Peak Rainfall Intensity Research Articles

Geomorphic effects of a large flood on fluvial fans

AbstractThe response of 12 fluvial fans near Sydney, Australia to a large storm between 2 and 4 February 1990 was determined by repeating previously surveyed longitudinal profiles and by undertaking detailed field observations of erosion and deposition. Peak rainfall intensities occurred on 3 and 4 February when between 173 and 193·8 mm were recorded. Return periods for 24 h duration peak rainfall ranged between 5·7 and 11·0 years on the annual maximum series at six stations within the study area and return periods for 48 h peak rainfall ranged between 13·5 and 29·4 years.Of the 12 fans, seven were trenched and five untrenched. The most significant geomorphic effects of the storm were recorded on the proximal region of the fans. However, fan response was highly variable, with one fan exhibiting no detectable change, three fans localized deposition, two fans spatially disjunct erosion and deposition, two fans channel avulsions, and seven fans fanhead trench reworking. Some fans exhibited more than one type of response.A four‐stage, tentative cyclical model of fanhead development was constructed from the field data. Stage 1 refers to the episodic aggradation of the fanhead by localized deposition, spatially disjunct erosion and deposition and/or channel avulsions. Stage 2 represents the initiation of a fanhead trench when progressive aggradation locally exceeds a threshold slope leading to localized erosion. This erosion initially creates one or more discontinuous flow‐aligned scour pools. Over time, the scour pools widen, deepen and extend both up‐ and downfan. Stage 3 refers to the coalescence of discontinuous scour pools into a continuous trench by the removal of intervening log and boulder steps. Stage 4 represents the backfilling phase of the trench once it has been overwidened and/or slope reduced. Aggradation then continues as for stage one.

SENSITIVITY ANALYSIS OF THE WEPP HILLSLOPE PROFILE EROSION MODEL

ABSTRACT Sensitivity analysis was performed on the hillslope profile erosion model developed by the USDA-Water Erosion Prediction Project. The erosion model calculates soil loss and sediment yield caused by rill and interrill erosion on complex shaped hillslope profiles. Sensitivity analysis of a physically based simulation model is used for assessing the rationality of the model, to provide insight into the overall physical system which the simulation model represents, and to help identify research needs. Changes in predicted soil erosion and sediment yield as a function of changes in soil, plant residue and canopy, hillslope topography, and hydrologic input variables were assessed. Dominant factors related to model response were precipitation, rill erodibility, rill residue cover, and rill hydraulic friction factors. Saturated hydraulic conductivity and interrill erodibility were moderately sensitive parameters. Other factors which had less influence on output were canopy height, interrill cover, soil bulk density, antecedent moisture, peak rainfall intensity, time to pe^ rainfall intensity, rill width and spacing, and sediment characteristics. Slope length, gradient, and slope shape effects on soil loss and sediment delivery were also discussed.

EVALUATION OF CONCURRENT CHARACTERISTICS OF RAINFALL AND STORM SURGE BY A STOCHASTIC TYPHOON MODEL

A stochastic typhoon model with three properties of central pressure, moving direction and moving velocity is constructed by using B-spline functions and multi-variate autoregressive models. This stochastic typhoon model generates hourly data of typhoon location, central pressure, moving direction and velocity. Predictors against the rainfall intensity and the storm surge at Osaka are selected by using a stepwise regression analysis. About 2820 typhoons for 600 years are generated, and rainfall intensities and storm surges associated with the simulated typhoons are generated at Osaka. The concurrent characteristics of rainfall and storm surge; such as the frequency distribution of time lags, the spatial distributions of typhoon locations, when the peak rainfall intensities and the maximum storm surges occurred. At the same time the joint return periods of heavy rainfall intensities and large storm surges, are evaluated by using observed and simulated data.

