Flood protection
Proceedings NCR-days 2006 68
-Building on piles in floodplains
J. Harke1,2, A.J.G. van der Maarel 2, R.M.J. Schielen 1,3, J.S. Ribberink 1
1 Water Engineering and Management, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands;
j.harke@alumnus.utwente.nl
2 Consultancy and engineering company Tauw bv, P.O. Box 133, 7400 AC Deventer, The Netherlands
3 Institute for Inland Water Management and Waste Water Treatment (RIZA), P.O. Box 9072, 6800 ED Arnhem, The
Netherlands
Introduction
Last year in the Netherlands 15 locations were allocated along the Rhine branches where – under strong restrictions - it was allowed to build in floodplains. Building in floodplains may lead to a water level rise during floods and moreover, the river bed morphology may be disturbed (erosion/sedimentation). A potential building location on a floodplain of the river IJssel near Deventer (Wilpsche Klei) is used as a fictitious case to investigate these processes (Fig. 1).
Figure 1. Potential building location.
Method
Hydraulic and morphological calculations are carried out with a simple analytical model and with a 1D SOBEK model. Hydraulic
calculations are also carried out with a 2D WAQUA model. The flow obstruction of the piles in the floodplain is represented as an additional roughness in the floodplain. The change in roughness is calculated using a force balance taking into account the gravity force, the bottom friction and the drag force of the pile elements on the water. The following expression for the representative Chezy coefficient, Cr, is applied:
Figure 2. Effect on waterlevel and backwatercurve, design-discharge 2356 m3/s. p D r gA N h D C C C 2 1 1 2 0 + = (1)
in which C0 is the Chezy coefficient of the
floodplain bed roughness (without piles) estimated with the White-Colebrook relation based on Nikuradse roughness, kn, N the number of piles, D the diameter of the piles (0.5 m), h the water depth, g the acceleration of gravity, Ap the area in which piles are placed, and CD the drag coefficient (Van Velzen et al., 2003; Huthoff et al., 2006).
Results
In the building region, the river shows a maximum water level rise of several
centimetres during the design discharge (Fig. 2; Ribberink and Hulscher, 2003). The maximum water level rise calculated with the simple analytical model is about 3.0 cm and about 3.2 cm calculated with the 1D model. 2D model calculations show a maximum water level rise of 1.3 cm in the middle of the river to 2.8 cm in the floodplain.
The influence of a number of parameters such as the number of piles, the diameter of the piles, the roughness between the piles and the drag coefficient of the piles (Fox and McDonald, 1994) on the water level rise is investigated using the analytical model and SOBEK (Fig. 3). The calculated water level rise ranges between 1 and 5 cm.
Simple analytical calculations show that, due to a shift of the river discharge from the floodplain
8.30 8.32 8.34 8.36 8.38 8.40 -5000 0 5000 10000 15000 20000 25000 Distance to intervention (m) W at er leve l ( N A P ) 3000 houses 150 ha Deventer Gorssel Wilp
Flood protection
Proceedings NCR-days 2006 69
-to the main channel in the region where the buildings are planned, the main channel shows a bed level erosion of ca. ½ cm per day during a flood (Ribberink, 2004). Upstream
sedimentation due to backwater effects is not significant.
SOBEK computations show that the
riverbed in the main channel recovers from this erosion during the longer dry periods between the floods when the flow is confined again to the main channel. In the long term there is erosion as well as sedimentation of the river bed along the river.
Model comparison
In general, the hydraulic results obtained with the simple analytical model and SOBEK show a good correspondence. The water level rise as computed with the 2D WAQUA model is slightly smaller. This difference is probably due to 2D flow effects, which are very relevant in the river area considered and cannot be represented in the other 1D approaches. Further investigations with WAQAU are required.
The morphological calculations, as carried out with the analytical model and SOBEK, should be considered as indicative. The SOBEK computations show a dominant influence of long sand waves which interfere with the morphological effects caused by the floodplain intervention. Further investigation with a 2D morphological model is
recommended for more reliable predictions.
Recommendation
Although the present feasibility study provides a good first impression of the possible
hydraulic-morphological impacts of building on piles in floodplains, it is recommended to investigate other locations with different riverbed slopes and floodplain levels / widths in order to get more insight in the effects of piles in floodplains.
References
Fox, R.W., A.T. McDonald (1994) Introduction to Fluid Mechanics. John Wiley & Sons, Inc, New York. Huthoff, F., D.C.M. Augustijn, S.J.M. H. Hulscher (2006)
Depth-averaged flow in presence of submerged cylindrical roughness elements. Riverflow 2006, September 6 - 8, 2006, Lisbon, Portugal.
Ribberink, J.S., S.J.H.M. Hulscher (2003) Dictaat Ondiepwaterstromingen. Universiteit Twente. Ribberink, J.S. (2004) Dictaat Transportverschijnselen en
morfologie. Universiteit Twente.
Velzen, E.H. van, P. Jesse, P. Cornelissen, H. Coops (2003) Stromingsweerstand vegetatie in uiterwaarden, deel 1 Handboek, versie 1-2003, RIZA rapport 2003.028. Ministerie van Verkeer en Waterstaat, Directoraat-Generaal Rijkswaterstaat, RIZA.
0.0 1.0 2.0 3.0 4.0 5.0 6.0 0 5000 10000 15000 20000 25000 30000 35000 40000 Number of piles W at er le ve l ris e ( cm )
SOBEK Simple analytical calculation Difference S-H
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Drag coeficient W at er l evel r ise (cm )
SOBEK Simple analytical calculation Difference S-H
0.0 1.0 2.0 3.0 4.0 5.0 6.0 0 0.2 0.4 0.6 0.8 1 1.2
kn-roughness between the piles (m)
W at er l evel r ise ( cm )
SOBEK Simple analytical calculation Difference S-H
