Employing Appropriate Software for Dam break Analysis and Flood Line rendering

Transcription

1 Employing Appropriate Software for Dam break Analysis and Flood Line rendering 1. Background The specialist field of Dam Safety has been developing in SA since about the 80 s. This has led to a well-developed statutory arrangement at present. Apart from the dam structural safety and (spillway) operational adequacy reevaluation required for DAM SAFETY investigations, the hazard that such a dam hold when it would fail has become one of the prominent parts of this field of engineering. Techniques to analyse the extent of damage that may occur due to the (dam break) flood released out of the dam basin has been developing continuously.

2 Two Scenarios of possible dam-break situations have been recognised. One being the obvious cause of spillway inadequacy, i.e. flood induced failure, and subsequent washing out of the structure due to erosional action or high dam level hydrostatical (concrete wall) structural collapse. This is thus associated with an extreme incoming flood. Another potential dam-break situation, the so-called sunny-day or clear-sky situation, would be associated with a developing weakness of the dam during normal full-dam operational conditions. Examples are the piping failure of earthfill dams or exceptional earthquake accelerations on concrete dam walls. The latter scenario may be associated with more of a lack of pre-warning of the imminent dam collapse.

3 Dam-break flood lines are required over the range of river downstream over which it has a significant influence. The criteria typically used to define the end of influence downstream, are as follows: o The position where the flood level do not substantially overtop the river bank o The dam-break flood attenuated to less than the general maximum allowed flood level for typically a new town development, i.e. typically the 100-year flood for the overall catchment. Another related criterion often used is the position where the dam-break peak flow has attenuated to the dam basin incoming flow peak typically say the RMF. o The position from where no more substantial development occurs downstream in the potentially flooded area. Constraints to the level of analysis to be done is created by o Available time or budget. o The degree of development downstream of the dam will also indicate what level of analysis (or depth of study) needs to be done. Smaller Category II dams would typically require only a simplified analysis say with SMPDBK.

4 o The great uncertainties involved in the estimation of the magnitude of dam-break that may occur (water levels downstream found to be insensitive to flow variations in any way) o The low resolution of topographic maps that is often only available. o The analysis described below could be considered more suitable for higher levels of analysis. The aim of this presentation is to provide a basic framework and understanding for the reader to start a dam break analysis with HEC-Ras. It is intended to be useful for both the beginner and the more experienced dam break analysis.

5 2. Software & Data Software has been developed to give increasingly accurate and timeefficient results. The mechanistic SMPDBK software has been the most popular so far for smaller dams. Full hydrodynamic software is also been developed and generally applied. These include DAMBRK/FLDWAV, Mike 11 and HEC-Ras. At present the inclusion GIS/DTM utility software*) is becoming increasing popular due to the time-efficiency thereof. Though 2D hydrodynamic software is becoming more feasible within the constraints mentioned above, the focus here is on the 1D analysis done by the software above. A few situations have been identified where a 2D analysis could be required. *) GeoRas, RiverCAD, Aquaveo WMS etc.

6 Available DATA in terms of image or digital maps, varies from: 1:50000 (20m contour) maps (- employ for level calibration of SRTM), 5 m contour Orthophoto image maps, 5 m DTM digital files (available in shp format), 25 m grid DEM files, 90 m grid satellite DEM files (SRTM) level needs calibration (typically 3 to 6 meters), to aerial surveyed maps (Lidar for example). Where only a single case needs to be analysed it could be worthwhile to employ CAD software to abstract cross-section dimensions and manually read in the figures into HEC-Ras Geometric window as shown in the abstract below. Otherwise there is efficient GIS/DTM software available to enhance the abstraction of the river data for direct input into HEC-Ras. The same software is then again employed to plot flood lines from the resulting flood levels from the (HEC-Ras) hydrodynamic analysis.

7 3. Features of HEC-Ras employed The analytical procedure for various dam cases is usually very casespecific, but the general approach thereof is presented below. A General Basic procedure involved in doing a dam-break analysis is first shown below. With this presentation it is assumed that the reader has adequate knowledge of doing at least a Steady State Analysis with HEC-Ras. The use of GIS/DTM utilities does not fall in the scope of this presentation, but is only referred to occasionally. The focus here is on Dam-break analysis only. Additional features found useful will be presented afterwards.

