Morphological processes in lowland streams : implications for stream restoration

Waterschappen in Nederland zijn constant op zoek naar kostenefficiënte alternatieven voor beekherstel. Eén van deze methoden is het verwijderen van oeverbeschoeiing, vervolgens zouden autogene morfologische processen een kronkelende loop moeten vormen. In een grootschalig veldexperiment (Hooge Raam) zijn deze morfologische processen onderzocht. De invloed van exogene processen op de initiatie van meandering zijn in detail bestudeerd in het Gelderns-Nierskanaal. In de Lunterse beek speelden stuweffecten een subtstantiële rol in de morfologische ontwikkelingen die uiteindelijk tot een bochtafsnijding hebben geleid. De term ’hermeandering´ is misleidend, vanwege de suggestie dat laaglandbeken de kenmerken zouden hebben van actief meanderende rivieren. Dit onderzoek heeft zich met name gericht op korte morfologische tijdschalen vanwege de relevantie voor het Nederlandse waterbeheer

Morfodynamiek van Nederlandse laaglandbeken

In de eerste helft van de 20ste eeuw zijn veel laaglandbeken in Nederland gekanaliseerd. Dit heeft vaak geleid tot grote veranderingen in de hydrologie en morfologie van beken, met als gevolg een sterke achteruitgang van de bijbehorende karakteristieke aquatische en terrestrische ecosystemen. In de afgelopen 25 jaar zijn de Nederlandse waterschappen begonnen met beekherstel om wateroverlast te voorkomen, verdroging terug te dringen en de ecologische (water)kwaliteit van beeksystemen te verbeteren (KRW). Eén van de meest toegepaste maatregelen hierbij is hermeandering

Het Geldernsch-Nierskanaal: hoe een recht kanaal gaat meanderen als gevolg van kwel

Aan het eind van de 18de eeuw is tussen de rivieren de Niers in Duitsland en de Maas in Nederland het Geldernsch-Nierskanaal gegraven, om piekafvoeren in het benedenstroomse deel van de Niers af te vlakken. Het Duitse deel van het kanaal is in de loop van de tijd gekanaliseerd gebleven door oeverbeschoeiing. Opvallend is dat het Nederlandse deel, dat de vrije loop werd gelaten, actief is gaan meanderen. Onderzoek laat zien dat kwel de aanzet gaf voor het meanderen

Wat doet een beek zelf na een project van beekherstel?

Al 25 jaar voeren waterbeheerders projecten uit waarbij laglandbeken een kronkelende loop terugkrijgen. Maar wat gebeurt er precies met de vorm van zo'n beek na uitvoering van het project? Hoe lang duurt de aanpassingsperiode en welke factoren hebben invloed? De conclusies van een onderzoek in de Lunterse Beek

Crop coefficients parameterization using remote sensing in basin-scale hydrological modelling

[SPA] Se utiliza un modelo hidrológico distribuido para evaluar cómo diferentes métodos influyen en la estimación de la evapotranspiración (ETc) y el balance de agua a escala de cuenca. La zona de estudio se ubica en la cuenca alta del Segura (~ 2.500 km2) en el Sureste español, zona caracterizada por una elevada heterogeneidad de condiciones del terreno y usos del suelo. El modelo hidrológico SPHY fue desarrollado y calibrado para un período de simulación de 15 años. Se emplearon cinco métodos para parametrizar el coeficiente de cultivo y se compararon los patrones espaciales y las dinámicas temporales simuladas para la evapotranspiración, la humedad del suelo y los caudales. Tres de los cinco métodos utilizan información de satélite, otro los valores del coeficiente de cultivo establecidos por FAO, y el último asume un valor constante para toda la cuenca y periodo de simulación. El análisis muestra que la generación de caudales apenas se ve afectada por la selección del método de parametrización, aunque sí es importante a la hora de calcular la evapotranspiración real, especialmente durante épocas húmedas y para los valores tabulados de FAO. [ENG] A distributed hydrologic model is used to evaluate how different methods to estimate evapotranspiration (ETc) influence the water balance and hydrologic response of basins. The study site, the upper Segura basin (~2500 km2) in Spain, is characterized by a wide range of terrain, soil, and ecosystem conditions. Input and calibration data for the hydrological model SPHY are obtained from best available data sources. The model was setup for a period of 15 year. Five crop coefficient parameterization methods are compared to explore the impact of spatial and temporal variations in these input datasets on actual evapotranspiration, streamflow and soil moisture. Methods include three that are based on remote sensing information; one based on FAO literature, and another that takes the crop coefficient equal to unity for the entire basin. The analysis shows that basin-level streamflow is hardly influenced by the choice in parameterization, but actual evapotranspiration and soil moisture are quite different, especially in the wet season and for the FAO-based method

Chute cutoff as a morphological response to stream reconstruction : The possible role of backwater

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

Morphodynamic regime change induced by riparian vegetation in a restored lowland stream

With the aim to establish and understand morphological changes in response to stream restoration measures, a detailed monitoring plan was implemented in a lowland stream called Lunterse Beek, located in the Netherlands. Over a period of 1.5 yr, the monitoring 5 included serial morphological surveys, continuous discharge and water level measurements and riparian vegetation mapping. Morphological processes occurred mainly in the initial period, before riparian vegetation development. In the subsequent period, riparian vegetation started to emerge, with a maximum coverage halfway the survey period, which coincides with the end of the summer period. Detailed morphological 10 and hydrological data show a marked difference in behaviour between the unvegetated initial stage and the vegetated final period. The riparian vegetation cover, obtained from an aerial photo, shows a strong correlation with inundation frequency. We applied linear regression to relate morphological activity to time-averaged bed shear stress. In the initial stage after construction, with negligible riparian vegetation, chan15 nel morphology adjusted without a clear response to the discharge hydrograph. In the subsequent period, morphological activity in the channel bed and bank zones showed a clear response to discharge variation. The two stages of morphological response to the restoration measures reveal the role of riparian vegetation, which acts to focus the morphodynamic developments in the main channel

Chute cutoff event in response to stream restoration

Introduction In the Netherlands, stream restoration generally refers to the construction of low-sinuosity channels. After construction, these channels typically show little morphological changes in time, mainly due to oversized cross-sections. Occasionally, pronounced morphological changes do occur, including meander cutoffs. In literature, two main types of meander cutoffs have been identified. Neck cutoffs can develop by the progressive migration of an elongated bend into itself. Chute cutoffs are shortcuts developing over a point bar within one bend (Lewis and Lewin, 1983). Neck cutoffs are more likely to occur in high-sinuosity meander bends, whereas chute cutoffs appear in meander bends featuring a moderate to low sinuosity. Here we report on a recent cutoff event that occurred right after the realization of a stream restoration project, showing a chute cutoff can develop within less than four months

Historical analysis indicates seepage control on initiation of meandering

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