## 2 Materials and Methods  
### Study Area  
The Porong Delta, as presented in Figure 1 is located in East Java Province, Indonesia (7.569 S, 112.872 E). It is approximately 37 km southeast of Surabaya, the second-largest city in Indonesia. It has a monsoon climate with a dry (April-September) and wet (October-March) season [29], in which the high precipitation contributes up to 80% of the mean annual precipitation [30]. The Porong River is one of the two major branches of the Brantas watershed. The watershed is regulated with several large dams, barrages, and flood gates. During the dry season, the flow is diverted to Surabaya, and therefore the flow in the Porong River is often reduced to almost zero [31]. During the wet season flood discharge is diverted from the upstream barrage and high precipitation contributes to high river flow that discharges into the Madura Strait [24; 25]. Madura Strait has a micro to the meso-tidal range and is categorised by a mixed diurnal-semidiurnal tide [32]. The Brantas river originates from the volcanic complex of the Semeru and Arjuno Mountains [32]. Erosion rates on the slopes of Mount Semeru are among the highest recorded in the world (105-106 m\({}^{3}\) km\({}^{-2}\) a\({}^{-1}\)) [33]. Sediment yield in several drainage basins of Mount Semeru is dominated by rain-triggered events during the wet season [30; 33]. The Brantas watershed is densely populated and affected by anthropogenic activities such as deforestation, intensive agriculture (mainly rice cultivation), and industries [34]. Due to the geological conditions consisting of the presence of easily erodible soils and high anthropogenic activities, surface erosion is high [31]. The Porong River drains off high sediment loads, causing a prograding delta [34; 35] with a progradation rates of approximately 0.4 \(\times\) 10\({}^{6}\) m\({}^{2}\) y\({}^{-1}\) over the 1935-1981 period [21; 31]. In addition to the sediment load due to runoff from the hinterland, the Porong River has been experiencing an extreme sediment load due to the mud volcanic eruption in Sidoarjo, Indonesia. The LUSI mudflow is reported to be the 'largest mud eruption in the world' [36], about 18 km west of Porong Delta (Figure 1). On 29 May 2006, the boiling mud erupted at a peak flow rate of up to 180,000 m\({}^{3}\) day\({}^{-1}\)[22] which declined to 50,000 m\({}^{3}\) d\({}^{-1}\) in September 2011 [23]. Sixty thousand residents were forced to evacuate, and 7 km\({}^{2}\) of residential area was submerged with mud [36] (Figure 2a). The excessive LUSI is still  
Figure 1: The study area is described in a sequence: (**A**) Indonesian border (light green) with East Java Province depicted in dark green, (**B**) East Java Province and the capital city Surabaya (represented as a red circle), (**C**) LUSI (lumpur (mud) and sidoarjo (the regency name)) mud volcano represented as a black polygon, and the Porong River as the black line flowing from the west to the east part of the map, and (**D**) Porong Estuary with LUSI island and the delta lobes. actively erupting material, gas, water, clasts, and oil, albeit at a considerably reduced rate [36]. The continuous discharge of mud has been diverted to the Porong River since 2007 [23]. To reduce damage to the nearby community and environment, the mud in LUSI is first stored in a reservoir contained by 10-m-tall dyke and then diluted and disposed of by pumping to the river [21; 36]. This operation has increased sediment concentration and loads of the Porong River by a factor of 3-4 compared to pre-LUSI conditions [21; 23; 34]. As shown in Figure 2b, due to the mudflow, the delta is rapidly prograding along with the development of mangrove belts. To date, the LUSI mud volcano is still erupting with no end in sight [36].