From hyporheic science to river restoration:the contribution of physically-based hydrological models

River managers need practical tools to promote appropriately hyporheic functions in restoration schemes. Hyporheic ecosystems are largely controlled by hydrological processes, which may be simulated through physically-based models. Despite their apparent limitations (sophisticated, data-hungry, and computationally demanding), these models offer substantial advantages that may pertain to the operational level of river management: representation of specific landscapes, process-based sensitivity analyses, and alternative restoration scenario testing. This presentation builds on idealized and field-based studies, as well as literature examples, to discuss how physically-based models of hyporheic exchange can be fully exploited for restoration purposes. Results suggest that these models are best suited to map spatial patterns of exchange at the sediment-water interface, rather than to estimate HEF fluxes and residence times. Because subsurface data are typically unavailable or sparse, an approach combining high-resolution topographic data combined with a sensitivity analysis appears as a promising approach for: (a) delineating potential areas of upwelling and downwelling along longitudinal and lateral channel sections; and (b) highlighting potential differences in HEF characteristics between reaches. To achieve satisfactory simulations, however, the challenge lies in the topographic representation of the channel and the discretization of the mesh. Here, common pitfalls are identified, and guidance to overcome these is provided

Typology and metrics in hyporheic hydrology

This paper evaluates the perceptions and conceptual basis in hyporheic flow (HF) modeling. The combination of flow processes at several scales may currently not be well described by mechanistic models which are often limited by the calibration data and the modeler's understanding of the system. For example, transient storage models calibrated on tracer tests may underestimate long residence time (days) flow paths; the Darcy approach based on hydraulic heads tends to underestimate the shorter ones (hours). This has important implications for predicting contaminant fluxes through the hyporheic zone at the catchment scale. A set of standard measurements (metrics) in the hyporheic physical environment, combined with a typology of HF systems are discussed as promising lines to predict the relative magnitude of changes of residence time, flow budgets and spatial extent of flow paths. Since models do not need to be strictly realistic in terms of structure, the potential for hyporheic metrics to inspire simplified models is stressed. Such an approach may be appealing to ecologists who must integrate the hydrological component in more complex systems. These metrics are discussed on the basis of both experimental evidence and theoretical studies. Because some metrics may only apply to specific environments, a typology of hyporheic systems is presented which is a function of the physiographic setting, including scale and dynamics, and the biogeochemical problem addressed. There has been speculation that stream morpho-dynamic controls on HF deal more with shallow exchange, which is often easier to measure. Such a bias may be addressed by building on the hydrogeological experience and concepts in dealing with heterogeneous media, in particular in groundwater contaminant literature. More widely, environmental modelling principles such as equifinality and predictive uncertainty, which are often ignored in hyporheic studies, are discussed on the basis of generic HF models. The scope for hyporheic metrics and a physical typology of hyporheic systems is illustrated by the study case of a groundwater-fed river in the UK. A range of methodological tools, such as electrical resistivity profiles, environmental and artificial tracers, piezometric network development, has been applied from the bedform to the kilometre scale

Elevation and volume changes of seven Dickson Land glaciers, Svalbard, 1960–1990–2009

Melting Svalbard glaciers have been recognized as an early indicator of climate change. Large parts of Svalbard remain insufficiently investigated, including Dickson Land, in the quasi-continental interior of Svalbard. In this study, elevation and volume changes of seven glaciers located in the Pyramiden region are assessed by analysing contour lines from 1960 topographic maps and photogrammetrically derived 1990 and 2009 digital elevation models. Mass loss was documented for all seven glaciers. In the period 1960–1990, their average elevation change rate was −0.49 m a−1, while in the more recent period, 1990–2009, it was more negative at −0.78 m a−1, caused by a significant equilibrium line altitude shift with post-1990 rise in summer temperatures. Large variation in elevation change rates between individual glaciers was found and is attributed mainly to aspect and hypsometry. This highlights the importance of choosing a representative sample when investigating mass balance of whole regions. Evidence of a rapid increase in thinning rates in the upper parts of the studied glaciers, linked to decreasing albedo in former accumulation zones, was also found