Monitoring river island dynamics using aerial photographs and lidar data: the tagliamento river study case L. Picco, A. Tonon, D. Ravazzolo, R. Rainato & M. A. Lenzi
Applied Geomatics ISSN 1866-9298 Appl Geomat DOI 10.1007/s12518-014-0139-7
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Author's personal copy Appl Geomat DOI 10.1007/s12518-014-0139-7
ORIGINAL PAPER
Monitoring river island dynamics using aerial photographs and lidar data: the tagliamento river study case L. Picco & A. Tonon & D. Ravazzolo & R. Rainato & M. A. Lenzi
Received: 3 June 2013 / Accepted: 25 August 2014 # Società Italiana di Fotogrammetria e Topografia (SIFET) 2014
Abstract River islands are defined as discrete areas of woodland vegetation located in the riverbed and surrounded by either water-filled channels or exposed gravel. To be defined as such, they also have to exhibit stability over time and remain exposed during bank-full flows. River islands are very important from both a morphological and an ecological point of view, as they represent the most natural state of a fluvial system and are to a large degree influenced by human pressure. This study aims to analyze morphological and vegetation characteristics of three types of river islands, pioneer, young and stable, and their dynamics after different flood events in the Tagliamento River, a natural gravel bed river with highly natural conditions. The Tagliamento River, with a length of 178 km, is the main river of the Friuli-Venezia Giulia region, and it represents a link between Alpine and Adriatic zones. Because of the limited human pressure, the main course of Tagliamento is characterized by highly complex fluvial dynamics. The analyses were carried out in two sub-reaches characterized by different dominant morphology but equally low human pressure. Island dynamics, in relation with flood events, have been analyzed using aerial photos taken over two consecutive years, 2010 and 2011. Mean elevation of surface and maximum height of vegetation were estimated for each river island using digital models obtained from two LiDAR datasets. The results suggest that river islands lie at different elevations, and this implies a different resistance capacity during flood events. Pioneer river islands and young river islands lie at lower elevations than stable islands causing a total or partial incapacity to survive during considerable flood events. In most cases, the islands’ typology (pioneer, young, or stable) is cancelled by ordinary floods. Stable islands lie at L. Picco (*) : A. Tonon : D. Ravazzolo : R. Rainato : M. A. Lenzi Department of Land, Environment, Agriculture and Forestry, University of Padova, Viale dell’Universita 16, Legnaro 35020, Italy e-mail:
[email protected]
higher elevations, and only intense and infrequent flood events, i.e. recurrence interval (RI) >10–15 years, are able to determine substantial erosions. The presence of high vegetation, in some of our study cases higher than 30 m, contributes to increase the resistance of stable islands. River islands are very dynamic elements, strictly associated with the occurrence of major floods and to the morphological characteristics of the fluvial system. The main morphological changes have been documented in the braided sub-reach; the highly dynamic response to changes of the fluvial reach also affects the islands stability. However, the different resistance capacity during flood events of pioneer, young, and stable islands as well as the different bank erosion capacity are important aspects to better understand dynamics of fluvial islands. In this sense, the study could be helpful to predict the hydraulic hazard linked to the morphological changes and to formulate more effective flood management programs. Keywords Fluvial islands . Tagliamento river . Morphological changes . Photo interpretation . LiDAR applications
Introduction Braided rivers are characterized by flowing at least in two alluvial channels, separated by bars and islands as to produce a multi-threaded planform (Leopold and Wolman 1957). These fluvial systems are the most dynamics between alluvial rivers (Brierley and Fryirs 2005). Gravel-bed braided rivers are very dynamic systems, and in this context, even ordinary flood events can trigger morphologically active processes. Channel evolution is a response to fluctuations and changes of runoff and sediment supply involving mutual interactions among channel form, bed material size, hydraulic forces (Lisle
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et al. 2000), riparian vegetation (Picco et al. 2012b), and islands (Picco et al. 2012a; Picco et al. 2014). River islands are defined as discrete areas of woodland vegetation either surrounded by water-filled channels or exposed gravel (Ward et al. 1999) which exhibit some stability (Osterkamp 1998) and are not submerged during bank-full flows. An early classification of straight, braided, and meandering channel patterns (Leopold et al. 1964) implicitly incorporates island development through two different processes: (a) the evolution of relatively stable medial bars on which vegetation can establish itself within braided channels; (b) the isolation of a section of vegetated floodplain through avulsion and cutoff along meandering channels. Kellershals et al. (1976) further discriminated between occasional, frequent, split, and braided island patterns. The downstream changes in tree species, sediment, climate, and subsurface hydrology dictate