Study Report 3

M2 - Putika Ashfar Khoiri

Water Engineering LaboratoryDepartment of Civil Engineering

October 17th , 2017

General Purpose:Simulate past flood event in Surabaya City to derive probabilistic flood maps in flood prone area by assessing several uncertainties in hydraulic modelling and flood mapping.

Problems to be discussed :1. How river discharge and other parameters have influence on flood simulations

(How the model results change depending on the input data)2. What is the problems while simulating floods?

Objective

Outline

1

Previous Task

• Development of flood scenarios and model simulations

Perform flood inundation modelling for the study area NAYS 2D Flood for 1 flood event

Inflow discharge hourly observed discharge data from February 11st,

2015 to February 13rd, 2015

water surface at downstream free outflow

Initial water surface Depth = 0

Calculation time 42 hours

Time step 2 mins

Calculation Condition

Parameters which are included :

1. Determination of grid shape2. Roughness coefficients of the floodplain

2

Current Progress

Modules Evaluation tasks

Hydrological AnalysisFlow hydrograph shapePeak discharge estimationwater-elevation and distance relationship curve

Rating-curve rating curve estimationFlood routing Flow hydrograph shape

Flood inundations were simulated using the flow conditions on the past flood events between January to March in 2014, 2015 and 2016

Reported flood events obtained from National Agency for Disaster Management Data

Date Inundation Height (cm) Duration

23-01-2014 (event 1) 50-70 2 to 3 days

20-02-2015 (event 2) 10-30 2 days

24-02-2016 (event 3) 20-30 2 days

The annual maximum peak discharge during the flood events are observed in 3 weeks around

flood peak

• Initial condition : 2 weeks before the flood occurs

• Simulation period : 100 hours 3

Calculation setting

River length 8.64 kmSize of W 3000 meterGrid size 30 m x 30 m

number of river cross section data

9 points

upstream

downstream

inflow

Name of polygon Description Roughness Coefficient

Agricultural area Grass, agricultural site 0.030

Low density area Citizen house 0.040

High density area Office, school and

public facility

0.070 – 0.80

River River 0.01

(*)IRiC User Manual 4

Initial Condition (2 weeks before flooding inundation simulation) based on the observation data

Date Average discharge (m3.s-1) Maximum Discharge (m3.s-1)

9-01-2014 to 22-01-2014 (IC 1) 79.54010976 181.22

5-02-2015 to 19-02-2015 (IC 2) 66.41162405 182.22

9-02-2016 tp 23-02-2016 (IC 3) 60.37975238 180.1

-50.00

0.00

50.00

100.00

150.00

200.00

0 50 100 150 200 250 300 350 400

Dis

char

ge (

m3

.s-1

)

Period (Hours)

IC 1

IC 2

IC 3

Calculation setting (initial condition)

The discharge on event 1 is relatively low before the flood events 5

Result (initial condition)

1. The value of river depth are varies from 0.5 m to 4 m -> the depth value need to calibrate with the datum to produce water elevation.

2. In the middle stream area, the water depth become higher, probably due to the effect of topographic changes or narrow changes in river cross-section width.

Water depth

6

Result (initial condition)-velocity

1. The velocity magnitude are varies between 0.784 m/s to 1.57m/s along the centerof the stream line.

2. Higher velocity occurs due to the meandering of the river

Velocity

7

Discharge routing methodThe Muskingum channel routing method is employed to route the flow through the streams

𝑄𝑢 − 𝑄𝑑 =∆𝑥

𝑐3

𝜕

𝜕𝑡𝑄𝑑 +

1

2−

𝐿

∆𝑥(𝑄𝑢 − 𝑄𝑑)

