Case Study of the Chaq-Chaq Dam Failure: Parameter Estimation and Evaluation of Dam Breach Prediction Models

Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116

RESEARCH ARTICLE

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Case Study of the Chaq-Chaq Dam Failure: Parameter Estimation and Evaluation of Dam Breach Prediction Models Dr. KawaZedanAbdulrahman Abstract On 4th of February, 2006 at about 10:00 pm.Chaq-Chaq dam failed due to overtopping. The fall of 131.2 mm of rain over a 24-hour period was recorded at Sulaimani metrological gage station, which is located about 7.5Kmsouth-east of the dam. As a result, the reservoir level rose, the dam has been overtopped and finally breached near the spillway at the right abutment. Fortunately no human lives loss nor important structure destruction were reporteddue to the dam failure. The aim of this paper is to estimate the flood hydrograph passing through Chaq-Chaq dam breach using measured breach geometry as input to unsteady option of HEC RAS 4.1.0 and calibrating the breach formation time to obtain the measured maximum water surface at ChaqChaq Bridge (1.36 km downstream of dam axis). In addition the recent breach prediction models were evaluated to check their accuracy in predicting the breach geometry, breach formation time and peak breach discharge.

I. Introduction Chaq-Chaq dam is located about 2 km NE of Sulaimani city (Iraq). Fig. 1 shows a satellite image of the area between Chaq-Chaq dam and Chaq-Chaq Bridge.Chaq-Chaq dam is a zoned earth dam of central clay core and gravelly shell as shown in Fig.2. Chaq-Chaq dam was designed and built by engineers of little experience in the field of dam design and construction. As a recognized design problem,one of the major mistakes was the building of the spillway beside the dam in the same valley not as a separate structure. The spillway wall has been made vertical. Compaction of an embankment near a vertical wall is notrecommended in constructing embankmentdams because this procedure will produce a weak bond at the interface of the wall and the embankment(FEMA, 2005).In addition; the required compaction for the materials close to the vertical wall will not be gained. This weak-compacted portion will be weaker compare to the other well-compacted portions of the dam. Therefore, the dam breached close to the spillway rather than other locations. In order to check the accuracy of existing breach prediction models in predicting the breach geometry, breach formation time and peak breach discharge; a bathymetric survey after the dam failure has been carried out to obtain the breach geometry. Extensive interviews with the surrounding habitants, owners of the tourism cabinets, and directorate of security have been done to gather information about the breach formation time and the highest water level at Chaq-Chaq Bridge.The breach formation time and

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the highest water level at Chaq-Chaq Bridge is used as input to calibrate the HEC RAS 4.1 (Brunner, 2010 a,b) to achieve the maximum flood discharge passing through the dam breach as it will be presented as the followings.

Figure 1: Satellite image showing the area between Chaq-Chaq dam and Chaq-ChaqBridge.

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Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116

Spillway

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Dam Body

Core

Flow Direction

Shell

Figure 2: Photo of Chaq- Chaq dam after failure.

II. Breach GeometryData Bathymetric survey has been carried out to obtain the breach geometries;Table 1 shows the geometry parameters of Chaq-Chaq dam and its breach. Table 1: Geometry parameters of Chaq-Chaq dam. Parameter Height Top Upstream Downstream Breach Breach Breach Dam of dam width slope (v:h) slope (v:h) Bottom Average Top crest H (m) (m) width width width level (m) (m) (m) (masl) Value 14.5 9 1:3 1:2 29.6 38 46 780

III. Breach hydraulic data Due to insufficient spillway capacity ChaqChaq dam was overtopped and then failed. According to a local witness (who was the formal responsible of the dam and his house was located about 100 m far from the dam) the maximum depth of water above the dam crest was between 0.5 − 0.6 m. So, he was also estimated the breach formation time to be between 1 to 1.5 hours. In addition the maximum water level due to the dam failure flood at Chaq-Chaq bridge which is located about 1.36 km downstream of

Para meter

Value

the dam has been decided based on eyewitness accounts. The maximum water level at the bridge was around 759.4-759.5maslas corresponded to 20-30 cm below the lower cord of the bridge. There was a security team at the bridge to prevent peoples from passing the bridge because there was a potential of bridge failure due to the flood before and during the dam failure. The flood extent at the bridge was seen by the security team. Table 2 shows some of the hydraulic parameters of Chaq-Chaq dam and ChaqChaq Bridge.