the strategies available for vegetation establishment and the rate at which the vegetation can develop. Islands can remain in place over a decadal time scale, being defined as stable if they persist after high flows. The type of islands that characterize a riverine system can also help to describe the river processes present. Gurnell and Petts (2002) determined that most European rivers were once island-dominated (pre-1900), but have become devoid of islands due to human interference. Away from areas of agricultural or urban development in Europe, islands remain a common feature of riverine landscapes, such as the Fiume Tagliamento in northeast Italy (Ward et al. 1999). The presence of a certain species of plant on an island can help determine the flow conditions in the area. Some plant species require specific growth conditions, such as inundation duration, gradient, and particle size (Picco et al. 2012b). Nearly all large European rivers are flow-regulated to some degree. This can have implications for fluvial island development and stability. Dams reduce flood peaks, increase base flow, and store sediments (Kondolf 1997; Braatne et al. 2003). The sediment transported downstream from a dam can be only a fraction of the sediment transported upstream. The reduced flow-peaks downstream in a dam eliminate most processes of channel erosion, overbank deposition, and sediment replenishment. This also generally reduces the habitat, biodiversity, and interactions between biotic and hydrologic processes (Poff et al. 2007). While dams can reduce the erosion and destruction of fluvial islands, they also promote bank attachment by decreasing the sediment supply and reducing the downstream transport capacity which leads to deposition of tributary input sediment. Osterkamp (1998) and Wyrick (2005) described several scenarios in which islands could disappear or can be eliminated. This fact can be connected to the erosion of the vegetated area, during consistent floods, or by the connection between this area and the surrounding floodplain, typically during low flow periods.
The aim of the present study is to analyze the shortterm dynamics of stable, young, and pioneer islands along two sub-reaches of the Tagliamento River, as to be able to better understand the fluvial island dynamics. The shortterm analysis (2010–2011), takes advantage of two aerial photographs sets, and of two subsequent LiDAR datasets that permit to recognize the differences between the development stages and the surface characteristics of the proposed island types.
Study area The Tagliamento River is located in the southern Alps of northeast Italy. It originates at 1,195 m a.s.l. and flows for 178 km to the northern Adriatic Sea, thereby forming a corridor linking the Alpine and the Mediterranean zones. Its drainage basin covers 2,871 km2 (Fig. 1). The river has a straight course in the upper part, while most of its course is braided shifting to meandering in the lower part where dykes have constrained the lower 30 km; thus, it is now little more than an artificial channel about 175-m wide. However, the upper reaches are more or less intact; thus, the basic river processes, such as flooding and the erosion and accumulation of sediment, take place under near natural conditions. A strong gradient exists along the length of the river which has a big influence on precipitation, temperature, humidity, and consequently vegetation patterns (Fig. 2). Because of the climate gradient, the flood plain of the Tagliamento is an important geographical corridor with a strong longitudinal, lateral and vertical connectivity, high habitat heterogeneity, a characteristic sequence of geomorphic types, and very high biodiversity (Tockner et al. 2003). The hydraulic regime, due to the climatic and geologic conditions of the upper part, is characterized by flashy pluvio-nival flow conditions (Tockner et al. 2003). The Tagliamento River is considered by different authors as the most intact and natural river in the Alps (Müller 1995; Lippert et al. 1995; Ward et al. 1999), and it can be considered as “a reference ecosystem for the Alps but also as a model ecosystem for large temperate rivers” (Tockner et al. 2003). The extensive vegetated islands and gravel bars are key indicators of its natural conditions while engineering works for flood control or navigation have lead to their disappearance of these in most comparable European water courses. The Tagliamento River is clearly an ecosystem of European importance, because it constitutes a unique resource as a model reference catchment (Tockner et al. 2003). The basins of the main tributaries of the upper catchments lie in one of the wettest region of Europe where annual precipitation can reach 3,000 mm. The Tagliamento River is characterized by a bimodal flow regime with maximum discharges in spring and autumn due to snowmelt and intense
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reach the village of Latisana (on the regulated lower part) in just 12 h. Upstream, the height of the river rises by only 2 m while near the Latisana village, the river is forced into a narrow channel and its level can arrive at levels higher than 7 m. Flood events along the Tagliamento River were recorded during the study period. Between the two data acquisitions, there were four significant floods having a recurrence interval (RI) of 1 year, with two of them with water level higher than the bankfull level (Fig. 3) (Picco et al. 2013).
