Riparian zones fulfill diverse ecological and economic functions. Sustainable management requires detailed spatial information about vegetation and hydromorphological properties. In this study, we propose a machine learning classification workflow to map classes of the thematic levels Basic surface types (BA), Vegetation units (VE), Dominant stands (DO) and Substrate types (SU) based on multispectral imagery from an unmanned aerial system (UAS). A case study was carried out in Emmericher Ward on the river Rhine, Germany. The results showed that: (I) In terms of overall accuracy, classification results decreased with increasing detail of classes from BA (88.9%) and VE (88.4%) to DO (74.8%) or SU (62%), respectively. (II) The use of Support Vector Machines and Extreme Gradient Boost algorithms did not increase classification performance in comparison to Random Forest. (III) Based on probability maps, classification performance was lower in areas of shaded vegetation and in the transition zones. (IV) In order to cover larger areas, a gyrocopter can be used applying the same workflow and achieving comparable results as by UAS for thematic levels BA, VE and homogeneous classes covering larger areas. The generated classification maps are a valuable tool for ecologically integrated water management.

Sustainable management of riparian zones requires detailed spatial information about vegetation and hydromorphological properties.Uncrewed aerial systems (UAS) or gyrocopters equipped with multispectral cameras yield imagery of small to intermediate scale areas.Machine learning classification workflows (object based, random forest) including additional geodata and trained with in-situ data allow to map classes of vegetation and hydromorphological substrate types with different level of detail.A case study was carried out in a floodplain area along the River Rhine, Germany, resulting in overall accuracies for UAS data of 89% for basic surface types, 88% for vegetation units, 75% for dominant stand, and 62% for substrate types.Classification probability maps helped to identify areas of lower classification performance, as e.g.vegetation within the transition zone, thus allowing for a subsequent, more focused and effective site inspection.In combination, this workflow provides a valuable tool for monitoring and ecologically integrated water management.

Up-to-date information about vegetation types and hydromorphological structures and features are essential for the management of waterways. They are e.g. used for the monitoring and reporting of riparian statuses and their changes e.g. after river restoration and consequently, numerous man-days are spent on field surveys. To allow for an effective survey of vegetation and hydromorphology in large or even inaccessible areas, a data acquisition and processing workflow is being developed complementing in-situ methods with remote sensing techniques. This is part of the joint research project “mDRONES4rivers” funded by the German Federal Ministry of Transport and Digital Infrastructure (19F2054A). Aerial surveys by unmanned aerial systems (UAS) and a gyrocopter are combined with ground measurements of hyperspectral reflectance signatures as well as with field mapping of vegetation types and hydromorphological structures and features. The remote sensing data is classified with an object based image analysis and classification algorithm. The mobile and (at selected sites) permanent measurements of hyperspectral field data and the typical field surveys provide data for calibration. Contrary to other approaches that focus on what can be detected and classified with certain sensor systems and datasets, the project addresses equally the user needs to obtain certain classes for monitoring and reporting. The intended results are (i) data acquisition, correction and classification workflow combining remote sensing and field data, identification and change detection (ii) of important vegetation and biotope types and (iii) of hydromorphological structures and substrate as well as indicators necessary for the evaluation of the hydromorphological quality. The preliminary results to be presented include datasets from UAS, gyrocopter, and field surveys, an outline of processing workflow and classification algorithm based on Python scripts and eCognition software and first vegetation and hydromorphological classification results from spring and summer datasets. In conclusion, procedures and algorithms are developed to use remote sensing in combination with and for the reduction of time-consuming traditional field surveys as a future operational tool for monitoring riparian vegetation and structures.

Rivers and floodplains are hotspots of biodiversity and provide an exceptional number of ecological functions. However, they are negatively affected by human impact worldwide. The need for restoration is high, but its realization is often hampered by antagonistic human interests. Replacing artificial riverbank protection with bioengineering techniques can be a first and straightforward step to restore riparian ecosystems. However, bioengineering still plays a marginal role in river management. We aim to introduce new arguments for bioengineering along riverbanks by applying the ecosystem service approach. We focus on major regulating services usually provided by floodplains. Denitrification and phosphorous retention were estimated by applying proxy-based models. Carbon sequestration within vegetation was calculated using biomass equations. Our study clearly indicates an increase of ecosystem services by bioengineering measures compared to conventionally fixed riverbanks. The dismantling of riprap removes up to 30 times more nitrogen and 20 times more phosphorous from the river load. Additional slope lowering increases both values up to 50-fold. The carbon storage capacity is four times higher in reed beds and 30 times higher in willow-brush mattresses. Our results show that bioengineering techniques for riverbank protection can be a feasible tool to support general efforts towards enhancing the self-purification of rivers and contribute to mitigating climate change, especially if conducted on a larger scale.

