KEEPING DRAFT DEPTHS AND CONTROLLING SEDIMENTATION BY
MAKING USE OF INNOVATIVE GEOTEXTILE CONSTRUCTIONS TO STEER
FINES AND STORE SEDIMENT
F.M. Lauwerijssen (1), P.J. Stook(2), G.A.R. Egbring (3) and M.H.P. Jansen (4)
Abstract: Due to wave action and water currents unprotected shorelines erode and natural habitats are lost.
Sediment is kept in suspension, thereby inhibiting growth of higher order aquatic plants and natural water quality
improvement. In a protected wetland area innovative and sustainable measures have been undertaken to reduce
hydrodynamics, sediment resuspension and turbidity in order to keep draft depths, enhance biodiversity and
improve water quality.
Two types of light weight geotextile constructions were built to steer fines and to store sediment: the Sediment
Settler and Sediment Storer. Due to the Sediment Settler fines transport decreased by 20-40 %. One year after
installing the Sediment Settler a few decimeters thick sediment layer had already been formed at its lee side. Also
about 15.000 cubic meters of slightly polluted sediment was dredged and transported to Sediment Stores in order
to alter hydrodynamics and induce nature development. By doing so, maintenance costs were minimized and
sediment had been beneficially re-used. By the end of the first growing season the sediment top layer was covered
by a pioneer vegetation of reedmace. Even species that have disappeared 30 years ago returned.
This pilot study has shown that it is possible to control sediment settling and transport and improving recreational
and ecological quality of this wetland area. Nautical bottlenecks were reduced and in lee areas vegetation
develops. So, synergies were realized between remediation, nature restoration and (at some locations) shore line
protection. Dredged material can easily be kept and stored in submerged basins.
Key words: Geotextile construction, sediment storage, shoreline protection
1-3Tauw
Group, The Netherlands, e-mail [email protected]; [email protected]; [email protected]
The Netherlands, e-mail [email protected]
4Witteveen+Bos,
1
INTRODUCTION
The Wormer- and Jisperveld area is the largest uninterrupted peaty grassland in Western Europe and one of the
most important areas for wetland birds in the Netherlands (Niedekker, 2006). It is under European Natura2000
protection and under the EU Water Framework Directive (WFD) classified as surface water body. This nature
area is located 20 kilometers north of the city of Amsterdam and has a surface area of about 2500 hectare. There
are three shallow lakes that are connected to each other by small channels. Most ditches are manmade and left
overs of peat excavation in the past. Peat was harvested and spread over rectangular shaped parcels to dry
characterizing peat lands in the Netherlands. In figure 1 an aerial photograph is shown of the study area. During
summertime these lake are intensively used by small motor boats, sailing vessels and canoes.
Figure 1. Aerial photograph of Wormer- and Jisperveld area with is shallow lakes and small channels.
1.1
Sediment quality
As a result of industrial activities in the 17th and 18th century heavy metals (copper, lead and mercury) have
accumulated in the sediment. Wind mills were used for paper and paint industry and industrial wastewater was
discharged directly into the surface water. The shallow lakes formed a natural sink for this pollutants. Some
sediments still contain high amounts of heavy metal concentrations.
Dutch policy and legislation stimulates re-use of soil and sediments. The application possibilities are mainly
determined by the concentration of pollutants in the sediment. Usually in this type of areas a small hopper
excavates the dredged material and transports it to a field depot for ripening. However sediments within the
shallow lakes are slightly polluted. Due to high heavy metal concentrations it is prohibited to spread it out over
nearby parcels. Transporting it by ships or trucks, on the other hand, will result in high costs.
Figure 2. Picture from former mills near the southern lakes in the Wormer- and Jisperveld area.
1.2
Surface water quality
The water quality of the lake is poor. It is rich of nutrients and due to water movement fine sediment particles
are kept in suspension and impede sunlight penetration to the bottom (Witteveen+Bos, 2013a). These fines
inhibit the growth of higher order aquatic plants and natural water quality improvement. Shorelines and
channels erode and fine suspended particles settle down in lee areas where the water currents are slow. Since all
waterways are connected fine particles are easily transported by channels. Near harbors and channel mouths
sediments settle and nautical bottlenecks arise. To make a change in this negative pattern, innovative and
sustainable measures needed to be taken to reduce hydrodynamics, sediment resuspension and turbidity in order
to improve water quality and keep draft depths at nautical bottlenecks.
