Identification, localization and numbering of islets and geomorphological units of the Betsiboka delta (northwestern Madagascar) on Sentinel 2 satellite images (2020, 2021) (doi:10.23708/COLVLV)
(Identification exhaustive et numérotation des îlots et unités géomorphologiques du delta du Betsiboka (Nord ouest Madagascar) sur images satellitaires Sentinel 2, 2020, 2021)

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Identification, localization and numbering of islets and geomorphological units of the Betsiboka delta (northwestern Madagascar) on Sentinel 2 satellite images (2020, 2021)

doi:10.23708/COLVLV

DataSuds

2024-02-16

1

Hervé, Dominique; Randrianatoandro, Ravakiniaina; Delaitre, Eric; Robison, Laurent; Razanaka, Samuel; Ratiarson, Venot, 2024, "Identification, localization and numbering of islets and geomorphological units of the Betsiboka delta (northwestern Madagascar) on Sentinel 2 satellite images (2020, 2021)", https://doi.org/10.23708/COLVLV, DataSuds, V1

Identification, localization and numbering of islets and geomorphological units of the Betsiboka delta (northwestern Madagascar) on Sentinel 2 satellite images (2020, 2021)

Geomorphological units of the Betsiboka delta embedded in its estuary, including islets, islets merged with other islets, right and left bank units, shoals and traces observed in dry season, low tide images

Identification exhaustive et numérotation des îlots et unités géomorphologiques du delta du Betsiboka (Nord ouest Madagascar) sur images satellitaires Sentinel 2, 2020, 2021

doi:10.23708/COLVLV

Hervé, Dominique (UMR SENS - CIRAD, IRD, Université Paul-Valéry Montpellier 3 - France)

Randrianatoandro, Ravakiniaina (CNRE, Univ. Antananarivo - Madagascar)

Delaitre, Eric (UMR ESPACE-DEV - IRD, Univ.Montpellier, Univ.La Réunion, Univ.Guyane, Univ.Antilles - France)

Robison, Laurent (CNRE - Madagascar)

Razanaka, Samuel (CNRE - Madagascar)

Ratiarson, Venot (ENI Ecole Nationale d'Informatique, Univ. Fianarantsoa - Madagascar)

RANDRIANATOANDRO Ravakiniaina, CNRE-Univ. Antananarivo

RATIARSON Venot, ENI, Univ. Fianarantsoa

ROBISON Laurent, CNRE

DELAITRE Eric, IRD-Espace-Dev

HERVE Dominique, IRD-SENS

2023-08-24

DataSuds

Hervé, Dominique

Hervé, Dominique

2023-03-15

The islets of the Betsiboka delta

https://doi.org/10.23708/COLVLV

Agricultural Sciences, Earth and Environmental Sciences, Delta, Estuary, Sedimentation, Islets, Inventory, Geomorphological units, low tide

