Dam break simulation and downstream flood routing using HEC-RAS

Dam Break

Excutive Summary

Analytical tools and techniques used in approximating dam breakages are usually evaluted and comparison made in orders to ascertain there effectiveness. During dam construction, it is necessary to evaluate the potentiality of dam failure modes, breach the necessary paramenters related to failuere modes, and define routing and the map of the consequent discharge hydrograph. This paper outlines how mapping of propable inudation emanating form the dam failures needs numerous elements i.e. likely dam failures, break parameters, hydrologic scenario that are connected to failure models, including routing and mapping of the consequent discharge hydrograph. The unsteadily flowing routing models i.e HEC-RAS are oftenly applied in computation and rfelction of the dam's downstream consequences emaneting from dam failures. Approximation of the break elements i.e. time of development and width is carried out externally to the model in use.

Introduction

In the recent past most agencies are enganging themselves in reviewing and updating safety measures concerned with dam safety and risk management. Studies relating to Dam break, involves updates on likely maximum flood inflow hydrographs and routing of most probable dam break floods to evaluate downstream impacts . The approximation of these estimate dimensions and developments is essentail in investigation of the dam safety. The flows in the downstream location and other inundated areas of the dam is directly proportional to the breach parameters, other factors tat influence include; the breach location, time of formation and the size (USACE, 1980). All these parameters are essentail in dam failure analysis.

The geometric design of a dam break ought to be approximated in order to reflect the resultant flood wave and down stream impacts . In this case an example of models that can be used to demonistrate dam breach outflow hydrograph computing and downstream routing include HEC-HMS (HEC, 2006b), HEC-RAS (HEC,2006a), NWS-DAMBRK (Fread, 1988b) amon others. The models requires the probable breach properties to be approximated outside the model. During investigation of dam break, breach dimenisions and time taken in develoment must be put in consideration in estimation of the failure scenarios (Washington State Dept. Of Ecology 1992). The necessitaties include various failure modes and different hydrologic events . The breach parameters related to hydrologics of the level of the likely maximum flood level may be different from that of sunny day failures broght about, likely, by a seismic event. Hence, anumber of breach parameters ought to be made for each connection of hydrologic event and failure scenario

Dam Break Simulation with HEC-RAS model:

Dam break tests are performed with HEC-RAS model for dam safety study and also flood damage analyses for circumstances that involves likely levee breaks. These studies in most cases need to ascertain the following circumstances and their associated uncertainties.

Hydrologic Scenarios (USACE, 1980).

Sunny day failure and,

Large hydrologic event

Failure Modes, they include (USACE, 1980).;

Overtopping

Piping

Breach definition input data and information (USACE, 1980).

Failure mode and initiation specification

Breach growth progression

Breach side slopes

Breach development time

Ultimate breach bottom elevation

Maximum breach bottom width

• Failure initiation

Selected time

Reservoir stage

Reservoir stage and duration

Data Required for Breach Simulation with HEC-RAS model .

The model employs the following parameter to define dam break (Froehlich, 1995);

Formation Time: critical breach development period

Trigger Condition: pool elevation, or clock time pool elevation and duration

Weir and Pipe Flow Coefficients: weir coefficients are applied in computation of weir flow / overtopping, whereas orifice coefficient is aplied in computation force flow./piping (Washington State Dept. Of Ecology 1992).

Location: transverse location of the centerline of the breach in the dam

Failure Mode: overtopping or piping

Shape and Progression: bottom width, bottom elevation, left and right side slopes. Linear or non-linear time formation

The dam break charactericts can be estimated in various ways which include;

Comparative analysis: this study involves making comparisons between historical failures of dams of similar sizes, water volume and materials. This is done through regression equations derived from past dam failures in order to estimate the peak outflow or breach size and time of development (Froehlich, 1995). Also computer-based models i.e computer programs that are used to develop models thus physically breaching the system by use erosion equations, priciples of hydrolics and soil mechanics equations.

Failure Location:

The location of the breach failure depends on several factors i.e. shape, type, structural elements of the dam and failure. All the factors relating to the dam, specifically historical record of the seepage and problems concerned with the foundation, ought to be put in consideration so that to place the breach in the most likely point of each failure mode. If the likely point is not identified, the centreline on the breach ought to be set to the centerline of the downstream mainchanel

Determination of the size and growth rate of dam breaks

Estimation of the growth rate and size for dam breaks is not an easy task. Hence, simulations models facilatates the user to estimate the effects of a range of parameters from the data obtained from the examination. There are number of equations involved:

Regression Equations

There are number of regression equations that have been in the pasted applied in dam safety research obtained in literature and still are being put in consideration in making conclusion concerning guidance for the corps of engineers portfolio risk assessment studies i.e.Von Thun and Gillette, Macdonald and Langridge-monopolis and Froahlich (Froehlich, 1995). From these equations we obtain an standard literature review of this subject .

