary georeferencing of these historical maps, the resulting accuracy does not meet the requirements for fine-scale landscape morphological analysis. Therefore, this study used the already-registered CORONA imagery as a reference, and applied a separate control-point-based georeferencing process to the military survey maps of Yangzhong. This approach ensures spatial consistency and improves the interpretive accuracy of image-based landscape analysis.

Results show that in the riverside area, ring-shaped polder units and their peripheral dike structures can be clearly identified through image interpretation. In the inland regions, however, the dikes appear more obscure in the imagery, with only segments along major rivers and channels retaining discernible dike lines. Nonetheless, a large number of ring-shaped farmland units labeled with the character of polder in the military maps spatially correspond closely with the field patterns observed in the CORONA imagery, revealing the extensive distribution of sandbar polder landscapes and their distinctive naming conventions. Although this stage of image interpretation does not yet allow for a detailed reconstruction of the internal spatial pattern of the polder units, it sufficiently confirms the Yangzhong area as a representative reclamation-based agricultural region, providing a foundation for further morphological identification.

2.3.2. Reconstruction of the Spatial Structure of Typical Polder Units

From a morpho-typological perspective, the formation of human-built environments is a continuous process closely tied to intrinsic spatial characteristics—traits that are identifiable and classifiable [26]. In the lower Yangtze River region, the morphological features of traditional sandbar polders are clearly discernible in the 1960s CORONA satellite imagery. Leveraging the powerful image-recognition capabilities of machine-learning-based artificial intelligence [27], this study employed the SegNet semantic segmentation network, a convolutional neural network (CNN), to detect polder landscapes. The CORONA imagery covering the Yangzhong area was manually labeled to identify polder zones and underwent data augmentation before being used for training the network. Once trained, the model was applied to CORONA imagery of the entire Jiangsu section of the lower Yangtze River to generate a preliminary distribution map of sandbar polders. To address occasional misclassifications, a confidence-based clustering analysis of the model predictions was conducted. In combination with manual visual interpretation, this allowed for targeted refinement of the results. The final output is a spatial distribution map of sandbar polders in the Jiangsu section, covering an estimated area of approximately 2180 km² (Figure 4).

The identification results include the Jiangyin riverside area, whose traditional landscape fabric closely resembles that of the Yangzhong region. Notably, this area is also documented in the 1880s atlas Map of Newly Purchased Polders on the Jiangyin Sandbars, produced using the traditional bow-step surveying method. The atlas offers detailed depictions of polder dike boundaries, water inlets, canal systems, and the positions of tidal flats and grass-covered shoals, outlining a reclamation process analogous to that of Yangzhong. By georeferencing and aligning this historical atlas with CORONA satellite imagery of the Jiangyin riverside, we clearly identified the characteristic dike–water–field–settlement spatial structure of this polder type: the dike forms the outer boundary of each farmland unit, interior drainage canals run parallel to the dike, and settlements are linearly distributed along its crest. This cross-verification confirms the feasibility of using CORONA imagery to recognize traditional polder landscape patterns and provides a robust methodological foundation for extrapolating the spatial structure of the Yangzhong area.

2.3.3. Regional-Scale Reconstruction of Landscape Structure

Building on the identification of typical polder unit structures, this study applies the dike–water–field–settlement landscape model across the entire Yangzhong area using CORONA satellite imagery to systematically reconstruct the spatial pattern of sandbar polder landscapes. The operational procedure includes identifying polder units with distinct structural characteristics based on features such as farmland textures, canal orientations, and settlement distribution in the satellite imagery; restoring the spatial relationships among dikes, canals, settlements, and fields on the imagery according to the interpretation logic; and deducing the locations of sandbar cores and the expansion directions of polder development through elements such as settlement density, dike alignment, and water system layout. The entire reconstruction was carried out through image interpretation and GIS operations: key elements of the polder landscape—dikes, water systems, and settlements—were extracted from CORONA imagery of the Yangzhong area in vector format to create shapefiles.

