Quantification of the temporal storage at characteristic cross-sections at the Isabena river in the Pre-Pyrenees of Spain Anita Becker

Transcription

1 Quantification of the temporal storage at characteristic cross-sections at the Isabena river in the Pre-Pyrenees of Spain Anita Becker Tutor: Dr. Eva-Nora Müller Co-tutor: Dr. Ramon J. Batalla Villanueva University of Potsdam/ Germany; Institute of Geoecology Potsdam,

2 Index 1. Introduction Study area The Isabena Catchment Transects: Methods Field work Correction of field measurement Explorative statistics Calculation of the sediment storage Results Statistical evaluation Analysis of the sediment concentration Volume calculation Discussion and conclusion References Appendix Data of the cross-sections Aerial views

3 Quantification of the temporal storage at characteristic cross-sections at the Isabena river in the Pre-Pyrenees of Spain Abstract: The objective of the project is the estimation of the temporal variations of fine sediments stored within characteristic river reaches of the Isabena River. For this purpose a new method has been evaluated to quantify the temporal storage alternatively the degradation. Steel bars were installed within the riverbed to mark the current height of its temporary sedimentation. The height of riverbed sedimentation was measured from the beginning of September to the end of December before and after flood events in order to understand the dynamic of the sediment fluxes. This data has been related to the measurements of discharge and suspended sediment at the Capella gauging station. 1. Introduction The study investigates the sediment transport of a Mediterranean mountainous dryland river within the meso-scale Isabena catchment of the Pre-Pyrenean region in Aragon, NE Spain. The study area is affected by intense soil erosion from so called badland areas that leads to severe sedimentation of a downstream reservoir, the Barasona Reservoir, thus threatening future water supply for the region. According to Sanz Montero et al. (1996), the reservoir had lost about one third of its initial water storage capacity over a time period of ca. 20 years due to sediments from the badland areas. The badlands are thought to be the major cause for the sedimentation of the Barasona Reservoir (Fargas et al. 1996). Badlands are a typical landform of this region within the upper middle part of the Isabena catchment, dominated by Mesozoic carbonate rocks and marls and lacking any vegetation cover. Badlands are vulnerable to flash flood erosion and produce large amounts of eroded material, that are then transported downstream as suspended sediments in the Isabena Rivers. Preliminary field investigations (Batalla 2005, personal communication) showed that during these flood events the Isabena riverbed plays a significant role as a temporary storage of fine sediments in between consecutive flood events before the sediments are transferred further downstream to the Barasona Reservoir. However, not much is known on the magnitude and temporal regime of the temporary sediment storage along the riverbed (E.N. Müller et al. 2006). Therefore, between the and the field work was carried out. It was focused on a quantification of the fine sediment which is stored alternatively transported due to flood events in the riverbed. Respecting the variation of the Isabena five characteristic cross-sections were selected. Four profiles were already analysed in previous researches 1. One cross-section was chosen for the first time. At this cross-sections a total of 36 metal bars were installed with a hammer. The metal bars have a length of 1 m and a diameter of less than 1 cm. For the four flood events which occurred in the test period readings of the sediment heights had been carried out before and after each event. 2. Study area 2.1 The Isabena Catchment The Isabena Catchment is located at N and 0 20 E and covers an area of ca. 435 km 2. The climate is a typical Mediterranean mountainous type with a mean annual precipitation rate of 600 to mm and an average potential evaporation rate of 550 to 750 mm. Both rates show a strong south-north gradient, which can be related to the strong topographical difference of altitude within the watersheds ranging from 430 m to m over MSL. The major rivers in the catchment never dry up, although flows are low during the 1 Juan Verdu s river survey (2001); SESAM river survey (2005) 3

4 summer and some of the tributaries of the Isabena Rivers exhibit ephemeral behaviour with no flow at the end of the dry season. Most floods occur due to the passing of cold fronts in spring and to local thunderstorms in autumn and early winter. The vegetation includes evergreen oaks and pines in the valley bottoms and deciduous oaks in the upper areas. The lower parts of the Isabena catchment are mainly dominated by Miocene continental sediments, the middle part by carbonate rocks and marls forming the characteristic badland areas, and the upper part by Paleozoic rocks. The Isabena River is characterised by a very heterogeneous spatial distribution of river forms and properties. Steep, narrow, deep incised mountain torrents with rocky, gravely riverbeds in the upper parts of the catchment alternate with shallow, plain and very wide riverbeds and large floodplains with silty riverbed materials in the lower catchment area, with parts of the river system having an ephemeral flow regime. (E.N. Müller et al. 2006). The fieldwork was carried out at five characteristic river-bed stretches along the lower part of the Isabena River (three upstream and two downstream the measuring station of Capella, see figure 1). The river reach which is under examination is about 5 km long. 2.2 Transects: Figure 1: On a scale of 1:58.000; Data origin: 4

