Flood Levee Research Articles

Overtopping Reliability Models for River Levee

Wind setup and wave run-up heights are important considerations in determining the total height of a river levee system. In this study, two generalized reliability models for a levee system are developed considering the inherent uncertainties associated with wind as well as hydrologic and hydraulic processes. The generalized models can be simplified to reliability models when only hydrologic and hydraulic uncertainties are incorporated. By using the concept of a wind-flood damage ratio, reliability values of a levee system can be obtained and discussed on the same economic basis. A higher ratio of wind-flood damage requires a higher total levee height to obtain the same reliability value as for a lower wind-flood damage ratio. The total reliability decreases with an increase in service period of the hydraulic structure and/or the wind-flood damage ratio if the total levee height remains the same. The models enable the performance of a levee system to be predicted in terms of reliability. When the total levee height is specified, different options exist for the flood levee height to protect flood overtopping and for the freeboard allowance to withstand wind-generated wave overtopping. The best option can be chosen on the basis of a maximum reliability value. An example application of the models shows how the flood levee height and freeboard allowance may be determined to yield maximum reliability.

Bedrock benches and boulder bars: Floods in the Burdekin Gorge of Australia

The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability. Slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1,200 yrs. These floods range in magnitude from 11,000 to 30,000 m 3 s -1 and are characterized by large downstream variations in hydraulics. The floods generate high values of velocity, boundary shear stress, and stream power per unit area. Downstream fluctuations in these variables help to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels. Boulder bars and flood levees form where shear stress and stream power decrease due to channel widening. Small-scale erosional features, such as potholes and troughs, are best developed at sites of channel constriction and associated increases in shear stress and stream power. The development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure, and by flow hydraulics. All of the channel features are related to the hydraulics of large floods, which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge.

RELIABILITY‐BASED DESIGN CONCEPTS IN HYDRAULIC ENGINEERING1

ABSTRACT: Different probability‐based concepts for design of hydraulic structures are presented that could replace or complement traditional designs based on the concept of design load plus a safety margin. The traditional concept (here called Level I design) does not permit the assignment of the correct probability to the failure event. Concepts which are based on the correct probability of failure are based on descriptions of resistance and load as random variables. Level II design assumes these random variables to be Gaussian distributed. Level III is based on arbitrary distributions of load and resistance. Level II and III concepts are appropriate for evaluation of a design's reliability. Design Level IV is also based on the joint probability density function for loads and resistances; in addition, it requires the assignment of a consequence function to each combination of resistance and loads. The design concepts are illustrated with the design of a flood levee on a river.

Some new perspectives on the probabilistic modeling of floods

In the design of a flood control system a flood hydrograph realization, corresponding to a stated risk is taken as the hydrograph to be routed through the system. In general, floods come in groups and form multipeaked hydrographs. A multipeaked flood hydrograph (or a group of floods) when routed through a reservoir-levee-wall flood control system may yield a more critical result than routing a single-peaked flood hydrograph even if each of the peaks in the multipeaked hydrograph is smaller than the peak of the single-peaked hydrograph. This is due to the fact that while the earlier floods within a flood group may not be critical for the system, they may still fill the reservoir to a critical level so that the later floods in the flood group are passed by the reservoir directly downstream and, thereby, cause damage downstream. Similar arguments also follow for the flood levees and flood walls on the river reach downstream of a reservoir, since there is a channel storage during the passage of a flood wave through the river channel. Also due to storage in the system, not only the peak discharge stochasticity but the stochasticity of the complete flood hydrograph shape needs to be described. Instead of basing the probabilistic description of flood realizations on a single peak discharge, this paper presents in a mostly graphical fashion some new stochastic models for the complete shapes of generally multipeaked continuous-time flood realizations. First, a point stochastic model of the multistation precipitation process is presented. Using precipitation as the driving process of floods, a three-dimensional point stochastic (3-DPS) model for flood starting times, times to peaks and peak magnitudes is presented. Then a stochastic model for the continuous-time flood realizations, as an extension of the previous 3-DPS model, is discussed.

Hydraulic Uncertainties in Flood Levee Capacity

This paper explores hydraulic uncertainty in flood levee capacity, which results from the inability of mathematical models and/or empirical equations to describe completely the physical flow process of floods. The hydraulic uncertainty is evaluated through a first‐order analysis of uncertainties of Manning's equation. This study shows that the roughness coefficient and the friction slope dominate the variation in hydraulic uncertainty in that they account for 95% of the hydraulic uncertainty. A procedure is developed for determining the hydraulic effect of friction slope uncertainty.

