Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters

The Lippensbroek polder as a case study for the inner dike restoration of ecosystem structures and functions

In the framework of the SIGMA plan, Flemish authorities (Waterways and Maritime Affairs Administration, division Sea Scheldt) plan a large controlled inundation area (CIA) in polders (Kruibeke-Bazel-Rupelmonde, 578 ha) along the freshwater - oligohaline border of the Scheldt estuary. Beside fulfilling an important role in enhancing safety against flooding, a CIA could contribute to the restoration of degraded ecosystem functions and habitats by implementing specific management options. However, as the outcome of some management types on ecosystem structures and functions are still uncertain, preliminary projects on a smaller scale must yield adequate information to allow, if necessary, appropriate adaptations. In the pilot project ‘Lippensbroek’, a management scenario for the northern part of the future Kruibeke-Bazel-Rupelmonde CIA will therefore be tested beforehand on an innerdike area of 10 ha. Carefully designed sluices will allow the exchange of limited amounts of Scheldt water, causing a semidiurnal submersion of ca. 0.5 m. Additionnaly, submersions with larger water quantities will occur on a less regular base during storm floods. We discuss the present and future hydrologic regime of the Lippensbroek polder, together with the necessity of a multidisciplinary program to monitor the restoration of ecosystem structures (fresh water tidal marshes, mudflats, creeks, etc.) and functions (nutrient cycling, storage capacity enhancement, sediment retention, biodiversity and habitat support, etc.)

De inpassing van de Barbierbeek in het gecontroleerd overstromingsgebied Kruibeke-Bazel-Rupelmonde

This report deals with the possibilities to fit the Barbier-brook in the Controlled lnundation Area of Kruibeke, Bazel en Rupelmonde that was planned according the Sigmaplan. The so called CIA-KBR is situated upstream Antwerp. The CIA-KBR will be inundated when a certain flood on the river Schelde overflows the lowered river dike. The result is a decrease of the peak of the flood. A dike at the polder side will fulfill the defensive function during the period of inundation. This dike is called the "ringdike". The Barbier-brook cuts through the CIA-KBR. Within the CIA the Barbier-brook is diked in since the 13th century. Further on in this text these dikes will be called Barbier-brook dikes. Nowadays, the area between these dikes forms the storage for the Barbier-brook.The study participates in the preparations for the CIA-KBR. Co-ordinated by the Institute of Nature Conservation in Brussels some studies for possibilities of nature development within the CIA-KBR are carried out.In this report the technical possibilities to fit the Barbier-brook in the CIA-KBR as weIl as the perspectives for nature developrnent are studied. The perspectives for nature development can be split in:.nature development influenced by the Barbier-brook.nature development influenced by the Barbier-brook and a controlled reduced tide, controlled reduced tide stands for: a tide controlled by culverts for the in- and outlet of water from the river Schelde with a reduced tide amplitude with an average of 0,25 to 0,5 m.This last perspective corresponds most to the historical natural situation of the area, which consists of freshwater marshes and mud plains.To be able to work out these aspects it is necessary to carry out also a study ofthe water quality and quantity of the Barbier-brook.No discharge data from the Barbier-brook are known at all. Based on the discharge data of the Kleine Molenbeek near Liezele, which has a similar catchment area except the size, the discharge frorn the Barbier-brook could be abstracted. To do so, the surface area of the Kleine Molenbeek is multiplied by a factor 1,43. Frorn these discharge data follows a mean discharge of the Barbier-brook which varies seasonally between 3 and 5 m3/s. These data show also an increase in the annual maximum 24h-discharges starting from approximately 1980.The quality of the water of the Barbier-brook off the CIA-KBR is very bad. The brook is heavily polluted with domestic, industrial and agricultural wastewater. Nearby the site where the brook discharges into the Schelde it is characterised as "biologically dead". The involved communities have made sewerage plans which will decrease the pollution of the Barbier-brook with dornestic wastewater with about 80%. Concerning the houses which are not covered by the sewerage plans the following measures are given: 1) yet inc1ude into the sewerage plans, and 2) provide small-scale biological treatment.After completion of the sewerage plans and the extra measures the following reductions of the pollution by wastewater on the Barbier-brook could be reached:dornestic wastewater: 92%; industrial wastewater: 90%; agricultural wastewater: 31 %To make it technically possible to fit the Barbier-brook in the CIA-KBR it is necessary to compensate the storage of the Barbier-brook in order to prevent the brook to cause trouble at the houses of Bazel that stand close to the brookside. For this reason the following measures will be elaborated:The realisation of a storage along the ringdike which is connected with the CIA by means of a culvert. The surface needed for the storage is regulated by the maximum water level that is tolerated in the storage. From calculations follows that there is enough place to realise a storage that can guarantee a safety for a situation with a return period of 1 per 1000 years. No houses need to be removed.The realisation of a storage in the natural valley of the Barbier-brook situated upstream Bazel and the CIA. This storage needs to be fit in the protected landscape without large works. From calculations follows that the available storage volume is insufficient to guarantee a safety for a situation with a return period of 1 per 1000 years.Placing a pumping station at the ringdike with sufficient capacity to pump over a discharge of the Barbier-brook with a return period of 1 per 400 years which is 8,7 m3/s. It is technically possible to reach this with a "screw-up-pumping station" as well as with a "screw-centrifugal-pumping station”Not until the quality of the water of the Barbier-brook improves strongly, nature development can be effectively worked out. In this study the following perspectives for nature development are elaborated in order to increase the value of nature in that part of the CIA that can be influenced by the Barbier-brook:.Concerning the area between the Barbier-brook dikes possible measures are worked out to improve the present potential values of nature. Thus rich zones along the Creek of Kruibeke can be expected and the higher grounds will be rich in flowering plants. When the dikes around the Barbier-brook are preserved the introduction of a controlled reduced tide is of no influence concerning nature development of the Barbier-brook..Concerning a part of the polder of Kruibeke space will be created for development of a more natural Barbier-brook. The agricultural activities on the other grounds will be preserved. Different measures will make this possible of which one is the removal of the Northern Barbier-brook dike. There will be aimed towards broad and rich developed zones along the brook with gradation in vegetation types. The introduction of a controlled reduced tide is not considered because of the present function of agriculture in the polder ..Concerning the polders of Kruibeke and Bazel measures are worked out to provide a total free development of the Barbier-brook. The most effective measure to reach this is the removal of the Barbier-brook dikes except that part of the dikes between the new ringdike and the natural height in the polder of Bazel. There will be a striving towards a large scaled landscape dominated by reeds and rich developed wetlands. The introduction of a controlled reduced tide is of great influence of the nature development in this situation. It might even re sult in freshwater marshes and mud plains in the CIA-KBR