Michiharu Shiiba

Incorporation of the effect of concentration of flow into the kinematic wave equations and its applications to runoff system lumping

Abstract The usual kinematic wave equations are revised to consider the interaction between surface flow and subsurface flow in mountainous watersheds having curved surfaces covered with A-layers of uniform thickness. A function which represents watershed surface geometry, which is called the geometric pattern function in the paper, is incorporated into the basic equations of the kinematic wave model, and the depth-flow relation equation for the surface-subsurface flow system is derived. When the watershed surface is linearly converging or diverging, its geometric pattern function has a linear form, and numerical simulations for such cases are given here. If the geometric pattern function is regarded as a new parameter of the kinematic wave model, then the kinematic wave flow model becomes very flexible. In fact, when the lateral inflow is spatially uniform, the model may be used as a simple model of a stream network system. Using this simplified model, a method of transformation of the kinematic wave flow model of a stream system into a lumped runoff model, which we call a “reservoir cascade” model, is developed.

Real-time stage and discharge estimation by a stochastic-dynamic flood routing model.

A dynamic-stochastic model for flood forecasting based on the Kalman filtering theory is described. To reduce the computational time that the Kalman filter involves, this model is performed using the reduced rank square root algorithm, which approximates the covariance matrix by a matrix of lower rank. By using the special banded structure of the Jacobian matrix of the discretization of the Saint Venant equations by the four points Preissman scheme, this algorithm is made computationally efficient for a flood routing model. The noise terms are treated as coloured noise processes, with one term for every grid element. An application to estimate the state of a one-channel reach is described. Copyright

Ensemble Kalman Filtering and Particle Filtering in a Lag-Time Window for Short-Term Streamflow Forecasting with a Distributed Hydrologic Model

AbstractThe performance of the ensemble Kalman filter (EnKF) and the particle filter (PF) is assessed for short-term streamflow forecasting with a distributed hydrologic model, namely, the water and energy transfer processes (WEP) model. To mitigate the drawbacks of conventional filters, the ensemble square root filter (EnSRF) and the regularized particle filter (RPF) are implemented. For both the EnSRF and the RPF, sequential data assimilation is performed within a lag-time window to consider the response times of internal hydrologic processes. The proposed methods are applied to two catchments in Japan and Korea to assess their performance. The model ensembles are perturbed by the noise of the soil moisture content and are assimilated with streamflow observations. The forecasting accuracy of both the EnSRF and the RPF is improved when sufficient lag-time windows are provided. The EnSRF is sensitive to the length of the lag-time window and has a limited ability to forecast within short lead times, wherea...

Assessment of the caesium-137 flux adsorbed to suspended sediment in a reservoir in the contaminated Fukushima region in Japan.

We estimated the flux of caesium-137 adsorbed to suspended sediment in the Kusaki Dam reservoir in the Fukushima region of eastern Japan, which was contaminated by the Fukushima Nuclear Power Plant accident. The amount and rate of reservoir sedimentation and the caesium-137 concentration were validated based on the mixed-particle distribution and a sediment transport equation. The caesium-137 and sediment flux data suggested that wash load, suspended load sediment, and caesium-137 were deposited and the discharge and transport processes generated acute pollution, especially during extreme rainfall-runoff events. Additionally, we qualitatively assessed future changes in caesium-137 and sediment fluxes in the reservoir. The higher deposition and discharge at the start of the projection compared to the 2090s are most likely explained by the radioactive decay of caesium-137 and the effects of reservoir sedimentation. Predictions of the impacts of future climate on sediment and caesium-137 fluxes are crucial for environmental planning and management.

A Real-Time Estimation of the Accuracy of Short-Term Rainfall Prediction Using Radar

The short-term rainfall prediction method proposed by the authors is extended to a stochastic method so that the method could provide in real-time the accuracy of the areal rainfall predicted using radar information. The extension was carried out based on the investigation on the stochastic properties of model parameters of the basic prediction model. First, we present results of theoretical analyses on the features of analytically predicted accuracy relating to the patterns of the movement of rainfall distribution. Next, we present a case study using actual rainfall distribution observed by radar during a Japanese typhoon. The results show that we can predict the mean square error of predicted areal rainfall at least within 1 hour ahead by the method proposed here.

