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7/27/2019 Full Report2

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1. Model Calibration-model represent the real case and essential for accurate flow

resistance forecasting

Model calibration was attempted by simulate the autumn 2000 flood and then verified with

the available recorded information. Based on mannings equation (Equation 1), the floodplain

roughness of the model was adjusted until the simulatedvalue (flood depth and velocity)was fitted well with recorded value (n=0.07).

Equation 1

Table 1.1 shows Mannings value based on Chow, 1959 whilst table 1.2 indicates the

simulated velocity and depth based on trial and error to fit the observed data from farmer A.

Table 1.1: n value for floodplains

Table 1.2: Observed (farmer A) and simulated velocity and depth with different mannings value

Simulated results show good correlations with observed data for Farmer A with n value=0.07(figure 1.1).

Figure 1.1: Good correlation between observed and simulated data


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Graph 1.1, 1.2 and Table 1.3 illustrates simulated and observed value using recorded datafrom Farmer A, Farmer B and willow.

Graph 1.1: Simulated and observed velocity and depth at farmer A logging equipment at point 384756.847,

243710.625

Graph 1.2: Simulated and observed depth at farmer B CCTV at point 385958.536, 239416.722

Table 1.3: Simulated and observed time that flood water reach tree and time of maximum depth at willow at point

384902.658, 241679.317

-0.5

0

0.5

1

1.5

2

2.5

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0:00:00 4:48:00 9:36:00 14:24:00 19:12:00 0:00:00 4:48:00 9:36:00 14:24:00

Depth(m)

Velocity(m/s)

Time from start of simulation (hh:mm:ss)

Observed Velocity V - calibration run 0.03 V - calibration run 0.04

V - calibration run 0.05 V - calibration run 0.06 V - calibration run 0.07

Observed Depth Depth - calibration run 0.03 Depth - calibration run 0.04

Depth - cal ibration run 0.05 Depth - calibration run 0.06 Depth - cal ibration run 0.07

0

0.5

1

1.5

2

2.5

0:00:00 12:00:00 0:00:00 12:00:00 0:00:00

Observed depth

Depth - calibration run 0.03





Time into simulation Comment 0.03 0.04 0.05 0.06 0.07

09:10:00 time that flood waters reach tree 08:55:00 09:05:00 09:05:00 09:10:00 09:15:00

14:00:00 time of maximum depth, 2.20 m 12:50:00 13:05:00 13:30:00 13:40:00 13:50:00


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Figure 1.2: Comparison flood inundation area between satellite image and calibrated model based on autumn

2000 flood events

There is no much different between flood inundation areas for the model based on autumn

2000 flood events with satellite image flood inundation area. With respect to flow depth, flow

velocity and flood area extent, the model is good calibrated to represent the reality.

Figure 1.3 illustrates flood extent, area affected and total volume at different time based on

different return period (50, 100 200). Based on the area, the flood extent and volume

increase with the increased in return period.


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Figure 1.3: Location liable to flooding at a return period of 1 in 50, 100 and 200 years with no flood defences


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2. Flood Defence Measures: Flood protection level is set to 100yrs return period at Upton

upon Severn town.

Simulation of Existing River Section (ISIS 1D)-Increase conveyance capacity

Purpose: To obtain design flood hydrographs for the rivers

To obtain design flood levels along the river system

To determine the flood protection levels at various stretches of rivers

Water level profile of 100yrs return period for long section of River Severn

Water level profile of various ARI (50, 100, 200) at cross section M37


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Simulation of Floodplain (ISIS 2D)-control flooding


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PossibleOptions

Advantages Disadvantages

Gazetting thelow-lying areasas floodplain

corridor andonly controldevelopment ispermissible

Urbanisation is inevitable

Development pressure

Floodforecasting andwarningsystems: toprovidesufficient leadtimes forrelevant

authorities toevacuatepotential floodvictims to safeareas.