Table 2: Hydraulic parameters of the Chaq-Chaq dam and bridge. Depth Breach Reservoir Reservoir Reservoir Spillway Spillwa Minimum of formati storage at storage at storage at length y crest stream bed overt on NPL El. El. 780.0 El. 780.6 (m) level level at the oppin time 777.5 MCM MCM (masl) bridge g (m) (hr) MCM (masl)

0.6

1-1.5

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1.4

2.344

2.55

15

777.5

754.4

High cord level of the bridge (masl)

low cord level of the bridge (masl)

761

759.7

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Maxim um water level at the bridge (masl) 759.6

Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116 IV. Upstream and downstream cross sections data One-dimensional dam breach hydraulic modelof HEC-RAS is used frequentlyto predict the flood inundation area due to a dam breachflood through the downstream valley. It was found thatHEC-RAS performed well, with relatively good agreement between predicted and measured water levels(Yochum etal.,2008) and(Gee, 2010). HEC-RAS modeling system is a public domain model developed by the US Army Corp of Engineers (Brunner, 2010 a,b). It performs onedimensional (1D) steady and unsteady flow simulations on a full network of natural or man-made open channels. Additionally, it has the ability to model storage areas and dam break problems as well as bridges and culverts hydraulics. In order to model the flooding in the stream valley using HEC‐RAS; cross sections data are required. In this study a topographic map of 1m interval is obtained in AutoCAD format from the municipality of Sulaimanya.Then, the river reach in the Chaq-Chaq system extending over a length of 4.15 km from upstream end of the reservoir to the downstream portion of the damis considered for analysis. The cross sections data of the river reach aredeveloped by AutoCAD Civil 3D 2013, by using this software the main channel as well as right and left overbank have beennoted and coded in the hydraulic model. A number of 21 cross sections at the upstream of the dam are used to model the reservoir area and19 cross sections were developed at the downstream portion. Extra cross sections were added by interpolation at a maximum distance of 75 m. The values of Manning’s roughness coefficient were entered directly into the cross section editor to describe the channel and overbanks. These values were determined by visual inspection and satellite imagesbased on guidance fromChow (1959). The Manning’s roughness coefficient values were set at 0.028 for main channel and the two overbanks. These values have been assumed because the stream reach under study is clean with stones and high flow stages are expected during the dam break analysis (Parhi etal, 2012).

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V. HEC-RAS Model The unsteady option of HEC-RASrequires the breach geometry and breach formation time as input in order to model a dam breach flood.The breach geometry is readily available from the bathymetric survey but breach formation time is still a matter of uncertainty (1-1.5 hrs). Breach formation time is the most sensitive parameter in developing a hydraulic model for dam break problems and breach hydrograph development. Therefore, in this study it is attempted to calibrate the breach formation time through simulation of breach flood using HEC-RAS 4.1 unsteady model. For calibration of Breach formation time value; the observedWSE at the downstream bridge has been considered. A weir coefficient of 1.1 was used in this analysis; the trigger time of breach is set such that it corresponds to the time of peak of a developed inflow hydrograph as it will be explained in the next paragraph. At that time the water surface elevation was equal to 780.57 m which is close to the observed water surface elevation ( 780.5 − 780.6 m). This equality in the simulated and the observed WSE proves that the developed inflow hydrograph is accurate and that there was under-estimate for the inflow hydrograph in the design of Chaq-Chaq Dam.

