Америкийн Нэгдсэн улсын Инженерийн армийн корпсын хийсэн Туул голын үерийн тархалтын загварчлалын тайланг оруулж байна. Сургалтаар материал нь тараагдсан боловч зарим хэрэгтэй хүндээ хүрээгүй байж болно. Та бүгдэд хэрэгтэй мэдээлэл байж болох юм. Гидравлик, гидрологийн нэг хэмжээст шугаман загварчлалыг залуучууд маань эхнээсээ хийдэг болж байна.
Tuul River Flood Analysis
Introduction
This documents the development of the Tuul
River, Mongolia hydrology and hydraulics model development for the Tuul River
Flood Analysis Workshop. The models are
intended to assist local floodplain management officials to begin the
evaluation of building a dam on the Tuul River for water supply and floodplain management. The HEC-HMS and HEC-RAS models should be
considered preliminary for the watershed and river analysis. Limited observed precipitation, river flow,
and river stage data is available for calibration and verification of model
results. Further, the terrain data used
for geometric input is not accurate in the main channel; therefore the
conveyance (or carrying capacity) of the main channel is not clearly known. Further, the accuracy of the ground surface elevation
data does not provide enough accuracy in
the floodplain to develop a highly accurate river hydraulics model or
floodplain boundaries. Therefore, this
model should only be used for preliminary assessment – high accuracy terrain
data should be acquired and used in the model before significant conclusions
are reached.
Background
The Tuul River watershed is approximately 6750
square kilometers for the study area. The
Tuul River was modeled from the Bosgo Gage location downstream approximately
134 km to below the Chinggis Khan International Airport. The river is a highly braided floodplain with
several bridge crossings as the river nears Ulaanbaatar City that affect the
higher flows. Further, there is a levee
system that directs floodplain flow both for the Tuul River and the Selbe
River, however, the exact location or elevations of the levees are not fully
understood.
The primary goal of this analysis was to
identify the affects of constructing a dam on the Tuul River. Three proposed dam locations were evaluated
in the river hydraulics model and the affects of the dams hypothetical
breach. The reason for the breach was
not identified, however, it was assumed that the failure mechanism would be due
to overtopping of the dam.
Hydrologic Model Development
A hydrologic model of the Tuul River
Watershed, upstream of Ulaanbaatar, was developed to support a preliminary
analysis of proposed reservoir locations.
The hydrologic model was used to estimate the precipitation-runoff
response at numerous locations within the watershed. In addition, the model was used to provide
boundary condition data for a river hydraulics model. A hydrology model is typically used to
estimate the volume and magnitude of runoff, given a precipitation event, while
a hydraulics model is used to determine the depth of flooding. The Hydrologic Engineering Center’s
Hydrologic Modeling System (HEC-HMS) was used as the hydrology model for this
analysis. This software contains a user
interface that allows the engineer to define the stream and subbasin network,
input parameter data, define boundary conditions, simulation historic and
hypothetical precipitation-runoff events, and evaluate simulation results. This report describes the following steps for
developing the HEC-HMS model of the Tuul River Watershed.
1)
Gather terrain data and
delineate stream and subbasin network using HEC-GeoHMS,
2)
Import preliminary HEC-HMS
model structure generated by HEC-GeoHMS,
3)
Import precipitation and flow
data into HEC-DSS,
4)
Create the model to the 1993
flood event,
5)
Perform a statistical analysis
to estimate the 1-percent precipitation and flow at the UB gage,
6)
Model the 1-percent flood given
the 1-percent precipitation within the calibrated HEC-HMS model (compare
results to the statistical analysis),
Gather terrain data and delineate stream and subbasin network using HEC-GeoHMS
The Hydrologic Engineering Center’s
Geospatial Hydrologic Modeling Extension, HEC-GeoHMS, is a tool that can be
used within ESRI’s ArcMap program for delineating stream and subbasin networks
and generating input files for an HEC-HMS project. It is not necessary to use HEC-GeoHMS for
creating the initial HEC-HMS project; however, it can save considerable time
defining the stream and subbasin network for larger watershed studies. Terrain data, in the form of a raster dataset,
is required by HEC-GeoHMS for developing the stream and subbasin network. The Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) was
downloaded for Mongolia. The ASTER data has
a 30 meter resolution (one elevation value for each 30 meter by 30 meter grid
cell). Figure 1 shows
the elevation data and the watershed boundary upstream of the Ulaanbaatar
stream gage.