Use of weighted integral variables to determine the relation between rainfall intensity and storm flow and peak flow generation

Hewlett's (1977) work using his storm flow separation technique has established that peak rainfall intensities have little predictive value in the determination of storm flow depth or storm peak flow from small upland streams emanating from forested catchments. A logical but disconcerting inference is that storm rainfall intensities have little influence on either the volume of storm flow or peak flow. To obtain additional information a variant of the technique was applied to excellent records from two separate, steep, upland catchments in southern Australia. The catchments were regarded as having quite distinct rainfall response characteristics by hydrologists involved in data collection. Antecedent flow, storm rainfall, and measures of maximum rainfall intensity were supplemented by weighted integrals of rainfall intensity with time to test whether they gave additional insight into the influence of rainfall intensity. As expected, regression analysis showed that knowledge of maximum intensities gave little increase in predictive ability over storm rainfall. However, the time integral of rainfall intensity to the power of 1.5 was better correlated with peak flow than any other measure on both catchments. The results suggest that rainfall intensities are more important in the generation of peak flows than indicated by regression analysis using only peak rainfall intensities. Comparison of independent measurements of the variables using separated rain gauges showed that independent measures of the weighted variables were better correlated than measures of storm maximum intensities, indicating that they are also a more precise measure.

STOCHASTIC FORMULATION OF DESIGN STORM PATTERN AND PROPOSAL OF CONDITIONAL PROBABILITY-RAINFALL INTENSITY FORMULA

A stochastic single storm pattern with a peak of rainfall is theoretically derived from the bivariate exponential probability density function defined by Freund. Two typical design storm patterns: backward peaked type and central peaked type are prescribed by three parameters: the reduced variate yP of the peak rainfall intensity, the autocorrelation index k, and the conditional probability F. A new conditional probability intensity-duration formula is proposed by integrating the given design storm hyetograph. Furthermore, a practical estimation method of the three parameters prescribing the design storm and the intensity-duration curve is clearly shown and demonstrated using actual rainfall data.

STUDY ON CHARACTERISTICS OF CONCURRENCE OF RAINFALL AND STORM SURGE ASSOCIATED WITH TYPHOON

The characteristics of concurrence of rainfall and storm surge at Osaka are made clear by analyzing the available data of typhoons for the period 1900-1980; Distributions of typhoon positions when maximum storm surge ηmax or peak rainfall intensity γp occurred, a histogram of time-lag between ηmax and γp, and relationships between ηmax and γp, and between ηmax and total rainfall RT, are shown. Predictors of rainfall and storm surge are selected by using the stepwise regression analysis, and the probability distributions of the residuals are revealed. It is probable that the flood wave following the rainfall moves downstream in a river and the storm surge invades the river, with a considerably short time-lag. On these occasions, the high water levels along the river change, depending on the length of the time-lag. Such features of high water levels are examined in terms of the behavior of storm surge propagation in the lower reach with mild slope.

A Preliminary Numerical Simulation of a Shower

A quasi-one-dimensional, time-dependent and precipitating cumulus cloud model incorporated with a time-dependent PBL model has been used to simulate the precipitation record of a local summer afternoon shower induced by sea breeze. The system is so designed that it is subjected only to the variations of the parameters pertaining to the PBL. Through the use of this system, we have obtained relatively good agreement between the observed and model produced rainfall pattern, peak rainfall intensity and total rainfall; we are also confident of reproducing the time of onset of the shower. It has also been found that the PBL influences the precipitation characteristic of a shower in a complicated way. For a given water vapor content, greater thickness of the PBL, which implies a greater heat supply to the cloud activity above, will delay the onset of a shower, reduce its total rainfall and produce multi-peaked intensities. When the thickness is increased to a critical value, no shower can be produced. If the thickness is increased further, a shower can be produced again. For a given thickness, increasing water vapor will greatly expedite the onset of a shower which also has greater total rainfall and multi-peaked rainfall intensities. Only when the heat content is accompanied by a proper water vapor content can a shower of single-peaked rainfall intensity be produced. In this model, we have introduced an immediate environmental (IE) region which allows the cloud to generate its own inner or immediate environment, enriched by water mass in both liquid and vapor form. Our work has shown that the IE region plays an important role in enhancing cloud development, delaying the showers to afternoon hours and in providing a recycling process of water mass so that heavy rainfall can be produced by this simple model.