Flood Inundation Mapping of Tadi River

CE 547 GIS in Water Resource Engineering

Final Report

Submitted By:

Aayush Piya

May 5, 2017

Contents 1 Motivation & Background .................................................................................................................... 3

2 Introduction ........................................................................................................................................... 3

3 Objective ............................................................................................................................................... 3

4 Methods................................................................................................................................................. 4

4.1 Data sources .................................................................................................................................. 4

4.2 Projection used .............................................................................................................................. 4

4.3 Analysis......................................................................................................................................... 4

4.3.1 Delineating Watershed Area ................................................................................................. 4

4.3.2 Hydrological Analysis ........................................................................................................... 5

4.3.3 Flood Inundation Mapping .................................................................................................... 5

5 Results ................................................................................................................................................... 7

5.1 Watershed Delineation .................................................................................................................. 7

5.2 Hydrological Analysis................................................................................................................... 8

5.2.1 Mean monthly flow and Flow Duration Curve ..................................................................... 8

5.2.2 Rating Curve ......................................................................................................................... 9

5.2.3 Flood Forecast Analysis ........................................................................................................ 9

5.3 Flood Inundation Mapping.......................................................................................................... 10

6 Conclusion .......................................................................................................................................... 11

7 Future work ......................................................................................................................................... 11

APPENDIX

1 Motivation & Background Nepal with its fragile geology, steep slopes, high relief, and variable climates, is prone to water

induced disasters such as floods and landslides. Over the last twenty years from 1983-2002,

floods and landslides caused 6,466 deaths and more than US $ 200 million in damage. In the

absence of information about the nature of flood events, exposure of life and properties and

capabilities to cope with disasters, it is difficult to prepare and implement pre-disaster activities.

Lack of information is likewise a major constraint in implementing and coordinating the rescue

and post-disaster management activities effectively. The necessity of understanding the

phenomenon of flooding of the river and to identify and map vulnerable areas for proper

management and mitigation of floods is becoming essential to minimize the damages incurred

annually.

2 Introduction Flood inundation mapping(FIM) is required to understand the affects of flooding in an area and

on important structures such as roadways, railways, streets, buildings and airport. FIM provides

important information, like depth and spatial extent of flooded zones, required by the municipal

authorities to inform the citizens about the major flood prone areas and adopt appropriate flood

management strategies. In this Project, the catchment area of a river at site location is determined

and a flood inundation map has been developed for the river within the catchment area caused by

the 100-year return period flood.

3 Objective The main objective of this project was to explore and learn the basic function of HEC-RAS and

HEC-GeoRAS and prepare a flood inundation map using this modelling tools. The specific

objectives were as follows:

i. Delineate watershed area of a section of Tadi river

ii. Perform Hydrological calculation

iii. Use HEC-RAS and HEC-HEC-GeoRAS to develop flood inundation map of the section

of river in ArcGIS

4 Methods

4.1 Data sources i. Hydrological data (daily flow records) from the existing Station No. 448 at Tadipul,

Belkot, Nuwakot in Tadi Khola published by Department of Hydrology and Meteorology

(DHM), Government of Nepal (GoN).

ii. Digital Maps of Topography Base Map with Index sheet no. 2785 02B and 2885 14D

4.2 Projection used

A modified projection named Nepal Central Projection was used. It is CGS Everest Bangladesh

1937 modified to following parameter. This projection overcomes the inconveniences caused by

the negative numbers in Rectangular Coordinate system.

Projection: False_Easting: 500000.0

False_Northing: 0.0

Central_Meridian: 84.0

Scale_Factor: 0.9999

Latitude_Of_Origin: 0.0

Linear Unit: Meter (1.0)

4.3 Analysis

4.3.1 Delineating Watershed Area

Tadi Khola is located in Rautbeshi, Shikarbeshi and Ghyanphedi Village Development

Committees (VDCs) of Nuwakot District in Central Development Region of Nepal. Digital maps

were used to determine the catchment area. However, only two digital maps were available

which entailed only upper section of the river. Because of unavailability of all the digital maps

only a section of the river was considered for this project. A point with coordinates 85° 25′ 3.48″

E and 27° 57′ 45″ N was taken as reference to delineate the watershed area.

