54 research outputs found
Frictional interactions between tidal constituents in tide-dominated estuaries
When different tidal constituents propagate along an estuary, they interact because of the presence of nonlinear terms in the hydrodynamic equations. In particular, due to the quadratic velocity in the friction term, the effective friction experienced by both the predominant and the minor tidal constituents is enhanced. We explore the underlying mechanism with a simple conceptual model by utilizing Chebyshev polynomials, enabling the effect of the velocities of the tidal constituents to be summed in the friction term and, hence, the linearized hydrodynamic equations to be solved analytically in a closed form. An analytical model is adopted for each single tidal constituent with a correction factor to adjust the linearized friction term, accounting for the mutual interactions between the different tidal constituents by means of an iterative procedure. The proposed method is applied to the Guadiana (southern Portugal-Spain border) and Guadalquivir (Spain) estuaries for different tidal constituents (M2, S2, N2, O1, K1) imposed independently at the estuary mouth. The analytical results appear to agree very well with the observed tidal amplitudes and phases of the different tidal constituents. The proposed method could be applicable to other alluvial estuaries with a small tidal amplitude-to-depth ratio and negligible river discharge.info:eu-repo/semantics/publishedVersio
Extension of the general unit hydrograph theory for the spread of salinity in estuaries
From both practical and theoretical perspectives, it is essential to be able to express observed salinity distributions in terms of simplified theoretical models, which enable qualitative assessments to be made in many problems concerning water resource utilization (such as intake of fresh water) in estuaries. In this study, we propose a general and analytical salt intrusion model inspired by Guo's general unit hydrograph theory for flood hydrograph prediction in a watershed. To derive a simple, general and analytical model of salinity distribution, we first make four hypotheses on the longitudinal salinity gradient based on empirical observations; we then derive a general unit hydrograph for the salinity distribution along a partially mixed or well-mixed estuary. The newly developed model can be well calibrated using a minimum of three salinity measurements along the estuary axis and does converge towards zero when the along-estuary distance approaches infinity asymptotically. The theory has been successfully applied to reproduce the salt intrusion in 21 estuaries worldwide, which suggests that the proposed method can be a useful tool for quickly assessing the spread of salinity under a wide range of riverine and tidal conditions and for quantifying the potential impacts of human-induced and natural changes.51979296; 52279080; 2019ZT08G090; 440001-2023-10716; LA/P/0069/2020info:eu-repo/semantics/publishedVersio
Seasonal behaviour of tidal damping and residual water level slope in the Yangtze River estuary: identifying the critical position and river discharge for maximum tidal damping
As a tide propagates into the estuary, river discharge affects tidal damping, primarily via a friction term,
attenuating tidal motion by increasing the quadratic velocity in the numerator, while reducing the effective friction
by increasing the water depth in the denominator. For the
first time, we demonstrate a third effect of river discharge
that may lead to the weakening of the channel convergence
(i.e. landward reduction of channel width and/or depth). In
this study, monthly averaged tidal water levels (2003ā2014)
at six gauging stations along the Yangtze River estuary are
used to understand the seasonal behaviour of tidal damping and residual water level slope. Observations show that
there is a critical value of river discharge, beyond which
the tidal damping is reduced with increasing river discharge.
This phenomenon is clearly observed in the upstream part
of the Yangtze River estuary (between the Maanshan and
Wuhu reaches), which suggests an important cumulative effect of residual water level on tideāriver dynamics. To understand the underlying mechanism, an analytical model has
been used to quantify the seasonal behaviour of tideāriver
dynamics and the corresponding residual water level slope
under various external forcing conditions. It is shown that a
critical position along the estuary.info:eu-repo/semantics/publishedVersio
Learning to Break the Loop: Analyzing and Mitigating Repetitions for Neural Text Generation
While large-scale neural language models, such as GPT2 and BART, have
achieved impressive results on various text generation tasks, they tend to get
stuck in undesirable sentence-level loops with maximization-based decoding
algorithms (\textit{e.g.}, greedy search). This phenomenon is counter-intuitive
since there are few consecutive sentence-level repetitions in human corpora
(e.g., 0.02\% in Wikitext-103). To investigate the underlying reasons for
generating consecutive sentence-level repetitions, we study the relationship
between the probabilities of the repetitive tokens and their previous
repetitions in the context. Through our quantitative experiments, we find that
1) Language models have a preference to repeat the previous sentence; 2) The
sentence-level repetitions have a \textit{self-reinforcement effect}: the more
times a sentence is repeated in the context, the higher the probability of
continuing to generate that sentence; 3) The sentences with higher initial
probabilities usually have a stronger self-reinforcement effect. Motivated by
our findings, we propose a simple and effective training method \textbf{DITTO}
(Pseu\underline{D}o-Repet\underline{IT}ion
Penaliza\underline{T}i\underline{O}n), where the model learns to penalize
probabilities of sentence-level repetitions from pseudo repetitive data.