Comment on ‘Optimal risk‐based design of flood levee systems’ by Yeou‐Koung Tung and Larry W. Mays

Water Resources ResearchVolume 18, Issue 6 p. 1745-1746 CommentsFree Access Comment on ‘Optimal risk-based design of flood levee systems’ by Yeou-Koung Tung and Larry W. Mays Istvan Bogardi, Istvan BogardiSearch for more papers by this authorLucien Duckstein, Lucien DucksteinSearch for more papers by this author Istvan Bogardi, Istvan BogardiSearch for more papers by this authorLucien Duckstein, Lucien DucksteinSearch for more papers by this author First published: December 1982 https://doi.org/10.1029/WR018i006p01745AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Bogardi, I., Flood exposure recommended as a parameter for describing the fatigue loading on flood control structures, Bull. IAHS, 3, 19– 27, 1968. Bogardi, I., A. Casti, J. Casti, L. Duckstein, Optimal flood levee designs by dynamic programmingRep. RM-77-12, 4– 13Int. Inst. for Appl. Syst. Anal., Laxenburg, AustriaMarch, 1977. Duckstein, L., M. Gablinger, The dynamic value of conjunctive storage of water, Appl. Math. Model., 5, 394– 398, 1981. Larson, R., J. Casti, Principles of Dynamic Programming, Part 1, Basic Analytic and Computational Methods, Marcel Dekker, New York, 1978. Linsley, R. K., M. A. Kohler, J. L. H. Paulhaus, Hydrology for Engineers 3, McGraw-Hill, New York, 1982. Tung, Y.-K., L. W. Mays, Optimal risk-based design of flood levee systems, Water Resour. Res., 17(4), 843– 852, 1981. Volume18, Issue6December 1982Pages 1745-1746 ReferencesRelatedInformation

Risk models for flood levee design

In the design of flood levee systems there are many parameters and variables with associated uncertainties. This paper presents models which systematically analyze the various types of uncertainties in the hydrologic aspect as well as hydraulic aspect of design and analysis to define the risk and reliability of overtopping. Both static and time dependent risk models are developed. Results show that risk evaluated by the simple return period method can be underestimated by 10% to 50%, depending on the loading distribution model used. The risk‐safety factor relationships are shown to be very sensitive to the hydrologic loading probability model adopted, so that composite models are suggested for risk analysis.

Optimal risk‐based design of flood levee systems

Methods for risk analysis of flood levee systems that account for hydraulic and hydrologic uncertainties are incorporated into optimal design procedures. The uncertainties are considered in the expected damage functions so that the objective is to determine the optimal flood levee system design that minimizes the total expected cost. Dynamic programing (DP) and discrete differential dynamic programing (DDDP) models are developed for the optimization. The DDDP approach tremendously reduces the computation effort and provides more accurate results when compared to the DP. Through the use of an example, different design philosophies (level of risk consideration) are examined.

Application of Reliability Theory to Hydraulic Engineering Design

The reliability of two hydraulic engineering systems with random resistance and/or loading is estimated. The first system is a flood levee reach that is subject to four failure modes: overtopping, boiling, slope sliding and wind wave erosion. The second system is a flood levee system along a confluence reach. For this levee, failure by overtopping depends on the bivariate distribution of water stages in the tributary and the main river, linked by a backwater curve. The failure probability, which is the complement of reliability, is estimated analytically for the first system, and then by hydraulic calculations and simulation for the second system. In each case, estimated reliabilities agree well with experimental observations.

An analysis of flood levee reliability

In most studies of flood levee reliability the probability of failure is taken to be equal to the probability that the flood peak discharge will exceed the capacity of the channel‐levee system. Such an analysis assumes that levees fail only by overtopping, whereas in many cases, levees fail at discharges much below the channel capacity. In the present paper the discharge at which levees fail is assumed to be a random variable with a particular distribution. The results on the flood frequency curve are presented for two conditions: one for the failure discharge having a uniform probability density function and the other for failure discharges having a quadratic distribution. In a similar manner the effects on the net benefits from levee construction were also analyzed and results are given.