Materials and methods
Fig. 1 The Tagliamento River basin (top) and its localization in the eastern part of Italy (bottom)
rainfall in autumn, respectively. The catchment is mainly mountainous, the slopes are very steep, leading to high peak flows, and sediment loads in the central and lower part of the basins. The flood peak moves downstream very fast and can
The analysis was done on two subsequent datasets, 2010 and 2011, using aerial photographs and LiDAR data. The aerial photos range in scale from 1:8,000 to 1:33,000. Photos were scanned at a resolution of 600 dpi in order to obtain an average virtual resolution of 1 m or smaller. Aerial photographs were rectified and co-registered to a common mapping base at 1:5,000 by a GIS software (Esri ArcGIS 9.2). Approximately, 30 ground-control points were used to rectify each single frame, and second-order polynomial transformations were then applied, obtaining root mean square errors (RMSE) of about 1 m. The active channel area and width were calculated using the different aerial photographs and correspond to the area of water and un-vegetated sediment bars. Assuming the conceptual model of island dynamics proposed by Edwards et al. (1999), vegetated bars and pioneer islands were classified as distinct morphological units, the latter being areas with vegetation higher than 3 m. Pioneer, young, and stable islands were distinguished based on the maturity and size of the vegetation (Kollmann et al. 1999). In the aerial photos, distinction between arboreal and shrubby vegetation was made by estimating vegetation height based on canopy texture, shape, and shadows. Pioneer islands were defined as surfaces on bars with patchy vegetation covered with spots of vegetation 1–5 m high. Young islands, which are an intermediary step towards the consolidation as stable islands, were distinguished because of the higher extension and ground elevation, whereas stable islands were defined as older (15–20 years) areas with high and dense vegetation cover (Gurnell and Petts 2002). Previous analysis on the Tagliamento River had shown that in a natural braided gravel bed river, these landforms rarely survive more than 20 or 25 years (Gurnell et al. 2001). Canopy height derived from the LiDAR was used to complement the aerial photographs (Zanoni et al. 2008). Two airborne LiDAR flight, carried out in August 2010 and April 2011 allow to analyze vertical characteristics of ground and
Author's personal copy Appl Geomat Fig. 2 The study area localization (top) of the two subreaches, the upper sub-reach called Cornino (on the left) and the lower sub-reach called Flagogna (on the right)
Cornino Flagogna
vegetation of the islands. Orthometric elevations were used with estimated vertical error ±20 cm. After filtering a point density of 2–3 points/m2 was obtained. In order to create an accurate DTM, an analysis of the effective density of the dataset’s points was done using of “Spatial Analyst Density” module of ArcMap 9.2 software. The digital terrain model (DTM) and the digital surface model (DSM) were created at 1m resolution using the tool “3D-analyst” of ArcGIS 9.2. Based on the 0.5-m LiDAR resolution, the raster subtraction of the original DTM layer and the DSM layer generated the canopy height model (CHM). LiDAR technology is particularly appropriate for this type of process because it penetrates canopy gaps (Alberti et al. 2013) and samples the bare ground plane below with enough energy to return information allowing estimation of DTM with high accuracy also under relatively dense vegetation using airborne (Pirotti 2010) or terrestrial (Pirotti et al. 2013) laser scanners. This information
was used to obtain the maximum, minimum, and mean elevation of island surfaces and the maximum and mean height of the island vegetation (Picco 2010).