2
WATER MOVEMENT AND SEDIMENT TRANSPORT
In order to get insight in nautical bottlenecks, water movement and sediment transport first a multidisciplinary
study was conducted.
2.1
Water depth and nautical bottlenecks
To prepare this dredging operation the amount of sediment at nautical bottlenecks needed to be determined first.
To do so a multibeam echosounder (type of sonar) was used to map the water bottom. By making use of
different frequencies different soil densities can be distinguished. Water depth is determined by measuring the
time difference between emitting and receiving. Since peat might disrupt the signal, the measurements needed to
be verified. In the lakes and channels point measurements were conducted by hand. All measurements were then
analyzed and used to create a Digital Terrain Model (DTM) in ArcGIS.
Second, sailing routes and preferred draft depths were used to calculate the amount of sediment at nautical
bottlenecks. Figure 3 shows the output of nautical bottlenecks and shallow banks. Most of the sediment settles
down in the south-western parts. At the northern banks, on the other hand, significantly less sediment is present
and shorelines are more damaged by waves.
Figure 3. Nautical Bottlenecks (circled) and differences in water depths (Tauw, 2013)
2.2
Hydrodynamic modelling
A good morphodynamic numerical model describes the water levels, currents, waves and sediment transport in
lakes and the exchange of fines with the surrounding area. Every lake has its own hydrodynamic and
morphologic behavior. Therefore the kind of numerical modelling is different for each lake. In long shaped
lakes the wave growth in the length of the lake is important (focus on waves), while in round lakes the wind
induced flow is more important (focus on currents and waves). In deep lakes waves are less important than
currents (focus on currents), while in shallow lakes both waves and currents are more important (Soulsby et al.,
1993). In lakes with a varying depth, density flows can be important and three-dimensional hydrodynamic
modelling is necessary. In order to get insight in wave pattern and existing transportation patterns, a numerical
hydrodynamic model (Witteveen+Bos, 2013b) was created (programs Delft3D and SWAN).
This model made it possible to visualise water movement and overall transportation routes through channels and
lakes for the most important wind directions (Jansen et al., 2015). Special attention was paid to the grid size in
the lakes and the channels. In the lakes a grid size of 10 m was applied. Also the orientation of the grid referring
to the channels was improved, to avoid irregular boundaries (Weare, 1979).
The method presented in Dusseljee et al. (2012) has been extended to get in the wave attack and present
transport patterns of fines. Wind data from KNMI, location Berkhout was used to derive the local wind climate
at Wormer- and Jisperveld (Figure 4). Wind direction 210°N is the most important wind direction.
Figure 4. Wind conditions at Wormer- and Jisperveld (source: KNMI).
Based on this wind climate the main wind directions and wind speeds with occurrence of 10%, 20% and 50%
were selected and used to calculate the flow and waves in the Wormer- and Jisperveld. These calculations show
the most important flows and waves in the area.
The model also shows the sources of fines. The most important sources are the south-west oriented banks and
the channels (Figure 5).
Figure 5. Wind-induced and wave-induced flow currents for the reference situation during wind from 210°N at
about 20 m/s wind speed.
3
INNOVATIVE GEOTEXTILE CONSTRUCTIONS
In this area it is impossible to use heavy machinery to build constructions, to excavate dredged material or to
transport heavy materials. Since the area is under European protection as well the impact of dredging activities
must be minimized. So there was a need for a light weight construction that was easy to install and to remove
without damaging the environment. In order to store sediment a screen was developed that was strong enough to
withstand pressure and filter sediment form water. This made gravity and buoyancy the decisive elements in the
design.
Over the last decade geotextiles (TenCate Geotube® containers) are used for hydraulic & marine and
environmental dewatering applications. These materials have proven themselves to be strong and to have a long
lifespan. Together with TenCate Geosynthetics B.V. we developed a new type of construction (see figure 6). A
modified prefabricated Geotube® unit was linked to a geotextile screen and a floater. The small Geotube® unit is
filled with sand and used as an anchor to keep the structure in place. A floater and propylene lines are used for
tensioning the screen to wooden anchor poles. The enclosed water surface behind the construction could then be
used to store sediment. The water depth after filling determines the kind of vegetation that will develop over
time (emergent or submersed vegetation)
Figure 6. Schematization of Sediment Storer.
