Maximum Hourly Rainfall Research Articles

Impacts of Anthropogenic Heat Flux and Urban Land-Use Change on Frontal Rainfall near Coastal Regions: A Case Study of a Rainstorm over the Pearl River Delta, South China

AbstractThis study investigated heavy frontal rainfall that occurred on 13–14 October 2011 over the Pearl River Delta (PRD) in China. The frontal rainstorm was simulated using the WRF-ARW Model (version 3.3), which included its urban canopy model. Although the model-simulated convection occurred 2 h early and the second precipitation peak was underestimated, the model represented the formation, development, and extinction of the frontal rainfall and captured the distribution of the peak value. In addition, the averaged value of 49.7 W m−2was taken as the anthropogenic heat flux (AHF) of the PRD, and two land-use datasets were adopted: one for 1992 and the other for 2011. The simulation revealed that AHF and urban land-use change (ULUC) increased the total rainfall over the PRD by 6.3% and 7.4% and increased the maximum hourly rainfall intensity by 24.6% and 21.2%, respectively. Furthermore, to elucidate the mechanism of AHF and ULUC influence, the rainstorm structure, low-level jet (LLJ), and CAPE of the rainfall event were analyzed. It was found that AHF and ULUC enhanced two strong southward LLJs located over the urban areas, which carried abundant water vapor to the PRD and generated additional upper-level CAPE. This not only sustained steady ascent of the air, but it also created conditions favorable for downward motion, resulting in large persistent convective clouds and heavy frontal rainfall.

On the Extreme Rainfall Event of 7 May 2017 over the Coastal City of Guangzhou. Part I: Impacts of Urbanization and Orography

Abstract In this study, a nocturnal extreme rainfall event induced by the urban heat island (UHI) effects of the coastal city of Guangzhou in South China on 7 May 2017 is examined using observational analyses and 18-h cloud-permitting simulations with the finest grid size of 1.33 km and the bottom boundary conditions nudged. Results show that the model reproduces convective initiation on Guangzhou’s downstream side (i.e., Huashan), where a shallow thermal mesolow is located, the subsequent back-building of convective cells as a larger-scale warm-moist southerly flow interacts with convectively generated cold outflows, and their eastward drifting and reorganization into a localized extreme-rain-producing storm near Jiulong under the influences of local orography. In particular, the model produces the maximum hourly, 3- and 12-hourly rainfall amounts of 146, 315, and 551 mm, respectively, at nearly the right location compared to their corresponding observed extreme amounts of 184, 382, and 542 mm. In addition, the model reproduces an intense meso-γ-scale vortex associated with the extreme-rain-producing Jiulong storm, as also captured by Doppler radar, with organized updrafts along cold outflow boundaries over a semicircle. A comparison of sensitivity and control simulations indicates that despite the occurrence of heavier rainfall amounts without the UHI effects than those without orography, the UHI effects appear to account directly for the convective initiation and heavy rainfall near Huashan, and indirectly for the subsequent formation of the Jiulong storm, while orography plays an important role in blocking cold outflows and enhancing cool pool strength for the sustained back-building of convective cells over the semicircle, thereby magnifying rainfall production near Jiulong.

Molecular fingerprinting of particulate organic matter as a new tool for its source apportionment: changes along a headwater drainage in coarse, medium and fine particles as a function of rainfalls

Abstract. Tracking the sources of particulate organic matter (POM) exported from catchments is important to understand the transfer of energy from soils to oceans. The suitability of investigating the molecular composition of POM by thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide directly coupled to gas chromatography and mass spectrometry is presented. The results of this molecular-fingerprint approach were compared with previously published elemental (% C, % N) and isotopic data (δ13C, δ15N) acquired in a nested headwater catchment in the Piedmont region, eastern United States of America (12 and 79 ha). The concordance between these results highlights the effectiveness of this molecular tool as a valuable method for source fingerprinting of POM. It emphasizes litter as the main source of exported POM at the upstream location (80±14 %), with an increasing proportion of streambed (SBed) sediment remobilization downstream (42 ± 29 %), specifically during events characterized by high rainfall amounts. At the upstream location, the source of POM seems to be controlled by the maximum and median hourly rainfall intensity. An added value of this method is to directly investigate chemical biomarkers and to mine their distributions in terms of biogeochemical functioning of an ecosystem. In this catchment, the distribution of plant-derived biomarkers characterizing lignin, cutin and suberin inputs were similar in SBed and litter, while the proportion of microbial markers was 4 times higher in SBed than in litter. These results indicate that SBed OM was largely from plant litter that has been processed by the aquatic microbial community.