𝑄𝑑,𝑗+1 = 𝐶1𝑄𝑢,𝑗+1 + 𝐶2𝑄𝑢,𝑗+1 + 𝐶3𝑄𝑢,𝑗+1

Where the coefficients C1,C2,C3 are expressed as

𝐶1 =−𝐾𝜃 + 0.5∆𝑡

𝐾 1 − 𝜃 + 0.5∆𝑡

𝐶2 =−𝐾𝜃 + 0.5∆𝑡

𝐾 1 − 𝜃 + 0.5∆𝑡

𝐶3 =𝐾 1 − 𝜃 − 0.5∆𝑡

𝐾 1 − 𝜃 + 0.5∆𝑡

K= travel time

θ = weighing parameter ( 0 to 1)

Time needed from the water at the

upstream reach the downstream end at the

simulation

∆𝑡 >∆𝑥𝑗

𝑐𝑗𝑛 + 𝑢𝑗

𝑛

∆𝑥𝑗 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑖𝑣𝑒𝑟 𝑟𝑒𝑎𝑐ℎ

𝑐𝑗𝑛 = 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 𝑤𝑎𝑣𝑒 𝑐𝑒𝑙𝑒𝑟𝑖𝑡𝑦

𝑢𝑗𝑛 = 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑒𝑎𝑐ℎ 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 8

Calculation setting (2)Inflow hydrographs used for the simulations boundary conditions

Hourly discharge data are provided at the upstream area of Surabaya River.

Date

Maximum Discharge

(m3.s-1)

Tp

(hours) T0.3 (hours)

23-01-2014 to 27-01-2014 (event 1) 367.55 28 18

20-02-2015 to 24-02-2015 (event 2) 337.53 46 32

24-02-2016 to 28-02-2016 (event 3) 335.9 35 27

Tp (lag time) = the amount of time it will take for a river to flood after a period of heavy precipitationT0.3 = the amount of time need from the peak of discharge to 30% peak of discharge

Event Initial Water Surface (m)

time step

(sec)

Initial manning roughness

channel floodplain slope

1 17.48514757 120 0.01 0.001

2 17.71327015 180 0.01 0.001

3 17.672942 120 0.01 0.001

A uniform routing time interval, Δt = 30min and Δx = 30 m was used for all the test runs.

9

Routing results (event 1)

1. The hydrograph peak theoretically took 72 minutes to travel between 2 gauges and relatively little

attenuation occurred. With the estimation of velocity between 0.74 m/s to 1.8 m/s

2. Peak flow occurred at the upstream end at 30 hours from the initial condition, and at downstream

end on the same day within 124 minutes

3. The downstream discharge is slightly unstable on the peak of the hydrograph

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120

Dis

char

ge (

m3

/s-1

)

Time (hours)

upstream boundary

downstream discharge

downstream discharge(simulation)

10

Routing results (event 2 and 3)

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120

Dis

char

ge (

m3

/s-1

)

Time (hours)

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120

Time (hours)

upstream boundary

downstream discharge

downstream discharge(simulation)

1. Event 2 and 3 relatively have long peak hydrograph with duration of 4 to 6 hours 2. Using the same travel time estimation (K) for the routing calculation on event 2 and 3,

the downstream simulation result has a close relation with the calculated downstream discharge

3. But, we still need to evaluate the relative error in the time- to peak of the routed discharge hydrograph

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Rating-curve rating-curve estimation (tentative)

Flood dynamics Parameter estimation

Grid-shape accuracy

Damage estimation

Probabilistic flood Map

water elevation-damage relations

building and land-use value

Methodology

Hydrological Analysis

Flood routing

Damage estimation

uncertain

12

Setting of initial conditionAnalysis of hydrograph shape and water elevation profile

Model Parameters

The table lists the factors which are used as calibration factors in each model implementation.

Parameter Sampling range

Manning roughness channel 0.01-0.04

Floodplain slope 0.001-0.015

Initial water surface at downstream boundary 17.5 m – 17.8 m

Eddy viscosity coefficient 0 to 1

Calibration parameters

1. Capability of the model reproduce hydraulic behaviour of the river and

inundated floodplain

2. Time stationary of model parameter (i.e. roughness parameter)

3. Hydraulic information (i.e. flow hydrograph)

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Future Tasks

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