Institute of Water structures

Failures of small dams can pose a serious threat to people and property even if the size of the schemes is relatively low. In many cases, small dams are situated in a cascade along streams, meaning that the failure of the uppermost dam may cause the dams downstream to fail. In this paper, a cascade of three small reservoirs, Lichnov II (14.6 m high), Lichnov III (10 m high), and Pocheň (8.5 m high), is the subject of the dam break analyses carried out via various methods such as empirical formulae, analogy, and hydraulic modeling. The dam-break flood routing was simulated using a shallow water flow hydraulic model. The simulations confirm that the attenuation effect of the peak discharge is governed by the flood volume, slope, and morphology of the floodplain and increases with the distance from the breached dam following an approximately exponential trend. When estimating peak discharge, empirical formulae derived for a single dam break should be applied carefully as they may under...

The Influence of Current on the Height of Wind Wave Run-Up: A comparison of experimental results with the Czech National StandardOne of the basic questions related to the safety of dikes and river levees is the size of the freeboard. One of the important parameters for freeboard design is the height of waves and wave run-up on levee slopes. Routine and standardised calculations of wave run-up deal with the freeboards of dams where wind waves originate on the still water of the reservoir. In the case of running water in streams (thereinafter only "currents") the effect of wave and current interaction on wave run-up is usually not taken into account due to the lack of reliable knowledge regarding the phenomenon. In the Czech Republic this question is topical in the case of large rivers such as the Elbe, the Vltava and the Morava. Within the framework of the projects Hydralab III and NAZV QI 92A139, hydraulic research and further analysis focused on wave run-up as a result of...

The hole erosion test (HET) is used in the study of soil erosion in the case of what is known as “piping” when concentrated leaks occur. The HET enables the determination of soil erosion characteristics such as the critical shear stress along the pre-formed hole (pipe) and the coefficient of soil erosion. Normally, in the HET, the hydraulic gradient is determined from the difference between the piezometric heads measured at the inflow and outflow chambers (upstream and downstream of the soil specimen). Hydraulic analysis shows that such measurements ignore losses at the entrance and exit of the hole, causing the overestimation of the hydraulic gradient along the length of the hole, and thus the calculated shear stress. In this technical note, the results of preliminary analysis using the Bernoulli principle and of numerical study of the pressure conditions in the HET apparatus are shown. The turbulent flow in the HET apparatus was calculated using ANSYS commercial CFD (computational...

In this paper a model for the estimation of the number of potential fatalities is proposed based on data from 19 past floods in central Europe. First, the factors contributing to human losses during river floods are listed and assigned to the main risk factors: hazard-exposure-vulnerability. The order of significance of individual factors has been compiled by pairwise comparison based on experience with real flood events. A comparison with factors used in existing models for the estimation of fatalities during floods shows good agreement with the significant factors identified in this study. The most significant factors affecting the number of human losses in floods have been aggregated into three groups and subjected to correlation analysis. A close-fitting regression dependence is proposed for the estimation of loss of life and calibrated using data from selected real floods in central Europe. The application of the proposed model for the estimation of fatalities due to river floods is shown via a flood risk assessment for the locality of Krnov in the Czech Republic.

Intention of the project FlowDike-D is to quantify the impacts of current and wind on wave run-up and wave overtopping and to consider these processes in existing design formulae for estuarine, river and sea dikes. Physical model tests were carried out in the shallow water basin at DHI (Hørsholm/Denmark) for two different dike geometries (1:3 and 1:6 sloped dike). The paper introduces the model setup and test programme followed by a short description of the applied instrumentation. The test results for wave run-up and wave overtopping with oblique and non-oblique wave attack, but without current, correspond well with existing formulae from the EurOtop-Manual (2007). The influence of current parallel to the dike combined with different angles of wave attack on wave overtopping and wave run-up has been quantified. A distinction was made between wave attack with and against the current.