HEC-GeoHMS uses an 8-point pour model to
define the direction water moves across the terrain grid; it is assumed water
moves in the direction of steepest slope.
An intermediate grid is created where each grid cell is assigned a value
that indicates the direction of the steepest slope. Once that is known, another intermediate grid
is created that accumulates the number of grid cells flowing into each grid
cell. Grid cells with a value of 0
indicate those grid cells located along the terrain boundary, while those grid
cells with a large value indicate grid cells that are part of the stream
network. This flow accumulation grid can
be used to define drainage area at any point within the grid by multiplying the
flow accumulation value times the area of the grid cell. For example, if the grid cell is 30 meters by
30 meters and the flow accumulation had a value of 1,000, then the drainage
area would be 0.9 square kilometers.
These flow direction and flow accumulation grids make it possible to
delineate subbasin boundaries at any point along the stream network. Figure 2 shows
the subbasin boundaries; note subbasin outlets were defined at proposed
reservoir locations and stream gage locations, indicated by the triangles in Figure 2.
Figure 1. Terrain data used for defining the stream and
subbasin network.
Figure 2. Subbasin delineation for the Tuul River
HEC-HMS Model.
It’s important to verify the delineation
from HEC-GeoHMS with other published information. Figure 3 shows
the subbasin delineation generated by HEC-GeoHMS (red line) and a subbasin
delineation provided from another source (black line). Notice the discrepancy within the circled
area. Figure 4 shows
an aerial image within this same area along with the delineation from
HEC-GeoHMS (red line) and from another source (black line). Based on the aerial image, the delineation
from HEC-GeoHMS correctly defines the drainage in this headwater basin.
Figure 3. Discrepancy between subbasin delineation from
HEC-GeoHMS and published boundary.
Figure 4. Aerial image showing the subbasin delineation
for HEC-GeoHMS is correct.
After the subbasin and stream network were
defined, HEC-GeoHMS was used to define physical characteristics of the
watershed, such as the longest flow path and stream slope. These data were saved to attribute tables for
the subbasin and river layers. Finally,
HEC-GeoHMS was used to create files that could be read by HEC-HMS. These files contain the connectivity of
subbasins and streams along with the parameters estimate from GIS
datasets. Figure 5 shows
the fin
al
configuration of the HEC-GeoHMS project.
Figure 5. Final HEC-GeoHMS model.
Import preliminary HEC-HMS model structure generated by HEC-GeoHMS
The files generated by HEC-GeoHMS were
imported into an HEC-HMS project. The
HEC-HMS model was modified by incorporating the previously developed model of
the Selbe River and subbasin and river reaches were renamed to reflect either
gage locations or tributary names. Figure 6 shows
the schematic of the HEC-HMS model, with updated names. Table 1
contains a list of subbasins and drainage area.
The main components of an HEC-HMS model are
the basin model, meteorologic model, and input data, like precipitation and
flow data. As mentioned, the basin model
was initially developed using HEC-GeoHMS.
Once this was imported into the HEC-HMS project, additional information
was required. This additional
information includes parameters that are used to model infiltration, surface
runoff, baseflow, and channel routing.
The deficit and constant method was used to model infiltration, the
Clark unit hydrograph method was used to model surface runoff, the recession
method was used to model baseflow, and the Modified Puls and Muskingum methods
were used to model channel routing. More
information about these modeling methods can be found in the HEC-HMS User’s
Manual and Technical Reference Manual. Table 2
contains a list of the more sensitive parameters used to populate the Tuul
River HEC-HMS model. Some model
parameter can be estimated using GIS datasets, for example the time of
concentration can be estimated using the longest flow path (length) and an
estimate of water velocity.
It is necessary to calibrate the model to
observed precipita
tion and
flow. As discussed later in the report,
the model was calibrated to the 1993 flood event. The constant loss rate and Clark unit
hydrograph parameters were modified so that the model could reproduce the
measured flow for this event.