3. The Morphological Evolution Characteristics of Sandbar Polder Landscapes in the Yangzhong Area

3.1. Reclamation Background

The Yangzhong area is located on the convex bank of the mid- to lower reaches of the Yangtze River. The fluvial environment on the convex bank tends to be relatively stable, which facilitates sediment deposition [28]. During the Eastern Jin Dynasty (317–420 AD), changes in the boundary conditions of the Yangtze River led to a decrease in flow velocity, creating favorable conditions for continuous sediment accumulation, and sandbars began to emerge above the river surface. By the mid-to-late 13th century, small sandbars gradually merged into larger landmasses, forming inter-channel islands that diverted river flow, historically referred to as Small Sandbars [29]. In the 8th year of the Hongzhi reign of the Ming Dynasty (1495), the Yellow River diverted through the Huai River and entered the Yangtze River west of Yangzhong area, accelerating the river’s flow and causing the previously formed sandbars in the Yangzhong area to be largely submerged back into the river.

In the fifth year of the Xianfeng reign of the Qing Dynasty (1855), the Yellow River shifted northward, resulting in a decreased flow velocity of the Yangtze River in the Yangzhong area. This change created favorable conditions for stable sediment deposition, leading to the gradual emergence of sandbars above the water surface and the formation of the main contours of Yangzhong’s sandbar landscape. Large-scale migration from both banks of the Yangtze brought more advanced polder reclamation techniques, which further accelerated shoreline formation in the region. During this period, due to the frequent battles between the Taiping forces, who had established their capital in Nanjing, and the Qing army, the Yangzhong area—located midstream in the river—remained relatively untouched by war, attracting many refugees. The influx of migrants not only caused a substantial population increase in the Yangzhong area but also promoted further development of the sandbars. By the early 20th century, with continuous sediment accumulation, the rudimentary landform of Yangzhong area had taken shape, comprising 58 large and small sandbars [30]. It was not until the 30th year of the Guangxu reign (1904) that the Yangzhong area transitioned from a scattered sandbar region to an officially established administrative unit [31] (Figure 5).

3.2. Reclamation Pattern

The geomorphological structure of the Yangzhong area exhibits typical characteristics of mid-channel sandbars in the Yangtze River. Each sandbar unit contains a relatively elevated sandbar core. The elevation of these cores generally aligns with the average tidal level of the Yangtze River, typically ranging from 4 to 5.5 m and is composed primarily of heavy silts and clays [16]. As the sandbar cores stabilized, floodwaters slowed rapidly at the bank margins, causing coarser sediment to accumulate around the core, gradually forming slightly elevated ridges surrounding the sandbar. These ridges are usually about 1 m higher than the sandbar cores, with widths varying depending on hydrological conditions [35]. Once the sandbar emerged stably above the water level, the ridges closest to the core were first enclosed with dikes. Over time, additional sediment accumulated beyond the initial ridges, forming grasslands and new ridges, thus expanding the sandbar outward. As new ridges emerged, the same method of reclamation was applied, with successive layers of dikes moving outward, resulting in a clearly concentric spatial pattern (Figure 6).

The development of polder fields in the Yangzhong area was initiated spontaneously by local residents who applied for land reclamation rights. During the Qing Dynasty and the Republican period (1644–1949), dedicated Sandfield Bureaus were established in Yangzhong to manage administrative procedures such as application, allocation, surveying, and mapping of tidal flats located outside the dikes. The reclaimed sandfields were categorized into three types: rising shoals, referring to grassy tidal flats adjacent to existing dikes; sudden-rising shoals, denoting newly formed tidal flats in the river at a distance from the old dikes; and shadow shoals, which had not yet surfaced but were identifiable by water coloration or pole probing. Applicants were required to pay a deposit in advance. Upon survey approval, the tidal flat owner would be notified to pay the full fee based on the measured sand area, after which a license would be issued, allowing independent dike construction and development. In cases where storm tides breached dikes and submerged the reclaimed fields into the Yangtze River, the loss was borne entirely by the applicant. This illustrates the harsh reality of transforming watery wilderness into farmland [29].

