Peak flood volume and its suspended sediment at various rainfall in Kedungbulus catchment in Gombong, Central Java, Indonesia

Flood is a natural disaster that frequently happens and causes many material and immaterial losses. During flooding, the suspended sediment is carried along by the streamflow. The amount of sediment transported varies and depends on natural and anthropogenic factors. Limited studies have been conducted regarding the relationship between peak flood volume and its sediment content. Therefore, a study with the purpose to understand the relationship of rainfall characteristics, peak flood volume, and suspended sediment was undertaken in Kedungbulus Catchment in Gombong, Central Java, Indonesia. The size of Kedungbulus catchment is 37.8 km2. To collect the required data, an automatic stream water level recorder was installed in the outlet of the catchment. In addition, an automatic and two conventional rain gauges were set up inside the catchment. Hydrograph and statistical analysis were conducted on 2016-2017 data. The results showed that during the study period, the highest peak flood volume occurred on October 8, 2016. The flood duration was 490 minutes, with the time to peak was 135 minutes. At the highest peak flood volume, the stream water was 5,091,221 m3, and the suspended sediment was around 2,394 tons. Rainfall depth significantly affects the peak flood volume and its suspended sediment. The rainfall intensity and Antecedent Soil Moisture Content (ASMC) weakly correlate with peak flood volume and its suspended sediment content.

Risk-Based Warning Decision Making of Cascade Breaching of the Tangjiashan Landslide Dam and Two Smaller Downstream Landslide Dams

Mega earthquakes or serious rainfall storms often cause crowded landslides in mountainous areas. A large part of these landslides are very likely blocking rivers and forming landslide dams in series along rivers. The risks of cascading failure of landslide dams are significantly different from that of a single dam. This paper presented the work on risk-based warning decision making on cascading breaching of the 2008 Tangjiashan landslide dam and two small downstream landslide dams in a series along Tongkou River. The optimal decision was made by achieving minimal expected total loss. Cascade breaching of a series of landslide dams is more likely to produce a multi-peak flood. When the coming of the breaching flood from the upstream dam perfectly overlaps with the dam breaching flood of the downstream dam, a higher overlapped peak flood would occur. When overlapped peak flood occurs, the flood risk would be larger and evacuation warning needs to be issued earlier to avoid serious life loss and flood damages. When multi-peak flood occurs, people may be misled by the warning of the previous peak flood and suddenly attacked by the peak flood thereafter, incurring catastrophic loss. Systematical decision making needs to be conducted to sufficiently concern the risk caused by each peak of the breaching flood. The dam failure probability Pf linearly influences the expected life loss and flood damage but does not influence the evacuation cost. The expected total loss significantly decreases with Pf when the warning time was insufficient. However, it would not change much with Pf when warning time is sufficient.

Do megafloods reset mountain valleys?

<p>Quaternary megafloods (10<sup>6</sup> m<sup>3</sup>/s) sourced from valley blocking glaciers on the Tibetan Plateau have long been implicated in the evolution of Yarlung-Tsangpo Gorge on the Yarlung-Siang River. However, past estimates of megaflood erosion in this region have relied on back of the envelope estimates of peak discharge and shear stress. This makes it difficult to fully understand how megafloods shape the landscape. Here, we use 2D numerical simulations of megaflood hydraulics over 3D topography to examine the legacy of these massive floods on this confined, sinuous mountain river. First, to assess erosional potential in the Gorge, we calculate flood power and compare it to measurements of annual stream power. We find that the simulated megaflood produces peak flood power up to three orders of magnitude higher than the stream power of the annual river. Compared to stream power, flood power in the Gorge is disproportionately higher than it is downstream of the Gorge. Additionally, in the Gorge, a larger proportion of the inundated valley experiences high flood power and shear stress for long periods of time (5-10 hrs) compared to the valley downstream of the Gorge. These results support previous hypotheses that megafloods can erode more material (both alluvium and bedrock) than the annual monsoon—potentially enough to “reset” the mountain valley by removing most of the sediment and fractured bedrock in the system. However, we hypothesize that this erosional effect is felt primarily in the Gorge region. In contrast to the erosive power in the Gorge, there is an order of magnitude decrease in average peak flood power downstream of the Gorge. We hypothesize that megafloods are predominantly depositional in this downstream domain. Here, we observe few locations that experience sustained (>5 hrs) high (>10 kPa) shear stress and those locations are often isolated and vary through time. At locations that do experience these higher shear stresses, megafloods could move and deposit large (>3 m) boulders, which subsequent annual flows or smaller historical outburst floods would be incapable of moving. These large boulders could then armor the bed and prevent erosion, which could have lasting consequences for the modern river. Most of the shear stress and flood power of the simulated megaflood outside of the modern channel boundaries are much lower, capable of moving gravel to sand sized sediment at most. This is particularly true where we observe significant amounts (>10 km) of megaflood backflow up tributaries. Instead of resetting the system, we predict our megaflood will overwhelm this downstream flood domain with the deposition of coarse- and fine-grained sediment. For the Yarlung-Siang River to incise into the bedrock in a post-megaflood landscape, it must first make its way through these megaflood deposits. Together, our results suggest that the legacy of a megaflood in the region is both erosional and depositional. We predict wide-spread megaflood erosion in the Gorge, potentially enough to reset the system, but would expect exceptional deposition downstream of it, possibly enough to overwhelm this downstream domain.</p>