Remote sensing

Long time series (more than a century) are difficult to construct but they are useful for understanding hydro-geomorphological phenomena that take place over the long term and anticipating future risks.<br> The <a href="https://en.wikipedia.org/wiki/Betsiboka_River">Betsiboka River</a> into which the Ikopa flows from Antananarivo, capital of Madagascar, is characterized by hyper-sedimentation which gives it a red color. Downstream, at Mahajanga, the river forms a delta nested in its estuary and influenced by the tides, which becomes blocked over the years, then flows into Bombetoka Bay and the Mozambique Channel. This clogging takes the form of islets which are colonized by the mangrove, once emerged. Monitoring a population of islets and their cover by mangroves makes it possible to evaluate the sedimentation speed of the delta and the dynamics of the mangroves (colonization, growth, mortality).<br> These data - and the associated article - illustrate the stages of constructing a map representing all of these islets, located and numbered in 2021 (Satellite Sentinel 2 image of 04/28/21, low tide = -1.86) , the delimitation of the nested delta, the location of the islets according to an upstream-downstream gradient, finally the contours and numbering of each islet. This inventory can thus serve as a reference, as an end point for historical studies, or as a starting point for multi-year monitoring enabled by the Sentinel 2 satellite. <hr> Les séries chronologiques longues (plus d’un siècle) sont difficiles à constituer mais elles sont utiles pour comprendre les phénomènes hydro-géomorphologiques qui se déroulent sur le temps long et anticiper les risques à venir.<br> Le fleuve Betsiboka dans lequel se jette l’Ikopa depuis Antananarivo, capitale de Madagascar, se caractérise par une hyper-sédimentation qui lui donne une couleur rouge. En aval, au niveau de Mahajanga, le fleuve forme un delta emboité dans son estuaire et influencé par les marées, qui se colmate au fil des années, puis se déverse dans la baie de Bombetoka et le canal du Mozambique. Cette colmatation prend la forme d’îlots qui sont colonisés par la mangrove, une fois émergés. Le suivi d’une population d’îlots et de leur couverture par des palétuviers permet d’évaluer la vitesse de sédimentation du delta et la dynamique des mangroves (colonisation, croissance, mortalité).<br> Ces données -et l'article associé- illustrent les étapes de construction d’une carte représentant la totalité de ces îlots, localisés et numérotés en 2021 (image Satellite Sentinel 2 du 28/04/21, marée basse = -1,86), la délimitation du delta emboité, la localisation des îlots selon un gradient amont-aval, enfin les contours et numérotation de chaque îlot. Cet inventaire peut ainsi servir de référence, comme point d’aboutissement d’études historiques, ou comme point de départ d’un suivi pluri-annuel permis par le satellite Sentinel 2.

2020-01-01-2021-12-31

Madagascar

Boeny, Mahajanga, Delta of Betsiboka river

• West Bounding Longitude: 44.8689798972046
• East Bounding Longitude: 47.4365028557223
• South Bounding Latitude: -17.3646966528282
• North Bounding Latitude: -15.200936926398

Sentinel 2 image, 2020, Dry season, Low tide Sentinel 2 image, 2021, Dry season, Low tide

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DIDEM_Betsiboka_Delta_01_README.txt

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Dataset description

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DIDEM_Betsiboka_Delta_02_Authors_Contributions_EN.pdf

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Co-author contribution details.

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DIDEM_Betsiboka_Delta_03_Downstream_LongLimit_1.pdf

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Detail of the downstream limit of the Betsiboka delta, between Boanamary in the right bank and a river in the left bank.

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DIDEM_Betsiboka_Delta_04_LargeLimit_2.pdf

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Large limits of the delta are defined by the extension of mangrove soils in green (ORSTOM, 1952) and the maximum extension of flooded area in red (flooded area in 2015-2016 by Catry et al., 2017).

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DIDEM_Betsiboka_Delta_05_Limits_3.jpg

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The definitive limits of the delta are defined by the downstream long limit (Boanamary), the upstream first right channel, and the large limits defined according to mangrove and flooded area.

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DIDEM_Betsiboka_Delta_06_Axis_Equidistants_1.jpg

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Course of the main axis of the Betsiboka river in its delta, with equidistants points (Sentinel 2, 2021).

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DIDEM_Betsiboka_Delta_07_Transects_2.jpg

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13 equidistants transects in the Betsiboka delta (Sentinel 2, 2021).

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DIDEM_Betsiboka_Delta_08_Up-Mid-Down-Stream_3.jpg

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13 transects in the Betsiboka delta, grouped in Upstream, Midstream and Downstream sections (Sentinel 2, 2021).

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DIDEM_Betsiboka_Delta_09_GeoUnits_2020_1.pdf

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Grid with 4km by 4 km boxes applied on the Betsiboka delta, numbered to enable localization of the 186 geomorphological units identified in the Sentinel 2 image of 2020 (dry period, low tide).

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DIDEM_Betsiboka_Delta_10_GeoUnits_2021.pdf

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Final map of numbered islets (Sentinel 2 image April 28, 2021 at low tide -1.86).

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