Minimise flood damage Proper planning and failure of itcan cause severe damage andflooding

From the possible options with advantages and disadvantages, in order to increase

conveyance capacity approaches for waterfronts, buildings floodwalls in the cities destroy

the citys window on the river. In this manner, it is possible to employ earth bund

1) Floodplain: construction of earth bunds at right bank to act as flood storage or build

storage at upstream for higher ARI when the existing or proposed river channels areinsufficient to cater the peak discharge (at source control)

Construct earth bunds at both banks with slope protection. Proposed earth bund (at both

sides of river back) along the lower reaches of the River Severn extending from M13 to M25

and M35 to M38 to provide 100yrs flood protection for present settlements.

3. The principles, assumption and uncertainties of error

The area is modelled using integrated 1D and 2D hydrodynamic model. By using iterative

procedures, it can be used to solve equations that are impossible to solve by normal

mathematical manipulation. It involves some element of simplification. 1D model is used for

in-bank modelling for predicting the conveyance capacity. Meanwhile, 2D modelling is used

to model the behaviour of flow movement in the floodplain (out-bank). 1D model solves St

Vernant and energy equations. The 2D model used shallow water solver for flows in

floodplain environments. ISIS 2D use the numerical schemes, ADI (Alternating Direction

Implicit), that are specifically developed to tackle for subcritical flows.

Assumptions:

Shear or turbulence and secondary currents are negligible (1D)- Resistance is

constant with time

Neglect turbulence/3D flow affects (2D)-Resistance is constant with time Mannings dependant for frictional resistance to flow (1D and 2D)


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Data quality from farmer A, Farmer B and Willow are acceptable.

Flood hydrograph for different return period have same shape and time to peak with

the 2000 flood hydrograph

Uncertainties

Error in data input

o Data (Flow, channel geometry)

o Coefficient (Roughness, discharge)

Effect of Vegetation, seasonal effects (Summer and winter)-roughness value

Topography data resolution (not capture important information such as river

crossings, height, etc.)

Grid size of flood extent

How could be reduced

Site investigation (verification process) Calibration and verification (Calibrate by comparing model output with observed data)

Land use map (In hydrodynamic modeling, land cover maps are commonly used to

parameterize floodplain roughness by assigning a roughness coefficient to each land

cover type (Chow, 1959).

Use high resolution data

Graph 2.1: Flood hydrograph for different return period

A discussion which considers the best modelling approach or type to use for this site.

Is the model you have used the most appropriate for this area or would yourecommend changes to the modelling approach? You should give convincing


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scientific/engineering reasons for your recommendations and discuss the

implications in term of data requirements.

For this site, the best modelling approach is 3D modelling since the study involved flood

investigation and need to investigate the conveyance capacity (1D) and the behaviour of

flow movement in the floodplain (2D). The model in this study is appropriate. There is nodifferent between the water levels on the floodplain from the 2D model with water levels in

the channel from the 1D model. and but in order to build the storage, the actual volume

need to take into account and if flood forecasting system need to be installed

The figure XX shows user that the time step is too small or too big for the simulation, also it

can implicate that the model is diverging. Poor Model Convergence: the values of model

convergence and iterations are not between tolerant boundaries. The model is not stable.

identify reasons for discrepancies based on what you have learnt about modelling

uncertainties.


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Uncertainty regarding roughness value for floodplain modelling

Design flood hydrographs input into hydraulic modeling in order to evaluate the capacity

and conveyance of the rivers;

Finally, design discharge information for the river systems are used to develop and test

various flood mitigation option.

Channel and floodplain roughness is 0.07 (global roughness value for the entire area rather

than spatially varying roughness)

Flood Mitigation Engineer has to estimate the flood discharge and volume to size a

flood storage pond and to determine channel capacity.

Spatial resolution

Grid size

Decrease time step

Flood protection level should be 100-year ARI


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PMF

May need to wait for a flood to calibrate models correctly

Design Flood Hydrograph will be used the flow in autumn 2000 flood event by scaling the

flow using flow multiplier. In the real case time to peak and flood volume may differ.

It is used to measure the 'characteristics' of a river eg the gradient, the velocity etc. The

flood hydrograph is basically a dual plot of river discharge (line) and rainfall (bars) over

time.A large range of factors control the shape of hydrographs. These include: precipitation

type and intensity, catchment shape, catchment gradient, land use and vegetation (eg

afforestation, urbanisation), soil type, geology etc

Basic Data

Parameters

Boundary Data-upstream and downstream conditions

Calibration and verification data

The digital elevation model (DEM) used in the model is from LiDAR

River and reservoir routing may need inputs from groundwater, irrigation canal diversions (if

ada), public and industrial water supply

Inadequate

Bridge?