Estimated Flood Discharge in Downstream Krueng Meureubo of Pasi Pinang Section West Aceh Regency

The phenomenon of floods in Indonesia has become a routine thing that happens every year, almost all areas that are lowlands often flood when the rainy season arrives. Flooding is caused by high rainfall where the ability of the soil to absorb water decreases along with the rapid development of land that was once a rainwater catchment area, consequently surface runoff becomes high. Pasi Pinang Village can be categorized as a very flood prone village in Meureubo Subdistrict, due to the low condition of this village and directly borders the Meureubo river. Many adverse effects caused by floods and cause losses to the local community. The phenomenon of floods in Indonesia has become a routine thing that happens every year, almost all areas that are lowlands often flood when the rainy season arrives. Flooding is caused by high rainfall where the ability of the soil to absorb water decreases along with the rapid development of land that was once an area. In this case a study on flood discharge analysis needs to be done in order to reduce the incidence of flooding in the village of Pasi Pinang. The method used in this study is the Nakayasu HSS method, aim of this study is to determine the amount of flood discharge that occurred in the Krueng Meureubo River Basin in Pasi Pinang Village. Total hydrograph discharge calculated using the Nakayasu HSS method is the watershed area (A) = 1961.53 km, length of main river (L) = 157.02 km, time delay (tg) = 9.507 hours, duration of rain (Tr) = 7 , 13 hours, the time from the beginning of the flood to the peak of the flood hydrograph (tp) = 15.21 hours, the time of discharge 0.3 times the peak flood discharge (t0.3) = 19.01 hours and the peak flood discharge (Qp) = 23.109 m3 / sec then the total hydrograph discharge obtained by the Nakayasu method is equal to = 2040.26 m3/sec.

Features of the Maximum Water Flows of the Arpa River Under Modern Conditions

The paper discusses the main regularities of the peak flood discharge distribution in modern conditions, using actual data of Hydrometeorology and Monitoring Center SNCO with Ministry of Environment of the Republic of Armenia on the peak flood discharge of the river Arpa.

Flood discharge analysis of Brang Ode River as a disaster mitigation effort in Kalimango Village

Flood is a hydrometeorological disaster that often occurs in West Nusa Tenggara, especially in the Brang Ode River, Kalimango Village, Alas District, Sumbawa Regency. One of the worst floods ever happened was on December 12th, 2016, which caused several villages to be inundated and houses along the river to wash away. This study aims to obtain the peak discharge from the worst flood that has ever occurred. This model is simulated using HEC-RAS 5.0.7 and QGIS for mapping the flood inundation area. Terrain data used DEMNAS. The peak discharge is obtained from the modeling results based on the flood inundation area, validated with the flood map from the DESTANA (disaster resilient village) Community of Kalimango Village. The modeling results showed that the peak flood discharge is 950 m3/s, with the inundation area 150,752.07 m2. The actual peak flood discharge can be smaller or larger than the modeling results. It may be affected by the DEMNAS raster data accuracy.