|Title||Water level and discharge dynamics in human-affected lowland rivers|
|Author(s)||Geertsema, Tjitske J.|
|Source||Wageningen University. Promotor(en): A.J.F. Hoitink; R. Uijlenhoet, co-promotor(en): A.J. Teuling. - Wageningen : Wageningen University - ISBN 9789463951043 - 163|
Hydrology and Quantitative Water Management
|Publication type||Dissertation, internally prepared|
|Availibility||Full text available from 2020-12-20|
The current water policy aims to accelerate discharge, which is causing problems, such as eutrophication, reduced (aquatic) biodiversity, drastic decline in groundwater levels, increased water levels during floods and prolonged droughts, predominantly in urban areas and densely populated lowland areas. To address and prevent these problems, the water policy should concentrate more on water retention. Few of the existing strategies towards water retention have been evaluated regarding their functionality and quantified impacts on the regional water system. Specifically, regional water systems in lowland areas are defined as areas that freely drain in large rivers and where groundwater is an important component in the water system. In this context, the research focusses on the local and regional effects of increasing water levels and delaying discharge in regional water systems. The current practice of stream restoration involves the construction of meander bends and insertion of wood in streams, asserting local controls on stream conveyance capacity and on water level and discharge dynamics. Backwater effects resulting from sharp bends and the obstruction by wood in streams are currently poorly understood, and are considered key sources of uncertainty in predicting the effects of stream restoration and water retention measures. Regionally, streams are typically analysed in isolation from the downstream river system, with the risk of simultaneous occurrence of discharge peaks in the river and its tributaries. Within a catchment, groundwater variation near rivers is rarely being analysed as a response to surface water dynamics. Progress in understanding the impacts of stream restoration on discharge dynamics requires more knowledge about simultaneous occurrence of discharge peaks in a river and its tributaries, and a deeper understanding of response time dependence on open water-subsurface water exchange. This thesis discusses the topics wood in streams (chapter 2), flow in sharp bends (chapter 3), groundwater reaction as result of instant lowering of river water levels (chapter 4) and the simultaneous occurrence of discharge peaks in a main river and its tributaries (chapter 5).
Wood-induced backwater effects are influenced by discharge, wood configuration and channel geometry. Model simulations and field experiments in chapter 2 show that the backwater effect increases with increasing discharge up to a maximum, after which the backwater effect decreases with discharge. A sensitivity analysis of this model showed that the width of the wood patch is responsible for the peak in the backwater elevation, and the height of the wood is responsible for the discharge at which the highest backwater effect is observed. Moreover, the characteristics of this relation depend on wood configuration and channel geometry. These two components may change over time, exerting an impact on the backwater response to discharge condition. Significant morphological adjustments and wood degradation were observed within two to five years after wood placement.
Horizontal recirculation zones are common flow patterns in sharp river bends. Chapter 3 shows that these recirculation zones are not stable with increasing discharge. The Essche Stroom showed similar flow patterns for similar discharge conditions. The Dommel River, on the contrary, showed two distinct flow patterns, one with and one without flow recirculation. With a simple model for bend flow and the one-dimensional depth-\\averaged momentum equation, it is shown that for the flow pattern without flow recirculation, the flow decelerates according to what may be expected from channel curvature and cross-section variation. For the flow pattern where flow recirculation is present, the model is less easily applicable. Chapter 3 discusses the delicate processes that may or may not result in flow reversal in the Dommel River, highlighting the instability of flow recirculation zones. The water surface topography substantially influences the flow patterns in bends, however, it is too small to be directly measured with standard instrumentation, and is only locally observed in the bends. Two and three dimensional effects, such as lateral redistribution of momentum and the interaction with scour holes, will influence the horizontal flow recirculation. These effects should be incorporated in future analysis to explain the unexpected transience of flow recirculation in the Dommel River.
When the Maas water levels were instantaneously reduced, the groundwater levels responded immediately. Chapter 4 shows that the groundwater levels close to the river were reduced by 1.5 meters within 3 days. The groundwater level reduction was negligible at 2 km distance from the river. The rapid response of groundwater levels in the Maas area is encouraging to rapidly create additional groundwater storage. Groundwater storage can be created by lowering river water levels, and thereby lowering groundwater levels, before an intense rainfall event. When the groundwater storage is dynamically managed, the timing and height of the discharge peaks can be controlled, and water levels can be maintained at higher levels during periods without intense rainfall. In addition to groundwater storage, the scaling effects of hydraulic soil parameters were investigated in chapter 4, by comparing the hydraulic soil parameters of local pumping tests and of the regional river lowering in the Meuse. The available soil parameters of the pumping test are preferably used to understand the regional groundwater response using the physically-based Boussinesq equations. Chapter 4, however, shows that the measured specific yield from the pumping tests is not comparable with the specific yield obtained from the regional lowering of the Meuse water levels. The currently available data can therefore not be easily used to understand river-groundwater exchange. It can be concluded that the rapid response of groundwater levels to an instantaneous lowering of the river water levels is promising to store large amounts of water, but scale effects have to be taken into account for application at regional scale.
Extreme discharge events at the confluence of the River Meuse and two joining lowland tributaries are studied, introducing a new analysis method based on dynamic time warping. The method offers robust means of tracing individual discharge waves in discharge time series collected throughout a catchment. Chapter 5 shows that the precipitation patterns in the catchment areas are spatially correlated. Spatial correlation of the precipitation patterns is a prerequisite for simultaneous occurrence at the confluence. From a comparison of the nine highest discharge waves in the main stream and the joining lowland tributaries, it follows that the exact timing of the discharge peaks and the probability of simultaneous occurrence of discharge peaks are of little relevance to flood risk. The duration of the discharge wave in the main stream is larger compared to the time lags between discharge peaks in the main channel and the tributaries. Simultaneous occurrence of discharge peaks result in the backwater effects in the tributaries. The backwater height can increase to 1.5~m over 4~km from the confluence in the Dommel and Aa rivers.
Water level and discharge dynamics can change as result of human intervention in lowland river systems. It is important to understand the dynamics in detail before interventions are applied in order to improve, for instance, water retention. A number of interventions have been studied in detail in this thesis. It can be concluded from these studies that:
With the help of these insights, local and regional effects on water level and discharge dynamics can be better understood, and by continuous learning from the water system, a liveable environment can be maintained for humans and other biota.