VI. Boundary conditions The upstream boundary condition is modeled using the flood hydrograph corresponding to the measured 131.2 mm rainfall depth during 24 hrs on a 151 𝑘𝑚2 of catchment area. The flood hydrograph is developed from contributing catchments using NRCSunit hydrograph (UH) method. The NRCS dimensionless UH is a synthetic unit hydrograph in which the discharge is expressed by the ratio of discharge to peak discharge and the time by the ratio of time to the time of rise of the unit hydrograph(Chow etal., 1988). Fig.3 shows the developed inflow flood hydrograph. Details on how to develop flood hydrograph using NRCS UH can be found in McCuin (2005). The downstream boundary condition is set to normal depth and an approximate water surface slope is assumed for the friction slope.

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Flood (m3/sec)

Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116

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200.00 180.00 160.00 140.00 120.00 100.00 80.00 60.00 40.00 20.00 0.00 0

10

20

30

40

50

60

70

80

90

Time (hrs) Figure 3: Inflow flood hydrograph. 1.60hrs.Fig. 4 provides a plot of modeled water surface profiles at different times of the simulation and Fig. 5 shows the outflow flood hydrograph through the dam breach. Each hydrograph was routed through the downstream reach to produce different water surface elevations at the downstream bridge; the results of the model at the bridge location are shown in Table 3. The percentage of errorsbetween the predicted water surface elevation and theobservedwater surface elevation at the downstream bridge are depicted in the Table 4.

VII. Initial conditions The WSE upstream of the dam is set to 780 m which is the crest elevation of the dam; while WSE at the downstream reach is set such that 2 m depth of water is existing.

VIII. Hydraulic model result Using of surveyed dam breach geometry combined with standard approaches for developing the upstream hydrograph boundary conditiona HEC RAS model was developed to generate different breach hydrograph corresponding to different breach formation times, namely 1.25 hrs, 1.50hrsand Chaq Chaq Ch1

Legend WS 04FEB2006 2340 WS 04FEB2006 2200

780

Elevation (m)

Ground

770

0

First S ection at upstream

750

Dam Axis

Bridge section

760

1000

2000

3000

4000

Main Channel Distance (m)

Figure 4: water surface profiles at different times of simulation corresponding to 1.6 hrsof breach formation time.

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Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116

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Plan: 1.1 weir 1.5 River: Chaq Chaq Reach: Ch1 RS: 19.5

1200

Legend

Flow (m3/s)

1000

Flow Beginning of dam failure @time of 2200

800 600

Peak outflow@ time of 2340

400 200 0 1200

1800 04Feb2006

2400

0600 05Feb2006

Time Figure 5: Outflow flood hydrograph due to Chaq-Chaq dam breach. Table 3: Results of the model at the bridge location at different breach formation time. Breach River Time Q Min W.S. Crit E.G. E.G. VelC formati Sta of peak Total Ch Elev W.S. Elev Slope hnl. on time (hrs) El (m) (m) (m) (m) (m/s) (𝒎𝟑 /s (hr) (m) )

1.25

1.5

1.6

Just upstrea m of bridge Just upstrea m of bridge Just upstrea m of bridge

Flow Area (𝒎𝟐 )

Top Width (m)

Froud e# Chl

4FEB200 62325

929.5

754.4

760.60

758.4

760.8

0.0005 17

2.37

492.4

287.2

0.32

04FEB20 06 2335

919.1

754.4

759.92

758.4

760.2

0.0009 71

2.94

392.1

208.1

0.43

04FEB20 06 2340

915

754.4

759.55

758.4

760

0.0012 81

3.24

352.9

165.3

0.48

Table 4: Departures of estimated and observed water surface elevations at Chaq-Chaq Bridge corresponding to different BFT. Breach formation time Estimated WSE at ObservedWSE at ChaqDifference between (hr) Chaq-Chaq bridge Chaq bridge (m) estimated and measured using HEC RAS (m) WSE (m) 1.25 760.60 759.50 1.10 1.5 759.92 759.50 0.42 1.6 759.55 759.50 0.05 A comparison of the predictedWSE with the observed WSE at the bridge indicates that a breach formation time of 1.60 hrs may be considered the most accurate value, with a differenceof 0.05 m in WSE.The modeling indicates a peak breach flood discharge of979.2 m3 s and this value attenuates at the bridge to 915.4 m3 s.