SAMPLE UNCERTAINTY IN FLOOD LEVEE DESIGN: BAYESIAN VERSUS NON‐BAYESIAN METHODS1

ABSTRACT: Bayesian and non‐Bayesian flood levee design methods that account for the uncertainty due to limited record length are compared using a case study. The first method, Bayesian decision theory (BDT), imbeds the uncertainty in the parameters of the yearly peak stage into a loss function. The optimum design of the flood levee, called Bayes design, corresponds to the minimum expected loss, called Bayes risk. The second method, induced safety algorithm (ISA), computes a margin of safety to be added to either an existing levee or a levee designed by classical benefit‐cost analysis. The design decision is shown to fluctuate as different record lengths are considered. For short record lengths, BDT, which takes small sample bias into account, appears to yield a more conservative design than ISA. On the other hand, ISA, which is simple to implement, seems to be preferable to BDT for longer record lengths.

Bayesian decision theory applied to design in hydrology

The role of Bayesian decision theory in hydrologic design problems is presented both in theory and by example. The theory is applied to an actual flood levee design problem on the Rillito Creek floodplain in Tucson, Arizona. Computer solutions provide a basis for judging the costs of overdesign in the face of uncertainty in the parameters of the flood frequency model (log normal) and for determining the worth of hydrologic data. One conclusion is that decision theoretic analysis looks at the decision situation from the standpoint of the engineer: how can one best decide in the face of limited data and the present knowledge about system behavior?

FLOOD EXPOSURE RECOMMENDED AS A PARAMETER FOR DESCRIBING THE FATIGUE LOADING ON FLOOD CONTROL STRUCTURES

Abstract Highwater stages of frequently very long duration which commonly develop on plain-land watercourses having flat slopes, constitute a fatigue loading on the flood levees built along such streams. In order to describe by a single parameter the magnitude of this fatigue load, the concept of flood exposure has been introduced and defined as the area under the flood hydrograph exceeding a certain stage (the toe of the levee). Consequently both stage and duration of highwater are taken into consideration. Positive relationships have been found to exist between the magnitude of flood exposure and adverse phenomena associated with highwaters (groundwater emergence, underseepage, leakage, boil formation, saturation slumping and wave action). The magnitude of the labour force, fleet of mechanical equipment and materials used in flood fighting, consequently the costs thereof depend to a significant extent on the magnitude of flood exposure, which can thus be used conveniently for economic analysis as well. Mathematical statistical analysis has shown the logarithmic normal distribution to fit best to the empirical distribution of flood exposure. The full set of data was found to be homogeneous in the gaging sections examined on the Danube and Tisza Rivers. Sets of data have been grouped according to the dates of major interference into the life of the watercourse. It was found further that in the sections considered the duration of highwaters could also be regarded as a homogeneous random variable.

Field and Laboratory Behaviour of a Lightly Overconsolidated Clay

Synopsis An extensive series of flood levées up to 10 ft high is being constructed in Launceston, Australia. The levées rest on a soft clay and a major failure occurred on raising the levées to 7 ft height. An investigation has been made to analyse the failure and recommend on future construction procedure. Laboratory testing showed the soil to be lightly overconsolidated and observations on a full size instrumented test levée indicate that the degree of overconsolidation, although small, influences field behaviour to a marked extent. Total stress analyses gave a factor of safety against general shear failure of 1·60, and an effective stress analysis gave slightly below unity assuming c′ = 0, indicating that critical state conditions may apply. A four stage construction procedure has been recommended on the basis of results from the investigation. Une série étendue de digues de hauteurs allant Jusqu'à 3,04 m (10 pieds) est en train d'être construite à Launceston en Australie. Les digues reposent sur un argile mou et une rupture de première importance s'est produite lorsque l'on éleva les digues à une hauteur de 2,13 m (7 pieds). Une enquête a été menée pour faire une analyse de la rupture et pour faire des recommendations sur les procédés futurs de construction, Des essais en laboratoire ont fait apparaitre que le sol était légèrement trop consolidé et des observations portant sur une digue d'essai de grandeur nature pourvue d'instruments indiquent que le degré de surconsolidation, quoique réduit, a une influence sur le comportement sur place à un degré notable. Des analyses de la contrainte totale donnèrent un facteur de sécurité par rapport à un défaut de cisaillement général s'élevant à 1,60 et une analyse de la contrainte effective a produit un chiffre légèrement inférieur à l'unité en supposant que c′ = 0, indiquant que des conditions d'état critique e peuvent s'appliquer. Un procédé de construction en quatre stages a été recommandé à partir des résultats de l'enquête.