Figure 6. HEC-HMS model of the Tuul River Watershed
after renaming subbasins and river reaches.
Table 1. Subbasin Names and area.
Subbasin Name
|
Area (square km)
|
Sub_Tuul10
|
1380.1
|
Sub_Galttayn
|
473.06
|
Sub_Tuul20
|
408.54
|
Sub_BaruunBayangiyn
|
413.31
|
Sub_Tereljiyn
|
1218.41
|
Sub_Tuul30
|
213.44
|
Sub_Tuul40
|
807.82
|
Sub_Nalayhyn
|
284.72
|
Sub_Tuul50
|
557.00
|
Sub_Uliastayn
|
314.27
|
Sub_Bymbatyn
|
281.43
|
Sub_Selbe10
|
45.413
|
Sub_Selbe20
|
38.477
|
Sub_Selbe30
|
110.33
|
Sub_Selbe40
|
65.48
|
Sub_Selbe50
|
56.80
|
Sub_Tuul60
|
84.29
|
Sub_Tuul70
|
545.68
|
Table 2. Parameters defined in HEC-HMS model.
Subbasin Name
|
Constant Loss Rate (mm/hr)
|
Time of Concentration (hr)
|
Storage Coefficient (hr)
|
Sub_Tuul10
|
0.5
|
10.2
|
40.8
|
Sub_Galttayn
|
0.5
|
6.5
|
26.0
|
Sub_Tuul20
|
0.5
|
5.4
|
21.6
|
Sub_BaruunBayangiyn
|
0.5
|
6.5
|
26.0
|
Sub_Tereljiyn
|
0.5
|
11.9
|
47.6
|
Sub_Tuul30
|
0.5
|
4.1
|
16.4
|
Sub_Tuul40
|
1
|
8.4
|
33.6
|
Sub_Nalayhyn
|
1
|
4.8
|
19.2
|
Sub_Tuul50
|
1
|
7.0
|
28.0
|
Sub_Uliastayn
|
1
|
6.4
|
25.6
|
Sub_Bymbatyn
|
1
|
5.0
|
20.0
|
Sub_Selbe10
|
1
|
4.4
|
17.6
|
Sub_Selbe20
|
1
|
2.7
|
10.8
|
Sub_Selbe30
|
1
|
6.1
|
24.4
|
Sub_Selbe40
|
1
|
4.6
|
18.4
|
Sub_Selbe50
|
1
|
5.1
|
20.4
|
Sub_Tuul60
|
1
|
2.4
|
9.6
|
Sub_Tuul70
|
1
|
6.5
|
26.0
|
Import precipitation and flow data into HEC-DSS
Precipitation and flow data were provided
for the Ulaanbaatar and Terelj gages.
Additional flow data was provided for the Bosgo gage. In order to use this data within the HEC-HMS
model, it was imported into the Hydrologic Engineering Center’s Data Storage
System Visualization Utility Engine (HEC-DSSVue). HEC-DSSVue can be used to easily visualize,
compare, and edit the time-series data. Figure 7 shows
the editor used to import the data into HEC-DSSVue, the data was copied and
pasted from Excel files into this editor.
Figure 8 shows
a plot in HEC-DSSVue comparing flow at the Ulaanbaatar and Terelj gages for the
1993 flood.
Figure 7. Entering time-series data into HEC-DSSVue.
Figure 8. Comparison of flow hydrographs at the Ulaanbaatar
and Terelj Gages.
The time-series data was accessed from the
HEC-HMS model by creating flow and precipitation gages. Figure 9 shows
the precipitation gage for the Terelj gage.
Notice the gage is pointing to the Tuul Data.dss file. This is the file where all data from the Excel
spreadsheets was imported. The DSS
Pathname field shows the Terelj precipitation record was selected. When a simulation is computed within the Tuul
River HEC-HMS model, the program will access this DSS file when gathering data
for the Terelj gage.
Figure 9. Link time-series gage in the HEC-HMS model to
DSS records.