Fig. 3 Flood events occurred during the study period along the two studied sub-reaches on the Tagliamento river
Author's personal copy Appl Geomat Table 2 Variation in extension, relating to area unit, for every different Island typology along the Cornino and Flagogna sub-reaches
Results Islands dynamic
Cornino (m2/ha)
As shown in Table 1, from 2010 to 2011, there is a significant reduction in the number of islands along the sub-reaches under consideration. In both sub-reaches, the higher levels of reduction from 2010 to 2011 were related to the pioneer islands (41 % in Cornino sub-reach and 33 % in Flagogna sub-reach), but also the young islands were affected by strong decrease (31 and 23 %, respectively). Only the number of stable islands did not change significantly during the study interval, with a reduction of about 1 and 4 % for the Cornino sub-reach and the Flagogna sub-reach, respectively. As it is possible to see in Table 2, during the study period, there were reductions in the extension (related to area unit) of all the three different typologies of islands. Maximum values of reduction were observed for the pioneer islands with values of 38.9 and 26.7 % into the Cornino sub-reach and the Flagogna sub-reach, respectively.
2010
Ground characteristics Looking at Fig. 4, it is possible to appreciate the distinction of the ground elevation for the three different typologies of islands along the Cornino sub-reach for 2010 and 2011. It is possible to see that, in both years, there is a great distinction between the ground elevation of the stable islands and the other two typologies of islands. It is also interesting to see the distinction between the stable island ground elevation in 2010 and in 2011; in fact, it is possible to observe a decrease of the median value of about 1 m. Finally, it is possible to see that there is a constant difference between the median ground elevation value of about 0.20 m between the pioneer typology and the young one. Looking at the Flagogna sub-reach (Fig. 5), it is possible to see that there is a lower distinction between the ground
Table 1 Variation in the numerousness, relating to area unit, for every different typologies of islands along the Cornino and Flagogna subreaches Cornino (N/km2)
Pioneer Young Stable Totale
Flagogna (N/km2)
2010
2011
%Δ
2010
2011
%Δ
667.8 109.4 1.96 779.2
394.6 75.3 1.93 471.8
−41 −31. −1. −39.
738.9 375.8 15.3 1,130.0
497.4 287.4 14.7 799.5
−33. −23. −4. −29.
Flagogna (m2/ha)
2011
%Δ
2010
2011
%Δ
Pioneer
144.7
88.3
−38.9
258.3
189.4
−26.7
Young Stable
81.0 214.6
61.0 186.9
−24.6 −12.9
284.9 381.9
251.9 341.5
−11.6 −10.6
elevation values of the three different typology of islands. In fact, the stable islands present lower median values of about 1.4 and 1.2 m for 2010 and 2011, respectively. The ground elevation median values of pioneer and young islands are, instead, comparable with the values that were observed along the Cornino sub-reach.
Vegetation characteristics Analyzing the height of the vegetation (Fig. 6), it is possible to appreciate the distinction between the three different typologies of islands along the Cornino subreach. It appears clearly that there is a strong distinction between the three classes. In fact, it is possible to see that the stable islands present a median value of the height of the vegetation of around 19 m in 2010 and around 13 m in 2011. The young islands have lower values of around 6 m in 2010 and 2.5 m in 2011. Finally, it is possible to observe that the pioneer islands present a median value of the height of the vegetation around 2 m in 2010 and 1 m in 2011. Looking at the maximum values (not considering the outliers and extremes when presents) it is possible to see, again, a clear distinction between the typologies and, again, the higher values are in 2010 than in 2011. The vegetation data related to the Flagogna sub-reach are shown in Fig. 7. Also in this sub-reach, it is possible to observe a clear distinction between the three different typologies of islands. In 2010, the stable islands present maximum values (not considering the outliers and extremes when presents) of the vegetation around 32 m and median values of about 16 m. The young islands are characterized by maximum values of about 16 m and median of about 6 m and, finally, the pioneer islands are characterized by values of about 8 m and 4 m, of maximum and median height, respectively. Looking at the 2011 there were, again, general reductions in the values. The stable island present maximum values of about 28 m and median of about 9 m, the young island are characterized by maximum height of 9 m and median of 3 m and, finally, the pioneer present maximum height of about 5 m and median of 1.5 m.