Frequency analysis of annual maximum hourly precipitation and determination of best fit probability distribution for regions in Japan

In the design of irrigation and other hydraulic structures, evaluating the extreme rainfall for a specific probability of occurrence is important. The capacity of such structures is usually designed to cater to the probability of occurrence of extreme rainfall during its lifetime. In this study, a frequency analysis of annual maximum hourly rainfall for 15 locations in Japan was carried out using the Expanded Automated Meteorological Data Acquisition System (EA) weather data of 20years from 1981 to 2000. Eight formulas were used to expect the return period in years of annual maximum hourly rainfall. Five different probability distribution functions (PDFs) were adopted to predict the probability distribution of occurrence of annual maximum hourly rainfall. The goodness of fit was evaluated using Chi-square test. It indicated that the Log-Pearson type 3 (LP3) distribution is the overall best fit PDF for annual maximum hourly rainfall at most locations of Japan.

QUANTITATIVE EVALUATION OF ENGINEERING TREATMENTS FOR PREVENTION OF DEBRIS FLOW AT WENJIA GULLY

After Wenchuan Earthquake in China, debris flows as a kind of geohazard occur frequently in Wenjia Gully. The debris flows have already cost millions of property and casualty losses to the local community. For the purpose of preventing debris flows, geological engineering treatments have been implemented in multiple locations of Wenjia Gully. The adopted preventive measures include water and sediment separation upstream, reinforce bottom of channel reinforcement and slope protection midstream, damming against and silting downstream. To evaluate the effectiveness of these treatments, this paper presents a data-driven method to conduct this research by collecting data during five seasons of heavy rainfalls. Survival analysis, Bootstrap method and Extreme Learning Machine(ELM) are selected to build data-driven models. By implementing survival analysis models, the survival probabilities of locations without treatment decrease to 0%.The survival probability of the locations with treatment stay at 55.6%after the five rainfall seasons. Meanwhile, the maximum hourly rainfall in the post-treatment period is 2.571 times as the one in pre-treatment period. Total rainfall volume of post-treatment period is 1.232 times as the one in pre-treatment period. Total time period of rainfall of post-treatment is 5.435 times as the one of pre-treatment. Comparing the survival probabilities, the effect of treatments are significant in the prevention of debris flows. After resampling by bootstrap method, the predictive results from Extreme Learning Machine indicate that, without geological treatment, the probability of having debris flow under 10 times of heavy rainfall is 100%.It is much higher than the observed 30% between the year of 2011 and 2015. The predicted debris flows magnitudes are also significantly higher than the observed ones with treatment. Hence, geological engineering treatment is crucial in reducing and preventing geohazards.

Flood risk reduction and flow buffering as ecosystem services – Part 2: Land use and rainfall intensity effects in Southeast Asia

Abstract. Watersheds buffer the temporal pattern of river flow relative to the temporal pattern of rainfall. This ecosystem service is inherent to geology and climate, but buffering also responds to human use and misuse of the landscape. Buffering can be part of management feedback loops if salient, credible and legitimate indicators are used. The flow persistence parameter Fp in a parsimonious recursive model of river flow (Part 1, van Noordwijk et al., 2017) couples the transmission of extreme rainfall events (1 − Fp), to the annual base-flow fraction of a watershed (Fp). Here we compare Fp estimates from four meso-scale watersheds in Indonesia (Cidanau, Way Besai and Bialo) and Thailand (Mae Chaem), with varying climate, geology and land cover history, at a decadal timescale. The likely response in each of these four to variation in rainfall properties (including the maximum hourly rainfall intensity) and land cover (comparing scenarios with either more or less forest and tree cover than the current situation) was explored through a basic daily water-balance model, GenRiver. This model was calibrated for each site on existing data, before being used for alternative land cover and rainfall parameter settings. In both data and model runs, the wet-season (3-monthly) Fp values were consistently lower than dry-season values for all four sites. Across the four catchments Fp values decreased with increasing annual rainfall, but specific aspects of watersheds, such as the riparian swamp (peat soils) in Cidanau reduced effects of land use change in the upper watershed. Increasing the mean rainfall intensity (at constant monthly totals for rainfall) around the values considered typical for each landscape was predicted to cause a decrease in Fp values by between 0.047 (Bialo) and 0.261 (Mae Chaem). Sensitivity of Fp to changes in land use change plus changes in rainfall intensity depends on other characteristics of the watersheds, and generalisations made on the basis of one or two case studies may not hold, even within the same climatic zone. A wet-season Fp value above 0.7 was achievable in forest–agroforestry mosaic case studies. Inter-annual variability in Fp is large relative to effects of land cover change. Multiple (5–10) years of paired-plot data would generally be needed to reject no-change null hypotheses on the effects of land use change (degradation and restoration). Fp trends over time serve as a holistic scale-dependent performance indicator of degrading/recovering watershed health and can be tested for acceptability and acceptance in a wider social-ecological context.