Calibrate the model to the 1993 flood event
The 1993 flood event was used to calibrate
the HEC-HMS model. The simulation time
window was from June 20, 1993 through August 15, 1993. A 1-hour simulation time-step was selected. The precipitation data provided was at a
12-hour interval (includes the accumulated precipitation for a 12-hour
period). When running the simulation at
a 1-hour time-step, HEC-HMS will proportion the 12-hour data equally among the
12 hours. For example, if the data
contained 12 millimeter of precipitation for a 12-hour period, then HEC-HMS
will apply 1 millimeter of precipitation for each hour within the 12-hour
period.
Both the Terelj and Ulaanbaatar
precipitation data was used for the 1993 event simulation. The model was set up so that all subbasins in
the upstream portion of the watershed, upstream of the Terelj gage and the
upper proposed reservoir, used the Terelj gage and all the remaining subbasins
used the Ulaanbaatar gage. Figure 10 shows
the precipitation from both these gages for the 1993 event. The Terelj gage measured 330 millimeters and
the Ulaanbaatar gage measured 200 millimeters.
Figure 11 shows
a comparison of simulated (blue line) and observed (black line) flow at the
Terelj gage for the 1993 simulation. Figure 12 shows
the comparison at the Ulaanbaatar gage.
Results show fairly good agreement in magnitude and volume of runoff
(HEC-HMS model parameters were adjusted, within reason, to improve model
results). At the Ulaanbaatar gage the
model simulated 204.6 millimeters of runoff and the observed discharge is 206.0
millimeters. One item to note is that
the observed streamflow data does not capture the peak flow or the daily
average flow of the event. It only
represents the instantaneous flow measured once per day. In order to accurately model this historic
event, more detailed measurement of flow data is necessary.
Figure 10. Comparison of precipitation at the Ulaanbaatar
and Terelj gages for the 1993 event.
Figure 11. Computed and observed hydrographs at the
Terelj gage for the 1993 simulation.
Figure 12. Computed and observed hydrographs at the Ulaanbaatar
gage for the 1993 simulation.
The deficit and constant loss method does
support continuous simulation of the hydrologic cycle. During periods without precipitation, the
program will remove water from the soil, creating a moisture deficit. Figure 13 shows
results from one of the subbasin elements.
The first plot shows precipitation, the second is the moisture deficit,
and the third is the flow from the subbasin element. Notice how the deficit grows when there is no
precipitation and becomes zero during large precipitation events.
Figure 13. Example of continuous simulation in HEC-HMS
model.
Perform a statistical analysis to estimate the 1-percent precipitation and flow at the UB gage
A statistical analysis of precipitation and
flow data was completed to estimate the 1-percent 24-hour rainfall and the
1-percent flow at the Ulaanbaatar gage.
This information was used to create simulations within HEC-HMS that
evaluated the impact of a reservoir on the flow at Ulaanbaatar.
Precipitation and Flow data found in the Preliminary Hydrological and Hydraulic
Investigation Report, written in November 2010 for the existing and
proposed Ulaanbaatar Bayanzurkh Bridge project, was imported into the
Hydrologic Engineering Center’s Statistical Software Package (HEC-SSP). Figure 14 shows
the annual maximum peak flows from the Tuul-Ulaanbaatar gage. These peak flows are the maximum flow
measured at the gage, but do not reflect the maximum flow for the event. Figure 15 contains
the annual maximum 24-hour precipitation from the Ulaanbaatar gage. These values have been adjusted to better
reflect a maximum over any 24-hour period (the original data was maximum
precipitation at standard clock intervals, not maximums over 24-hours).
HEC-SSP contains a general frequency
analysis that fits an analytical frequency curve to the annual maximum
data. For this analysis, the log Pearson
type III curve was fit to the data.
HEC-SSP follows Guidelines for Determining Flood Flow Frequency,
Bulletin 17B, when computing the frequency curve. Figure 16 shows
a graphical plot and Figure 17 show
the table of the precipitation frequency curve.
Notice the 1-percent precipitation is 81 millimeters. Figure 18 shows
a graphical plot and Figure 19 shows
the table of the flow frequency curve.
The 1-percent flow is approximately 2094 cms.
Figure 14. Modified peak flows measured at the Ulaanbaatar
gage.
Figure 15. Modified peak 24-hour precipitation measured
at the Ulaanbaatar gage.
Figure 16. Graphical precipitation frequency curve for
maximum 24-hour data.
Figure 17. Tabular precipitation frequency curve for
maximum 24-hour data.
