Rapid morphological evolution during beach breaching and closure at a bar-built estuary

Mapping Intimacies ◽

10.5194/egusphere-egu21-219 ◽

2021 ◽

Author(s):

Mara Orescanin ◽

Tyonna McPherson ◽

Paul Jessen

Keyword(s):

Sediment Transport ◽

River Discharge ◽

Morphological Evolution ◽

Water Levels ◽

Low Flow ◽

Long Term Stability ◽

Erosion Rates ◽

Discharge Rates ◽

Elevation Changes ◽

Elevated Water

<p>The Carmel River runs 58 km from the Santa Lucia Mountains through the Carmel Valley eventually entering a lagoon at Carmel River State Beach near Carmel, California, USA. During the dry summer months, the lagoon is closed, with no connection to the coastal ocean.&#160; However, during the wet winter months, the river often breaches through the lagoon allowing water to freely flow between the river and Carmel Bay. Sediment transport, in part owing to river discharge and in part owing to ocean forcing (tides and waves), contributes heavily to whether the lagoon is open or closed: when there are low flow conditions, waves and tides can decrease flow rates in the breach, allowing sediment to settle. The sediment budget is expected to be a closed system, owing to the rocky headlands and long-term stability (no yearly regression or transgression) of the shoreline, despite managed attempts to control breach and closure timing. However, it is currently unknown 1) how velocity profiles evolve during breaching, and 2) how much sediment moves during such an event. The hypothesis is that the breach mouth can completely disappear and re-emerge over a single breach-closure cycle, leading to meter-scale daily accretion and erosion rates of berm height if berm elevation is significantly lower than the expected steady-state berm height. Furthermore, it is hypothesized that during active breaching, discharge rates through the breach channel are larger than upstream river discharge rates owing to elevated water levels within the back lagoon. This study uses a RiverSurveyor M9 Acoustic Doppler Profiler to measure outflow discharge and GPS topographic surveys to quantify elevation changes. A velocity profile can be built which will estimate the sediment transport potential within the breach. The information obtained will help identify and better understand the river discharge thresholds which contribute to frequent breaching as well as estimates of morphological evolution during breaching, which are currently unknown, and can assist in determining likelihood of successful managed breaching and closure events.&#160;</p>

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Seasonal Tidal Dynamics in the Qiantang Estuary: The Importance of Morphological Evolution

Frontiers in Earth Science ◽

10.3389/feart.2021.782640 ◽

2021 ◽

Vol 9 ◽

Author(s):

Dongfeng Xie ◽

Zheng Bing Wang

Keyword(s):

River Discharge ◽

Morphological Changes ◽

Morphological Evolution ◽

Sediment Accumulation ◽

Water Levels ◽

Tidal Range ◽

Tidal Dynamics ◽

Bathymetric Data ◽

Bed Erosion

Despite the increasing number of studies on the river-tide interactions in estuaries, less attention has been paid to the role of seasonal morphological changes on tidal regime. This study analyzes the seasonal interplay of river and tide in the Qiantang Estuary, China, particularly focusing on the influences of the active morphological evolution induced by the seasonal variation of river discharge. The study is based on the high and low water levels at three representative stations along the estuary and daily river discharge through 2015, an intermediate flow year in which a typical river flood occurred, as well as the bathymetric data measured in April, July and November, 2015. The results show strong seasonal variations of the water level in addition to the spring-neap variation. These variations are obviously due to the interaction between river discharge and tide but can only be fully explained by including the effect of morphological changes. Two types of the influences of the variation of the river discharge on the tidal dynamics in the estuary can be distinguished: one is immediately induced by the high flow and the other continues for a much longer period because of the bed erosion and the following bed recovery. Tidal range in the upper reach can be doubled after the flood because of bed erosion and then decrease under normal discharge periods due to sediment accumulation. Over a relatively short term such as a month or a spring-neap tidal cycle, there exist good relationships between the tidal range, tidal amplification in the upper reach and the tidal range at the mouth, and between the hydraulic head over the upper and lower reaches. Such relationships are unclear if all data over the whole year are considered together, mainly because of the active morphological evolution.

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The Effect of Wave-Induced Current and Coastal Structure on Sediment Transport at the Zengwen River Mouth

Journal of Marine Science and Engineering ◽

10.3390/jmse9030333 ◽

2021 ◽

Vol 9 (3) ◽

pp. 333

Author(s):

Chun-Hung Pao ◽

Jia-Lin Chen ◽

Shih-Feng Su ◽

Yu-Ching Huang ◽

Wen-Hsin Huang ◽

...