Advanced use into rainfall prediction of three-dimensionally scanning radar

A computational method for the determination of rainfall distribution for applications in short term rainfall prediction is presented here. The method is strongly influenced by the experience gained from the observation and analysis of data gathered on a heavy rainfall event in 1986 that occurred during the Baiu Season in Japan. The method is based on the concept that rainfall occurs as an interaction between an instability field, appropriately modeled, and a field of water vapor under the influence of topography. The results from this computational method showed good agreement with the temporal variation in the rainband that moved across the observation field in 1986. Towards determination of the parameters in the computational model, another method for the determination of the rainfield is also developed. This second method determines the rainfall distribution from estimation of the conversion rate of water vapor to liquid water through use of data from a three dimensional scanning radar. The results are consistent with those obtained from the first method.

Estimating the collapse of aggregated fine soil structure in a mountainous forested catchment.

This paper describes the relationship of forest soil dryness and antecedent rainfall with suspended sediment (SS) yield due to extreme rainfall events and how this relationship affects the survival of forest plants. Several phenomena contribute to this relationship: increasing evaporation (amount of water vapour discharged from soil) due to increasing air temperature, decreasing moisture content in the soil, the collapse of aggregates of fine soil particles, and the resulting effects on forest plants. To clarify the relationships among climate variation, the collapse of soil particle aggregates, and rainfall-runoff processes, a numerical model was developed to reproduce such aggregate collapse in detail. The validity of the numerical model was confirmed by its application to the granitic mountainous catchment of the Nagara River basin in Japan and by comparison with observational data. The simulation suggests that important problems, such as the collapse of forest plants in response to decreases in soil moisture content and antecedent rainfall, will arise if air temperature continues to increase.

COST-BENEFIT ANALYSIS OF LANDUSE REGULATION STRATEGY BASED ON FLOOD RISK ASSESSMENT USING A LOCATION EQUILIBRIUM MODEL INCORPORATING HOUSEHOLD INCOME DISTRIBUTION

流域管理的治水対策の費用便益評価を行う枠組みの中で，世帯の所得分布を考慮できるように立地均衡モデルを拡張し，これを用いて寝屋川流域における土地利用規制の費用便益評価を行うとともに，土地利用規制が世帯に与える影響を所得の違いに応じて分析した．その結果，土地利用規制の影響は各所得層で異なることが明らかになった．所得に対する費用の割合は所得が低いほど大きく，低所得層の負担が大きくなることがわかった．また，再現期間で5年（時間45mm程度）相当の弱いレベルの土地利用規制で総便益が最大となり，再現期間で25年以上の強い土地利用規制を実施した場合は，費用が便益を上回る結果となった．以上より，本流域では，比較的高い頻度で浸水する地区の住宅地としての利用は避けた方がよい可能性のあることが明らかとなった．

COST-BENEFIT ANALYSIS OF BUILDING REGULATION BASED ON FLOOD RISK ASSESSMENT AND ITS COMPARISON TO LAND USE REGULATION

本研究では，大阪地域と東京地域を対象として，水災害危険度に基づいて建築規制を実施した場合に生じる費用と便益を比較することで，建築規制の利害得失や適用性について検討した．その結果，いずれの地域においても，今回検討した範囲では建築規制の総便益は正となり，建築規制は水防災対策として一定程度の適用性を有していること，大阪地域のほうが東京地域より建築規制の総便益が大きくなり，比較的少ない負担で効果的に水災害被害額を減少させうることが明らかとなった．さらに，同じ地区を規制対象とする建築規制と土地利用規制を比較したところ，建築規制のほうが土地利用規制より総便益が若干大きいか同程度となることがわかった．このことから，建築規制は床下浸水を許容する土地利用規制とほぼ同程度の適用性を有していると考えられる．

A REAL TIME DAM INFLOW PREDICTION SYSTEM BASED ON DISTRIBUTED RAINFALL-RUNOFF MODEL

Today, we can get high quality information about spatial distribution of rainfall observed by radar on real time. In order to make effective use of such a spatial rainfall information, and to improve the flood forecasting accuracy for dam operation, we developed a real time dam inflow prediction system based on distributed rainfall-runoff model. In our system, channel flow is calculated by using the integrated kinematic wave model for channel networks, and the state of channel flow is updated with Kalman filter. Simulations are carried out with dam inflow data and precipitation data observed by rain gauges, and it was confirmed thatthe prediction system performs well.