Figure 18. Graphical flow frequency curve for the Tuul
River at Ulaanbaatar.
Figure 19. Tabular flow frequency curve for the Tuul
River at Ulaanbaatar.
Model the 1-percent flood given the 1-percent precipitation within the calibrated HEC-HMS model (compare results to the statistical analysis)
The 1-percent precipitation value computed
using HEC-SSP was specified in the HEC-HMS model and a “hypothetical” time
pattern was applied (the SCS type II pattern).
This was necessary to distribute the 24-hour precipitation into 1-hour
increments. Another option was to
perform the frequency analysis on 12-hour data, then manually configure the
storm given the 24-hour and 12-hour values.
Figure 20 shows
the precipitation hyetograph computed by the SCS type II storm (the maximum
1-hour intensity is 30 mm). As shown
above, the statistical analysis estimated 81 inches for the 1-percent precipitation. This represents precipitation measured at one
location and does not represent the precipitation intensity for a storm
occurring over the entire Tuul River watershed.
One assumption typically made when modeling frequency events is to
reduce the “point” precipitation for the area the storm is being applied. For the HEC-HMS model, the 81 millimeters was
reduced to 70 millimeters to reflect a storm being applied to the entire Tuul
River Watershed upstream of Ulaanbaatar.
A more in-depth analysis should be completed where historical storms are
used to develop depth-area reduction factors.
Figure 20. Precipitation hyetograph for the 1-percent
event simulation.
Figure 21 shows
the simulated hydrograph at the Ulaanbaatar gage for the 1-percent event
simulation. The peak flow is 2036 cms
and the total volume of runoff is 104 millimeters. The computed peak flow is similar to the
1-percent flow generated by the statistical analysis of observed data, 2094
cms.
Figure 21. Runoff hydrograph at Ulaanbaatar for the
1-percent hypothetical flood.
Set up a simulation that can be used to illustrate the benefits, reduction of peak flow and volume, of a reservoir located upstream of Ulaanbaatar on the Tuul River
The final analysis completed with the
HEC-HMS model was to determine the effect of an upstream reservoir on the
1-percent flood event. This analysis was
completed by duplicating the basin model used to compute the 1-percent
hydrograph. Then, at the location of the
upper reservoir, the junction was disconnected from the stream network, see Figure 22. Effectively, this removed 2262 square miles
from the watershed. A source element was
added to the model and was configured to supply a constant 5 cms (this
simulates a minimum reservoir release).
A simulation was created that used the same 1-percent precipitation and
the modified basin model. Figure 23 shows
a comparison of hydrographs at the Ulaanbaatar gage for the 1-percent
simulation, current conditions, and the 1-percent simulation with an upstream
reservoir. The peak flow drops from 2036
cms to 1287 cms and the total runoff volume (for the event time-window)
decreases from 104 millimeters to 71 millimeters.
Additional basin model could be configured
to determine the effects of different reservoir locations on the peak flow at Ulaanbaatar. In addition, a detailed reservoir simulation model
could be used to determine an operation criteria for larger storm events to
minimize downstream impacts and retain water for water supply.
Figure 22. Basin model schematic showing where upstream subbasins
we
re disconnected
from the drainage network.
Figure 23. Comparison of 1-percent hydrograph for the
current conditional scenario and the upstream reservoir scenario.
Hydraulic Model Development
An HEC-RAS model was created to evaluate hypothetical
dam breaches for three proposed dam locations.
Therefore, many of the assumptions used in model development consider
that modeling a very large flow is the intended purpose of the model. This
report describes the following steps for developing the HEC-RAS model of the
Tuul River and applying it to hypothetical dam breach scenarios.
1)
Gather elevation data
2)
Develop model geometry
3)
Establish flow data and
boundary conditions
4)
Enter dam information and
breach parameters
5)
Hydraulic simulation and
analysis of results
Elevation Data
THE HEC-RAS model was constructed based on
SRTM (Shuttle Radar Topography Mission) data.
