Erosion and deposition vulnerability of small (<5,000 km2) tropical islands

The tropics are naturally vulnerable to watershed erosion. This region is rapidly growing (projected to be 50% of the global population by 2050) which exacerbates erosional issues by the subsequent land use change. The issue is particularly of interest on the many (~45,000) small tropical (<5,000 km2) islands, and their >115M residents, where ecotourism and sediment intolerant ecosystems such as coral reefs are the main driver of their economies. However, vulnerability to erosion and deposition is poorly quantified in these regions due to the misclassification or exclusion of small islands in coarse global analyses. We use the only vulnerability assessment method that connects watershed erosion and coastal deposition to compare locally sourced, high-resolution datasets (5 x 5 m) to satellite-collected, remotely sensed low-resolution datasets (463 x 463 m). We find that on the island scale (~52 km2) the difference in vulnerability calculated by the two methods is minor. On the watershed scale however, low-resolution datasets fail to accurately demonstrate watershed and coastal deposition vulnerability when compared to high-resolution analysis. Specifically, we find that anthropogenic development (roads and buildings) is poorly constrained at a global scale. Structures and roads are difficult to identify in heavily forested regions using satellite algorithms and the rapid, ongoing rate of development aggravates the issue. We recommend that end-users of this method obtain locally sourced anthropogenic development datasets for the best results while using low resolution datasets for the other variables. Fortunately, anthropogenic development data can be easily collected using community-based research or identified using satellite imagery by any level of user. Using high-resolution results, we identify a development trend across St. John and regions that are both high risk and possible targets for future development. Previously published modeled and measured sedimentation rates demonstrate the method is accurate when using low-resolution or high-resolution data but, anthropogenic development, watershed slope, and earthquake probability datasets should be of the highest resolution depending on the region specified.

Vulnerability to watershed erosion and coastal deposition in the tropics

Over half of the global population is projected to live in the tropics by 2050. Sustainable land development will be challenged by enhanced sediment erosion and deposition, which can negatively impact water quality and ecosystem services in inland and coastal waterways. Existing erosion assessments treat watersheds and coastal zones separately, but we connect them in a two-part vulnerability index to watershed erosion and coastal deposition at 0.0004° (~ 45 m) resolution throughout the tropics. We use open-source datasets and a simple, GIS-based method geared toward tropical, novice end-users. Part 1 of the index reveals a majority of the tropics is vulnerable to erosion. Vulnerability is highest where there are co-occurrences of earthquakes, steep slopes, and high precipitation such as the Caribbean and Southeast Asia. In Part 2, we assess erosion vulnerability at 4 watersheds and include their coastal systems, which can enhance or diminish vulnerability of the entire system to coastal deposition.

Studi Sedimentasi di Sungai Batang Lampasi Sedimentation Study at Batang Lampasi River

BatangLampasi River was located in Talawi village, Nagari Koto Nan GadangPayakumbuh, about 30 km from Bukittinggi, with 48.19 km river length and around 226 km2 Cathment Area. The river flow carries a lot of sediment material, causing siltation on the riverbed which causes the river to overflow in the surrounding area. Sediment transport was triggered by Lampasi watershed erosion, that especially in the upstream section and settling on the riverbed. Prediction of depth decrease caused by sedimentation was conducted. Theoretical flood discharge determined used Nakayasu method. Sediment yield caused by land erosion determined by USLE equation and will compared with total sediment transport that calculated for 5 years return period with Yang's, Engelund Hansen, Ackers and White's methods. HEC-RASS 4.0 was used to find the high of flood water level and also as input hydraulics term to calculating transport sediment. The equivalent value approaching sediment yield was Yang's method where the sediment transport value is 21294,763 tons / year with sediment thickness of 27.24 cm / year. With these deposits thickness we can see from HEC-RASS 4.0 modeling the elevation of high flood water during normal conditions and the presence of thick sediment deposits.