Influenced on Analysis of Characteristics of Forest Environmental Factors on Debris Flow Occurrence

This study was conducted to analyze the forest environmental characteristics on a total of 20 forest environmental factors affecting the debris flow against 272 sites of risk areas. In the case of environmental factors, it showed the high risk of debris flow under the following conditions such as soil depth of less than 30cm, west slope, altitude of 200~300 m, mountain average slope of , sandy loam, igneous rocks, and composite slope. Among the rainfall factors, 50~100 mm of maximum hourly rain fall and 300 mm of maximum rain fall per day have been shown the high risk of debris flow. Furthermore, the high risk of debris flow was related to the river-bed average slope of , the river-bed average width of >10 m, the small amount of debris in river-bed (less than 20% of river-bed structure), the drainage density of >, the 40~60% of area with more than slope, and the 40~60% of areas with risk grade 2 of landslide. In addition, forest environmental factors including the driftwood, soil erosion control structures, age-class 3, crown density (density), and mixed forest were important factors causing the high risk of debris flow.

The October 16, 2013 rainfall-induced landslides and associated lahars at Izu Oshima Volcano, Japan

Intense rainfall related to the typhoon T1326 on October 15–16, 2013 (total 824mm; maximum hourly rainfall 118.5mm) triggered numerous landslides and associated lahars at Izu Oshima Volcano, the northernmost part of Izu Mariana volcanic arc, Japan. The landslides were concentrated mainly in a 2-km2 area located on the western slope of Izu Oshima Volcano. Most of the landslides were shallow soil slips (<2m thick) in unconsolidated fallout tephra layers overlying lava flows and pyroclastic rocks. The rupture surfaces of them were located near the base of Y1 tephra (AD 1777–1778) and/or the base of Y4 tephra (AD 1421). The Y1 and Y4 tephras differ from the underlying paleosols in permeability, grain size and degree of compaction. The saturated hydraulic conductivities of the paleosols were one to two orders of magnitude smaller than those of the overlying Y1 and Y4 tephras. Most landslides mobilized completely into lahars, traveling along stream channels or flat slopes and flooding at the foot of the volcano. The associated lahars severely damaged inhabited areas and caused thirty five fatalities. Although the lahars eroded slopes and transported boulders up to 1m in diameter and a large amount of woody debris, they contained more than 90% of sand-to-silt-size particles, similar in composition to the original sliding materials. Sediment discharge volumes from three basins were estimated at 1.8–4.1×104m3/km2, based on debris volumes trapped by sediment retention dams. The characteristics of rainfall-induced landslides and associated lahars at Izu Oshima Volcano in 2013 provide an important lesson about future non-eruption-related landslide and lahar hazards at tephra-rich volcanoes.