According to Froelich equation, Froelich used: Zoned earthen, rockfall data set and 63 earthen to come up with a set of equation s that were to be applied in estimation of the mean failure time, width and side slopes of dam break. During actual application, he calculated the height of the break by assuming that the break gegins from the top of the dam to the natural ground elevationbeing at the centreline of the break point unlike MacDonald and Langridge -- Monopolis (Froehlich, 1995)

Von Thun and Gillette (Von Thun, 1990)

He used a total of 57 dams from both the MacDonald and Langridge -- Monopolis and Froehlich researcg to come up with thier methodology. These methods advocates application of break slopes of 1.2H:1.2V; excepting dams that have cohesive layer of soil, whereas the side slopes of thedams ought to be arranged in the manner 0.6V:1.0V to 0.34:1.1V (Von Thun, and Gillette, 1990). Von Thun and Gillette come up with two dismillar set of equations to be used in break development time considering embankment matter.

NWS-BREACH (Fread, 1988a)

This model depends on the sediment mechanics and locomotion connections to simulate the dam break growth. In this model, the sediment transport method is employed to calculate the erosion pace and the size of the break in regard to the data pertaining the soil aspects of the dam and hydrograph inflow (Washington State Dept. Of Ecology 1992). Development of the break with time is computed by sediment transport equation, drastic fall of the dam as result of excessive hydrostastic force and width enlargement by slope stability. Sediment transport in this concept is in application with either cohesive or non-cohessive materials within the dam environment. Other models that can be employed in detemination of the dam break include SIMBA and HR-BREACH though they are still under development.

DAM BREACH FLOW SIMULATION USING HEC-RAS

While dams provide many desirable benefits to society, they also are a major hydro-modification to ecosystems, can be safety and boating hazards, and may degrade water quality of the river. In the Great Lakes District, dams that exceed a height of 6ft 2m and a pool volume 50 ac-ft are inspected and require a state permit to ensure they are properly maintained. For dams that fail inspections, dam owners are faced with four options: (1) do nothing, (2) modify the dam to such an extent that it is not subject to the regulations, (3) rehabilitate the dam to meet the regulatory guidelines of the permit, and (4) remove the dam. The chosen option often depends on the outcome, its economics, and the environmental and political pressures associated with the option (Gee and Brunner, 2007).

The modeling scheme in HEC-RAS is illustrated on graphical illustration of the breach dimension, location and the development which is connected with the computed upstream and downstream water surface profiles gives practical information for the expert and the customers of the learn. HECRAS applies dam breach parametric quantity formulated outwardly (applying the methods described above or any others considered suitable) to calculate the temporary development of a break in an lined up structure (dam) (Froehlich, 1995). The movements through that arrangement are calculated examining breach flow, overtopping flow, spillway discharges, gated flow, and sinking outcomes as a result of downriver backwater. Those numerous flow sections are used as an inner border line state for unfirm flow shaping of the pool and the downriver reach. The pool may be examined using either uncomplicated level pool direction-finding or as a wavering flow reach using cross sections. Variations in breach outflow hydrographs caused by different breach parameters will reduce as the submerge wave is routed through the downriver reach.

Figure 1. HEC-RAS Dam Breach Model (USACE, 1980)

SOME COMPARISONS

Currently, the breach parameter assessment techniques outlined above have been used in five circumstances. Two are theoretical failures at real schemes. One of these has computed cross sections in the pool and the downriver reach so that the results of in-pool and downstream course-plotting can be analyzed. Three actual remarkable failures are also repeated; one of which was a designed research. All of the cases talked about here are dominating failures. In all cases the breach progress was understood to be linear in time. The theoretical cases were presumed to initiate breaching when the overtopping was about 0.3 m; the described beginning time of failure was applied in the notable cases. Only the applications to the two historic breakdowns are offered here because of space limitations; complete effects can be found in (Gee and Brunner, 2007).

APPLICATIONS TO SOME HISTORIC OVERTOPPING FAILURES

Oros Dam

Oros dam (Brazil) was being constructed when it broke down by overtopping in March of 1960 (CEATI). Oros dam was constructed of a clay core with sand and rock shoulders and the height was about 35.5m. The experimental techniques of MacDonald, Froehlich and VonThun were used to this construction along with the BREACH process theory. The quantity of water on the rampage was projected to be 660*106 m3 (1).

The breach hemorrhage hydrographs calculated using these parameters are shown on Fig. 2 with the approximated outpouring hydrograph. The outflow hydrograph was derived from the record of water height during the occurrence (CEATI).