4.3.2 Hydrological Analysis

In this project, the hydrological analysis deals with the flow analysis to obtain the mean monthly

flow at the provided point of reference using catchment correlation, flood at different return

periods and in short it provides a basis of forecasting. If two basin are hydro-meteorologically

similar, data extension may accomplished simply by multiplying the available long term data at

the Hydrometric station with the ratio of the basin areas of the base station (proposed site under

study) and the index Hydrometric station. In this contest, more accurate results can be obtained

using Dicken’s formula. 𝑄 = 𝑄𝑜 ∗𝐴

𝐴𝑜 Where, Q and Qo are the discharge at the base and index

stations, respectively, and A and Ao are the corresponding basin areas. Using this method, flow

duration curve (FDC) and rating curve at the proposed site was developed. For flood frequency

analysis Gumbel’s extreme value distribution was used.

4.3.3 Flood Inundation Mapping

The flood inundation map was developed for a section of Tadi river. For this work, the HEC-

RAS was used to calculate water-surface profiles; ArcGIS was used for GIS data processing. The

HEC-GeoRAS for ArcGIS was used to provide the interface between the systems. HEC-

GeoRAS is an ArcGIS extension specifically designed to process geospatial data for use with

HEC-RAS. The extension allows users to create an HEC-RAS import file containing geometric

attribute data from an existing digital terrain model (DTM) and complementary data sets. HEC-

GeoRAS automates the extraction of spatial parameters for HEC-RAS input, primarily the three-

dimensional (3D) stream network and the 3D cross-section definition. Results exported from

HECRAS are also processed in HEC-GeoRAS. The general procedure adopted for inundation

modelling consists basically of five steps: i) preparation of terrain (DEM or TIN) in ArcGIS, ii)

HEC-GeoRAS for pre-processing to generate a HEC-RAS import file, iii) running of HEC-RAS

to calculate water-surface profiles, iv) post-processing of HEC-RAS results, and v) floodplain

mapping.

4.3.3.1 Data in HEC-RAS

The geometric data were imported from HEC-GeoRAS. The imported file includes river

streamline along with the bank lines and flow path. The file also includes cross section of the

river. Geometric data also requires Manning’s roughness coefficient. For simplicity, a constant

Manning’s coefficient was used. Tadi river is a mountain stream with no vegetation in channel

and large boulder at bottom. The n value fitted for this description was 0.04, 0.05 and 0.07 for

minimum, normal and maximum respectively.

Similarly, for flow data, a constant flow was assumed throughout the stream. A peak flood

discharge for 100-year return period was estimated using Gumbel’s method. This discharge was

used as constant flow for steady flow analysis. For boundary condition, rating curve was used.

5 Results

5.1 Watershed Delineation

The total catchment area of the basin at the point of reference was calculated 101Km2.

5.2 Hydrological Analysis

5.2.1 Mean monthly flow and Flow Duration Curve

Month Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Mean monthly

flow at St. No.

448, m3/s

9.63 7.29 5.23 5.62 9.94 34.30 99.15 129.43 92.30 43.22 21.76 13.14

Estimated

mean monthly

flow at

proposed site,

m3/s

1.52 1.18 0.89 0.93 1.44 5.48 15.33 20.29 14.95 6.81 3.51 2.13

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

22.00

24.00

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Flo

w, m

3/s

Exceedence Level Of Flow, %

FLOW DURATION CURVE

5.2.2 Rating Curve

5.2.3 Flood Forecast Analysis

Gumbel Parameters Mean (x) 96.20269231

Std deviation(σ) 51.43283741

Reduced Mean (y) 0.532

Reduced stdev s 1.0961

Return Period (yrs) Reduced Variate (yt) Frequency Factor (K) Estimated Discharge (Xt) (m3/s)

2 0.366512921 -0.150978085 88.43746099

5 1.499939987 0.88307635 141.6218146

10 2.250367327 1.567710362 176.8344844

20 2.970195249 2.224427743 210.6113227

50 3.901938658 3.074481031 254.3319753

100 4.600149227 3.71147635 287.094452

1000 6.907255071 5.816307883 395.35191

2000 7.600652407 6.448911967 427.888533

5000 8.517093183 7.285004272 470.8911326

0

1

2

3

4

5

0 25 50 75 100 125 150 175 200 225 250 275 300

Stag

e (m

)