Many studies have illustrated the elevational gradient of plant zonation in estuarine marshes. Nevertheless, little is known about the elevational patterns of plant zones and marsh edge existing along deepened navigation channels compared to their shallow anabranches. Our study analyzed the factors controlling these patterns. This knowledge is required when restoring tidal marshes to compensate for anthropogenic impacts. We determined the species-elevation-pattern in the navigable German Elbe and Weser estuaries. Using the key species Scirpus tabernaemontani, Scirpus maritimus, and Phragmites australis, we compared (1) marshes along navigation channels and anabranches and (2) their marsh edges. (3) Additionally, we tested how tidal range affects the lowest elevation of marsh edges. We analyzed data derived from digital vegetation maps and digital elevation models, and then normalized the elevation data using the mean tidal range. We determined the elevational distributions of plant zones on regularly flooded estuarine marshes and quantified the lowest elevation as an important niche boundary which reflects the resistance against physical stress. Our findings indicate that the patterns result not only from tidal range but also from hydrodynamics due to the dredging history, diversely shaped channels, bank morphologies, and vessel frequencies. The patterns found and the species-specific elevations need to be considered when choosing suitable elevations when restoring tidal marshes in dredged estuaries. The results might also inspire reconsidering the reference plants zonation while monitoring ecological potential of severely modified estuaries under Water Framework Directive.

The European Water Framework Directive requires a good ecological potential for heavily modified water bodies. This standard has not been reached for most large estuaries by 2015. Management plans for estuaries fall short in linking implementations between restoration measures and underlying spatial analyses. The distribution of emergent macrophytes - as an indicator of habitat quality - is here used to assess the ecological potential. Emergent macrophytes are capable of settling on gentle tidal flats where hydrodynamic stress is comparatively low. Analyzing their habitats based on spatial data, we set up species distribution models with 'elevation relative to mean high water', 'mean bank slope', and 'length of bottom friction' from shallow water up to the vegetation belt as key predictors representing hydrodynamic stress. Effects of restoration scenarios on habitats were assessed applying these models. Our findings endorse species distribution models as crucial spatial planning tools for implementing restoration measures in modified estuaries.

Climate change and engineering activities have modified the hydrology and morphology of estuaries. However, the potential effects of these modifications on vegetation succession in estuarine marshes are still poorly understood. Therefore, we studied temporal changes in tidal habitats of the Elbe estuary over a period of 30 years. We compared vegetation maps from 1980 to 2010 and calculated the change in area of habitats with respect to three salinity and three elevational zones. To analyze the direction of the temporal change, we differentiated between progressive and regressive succession. By using regression tree models (conditional inference trees), we identified the most influential factors determining progressive or regressive succession of low marshes. The total area of the estuarine tidal marshes at the Elbe increased by 2 % from 1980 to 2010, but changes were unequal among the salinity zones. In the salt and brackish zones, the area covered by high marshes increased substantially but decreased in the tidal freshwater zone, while that covered by low marshes decreased in all the salinity zones. Additionally, we determined high persistence of tidal flats and high marshes, whereas only 19 to 28 % of the low marshes found in 1980 remained in 2010. In salt and brackish marshes, more than two-thirds of the area that had been identified as low marshes in 1980 had progressively developed into high marshes. In contrast, 44 % of the area of low marshes in tidal freshwater marshes showed regressive succession back into tidal flats. The distance to the navigation channel was the main factor determining successional direction in salt and brackish marshes. Here, greater proximity to the channel was correlated with higher risk of regressive succession. In tidal freshwater marshes, we identified both the distance to the navigation channel and the situation on the river shore (i.e. inner bank, outer bank or straight bank) as the main factors for marsh succession. Here, considerable engineering activities in the channel had simultaneously decreased the mean low water level and increased the mean high water level between 1980 and 2010, which led to an increase in tidal amplitude. It is quite likely that these changes negatively modified marsh distribution, increased regressive succession and, thus, lowered the quality of tidal freshwater marshes.

Recent research indicates that many ecosystems, including intertidal marshes, follow the alternative stable states theory. This theory implies that thresholds of environmental factors can mark a limit between two opposing stable ecosystem states, e.g. vegetated marshes and bare mudflats. While elevation relative to mean sea level is considered as the overall threshold condition for colonization of mudflats by vegetation, little is known about the individual driving mechanisms, in particular the impact of waves, and more specifically of wave period. We studied the impact of different wave regimes on plants in a full scale flume experiment. Seedlings and adult shoots of the pioneer Scirpus maritimus were subjected to two wave periods at two water levels. Drag forces acting on, and sediment scouring occurring around the plants were quantified, as these are the two main mechanisms determining plant establishment and survival. Depending on life stage, two distinct survival strategies emerge: seedlings present a stress avoidance strategy by being extremely flexible, thus limiting the drag forces and thereby the risk of breaking. Adult shoots present a stress tolerance strategy by having stiffer stems, which gives them a higher resistance to breaking. These strategies work well under natural, short period wind wave conditions. For long period waves, however, caused e.g. by ships, these survival strategies have a high chance to fail as the flexibility of seedlings and stiffness of adults lead to plant tissue failure and extreme drag forces respectively. This results in both cases in strongly bent plant stems, potentially limiting their survival.

