5.1 SEDIMENT TRANSPORT LIMITATION
Marshland topography (except artificial relief elements like Warften and dykes) is a result of spatial highly variable sediment depositions by reason of inhomogeneous transport processes. At natural marshlands, sediments are distributed via the marshland edge and a wide branched tidal channel system (D’Alpaos et al. 2007, Bartholdy et al. 2010a). Therefore accretion rates are highest adjacent to channels and margins, while rates are decreasing more distant to the sediment source. DGMs based on airborne LIDAR measurements of the year 2005 (fig. 5-1) reveal clear relief elements on the Halligen. Beside characteristic wide banks alongside of the major tidal channels (lateral extension 20 – 50 m) a distinct elevation rise within 100 to 200 m distance to the marshland edge is visible. The latter shapes the cross section profile of the Halligen like a flattened bowl with a decrease in surface elevation from the edge to the hinterland. Therefore, the general topography of the Halligen is comparable to those of uninhabited salt marshes in a natural state (cf. Temmerman et al. 2003, D‘Alpaos et al. 2007, Bartholdy et al. 2010a). Analyses on the variability of short-term (2010 – 2013) spatial sediment accretion (chapter 4.5.3,
Figure 5-1: DGMs of Hooge, Langeness and Nordstrandischmoor based on LIDAR data (2005) with a spatial resolution of 1 x 1 m.
CHAPTER 5 Schindler et al. 2014b) also reveal highest marshland accretion rates to be consequently attached to the marshland edge. Nevertheless, observation points distant to the Hallig margins but adjacent to a major channel system do not show higher accretion rates than observation points which are both, distant to marshland margin and channel system. Furthermore field measurements of the small scale sediment deposition variability along tidal channels and the marshland edge (fig. 5-2) indicate that variations in sediment deposition are recently limited to the first meters adjacent to the possible sediment source. At Nordstrandischmoor higher sedimentation rates occur obviously in a distance of 0 to 14 (max. 24 m) to the block revetment.
For the tidal channel at Langeness, an interaction between sedimentation and the channel structure seems to be limited to a distance of 5 m. Due to this limitation of sediment transport, the broader relief structures as described before are nowadays not supported by sediment deposition. Hence, their origin has to be of an older date. These observations confirm the assumption to regard the Halligen as “transport limited landscapes” (D’Alpaos et al. 2007).
Insufficient sediment transport mechanisms by an obstructed channel system result in a lag of marshland accretion distant to the sediment source (Schindler et al. 2014b).
Figure 5-2: Clastic sediment deposition (Mc) during the stormy season from October 2011 to March 2012 as a function of: (a) increasing distance to the block revetment on the marshland edge of Nordstrandischmoor, (b) increasing distance to the tidal channel “Leye” on Langeness.
Deposition data is calculated as average values on (a) three lines of sediment traps type A (bottle) and (b) six lines of sediment traps type A (bottle).
5.2 REACTIVATION OF A TIDAL CHANNEL SYSTEM (LANGENESS)
5.2.1 METHODS
With regard to the transport limitation, it is obvious to test if the reactivation of the tidal channel system during high or storm tides could be an adequate measure to enhance sediment transport to the inner marshland. For that purpose, the field experiment “Osterwehl” was established.
Between 25th to 27th of March 2013 and 09th to 11th of January 2014 the tidal gates at the
“Osterwehl” on Hallig Langeness were kept open (fig. 5-3) and the suspended sediment concentration was measured. For that purpose an ASM-IV-L (Argus GmbH) optical backscatter devise was mounted in the channel ~ 50 m landwards of the tidal gates. It contains 192 infrared emitters and sensors with a vertical distance of 1 cm fitted into a pale of stainless steel. By reason that higher sediment concentrations were expected during measurements conducted in January 2014, the sensor sensitivity was changed from 0 – 500 mg/l to 0 – 5000 mg/l. Measurement cycles were set to 30 seconds. To calculate average sediment concentrations, the ASM-IV-L measurements were filtered for artefacts due to not submersed sensors or blind sensors. The filtering was conducted using an Excel-macro-routine by V. Karius (see affiliation of co-authors).
The algorithm to distinguish between data and artefacts is based on appropriate threshold values for the standard deviation of ten moving averages of the order 10 of each sensor time series.
Figure 5-3: Open gates at the tidal channel system “Osterwehl” (Langeness). Gates are fixed by wooden beams against a closing by water inflow. Photo by M. Deicke (2013).
CHAPTER 5 5.2.2 RESULTS AND DISCUSSION
During the first measurement campaign (25th – 27th of March 2013), five tidal cycles ranging from neap to normal MHW were measured. Sediment concentrations of the seawater streaming into the channel system were within a range of 100 to 200 mg/l. During the second measurement campaign (09th – 11th of January 2014), conducted for four tide cycles, the first two tides were exceeding the normal MHW level by ~ 0.5 m due to heavy wind from westward directions.
Thus, a higher volume of water passed the gates towards the Hallig. Tide number 3 and 4 reached comparable gauge levels (around MHW) to the first measurement campaign conducted in March 2013. Figure 5-4 illustrates the suspended sediment concentrations measured by the ASM-IV-L backscatter device. The colour image illustrates the temporal as well as vertical backscatter variability (concentration of solids in mg/l). Yellow to red bands of highest reflections occur due to organic flotsam caught by the sensor-pale. The shift from bluish (water) to solid green colours of medium reflections (air) are tracing the changing water level due to the tide (left y-axis). The black signatures indicate for the average concentration of solids in the water column (right y-axis) calculated on the processed and corrected raw data (colour image).
Figure 5-4: Mean suspended sediment concentration (black signatures) at the tidal channel system “Osterwehl” (Langeness) in mg/l during four tide (09th to 11th of January 2014). The coloured image illustrates the temporal backscatter variability (also given in mg/l) due to sensor height above the channel bed. Measurements were conducted by an ASM-IV-L (Argus GmbH) optical backscatter device. The measurement cycle was set to 30 seconds with a sensor sensitivity of 0 – 5000 mg/l.
It is apparent that flood stream sediment concentrations during the raised tides no. 1 and 2 are noticeable higher (400 – 500 mg/l) compared to measurements conducted by MHW-conditions.
Eye-catching are low transport rates (max. 50 mg/l) during tide no. 3 after the weather has calmed down. Water level and sediment concentrations of tide no. 4 (approx. 200 mg/l) are well comparable to values measured during the first campaign in March 2013. Furthermore, sediment concentrations tend to increase fast at the beginning of the flood stream. When the reversal point of the tide is reached and the flood stream turns to ebb tide, sedimentation processes are decreasing the suspended sediment concentrations abruptly. Therefore a sediment input to the channel system takes place, which increases in dependence to rising tides. Regarding those observations it seems to be obvious to expect further increasing sediment concentrations during storm surge conditions. Measurements above the marsh surface of Hallig Nordstrandischmoor during a storm surge in November 2011 revealed suspended sediment concentrations of
> 400 mg/l, being comparable to the “Osterwehl” data. Nevertheless, the measured sediment concentrations during spring tide as well as storm surge conditions, are fare exceeding the threshold of Kirwan et al. (2010) (being 30 mg/l), for marsh sustainability during a high rate of SLR (> 20 mm/a). Summing up, it can be reasoned that the suspended sediment concentration seems to be appropriate to enhance marshland accretion by reactivation of the tidal channel system during advantageous weather conditions. To evaluate this measure, a field survey to quantify sediment deposition rates adjacent to active channel systems is highly recommended.