3.3. Landscape Structure

Centered around the sandbar core, the polder fields in the Yangzhong area expanded outward in concentric layers. In the process of this radial expansion, drainage channels connecting to the Yangtze River were deliberately left between successive dike rings. Culverts were installed within the dikes, allowing internal drainage ditches to connect with external drainage channels. Before the mid-20th century, this system enabled gravity-fed irrigation: during the flood season from May to September, the Yangtze River’s water level was higher than the interior fields, allowing river water to flow naturally through these culverts and channels into the polder for irrigation and storage. During the dry season from October to April, when the river level dropped below that of the internal fields, the same culverts allowed excess water to drain outward into the drainage channels. In times of drought when the river remained at a low level and gravity irrigation became infeasible, farmers resorted to manual irrigation using traditional longguche, though these methods were significantly less efficient [29]. Surrounded by numerous river-connected ports and characterized by winding channels and dense ditches, the Yangzhong area featured over 270 internal and external drainage waterways and more than 80 drainage channels connecting to the Yangtze River by the early 20th century. This intricate water network remained largely intact through the mid-20th century (Figure 7).

The Yangzhong area was formed by the merger of multiple smaller sandbars, each with its own sandbar cores, resulting in a complex dike system composed of multiple concentric structures. Based on the reclamation pattern of sandbar polders in the Yangzhong area and the ring-shaped ridge textures observed in CORONA satellite imagery, it is possible to trace the locations of the original sandbar cores. In some areas, the spatial structure of these sandbar cores remains well preserved, displaying a complete circular configuration (Figure 8).

In the Yangzhong area, the polder fields on the river sandbars expanded outward layer by layer from the sandbar cores. In the absence of external disturbances, this expansion typically followed a consistent rate, which is reflected spatially by the alignment and uniform spacing of successive polder dikes (Figure 9). However, even though Yangzhong lies in a relatively stable sedimentation zone, certain deviations from the expected sandbar reclamation patterns can be observed in the landscape morphology. For instance, in areas where multiple sandbars converge, the directions of the dikes appear skewed, and the arrangement of polder fields is relatively cramped. These sandbars, having merged during the development process, left only limited intertidal space at their junctions, resulting in narrower polder fields and more closely spaced dikes. In other cases, several sandbar cores are present within a single region, yet the characteristic concentric expansion pattern centered on a single core is absent. This suggests that multiple similarly sized and closely located sandbars emerged simultaneously, each forming a core suitable for reclamation. As they rapidly merged, there was insufficient space to develop the ideal concentric ring structure. Although the Yangzhong section of the Yangtze River provided a relatively stable environment for the reclamation and development of sandbar polders, the formation of the ideal concentric dike pattern centered on a sandbar core remained contingent on environmental conditions. Any changes in external factors were directly reflected in the morphology of the dike system.

3.4. Farmland Settlements

In the Yangzhong area, dikes served not only as critical flood control structures but also fulfilled dual roles as residential foundations and transportation routes. Settlements were typically built on the landward side of the dikes, with houses oriented toward the Yangtze River. To enhance the structural stability of the dikes, residents commonly planted plants such as willow trees and reeds, which helped reinforce the banks against erosion and improved the defensive resilience of the reclamation system [37]. These plants were usually located behind the houses. Although the inhabitants lived in low-lying areas, they skillfully utilized micro-topographic features and vegetation to create a spatial configuration in line with the traditional Chinese ideal of “facing water with hills at the back” (Figure 10).

