CSDMS 2013 annual meeting poster Jun Cheng: Difference between revisions

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'''Abstract:'''
'''Abstract:'''


On earth, landscape morphology is mainly controlled by rivers evolutions and their interactions with hillslopes. But hydrographic network may be re-organized by stream capture and modify deeply the relief. This transition may be induced by several mechanisms (diversion, headward erosion, avulsion, or subterranean filling up). It has interested numerous scientists since a long time (Davis 1895, Blache 1943, Lesson-Quinif 2001 & Le Roux-Harmand 1997-2009…). Here we focus on stream piracies by headward erosion, when an actively eroding low level stream (called the captor) encroaches on the drainage of a nearby stream flowing at a higher level (called the diverter) and diverts part of the water of the higher stream.
Breaking waves, especially plunging breakers, generate intense turbulence and is crucial in dissipating incident wave energy, suspending and transporting sediment in the surf zone. Therefore quantifying breaking-induced turbulence kinetic energy (TKE) is essential in understanding surf zone processes. Surf zone hydrodynamic data collected at the Large-scale Sediment Transport Facility (LSTF) at the U.S. Army Engineer Research and Development center were used here. One LSTF case, with irregular waves (3 s peak period), is examined here. This case resulted in dominantly plunging type of breaker. Waves and currents were measured simultaneously at 10 cross-shore locations and throughout the water column, with a sampling rate of 20 Hz. In order to separate orbital wave motion from turbulent motion, an adaptive moving average filter is developed, involving a 5-point moving average, with additional 3-point moving average at sections with more fluctuations. This adaptive moving average filter is able to maintain more wave energy as compared with the results from 7-point moving average, while resolve more turbulence energy as compared with the result from 5-point moving average. 
During the last decades, several landscapes evolution models (LEM) have been developed to quantify the topography evolution with diffusion and advection equations. These models play an important role in sharpening our thinking to better understand the interaction between landscape evolution processes. LEM were developed basically to simulate erosion, tectonic and climate at different scales of time and space. But, these models were not designed to describe specific mechanisms as the stream capture. It’s one of the aims of this work to evaluate LEM for this purpose.
    The TKE was calculated based on the resolved turbulence. Large TKE was generated at the water surface associated with wave breaking and dissipated rapidly downward. The TKE decreased nearly one order of magnitude downward within 15 cm. The TKE reached a minimum value at approximately 50%-80% of the water depth, and increased towards the bottom due to the generation of bed-induced turbulence. The TKE flux during wave crest and tough indicate that, at the bottom and middle layers of the water column, the TKE is transported dominantly onshore, while for the top layer, it is transported mostly offshore.
In this paper, we develop a 1D model based on LEM equations to investigate the stream piracy by headward erosion responses to climatic or tectonic changes. This model incorporates the most common equations used in quantitative geomorphology; diffusion in hillslope, advection in river (detachment-limited mode) and an inequality based on slope and drainage area for the limit between these two domains (Montgomery and Dietrich, 1988). First, simulations on analytical cases highlight the stream head progression mechanism, and the results indicate that this progression rate is mainly controlled by the slope at the captor source. Consequently, the aggradation of the diverter or (and) the incision of the captor accelerate the process. Then, a predictive study with an improved version of GOLEM (software developed by Tucker & Slingerland in 1994) on the Meuse basin shows that several piracies may probably occur in the future. A comparison with the 1D model gives similar results.  
The simplicity and the flexibility of the 1D model allow complex simulations in the Meuse basin taking into account: lithological differences of outcropping layers, Meuse deposition tendency, etc. Once the 2D simulations or topography analysis locate potential captures, 1D simulation may intensively be used, as it presents many advantages; weak execution time, simple limits conditions setting, less time for data preparation, etc. Consequently, a sensitivity analysis to estimate piracies ages is realized with the developed 1D model.


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Revision as of 08:58, 28 March 2013

Presentation provided during CSDMS annual meeting 2013

Cross-shore and Vertical Distribution of Turbulence Kinetic Energy in the surf zone generated by plunging breakers

Jun Cheng, The Universtiy of South Florida Tampa Florida, United States. jun@mail.usf.edu
Ping Wang, The University of South Florida Tampa Florida, United States. pwang@usf.edu

Abstract:

Breaking waves, especially plunging breakers, generate intense turbulence and is crucial in dissipating incident wave energy, suspending and transporting sediment in the surf zone. Therefore quantifying breaking-induced turbulence kinetic energy (TKE) is essential in understanding surf zone processes. Surf zone hydrodynamic data collected at the Large-scale Sediment Transport Facility (LSTF) at the U.S. Army Engineer Research and Development center were used here. One LSTF case, with irregular waves (3 s peak period), is examined here. This case resulted in dominantly plunging type of breaker. Waves and currents were measured simultaneously at 10 cross-shore locations and throughout the water column, with a sampling rate of 20 Hz. In order to separate orbital wave motion from turbulent motion, an adaptive moving average filter is developed, involving a 5-point moving average, with additional 3-point moving average at sections with more fluctuations. This adaptive moving average filter is able to maintain more wave energy as compared with the results from 7-point moving average, while resolve more turbulence energy as compared with the result from 5-point moving average. 

    The TKE was calculated based on the resolved turbulence. Large TKE was generated at the water surface associated with wave breaking and dissipated rapidly downward. The TKE decreased nearly one order of magnitude downward within 15 cm. The TKE reached a minimum value at approximately 50%-80% of the water depth, and increased towards the bottom due to the generation of bed-induced turbulence. The TKE flux during wave crest and tough indicate that, at the bottom and middle layers of the water column, the TKE is transported dominantly onshore, while for the top layer, it is transported mostly offshore.

* Please acknowledge the original contributors when you are using this material. If there are any copyright issues, please let us know and we will respond as soon as possible.

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