The Effects of Exchange Flow on the Karst Spring Hydrograph under the Different Flow Regimes: A Synthetic Modeling Approach

In this study, a synthetic modeling approach is proposed to quantify the effect of the amount and direction of the exchange flow on the karstic spring discharge fluctuations under different hydrologic conditions corresponding to high and low flow conditions. We hypothesis that the spring discharge fluctuations constitute a valuable proxy to understand the internal processes of the karst system. An ensemble of spring hydrographs was synthetically produced to highlight the effect of exchange flow by exploring the plausible range of variability of coefficients of exchange flow, conduit diameter, and matrix hydraulic conductivity. Moreover, the change of the rate of point recharge through the karst conduit allows for the quantifying of the sensibility of the spring hydrograph to the directions of exchange flow. We show that increasing the point recharge lies to a remarkable linear recession coefficient (β) as an indication of the conduit flow regime. However, a reduction in and/or lack of the point recharge caused the recession coefficient to change to exponential (α) due to the dominant effect of the matrix restrained flow regime and/or conduit-influenced flow regime. The simulations highlight that the exchange flow process from the conduit to the matrix occurred in a short period and over a restricted part of the conduit flow regime (CFR). Conversely, the exchange flow dumped from the matrix to the conduit occurs as a long-term process. A conceptual model is introduced to compare spring hydrographs’ characteristics (i.e., the peak discharge, the volume of baseflow, and the slope of the recession curve) under the various flow conditions with the directions of the exchange flow between the conduit and the matrix.

Biogeomorphodynamic of fluvial-tidal levees and accommodation space infilling

<p>Densely populated low-lying areas are under pressure of relative sea level rise and human impacts. Low-lying areas like most of The Netherlands were built with fluvial-marine sediment supply interacting with peat and vegetation. The morphology and sedimentological architecture of such areas is controlled by initial conditions (e.g. accommodation space), boundary conditions (fluvial-tidal discharges) and internal biogeomorphodynamic feedbacks. The relative importance of these controls varies per system and we need generic rules to better understand the past and future delta and alluvial plain evolution. Here we setup novel long-term idealized morphodynamic models including stratigraphy and vegetation to unravel the effect of initial and boundary conditions in building landscape and creating complex depositional environments. Larger accommodation space creates and preserves a bayhead delta while limited space resulted in ebb-delta growth. Fluvial-tidal discharge fluctuations promote larger levees and more crevasses, contributing to floodplain accretion. The presence of sparse vegetation (i.e. trees) also contributed to floodplain infilling and created wide levees and more crevasses. On the other hand, dense vegetated floodplain inhibits levee widening and the formation of crevasses leaving the floodplain rather starved. Our results agree with the dimensions and evolution from geological reconstructions of the Rhine Delta in The Netherlands. In general, discharge fluctuations by rivers and tides, sediment delivery and (sparse) vegetation are crucial to create more land. These findings are important for the reconstruction of past environments and sediment budget estimative as well to future management of low-lying areas where raising the land-level is a challenge.</p>

Solute Transport and Transformation in an Intermittent, Headwater Mountain Stream with Diurnal Discharge Fluctuations

Time-variable discharge is known to control both transport and transformation of solutes in the river corridor. Still, few studies consider the interactions of transport and transformation together. Here, we consider how diurnal discharge fluctuations in an intermittent, headwater stream control reach-scale solute transport and transformation as measured with conservative and reactive tracers during a period of no precipitation. One common conceptual model is that extended contact times with hyporheic zones during low discharge conditions allows for increased transformation of reactive solutes. Instead, we found tracer timescales within the reach were related to discharge, described by a single discharge-variable StorAge Selection function. We found that Resazurin to Resorufin (Raz-to-Rru) transformation is static in time, and apparent differences in reactive tracer were due to interactions with different ages of storage, not with time-variable reactivity. Overall we found reactivity was highest in youngest storage locations, with minimal Raz-to-Rru conversion in waters older than about 20 h of storage in our study reach. Therefore, not all storage in the study reach has the same potential biogeochemical function and increasing residence time of solute storage does not necessarily increase reaction potential of that solute, contrary to prevailing expectations.

Changes in the Nature of Long-Term Fluctuations of Water Flow in the Subarctic Region of Yakutia: A Global Warming Perspective

Global warming has begun to affect Yakutia, an area recognized as the coldest region of the Northern Hemisphere. Previous research has indicated that the effects of global warming will be long-term. When modeling oncoming climatic changes, researchers often forecast the related water flow changes in various water bodies as well. However, these evaluations frequently differ from the actual water flow data. Thus, the current study identifies and assesses the trends in long-term flow fluctuations in the current context of global warming. This is particularly relevant in the subarctic region of Yakutia, because the local climate is not significantly influenced by anthropogenic factors. The region has an essentially uniform climate, and the river basins within the subarctic zone flow in the same direction. Thus, the study parameters can be adequately compared. Analysis of changes in the water regimen parameters of the rivers in this region is of particular importance. This study demonstrates that the changes in the long-term river regimen in the region, within approximately equivalent climate zones, have been highly and locally variable indifferent areas and time periods. However, we were unable to detect any specific consistency in these changes. The water content of almost all rivers in Yakutia has increased in the last 30 years (approximately), thus confirming general assumptions based on predictive models of climate changes; however, in most cases, such changes were the result of reaching the high-water stage of established long-term cycles. The nature of long-term fluctuations in the water flow of rivers did not change in about half of the Yakutia rivers. One water body showed a further decrease in the water content from the norm, both in terms of duration and water flow rate. Meanwhile, specific water bodies exhibited extreme long-term fluctuations, which are predicted to be a reaction to global warming. Prior to the onset of significant warming in the region, the trends of long-term water discharge fluctuations were stationary. Then, the trends of certain rivers became non-stationary due to the reasons indicated above. On their own, quantitative characteristics are insufficient to evaluate actual changes in water regimens. Moreover, evaluations obtained in the absence of a trend analysis of specific long-term discharge fluctuations, which can only be performed via graphic visualization, are most likely to be inaccurate.

Introduction

In this chapter a summary of the historical record of dam and gate failures is compiled. Once the reader is familiar with the failure statistics, a concise summary of the prerequisite topics such as press-shut and press-open devices needed to understand the analysis of gate failures is presented. Subsequently, a non-mathematical conceptual descriptions of various excitation mechanisms, such as discharge fluctuations, shear layers instability, nappe instability, coupled-mode long-span gate instability, eccentricity instability and coupled-mode Tainter gate instability that excite gate vibration are presented to set the stage for more analytical treatment in subsequent chapters.