5 The five transects shown in the above illustration were chosen because of their different characteristics ranging from very narrow to very wide, shallow stretches. Moreover previous examinations of four of these selected cross-sections enable comparisons. The location is also advantageous because of the adjacency to the Capella gauging station whereby data of the discharge and sediment concentrations are available. Prior to the field survey the Sesam profiles were described by the SESAM-Project 2005 and Capella 1 and 2a were selected by Juan Verdu s river survey in Capella 2b was studied for the first time. To enable the special characterisation of river reaches, one state-of-the-art river classification system is employed: the descriptive classification after Rosgen (1994). Overview Capella 1 Capella 2a Capella 2b Figure 2 On the scale of 1: ; Data origin: Description of the location Very wide vegetated floodplain with one narrow major river reach and a second dominant one, although currently without water. In main reach: with rather high flow covering only a small fraction of the apparently frequently flooded floodplain. With stable stream banks. Very wide vegetated floodplain with one major river reach and a second dominant one, although currently without water. In main reach: covering only a small fraction of the apparently frequently flooded floodplain. With stable stream banks within the floodplain. Rather narrow and deep part of the Isabena, with high accumulation rates of fine sediment. Trees and shrubs on both sides. Classification after Rosgen Entrenchment factor: D, possibly C Longitudinal profile D Bed material Boulder, cobble, silt/clay Valley form 5 Entrenchment factor D, possibly C Longitudinal profile D Bed material Boulder, cobble, silt/clay Valley form 5 Entrenchment factor - BC Longitudinal profile - BC Bed material gravel, silt/clay Valley form - 5 Vegetation type Above floodplains tall shrubes and trees, few vegetation in main river stretches. Above floodplains tall shrubes and trees, no vegetation in main river stretches and few within the floodplain. On the hillslopes trees, on the left side shrubs and on the right side next to the shore reeds. 5

6 Capella 1, Capella 2a, Capella 2b, Figure 3-5 6

7 Overview Sesam 2 Sesam 3 Figure 6 On the scale of 1: Data origin: Description of the location Classification after Rosgen Vegetation type Sesam 2, Lower part of the Isabena River. Very narrow, and rather deep, with trees at both sides and steep banks both sides. Entrenchment factor - BC Longitudinal profile - BC Bed material Cobble, gravel, silt/clay Valley form 5 On the hillslopes trees, on the left side next to the shore reeds. Very wide floodplain with one major river reach and a second dominant one, although currently without water; in main reach: with low flow covering only a small fraction of the apparently frequently flooded floodplain. Entrenchment factor- C, possibly D Longitudinal profile F, possibly DA Bed material Boulder, cobble, silt/clay Valley form - 5 Above floodplains tall shrubs and trees, no vegetation in the two main river stretches, but extensive vegetation of floodplain,including small trees, reed, grasses, herbs, in patches variable, ca. 60 % cover. 7

8 Sesam 3, Figure Methods 3.1 Field work The fieldwork lasted over three months from the to the Initially five characteristic cross-sections were selected. Above mentioned previous surveys facilitated the finding. A total of 36 metal bars (1m length) were located at significant points such as the bankfull stage. Using a hammer, the steel bars were fixed more or less rectangular in the riverbed or bank as deep as possible. Limitative was the pebbly substrate that precludes the installation of the metal bars on a couple of sites. After the first installation of the metal bars the height of the bars (minus the part covered by sediment) were scaled with a measuring tape and numbered as stated below. After installing the metal bars each cross-section was mapped by means of GPS. At the second reading half of the metal bars were lost after a period of torrential rain. Presumably many bars were pulled out by flotsam which winded around the bars2. Subsequently installed metal bars were fit inclined by reason that flotsam can not wind around and pull them out that easy. On the seven and on the twelve steel bars were reinstalled. For the inclined metal bars in addition the angle was measured with a level and a measuring tape. Considering the metal bar as the hypotenuse the length of the side adjacent to the hypotenuse and of the opposite leg were measured after adjusting the right angle (via level) in this triangle. The angle of the inclination is important to adapt the sediment heights before and after the inclination of the steel bars. (See the calculation in chapter 3.2) At the end of the test period on the the cross-sections and the metal bars were mapped with a total station. Figure 9: Nomination of the metal bars 2 Figure 18, page 17 8