Keyword(s):

Sediment Transport ◽

Finite Difference ◽

Finite Volume ◽

Morphological Evolution ◽

River Mouth ◽

Water Levels ◽

Coastal Structures ◽

Sea Bed ◽

Residual Transport ◽

Wave Induced

The mechanisms that control estuarine sediment transport are complicated due to the interaction between riverine flows, tidal currents, waves, and wave-driven currents. In the past decade, severe seabed erosion and shoreline retreat along the sandy coast of western Taiwan have raised concerns regarding the sustainability of coastal structures. In this study, ADCPs(Acoustic Doppler Current Profiler) and turbidity meters were deployed at the mouth of the Zengwen river to obtain the time series and the spatial distribution of flow velocities and turbidity during the base flow and flood conditions. A nearshore circulation model, SHORECIRC, has been adapted into a hybrid finite-difference/finite-volume, TVD (Total Variation Diminishing)-type scheme and coupled with the wave-spectrum model Simulating Waves Nearshore (SWAN). Conventional finite-difference schemes often produce unphysical oscillations when modeling coastal processes with abrupt bathymetric changes at river mouths. In contrast, the TVD-type finite volume scheme allows for robust treatment of discontinuities through the shock-capturing mechanism. The model reproduces water levels, waves, currents observed at the mouth of the Zengwen River reasonably well. The simulated residual sediment transport patterns demonstrate that the transport process at the river mouth is dominated by the interaction of the bathymetry and wave-induced currents when the riverine discharge was kept in reservoirs. The offshore residual transport causes erosion at the northern part of the river mouth, and the onshore residual transport causes accretion in the ebb tidal shoals around the center of the river mouth. The simulated morphological evolution displays significant changes on shallower deltas. The location with significant sea bed changes is consistent with the spot in which severe erosion occurred in recent years. Further analysis of morphological evolution is also discussed to identify the role of coastal structures, for example, the extension of the newly constructed groins near the river mouth.

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Dams induced stage–discharge relationship variations in the upper Yangtze River basin

Hydrology Research ◽

10.2166/nh.2015.010 ◽

2015 ◽

Vol 47 (1) ◽

pp. 157-170 ◽

Cited By ~ 17

Author(s):

Xuefei Mei ◽

Zhijun Dai ◽

Wen Wei ◽

Jinjuan Gao

Keyword(s):

River Basin ◽

River Discharge ◽

Yangtze River ◽

Yangtze River Basin ◽

Back Propagation ◽

Water Levels ◽

Low Flow ◽

Upper Yangtze River ◽

Rating Curves ◽

The Upper Yangtze River

Although stage–discharge relationships are crucial for discharge estimations and hydrological analyses, few efforts have been taken to assess their temporal alterations in the context of dam regulation. Here, the upper Yangtze River basin serves as an example to demonstrate the influence of hydraulic structures on stage–discharge relationships evolution. Daily records of water level and river discharge from 1950 to 2013 at Yichang hydrometric station were grouped and analyzed. Back-propagation artificial neural network was used to model the stage–discharge relationships. The obtained curves revealed substantial shifts since the Gezhouba Dam (GD) and Three Gorges Dam (TGD) were put into practice sequentially. In low flow scenarios, the decline of water levels due to GD and TGD regulation were variable with river discharge, whereas in normal flow scenarios, the rating curves indicate equilibrium state with almost the same slopes regardless of GD and TGD influence. In high flow scenarios, the rating curves representing natural condition, GD, and TGD regulation intersect with each other. Moreover, the detected changes in stage–discharge relationship were mainly in response to dam regulation, channel erosion and sand exploitation, while irrelevant to precipitation variability. The contribution of sand mining, GD regulation, and TGD regulation on rating curve variations at Yichang station were 36%, 11%, and 53%, respectively.

