Third National Dam Safety Conference IIT Roorkee 18-19 February, - PowerPoint PPT Presentation

Third National Dam Safety Conference IIT Roorkee 18-19 February, 2017 PERFORMANCE EVALUATION OF THREE DAM FAILURE ANALYSIS MODELS by Aravind V. , Asst. Engineer , PWD, Govt. of Kerala Muthiah Perumal, Professor , Deptt. of Hydrology, IIT Roorkee N.N.Rai, Director , CWC, New Delhi

O RGAN ISATIO N O F PRESEN TATIO N  Introduction  O bjectives  Models considered  Features of models  Considered case study  Results  Performance evaluation  Conclusions

Introduction Necessities: • For predetermination of the warning time and probable maximum water level downstream of dam for various likely dam failure scenarios to plan evacuation measures • To simulate the flood propagation of a recorded failed dam scenario

Approach followed for dam failure analysis  Development or identification of the inflow hydrograph to the reservoir at the time of failure.  Routing that hydrograph through the reservoir  Development of failure condition of the structure  Calculating the outflow hydrograph from the failed structure  Modeling the propagation of the flood wave of the failed dam downstream to determine travel time, maximum water level reached , inundated areas etc

Objectives 1. Reconstitute the flood wave resulting from the failure of Machhu dam II which happened on 11 th August 1979 using the models NWS-DAMBRK(1990), MIKE11(2008) and HEC-RAS(2008) 2. Compare the results of each of these models with the observed values and analyse the results. 3. Evaluate the performances of the above mentioned dam failure analysis models.

Models for analysing dam failures  The U.S.National Weather Service (NWS) Dam-Break Flood Forecasting Model(DAMBRK) by Dr. D.L. Fread (1984)  MIKE 11 by Danish Hydraulic Institute (DHI).  HEC-RAS of U.S.A.C.E

Major Components of Analysis 1. Reservoir routing a) Storage Routing b) Dynamic Routing 2. Breach simulation a) Overtopping failure b) Piping failure 3. River routing a) One-dimensional b) two-dimensional

NWS-DAMBRK Model features Reservoir routing  by Puls Method or Dynamic routing Breach Description a) Breach due to overtopping b) Breach due to piping  Simulating breach in the form of rectangular, triangular or trapezoidal shapes.  Assumptions are to be made to approximate the actual breach profile to correspond to any of these breach profiles.  By assuming equal area the breach is approximated to a trapezoidal form as shown:

Breach due to overtopping Breach due to piping River routing  One-dimensional routing using the St. Venant’s equations based on four point-implicit finite difference scheme i. Conservation of mass (continuity) equation: ii. Conservation of momentum equation:

MIKE 11 MODEL FEATURES  Core of MIKE11 system is the HD module.  MIKE11 HD module is an implicit, finite difference model which solves Saint Venant’s equations using 6-point Abbott finite difference scheme.  HD module utilizes a space staggered grid consisting of alternate h & Q points

 During simulation, the St. Venant equations are solved numerically at the grid points at specified time interval for the given B.C.  The i/p files used in dam failure analysis are: Network file • Cross-section file • Boundary file • HD parameter file • Simulation file •  Results of HD simulation consists of a time series of water levels and discharges along the reach d/s of the failed dam.  Results can be viewed by MIKE View

HEC-RAS Model features  Dams are modeled using the Inline Structure editor.  Inline Structure editor allows the user to input an embankment, define overflow spillways and weirs, and gated openings.  Gate openings can be controlled with time series of gate openings or using the elevation control gate operation features in HEC-RAS.  Solves the St.Venant equations using the four-point implicit scheme.

 Two c/s should be given before the dam.  The routing reach is hydraulically connected to the reservoir with the 1 st c/s.  The 2 nd c/s is required as a B.C. for the inline structure (the dam)

Considered case study Machhu Dam-II in Gujarat State failed on 11 th August 1979

 Relevant design aspects of Machhu Dam-II before the failure Type of dam Masonry spillway with earth dam flanks on either side Length of the earthen dam on left 7689 ft (2343.61m) Length of earthen dam on right 4588 ft (1398.42m) Length of non-overflow masonry portion 272 ft (82.91m) Spillway length 676 ft (206.0448m) Shape of spillway Ogee Crest of spillway RL 168 ft (51.21m) Spillway design flood for avg. annual rainfall of 2,18,330 cusecs (6182.42 cumec ) 558.8 mm Details of radial gates of the spillway 18 gates of 30 ft (9.144m) Long and 20 ft (6.096m) high Low water level RL 155 ft (47.244m) 7926 ac.ft. (9776923.91m 3 ) Dead storage Full reservoir level(FRL) 188 ft. (57.30m) 81520 ac.ft (100557006.9m 3 ) Gross storage Flood cushion 1 ft. (0.3048m) High Flood Level 189 ft. (57.61m) Free Board 8 ft. (2.44m) Top of the dam RL 197 ft. (60.05m)

 The observed spillway flood which caused the failure was 13139 cumecs due to 711.2 mm rainfall recorded in less than 24 hours.  The water level has risen 1.5 ft. above the dam crest level which leads to the overtopping failure at 1:30 pm. 1850 ft 3600 ft

Adopted final breach size Adopted design inflow hydrograph into the reservoir of Macchu Dam-II (source: Jagdish Narain et al. (1981))

Adopted Manning’s n values and contraction-expansion values d/s

Results 1. DAMBRK Model simulations Simulated discharge hydrographs downstream of Machhu Dam-II using DAMBRK

2. MIKE 11 simulations Simulated discharge hydrographs downstream of of Machhu Dam-II using MIKE 11

3. HEC-RAS simulations Simulated discharge hydrographs downstream of Machhu Dam-II using HEC-RAS

Performances evaluation Comparison of breach outflow

Comparison of time to peak stages at downstream of Machhu Dam-II

Comparison of simulated and observed peak stages at d/s reach

Comparison of computed and observed peak flood water levels at d/s reach due to dam failure

Conclusions  On the basis of the simulated peak stage profiles with the observed values, the results from MIKE11 are much closer to the observed values along the downstream channel than the other two models.  DAMBRK Programme shows high rate of attenuation, while the MIKE11 shows the least rate of attenuation and HEC-RAS shows moderate rate of attenuation.  MIKE11 is more user friendly without involving trial and error approach in the use of spatial and temporal step sizes in comparison with the HEC-RAS usage.  In DAMBRK Programme the cross-sections at which discharge hydrographs can be displayed is limited to six.  Results can be represented in a more user friendly form such as by the use of animation both in MIKE11 and in HEC-RAS models.