3.1 Pilot Sediment Storer
First, a pilot was conducted to store 500 cubic meters of sediment behind a Sediment Storer of 50 meters in
length (Stook et al., 2013). Several weeks after installation of the Sediment Storer in March (2013) vegetation
started to develop. By the end of the growing season the sediment top layer was totally covered by a pioneer
vegetation of reedmace (Typha). Roots provide structure and induce dewatering of sediments by plant
transpiration. Also, water birds and small mammals found their habitat in this wetland. In winter time the
Sediment Storer withstands ice coverage and storm surges (wind speeds of 140 km/h).
Monitoring the sediment layer around the construction has shown that the whole construction is strong enough
to face the expectations. However, aspects that needed attention were the floater and the screen. The PVCfloater was not easy to handle and could not resist frost well. Besides, some of the fines seemed to pass through
the screen, since the water level had risen within the enclosed water surface. Lab experiments show that the kind
of geotextile we chose could not maintain the sediment particles under extreme stress.
Figure 7. Photo of pilot Sediment Storer (August 2013)
3.2 Design Sediment Settler
Further improvement of the Sediment Storer resulted in the innovative design of the Sediment Settler. The
Sediment Settler now reduces the intensity of wave action (like a groyne) and prevents sediment resuspension.
Besides the construction should be save for water users if collision occurs. This time, the floater was not made
from PVC but Styrofoam® (fabricated by DOW Chemicals), which is stronger. Since vegetation rapidly
developed behind the Sediment Storer two floaters were used to create a basket that could keep sediment inside.
Figure 8. Schematisation of Sediment Settler
Testing several geotextiles and non woven materials under extreme stress in lab experiments, resulted in a
suitable thin non-woven layer that was both permeable by water and able to maintain fine silt particles. After
filling the basket with sediment, the construction was covered by a mesh.
4
IMPLEMENTING CONSTRUCTIONS IN NUMERICAL MODEL
In order to define the most effective positions to reduce eddies and to create lee areas, both constructions were
fit in the numerical model to examine the effects. To do so, several positions were examined at different wind
directions. The combination of measures seemed to most effective. For example, in the northern lake large lee
areas were created by Sediment Storers at its west bank. At the east bank, smaller Sediment Stores were
installed to restore damaged shorelines. Near the centre of the lake Sediment Settlers were installed at the
location where strong eddies occurred. Figures 9 and 10 show the influence of these measures on
hydrodynamics (Witteveen+Bos, 2013b). White polygons indicate Sedimant Storers and line objects indicate
Sediment Settlers.
Figure 9. Wind-induced and wave-induced flow currents with (left) and without the design variant during wind
from 210°N at about 20 m/s wind speed for the northern lake..
In the southern lake Sediment Stores are able to reduce the width of the mouth of channels to prevent further
erosion and reduce water currents. Sediment Settlers seemed to be most effective in reducing hydrodynamics at
locations where strongest water currents are present.
Figure 10. Overview of wind-induced and wave-induced flow currents for the design variant during wind from
210°N at about 20 m/s wind speed.
The bottom shear stress gives a good indication of the effects of measures on erosion and sedimentation. Figure
11 gives the relative effect of measures in relation to the reference situation. It shows a decrease of 20% to 50%
in the northern lake and in the northern part of the southern lake. In the middle of the southern lake, in the
navigation channel the shear stresses will increase, which will result in erosion.
Figure 11. Difference in maximum bed shear stress between the design variant and the reference situation for a
wind speed of about 20 m/s from 210°N.
5
MONITORING
In January 2014 four Sediment Settlers (25 meters each) were placed perpendicular to water currents in order to
reduce those and induce settlement of sediment on the lee side. Since then the fines transports decreased by 2040% and on top of the construction vegetation which was missing for the last 30 years returned. Furthermore, a
few decimeters thick sediment layer has already been formed at the lee side over a surface area of one hectare.
Figure 12. Photo of vegetation development (August 2014)
Though, this type of construction is not meant to storage sediment, it collects sediments on its lee side (western
side). In figure 13 colors indicate differences in water depth. At dark green locations sediment has settled down.
Between the Sediment Settlers erosion occurs. A better solution would be one large screen, instead of three short
screens.
Figure 13: Differences in water depth near Sediment Settlers (September2014)
So far, lee areas can be created by Sediment Settlers and Sediment Storers and in these areas sediment settles.
6
FROM PILOT TO LARGE SUBMERGED DEPOTS
In the winter of 2014/2015 more Sediment Storers have been installed to remediate slightly polluted sediment.
Due to pilot results small modifications were made. The same non-woven material of the Sediment Settler was
now placed behind the geotextile screen to prevent sediment from passing through the screen. Also, the floater
was covered by UV-resistant material so a long lifespan can be guaranteed.
