fluvial hydraulic modelling report 1. basic model information · hydraulic model summary report –...

Hydraulic Model Summary Report – Ellington Brook 1

Fluvial Hydraulic Modelling Report

1. Basic Model Information

Model Name: Ellington Brook

Primary Watercourses / Water Bodies Ellington Brook/Alconbury Brook

Designation Main river

Model ID Scenario Flood Event

Alconbury_and_Ellington_JE2014.dat

Alc_Eli_0025yr_23hr_28_JE2014.ief

Alc_Eli_0025yr_23hr_28_JE2014.tcf

Existing Model 4% AEP (1 in 25 years)




1% AEP (1 in 100 years)


Alc_Eli_0100yrCC_23hr_28_JE2014.ief

Alc_Eli_0100yrCC_23hr_28_JE2014.tcf

1% AEP +CC (1 in 100 years + 20%)

Alconbury_and_Ellington_JE2014_Design.dat

Alc_Eli_0025yr_23hr_28_JE2014_Design.ief

Alc_Eli_0025yr_23hr_28_JE2014_Design.tcf

Scheme Model 4% AEP (1 in 25 years)


Alc_Eli_0100yr_23hr_28_JE2014_Design.ief

Alc_Eli_0100yr_23hr_28_JE2014_Design _v3.tcf

1% AEP (1 in 100 years)


Alc_Eli_0100CCyr_23hr_28_JE2014_Design.ief

Alc_Eli_0100CCyr_23hr_28_JE2014_Design.tcf

1% AEP +CC (1 in 100 years + 20%)

2. Survey Data and Base Mapping

2.1 Base Mapping: 1 to 10,000 Scale Raster

Reference:

TL1070, TL2070

2.2 DTM for 2D Model domain:

As per Environment Agency 2014 Alconbury Brook model

2.3 River channel/Structures survey

As per Environment Agency 2014 Alconbury Brook model


3. Baseline Hydraulic Model Schematisation

3.1 Software: 1D domain: Isis Version 3.7.0.233 (64 bit - Single Precision)

2D domain(s): 2012-05-AE-iSP-w64

3.2 Baseline model: Environment Agency 2014

3.3 Baseline Model Reference

Alconbury_and_Ellington_JE2014.DAT



Alc_Eli_0100yrCC_23hr_28_JE2014.ief




3.4 Model area / extent: The Ellington Brook model covers the northwest extent of the Great Ouse model study area and includes Ellington Brook and Alconbury Brook watercourses, running along the A14 and A1 respectively. The 1D model includes the Ellington Brook for approximately 10 km and Alconbury Brook for 17 km. The 2D model boundary follows the shape of the two watercourses and includes the area of Huntingdon downstream of the 1D model boundary.

See Figures 1, 2 and 3 below for study area and model extents.


Figure 1: Areal Extent of Ellington Brook


Figure 2 Model Schematisation

Alconbury Brook

Ellington Brook


Figure 3: Model schematisation in the proximity of the proposed scheme

Alconbury Brook

Ellington Brook

Alconbury Brook Realignment


3.5 Model reaches: The following model reaches as shown on the maps above have been defined in the model:

Watercourse name Upstream model node Downstream model node

Ellington Brook E12.80 E0.00

Alconbury Brook Al_2050 Al_0000

Total model length (km): 10km (Ellington Brook), 17km (Alconbury Brook)

3.6 Scheme Structures: There are no structures in the baseline model which are affected by the proposed A14 scheme.

3.7 Floodplain schematisation

Floodplain areas have been modelled in 2D using a linked Isis – Tuflow schematisation.

3.8 Model Boundaries - Inflows

ReFH units ‘Al_2050’ and’E12.80’ are input into the model as point inflows at the upstream end of Alconbury Brook and Ellington Brook respectively. Re-FH units for downstream sub-catchments ‘13.2000hyd’ and ‘al_0481hyd’ on Alconbury Brook and ‘E5.00hyd’ on Ellington Brook are entered as distributed lateral type.

Peak runoff (m3/s) for each unit is provided in the table below.

Input Node in the Hydraulic Model

Re-FH peak runoff (m3/s)

4%AEP 1% AEP 1%AEP + CC

Al_2050 13.329 16.155 19.269

E12.80 18.121 21.902 26.123

13.2000hyd 28.807 33.904 40.429

al_0481hyd 4.515 5.175 6.210

E5.00hyd 13.097 15.783 18.831

3.9 Model Boundaries – Downstream Conditions

The outflow at the downstream end of the 1D model extent is modelled using a normal depth boundary applied at node ‘al_0000’. The boundary is significantly remote from the area of interest for the proposed scheme, and therefore is not considered to influence the scheme scenario results.


4. Model Build – Scheme Model

4.1 Scenario Definition

The proposed scheme involves the widening of the A1 adjacent to Alconbury Brook and Ellington Brook, as well as the addition of a link road between the A14 and A1 highways.