Although our method is motivated by mitigating repetitions, experiments show
that DITTO not only mitigates the repetition issue without sacrificing
perplexity, but also achieves better generation quality. Extensive experiments
on open-ended text generation (Wikitext-103) and text summarization
(CNN/DailyMail) demonstrate the generality and effectiveness of our method.Comment: Accepted by NeurIPS 2022. Code is released at
https://github.com/Jxu-Thu/DITT
Effects of tidal-forcing variations on tidal properties along a narrow convergent estuary
A 1D analytical framework is implemented in a narrow convergent estuary that is 78 km in length (the Guadiana, Southern Iberia) to evaluate the tidal dynamics along the channel, including the effects of neap-spring amplitude variations at the mouth. The close match between the observations (damping from the mouth to ā¼ 30 km, shoaling upstream) and outputs from semi-closed channel solutions indicates that the M2 tide is reflected at the estuary head. The model is used to determine the contribution of reflection to the dynamics of the propagating wave. This contribution is mainly confined to the upper one third of the estuary. The relatively constant mean wave height along the channel (<ā10% variations) partly results from reflection effects that also modify significantly the wave celerity and the phase difference between tidal velocity and elevation (contradicting the definition of an āidealā estuary). Furthermore, from the mouth to ā¼ 50 km, the variable friction experienced by the incident wave at neap and spring tides produces wave shoaling and damping, respectively. As a result, the wave celerity is largest at neap tide along this lower reach, although the mean water level is highest in spring. Overall, the presented analytical framework is useful for describing the main tidal properties along estuaries considering various forcings (amplitude, period) at the estuary mouth and the proposed method could be applicable to other estuaries with small tidal amplitude to depth ratio and negligible river discharge.info:eu-repo/semantics/publishedVersio
A data-driven model to quantify the impact of river discharge on tide-river dynamics in the Yangtze River estuary
Understanding the role of river discharge on tide-river dynamics is of essential importance for sustainable water management (flood control, salt intrusion, and navigation) in estuarine environments. It is well known that river discharge impacts fundamental tide-river dynamics, especially in terms of subtidal (residual water levels) and tidal properties (amplitudes and phases for different tidal constituents). However, the quantification of the impact of river discharge on tide-river dynamics is challenging due to the complex interactions of barotropic tides with channel geometry, bottom friction, and river discharge. In this study, we propose a data-driven model to quantify the impact of river discharge on tide-river dynamics, using water level time series data collected through long-term observations along an estuary with substantial variations in river discharge. The proposed model has a physically-based structure representing the tide-river interaction, and can be used to predict water level using river discharge as the sole predictor. The satisfactory correspondence of the model outputs with measurements at six gauging stations along the Yangtze River estuary suggest that the proposed model can serve as a powerful instrument to quantify the impacts of river discharge on tide-river dynamics (including time-varying tidal properties and tidal distortion), and separate the contribution made by riverine and tidal forcing on water level. The proposed approach is very efficient and can be applied to other estuaries showing considerable impacts of river discharge on tide-river dynamics.info:eu-repo/semantics/publishedVersio
GPT4Video: A Unified Multimodal Large Language Model for lnstruction-Followed Understanding and Safety-Aware Generation
While the recent advances in Multimodal Large Language Models (MLLMs)
constitute a significant leap forward in the field, these models are
predominantly confined to the realm of input-side multimodal comprehension,
lacking the capacity for multimodal content generation. To fill this gap, we
present GPT4Video, a unified multi-model framework that empowers Large Language
Models (LLMs) with the capability of both video understanding and generation.
Specifically, we develop an instruction-following-based approach integrated
with the stable diffusion generative model, which has demonstrated to
effectively and securely handle video generation scenarios. GPT4Video offers
the following benefits: 1) It exhibits impressive capabilities in both video
understanding and generation scenarios. For example, GPT4Video outperforms
Valley by 11.8\% on the Video Question Answering task, and surpasses NExt-GPT
by 2.3\% on the Text to Video generation task. 2) it endows the LLM/MLLM with
video generation capabilities without requiring additional training parameters
and can flexibly interface with a wide range of models to perform video
generation. 3) it maintains a safe and healthy conversation not only in
output-side but also the input side in an end-to-end manner. Qualitative and
qualitative experiments demonstrate that GPT4Video holds the potential to
function as a effective, safe and Humanoid-like video assistant that can handle
both video understanding and generation scenarios
- ā¦