Decision-tree analysis of factors influencing rainfall-related building structure and content damage

Abstract. Flood-damage prediction models are essential building blocks in flood risk assessments. So far, little research has been dedicated to damage from small-scale urban floods caused by heavy rainfall, while there is a need for reliable damage models for this flood type among insurers and water authorities. The aim of this paper is to investigate a wide range of damage-influencing factors and their relationships with rainfall-related damage, using decision-tree analysis. For this, district-aggregated claim data from private property insurance companies in the Netherlands were analysed, for the period 1998–2011. The databases include claims of water-related damage (for example, damages related to rainwater intrusion through roofs and pluvial flood water entering buildings at ground floor). Response variables being modelled are average claim size and claim frequency, per district, per day. The set of predictors include rainfall-related variables derived from weather radar images, topographic variables from a digital terrain model, building-related variables and socioeconomic indicators of households. Analyses were made separately for property and content damage claim data. Results of decision-tree analysis show that claim frequency is most strongly associated with maximum hourly rainfall intensity, followed by real estate value, ground floor area, household income, season (property data only), buildings age (property data only), a fraction of homeowners (content data only), a and fraction of low-rise buildings (content data only). It was not possible to develop statistically acceptable trees for average claim size. It is recommended to investigate explanations for the failure to derive models. These require the inclusion of other explanatory factors that were not used in the present study, an investigation of the variability in average claim size at different spatial scales, and the collection of more detailed insurance data that allows one to distinguish between the effects of various damage mechanisms to claim size. Cross-validation results show that decision trees were able to predict 22–26% of variance in claim frequency, which is considerably better compared to results from global multiple regression models (11–18% of variance explained). Still, a large part of the variance in claim frequency is left unexplained, which is likely to be caused by variations in data at subdistrict scale and missing explanatory variables.

Duration and seasonality of hourly extreme rainfall in the central eastern China

Compared with daily rainfall amount, hourly rainfall rate represents rainfall intensity and the rainfall process more accurately, and thus is more suitable for studies of extreme rainfall events. The distribution functions of annual maximum hourly rainfall amount at 321 stations in China are quantified by the Generalized Extreme Value (GEV) distribution, and the threshold values of hourly rainfall intensity for 5-yr return period are estimated. The spatial distributions of the threshold exhibit significant regional differences, with low values in northwestern China and high values in northern China, the mid and lower reaches of the Yangtze River valley, the coastal areas of southern China, and the Sichuan basin. The duration and seasonality of the extreme precipitation with 5-yr return periods are further analyzed. The average duration of extreme precipitation events exceeds 12 h in the coastal regions, Yangtze River valley, and eastern slope of the Tibetan Plateau. The duration in northern China is relatively short. The extreme precipitation events develop more rapidly in mountain regions with large elevation differences than those in the plain areas. There are records of extreme precipitation in as early as April in southern China while extreme rainfall in northern China will not occur until late June. At most stations in China, the latest extreme precipitation happens in August-September. The extreme rainfall later than October can be found only at a small portion of stations in the coastal regions, the southern end of the Asian continent, and the southern part of southwestern China.

Use of ensemble simulations to evaluate the urban effect on a localized heavy rainfall event in Tokyo, Japan

Recently, localized heavy rainfall over highly urbanized areas has caused severe damage in Japan. Although studies have indicated that the presence of urban areas can intensify rainfall, the chaotic noise caused by differences in initial conditions can change the urban effect. Therefore, the usability of ensemble simulation methods for urban effects on a single localized heavy rainfall event must be clarified to synthesize state-of-the-art observations and numerical model studies. This study examined the difference in a localized heavy rainfall event under different initial conditions. We demonstrated the advantage of ensemble experiments using a cloud-resolving model to evaluate the urban effect. In this study, we focused on a localized heavy rainfall event that occurred over Tokyo on 5 August 2008 and killed five people working in a drainpipe. Two ensemble experiments were performed according to the lagged average forecasting (LAF) method. Each experiment had six members, with initial conditions spanning 6 h from 02 to 07 Local Standard Time (LST) on 5 August 2008. The model was integrated continuously through 18 LST for each run. The model had a horizontal grid resolution of 2 km. The first experiment (CTRL) used the actual land cover, anthropogenic heat, and geometric structures. The second experiment (PDDY) used an imaginary land cover in which the urban area was changed to paddy fields. Comparison of the ensemble means showed that the peak value of the maximum hourly rainfall decreased and the peak time was delayed in PDDY compared with CTRL. The smaller peak value and delayed peak time of rainfall in PDDY seemed to be caused by the weaker updraft associated with near-surface convergence, which was caused by weaker surface heating. Regardless of the difference caused by the initial conditions, the decrease in the peak value and the delay in the peak time were significant according to the t-test. Although the area was limited compared with near-surface temperature and horizontal wind convergence, a significant reduction in rainfall was also seen over the heavily urbanized area of Tokyo in the horizontal distribution of the difference in accumulated rainfall. These results indicate that although differences in the initial condition can change the modeled urban effect on rainfall, ensemble simulation methods can be used to examine the significance of the urban effect on a single localized heavy rainfall event.