Discharge (m3/s)

Rating Curve

Gauge Height Log. (Gauge Height)

5.3 Flood Inundation Mapping

6 Conclusion

The flood inundation map was developed using HEC-GeoRAS and HEC-RAS. The flood map

covered area of 2km2 with depth ranging from 0 m to 31 m. This project helped to learn basic

function of these modelling tools. For any flow analysis, the flow data along with boundary

condition are very important. In this project, for simplicity, constant value were used whereas in

real life these value changes for each cross section. Selecting proper boundary condition is very

important. While developing the inundation map, there were several times that the output

polygon for flood area was not continuous. The main cause for this irregularity was found to be

cross-section. The cross section is very essential for developing these maps and it is imperative

that there is enough cross section provided in the geometric data.

7 Future work

While carrying out this project, only a section of the river was considered. It would be very

interesting to see how the map develops when whole river is considered for analysis. As

mentioned before, constant data such as constant flow and manning’s equation were used. An

only one boundary condition was applied. In future, this project can be conducted with detailed

flow data.

APPENDIX

Nepal Index Sheet

Site Location

Hydrological Analysis

Mean monthly flow

Year: Jan. Feb. Mar. Apr. Hay June July Aug. Sep. Oct. Nov. Dec. Year

1969 1.08 0.69 0.62 0.54 0.50 2.43 11.20 17.01 13.60 5.65 2.77 1.41 4.8

1970 0.94 0.72 0.61 0.49 0.72 4.79 17.63 24.90 13.07 7.41 4.32 2.46 6.5

1971 1.56 1.15 1.03 1.79 2.03 14.06 18.10 21.65 12.79 8.80 4.25 2.20 7.5

1972 1.46 1.33 1.14 1.04 0.94 3.17 17.17 18.25 18.56 7.05 4.75 2.51 6.4

1973 1.59 1.07 1.20 0.81 1.89 12.08 13.94 19.95 19.64 10.12 4.53 2.10 7.4

1974 1.35 0.82 0.55 0.59 0.88 3.85 14.51 25.21 21.50 9.53 4.72 2.91 7.2

1975 2.12 1.70 0.86 0.89 1.05 5.43 18.56 18.25 23.36 8.79 3.67 2.24 7.2

1976 1.69 1.30 0.80 0.95 2.68 7.18 11.94 17.48 15.93 7.28 3.82 1.93 6.1

1977 1.31 1.04 0.75 1.23 1.50 3.19 16.09 23.36 10.32 5.00 2.58 1.58 5.7

1978 1.15 0.86 0.83 0.89 1.76 9.17 26.45 30.93 14.03 8.32 3.40 1.93 8.3

1979 1.29 1.13 0.64 0.73 0.62 2.20 12.16 20.11 10.38 4.42 2.89 2.29 4.9

1980 1.62 1.28 1.14 0.94 1.18 5.92 18.56 24.28 10.29 4.25 2.35 1.53 6.1

1981 1.20 0.93 0.65 1.05 1.39 3.22 13.87 17.94 9.85 3.34 2.30 1.53 4.8

1982 1.15 1.10 0.86 0.85 0.72 2.47 11.04 13.70 8.83 3.31 2.41 1.59 4.0

1983 1.23 0.95 0.80 0.89 1.50 1.75 12.50 14.63 17.94 8.99 4.05 2.54 5.6

1984 1.87 1.32 0.85 0.94 1.90 4.78 16.09 15.93 15.17 4.27 2.64 1.79 5.6

1985 1.40 1.11 0.71 0.68 0.97 3.31 16.09 23.51 21.96 8.60 3.70 2.04 7.0

1986 1.42 1.01 0.62 0.82 1.11 7.59 18.25 16.09 16.40 8.27 3.91 2.63 6.5

1987 1.67 1.40 1.03 1.01 1.10 2.89 15.47 19.33 15.93 8.46

1988 9.28 22.58 26.14 14.40 5.78 3.65 2.55

1989 2.54 1.58 1.18 0.90 2.47 7.38 15.25 26.29 13.73 6.94 3.43 2.06 7.0