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Based on flow data from 1970-2009, Probable Maximum Flood, which is the estimation of

the greatest flood that could occur at Upton-upon Severn is 1200 m3/s. This PMF might be

appropriate if the dam are built on a river upstream or protection of nuclear plant or chemical

works.

Hydrological analysis is concern with

The estimation of magnitude of flood discharge, flood volume, rainfall intensity

The stage discharge relationship were established using the observed flood gauge and

discharge data

The best modelling approach

Type of Data Purpose (Type of Analysis)a) Hydrological Data- Rainfall- Streamflow / Discharge- River stage- Hydrology analysis, IDF curve, hydrological modelling andhydraulic modelling.- Hydrology analysis (hydrograph), rating curve, hydrologicalmodelling and hydraulic modelling.- Hydraulic analysis, rating curve, flood map and hydraulicmodelling.b) Hydraulic Data

- Sediment data- Tidal data- Hydraulic analysis and hydraulic modelling.- Hydraulic analysis and hydraulic modelling.c) River Characteristics- Cross-section and Longitudinal Section- River Geometry- River Structure- Hydraulic analysis, hydraulic modelling and detailed design.- Hydraulic modelling.- Hydraulic analysis, flood map, detailed design and hydraulicmodel.d) Maps & Plans- Topographic maps- Structural plan & drainage layout

- Local development plan- Master development plan- Landuse map (existing & future)- Soil map- Geology map- Contour map- Site survey plan- Cadastral plan/ Sheet- Hydrology analysis, hydraulic analysis and flood map.- Hydraulic analysis and flood map.- Hydraulic analysis, flood map and detailed design.- Flood map, detailed design and hydraulic analysis.- Hydrology analysis, hydraulic analysis, flood map andhydrological modelling.- Hydrology analysis, sedimentation and erosion analysis.- Detailed design.- Flood map and hydraulic modelling, hydrology analysis- Hydraulic analysis, flood map, hydraulic modelling and detailed


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design.- Hydraulic analysis and flood map, detail designe) River Section & Corridor Survey- River section- Corridor survey- Hydraulic analysis and hydraulic modelling.- Hydraulic analysis and flood map.

f) Satellites & Images- Images (Aerial photograph)- Digital Earth Model (DEM) & DigitalTerrain Model (DTM)- Satellite imagery (Earth Mapping)- Flood map.- Hydraulic analysis and flood map.- Flood map.g) Vulnerability Data- Socio-economy data- Physical Data- Flood damage assessment and ERP.- Flood damage assessment.h) Data for Previous Flood Events & RelatedStudies- Flood prone area

- Flood level- Extend of flooding- Flood depth- Flood damage- Flood history- Documented report- Oral/communication/interview- Built-up platform level and flood map.- Hydraulic analysis, flood map and hydraulic modelling.- Hydraulic analysis and flood map.- Hydraulic analysis and flood map.- Hydraulic analysis and flood map.- Hydraulic analysis and flood map.- Hydrology analysis, hydraulic analysis, flood map, flood damageassessment and detailed design.

- Hydrology analysis, hydraulic analysis, flood map, ERP and flooddamage assessment.i) Flood Properties - Flood risk and detailed design.j) Soil Investigation - Detailed design.k) Catchments Characteristics - Hydrology analysis, hydraulic analysis, hydrological modelling andhydraulic modelling.l) Data for Operation and Maintenance - As Built Plan- Standard Operation and Maintenance Procedure

An explanation of how the flood risk of the area is likely to change with time, and

make recommendations for any actions the local authorities (EA, local council, water

company, fire service, etc) can make.

A stream periodically changes by flowing over its banks.

Climate change may alter the flow characteristics. Pgv26 floodplain mgt


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References

Natural Resources Conservation Service, 2001. Stream corridor restoration.

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/ndcsmc/?cid=nrcs143_009158

Documents

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