The dikes also functioned as internal roads within the settlements and were typically located in front of residential buildings. On the inner side of the dikes, drainage ditches were arranged parallel to the dike; in some areas, these were widened due to excavation for dike and house construction. These ditches were connected to outer waterways via culverts embedded in the dikes. Residents often partitioned the section of the ditch in front of their homes to create private ponds for water storage, where poultry and fish could be raised (Figure 11). As the Yangzhong area is located in the middle of the Yangtze River, land development and formal administrative establishment came relatively late. Due to the region’s fertile soil and self-sufficient agricultural system, large-scale settlements had not yet emerged prior to the mid-20th century. Instead, small-scale market towns appeared along riverbanks, distributed linearly along the dikes, and were generally similar in size and form to ordinary polder villages.

4. Discussion

4.1. The Application Potential of Image Interpretation in Historical Landscape Research

In regional studies of traditional landscapes—particularly in areas lacking continuous written records and with limited public awareness—visual materials have become essential media for understanding historical spatial structures. Although the sandbar polder landscapes of the Yangzhong area retain abundant field evidence, including agricultural patterns and dike traces, their historical significance and spatial logic have not been fully recognized or interpreted. This study conducted a systematic interpretation of multi-source imagery—including historical maps, military survey maps, CORONA satellite imagery, and modern remote sensing data—using a time-series overlay approach. The result is an operational methodology suitable for regional landscape identification and structural inference, confirming the practical value of image interpretation in both reconstructing historical landscapes and enhancing their cultural recognition.

The historical map from the 1880s, the 1910s to 1940s military survey map, and the 1960s CORONA satellite imagery used in this study each possess strong spatial expressiveness and a high density of historical information, representing three distinct technological systems: traditional ground-based surveying, modern cartographic mapping, and aerial remote sensing. Despite differences in image resolution and symbol systems, spatial correspondence and feature tracking across time periods can be achieved within a GIS platform through georeferencing, feature extraction, and manual verification. This methodological approach not only enhances the interpretability of image data but also overcomes the representational limitations associated with single time-point imagery.

The key to image interpretation lies in extracting the underlying logic of landscape structure from visual features. This study adopts the analytical framework of dike–water–field–settlement to identify dikes, water systems, fields, and settlements in imagery, thereby constructing a spatial model of polder units. By recognizing agricultural textures in CORONA imagery of the Jiangyin area that match those in the Yangzhong area and correlating them with the 1880s historical map, the basic spatial structure of sandbar polder landscape in the Yangzhong area was indirectly reconstructed. This approach demonstrates that image interpretation is not limited to areas directly depicted. Instead, it can be expanded through comparative analysis, thereby enhancing the logical reconstruction capacity of image data in historical landscape research.

In this study, image interpretation functions not only as a technical means for data processing and feature identification but also as a historically reconstructive tool with integrative explanatory power. It establishes causal links between natural landforms, social development processes, and spatial organization patterns. Compared to traditional research approaches that rely on textual records and oral histories, image interpretation offers distinct advantages, including strong visual intuitiveness, broad spatial coverage, and seamless integration with GIS platforms. These qualities make it particularly well-suited for reconstructing complex landscapes such as reclaimed agricultural systems, ancient waterway systems, and traditional settlement patterns. The multi-source image interpretation framework developed in this study provides a viable paradigm for the historical reconstruction of the sandbar polder landscapes in the Yangzhong area and offers a methodological reference and conceptual expansion for regional agricultural landscape research in the Yangtze River Delta and other areas lacking systematic documentary records.

4.2. Morphological Structure and Adaptive Mechanism of Sandbar Polder Landscapes

As a representative area of the sandbars in the middle of the river section in the newly formed Yangtze River Delta, the sandbar polder landscape of the Yangzhong area developed through a long-term coupling of unique geomorphological conditions and sustained socio-spatial construction. From a natural perspective, sandbar cores gradually emerged above the water surface through sedimentation. Surrounding these cores, linear elevated ridges formed under flood scouring, providing the initial foundation for early dikes. With continued sediment accumulation, successive ridges and marginal shoals developed, prompting outward expansion of reclamation activities. This process resulted in a ring-shaped polder system radiating from the sandbar core. Such a reclamation pattern aligned with the natural rhythm of hydrodynamic processes and leveraged the elevation differences in coordination with flood levels, thereby reducing the engineering difficulty of dike construction.

