Papers by Joris Eekhout
Channelisation measures taken halfway the 20th century have had destructive consequences for the ... more Channelisation measures taken halfway the 20th century have had destructive consequences for the diversity of the ecology in the majority of the lowland streams in countries such as the Netherlands. Re-meandering is the common practice in restoring these lowland streams. Three reconstructed streams were monitored during the initial two years after construction of a new channel. The monitoring program included morphological surveys, sediment sampling, habitat pattern surveys, and discharge and water level measurements. Adjustments of the longitudinal bed profile formed the main morphological response. These adjustments were most likely caused by a lack of longitudinal connectivity of the streams as a whole, interrupting transport of sediment at locations of weirs and culverts. Bank erosion was observed only in a limited number of channel bends, and was often related to floodplain heterogeneity. Longitudinal channel bed adjustments and bank erosion were mainly caused by exogenous influences. In channel bends, the cross-sectional shape transformed from trapezoidal to the typical asymmetrical shape as found in meandering rivers. This behaviour can be attributed to an autogenous response to the prevailing flow conditions. Due to the prevailing fine sediment characteristics, bed material is readily set in motion and is being transported during the entire year. The existing design principles fail to address the initial morphological development after reconstruction. An evaluation of pre-set targets to realise water depth and flow velocity ranges shows the current procedures to be deficient. Based on this unfavourable evaluation, and the two-dimensional nature of habitat patterns needed to improve the conditions for stream organisms, we recommend to predict morphological developments as part of the design procedures for lowland stream restoration in the Netherlands.
Stream restoration efforts often aim at creating new unconstrained meandering channels without we... more Stream restoration efforts often aim at creating new unconstrained meandering channels without weirs and bank revetments. In reconstructed streams, the initial morphological response of the new streams is often rapid, until a dynamic equilibrium is reached. Here we report on a chute cutoff that occurred within 3 months after realization of a stream restoration project, caused by a plug bar that formed in response to a backwater effect. The temporal evolution of the morphology of both the new and the old channels was monitored over a period of nearly 8 months, including precutoff conditions. The observations can be separated into three stages. Stage 1 is the initial period leading to cutoff vulnerability, stage 2 is the actual cutoff, and stage 3 is the morphological adjustment in response to the cutoff. In stage 1, a plug bar was deposited in one of the channel bends. Hydrodynamic model results show the location of the plug bar coincides with a region where bed shear stress decreased in downstream direction due to backwater. Longitudinal channel bed profiles show that the channel slope decreased soon after channel reconstruction. Hence, sediment from upstream was available to form the plug bar. After the plug bar was deposited, an embayment formed in the floodplain at a location where the former channel was located (stage 2). The former channel was filled with sediment prior to channel construction. It is likely that the sediment at this location was less consolidated, and therefore, prone to erosion. The chute channel continued to incise and widen into the floodplain and, after 6 months, acted as the main channel, conveying the discharge during the majority of time (stage 3). The cutoff channel gradually continued to fill with sediment, from the moment the plug bar formed until the chute channel incised into the floodplain. Sedimentary successions of the deposited material show upward fining, which is in agreement with observations of chute cutoffs in rivers. Although the artificial setting limits the degree in which the observed processes can be projected on natural rivers, the observations prompt to investigate the role of backwater effects in natural chute cutoff initiation.
Alternate bars in rivers and streams develop as a result of differences in length scales involved... more Alternate bars in rivers and streams develop as a result of differences in length scales involved in the adjustment of flow and sediment transport to irregularities of the bed. The amount of field evidence supporting theoretical insights is highly limited. Here, we present results from a large-scale field experiment in a 600 m long straight reach. Over a period of almost 3 years, the channel was allowed to evolve autogenously from initially flat bed conditions, subject to discharge variation. Alternate bars developed within 8 months from the start of the experiment. The initial stages of bar development included bar growth, both in wavelength and amplitude, and bar migration. The latter was too limited to classify the bars as being migrating bars; therefore, we classify the bars as nonmigrating bars. Toward the end of the experiment, the regular alternate bar pattern evolved into an irregular pattern and bar amplitude started to decrease. From the start of the experiment we observed a declining channel slope, from 1.8 m km−1 initially to 0.9 m km−1 halfway the experiment, after which it stabilized. We applied two bar theories to establish their predictive capacity. Both bar theories predicted the development of alternate bars under the constructed channel conditions. In response to the declining channel slope, both theories predicted a decreasing likelihood for the development of alternate bars. Our study shows that under field conditions, the applied bar theories may predict the initial stages of bed development.
With the aim to establish and understand morphological changes in response to channel reconstruct... more With the aim to establish and understand morphological changes in response to channel reconstruction, a detailed monitoring plan was implemented in a lowland stream called Lunterse Beek, located in the Netherlands. Over a period of almost 2 years, the monitoring programme included serial morphological surveys, continuous discharge and water level measurements, and riparian vegetation mapping, from photographs and field surveys. Morphological processes occurred mainly in the initial period, before riparian vegetation developed. The initial period was largely dominated by upstream sediment supply, which was associated with channel incision upstream from the study area. Herbaceous vegetation started to develop approximately 7 months after channel reconstruction. The monitoring period included two growing seasons. A clear increase of riparian vegetation cover from first to the second year was observed. Detailed morphological and hydrological data show a marked difference in morphological behaviour between the pre-vegetation and post-vegetation stage. A linear regression procedure was applied to relate morphological activity to time-averaged Shields stress. In the initial stage after channel reconstruction, with negligible riparian vegetation, channel morphology adjusted, showing only a weak response to the discharge hydrograph. In the subsequent period, morphological activity in the channel showed a clear relation with discharge variation. The two stages of morphological response to the restoration measures may be largely associated with the upstream sediment supply in the initial period. Riparian vegetation may have played a substantial role in stabilizing the channel banks and floodplain area, gradually restricting the morphological adjustments to the channel bed.