9 The denomination of the metal bars result from the illustration above. Looking downstream the metal bars on the left side are shortened with L and those on the right side with R. The numbering begins at the riverbank counting inward. In case of loss of metal bars and subsequent reinstallation the abbreviations are additionally brand with.2 (For example L1.2). In some cases metal bars were installed during subsequent installations on significant points for the first time. Those metal bars are abbreviated with.1 (For example L1.1). At the first metering after the installation, the bars were subsequently provided with rings in order to get more detailed information about erosion and deposition processes during flood events. The assumption was that during one flood event degradation and deposition can occur consecutive. At the rising limb of the flood erosion processes due to the increasing discharge are followed by deposition processes at the decreasing limb of the flood. In this case the ring would cave in due to erosion processes. Proximate with decreasing discharge sediment could deposit on the ring. Therefore the ring is a tool in order to provide an idea about the erosion rate during the flood. Otherwise, because of the deposited sediment the true erosion rates are covert. During the whole period of examination five flood events occurred. Consecutively the first two flood events between the and the are considered as one event. The reason is that between both consecutive events no metering was possible. Considering those flood events as one, measuring data before and after each event is available (Figure 10). Besides two exceptions continuously data of the discharge and the sediment concentration is available. Between the up to the the discharge gauge at the Capella station malfunction. From the to the another error happened in measurement due to the discharge gauge at the Capella station. 3.2 Correction of field measurement After installing the bars the most intense flood event during the investigation period occurred. Thereby half of the metal bars were lost (16 of 32). Many of the remaining posed inclined. The calculation to deal with the inclination of the metal is as follows: S = Cosinus (alpha) * (L1-L2) S: vertical change of sediment in cm. L2: length of bars looking out of the ground at current date. L1: length at previous date S > 0: sediment deposition S < 0: sediment erosion The formula is valid assuming that the change of angle occurs instantaneously at the beginning of the flood due to the high water flow pressure at the rising limb of the flood. 3.3 Explorative statistics Consecutively statistical evaluation is summarized for the flood events as well as for the observed sediment processes. Each was analysed using the mean, maximum, minimum and the standard derivation. At first the sum of deposition and degradation at each cross-section per measuring date was listed (table 2). Thereafter the mentioned statistical parameters were applied as per particulars given below (table 3). Moreover the processes at the discrete metal bars were interesting. Therefore in table 4 the above parameters for the erosion and deposition rates at the discrete steel bars per measuring date were computed in order to get detailed information about the magnitude of those processes. Finally in table 5 those values are calculated for each cross-section to point their variability. 9

10 Hydrograph discharge (m3/s) no data time (days) Figure 10: The measuring dates (10.09, 01.10, 12.10, 07.11, 23.11, ) are red marked. Table 1: Analysis of the hydrograph Flood events Period Duration (in days) Readings before Readings after Max (in m 3 /s) Min (in m 3 /s) Mean (in m 3 /s) Sd (in m 3 /s) As shown in the above table the first intensive rainfall created the highest discharge in the whole test period with a maximum of 64.3 m 3 /s. The second and the third rain event constituted similar maximum discharge rates about 30 m 3 /s while the third flood event occurred in only one third of the time. The last rainfall caused the lowest discharge of 16.3 m 3 /s. 3.4 Calculation of the sediment storage Furthermore the volume of the eroded or deposited sediment has been calculated. Because the cross-section and nearly all metal bars were surveyed via total station on the (last measurement) total values of the sediment heights are existent. Besides the metal bars other characteristic points of the profile were mapped during the surveying of the crosssections. The cross-section was divided into several trapezoids for the further computation. A trapezoid results in each case from the distance of the neighbouring metal bars and the surveyed data of the cross-section. Problematical here is that the distances between the metal bars vary strongly. Thus distortions develop. If the distances between two metal bars for example amount to some meters, fluctuations in the sediment height generate a more 10