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Spatial and temporal variations of hydrodynamics and sediment dynamics in Indus River Estuary,Pakistan

Global NEST Journal ◽

10.30955/gnj.003333 ◽

2021 ◽

Keyword(s):

Sediment Transport ◽

River Discharge ◽

Saltwater Intrusion ◽

River Estuary ◽

Dry Season ◽

Water Levels ◽

Spatial And Temporal Variation ◽

Indus River ◽

Wet Season ◽

Dry Seasons

<p>Field investigations were conducted to study the seasonal variation of hydrodynamics and sediment transport in Indus River Estuary (IRE), Pakistan. The data of water levels, currents, salinity, and suspended sediment concentration (SSC) were collected hourly covering both wet and dry seasons. Tidal amplitudes were higher near the mouth than those at the middle and upper estuary. The ebb phase lasted longer than that of the flood during the wet season. The asymmetric tidal pattern with higher ebb velocity was observed during the wet season. A slight difference in current velocity was found during the dry season. The flood currents were higher at middle estuary than those in wet season. During the wet season, salinity variation within a tidal cycle slightly increased from the upper estuary to the mouth. Salinity was substantially higher during the dry season than the wet season at all three stations, with the absence of the flood-ebb variation, showing a strong saltwater intrusion. The SSC data revealed that the sediments were mainly brought into the estuary by freshwater discharge during the wet season. Sediment re-suspension process persists during the dry season, due to the tidal currents. A stronger saltwater intrusion occurred in the dry season due to weak river discharge. An estuarine turbidity maximum zone was formed near station-2 due to the combined effects of tides, river discharge and saltwater intrusion. Overall, field observations have shown a significant spatial and temporal variation in flood/ebb and wet/dry seasons for hydrodynamics and sediment transport in IRE.</p>

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The Response of Turbidity Maximum to Peak River Discharge in a Macrotidal Estuary

Water ◽

10.3390/w13010106 ◽

2021 ◽

Vol 13 (1) ◽

pp. 106

Author(s):

Yuhan Yan ◽

Dehai Song ◽

Xianwen Bao ◽

Nan Wang

Keyword(s):

River Discharge ◽

Recovery Time ◽

Morphological Evolution ◽

Spring Tide ◽

River Estuary ◽

Sediment Concentration ◽

Ocean Model ◽

Turbidity Maximum ◽

Sediment Supply ◽

Three Dimensional Model

The Ou River, a medium-sized river in the southeastern China, is examined to study the estuarine turbidity maximum (ETM) response to rapidly varied river discharge, i.e., peak river discharge (PRD). This study analyzes the difference in ETM and sediment transport mechanisms between low-discharge and PRD during neap and spring tides by using the Finite-Volume Community Ocean Model. The three-dimensional model is validated by in-situ measurements from 23 April to 22 May 2007. In the Ou River Estuary (ORE), ETM is generally induced by the convergence between river runoff and density-driven flow. The position of ETM for neap and spring tides is similar, but the suspended sediment concentration during spring tide is stronger than that during neap tide. The sediment source of ETM is mainly derived from the resuspension of the seabed. PRD, compared with low-discharge, can dilute the ETM, but cause more sediment to be resuspended from the seabed. The ETM is more seaward during PRD. After PRD, the larger the peak discharge, the longer the recovery time will be. Moreover, the river sediment supply helps shorten ETM recovery time. Mechanisms for this ETM during a PRD can contribute to studies of morphological evolution and pollutant flushing.

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Surface and subsurface flow effect on permanent gully formation and upland erosion near Lake Tana in the northern highlands of Ethiopia

Hydrology and Earth System Sciences ◽

10.5194/hess-14-2207-2010 ◽

2010 ◽

Vol 14 (11) ◽

pp. 2207-2217 ◽

Cited By ~ 94

Author(s):

T. Y. Tebebu ◽

A. Z. Abiy ◽

A. D. Zegeye ◽

H. E. Dahlke ◽

Z. M. Easton ◽

...

Keyword(s):

