Topo-bathymetric Surveys Research Articles

Optimizing the dataset size of a topo-bathymetric survey for Hammam Debagh Dam, Algeria

The capacity of water reservoirs may be considerably reduced by sediment transport and accumulation. This phenomenon may occur quickly in regions prone to erosion, such as Hammam Debagh Dam region in Algeria. For better management of water resources, it is important to estimate the volume of a reservoir by regular and frequent topo-bathymetric surveys. Topo-bathymetric surveys with a high point measurement density are expensive and time-consuming. To reduce this expense and to increase the frequency of topo-bathymetric surveys, it is required to optimize their size and density. The complete survey of the reservoir created by the Hammam Debagh Dam in Algeria was examined in comparison with 19 reduction subsets using geostatistics. The study showed that the complete dataset of 32,164 points, corresponding to a density of 42 points ha-1 could be reduced to 10,000 points, i.e. 13 points ha-1, without affecting the estimation of the water volume in the reservoir. This reduction could considerably simplify the work load, reduce the expenses to do these surveys, and, therefore, allow for an increased frequency of topo-bathymetric surveys.

Geomorphic effects of gravel augmentation on the Old Rhine River downstream from the Kembs dam (France, Germany)

During the last 30 years, river restoration activities aiming to improve the functionality of degraded fluvial ecosystems increased markedly. For large rivers, it remains difficult to evaluate restoration efficiency and sustainability due to the lack of standardized monitoring metrics. From 2010 to 2016, three gravel augmentations were performed on the Old Rhine, a by-passed reach downstream from the Kembs dam (France- Germany). A geomorphic monitoring combining topo-bathymetric surveys, bedload tracking and hydraulic modelling allows to evaluate the successfulness of these actions. Results show that, to be mobilized, artificial sediment deposit should be located in concavity rather than convexity areas, due to higher shear stresses for moderate floods (Q2). Sediment starvation appeared rapidly on the restored reaches once the sediment wave moved downstream, as a consequence of limited upstream sediment supply. Bathymetric homogenization was observed along and downstream from the restored reaches without creation of new fluvial forms. This research highlights that future actions should include channel enlargement downstream of gravel augmentations, which would promote sediment deposition and habitat diversification. Sediments excavated during artificial widening could be stored and injected progressively into the upstream part of the Old Rhine to benefit the downstream sections.

Caracterização de ambientes aquáticos no rio de Ondas, cerrado baiano

This work sought to characterize the types of environments present in the De Ondas River, with the aim of obtaining information on their structural complexity and supporting public policies for water resource management that will allow the shared water use and will protect the biodiversity of the river basin. The De Ondas River watershed is located in the Cerrado Region, in the western part of Bahia State, and covers an area of 5.559 km². In order to perform the characterization, two stretches were demarcated, one in the upper part of the basin and another in the lower. Afterwards, a topobathymetric survey was conducted, and characterizations of the bottom and of hydraulic properties were done for both the rainy and dry seasons. The topobathymetric survey in the upper course showed altitudes ranging from 699.82 m to 714.06, whereas in the lower course these varied from 439.45 m to 446.91 m. Bathymetric profiles allowed the visualization of the structure and distribution of the bottom of the channel environments. The granulometric analysis of the higher part of the basin showed that sand was predominant (88.94%), while in the lower part boulders (76.9%) and cobble (23.1%) predominated. In the higher part of the basin, the water flow varied from 5.20 m³s-1 (dry period) to 9.07 m³s-1 (rainy period); and in the lower part, from 25.73 m³s-1 (dry period) to 66.44 m³s-1 (rainy period). The complexity of the environments of these stretches favors biological diversity in the river, resulting in different environments that aid aquatic species in adapting to the channel structure. The study of the physical characteristics of the De Ondas River produced a great amount of information regarding the micro-environments of the stretches examined, and will support studies related to aquatic biota and water resource management.

TYPHOONS DRIVEN MORPHODYNAMICS OF THE WAN-TZU-LIAO SAND BARRIER (TAÏWAN)

This study focuses on the dynamics of the Wan-Tzu-Liao sand barrier when it is struck by one typhoon, a group of typhoons or the whole season of typhoons and monsoon. In the framework of the KUN-SHEN project, 7 months of monitoring (2011-2012) provided 20 topobathymetric surveys (within a 200 m long segment of the barrier from the subtidal zone to the back-barrier) and acquisitions of offshore, nearshore and shallow water hydrodynamics including velocity profiling, free surface measurement and absolute pressure. Offshore waves were extracted at Cigu buoy (18 m of water depth). Nearshore waves were acquired from the current profiler deployed 400 m off the coast in 4 m of water depth and water level on the subaerial beach were acquired from two pressure sensors deployed in the lower part of the subtidal zone and the dune crest. Morphologic changes of the emerged beach were monitored using D-GPS each week during winter monsoon season and just before and after each event during summer typhoons season. Amongst the eight studied typhoons, TALIM is the most energetic event observed. Offshore wave height reached H s = 10.34 m (T p = 14.6 s) at Cigu buoy and H s = 2.3 m (T p = 13.4 s) at the profiler. Surface waves dynamics throughout surf and swash zones are progressively dominated by infragravity motions. A part of the dune is surged at the storm apex. Morphological changes include 6.7 m of dunefoot retreat and a sand transfer from a dune breach to wash-over deposits in the lagoon. More surprisingly, the foreshore was nourished (2261 m3 +/- 268 m3 ) as well as the whole sand barrier (+1920 m3 +/- 1071 m3 ). The sand input is concomitant to the redistribution of sand from the intertidal sandbars interpreted as the ultimate stage of the subtidal bars landward migration. Although winter are erosive season (-4995 m3 +/- 1071 m3 ), the summer results in an accretion period (3556 m3 +/-1071 m3 ) with a shoreline seaward shift about 10.4 m. Over the annual time period, the sand barrier recorded 18.4 m of retreat coupled with a small sand loss (-1439 m3 +/- 1071 m3 ) without any significant abrasion of the dune-top.