11 substantial volume than distances within the centimetre range. Comparatively therefore an amended volume computation was accomplished. During the allocation of trapezoid also those points in the profile were considered which are none metal bar but surveyed by means of a total station. Thus the mentioned distortion should be avoided. All the contemplated calculations have been carried out using Excel. After the computation of the volume, the mass of the transported fine sediment was calculated. The density value out of the SESAM-Report 2006 was applied. Data thus obtained was applied for the length of the river reach between the cross-sections which is about 5 km. Finally the results of the changes of the stored fine sediment and its volume have been compared with the hydrograph and the sediment concentration measured at the Capella gauging station. 4. Results 4.1 Statistical evaluation Only during the first flood event in most of the stretches erosion occurred. After this most intense flood event in the sum sediment was accumulated even after subsequently flood events. After the first and most intensive rainfall the erosion rate of all cross-sections amount to cm (Table 2). On the other hand at the downward limb of the flood (between the and the ) occurred the highest amount of sediment deposition in the whole period of investigation. Total erosion and sedimentation of all cross-sections per measuring date cm measuring Sesam 3 Sesam 2 Capella 1 Capella 2a Capella 2b Figure 11: 1: / 2: / 3: / 4: / 5: / 6:

12 Table 2 Each cross-section with the total of all measured heights per measuring date net erosion/acc Sesam Sesam Capella Capella 2a Capella 2b sum With the exception of the trend concerning the discussed flood event, the different behaviour considering the erosion and sedimentation processes at the different transects is obvious (Figure 11). Similar rates show only Capella 1 and Capella 2a. For example the rates of erosion range between -1.9 to -5.2 cm (Table 3) deposition range between 3.9 and 4.4 cm respectively. Both cross-sections have a similar morphology of the riverbed: very wide floodplain. In spite of the adjacency between Capella 2a and Capella 2b the rates of accumulation and erosion are different. While the similarity between Sesam 2 and Capella 2b is much higher considering the amplitude of erosion or deposition processes, erosion rates show maximum data between and cm (Table 3) and maximum deposition rates between 9.9 and Here also the riverbed morphology is alike. Both cross-sections are narrow and rather deep. The mentioned similarities can be explained by the increasing velocity at smaller profiles which provokes more energy to erode. On the other hand in the very wide floodplains because of the lower velocity such high erosion rates can not occur. Furthermore within wide floodplains the dispersal of the sediment input is much higher. The outcome of this is a bias which must be taken into account and will be discussed in chapter 4.3 on the basis of the volume calculation. The main river reach of Sesam 3 contains riffle and pool processes which probably led to the total accumulation value of 39.5 cm measured at the A reason could be a sinking of the metal bar in the undercutting bank. Concluding those observations are a portent of the dependence between erosion and accumulation processes and the river morphology. The high variability of sediment fluxes is expressed considering the profiles Sesam 3 and Sesam 2 at intervals of 500 m. The erosion alternatively deposition value at Sesam 3 averages 8.7 cm whereas the mean at Sesam 2 amount to -4.3 cm. This shows the most opposed characteristic between the cross-sections concerning the erosion and deposition rates. In comparison the means of degradation and deposition at the Capella transects range between -0.5 and 4.5 cm. Table 3: Statistic of each cross-section of the totals of all measured heights at a measuring date max min mean sd Sesam Sesam Capella Capella 2a Capella 2b total

13 In the following figure all measured heights at all metal bars of the five cross-sections are diagrammed to get an idea about the variability within the metal bars of each profile. cm Capella 1 35,0 25,0 15,0 5,0-5,0-15,0-25,0-35, date cm Sesam 2 35,0 25,0 15,0 5,0-5,0-15,0-25,0-35, date cm Capella 2a date cm Sesam 3 35,0 25,0 15,0 5,0-5,0-15,0-25,0-35, date cm Capella 2b 35,0 25,0 15,0 5,0-5,0-15,0-25,0-35, date X-axis: Measuring dates 0: : : : : : Figure 12: Net erosion and accumulation of all metal bars and cross-sections In the following table the corresponding statistical values of erosion and accumulation processes which are diagrammed above are listed. The range between the maximum and minimum values of erosion alternatively deposition per date averages cm. Considered are the erosion or deposition rates of all metal bars per measuring. Table 4: Statistical values of erosion/ accumulation of all metal bars per date max min mean sd