In this process of human intervention driven by natural forces, the “dike–water–field–settlement” configuration emerged as the fundamental landscape unit of the sandbar polder system. The ridges rise approximately 1 m above the natural flood level. Dikes constructed atop these ridges not only enhance flood control and bank stabilization but also offer safer, elevated foundations for settlement construction. External channels outside the dikes and internal canals within them formed a multi-tiered water network responsible for irrigation and drainage. Farmland, enclosed by the water system, was arranged in fan-shaped strips, creating a distinct cultivation texture. Settlements were typically arranged along the dike lines, reflecting both defensive concerns and hydrological convenience. From a spatial structural perspective, this landscape unit reveals a nested hierarchy: dikes provide protection, water systems offer support, farmland forms the core, and settlements anchor the system. These functional components are interlocked based on variations in terrain elevation and hydrological adaptation, forming a multi-layered and composite spatial organization system.

For people living on sandbars during the traditional period, this landscape structure provided a certain degree of water retention capacity and risk control. On one hand, the multi-tiered water system functions to disperse water levels and enable internal drainage during flood seasons. On the other hand, the placement of settlements on elevated dike land reduces the risk of flood intrusion. The coupling between farmland layout and canal density reflects a high level of adaptation to both land-use efficiency and water resource management, embodying a site-specific logic of landscape construction. These landscape strategies—rooted in accumulated empirical knowledge—illustrate the traditional reclamation society’s precise understanding of microtopography, water level variation, and cultivation scales.

Overall, the spatial structure of the polder landscape in the Yangzhong area not only exemplifies the co-evolution of natural landforms and socio-technical interventions but also reflects the highly empirical knowledge systems of traditional agrarian societies in land administration, water infrastructure management, and settlement siting. The strong correlation between its morphological features and organizational patterns offers a critical reference for understanding the operational mechanisms of sandbar reclamation systems in historical periods.

4.3. Disruption of Urban–Rural Development and Lessons from Tradition

Although the traditional sandbar polder landscape of the Yangzhong area reveals a highly ordered spatial structure and ecological adaptability through image interpretation, this traditional pattern has gradually deteriorated amid modernization, particularly with urban and rural expansion in recent decades. While the Yangzhong area has not undergone intense urbanization, infrastructure development oriented around road networks and the scattered expansion of market towns have continuously disrupted the integrity of the sandbar polder landscape.

Firstly, the disruption of the polder texture is primarily reflected in the deconstruction of the water system and spatial organization. With the advancement of agricultural modernization, the traditional water system—characterized by dike-defined boundaries, interconnected ditches and creeks, and gravity-based irrigation and drainage—has been extensively filled in or transformed. The original multi-level irrigation–drainage network has largely disappeared, resulting in a significant decline in gravity-fed irrigation and water retention capacity. The reduced density and broken connectivity of the ditch network have not only weakened the polder system’s ability to manage stormwater but also increased its reliance on external energy and water sources for irrigation. Secondly, the coordinated relationship between settlement patterns and the dike system has gradually disintegrated. In the past, settlements were built along dikes, taking advantage of elevated terrain to resist flooding and forming a spatial pattern of "facing fields, backing water." Today, however, settlement expansion is more often guided by road networks rather than water logic, disrupting the coupling between settlements and water infrastructure and weakening the overall disaster resilience of spatial units. Moreover, although the existing market towns remain relatively small in scale, they are increasingly spreading in a scattered fashion across the polder area. This leads to land hardening and elevation of building platforms, which alters the original microtopographic hydrological rhythms and further undermines the retention and attenuation mechanisms of the traditional sandbar polder landscape.