Recently, the risk analysis of floodplain areas has been one of the most frequently used tools for assessment and design of flood protection measures. One of sub-tasks of risk analysis is to estimate probability of river channel overbanking. The basis of a probabilistic approach to evaluation of river channel capacity is shown in this paper. Probability of river channel overbanking

Dam-break due to overtopping is one of the most common types of embankment dam failures. During the floods in August 2002 in the Czech Republic, several small dams collapsed due to overtopping. In this paper, an analysis of the dam break process at the Luh, Velký Bělčický, Melín, and Metelský dams breached during the 2002 flood is presented. Comprehensive identification and analysis of the dam shape, properties of dam material and failure scenarios were carried out after the flood event to assemble data for the calibration of a numerical dam break model. A simple one-dimensional mathematical model was proposed for use in dam breach simulation, and a computer code was compiled. The model was calibrated using the field data mentioned above. Comparison of the erodibility parameters gained from the model showed reasonable agreement with the results of other authors.

At two streams in the Czech Republic, eight dye-tracing experiments were carried out during past two years. The purpose of these experiments was to collect data sets for the calibration and verification of both analytical and numerical solution of mathematical model of advective-dispersion problem. First part of the paper deals with the preparation, accomplishment and preliminary evaluation of the experiments. Preliminary evaluation deals with the assessment of stream discharge conditions and mass balance of the dye injected. Evaluation of tracing tests by modelling techniques was performed using both one-dimensional and two-dimensional analytical and numerical stream water quality models. Within the solution, the estimate of the coefficients of transversal and longitudinal dispersion was carried out together with the estimate of the lag coefficient using inversion algorithm. Obtained values of mentioned model parameters generally correspond with values referred to in available lite...

For a design of the volume of the rainfall infiltration facility, the coefficient of infiltration is determined by an infiltration test. The use of the coefficient of infiltration brings certain uncertainties into the solution, arising from different conditions in carrying out the infiltration test and in operating a real infiltration facility. In this study an analysis is carried out of factors that influence the process of infiltration and related uncertainties influencing the determination of the storage volume of the infiltration facility. The effect of the individual factors on the design of the infiltration facility was analyzed using numerical simulations by the software HYDRUS-2D and expressed using partial reliability factors. For their determination the nomographs were set up by extensive numerical computations.

The design and assessment of the slope stability of small embankment dams is usually not carried out using slope stability calculations but rather by the comparison of proposed or existing dam slopes with those recommended by technical standards or guidelines. Practical experience shows that in many cases the slopes of small dams are steeper than those recommended. However, most of such steeper slopes at existing dams do not exhibit any visible signs of instability, defects or sliding. For the dam owner and also for dam stability engineers, the safety of the slope, expressed e.g. via a factor of safety, is crucial. The aim of this study is to evaluate the safety margin provided by recommended slopes. The factor of safety was evaluated for several dam shape and layout variants via the shear strength reduction method using PLAXIS software. The study covers various dam geometries, dam core and shoulder positions and parameter values of utilised soils. Three load cases were considered: one with a steady state seepage condition and two with different reservoir water level drawdown velocities-standard and critical. As numerous older small dams lack a drainage system, variants with and without a toe drain were assessed. Calculated factors of safety were compared with required values specified by national standards and guidelines.

The reliability analysis of a flood protection dike depends on the probability of its failure. In this context, the surface lining covering the landside slope of the dike plays a considerable role, as it protects the dike against failure due to overtopping. A mathematical model describing the overtopping and erosion processes is proposed. The overtopping is simulated using simple surface hydraulics equations

Shallow-water models are standard for simulating flow in river systems during floods, including in the near-field of sudden changes in the topography, where vertical flow contraction occurs such as in case of channel overbanking, side spillways or levee overtopping. In the case of stagnant inundation and for frontal flow, the flow configurations are close to the flow over a broad-crested weir with the trapezoidal profile in the flow direction (i.e. inclined upstream and downstream slopes). In this study, results of shallow-water numerical modelling were compared with seven sets of previous experimental observations of flow over a frontal broad-crested weir, to assess the effect of vertical contraction and surface roughness on the accuracy of the computational results. Three different upstream slopes of the broad-crested weir (V:H = 1:Z1 = 1:1, 1:2, 1:3) and three roughness scenarios were tested. The results indicate that, for smooth surface, numerical simulations overestimate by abo...

