Sedimentation during a flood
5.1 Sedimentation during a flood
When developing a flood proof urban design, not only the aspects which occur during a flood should be taken into consideration, but also the effects of the flooding which arise after the actual flooding. When an area has been flooded, a residue due to sedimentation of clay, silt and garbage is left behind.
To gain an insight in the amount of sedimentation due to flooding in the Stadswerven area, preliminary calculations were conducted. Use was made of the SOBEK overland flow module (2D) in combination with the water quality module. Calculations representing the present situation were performed. This implies the present ground level and implementation of a traditional building layout. The design concepts ‘Integrated floodwall’ and ‘Watersteps’ for the Stadswerven were also evaluated. A flood was simulated which reaches a water level of NAP +3.4m, the predicted design water level for 2100. Because of the limitations in the availability of data of discharges and suspended particle matter (SPM) concentrations, it was decided to perform calculations for two different concentrations of SPM. The sediment loads for scenario 1 are 50 mg SPM/l and for scenario 2, 100 mg SMP/l. An extensive description of the calculations can be found in Appendix C.
The results for the calculations of scenario 2 are shown in Figure 5.1 to Figure 5.3. The figures show the thickness of the sediment layer (m) resulting from the flood event.
Figure 5.1
Thickness of the sediment layer (m) for the present situation.
Figure 5.2 Thickness of the sediment layer (m) for design concept Integrated floodwall.
Figure 5.3 Thickness of the sediment layer (m) for design concept Watersteps.
The results show that for the Stadswerven area only a thin layer of sedimentation, in most areas not more than 1 mm, is deposited. This is due to the fact that the area is only flooded for a short period. It is seen that the layer of sedimentation is slightly thicker for the Watersteps design. This is mainly seen in the areas where the water depths are larger and can be accounted to the fact that the deeper areas are flooded for
a longer period of time. Thicker layers of sediment also accumulate on the southern shore of the Stadswerven area at the mouth of the Wantij river.
The deposits could be slightly contaminated with phosphates, nitrates and heavy metals such as copper and zinc, although no exact information is available on the expected contaminants within the deposited matter.
During periods of low flow, SPM concentrations are low, but a lot of sources still introduce contaminants in the river system. Therefore, SPM quality can be poor. From research it is seen that during periods of high river flow, SPM concentrations increase and SPM quality can improve. This is attributed to dilution of the SPM with clean sediment material from the surrounding area (Asselman (2003), and Van der Heijdt and Zwolsman (1997)).
For areas which flood more frequently one should anticipate on the possibility of deposition of contaminated sediments. Each flooding event leaves a thin film of residue which could accumulate over time and become a thicker layer.