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Fig. 4 Ground elevation of the three typologies of island along the Cornino sub-reach in 2010 (top) and in 2011 (bottom)
Fig. 5 Ground elevation of the three typologies of island along the Flagogna sub-reach in 2010 (top) and in 2011 (bottom)
Discussions
et al. 2012b). This is also in accord with Bertoldi et al. (2011), they have demonstrated that, along the Tagliamento river, there is a strong distinction in morphology between the reaches that are characterized by the presence of big islands and the ones with limited vegetation cover. Thanks to the aerial photo interpretation that was possible to define the short-term dynamics for three different typologies of fluvial islands. There is a great distinction in the number and dimension of the three different kinds of islands, with maximum values of numerousity registered, as expected, for the pioneer islands and maximum values in dimension for the stable ones, as expected again. Results have shown a higher presence of stable islands along the Flagogna sub-reach revealing, once again, that the wandering morphology is less dynamic. Looking at the different results obtained from the analysis on 2010 and 2011, it is possible to observe that, during the flood events, there was great decrease in fluvial island extension along the Cornino sub-reach than the Flagogna one. It is possible to observe, also, that the much affected typology of island was, as expected, the pioneer one. It is
The flood events occurred during the study period have generated some important geomorphic variations along the two considered sub-reaches, where it was possible to observe an increase of the area with a maximum increase of about 0.04 km2 in Cornino. This sub-reach is characterized by a predominant braiding morphology therefore, naturally more active in respect to the wandering Flagogna sub-reach. Along the braided stretch, we registered some consistent bank erosion phenomena; on the other hand, along the Flagogna subreach, there was just a little migration of the active channel on the right bank. It is important to underline that, along the Flagogna sub-reach, there are many big stable islands that do not permit to the flow to move on this side, whereas the left bank is constituted by the Monte di Ragogna that constrains the Tagliamento River. This difference confirms, one more time, the higher instability of braided morphology in respect to the wandering one, revealing that along the braided river, it is possible to observe significant morphological changes also in occasion of not extraordinary floods (Picco
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Fig. 6 Height of the vegetation present on the three typologies of island along the Cornino sub-reach in 2010 (left) and in 2011 (bottom)
Fig. 7 Height of the vegetation present on the three typologies of island along the Flagogna sub-reach in 2010 (top) and in 2011 (bottom)
dynamism in a wandering environment that permits a more intense stabilization of in-channel vegetation. interesting to observe how the young islands were significantly affected by the flood events demonstrating that also this intermediate class of islands could be removed or remodeled by not extraordinary floods. These observations matched with those made by different authors (Bertoldi et al. 2009; Comiti et al. 2011; Picco et al. 2012b) wherein the island dynamics were found to be strictly associated to the occurrence of major floods (recurrence interval, RI>10–15 years), which are the only ones able to generate substantial island erosion. Thanks to the LiDAR data that was possible to analyze important morphometric characteristics of the different typologies of islands. As expected, stable islands lie at higher elevation. This is due to the fact that older vegetation growing on stable islands allows the deposition of fine sediments which, in turns, enhances the conditions for vegetation growth, providing even better circumstances for the vegetation to establish on stable islands. The results related to the vegetation demonstrated that, along the wandering sub-reach, trees reach higher height than along the braided sub-reach. Those results demonstrated once again that there is less
Conclusions The results here presented demonstrate that fluvial islands highly dynamic characteristics also in occasion of notextraordinary flood events. The process consists of a simple detection using aerial photographs. In the future new technologies such as unmanned aerial vehicles equipped with cameras will also allow to survey with aerial photographs with increased frequency (Coppa et al. 2009). This analysis can be helpful also to predict the hydraulic hazard linked to the morphological changes and to formulate more effective flood management programs. Analyzing the island dynamics is possible to observe the changes in the active channel area during different periods; this kind of analysis permit to define the potential increase of vegetation during no-flood periods and the concurrent decrease in the amount of exposed area where the flood can flow without particular obstructions. This
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observations permit to predict the increase in hydraulic hazards depending on the conditions and expansion of perifluvial vegetation and fluvial islands. Using LiDAR data, it is possible to predict the large wood (LW) characteristics, dimensions, and volumes that could be moved downstream during floods in case of island erosion processes. This capability allows localizing areas of storage that can be affected by the direct impact of LW (e.g., bridges, check dams, bank protections); on the other hand, it is possible to define the potential volume of sediment that can be released during island erosions. All this important information can be very useful to better define flood-management programs and precautionary actions such as the removal of the bigger trees from the islands located close to critical sections. Acknowledgments This research was founded by both the CARIPARO “Linking geomorphological processes and vegetation dynamics in gravel-bed rivers” Research Project, and the University of Padua Strategic Research Project PRST08001, “GEORISKS, Geological, morphological and hydrological processes: monitoring, modelling and impact in the North-Eastern Italy”, Research Unit STPD08RWBY-004.
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