IX. Existing Empirical Breach Prediction Models www.ijera.com

Simulation of dam breach floods is essential to characterize and identify hazards due to hypothetical dam failures. Hydraulic modelssuch as HEC-RASare often used for the analysis of downstream impacts resulting from potential damfailures. Estimation of the dam breach parameters, such as formation time, width and side slopes,has usually done external to the hydraulic model. If input breach parameters cannot be predicted with sufficient accuracy, more conservative parameters and associated increased costs may be 113|P a g e

Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116 required (Wahl, 1997).This paper aims to check the reliability of the existing breach prediction methodologies in estimating the breach parameters of Chaq-Chaq dam. Four important breach parameters namelytop width, average width, breach formation time and peak discharge pass through the beach are estimated by thefollowing breach prediction models Froehlich (1995, 2008), Xu and Zhang (2009) and Pierce etal. (2010)and the results are compared to the measured values (breach geometries) and HEC-RAS

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output values (breach formation time and peak discharge). Froehlich (1995) model was selected based on the results obtained by(Wahl, 2004) which showed that this model is more accurate than other existing prediction models up to the time the paper was published.Froehlich (1995) as cited in (Wahl, 2004), developed the following formulas based on 75, 34 and 31 case studies for Bavg , Tf and Q p ; respectively:

Bavg =0.1803× k o × Vw 0.32 × Hb0.19 … … … … … … … … … … . . … … … … … … … (1) Tf = 0.00254 × Vw0.53 × Hb−0.9 … … … … … … … … … … . . … … … … … … … … … (2) 1.24 Q p = 0.607Vw 0.295 Hw … … … … … … … … … … … . … … … … … … … … … … . . (3) Where K o = constant = 1.4 if there is overtopping and 1.0 if else, Z=1.4 if there is overtopping, otherwise Z=1.0, Vw = volume of reservoir at the time of failure, hb =height of breach, Bavg = average width, Tf = breach formation time and Q p = peak discharge. Froehlich (2008) developed the following formulas based on 74, 23 case studies for Bavg , and t f ; respectively: Bavg = 0.27K o Vw 0.32 Hb 0.04 … … … … … . . … … … … … … . … … … . . … … … … . . (4) Tf = 0.0175

Vw gHb 2

… … … … … … … … … … … … … … … . … … … … … … … … . (5)

Where K o = constant = 1.3 if there is overtopping and 1.0 if else, Z=1.0 if there is overtopping, if not Z=0.7. Xu and Zhang (2009) proved that his model is more accurate than other models. This model was developed using 182 case studies to estimateBt ,Bavg , Tf and Q p ; respectively: Bt Hd = 1.062 Hb Hr

0.092

1

0.508

Vw 3 Hw

eB 1 … … … … … … … … … … … … … … … … . (6)

With Bt = top width of the breach, Hd = dam height, Hr = 15m , Hw = height of water at the time of failure,B1 = b3 + b4 + b5 , in which b3 = 0.061, 0.088, and −0.089 for dams with core-walls, concrete faced dams, and homogeneous or zoned-fill dams, b4 = 0.299 and −0.239 for overtopping and seepage erosion or piping, b5 = 0.411, −0.062, and−0.289 for high, medium, and low dam erodibility Bave Hd = 0.787 Hb Hr