This data was found to be the best available data for the region. The SRTM collected radar data over the Earth
in February 2000 by Shuttle Endeavour and is process by the Jet Propulsion
Laboratory. SRTM data is expected to
have a horizontal accuracy of 20 m and vertical accuracy of 16 m; however,
verification of the SRTM data has shown a vertical accuracy closer to 9 m. For this study, SRTM data projected to the UTM
Zone 48 North at a nominal 30-m cell size was used for elevation data for all
geometry and inundation mapping.
Model Geometry
HEC-GeoRAS was used to extract geometric data
for HEC-RAS. HEC-GeoRAS is a GIS tool
that works with ArcMap and can extract elevation data (and perform mapping of
HEC-RAS results). Using GeoRAS, the
river alignment and cross sections were laid out based on the terrain
characteristics to model the floodplain.
Elevation information was then extracted from the SRTM terrain for
HEC-RAS cross sections. The HEC-RAS
cross sections required additional modification
to better represent the channel area and
shape; dimensions observed during field visits and using Russian Military
Topographic maps published in the 1960s and 1970s were used to modify the cross
sections. The channel dimensions were further
adjusted based on the measurement of field measurements of the bridge openings. The size and shape of the channel is highly
variable and the lack of high-resolution terrain data means that the cross
section geometry may not accurately represent the true ground surface
conditions. The river system and cross
section layout are shown in Figure 24 .
Figure 24. RIver and cross section layout.
Surface roughness values represented with
Manning n-values were set to 0.05 for the main channel and 0.1 for the overbank
areas. N-values were not varied
throughout the model as there was not data to support n-value refinement. These values were used based on estimates
from field visit and aerial imagery.
Four bridges were modeled (Yarmag, Zaison
(Peace Bridge), Marshalls, and Bayanzurh).
All bridge information was estimated from field visits for approximate
opening height, width, number of piers, and pier width. Elevations were tied into the terrain model
elevations which are known to not be highly accurate.
Levees
were used to model high ground – their location approximated from aerial
imagery and elevations taken from the SRTM data. Supplemental levee location and elevation
information was not available to refine the levee information used on the cross
sections. In the end, an elevation that
would consistently contain 2000 cms (an approximate 100-year flow as determined
through the hydrologic analysis) without overtopping was used as the final
levee height. An example of the levee
placement with cross sections is in shown Figure 25.
Figure 25. Levee locations shown along the river with
cross section locations.
Flow Data and Boundary Conditions
Flow developed from the HEC-HMS model were
used as inputs to the HEC-RAS unsteady-flow model. The estimated 1-pecent chance exceedance
flows were used as the flood scenario during the hypothetical dam
breaching. The normal depth boundary
condition was used for the downstream boundary condition (s= 0.0016) based on
the average land surface slope. The
downstream boundary condition did not affect hydraulic results in the study
area. Locations of inflow into the
HEC-RAS model are shown in Figure 26. Lateral (point inflow) and
uniform lateral (distributed inflow) inflow boundaries were used.
Figure 26. Flow input locations.
Dam Information
There were three proposed dam site
locations on the Tu
ul
River. Propose Site #1 was just
downstream o f the Bosgo Gage; Site #2 downstream of the confluence of the
Terelj River; and Site #3 about 10 km downstream from Nalayh. Proposed dam site locations are shown in Figure 27.
Figure 27. Proposed Tuul River dam site locations.
Site #1 is preferred proposed location and
was the focus for the analysis. The top
of dam elevation, reservoir storage volume and breach characteristics used for
the analysis are provided in Table 3.
Table 3. Dam Information.
Site #1
|
Site #2
|
Site #3
|
||
Dam Elevation (m)
|
1530
|
1490
|
1530
|
|
Total Volume (M m3)
|
274
|
405
|
400
|
|
Breach Bottom Width (m)
|
200
|
200
|
200
|
|
Side Slopes
|
1
|
1
|
1
|
|
Formation Time (hr)
|
2.65
|
3
|
3
|
|
Site #1 was modeled using an inline
structure to model the dam and a storage area object for the reservoir
volume. The elevation-volume information
for the storage area was developed using the terrain model. The curve was then adjusted to ensure that
274 million m3 corresponded to the max dam elevation of 1530 m. Site
#2 and Site #3 were modeled using an inline structure for the dam and cross
sections for the reservoir area. Breach
information was estimated using published regression equations for earthen
dams. These regression equations are
available within the HEC-RAS Breach Editor, shown in Figure 27 and an example of the Breach Editor is shown in Figure 28.