Seepage is the key factor in the safety of dikes and earth-fill dams. It is crucial to identify and localize the seepage excesses at the early stages before it initiates the internal erosion process in the structure. A proper seepage monitoring system should ensure a continuous and wide area seepage measurement. Here, continuous monitoring of seepage at the laboratory-scale is achieved by a passive optical fiber Distributed Temperature Sensing (DTS) system. An experimental model was designed which consists of initially unsaturated sand model, water supply, seepage outflow, optical fiber DTS system, and water and air temperature measurement. Initially, the sand temperature was higher than the temperature of the seepage water. An optical fiber DTS system was employed with a high-temperature resolution, short sampling intervals and short time intervals for temperature monitoring in the sand model. In the system, the small variation in the temperature due to groundwater flow was detecte...

Asphalt concrete linings (ACLs) are frequently used in pumped-storage hydropower schemes with frequently oscillating reservoir water levels. ACLs can be damaged during operation, which shortens their service life. One type of damage is the formation of blisters. In this study, the bacterial diversity in damaged and undamaged portions of the ACL of an upper water reservoir was evaluated. Matrix-Assisted Laser Desorption/Ionization–Time of Flight Mass Spectrometry (MALDI-TOF MS) was employed for the identification of the bacterial strains. No bacteria were detected growing inside the undamaged dense layer. Low bacterial diversity (2 and 4 morphotypes) was found in the upper part that has no contact with the reservoir water and in the lowest part that is permanently under water, respectively. More complex bacterial diversity (16 morphotypes covered by 2 different genera and 4 species) was found on the mastic coating of the ACL, which was alternately under and above the water. Very comp...

Seepage flow through embankment dams and their sub-base is a crucial safety concern that can initiate internal erosion of the structure. The thermometric method of seepage monitoring employs the study of heat transfer characteristics in the soils, as the temperature distribution in earth-filled structures can be influenced by the presence of seepage. Thus, continuous temperature measurements can allow detection of seepage flows. With the recent advances in optical fiber temperature sensor technology, accurate and fast temperature measurements, with relatively high spatial resolution, have been made possible using optical fiber distributed temperature sensors (DTSs). As with any sensor system, to obtain a precise temperature, the DTS measurements need to be calibrated. DTS systems automatically calibrate the measurements using an internal thermometer and reference section. Additionally, manual calibration techniques have been developed which are discussed in this paper. The temperatu...

Procjena onečišćenja podzemnih voda na području bivšeg vojnog aerodroma Rad se bavi analizom onečišćenja podzemnih voda na području bivšeg vojnog aerodroma uzrokovanog curenjem goriva iz spremnika te drugim slučajnim izlijevanjem onečišćivača, uglavnom policikličkih aromatskih ugljikovodika (PAH). Modeliranjem je zamijećeno da nakon prestanka crpljenja zagađivač zaobilazi brtveni zid oko njegovih rubova te se nastavlja kretati prema području gdje se zahvaća voda. Ovim modelom zaključeno je da postoji niska razina opasnosti od zagađenja, uglavnom zbog utjecaja raspada (razgradnje) zagađivača te sorpcije.

Recently, the risk analysis of floodplain areas has been one of the most frequently used tools for assessment and design of flood protection measures. One of sub-tasks of risk analysis is to estimate probability of river channel overbanking. The basis of a probabilistic approach to evaluation of river channel capacity is shown in this paper. Probability of river channel overbanking is directly connected to channel capacity, which is affected by a number of separate uncertainties, namely by input data uncertainty (e.g. hydraulic parameters, channel geometry and design discharge), mathematic model inaccuracy or statistical uncertainty. In the study presented, the Monte Carlo simulation was used for statistical evaluation of influence of the uncertainties mentioned above. The method proposed was applied and verified within the parametric study at the Svitava river channel.