In response to the spatial fragmentation and weakening of ecological functions outlined above, the structural wisdom embodied in traditional sandbar polder landscapes offers valuable insights for contemporary urban–rural spatial reconstruction: (1) The nested spatial organization centered on the dike–water–field–settlement model can serve as a planning prototype for green infrastructure in today’s low-lying waterfront areas. By aligning with micro-topographic conditions and organizing multi-level water systems, it enables coordinated rainwater retention and drainage while promoting moderate and orderly land use. (2) The multi-tiered water system within traditional sandbar polder landscapes demonstrates the capacity to build resilient irrigation and drainage structures upon natural terrain. This approach is particularly relevant for the revival of small-scale water cycles and the implementation of ecological retention strategies in current planning practices. (3) The spatial logic of siting settlements along dikes not only provided strong flood resilience but also reflected a comprehensive consideration of terrain, water systems, cultivation, and safety. It offers cross-scalar, multi-objective planning references for modern settlement siting and layout.

In summary, although the Yangzhong area has not undergone intense urbanization, the spatial structure and ecological mechanisms of its traditional sandbar polder landscape have nonetheless been continuously weakened by modern development. A comparative analysis of its historical patterns and contemporary transformations reveals that the landscape wisdom embedded in traditional polder systems still holds practical relevance and transformative potential for today’s urban–rural spatial restructuring.

5. Conclusions

This study focuses on the sandbar polder landscape of the Yangzhong area. Drawing on multi-source imagery—including historical maps, military survey maps, CORONA satellite images, and modern remote sensing data—and employing image interpretation and morphological analysis methods, the research systematically reconstructs the traditional dike–water–field–settlement structural unit of the polder system. It further reveals the formation mechanisms and ecological adaptation characteristics of this landscape. The main conclusions are as follows:

First, the spatial structure of the sandbar polder landscape is shaped by the dual influences of geomorphological evolution and human construction. In the Yangzhong area, the layout of the polder system follows the developmental trajectory of natural landforms such as sandbar cores, ridgels, and marginal shoals. By constructing dikes along the ridges, establishing settlements along the dikes, and subdividing farmland within the dike boundaries, a highly coupled landscape pattern was formed. This morphology not only exhibits a distinct concentric texture but also reflects adaptive strategies that respond to flood levels and make use of microtopography.

Secondly, the method of multi-source image overlay and interpretation demonstrates strong applicability and innovation in this study. Through spatial comparison and element tracking across different types of imagery, the study overcomes the limitations of scarce historical documentation, successfully extracting key components such as dikes, water systems, and settlements. By applying interpretative logic derived from the adjacent Jiangyin area’s historical maps, the study indirectly reconstructs the landscape units of Yangzhong area’s polder system, verifying the effectiveness of analogical image interpretation.

Thirdly, the spatial configuration of the sandbar polder landscape demonstrates intrinsic capacities for flood mitigation and hydraulic regulation. In traditional systems, multi-level water networks managed irrigation and drainage, strip fields were coordinated with canal density, and settlements were built atop dikes—together constituting a self-adaptive spatial framework that ensured agricultural efficiency and flood resilience. This landscape model reflects the profound empirical wisdom of agrarian societies in managing environmental risks.

Finally, in the context of ongoing urban–rural expansion and spatial restructuring, traditional polder systems face the risk of fragmentation and erosion. The dike–water–field–settlement pattern revealed in this study offers strategic insights for the planning of modern green infrastructure, ecological corridor restoration, and the development of low-lying areas. Traditional sandbar polder landscapes are not only agricultural heritage but also possess tangible potential for transformation from “landscape structure” to “infrastructure.”

In summary, this research demonstrates the methodological value of image interpretation in historical landscape identification, reveals the morphological logic and ecological intelligence embedded in sandbar polder landscapes, and proposes directions for their application in future urban–rural spatial governance.