Mapping Floods in Lowland Forest Using Sentinel-1 and Sentinel-2 Data and an Object-Based Approach

Mateo Gašparović,Damir Klobučar

doi:10.3390/f12050553

Abstract

The impact of floods on forests is immediate, so it is necessary to quickly define the boundaries of flooded areas. Determining the extent of flooding in situ has shortcomings due to the possible limited spatial and temporal resolutions of data and the cost of data collection. Therefore, this research focused on flood mapping using geospatial data and remote sensing. The research area is located in the central part of the Republic of Croatia, an environmentally diverse area of lowland forests of the Sava River and its tributaries. Flood mapping was performed by merging Sentinel-1 (S1) and Sentinel-2 (S2) mission data and applying object-based image analysis (OBIA). For this purpose, synthetic aperture radar (SAR) data (GRD processing level) were primarily used during the flood period due to the possibility of all-day imaging in all weather conditions and flood detection under the density of canopy. The pre-flood S2 imagery, a summer acquisition, was used as a source of additional spectral data. Geographical information system (GIS) layers—a multisource forest inventory, habitat map, and flood hazard map—were used as additional sources of information in assessing the accuracy of and interpreting the obtained results. The spectral signature, geometric and textural features, and vegetation indices were applied in the OBIA process. The result of the work was a developed methodological framework with a high accuracy and speed of production. The overall accuracy of the classification is 94.94%. Based on the conducted research, the usefulness of the C band of the S1 in flood mapping in lowland forests in the leaf-off season was determined. The paper presents previous research and describes the SAR parameters and characteristics of floodplain forest with a significant impact on the accuracy of classification.

Highlights

Floods are associated with high rainfall intensities and occur when recipients cannot receive all the water, causing them to spill over into the surrounding areas [1]
Visual interpretation and comparison of VV and VH imagery from two different periods show the identification of open water surfaces reflected in dark colors in both periods (Figure 8), which was to be expected given the introductory theory of synthetic aperture radar (SAR) technology and cited literature [23,27,38]
Open floods are shown in dark colors and are identified, while flooded forests are reflected in the light and light grey tones depending on the structure of the forest

Summary

Introduction

Floods are associated with high rainfall intensities and occur when recipients cannot receive all the water, causing them to spill over into the surrounding areas [1]. Flooded areas are land susceptible to being inundated by water from any source, which usually presents as low, frequently flooded habitats along the coast on river islands and reefs, usually in the river’s immediate vicinity depending on the micro-relief and in remote areas of the river valley [2]. The appearance of forest stands (morphology) and the spatial distribution of wood biomass (structure) are affected by the water regime and edaphic features. Any change in the water regime affects forest stands, especially their origin and dynamics.

Objectives

Methods

Results

Discussion

Conclusion