DOC

Failure Causes and Design Methods of River Levees

By Florence Gibson,2014-07-08 07:54
10 views 0
Failure Causes and Design Methods of River Levees

    Failure Causes and Design Methods of River Levees

    Kwangseok Yoon

    Korea Institute of Construction Technology

    Abstract

    In recent years, Korea has experienced damages due to the great floods such as Typhoon ‘Rusa’ and ‘Maemi’. Especially, levee failure caused the loss of lives and poverty around

    riverine area. It has known that there are several causes in levee failure in flood time. In this study, the analysis of the levee failure causes and its properties was conducted and measures for improving stability of the levees were suggested. In conclusion, the quantitative method should be adapted to ensure the levee stability. The way to do is to estimate the internal and external forces. Moreover, a scientific study, an experimental study and the field investigation survey are carried out to develop the design method in the future.

    Key Words: Levee Failure, Design Method, Levee Safety Improvement

    1. Introduction

    The levees have been considered as important flood protection structure. For that reason, it is clearly brought out that the standard levee design and the plan for stability of the levees should be examined and established. This study performs the analysis of the levee failure causes and its properties and suggests measures for improving stability of the levees.

2. Analysis of Levee Failure Causes

    The causes of the levee failure in flood time, in general, are classified into four cases which are overflow, erosion, instability of embankment, and failure by river structures. The overflow is caused by outflow over design flood and by debris which makes carrying capacity to decrease and the erosion arises from scouring of the levee slope and bank. The instability of an embankment is poor quality embankment materials and piping of embankment and foundation. The failure by river structures is rose from a failure of drainage structures through the levee and from a failure of a contact surface with other structure made of different

     74

materials from the levee.

    The levee failure related to seepage mainly divided into two types such as sliding and seepage which is classified to seepage through embankment and under seepage. Figure 1 shows the piping by the seepage.

    Figure 1. The levee failure by seepage

    The erosion of levee is sorted to scour of stream bank and direct erosion of slope as shown in Figure 2. In this manner, the erosion of levee occurs at the bank and around the river structures.

    Direct Erosion

    of Slope

    Main ChannelScour of Stream

    Bank `

    Figure 2. The levee failure by erosion

    The overflow causes the levee failure when a larger flood than the design flood arises so that the flood level is over the top of levee. It is because the landside slope and crown edge whose material mainly soil are eroded during overflow.

    Figure 3. The levee failure by overflow

     75

    Some levees collapse due to unequal settlement between a flexible embankment and a solid structure which are made-up with different materials that cause piping phenomenon. The failure processing by the piping phenomena is showed in Figure 4. Recently, there have been some failures of drainage structures through levees which commonly had pile foundation to

    Refilling Soilsupport them. Settlement

    EmbankmentSoil

    Sand Layer

    FrailClay Layer

    ? Right after setting up ? Occurrence of crack and the drainage structuresrelaxation by settlement

    Crack and HeavingRelaxation

    CavityCavity

    ? Extension of crack and ? Occurrence of cavity in relaxationembankment by leakage around Figure 4. The levee failure process around the drainage structure the drainage structures

    Distribution of levee failure patterns (Total : 758 events, Duration : 1987-2003)Failure by Instability ofstructuresembankmentnearby (87), 11.5%(76), 10.0%Overflow (300), 39.6%

    Erosion (295), 38.9%

     Figure 5. Levee failure causes

    The analysis of the levee failure reasons according to the failure patterns from 1987 to 2003 is presented in Figure 5. As the cause of the levee failure, the overflow has the biggest portion, 300 cases of the 758 cases (39.6%), the erosion has 295 cases(38.9%), the instability of embankment has 87 cases (11.5%), and the failure by structures nearby has 76 cases (10.0%).

3. Levee Design Trend at Domestic and Abroad

3.1 The design of levee cross section

     76

    The slope degree is an important factor to decide the size of levee cross section. Flatter slope improves the stability of the levee than steeper one under the same condition of levee height and crown width. Guidelines for levees in Japan, suggests a gentle slope under 1V on 3H slope, Guidelines in Korea was revised the design slope from 1V on 2H to 1V on 3H after damages from the large floods, Typhoon Rusa and Maemi in 2002 and 2003, respectively.

