Porcupines for river training

Porcupines for river training: A study on the near-field effect of porcupines

Nientker, Gustav (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)

Uijttewaal, Wim (mentor)
Sloff, Kees (graduation committee)
Mosselman, Erik (graduation committee)
Schuurman, F (graduation committee)

Delft University of Technology

2018-07-10

Braiding rivers are characterised as highly dynamic, and experience annual morphological changes in plan- form. This dynamic nature of the river leads to navigational hindrance and risk of unstable bifurcation points where discharge distributions might switch. In pilot studies, porcupines have shown promising results in re- tarding the flow and cause sediment deposition near river banks to prevent bank erosion. However the aim now becomes to apply the porcupines on a much larger scale to increase the channel roughness and influ- ence discharge distributions of bifurcation points such that the flow is mainly diverted to the channel where the highest discharge is required. This way the main channel receives the largest discharge and therefore sedimentation in these channels is prevented. Currently it is not known how porcupines should be modelled in a numerical model. It is simply assumed that porcupines can be modelled similar to vegetation which is schematised as rigid cylinders with a certain density, drag coefficient and resulting representative roughness. No measurements are available to validate the assumed roughness of porcupines and if the hydro- and mor- phodynamic behaviour, represented by the model, is true.

In this thesis laboratory experiments are conducted to assess the near-field hydro- and morphodynamic ef- fect of porcupines and generate more knowledge about their behaviour. Experiments with a concrete bottom give a detailed insight in the hydrodynamic behaviour of porcupines without the interference of bedforms and morphological developments. Experiments with a sediment bottom give more insight in the morpho- logical development and flow patterns over time which clearly influenced the initial hydraulic behaviour. Experiments are conducted in a 12 metre long and 0.8 metre wide flume with a recirculating pump. The wa- ter level, discharge, density of the porcupine field and configuration of the field are systematically varied to identify the dependences on the drag and sedimentation/erosion volumes in the near-field domain of the porcupines. Additionally, general observations are performed, describing the flow structures and sedimenta- tion patterns in and around the porcupine field.

From fixed-bed experiments it is observed how the flow is retarded by the presence of porcupines. Flow is pushed around the field in both transverse and vertical direction. Behind the porcupines, longitudinal flow vectors are downward directed and the flow velocity near the bed is significantly reduced. It is observed that staggered porcupine grids help to retard the flow stronger and captures sediment behind the field in wider strokes. Non-staggered grids only work effectively in the line of porcupines. Between those lines barely any retardation is observed and therefore only narrow strokes of sedimentation are observed behind the lines of porcupines. The reduction in flow velocity behind the porcupines is similar to the velocity reductions ob- served in experiments with vegetation. The velocity retardation is gradually restored in longitudinal direction where the effect of porcupines gradually diminishes. For different experiments this deceleration of the flow has been observed and it follows a linear trend line, that by means of extrapolation can be used to quantify the effective retardation length in longitudinal direction. Water level differences over the porcupine field are observed indicating loss of energy, and pushing up the water level upstream. Porcupines can effectively in- fluence bifurcation points in braiding rivers this way. The local water level gradient over the porcupine field, combined with the velocity measurements, is used to determine the drag and representative roughness of the porcupines by using the equations of Baptist. The obtained values for the roughness are validated by sim- ulating the flume in SOBEK with the corresponding roughness coefficients. Comparing the measured water levels with the computed water levels by the model gave a relatively good fit indicating that porcupines can be schematised by the equations of Baptist with a few adjustments. The Reynolds stresses give an indication of the height of the bed shear stresses. Measurements show that the shear stress in the near-bed region behind the porcupines is lower or the same compared to the undisturbed velocity profile. Lower bed shear stresses indicate a reduction in sediment transport and is therefore an important mechanism to reduce the bedload sediment transport.

Mobile-bed experiments show clear erosion patterns inside the porcupine field due to the increased turbu- lence intensity generated by the porcupines themselves. For the experiment with low water levels and high flow velocities erosion is observed to be most severe, whereas in experiments with lower field density an

overall sedimentation is observed inside the porcupine field. Due to the erosion porcupines sink into the bed significantly reducing their effectiveness on retarding the flow. To prevent scour larger spacing between the porcupines seems beneficial. A growing sedimentation ridge behind the porcupines is observed that influ- ences the flow retardation even stronger, enhancing further sedimentation. Once maximum sedimentation height is reached, the ridge will migrate downstream as a growing sedimentation bar gradually sloping down towards its initial bed level. This length scale of the sedimentation bar is comparable with the retardation length scale. Combined with the migration rate of the sedimentation bar an estimation of the time scales can be obtained. Based on the mobile-bed experiments it is concluded that least erosion within the field is beneficial and therefore low water levels and high flow velocities should be avoided.

Although initial results show that porcupines show similar behaviour compared to vegetation and that the roughness can be estimated by using the equations of Baptist it has become clear that there are still major differences between the behaviour of vegetation and porcupines. Therefore further research is required to improve schematisations of porcupine behaviour, especially to improve the schematisation of the sediment transport around porcupines since no descriptions are yet available.

Porcupines
Permeable structures
Braiding rivers
River training
Flexible solutions
Laboratory experiment
Bank erosion

http://resolver.tudelft.nl/uuid:28298b28-8b7a-4c64-a5be-9561bc1180a5

https://doi.org/10.4121/uuid:70ddd374-9e21-4a9c-8c19-7f63f20b7023

Student theses

master thesis

© 2018 Gustav Nientker