Slope management criteria for Alishan Highway based on database of heavy rainfall-induced slope failures

In this paper, a study of slope failures along the Alishan Highway (locally, known as “Tai-18”) is carried out. An innovative empirical model is developed based on 15-year records of typhoon rainfall-induced slope failures. This model is intended for assessing the likelihood of slope failure along Tai-18 in the future. The rainfall data considered in the proposed model include the maximum hourly rainfall and the effective cumulative rainfall. The effective cumulative rainfall is defined at the point when the curve of cumulative rainfall goes from steep to flat. Then, a simple criterion is established for assessing the potential of slope failure and issuing warning and/or closure for Tai-18 during a future extreme rainfall. Slope failures during Typhoon Saola in 2012 and those in Japan demonstrate that the new empirical model is effective and applicable to other regions with similar rainfall conditions.

Characteristics of rainfall triggering of debris flows in the Chenyulan watershed, Taiwan

Abstract. This paper reports the variation in rainfall characteristics associated with debris flows in the Chenyulan watershed, central Taiwan, between 1963 and 2009. The maximum hourly rainfall Im, the maximum 24 h rainfall Rd, and the rainfall index RI (defined as the product RdIm) were analysed for each rainfall event that triggered a debris flow within the watershed. The corresponding number of debris flows initiated by each rainfall event (N) was also investigated via image analysis and/or field investigation. The relationship between N and RI was analysed. Higher RI of a rainfall event would trigger a larger number of debris flows. This paper also discusses the effects of the Chi-Chi earthquake (CCE) on this relationship and on debris flow initiation. The results showed that the critical RI for debris flow initiation had significant variations and was significantly lower in the years immediately following the CCE of 1999, but appeared to revert to the pre-earthquake condition about five years later. Under the same extreme rainfall event of RI = 365 cm2 h−1, the value of N in the CCE-affected period could be six times larger than that in the non-CCE-affected periods.

A formation model for debris flows in the Chenyulan River Watershed, Taiwan

Many debris flows were triggered in the Chenyulan River Watershed in Taiwan in a rainstorm caused by the Typhoon Toraji. There are 117 gullies with a significant steep topography in the catchment. During this Typhoon, debris flows were initiated in 43 of these gullies, while in 34 gullies, it was not certain whether they have occurred. High-intensity short-duration rainfall was the main triggering factor for these gully type debris flows which are probably entrained by a “fire hose” mechanism. Previous research identified 47 factors related to topography, geology, and hydrology, which may play a role in the formation of gully type debris flows. For a better understanding of the probability of the formation of debris flows, it is proposed to represent the factors related to topography, geology, and hydrology by one single factor. In addition to the existing topographic and geological factor, a normalized critical rainfall factor is suggested with an effective cumulative precipitation and a maximum hourly rainfall intensity. In this paper, a formation model for debris flows is proposed, which combines these topographic, geological, and hydraulic factors. A relationship of these factors with a triggering threshold is proposed. The model produces a good assessment of the probability of occurrence of debris flows in the study area. The model may be used for the prediction of debris flows in other areas because it is mostly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new and exciting way to study the occurrence of debris flows initiated by a “fire hose” mechanism.

The asymmetry of rainfall process

Using hourly station rain gauge data in the warm season (May-October) during 1961-2006, the climatological features of the evolution of the rainfall process are analyzed by compositing rainfall events centered on the maximum hourly rainfall amount of each event. The results reveal that the rainfall process is asymmetric, which means rainfall events usually reach the maximum in a short period and then experience a relatively longer retreat to the end of the event. The effects of rainfall intensity, duration and peak time, as well as topography, are also considered. It is found that the asymmetry is more obvious in rainfall events with strong intensity and over areas with complex terrain, such as the eastern margin of the Tibetan Plateau, the Hengduan Mountains, and the Yungui Plateau. The asymmetry in short-duration rainfall is more obvious than that in long-duration rainfall, but the regional differences are weaker. The rainfall events that reach the maximum during 14:00-02:00 LST exhibit the strongest asymmetry and those during 08:00-14:00 LST show the weakest asymmetry. The rainfall intensity at the peak time stands out, which means that the rainfall intensity increases and decreases quickly both before and after the peak. These results can improve understanding of the rainfall process and provide metrics for the evaluation of climate models. Moreover, the strong asymmetry of the rainfall process should be highly noted when taking measures to defending against geological hazards, such as collapses, landslides and debris flows throughout southwestern China.