In analytical and numerical models of river meandering, initiation of meandering typically occurs... more In analytical and numerical models of river meandering, initiation of meandering typically occurs uniformly along the streamwise coordinate in the channel. Based on a historical analysis of the Nierskanaal, here we show how and under which circumstances meandering has initiated in isolated sections of a channel. The Nierskanaal was constructed by the end of the 18th century, as a straight channel between the river Niers and the river Meuse. The purpose of this measure was to reduce flood risk in the downstream reaches of the river Niers. The banks on the Dutch part of the channel were left unprotected and developed into a morphodynamically active channel, featuring a meandering planform and valley incision. The planform development and incision process is analysed using topographic maps and airborne LiDAR data. Meandering initiated in three sections of the channel, where the channel sinuosity developed asynchronously. Sedimentary successions in the study area show layers of iron oxide, indicating groundwater seepage from aeolian river dunes and river deposits located nearby. Only at the spots where meandering has initiated iron oxide is found close to the surface level. This provides a clue that seepage triggered bank erosion by increasing moisture content of the banks. The isolated meandering sections expanded in the longitudinal direction. Valley incision has developed in the first decades after the construction of the channel, and diminished after a gravel layer was reached. Gravel was deposited in the downstream half of the channel bed, acting as an armouring layer. The spatial variation in meandering behaviour, as observed in the Nierskanaal, justifies efforts to implement the influence of floodplain heterogeneity and the effect of seepage on bank erosion in meander models. Copyright © 2013 John Wiley & Sons, Ltd.
Within the Dutch research project 'Valley wide meander restoration' six restored streams will be ... more Within the Dutch research project 'Valley wide meander restoration' six restored streams will be monitored over a 2-year period. The monitoring program aims at understanding initial morphological processes and the associated ecological developments. The present study focuses on the morphodynamic developments that took place after the completion of one of the restoration projects. The morphology and hydraulics of the field sites are evaluated using sequential GPS-surveys, discharge records and information from sediment samples. The present contribution concerns the Hooge Raam, where a new channel is constructed parallel to the stream that was to be restored. The channel was designed to investigate the autogenous formation of a meandering channel from an initially straight reach, designed based on hydraulic geometry relations. Eight months after construction of the stream, alternating bars emerged in the downstream part of the stream. Analysis of ten successive bed level surveys about two months apart shows channel bar migration and an increase of both the bar height and their wavelengths. Bar wavelength is increasing in time until it reaches an equilibrium value. The dynamics of both migration speed and bar height respond to the dynamics in the varying discharge.
Conference Presentations by Joris Eekhout
Around 1770, a straight artificial canal (Gelderns-Nierskanaal) has been constructed between the ... more Around 1770, a straight artificial canal (Gelderns-Nierskanaal) has been constructed between the River Niers and the River Meuse, crossing the border between Germany and the Netherlands, with the purpose of reducing flood risk in the downstream reaches of the River Niers. Whereas the German part of the canal is kept straight throughout time, the Dutch part was left unprotected and developed into a morphodynamically active stream featuring a meandering planform. The current planform and in-channel morphology are analyzed using airborn LiDAR data and historical topographic maps. Around the turn of the 18th century, the first attempts were made to make detailed topographic maps. From this time on, at least 16 topographic maps of the area around the stream were made. With the use of these historical topographic maps, a reconstruction is made of the planimetric shape of the stream over a period of 240 years. The LiDAR data show old meander belts at several places around the stream. Those belts compare well with the topographic maps. The sinuosity increases from upstream to downstream. This could be a consequence of the valley slope, where the upper part is flat and the slope increases in downstream direction. Besides, the LiDAR data show that erosion resulted in an incised valley, with dimensions to 50 m in width and 6 m in depth. Both the datasets are combined to make an estimate of the historical sediment budget of the stream.
The present contribution concerns an lowland stream project (Hooge Raam), where a new channel is ... more The present contribution concerns an lowland stream project (Hooge Raam), where a new channel is constructed parallel to the stream that was to be restored. The channel was designed to investigate the autogenous formation of a meandering channel from an initially straight reach. The design was based on hydraulic geometry relations. Eight months after construction of the stream, alternating bars emerged in the downstream part. Detailed morphological surveys are performed in this section of the stream using RTK GPS-equipment. Furthermore, discharge and water level is continuously measured and sediment samples have been analysed on grain size distribution. Analysis of the morphological surveys shows the evolution of the bar wavelength into an steady state of about 70 m. The dynamics of the bar height shows correlation with the discharge hydrograph, i.e. bar height increases with increasing discharge, and contrary. Bars are migrating downstream with a speed in the order of 0.1-0.2 m/day, where migrating rates increase with increasing discharge. The field results will be evaluated using existing theory on migrating bar development. The models used to analyse migrating bar behavior will include linear stability analysis and Delft3D.
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Papers by Joris Eekhout
Conference Presentations by Joris Eekhout