Figure 28. Breach parameter calculator in HEC-RAS.
Figure 29. HEC-RAS Breach Editor.
Hydraulic Results
Hydraulic results for the proposed dam site
locations were evaluated based on stage, flow and warning time at the UB Gage
located at Marshalls Bridge. For the
three proposed sites, a breach on the most upstream location (Site #1) results
in an estimated maximum water surface of 1296.6 m while a breach at Site #3
results in a estimated water surface elevation of 1299.5 m for about 3 m difference. Further a breach at Site #1 would provide for
an estimated 25 hours of warning/evacuation time while a breach at Stie #3
would only provide an estimated 4 hours of warning. A summary of results is shown in Table 4 and illustrated in Figure 29.
Table 4. Summary of Dam Breach results.
Site #1
|
Site #2
|
Site #3
|
|
Max WSE (m)
|
1296.6
|
1297
|
1299.5
|
Peak Flow (cms)
|
13400
|
15500
|
29600
|
Flood Arrival Time (hrs)
|
25.5
|
11
|
4
|
Figure 30. Dam breach stage hydrograph results for the 3
dam sites.
The approximate 3 m difference in water
surface elevation resulting from the breaches and the propose dam sites
resulted in very little difference in the flood inundation. As shown in Figure 30, there is an almost indiscernible difference in floodplain boundary
around Ulaanbaatar City.
Figure 32. Estimated flood depths on
structures for dam breach of Site #1.
Sensitivity analysis for the breach
parameters was evaluated for the proposed Dam Site #1 location. The analysis proved to not be sensitive the
selection of the parameters.
Table 5. Summary of breach parameter
sensitivity for Dam Site #1.
Low
|
Selected
|
High
|
||
Breach Bottom Width (m)
|
125
|
200
|
300
|
|
Side Slopes
|
0.5
|
1
|
0.5
|
|
Formation Time (hr)
|
1
|
2.65
|
3
|
|
Peak Flow @ Dam (cms)
|
34300
|
36600
|
43200
|
|
Max WSE @ UB Gage (m)
|
1296.4
|
1296.6
|
1296.65
|
|
Peak Flow @UB Gage (cms)
|
12500
|
13400
|
13700
|
|
Flood Arrival Time @ UB Gage
(hr)
|
25
|
25
|
25.75
|
|
Figure 33. Hydrograph sensitivity to
breach parameters for Dam Site #1 at the UB Gage.
Conclusions
The HEC-RAS model was constructed from data
that was not well defined in the main channel.
It is uncertain of the flood carrying capacity the channel and
floodplain. However, each of the dam
breach scenarios shows overtopping of
the mod
eled
levee. If the channel and levee heights
used in this analysis is not correct, however, the result could be severely
incorrect. The levee information was also not accurate enough to provide a high
level of confidence in identifying the overtopping water surface
elevation. The levee system may have
been professionally engineered and has a variable top elevation. As a
result, it is prudent to improve the HEC-RAS model with accurate geometric
data: channel data and levee positions and elevations. Examples of the importance in using correct
levees in shown in Figure 33 and Figure 34.
Figure 34. Levee positions and elevations will greatly
affect the water surface profile results on cross sections; thereby affecting
the floodplain mapping.
Figure 35. Example of the importance of proper levee
position and elevation for inundation mapping.
Further, it is important to acknowledge the
numerous bridges on the Tuul River. The
HEC-RAS model assumes that the bridge function according to the observed
conditions. However, during a dam breach
flood event the structural response of the bridges to the flood wave are highly
uncertain. The bridges may be removed by
the flood, clogged with debris, or the channel may be scoured in the bridge
opening. It is recommended that
sensitivity analysis of the bridge openings based on probable scenarios be
evaluated. This is most important
for areas where the river is leveed and the water surface may overtopped the
levee and flood the protected area.
Lastly, because the terrain data is not
complete (leading to deficiency in HEC-RAS model construct), the inundation
extents are not mapped with high certainty.
It is recommended that mapping is
performed after acquiring improved terrain data with improved hydraulic model
geometry and a refined hydraulic analysis.
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