Study Report 2

M1 - Putika Ashfar Khoiri

Water Engineering LaboratoryDepartment of Civil Engineering

July 18th , 2017

To simulate past flood event in Surabaya City in order to identify flood routing and analyse;1. flood inundation area 2. velocity magnitude of the flooded area The outcome of this analysis can be used for flood prediction using theoretical data or data driven model

Objective

Outline

Contents :

1. Previous Tasks2. Nays2D Flood3. Proposed Method4. Model Conditions5. Boundary Conditions6. Calculation Conditions7. Results8. Future Task

Previous Tasks

• Inundation area can't represent well in 1D model because of the limitation of cross-sectional plane, although water stage elevation can be examine easily.

• Simulation can’t continue after the middle of hydrograph, some parameters need to examine :

1. Calculation tolerance in the iteration process2. Maximum error in water surface solution

Limitation

Problem :

We cannot validate the calculation results of water level in the middle stream area

Analyse river channel water level and capacity from each cross-sections in Surabaya City

Open source product by Foundation of Hokkaido River Disaster Prevention Research andU.S Geological Survey (USGS)

Nays2D Flood

Simulating past severe flood events of Chao Praya River Basin (CPRB), Thailand to evaluate the effectiveness of non-structural flood countermeasures.

The computation procedures using iRICused to calculate changes in the flow fields and floodplain configurations

Satellite MapiRIC result

Flood inundation result from iRIC tends to estimate flood extend identified by satellite map, especially in point A

Nays2D Flood

Nays 2D Flood Nays 2DHPrefer used for flood flow analysis A plane 2D solver for calculating flow, sediment

transport, bed evolution and bank erosion in

rivers

Equation of continuity

𝜕ℎ

𝜕𝑡+𝜕(ℎ𝑢)

𝜕𝑥+𝜕(ℎ𝑣)

𝜕𝑦= 𝑞 + 𝑟

Equation of continuity

𝜕ℎ

𝜕𝑡+𝜕(ℎ𝑢)

𝜕𝑥+𝜕(ℎ𝑣)

𝜕𝑦= 0

The model uses the same assumptions and

coordinate system as NAYS2DH but not include

any treatment of sediment transport or bed

evolution.

Include river morphology change calculation

Modification: Addition of weirs, culverts,

pumps and others structures

Modification: bed materials type, vegetation,

sediment transport type, turbulent model, etc.

Where :

h = water depth, ν = flow velocity in the y direction, H = water surface elevation, q = inflow through a box culvert, a sluice pipe, or a pump per unit area , r = rainfall

(*)In this case, I don’t include rainfall first

Methods

Determine model domain

Create model conditions

Create and setting grids, Courant number and projection

Set calculation conditions

Determine input data and roughness conditions

Evaluate Result 1

Input rainfall data, add pump data and obstacle

Evaluate result 2

Validate result

No error

unsatisfied result

Model Conditions

Topographic data origin SRTM 30 m resolution

Coordinate projection UTM-sone 49 South, WGS 84

Number of ni 300 grids

Number of nj 100 grids

Size of W 3000 meter

di x dj 30 m x 30 m

2D Unstructured grid

(reduce the domain)

Model ConditionsTime-steps

The numerical stability of 2D Cartesian uniform grid is controlled by Courant–Friedrichs–Lewy (CFL) conditions for predicting an appropriate time-step (Liang, 2010)

𝐶 =𝑢𝑥∆𝑡

∆𝑥+𝑢𝑦∆𝑡

∆𝑦

where C (0<C<1)= Courrant number and is set to 0.75 in this workAssumed the flow velocity are 1-2 m/s2

∆𝑡 = 𝐶𝑚𝑖𝑛(𝑚𝑖𝑛𝑖,𝑗∆𝑥

𝑢𝑖,𝑗 + 𝑔ℎ𝑖,𝑗,𝑚𝑖𝑛𝑖,𝑗

∆𝑦

𝑣𝑖,𝑗 + 𝑔ℎ𝑖,𝑗)

Boundary Conditions

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Dis

char

ge (

m3

/s)

Period (hours)

Inflow discharge hourly observed discharge data from February 11st, 2015

to February 13rd, 2015

water surface at downstream free outflow

Rainfall data none

Initial water surface Depth = 0

Calculation time 42 hours

Time step 2 mins

Calculation ConditionsFinite difference method:

Cubic Interpolation Pseudo-particle (CIP) method for linear wave propagation

Name of polygon Description Roughness Coefficient Note

Forest Forested mountains 0.030

Low density area Low building density

area

0.040 Residential area, etc

High density area High building density

area

0.080 Industrial building

River River 0.025

The bottom roughness coefficient other than that building is calculated by:

𝑛02 =

𝐴1𝑛12 + 𝐴2𝑛2

2 + 𝐴3𝑛32

𝐴1 + 𝐴2 + 𝐴3

n1= 0.06, n2 = 0.047 n3=0.050

A1 = area of each land use (building, farmland)

A2 = area of each land use (road)

A1 = area of each land use (farmland)

(*)Flood simulation manual (draft)- Guide for simulation and verification of new model (1996)

Roughness Conditions

Calculation Conditions

Land use Area (km2) (%)Residential area 42.190 59Agricultural area 22.408 31Industrial area and public Facilities 5.129 7River 1.462 2Total 71

Roughness Conditions

Results Original DEM with determination of roughness condition

• The flood routing can’t well represented along the river flow

• The result represent the effect of domain topography (elevation data) and roughness condition under the boundary condition

13 hours

21 hours

Results DEM with adjustment of river data and roughness condition

• The flow-routing can follow the river-stream line but in the end of hydrograph the simulation the water start to inundate the surrounding area

13 hours

20 hours

• Larger grid domain may need for mapping the water propagation after peak-hydrograph in the lower area

Results Flow Velocity

• In the first time step of the hydrograph, the flood water has velocities about 1-2 m/s

Results Flow Velocity

• As the water propagates, the velocity magnitude in the middle stream area become higher than 2 m/s at some points

Future Tasks

- Evaluate model result (time steps, roughness coefficient, flow)

- Searching and add input for rainfall data

- Calculate discharge and inundation area for another tributaries

- Validate the results and try to reduce bias

calculate 𝑥(𝑖−12)

𝑥(𝑖+12)𝑓𝑛𝜕𝑥 and ∆𝐹𝑖

predict 𝑓′(𝑛+1)

𝑓′(𝑛+1) ≡ 𝑓′(𝑥𝑖 , 𝑡𝑛+1)=𝑓′(𝑥𝑖-c∆𝑡, 𝑡𝑛)

calculate 𝑓(𝑛+1)

Re-calculate 𝑓′(𝑛+1) from 𝑓(𝑛+1)

𝑓′(𝑛+1) = (𝑓𝑖+1𝑛 −𝑓𝑖

𝑛)∆𝑥

if |𝑓𝑖+1𝑛 -𝑓𝑖

𝑛|<|𝑓𝑖𝑛-𝑓𝑖−1

𝑛 |

(𝑓𝑖𝑛−𝑓𝑖−1

𝑛 )

∆𝑥otherwise

The dashed line (----) is the analytical solution

Cubic Interpolation Pseudo-particle (CIP) method(H. Takewaki, 1984)