Lessons from inlet relocation: examples from Southern Portugal

Inlet relocation is a coastal management tool that, when applied to a migrating inlet, involves the artificial opening of a new tidal inlet along the historic migration path of the inlet. The old inlet is then artificially closed, or it is left open and will eventually close if the new inlet captures the entire tidal prism. Two inlets were relocated in such a manner in Southern Portugal and then were the subject of a monitoring program that included the acquisition of quantitative (topo-bathymetric surveys) data. Data was acquired for 4 years after the relocation at one of the inlets (Ancão Inlet) and for 2 years at the other (Fuzeta Inlet). The data obtained from the monitoring program were analysed together with the wave climate and then compared with historical information on the natural inlets, in order to assess the degree of success of the relocation actions. One of the relocations studied, Ancão Inlet, was considered to be successful even though an initial unexpected behaviour produced some material damage to property. On the contrary, the relocation of Fuzeta Inlet did not have the expected results, and the new inlet was affected by the same problems as the old one. It was found that the most important factor for a relocation action to succeed is the correct choice of the opening location. A theoretical procedure to enhance the possibilities of relocation success is suggested. (1) Hydrodynamic studies are needed in order to determine if the present conditions are similar to the historical ones. (2) The position for the inlet opening is chosen according to the hydrodynamic conditions, but there are other factors to be taken into account, i.e., the historical migration paths and typical inlet width of the natural inlet; the hydrodynamics of the backbarrier; the morphology of the backbarrier and, for multi-inlet barrier island systems, the proximity to adjacent inlets. (3) Once the position is chosen, environmental impact studies should be made in order to assess the risk of the relocation for the ecosystems of the area. Only if the environmental impact studies are favourable should a relocation action be performed.

Morphology Evolution of an Ebb-Tidal Delta Following a Storm Perturbation: Assessments from Remote Sensed Video Data and Direct Surveys

Abstract Tidal inlet morphology is strongly dependent on local hydrodynamic conditions that rely on wave/tide dominance. In some places, this hydrodynamic context exhibits a very strong seasonal pattern, showing tide-induced morphologies in summer and wave-induced ones in winter. The Barra Nova tidal inlet in Algarve, South Portugal, presents such characteristics. Short term morphological changes were investigated during an extensive field campaign carried on in February and March 1999, after the winter storm period. This study integrates both topo-bathymetric surveys and remotely sensed video data in order to assess the morphological evolution of the inlet system during the transitional period following storms. Comparison of datasets issued from these two direct and indirect methods has shown a very good agreement (errors usually less than 10 cm in elevation) that allows the quantification of morphological changes of the entire inlet system during the whole period of video observation. During a transitio...

The first two years of an inlet: sedimentary dynamics

A monitoring program, including the acquisition of a series of topo-bathymetric surveys and oblique aerial photos, has been carried out at Ancão Inlet since its artificial opening in June 1997. This paper presents morphological and volumetric results, including inlet channel evolution (in terms of width, depth and cross-sectional area) and tidal delta formation. The results are analysed in relation to the prevailing oceanographic conditions. This allows the development of a conceptual evolutionary model for the natural evolution of artificially opened inlets with four stages: Stage 1 is the artificially opened inlet; Stage 2 represents the inlet after reaching dynamic equilibrium for the channel while ebb- and flood-tidal deltas are still developing; Stage 3 is the “mature” migrating inlet; and Stage PS (stage post-storm) is the inlet situation after high-energy conditions. The transition from Stage 1 to Stage 3 is a continuous process with intermediate sub-stages. Transition to Stage PS is a discrete phenomenon, caused by high-energy events, after which the inlet gradually changes back to the previous stage. Ancão Inlet was in Stage 1 in June 1997. In April 1998 the inlet channel reached dynamic equilibrium, thus beginning Stage 2. One year after the opening (July 1998) the inlet showed fully developed ebb and flood deltas, reaching the end of Stage 2. Transition to Stage 3, with the beginning of the downdrift migration and bypassing processes, took place during the second year. Stage PS characteristics were observed in January 1999 after high-energy storm conditions, with strong evidence of landward sediment transport. Although the conceptual model was developed for Ancão Inlet, it is envisaged that it could be applicable to other mixed-energy migrating inlets opened in an artificial way.