Inundações na Cidade de Lisboa durante o Século XX e os seus factores agraventes

INUNDATIONS IN LISBON DURING THE 20TH CENTURY AND THEIR URBANAND PHYSICAL CAUSES. Lisbon often suffers inundations, which must be classified as urban inundations, because the flooded areas involved are impermeable due to urban development. These inundations are not related to the overflow of watercourses, but can be classified as flash inundations because of the speed with which they form and spread. The present study covers a time period from 1918/19 to 1997/98, during which 420 occurrences were recorded in two leading daily newspapers, «O Século» and «Diário de Notícias». The research based on these newspapers takes into account daily rainfalls above 10 l/m2 recorded by the Instituto Geofísico Infante D. Luís, as well as daily rainfall below this level but that have also caused inundations. The analysis of the frequency and evolution of these inundations has showed that they occur more often during the autumn (specially in November) and that from the seventies onwards, they have decreased considerably, while at the same time the total rainfall volume has increased, as well as the maximum hourly rainfall that bring them about. Fieldwork was very important in the definition of flooded sites. Research was carried out to specify the physical factors (relief and drainage basins) and urban network (squares and streets). The relationship between these urban factors and the old drainage network was also taken into account in order to distinguish increasing inundation factors. Apart from the description of the physical characteristics of these sites, they were also typified through factor analysis. A hazard factor was attributed to each site based on inundation frequency. The most endangered areas and sites are located mainly in the «Frente Ribeirinha» (along the river bank) built upon the terminal sectors of the natural watercourses, which consists of the receptor streets and receptor squares of the flow. These streets and squares lie perpendicular to the ancient watercourses, and in places where artificial barriers and micro depressions exist. The latter result from the artificial surface of the riverside embankment. Other critical sites exist throughout the town mainly because of obstruction problems or deficient drainage systems, often aggravated, by newly built up impervious areas.

Linking typhoon tracks and spatial rainfall patterns for improving flood lead time predictions over a mesoscale mountainous watershed

Typhoon rainfall characteristics over a mesoscale mountainous watershed (drainage area of 620 km2) located in eastern Taiwan were analyzed to fill the gaps in our knowledge concerning the linkage between typhoon track, rainfall patterns, and flood peak time. This study used spatially high‐resolution radar‐derived rainfall estimates from 38 storm events (∼2800 h) to investigate this linkage. The effect of spatial rainfall patterns on the timing of flood peak for the selected events was examined with the aid of a diffusive wave model. The results show that the typhoon rainfall was spatially aggregated and that the relative variations in the rainfall became smaller at higher rainfall rates. The maximum hourly rainfall was approximately twice the areal mean rainfall. Three major rainfall types were identified statistically, and different typhoon tracks appeared to have preferable rainfall types. This finding is presumably due to the interaction of the typhoon circulation and precipitation with the mountainous landscape. Flood lead times were derived for the different rainfall types, and it was found that differences in their lead times could be as large as ∼3 h over the studied mesoscale watershed. It is recommended that this empirical approach be incorporated into flood forecasting and warning systems.