0.133

1

0.652

Vw 3 Hw

eB 2 … … … … … . … … … … … … … … … . (7)

with B2 = b3 + b4 + b5 , in which b3 = −0.041, 0.026, and − 0.226 for dams with core-walls, concrete faced dams, and homogeneous or zoned-fill dams, respectively, 𝑏4 = 0.149 𝑎𝑛𝑑 − 0.389 for overtopping and seepage erosion/piping, respectively, 𝑏5 = 0.291, −0.14, and − 0.391 for high, medium, and low dam erodibility, respectively Tf Hd = 0.304 Tr Hr

0.707

1

1.228

Vw 3 Hw

eB 3 … … … … … … … … … … … … . … … … . (8)

withTr = 1 hr., B3 = b3 + b4 + b5 , in which b3 = −0.327, −0.674, and − 0.189 for dams with core-walls, concrete faced dams, and homogeneous/ zoned-fill dams, respectively, b4 = −0.579 and − 0.611 for overtopping and seepage erosion/piping, respectively, b5 =−1.205, −0.564, and 0.579 for high, medium, and low dam erodibility, respectively. Qp gVw 5/3

Hd = 0.175 Hr

0.199

1

Vw 3 Hw

−1.274

eB 4 … … … … . … … … … … . … … … (9)

withB4 = b3 + b4 + b5 , in which b3 = −0.503, −0.591, and − 0.649 for dams with core-walls, concrete faced dams, and homogeneous or zoned-fill dams, respectively, b4 = −0.705 and − 1.039 for overtopping and www.ijera.com

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Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116

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seepage erosion/piping, respectively, b5 = −0.007, −0.375, and − 1.362 for high, medium, and low dam erodibility, respectively. Pierce (2010) showed that his developed multiple-regression model using 87 case studies is more accurate than the Froehlich (1995) in predicting peak-discharge through an embankment dam breach. Q p = 0.038Vw 0.475 Hd 1.09 … … … … … … … … … … … … … … … . . . … … … … … (10)

X. Comparison with considered empirical breach prediction models Applying the above equations to Chaq-Chaq dam failure yields the results shown in Table 5. Table 5: Results of empirical models applied to Chaq-Chaq dam failure. Breach Observed HEC RAS Froehlich Froehlich Xu and Pierce parameter value Prediction (1995) (2008) Zhang(2009) (2010) Prediction Prediction 𝐏𝐫𝐞𝐝𝐢𝐜𝐭𝐢𝐨𝐧𝐛 𝑩𝒕 (m) 𝑩𝒂𝒗𝒆 (m) 𝑻𝒇 (hr)

46 38 1-1.5 N.A.

𝒎𝟑

N.A. N.A. 1.6 979.2

𝑸𝒑 ( ) 𝑺 1.13 N.A. Side slope Z a. Obtained by using values of Z and 𝐵𝑎𝑣𝑒 b. Medium dam erodibility is assumed.

57.25𝑎 47.1 0.57 1364

51𝑎 43.8 0.62 N.A.

54.5 38.4 1.17 1274

N.A. N.A. N.A. 809

1.4

1

N.A.

N.A.

Table 6: Percentage of errors between predicted and measured values. Breach Froehlich Froehlich (2008) Xu and Zhang Pierce parameter (1995) Prediction (2009) (2010) Prediction Prediction Prediction 𝑩𝒕 (m) 𝑩𝒂𝒗𝒆 (m) 𝑻𝒇 (hr)* 𝑸𝒑 (

𝒎𝟑 𝑺

)*

24 23.9 -64.4 39.2

10.8 15.2 -61.2 N.A.

18.5 1.0 -26.8 30.1

N.A. N.A. N.A. -17.3

*HEC RAS results are considered as measured values Generally, all the models over-predict the breach top width andthe averagewidth.This trend of the models to over-predict the breach size may be attributed to the fact that they are developed based on the assumption of breach forms in a shape of trapezoid, while Chaq-Chaq breach has a vertical side near the spillway which may be considered as an odd case. However,Xu and Zhang (2009) predicts the average breach width more accurate than others, where the percentage of the error between the predicted and the measured values is 1% as shown in table (6). While the predicted breach top width using Froehlich (2008) appears to be better than others with an error of 10.8% and Xu and Zhang comes in the second order with an error of 18.5%. All the used models under-predict the breach formation time, includingXu and Zhang (2009) who was the best where it gives an error of -26.8%. Froehlich (1995 and 2008) errors are -65.4% and 62.4%; respectively.