Water Table ◽

Gully Erosion ◽

Grazing Land ◽

Cross Sectional ◽

Lake Tana ◽

Erosion Rates ◽

Remotely Sensed Imagery ◽

Reservoir Siltation ◽

Upland Erosion ◽

Elevated Water

Abstract. Gully formation in the Ethiopian Highlands has been identified as a major source of sediment in water bodies, and results in sever land degradation. Loss of soil from gully erosion reduces agricultural productivity and grazing land availability, and is one of the major causes of reservoir siltation in the Nile Basin. This study was conducted in the 523 ha Debre-Mawi watershed south of Bahir Dar, Ethiopia, where gullies are actively forming in the landscape. Historic gully development in a section of the Debre-Mawi watershed was estimated with semi structured farmer interviews, remotely sensed imagery, and measurements of current gully volumes. Gully formation was assessed by instrumenting the gully and surrounding area to measure water table levels and soil physical properties. Gully formation began in the late 1980's following the removal of indigenous vegetation, leading to an increase in surface and subsurface runoff from the hillsides. A comparison of the gully area, estimated from a 0.58 m resolution QuickBird image, with the current gully area mapped with a GPS, indicated that the total eroded area of the gully increased from 0.65 ha in 2005 to 1.0 ha in 2007 and 1.43 ha in 2008. The gully erosion rate, calculated from cross-sectional transect measurements, between 2007 and 2008 was 530 t ha−1 yr−1 in the 17.4 ha area contributing to the gully, equivalent to over 4 cm soil loss over the contributing area. As a comparison, we also measured rill and interrill erosion rates in a nearby section of the watershed, gully erosion rates were approximately 20 times the measured rill and interrill rates. Depths to the water table measured with piezometers showed that in the actively eroding sections of the gully the water table was above the gully bottom and, in stable gully sections the water table was below the gully bottom during the rainy season. The elevated water table appears to facilitate the slumping of gully walls, which causes the gully to widen and to migrate up the hillside.

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Modelling of sediment transport and morphological evolution under the combined action of waves and currents

Ocean Science ◽

10.5194/os-13-673-2017 ◽

2017 ◽

Vol 13 (5) ◽

pp. 673-690 ◽

Cited By ~ 5

Author(s):

Guilherme Franz ◽

Matthias T. Delpey ◽

David Brito ◽

Lígia Pinto ◽

Paulo Leitão ◽

...

Keyword(s):

Sediment Transport ◽

Morphological Changes ◽

Morphological Evolution ◽

Research Effort ◽

Beach Erosion ◽

Model Complexity ◽

Invariant Equilibrium ◽

Waves And Currents ◽

Combined Action ◽

Modelling System

Abstract. Coastal defence structures are often constructed to prevent beach erosion. However, poorly designed structures may cause serious erosion problems in the downdrift direction. Morphological models are useful tools to predict such impacts and assess the efficiency of defence structures for different scenarios. Nevertheless, morphological modelling is still a topic under intense research effort. The processes simulated by a morphological model depend on model complexity. For instance, undertow currents are neglected in coastal area models (2DH), which is a limitation for simulating the evolution of beach profiles for long periods. Model limitations are generally overcome by predefining invariant equilibrium profiles that are allowed to shift offshore or onshore. A more flexible approach is described in this paper, which can be generalised to 3-D models. The present work is based on the coupling of the MOHID modelling system and the SWAN wave model. The impacts of different designs of detached breakwaters and groynes were simulated in a schematic beach configuration following a 2DH approach. The results of bathymetry evolution are in agreement with the patterns found in the literature for several existing structures. The model was also tested in a 3-D test case to simulate the formation of sandbars by undertow currents. The findings of this work confirmed the applicability of the MOHID modelling system to study sediment transport and morphological changes in coastal zones under the combined action of waves and currents. The same modelling methodology was applied to a coastal zone (Costa da Caparica) located at the mouth of a mesotidal estuary (Tagus Estuary, Portugal) to evaluate the hydrodynamics and sediment transport both in calm water conditions and during events of highly energetic waves. The MOHID code is available in the GitHub repository.

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Mass balance controls on sediment scour and bedrock erosion in waterfall plunge pools

Geology ◽

10.1130/g48881.1 ◽

2021 ◽

Author(s):

Joel S. Scheingross ◽

Michael P. Lamb

Keyword(s):

Sediment Transport ◽

Mass Balance ◽

River Discharge ◽

Sediment Load ◽

Sediment Flux ◽

Habitat Availability ◽

Flux Divergence ◽

Plunge Pool ◽

Bedrock Erosion ◽

Recurrence Intervals

Waterfall plunge pools experience cycles of sediment aggradation and scour that modulate bedrock erosion, habitat availability, and hazard potential. We calculate sediment flux divergence to evaluate the conditions under which pools deposit and scour sediment by comparing the sediment transport capacities of waterfall plunge pools (Qsc_pool) and their adjacent river reaches (Qsc_river). Results show that pools fill with sediment at low river discharge because the waterfall jet is not strong enough to transport the supplied sediment load out of the pool. As discharge increases, the waterfall jet strengthens, allowing pools to transport sediment at greater rates than in adjacent river reaches. This causes sediment scour from pools and bar building at the downstream pool boundary. While pools may be partially emptied of sediment at modest discharge, floods with recurrence intervals >10 yr are typically required for pools to scour to bedrock. These results allow new constraints on paleodischarge estimates made from sediment deposited in plunge pool bars and suggest that bedrock erosion at waterfalls with plunge pools occurs during larger floods than in river reaches lacking waterfalls.

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