14 To get a more detailed impression about the rates of erosion and deposition, regarding the discrete metal bars of every cross-section, in the following table statistical values during the test period are represented. Table 5: Statistical values of erosion/ sedimentation at the discrete metal bars max min mean sd Sesam Sesam Capella Capella 2a Capella 2b total Below exceptional deposition and degradation data are considered. Figure 13: Sesam 3 with metal bars L3 and L4 (left) Especially one cross-section (Sesam 3) features notably high accumulation data. Between the and the , 29.6 cm were deposited at one metal bar. This value brands the highest accumulation rate of all metal bars during the test time. The metal bar L4 is pictured adjoining. As you can see there this metal bar has been flashed after the installation. Acting on the assumption that with increasing undercutting this metal bar sank more and more in the sodden substrate this is a traceable value. For the other profiles the maximum deposition data range between 5.4 and 9.6 cm between two readings. The maximum erosion value is -18 cm. The concerned metal bar L4 at Sesam 2 is the most outward one (L1.1, L2.1, L3.1 were installed later on the ). This comparatively high erosion value has to be discussed. At the inwardly chained metal bar L5 in the same period no changes of the sediment height happened. In an understandable scenario erosion must have occurred at all metal bars inward L4 during the intense flood event between the and the On the decreasing limb of flood fine sediment must have been deposited. But probably because of the lower water level no sediment could have been deposited at L4. The rings which could have verified this scenario were installed after this flood event on the For the other cross-section the maximum erosion data range between -3.0 and -7.8 cm. The means range between -0.7 and 1 cm at each metal bar. 14

15 4.2 Analysis of the sediment concentration Concentration of suspended sediment at the Capella station g/l date concentration Figure 14 Comparing the sediment storage in the riverbed upstream and downstream the Capella station with the measured sediment concentration at the Capella station, an interdependence can be concluded (Figure 14 and 15-16). After the first flood event in the sum sediment was eroded. This is reflected in the highest sediment concentration measured at the Capella station during the period of exploration. After this flood event a phase of accumulation started considering the measured values at the Capella cross-sections. Therefore the sediment outlet at the gauging station is minimized. The same processes can be observed at the Sesam transects. At the same time when erosion occurs at the cross sections, the sediment concentration at the Capella gauging station is increasing, while low sediment concentrations involve accumulation phases. Erosion/ deposition at the Capella crosssections Erosion/ deposition at the Sesam crosssections 30,0 40,0 cm 20,0 10,0 0,0 1-10, ,0 Capella 1 Capella 2a Capella 2b cm 30,0 20,0 10,0 0,0-10,0-20,0-30, Sesam 3 Sesam 2-30,0-40,0 measuring measuring Figure 15-16, 1: / 2: / 3: / 4: / 5: / 6:

16 4.3 Volume calculation With the formula for a trapeze the calculation of the changes of the sediment volume within the riverbed was applied. The formula is: 0,5*(L2+L1)*y. For this volume calculation all surveyed points with a total station between the metal bars have been borne in mind, besides the metal bars also characteristic points like bankfull were taken into account. The appointed range of each metal bar is half the range to the left and half the range to the right metal bar characteristic points respectively. Therefore the specific formula of the volume is: V= (L1+L2/2)*s1 + (L2/2+L3/2)*s2 + (L3/2+L4/2)*s3 s>0 connoted deposition s<0 connoted erosion The diagrammed volume changes demonstrate the impact of the latitude of the crosssections on the volume dimension (Figure 17). For example Capella 2b shows the highest accumulation volume of all transects also the total accumulation considering the measured sediment height was higher at Sesam 3 (Figure 11). Volume changes to the previous measuring in m3 1,0 m3 0,5 0,0-0, Sesam 3 Sesam 2 Capella 1 Capella 2a Capella 2b -1,0 measuring Figure 17 After computing the sediment volume the mass was calculated for 1 m 3 assuming a density of 1.5t m -3 (Sesam annual report 2006). Subsequently the mass of the sediment fluxes were calculated for the river reach between Sesam 2 and Capella 1 which is about 5 km. For this calculation the means of the previous calculation for 1 m 3 were applied. As a result after the first flood event t of sediment had been eroded. This flood event occurred with a mean discharge of 9.6 m 3 /s (with a maximum of 64,3 m 3 /s) and lasted 36 days. The second flood event up to the lasted 21 days and had a mean discharge of 7.0 m 3 /s (with a maximum of 30.8 m 3 /s). But after the second mentioned flood event no erosion occurred but t of sediment were deposited. Concluding, there must be a threshold of the discharge from which erosion processes appear. 16

