EROSION AROUND KRALENDIJK, BONAIRE Nick Roos Master student VUA (Vrije Universiteit Amsterdam) Wayaká Advies BV 12/02/2018 2501655 Fig. 1: Saliña di Vlijt, Kaya Amsterdam, Klein Bonaire 2017. (Photo by N. Roos) This research has been carried out in the framework of project: ‘erosion control and nature recovery’ managed by among others Jan Jaap van Almenkerk (Wayaká Advies BV).The project serves as a course of the Msc. Hydrology programme at the VU university of Amsterdam. 1 Abstract On Bonaire, special municipality of the Netherlands, there is a big problem with erosion. In and around the capital Kralendijk, most areas have sparse vegetation and/or are paved. Intense rainfall events (such as those occurring on Bonaire) can loosen the soil material, after which it is transported away towards the ocean by (rain)water flowing over the surface (Koster, 2013). Not only causes this a loss of fertile soil, it also has consequences for aquatic life and plants along the coast, such as the coral reefs of Bonaire and the species which depend on them (Bak et al, 2005). An erosion (hotspot) map could be a start to determine why, where and how action could be taken to reduce erosion and gives input for the type of measure that may be suited for such an area. Using the model of Borselli, an hotspot map reckoning factors such as hydrologic and soil properties, land use, landcover and connectivity is created. Critical erosion areas (including urban/paved areas) are now imaged and measures can be undertaken or improved to enhance erosion management. 2 Table of content Tabellenlijst ............................................................................................................................................. 4 Figurenlijst ............................................................................................................................................... 4 1 Introduction ..................................................................................................................................... 6 1.1 Background .................................................................................................................................... 6 1.2 Problem statement ........................................................................................................................ 6 1.3 Goal ............................................................................................................................................... 7 2 Study area and previous research ................................................................................................... 8 2.1 Study area ...................................................................................................................................... 8 2.2 Previous research ........................................................................................................................ 12 3 Methodology ................................................................................................................................. 15 3.1 Land use types ............................................................................................................................. 15 3.2 Infiltrometer-test ......................................................................................................................... 20 3.3 Boiffin-method ............................................................................................................................ 22 3.4 Water-drop method .................................................................................................................... 24 3.5 TGA (Thermogravimetric analysis) .............................................................................................. 26 3.6 Grain size ..................................................................................................................................... 26 3.7 Porosity ........................................................................................................................................ 27 3.8 Vegetation logs ............................................................................................................................ 27 3.9 Connectivity ................................................................................................................................. 28 3.10 Soil crust .................................................................................................................................... 30 3.11 Hotspot determination (Borselli method) ................................................................................. 31 4 Results ........................................................................................................................................... 34 4.1 Results soil aggregates locations ................................................................................................. 34 4.2 Boiffin method ............................................................................................................................. 35 4.3 Infiltrometer ................................................................................................................................ 35 4.4 Water-drop method .................................................................................................................... 36 4.5 Thermogravimetric analysis ........................................................................................................ 37 4.6 Grain size results ......................................................................................................................... 38 4.7 Vegetation ................................................................................................................................... 39 4.8 Hotspots of erosion ..................................................................................................................... 41 4.8.1 W-factor map .................................................................................................................... 41 4.8.2 Erosion map ....................................................................................................................... 42 5 Discussion ...................................................................................................................................... 46 5.1 Erosion sensitivity ........................................................................................................................ 46 5.2 Management scenarios/Measures .............................................................................................. 48 3 5.2.1 Reservoirs .......................................................................................................................... 48 5.2.2 Overgrazing........................................................................................................................ 48 5.2.3 Reducing paved surface area............................................................................................ 49 5.2.4 Fertile soil .......................................................................................................................... 50 6 Conclusion ..................................................................................................................................... 50 7 References ..................................................................................................................................... 52 Tabellenlijst Tabel 1: LU (Landuse classes) ................................................................................................................ 16 Tabel 2: Main features and properties of the different types of surface crust in semi-arid zones ...... 31 Tabel 3: Soil properties .......................................................................................................................... 34 Tabel 4: Boiffin method results ............................................................................................................. 35 Tabel 5: Infiltrometer results................................................................................................................. 35 Tabel 6: Water-drop method results ..................................................................................................... 36 Tabel 7: TGA results ............................................................................................................................... 37 Tabel 8: Grain size results ...................................................................................................................... 38 Tabel 9: Results of vegetation logs (2.9) for each landuse type ........................................................... 40 Tabel 10: Erosion sensitivity per land use type ..................................................................................... 46 Figurenlijst Fig. 2.1: Digital elevation map (DEM) Bonaire ........................................................................................ 8 Fig. 2.2: Hydrogeology of the Island Bonaire ......................................................................................... 8 Fig. 2.3: Study area: Kralendijk catchment ............................................................................................. 9 Fig. 2.4: Sub-Catchments in Kralendijk Catchment, Bonaire……………………………………………………………..11 Fig. 2.5: Water flow/catchment map ...................................................................................................