    About freeboard, guidelines by US Army Corps of Engineers applies it differently with location such as agricultural area and urban area, while Korea and Japan apply it by the design flood without consideration of levee location. This presents that the freeboard is decided with considering importance in flood management.

    Table 2. Comparison in criteria of levee cross section between domestic and foreign countries

    US Army Crops Korea Japan of Engineers

    Classification Guidelines for Guidelines for River Guidelines for Design and

    River Design Design River Levee Design Construction of

    (2002) (Revised in Oct., 2003) (2000) Levees (2000)

    1H on 3V Slope 1H on 2V 1H on 3V 1H on 2V (super levee: 1/30)

    Depending on 0.61m Freeboard (unchanged) Same as Korea Flood Discharge (2ft)

    Depending on Minimum width Crown width (unchanged) Same as Korea Flood Discharge of 3.05-3.66m

3.2 The method of evaluating levee safety

    Deciding safety from design cross section is very important in the levee design. The safety evaluation should be done about flow characteristic, foundation, and embankment feature with accuracy. If there is an estimate that the evaluated levee is unsafe, the levee should be redesigned to improve the safety. Therefore, it is necessary to perform proper evaluation regarding purposes and methods. Table 3 shows the method of safety evaluation at domestic and foreign countries.

    Table 3 Comparison in method of evaluating levee safety

    Korea Japan US Army Crops of Engineers Classi-

    fication Guidelines for River Guidelines for River Levee Design and Construction of

    Design (2003) Design (2000) Levees (2000)

    1) Sliding 1) Overtopping- 1) Sliding, Piping

    2) Piping 2) Sliding and piping - During and End-of-

    3) Settlement 3) Erosion of slope Construction

    Factor 4) Drainage structures - Steady State Seepage

    Conditions - Sudden Drawdown Conditions

     77

4. Concepts of Improving Stability

    The levee design is carried out by performance criteria that are the safety evaluation with computation of internal and external forces in Japan, while it is conducted by geometry criteria in Korea.

4.1 The safety factor against levee failure by sliding

    In table 4, minimum safety factor of sliding in Korea and Japan is presented. According to the research results of the design examples, 1.3 has been applied as the safety factor for sliding in Korea. On the other side, the safety factor is weighted according to failure and embankment records in Japan. In fact, when the failure and embankment records are complicate, the minimum safety factor is about 1.6 which is larger than one set in Korea. Therefore, it is required to bring in the safety factor considered with failure and embankment records for improving levee stability.

    Table 4 Minimum safety factors for sliding

    Classification References Minimum safety Remarks

    factor

    Korea Guidelines for River Facilities (1993) 1.3

    Guidelines for River Design (2003) 1.3-2.0

    Japan Guidelines for River Levee Design Landside slope 1.2×α×α 12(2000) : weighting factor αα : 1.2(complicated) 11for embankment : 1.1(simple)

    α : weighting factor : 1.0(new levee) 2

    for failure α : 1.1(noteworthy) 2

    : 1.0(not notice)

    1.0 Riverside slope

4.2 Flood wave for unsteady seepage analysis

    In order to draw the design flood wave for seepage analysis, it is needed to determine the duration time of flood, the increasing and decreasing slope of hydrograph. The procedure of the method is described in Figure 6. Firstly, the envelope of the maximum duration time at regular intervals of water level should be drawn from all design flood hydrographs as shown in Figure 6(b). The next process is to integrate the envelope line. If the peak of hydrograph does not reach the design level of levee, the 1hr-point on the design level is the starting point of the envelope. The design recession slope can be determined by choosing the maximum recession slope among the hydrographs in Figure 6(a). The maximum duration time at the normal stage can be determined simply by drawing the rising slope to make the enveloped

     78

    Duration time of area of Figure 6(b) equal to that of Figure 6(c) in the end. Duration time of peak stage1hrpeak stageProject level of levee

    Peak stage = Project Peak stagelevel of leveeFlood ‘A’