In hydrodynamically stressful environments, some species—known as ecosystem engineers—are able to modify the environment for their own benefit. Little is known however, about the interaction between functional plant traits and ecosystem engineering. We studied the responses of Scirpus tabernaemontani and Scirpus maritimus to wave impact in full-scale flume experiments. Stem density and biomass were used to predict the ecosystem engineering effect of wave attenuation. Also the drag force on plants, their bending angle after wave impact and the stem biomechanical properties were quantified as both responses of stress experienced and effects on ecosystem engineering. We analyzed lignin, cellulose, and silica contents as traits likely effecting stress resistance (avoidance, tolerance). Stem density and biomass were strong predictors for wave attenuation, S. maritimus showing a higher effect than S. tabernaemontani. The drag force and drag force per wet frontal area both differed significantly between the species at shallow water depths (20 cm). At greater depths (35 cm), drag forces and bending angles were significantly higher for S. maritimus than for S. tabernaemontani. However, they do not differ in drag force per wet frontal area due to the larger plant surface of S. maritimus. Stem resistance to breaking and stem flexibility were significantly higher in S. tabernaemontani, having a higher cellulose concentration and a larger cross-section in its basal stem parts. S. maritimus had clearly more lignin and silica contents in the basal stem parts than S. tabernaemontani. We concluded that the effect of biomass seems more relevant for the engineering effect of emergent macrophytes with leaves than species morphology: S. tabernaemontani has avoiding traits with minor effects on wave attenuation; S. maritimus has tolerating traits with larger effects. This implies that ecosystem engineering effects are directly linked with traits affecting species stress resistance and responding to stress experienced.

Tidal marshes are increasingly valued for protecting shorelines against wave impact, but waves in turn may limit the initial establishment of tidal marsh pioneer plants. In estuaries, the shorelines typically experience a wide range of wave periods, varying from short period wind waves (usually of around 1–2s in fair weather conditions) to long ship-generated waves, with secondary waves in the order of 2–7s and primary waves with periods that can exceed 1min. Waves are known to create sediment scour around, as well as to exert drag forces on obstacles such as seedlings and adults of establishing pioneer plant species. In intertidal systems, these two mechanisms have been identified as main causes for limiting potential colonization of bare tidal flats. In this paper, we want to assess to which extent common quantitative formulae for predicting local scour and drag forces on rigid cylindrical obstacles are valid for the estimation of scour and drag on slightly flexible plants with contrasting morphology, and hence applicable to predict plant establishment and survival under contrasting wave conditions. This has been tested in a full-scale wave flume experiment on two pioneer species (Scirpus maritimus and Scirpus tabernaemontani) and two life stages (seedlings and adults of S. maritimus) as well as on cylindrical reference sticks, which we have put under a range of wave periods (2–10s), intended to mimic natural wind waves (short period waves) and ship-induced waves (artificial long period waves), at three water levels (5, 20, 35cm). Our findings suggest that at very shallow water depths (5cm) particular hydrodynamic conditions are created that lead to drag and scour that deviate from predictions. For higher water levels (20, 35cm) scour can be well predicted for all wave conditions by an established formula for wave-induced scour around rigid cylinders. Drag forces can be relatively well predicted after introducing experimentally derived drag coefficients that are specific for the different plant morphologies. Best predictions were found for plants with a simple near-cylindrical morphology such as S. tabernaemontani, but are less accurate for plants of more complex structure such as S. maritimus, particularly for long period waves. In conclusion, our study offers valuable insights towards predicting/modelling the conditions under which seedlings and shoots of pioneer species can establish, and elucidates that long waves are more likely to counteract successful plant establishment than natural short waves.

The tidal River Elbe, northern Germany, serves as an important international waterway feeding the port of Hamburg and thus indicates its function as a significant economic lifeline for that region. At the same time the estuarine riverscape is a valuable natural habitat, protected by national and European legislation. In recent centuries the estuary has been impacted by manmade changes, e.g. by ongoing river training. There are very limited data about the dynamic interaction between soils, sediment transport and budgets as well as vegetation development. This article compiles ongoing Elbe research that includes: (i) the response of tidal reeds and invasive plants to shifts in hydrodynamics and land use; (ii) the resilience of bank sediments against hydro-mechanical stress; and moreover (iii) the analyses of socio-economic aspects in the context of bank restoration by applying the approach of ecosystems services.