Dissertation

Morphological Processes in Lowland Streams: Implications for Stream Restoration

Eekhout, Joris P. C

2014

Overview

Halfway the 20th century, groundwater management in agricultural areas led to channelisation of the majority of lowland streams in the Netherlands. This has led to degradation of the aquatic and terrestrial ecosystems, characteristic for lowland streams. Over the past 25 years, water authorities in the Netherlands have aimed at restoring these degraded streams. Historical maps show that many lowland streams consist of a meandering planform. Re-meandering is the common practice regarding stream restoration in the Netherlands. Little is known about the morphological processes following the completion of such stream restoration projects. The aim of this thesis is to characterize the morphodynamic developments of restored lowland streams, with a focus on meander processes.Three traditional stream restoration projects (Hagmolenbeek, Lunterse beek and Tungelroyse beek) were monitored during the initial two years after construction of the new channel. In these projects, the former straightened channel was replaced by a re-meandered channel. A standardized monitoring plan was implemented, which included three morphological surveys (cross-sections, with a one-year interval), sed- iment sampling (from the initial and final channel bed), habitat pattern surveys (three times per year) and continuous discharge and water level measurements. The morphological measurements revealed that morphodynamic developments are mainly concentrated in the first year following construction. Adjustment of the lon- gitudinal bed profile was the main morphological response. Structures (e.g. bridges and weirs), channel width variation and heterogeneity of the channel substrate caused channel bed incision and aggradation, and hence, channel slope adjustment. Lateral development was observed only in a limited number of channel bends, and was mainly related to floodplain heterogeneity. The habitat surveys showed the occurrence of gravel, silt, vegetation and algae on the channel bed. Morphological developments at the scale of the channel cross-section were exceptional, despite the fine sediment characteristics (median grain size 125-250 μ m). Due to these channel bed characteristics, sediment is transported during the entire year.Water authorities are constantly looking for more cost-effective alternatives for Summary re-meandering. One such method is to remove bank protection and allow autoge- nous processes to develop a sinuous planform. To study these processes, a large- scale field experiment was performed in a 600 m long straight channel reach (Hooge Raam). Over a period of almost three years, the channel was allowed to evolve au- togenously from initially flat-bed conditions, in response to a variable discharge. Alternate bars developed within eight months after the start of the experiment. The initial stages of bar development included bar growth, both in wavelength and in amplitude, and limited bar migration. Towards the end of the experiment, the alternate bar pattern became increasingly irregular and bar amplitudes started to decrease. During the experiment, the channel slope declined from 1.8 m km − 1 to 0.9 m km − 1 . Two bar theories were applied to establish their predictive capacity. Both bar theories predicted the development of alternate bars under the constructed channel conditions and a decreasing likelihood for the development of alternate bars in response to the declining channel slope. This study shows that it is unlikely that the typical sinuous planform in lowland streams, as observed from historical maps, is the result from autogenous processes alone. Exogenous processes, such as a local seepage or floodplain heterogeneity, may be needed to achieve channel sinuosity.Processes of meander initiation were studied focussing on a man-made canal (Gelderns-Nierskanaal). The canal was constructed near the end of the 18th century, as a straight channel between the river Niers (Germany) and the river Meuse (The Netherlands). The banks on the Dutch part of the channel were left unprotected and developed into an active meandering channel, featuring meander development and valley incision. These processes were analysed using historical topographic maps and recent airborne LiDAR data. Meandering initiated in three sections of the chan- nel, where the channel sinuosity developed asynchronously over time. Sedimentary successions in the study area show layers of iron oxide, indicating groundwater seepage from nearby located higher elevated terrains. Only at the spots where meandering had initiated, iron oxide was found close to the surface level. This pro- vides a clue that seepage triggered bank erosion by increasing the moisture content of the banks. The spatial variation in meandering behaviour, as observed in this channel, justifies efforts to implement the influence of floodplain heterogeneity and the effect of seepage on bank erosion in meander models.Backwater effects played a substantial role in the development of a chute cutoff, which occurred after the realization of a stream restoration project in the Lunterse beek. In this stream restoration project, additional measurements were performed, next to the standardized monitoring plan. Over a period of almost 2 years, the additional monitoring included fourteen high-resolution morphological surveys and 156 riparian vegetation mapping. Prior to the cutoff, a plug bar was deposited in the bend to be cutoff. Hydrodynamic model results were used to infer that the plug bar was related to a backwater effect, causing a drop of sediment transport capacity. Upstream from the plug bar, an embayment formed in the floodplain at a location where the former channel was located. The former channel was filled with sediment prior to channel construction, resulting in a less consolidated area of the floodplain. Consequently, it was prone to erosion. The chute channel continued to incise and widen into the floodplain and, after 6 months, acted as the main channel. These results show how upstream sediment supply and backwater effects are involved in the processes that lead to the occurrence of a chute cutoff.The chute cutoff and several additional morphological adjustments occurred in a period without riparian vegetation in the Lunterse beek. Herbaceous vegetation started to develop approximately 7 months after construction, with a maximum coverage by the end of the summer period. Detailed morphological and hydro- logical data show a marked difference in morphological behaviour between the pre-vegetation and post-vegetation stage. A linear regression procedure was ap- plied to relate morphological activity to time-averaged Shields stress. In the initial stage after construction, with negligible riparian vegetation, channel morphology adjusted with a weak response to the discharge hydrograph. In the subsequent period, morphological activity in the showed a clear relation to discharge varia- tion. The two stages of morphological response to the restoration measures may be caused by riparian vegetation development, although additional field measure- ments are needed to substantiate these findings.Lowland streams are small rivers, consisting of a sinuous channel pattern, a sandy channel bed and a gentle slope (<1 m km − 1 ). It is likely that the sinuous planform observed on historical maps is a result from exogenous influences, rather than autogenous processes. In general, the studied lowland streams show little mor- phological activity. The observed morphodynamics occurred mainly in the first year after construction and were caused by backwater effects and floodplain heterogene- ity. After initial morphological adjustments, the channel planform remained stable. The rapid establishment towards an equilibrium state of the channel planform is at odds with the view on lowland streams as small rivers migrating actively in their own deposits. The term re-meandering may be misleading, because of the conno- tation with channel meandering in time. The steadiness of the channel planforms has not been extensively demonstrated over geological time scales. This study has focused on short-term observations, since engineering time scales are more relevant for stream restoration practitioner

• Publisher: ProQuest Dissertations & Theses
• Subject: 18th century / Aquatic plants / Construction / Creeks & streams / Flow velocity / Grain size / Groundwater / Hydrology / Marine ecology / Morphology / Rivers / Sedimentation & deposition / Sediments / Soil erosion / Vegetation / Water Resources Management
• ISBN: 9798708790071