Hybrid engineering effectivity evaluation according to the changes in mangrove area and sedimentary rate in the eroded area of Sayung Regency, Demak, Central Java

Gigantic abrasion occurred in the Sayung Coast had severely damaged the ecosystems and the coastal area reached 1.5 km and 6 m depth. One of the mitigation efforts that had applied in the Sayung Coast is Hybrid Engineering (HE), which is constructed in the front of the coastline. That structure can be slip off by the water flow yet cannot reflect the waves. The first HE was built in the Timbulsloko Village in 2013 that needs to be evaluated sustainably. The evaluation of HE effectivity is essential moreover these structures will be applied to the other areas impacted abrasion. The correlation between sedimentary rate data (obtained from²¹⁰Pb natural isotopes analysis) and the changes in mangrove area is the best way to assess the effectivity of HE. The average of sedimentary rate before HE construction (2011-2013) reached 0.195 cm/year and enhanced about 0.4 cm/year in 2016. Those data are in accordance with the addition of the mangrove forest area (from 409 Ha in 2013 becomes 455.79 Ha in 2015). The existence of HE proves that its effectivity to trap the sediment and to minimalize the physical pressure from oceanographic factors are the best way to mitigate erosion, resulting in restored natural green belt in Sayung coast.