17 Table 6 Mass calculation (in t) for 1m3 with a density of 1.5t m total Sesam Sesam Capella Capella 2a Capella 2b mean Table 7 Mass changes (in t) for the investigated river reach (5km) using the means of all cross-sections total Discussion and conclusion The applied method of the quantification of the temporal storage of sediment by means of metal bars had enabled us to evaluate a magnitude of the processes. Nevertheless, this method proved to be challenging. Initially the installation of the steel bars via hammer emerged difficult because of the pebbly material of the riverbed. Therefore the metal bar could not always be fixed on characteristic but rather on accessibly sites. Another outcome of this method is that the bars easily become lost. Especially at intense flood events with high appearance of flotsam lots of steals bars can be pulled out (Figure 18). Metal bar winded with flotsam. Figure 18: Capella 2a/

18 On the following figures the cross-section Capella 2b is depicted (Figure 19-21). Here shall be demonstrated another problem due to the installation of the metal bars. The bars were fixed on subjective chosen sites. But to cite Capella 2b as an example here an enormous deposition of fine sediment occurred at a patch without any steel bar. Hence an objective and continuously installation plan is recommended. For example the fixation of metal bars with a distinct interval would be an advancement just as a better anchorage. Thus errors due to the uncontrolled movement of the bars or the imprecise reading of the angles of the metal bars could be avoided. Sesam 3 on the first measuring date. Figure 19: Capellla 2b High accumulation rates on the second measuring date Figure20: Capella 2b Figure21: Capella 2b

19 6. References SESAM Sediment Export from large Semi-Arid Catchments: Measurement and Modelling (2006) Annual report E.N. Müller et al. (2006) Dryland River Modelling of Water and Sediment Fluxes using a representative River Stretch Approach Fargas, D. Martínez, J. A. Poch, R. M. (1996) Identification of critical sediment source areas at regional level. In: Annales geophysicae 14 (supplement 11), 314, European Geophysical Society Harrelson, C. C., C. L. Rawlins, J. P. Potyondy (1994) Stream channel reference sites: an illustrated guide to field technique. United States Department of Agriculture, Forest Service, Rocky Mountain Research Station, General Technical Report RM-245 Sanz Montero, M. E., Cobo Rayan, R., Avendano Salas, C., and Gomez Mantana, J. L. (1996) Influence of the drainage basin area on the sediment yield to Spanish reservoirs. Proceeding of the First European Conference and Trace Exposition on Control Erosion Aerial views available from: 19

20 7. Appendix 7.1 Data of the cross-sections Capella 1 GPS location (UTM European Datum 1950) / Amount of existing Metal bars of the crosssection 4 metal bars at the end of the investigation First installation at the metal bars Loss of metal bars due to the floods in 1 metal bar September Reinstallation at the metal bar Reinstallation at the Loss of metal bars after the Capella 2a GPS location (UTM European Datum 1950) / Amount of existing Metal bars of the crosssection 4 metal bars at the end of the investigation First installation at the metal bars Loss of metal bars due to the floods in 4 metal bars September Reinstallation at the Reinstallation at the metal bars Loss of metal bars after the Capella 2b GPS location (UTM European Datum 1950) / Amount of existing Metal bars of the crosssection 7 metal bars at the end of the investigation First installation at the metal bars Loss of metal bars due to the floods in 3 metal bars September Reinstallation at the Reinstallation at the metal bars Loss of metal bars after the

21 Sesam 2 GPS location (UTM European Datum 1950) / Classification after Rosgen Vegetation type Amount of existing Metal bars of the crosssection at the end of the investigation First installation at the Loss of metal bars due to the floods in September Reinstallation at the Reinstallation at the Loss of metal bars after the Entrenchment factor - BC Longitudinal profile - BC Bed material Cobble. gravel. silt/clay Valley form - 5 On the hillslopes trees. on the left side next to the shore reeds 8 metal bars 7 metal bars 4 metal bars 2 metal bars 4 metal bars 1 metal bar Sesam 3 GPS location (UTM European Datum 1950) / Amount of existing Metal bars of the crosssection at the end of the investigation First installation at the Loss of metal bars due to the floods in September Reinstallation at the Reinstallation at the metal bars 9 metal bars 4 metal bars 4 metal bars 2 metal bars 21

22 7.2 Aerial views Figure 22, Aerial view of Sesam 2 and 3, data origin: 22

23 Figure 23, Aerial view of the Capella cross-sections, data origin: 23