8 RELEVANT SEQUENTIAL MENU ITEMS FOR DAM BREAK ANALYSIS 1 Geometric Data 2 Steady Flow Input 3 Run Steady Flow Analysis 4 Unsteady Flow Input 5 Run Unsteady Flow Analysis 6 Stage & Flow Hydrographs 7 View (Longitudinal) Profiles 8 View Cross-sections 9 Summary Output tables

9 1. GEOMETRIC DATA PAGE/WINDOW Cross-Section Editor Upstream vs Downstream Section PONGOLAPOORT DAM MANUAL INPUT FROM CAD CROSS-SECTION MEASUREMENTS ON 20 METER BACKGROUND IMAGE

10 Berg River Dam: Illustration of Cross-section dimensions from CAD for FLDWAV

11 DE HOOP DAM AUTOMATIC RIVER CROSS-SECTION DATA IMPORT FROM GIS/DTM UTILITY SOFTWARE

12 1.2 PONGOLA DAM ARCH WALL -

13 1.2.1 Dam wall Definition Flood Gates detail

14 1.2.3 BREACH DATA

15 1.3 Tables/Friction definition Typical values recommended SMPDBK summarised. - The Manning roughness coefficient for out-of-bank flows may be estimated to be 0.04 to 0.05 for a cross section located in an area where the overbank is pastureland or cropland, 0.07 for a moderately wooded area, and 0.10 to 0.15 for a heavily wooded area, Built up areas about 0,10. Overall resistance of whole cross-sections can be simply selected as similar for the high flow stage usually involved here..

16 1.4 Tools/Cross-section Interpolation very useful to obtain stable solution.

17 2. Steady Flow Data Input to check data Boundary Conditions PONGOLAPOORT DAM Downstream Condition - Sea Level Maputu Bay

18 3. STEADY FLOW ANALYSIS

19 4.UNSTEADY FLOW DATA

20 5. UNSTEADY FLOW ANALYSIS RUN Options/Output Locations for hydrograph plots

21 6. STAGE & FLOW HYDROGRAPHS PLOT PONGOLA POORT DAM Cross-section upstream of wall

22 7. PROFILE PLOT CONDITION WITH UPSTREAM HEADING SURGE IN GORGE UPSTREAM

23 8. VIEW BREACH DEVELOPMENT SIMULATION CROSS SECTION-WISE

24 9. SUMMARY OF OUTPUT RESULTS BASIC SOURCE FOR DELIVERABLES FILE COPY TABLE => EXCEL FOR EXAMPLE OPTIONS INCLUDE INTERPOLATED CROSS-SECTIONS - DEFINE TABLE

25 DEFINE TABLE

26 Further Features to expedite the process and help with finding satisfactory results: In the case where cross-sections is extracted from DTM map files of relative large contour spacing, say 5 or 20 m, GIS utility software creates unrealistic flat terrain downstream between the contours. To manage this, the crosssection lines can be place just downstream of the position where the contour runs through the river thalweg. The interpolation facility of HEC-Ras can then be used to define the riverbed in between for an initial run. Ideally hard breaklines should be used to define the river thalweg between the contour river crossings in cases like the above-mentioned, else the pilot channel feature in HEC-Ras can be used to serve as guide to manually adapt the cross-section of the in-between sections. These would however need to be adapted again every time a new updated set of river data is imported from a GIS/DTM utility. GIS software procedures (for ArcGIS) has been developed elsewhere to expedite automatization of this riverbed generation.

27 When importing cross-sections from a DTM utility software Point Filtering would be mostly required to limit the volume of data used in the routing analysis.

28 Run a Steady State flow analysis with a number of flow value that cover the range of expected flow over the length of reach to check for any errors or unrealistic results. To expedite the analysis, leave out the bank station definition. Employ the River bank estimation feature in the Geometric Data page in the Tools/Channel bank stations tab. The water level from a low steady flow value which would remain inside the river channel, say the 2-year to 5-year flood, will then be used in the software to define the bank stations.

29 First only run the analysis without the Post Processing. Use the Hydrograph tab to test for reasonableness of results. Check for Smooth hydrographs up to the end of the model reach. When running the Post Processing part in the analysis, a longitudinal water level simulation run will be of value to identify where instabilities still exists.

30 In order to hydrodynamically include the dam basin in the overall model of the study, three approaches for dam pool routing can be used: 1. Ideally topographic surveys may be available from which appropriate crosssection can be extracted giving an accurate result of the basin depletion during the dam wall breaching and afterwards. 2. Such topographic maps of the dam basin is however often not available. A reasonable simplified representation of the basin which should have fairly satisfactory depletion reaction during the dam-break, can be made. Employing a simple geometric shape like a horizontally positioned pyramid can be used. The length (height) of the pyramid then equals the length of the basin and the end surface area near the dam wall is calculated according to the volume of a pyramid corresponding to the basin volume. A triangular shaped end shape is usually appropriate. See shape employed previously for De Hoop dam below. Other simplified shapes can be used to give a fair representation of the basin as shown in the second figure of the upper part of Pongolapoort dam basin.

