Foresigth of the compensation channel in the Hahnöfer Nebenelbe (side arm of the river Elbe)
After the Alte Süderelbe (Old South Elbe) was blocked off in 1962 and following construction of the Ness Guard wall in 1969 and 1970 together with earlier work to adapt the fairway of the Elbe before 1999, the area of Hahnöfer Nebenelbe/Mühlenberger Loch has suffered from sedimentation on account of reductions in current, shifts in tidal phases between the main course of the river Elbe and its side arms, together with the lack of filling and draining action in the Alte Süderelbe. In order to improve the aquatic flow and sustain the ecological system Hahnöfer Nebenelbe/Mühlenberger Loch in the long term, a channel was planned for the area Hahnöfer Nebenelbe/Mühlenberger Loch as a compensation measure in the context of the last phase of work involved in adapting the fairway of the Elbe in 1999. However, in 2001 and 2002, before the compensation measure could be implemented, part (approx. 160 hectares) of the Mühlenberger Loch (total surface area around 875 hectares) were filled in to meet the needs of the Airbus aircraft factory, causing considerable alterations in the tidal dynamics of the Hahnöfer Nebenelbe/Mühlenberger Loch. The compensation channel produced in 2003 subsequently witnessed severe sedimentation around the DASA/Airbus extension, particularly at the curve of the compensation channel (picture 1).
In order to restore and optimize the functionality of the compensation channel under the changed general conditions following the DASA/Airbus extension, the Project Office for adapting the fairway of the Lower and Outer Elbe in the WSA Hamburg initiated various corresponding studies which were contracted to the BAW by HPA as the legal successor responsible for the project "DASA Extension A3xx" and the organisation responsible for this project.
A preliminary study looked at a number of variations in order to reveal optimisation potential. Following extensive discussions, the project led to an in-depth analysis and forecast of the morphological stability of the compensation channel if the channel will be dredged again.
The main results of the project are as follows:
- All variations of the preliminary study show only a very slight positive effect with regard to the current situation in the compensation channel. Long periods of slack water remain the dominant feature, thus promoting sedimentation of suspended material in the water column. This will cause aggradation of the area in the long term. A one-channel system recommended by the BAW would help to delay the aggradation process.
- The analysis of the current velocities shows a decrease of about 10 cm/s or 12% in the maximum flood current velocity of the eastern compensation channel. The most common current velocities lie within a range of + 25 cm/s within the slack water (< 20 cm/s). The ebb current velocity appears to be unchanged, together with the currents in the remaining Hahnöfer Nebenelbe.
- An analysis of the sounding data shows that partial sections of the east compensation channel have silted up by more than 2 m within two to three years. A flow through the channel at low water is no longer possible.
- The pattern of suspended load in the Hahnöfer Nebenelbe/Mühlenberger Loch is independent of the discharge of the dreged material at Ness-Sand. However, the concentration was amplified which results in an increased deposition rate in the east compensation channel (see animation below).
- As an approximation and with all mentioned uncertainties and given all assumptions, on the basis of the model study it can be presumed that the channel will fill up up again within two to three years after being dredged again. Special events such as storm surges, high floods or high fresh water discharge events are not taken into account.
- The residual advective transport of suspended load (picture 2) calculated in the model study clearly shows a convergence zone that approximately follows the course of the Este fairway, with another transport path pointing towards the Mühlenberger Loch downstream from the DASA extension area.
In terms of the morphological stability of the overall system Hahnöfer Nebenelbe/Mühlenberger Loch and with regard to the current general conditions, it can be said that there is no indication of a change in the system that could lead to a sustainable situation which makes it possible to manage without human intervention.