About 2 kilometers of geotextile constructions have been installed to store 15.000 cubic meters of sediment. In
total 4 hectares of water surface is enclosed.
First wooden poles were put into place. Second, constructions were attached to the poles and tubes for anchoring
the Sediment Storers were filled by a sand-water pomp. Then sediments were dredged by a small hopper and
flocculant was added to speed up settling and the filling process.
Figure 14. Photograph of Sediment Storer with UV-resistant top layer
For future dredging operations the Sediment Storers provide enough space to store another 15.000 cubic meters
of sediment. With this solution remediation and maintenance costs are minimized and sediment can be beneficially
stored. The effects of Sediments Stores on vegetation development and nautical bottlenecks will monitored by
multibeam echo sounding and field inspections.
7
CONCLUSION
Erosion and sedimentation of fines are ongoing processes and many shallow lakes and harbors suffer from
accumulation of fine sediment. Moreover, high amounts of sediment negatively influence water quality and
recreation. This study has shown that it is possible to steer sediment settling and therefore recreational and
ecological quality of this wetland area can be improved. At nautical bottlenecks draft depths are kept and in lee
areas vegetation develops. Transportation of fines has decreased considerably and in some lee areas emerged
vegetation developed, demonstrating the potential of this approach to solve problems with fines and to
contribute to ecological recovery of the lakes.
The light weight geotextile constructions designed for this project were easy to handle and to install. Despite their
low weight they have proven to be both resilient and strong. In winter time the constructions withstand ice
coverage and storm surges (wind speeds of 140 km/h). Small modifications have been made in order to improve
its filtering capacity and enlarge its lifespan. Due to these constructions slightly polluted sediment could be stored
under water, thereby minimizing maintenance costs and CO2 emissions. Surprisingly, by the end of the first
growing season the sediment top layer was covered by a pioneer vegetation of reedmace. Even species that have
disappeared 30 years ago returned.
So, synergies were realized between remediation, nature restoration and shore line protection. Dredged material
can easily be kept and stored within the shallow lakes. It is expected that this geotextile construction can also be
used for shoreline protection or to prevent harmful algal blooms reaching shores.
ACKNOWLEDGEMENTS
This project was commissioned by Dutch waterboard ‘’Hoogheemraadschap Hollands Noorderkwartier
(HHNK)’’. We appreciate their confidence and support in the pilots conducted. Furthermore the valuable
contributions brought in by K. Hopman (Waterboard HHNK), R. Wortelboer (TenCate Geosynthetics B.V.) and
their colleagues during this project was of great importance in improving the constructions and making it suitable
for further applications.
REFERENCES
Dusseljee D.W., de Jongste A.L. and Jansen M.H.P. (2012), An efficient method to assess erosion risks in
tidal basins, Jubilee Conference Proceedings, 20th NCK-Days 2012, 131-135, ISBN 9789036533423.
Jansen M.H.P, De Jongste A.L., Klinge M., Schep S., Stook P. and Egbring G. (2015). Steering fines in
shallow lakes. E-proceedings of the 36th IAHR World Congress 28 June – 3 July, 2015, The Hague, the
Netherlands.
Niedekker D. (2006), Door riet omzoomd, Wormer- en Jisperveld, Stichting Uitgeverij Noord-Holland, in
Dutch
Stook P.J., Lauwerijssen F.M. and Egbring G.A.R. (2013). De baggerbuffer: ‘Bagger van last tot lust’. Land
and Water magazine. Issue 11, pages 18-19 (Dutch).
Soulsby R.L., Hamm L., Klopman G., Myrhaug D., Simons R. and Thomas G.P. (1993), Wave-current
interaction within and outside the bottom boundary layer, Coastal Engineering, 21, 41-69
Tauw (2013). Sediment analysis in small lakes and southern ditches. Report R002-1214668GUE-sbb-V04-NL
(in Dutch).
Weare T.J. (1979), Errors arising from irregular boundaries in ADI solutions of the shallow-water equations,
International Journal for Numerical Methods in Engineering,Volume 14, Issue 6, pages 921–931
Witteveen+Bos (2013a). Modellering nutriëntenbalans Wormer- en Jisperveld. Resultaten en advies. Report
R005-1214668GUE-sbb-V01-NL, 30 May 2013 (in Dutch).
Witteveen+Bos (2013b). Hydrodynamica varianten Wormer- en Jisperveld. Report R010-1214668FML-nijV01, 2 August 2013 (in Dutch).