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The predictedpeak flood discharge using the considered empirical models shows that most of these equations tend to over-predictthe value of this parameter; except Pierce (2010) which yields a value lower than that indicated by HEC RAS model. Pierce (2010)yieldsapeak flood discharge with an error of 17.3%, Xu and Zhang (2009)estimates the peak discharge with an error of 30.1% andFroehlich (1995)estimates the peak discharge with an error of 39.2%.

XI. Conclusions Simulation of dam breach floods is essential to characterize and identify hazards due to hypothetical dam failures. Hydraulic models such as HEC-RAS are often used for the analysis of downstream impacts resulting from potential dam failures. Estimation of the dam breach parameters, such as formation time, width and side slopes, has usually done external to the hydraulic model.

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Dr. K Abdulrahman Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 5( Version 1), May 2014, pp.109-116 Due to uncertainty in determining the exact value of the breach formation time; different values of breach formation time have been coded into the HEC RAS 4.1 to calibrate its valueby using the observed highest water level at Chaq-Chaq Bridge.In this context a breach formation time of 1.6 hrs was achieved. Themaximum flood discharge passing through the dam breach for the corresponding breach formation time was 979.2 𝑚3 𝑠for the corresponding breach formation times. The most competitive and recent breach prediction models were examined to determine the most accurate onein predicting the breach parameters.In this context; Xu and Zhang (2009) performs better than other in predicting the average breach width and the breach formation time. Froehlich (2008) predicts the top breach width more accurate than other models andXu and Zhang(2009) is in the second order. The peak flood discharge passing the breach of the dam is under-estimated by pierce (2010) with an error of 17.3%, while Xu and Zhang (2009) over-estimates the peak discharge with an error of 30.1%. As a conclusion Xu and Zhang (2009) can be considered as the most accurate breach prediction model because it was the best in predicting the breach width and the breach formation time.

[6]

[7]

[8]

[9]

[10] [11]

[12]

[13]

XII. Acknowledgments The writer acknowledges the support from the municipality of Sulaimani especially the GIS department (Shahlaa A. F., Azad A. H., and Mohammed H.). Thanks also go to Dr. RizgarS. and Dr. NihadB.for their valuable notations.

[14]

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Chow, V. T., Maidment, D. R., Mays, L. W. (1988). Applied hydrology. McGRAWHILL. FEMA. (2005). Conduits through embankment dams. Federal Emergency Management Agency. Froehlich, D. C. (2008). Embankment dam breach parameters and their uncertainties. Journal of Hydraulic Engineering, 134(12), 1708-1721. Gee, D. M. (2010). Dam breach modeling with HEC-RAS using embankment erosion process models. World Environmental and Water Resources Congress (pp. 1347-1356). Davis: ASCE. McCuin, R. H. (2005). Hydrologic analysis and design. New Jersey: Pearson. Parhi, P. K., Sankhua, R. N., Roy, G. P. (2012). Calibration of Channel Roughness for Mahanadi River, (India) Using HECRAS Model. Journal of Water Resource and Protection, 4, 847-850. Wahl, T. L. (2004). Uncertainty of predictions of embankment dam. Journal of Hydraulic Engineering, 130(5), 389-397. Xu, Y., Zhang, L. M. . (2009). Breaching parameters for earth and rockfill dams. Journal of Geotechnical and Geoenvironmental Engineering, 135(12), 1957-1970. Yochum, S. E., Goertz L. A., Jones, P. H. (2008). Case study of the Big Bay dam failure: accuracy and comparison of breach predictions. Journal of Hydraulic Engineering, 134(9), 1285-1293.

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