Controls of infiltration–runoff processes in Mediterranean karst rangelands in SE Spain

Semiarid karst landscapes represent an important ecosystem surrounding the Mediterranean Basin for which little is known on runoff generation. Knowledge of the sources and patterns of variation in infiltration–runoff processes and their controls is important for understanding and modelling the hydrological functions of such ecosystems. The objectives of this paper are to determine the infiltration rates and their controls in a representative mountain karst area (Sierra de Gádor, SE Spain) at micro-plots and to investigate the integrated response of rainfall on a typical hillslope. Rainfall simulations in micro-plots and natural rainfall-runoff monitoring on a hillslope were carried out complementarily. We investigated the role of soil surface components (vegetation, rock outcrop, fracture, and soil crust), topographic position, antecedent soil moisture, and rainfall characteristics in regulating infiltration–runoff processes. Results of rainfall simulation revealed the importance of vegetation cover and the presence of rock fractures in promoting the infiltration in the limestone karst landscape, while bare patches and rock outcrops acted as sources for runoff. All plots with > 50% vegetation cover had no runoff with up to 55 mm h− 1 of simulated rain. In contrast, nearly all bare plots had runoff under the same simulated rain, with runoff coefficients ranging from 3.1 to 20.6% on dry soil surface conditions, and from 2.0 to 65.4% on wet soil surfaces. Runoff coefficients amounted to 59.0–79.5% for rock outcrops without cracks, but were drastically reduced by the presence of cracks. The surfaces with rock fragments resting on the soil (generally located in the middle of the slopes) prevented more effectively the runoff generation than those surfaces where rock fragments were embedded in the top soil. Antecedent soil moisture had significant impact on runoff generation, with wet soil having doubled runoff coefficient, shortened time to runoff, and increased runoff rate compared to the same but dry soil. Linear regressions indicated that the main controls for constant infiltration rate were the cover percentages of vegetation and litter, plus rainfall intensity; while the major controls for runoff coefficient were the bare soil and vegetation coverage, plus rainfall intensity. High infiltration rates measured at the micro-plots agreed with low intra-event runoff coefficients (mostly below 1%) observed under natural rainfalls at the hillslope. Runoff depth and coefficient at the hillslope was significantly correlated with rainfall depth, maximum hourly rainfall intensity and antecedent precipitation over 20 days (AP20). During the 1.5-year monitoring period from Sep-2003 to Mar-2005, the overall infiltration was 41% of the total rainfall amount and the maximum infiltration rate was almost 94% of the largest single rainfall event. The results from this study contribute to improved understanding of the magnitude and controls of the surface runoff in semiarid karst mountain areas.

Sediment Disasters Caused by Typhoon No.14, 2005, in Miyazaki Prefecture

Typhoon No.14 struck Japan in September, 2005, subjecting large areas to danger for many hours. The maximum hourly rainfall was not so great, but the total rainfall for 72 hours was > 500 mm in areas which suffered considerable damage. The typhoon caused 116 debris flows, 185 slope-collapses, and 30 landslides. Concerning human and building casualties, twenty-seven people were killed in Kagoshima, Miyazaki and Yamaguchi Prefecture, and two people went missing in Oita Prefecture. Additionally, 1,178 houses were completely destroyed and 3,692 houses were partly destroyed in Japan by the typhoon. These damages were due to comparatively heavy rainfall which continued for many hours. Large scale slope-collapses occurred in particular, around Miyazaki Prefecture due to heavy continuous rainfall (862 1,321mm).

Linkage of sediment supply and transport processes in Miyagawa Dam catchment, Japan

Linkages between sediment supply by episodic geomorphic processes (obtained from aerial photographs and field surveys) and sediment transport (estimated from changes in sediment deposition behind a large dam) were investigated in the Miyagawa Dam catchment, central Japan. A total of 6667 landslides were confirmed in the period from 1965 to 2000 on the basis of seven temporal pairs of aerial photographs. Both the occurrence of landslides and discharge into the dam lake affect sediment yield, indicating that fluvial systems in Miyagawa Dam catchment are supply limited with respect to sediment. Sediment yields are affected by not only the initial failed volume of landslides but also the mobility of landslides and debris flows. In Miyagawa Dam catchment, percentages of landslides reaching channels varied from 56% in 1997–2000 to 75% in 1976–1981 and were correlated with maximum hourly rainfall. In addition, the mobility of debris flows was higher during periods with high maximum instantaneous discharge compared to lower discharge, suggesting that the water content both in initially failed materials and transported sediment controlled their mobility. Topography also affected the mobility of landslides/debris flows. For catchments &gt;0.1 km2, the percentage of channel network length impacted by debris flows decreased with increasing catchment area due to reduced channel gradient. Thus both the magnitude of rainfall‐runoff events and the catchment topography affect how landslide sediment contributes to sediment yield at the large catchment scale.