1990 1.31 1.39 1.17 1.15 2.04 7.98 17.01 18.56 14.26 7.72 3.63 2.21 6.5

1991 1.75 1.16 0.99 1.06 1.78 4.93 11.01 19.80 14.25 4.52 2.46 1.58 5.4

1992 1.40 1.08 0.48 0.33 0.96 2.91 9.61 18.25 15.47 8.62 4.52 3.06 5.6

1993 2.23 1.87 1.14 1.65 2.91 5.88 12.65 18.25 11.21 5.83 3.31 2.01 5.7

1994 1.69 1.39 1.14 0.81 1.50 4.62 10.90 17.63 15.93 5.69 3.56 2.47 5.6

1995 1.29 1.11

Average: 1.52 1.18 0.89 0.93 1.44 5.48 15.33 20.29 14.95 6.81 3.51 2.13 6.15

Mean Monthly Data (Correlated)

Gumbel’s Method for Flood Forecast Analysis

GUMBEL'S METHOD

Year Discharge Order no. (m) Flood Discharge Tp

1969 59.86 1 252.12 27

1970 129.31 2 232.01 13.5

1971 82.6 3 139.21 9

1972 232.01 4 129.31 6.75

1973 252.12 5 126.84 5.4

1974 71.15 6 126.84 4.5

1975 103.63 7 112.91 3.86

1976 34.96 8 103.63 3.38

1977 54.6 9 103.63 3

1978 112.91 10 98.99 2.7

1979 75.79 11 94.35 2.46

1980 126.84 12 89.71 2.25

1981 89.71 13 85.07 2.08

1982 37.9 14 82.6 1.93

1983 126.84 15 79.97 1.8

1984 45.32 16 75.79 1.69

1985 85.07 17 71.15 1.59

1986 139.21 18 68.83 1.5

1987 103.63 19 68.83 1.43

1988 94.35 20 68.83 1.35

1989 79.97 21 59.86 1.29

1990 68.83 22 58.01 1.23

1991 98.99 23 54.6 1.18

1992 58.01 24 45.32 1.13

1993 68.83 25 37.9 1.08

1994 68.83 26 34.96 1.04

Gumbel Parameters Mean (x) 96.20269231

Std deviation(σ) 51.43283741

Reduced Mean (y) 0.532

Reduced stdev s 1.0961

Return Period Reduced Variate (yt) Frequency Factor (K) Estimated Discharge (Xt)

2 0.366512921 -0.150978085 88.43746099

5 1.499939987 0.88307635 141.6218146

10 2.250367327 1.567710362 176.8344844

20 2.970195249 2.224427743 210.6113227

50 3.901938658 3.074481031 254.3319753

100 4.600149227 3.71147635 287.094452

1000 6.907255071 5.816307883 395.35191

2000 7.600652407 6.448911967 427.888533

5000 8.517093183 7.285004272 470.8911326

0

50

100

150

200

250

300

350

400

450

500

1 10 100 1000

Flo

w (

m³/

s)

Return Period (yrs)

Flood Forecast

Gumbel Method

Rating Curve

Stage (ft) Flow(cfs)

11.08924 2113.936

13.51706 4566.5396

12.04068 2916.9915

15.74803 8193.3559

16.07612 8903.5338

11.58137 2512.6386

16.07612 3659.6589

9.612861 1234.6008

10.66273 1928.1808

12.79528 3987.3791

11.81102 2676.4986

13.45144 4479.3124

11.05643 3168.0788

10.1706 1338.4259

13.45144 4479.3124

10.5643 1600.4607

12.13911 3004.2187

13.77953 4916.1548

12.79528 3659.6589

12.46719 3331.9388

11.94226 2824.1139

11.48294 2430.7085

12.63123 3495.7989

10.99081 2048.6038

11.48294 2430.7085

11.48294 2430.7085

0

1

2

3

4

5

0 25 50 75 100 125 150 175 200 225 250 275 300

Stag

e (m

)

Discharge (m3/s)

Rating Curve

Gauge Height Log. (Gauge Height)

Flood Inundation Mapping

Geometric Data

Data imported from HEC-Geo RAS

Assigning Manning’s Roughness Coefficient

Steady Flow Data

Assign a constant flow in cfs.

Modify Reach Boundary Condition

Rating curve was provided in this project.

Photos of site