31 Simplified Pyramidal dam basin representation at De Hoop dam Wide opening up dam basin of Pongola poort dam

32 It has been experienced that a simplified synthetic compilation of the dambreak flood hydrograph by empirical formulae determination of the peak flow and volume of basin (and flood) represented by a triangular hydrograph flow distribution, gives satisfactory results as well.

33 4. Sources of Dam break parameter estimation: a) SANCOLD Guideline

34 b) SMPDBK Peak flow calculation: Instantaneous failure:

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36 c) Empirical models Summarised and evaluated by Wahl (2001)

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39 Those models marked above that have been judged as well rated, has been applied with satisfactory results in various cases. In general the breach-width and failure time have been successfully used in HEC-Ras and the Peak flow estimations mentioned above used to verify the analytical results

40 5. Case Studies & Deliverables with Recommendations The items listed above have been taken from the HEC-Ras manual as well experience with applications in numerous cases over time. Case studies: Experience with challenges and practical application development over time. Impofu dam (2013)

41 Pyramidal dam basin shape

42 Spit at estuary downstream of marina. A 2D analysis may be worthwhile here. Simply assumed that sand spit would wash out rapidly.

43 Fika Patso (Phutadidjaba Qwa-Qwa) (2013) Pyramidal dam basin

44 Development close to river

45 Attenuation due to storage clearly evident in position downstream with relative large tributary. Compare Peak flow to flow at maximum water surface at chainage to m

46 De Hoop (2014) Complicated topographical model End of Influence 300 km downstream at Phalaborwa Barrage due to lack of substantial development downstream. Small volume of breached Phalaborwa Barrage plays insignificant role.

47 Nhlaralumi River (Timbavati NR) (2014) This was a set of four earthfill dams, 5 to 9 m high, situated over a reach of 10 km on the Nhlaralumi River which all failed due to flooding in This occurrence was simulated by a System analysis in one model. Relative good resolution topographic DEM maps at 25 m grid spacing was available. An Empirical parameter prediction according to the methods mentioned above proved to closely correspond to Surveys of the breached dam walls. The figure below show the HEC-Ras representation of the dam wall profiles with fully developed breaches.

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49 Resulting flood levels compare with observed without an iterative process. Surveyed marks at right flank of Argyle #2 probably left during dropping flow.

50 Small Length of dambreak flood influence downstream. (Increasing inflow from tributaries over reach assumed) All dams are planned to be rebuilt. The upper two was previously classified as Category I dams, while the lower two is recommended to be classified as Category II.

51 Shiloh (21 m) & Seroala (12 m) (2014) These studies served as trial run for first level analyses. Shiloh 90 m grid DEM Seroala (Thaba Nchu)

52 Topographical maps 90 m & 25 m DEM Downstream structures Development: Shiloh passing through town of Whittlesea. Loskop (2014) Topographical map 25 m DEM Bridge effect considered due to high development. Modelled as both bridge structure and in-line structure to employ dam-break feature of HEC-Ras. Minor increase of 6 to 17 % in flow obtained. Very little effect on water level. Effect at Flag Boshielo At entering this dam the peak is substantially reduced from m3/s to m3/s. The fuse plug breaching could however again cause a minor increase in flow. Dam level can rise to 0,6 m below NOC.

53 Doorndraai (2015) Triangular Dam-break flood synthetically compiled from empirical prediction and dam storage volume. Bridges assumed to play no role. Proportion of Flow area on overbank employed to judge end of influence. Little development far downstream.

54 Deliverables: The deliverables from the analysis can to a large extent be obtained from the output table. The required items would usually entail the following: a) Water level reached b) Water depth the accuracy of such values is very dependent on the quality of the topographic map. Quite often low accuracy/resolution of riverbed level prevent a reliable values. c) Velocity Due to highly two dimensional velocity distribution in river flow, 1D analyses cannot accurately predict the velocity distribution over the width of the section and especially so on the overbank flow. The average velocity output in the channel can however still serve as good indication. d) Travel time this is an important factor in the compilation of preparedness plans. The arrival time of the flood, i.t.o. for example reaching of the river bank level need to be determined from the hydrograph output at the cross-sections required.

55 e) A further related factor would be the period of inundation which also can be determined from the hydrograph output. The period is required to plan post-flood remedial activities to get services functional again. f) For assessing the possible damage and lives lost the rate of water depth rise at some characteristic depth, would be useful. g) Plotted flood lines. Mark out chainages. Various cases evaluated can be indicated in separate colours for distinction on the same map. Generation of.kmz Google Earth files is very useful as the development changes along the flooded area.