Figure 2. Breach Hydrographs for Oros Dam

Banqiao Dam (China)

Banqiao Dam broke down by looking down from a large aggressive state of weather with winds 64-72 knots (11 on the Beaufort degree) and rainfall and thunder and lightning in 1975 (CEATI). The Banqiao dam was built or erected using clay core containing shale. The upstream and downstream work was standardized earth. It can be presumed that, as a result of building processes (mostly non-mechanized), that the foundation was weakly compressed. The height of the Bangiao dam was about 24.5 meters high with a crest altitude at 116.34m. Crest width was 6m and length 2020m (Michael 1988). The upstream slope was 3H:1V and downstream 2.5H:1V. The capacity of the architectural plan for the channel that carries excess water over or around a dam or other obstruction and channel works was 1742 m3/sec; the approximated apex inflow was about 13,000 m3/sec when breaching took place. The approximated breach parameters are shown in Table 3.

The breach outflow hydrographs calculated applying these parameters are illustrated on Fig. 3 (Mohamed, 2002)

Field examinations for this occurrence consist of the pool altitude time history.These observations can be equated with the pool reduction in quantity calculated using HECRAS with the approximated breach parameters

Conclusions and observations

A number of methods are available for estimating the breach parameters taking place from dam overtopping and successive failure. These methods are mostly practical, based on fitting connections between major parameters for example water depth at the back of the dam and notable examinations. One practical theory, NWSBREACH, was also used for assessment. Estimation of dam breach parameters is an essential first step in carrying out the investigation of the downstream effects of possible dam failures (Michael 1988).. These parameters are employed to calculate breach outflow hydrographs using estimated inflow hydrographs, reservoir elevation-capacity data, and spillway and gate hydraulic capacities. Methods are being formulated to translate and use the breach parameters estimated by application of these ways in a wavering flow model; HEC-RAS. The processes foresee a wide collection of breach parameters and as a result, a large disparity in outflow hydrographs (Von Thun, and Gillette, 1990).

The MacDonald technique customarily created the major peak outflows. The only comparison to a projected notable outflow hydrograph (Oros) showed that all of the processes produced flows larger than those perceived. For the case in which the pool drawdown statistics were presented, all of the methods, when used in the HEC-RAS simulation model, shaped similar results. The techniques tried out recommend use of flatter breach side slopes than are usually observed. The foundation for development of the practical techniques must be considered. The breach configuration utilized in developing the weakening equations was characteristically the critical shape that was noticed at the after the incident. What is needed for calculating outflow hydrographs is the development of the hydraulic power; be it a weir flow like that of overtopping or an orifice flow like piping (Michael 1988). The hydraulic computations practiced in HEC-RAS believe that the hydraulic control progresses grounded on a failure time estimated from the process used. Downstream sinking of the control is likely and is integrated in the outflow computation. Process models are presently being constructed and tested (Wahl, et al., 2008). The predictable benefit of this type of model will be the capacity to communicate breach parameters to the materials and building of the structure of interest. Observing of the development of the hydraulic control for the period of breaching should also be enhanced

REFERANCES

Department of the Interior, Bureau of Reclamation, Dam Safety Office, July 1998.http://www.usbr.gov/pmts/hydraulics_lab/twahl/breach/breach_links.html

Froehlich, D.C. (1995), "Embankment Dam Breach Parameters Revisited," Water

MacDonald, T.C., and Langridge-Monopolis, J. (1984), "Breaching Characteristics of Dam Failures," ASCE J. Hydraulic Engineering, 110(5), 567-586,

Michael D ( 1988), Comparison of Dam Breach Parameter Estimators, 1 Senior Hydraulic Engineer, Corps of Engineers Hydrologic Engineering Center, 609 2nd St., Davis, CA 95616;

Mohamed, M. (2002), "Embankment Breach Formation and Modelling Methods." Ph.D. Thesis. The Open University.

Resources Engineering, Proc. 1995 ASCE Conf. On Water Resources Engineering, New York,887-891.

U.S. Army Corps of Engineers (USACE, 1980), Hydrologic Engineering Center, "Flood Emergency Plans -- Guidelines for Corps Dams," RD-13, June 1980.

Von Thun, J.L., and Gillette, D.R. (1990), "Guidance on Breach Parameters," Internal Memorandum, U.S. Dept. Of the Interior, Bureau of Reclamation, Denver, CO.

Wahl, Tony L. (1988), "Prediction of Embankment Dam Breach Parameters -- A Literature Review and Needs Assessment," DSO-98-004, Dam Safety Research Report, U.S.

Wahl, Tony L., et al. (2008), "Development of Next-Generation Embankment Dam Breach Models," United States Society on Dams, 28th Annual USSD Conference, Portland, OR, April 28-May 2, 2008, pp. 767-779

Washington State Dept. Of Ecology (1992), Dam Safety Guidelines, Tech. Note 1 Dam Break Inundation Analysis and Downstream Hazard Classification, Water Resources Program, July1992, http://www.ecy.wa.gov/programs/wr/dams/Images/pdfs/technote_1.pdf