The proposed link road crosses the Ellington Brook watercourse and also spans over the A14, requiring a bridge crossing to be constructed. This road and surrounding embankments represent a significant increase in ground levels in this area where the road slopes down for 450 metres from the A14 flyover to tie into the level of the A1.

The widening of the A1 requires the minor realignment of a meander in the Alconbury watercourse. This is considered to have insignificant impact on the existing flood risk and therefore was not represented in the scheme model. In addition the channel realignment and the increase in A1 embankment footprint are too small to be captured effectively by the model grid.

The scheme components are shown in the figure below.

4.2 Model Extent of the Affected area

Figure 4 below shows the Preliminary Design model schematisation.

Figure 4: Proposed road layout and model schematisation


ISIS 1D Model

4.3 Modelling approach for the new structures

A Bridge (USBPR1978) unit was added to the ISIS model to represent the road bridge across Ellington Brook (ISIS unit E1.10u). The opening span and soffit level have been chosen based on design specifications and preliminary run results.

A soffit of 14mAOD was selected to allow 600mm of freeboard above the baseline 1%AEP + CC results. The parapets of the bridge were specified to be set back 2m from the banktops on either side, which resulted in the bridge opening spanning15m.

4.4 Model Units added/removed

Two copies of ISIS cross section E1.0 were created and named E1.1u and E1.1d. These cross section units were placed at the proposed Ellington Brook bridge crossing location, between E1.2 upstream and E1.0 downstream. A Bridge (USBPR1978) unit was placed between E1.1u and E1.1d.

4.5 Culvert Inverts N/A

4.6 Hydraulic Roughness of proposed units

The reach Manning’s roughness coefficient of n = 0.04 has been maintained for the added sections.

Tuflow 2D Model

4.7 Modelling approach for the new structures

The scheme is modelled in Tuflow using ‘z-lines (thick)’ to represent the road and ‘z-shapes’ to represent the embankments surrounding the road.

4.8 Design Detail Sources

Scheme location, extent, and design road levels were taken from:

‘design model 150814.dwg’


5. Hydraulic Model Outputs

5.1 Model Simulation Water level outputs have been extracted for the existing and scheme model.

5.2 1D Water Levels - Ellington Brook A1 link Road

Existing Model Maximum Stage values for the Existing Model are provided in the table below at key locations.

Location Model node

Peak Water level (mAOD)

4%AEP 1% AEP 1% AEP + CC

1.05km u/s of proposed link road

E2.2 14.175 14.182 14.187

454m u/s of proposed link road

E1.6 13.549 13.556 13.573

Immediately upstream of link road

E1.2 13.130 13.178 13.221

Immediately downstream of link road

E1.0 12.955 13.057 13.120

566m d/s of proposed link road

E0.58 12.620 12.738 12.815

Preliminary Design Maximum Stage values for the Design Model are provided in the table below at key locations. The change in water level between the Existing Model and the Preliminary Design Model are also included.

Location Model node

Peak water level (mAOD) and water level change (m)

4% AEP 1% AEP 1% AEP + CC

(mAOD) (m) (mAOD) (m) (mAOD) (m)

1.05km u/s of proposed link road

E2.2 14.175 0.000 14.181 -0.001 14.187 0.000

454m u/s of proposed link road

E1.6 13.562 0.013 13.574 0.019 13.604 0.031

Immediately upstream of link road

E1.2 13.157 0.027 13.216 0.038 13.281 0.060

Immediately downstream of link road

E1.0 12.955 0.000 13.064 0.007 13.133 0.013

566m d/s of proposed link road

E0.58 12.620 0.000 12.741 0.002 12.818 0.003

Long Section

Figures 5, 6 and 7 present a comparison between maximum stage for the Existing and Scheme Models for the 4%,1% and 1% + CC AEP.


5.3 1D Water Levels - Ellington Brook A1 culvert


Location Model node



~530m upstream of A1,

u/s of proposed Ellington

Brook Bridge

E1.10u 13.016 13.090 13.142

Upstream of A1 culvert E0.58 12.620 12.738 12.814

Downstream of A1

culvert E0.48 12.556 12.657 12.705

176m downstream of A1 E0.40 12.476 12.585 12.635


Location Model node




~530m upstream of A1,

u/s of proposed Ellington

Brook Bridge

E1.10u 13.02 0.004 13.098 0.008 13.154 0.012

Upstream of A1 culvert E0.58 12.620 0.000 12.740 0.002 12.817 0.003

Downstream of A1

culvert E0.48 12.556 0.000 12.658 0.001 12.706 0.001

176m downstream of A1 E0.40 12.476 0.000 12.586 0.001 12.636 0.001


5.4 1D Water Levels - Alconbury Brook at channel re-alignment location


Location Model node



Alconbury Brook at

channel re-alignment

location

al_0760ds 11.312 11.288 11.252

al_0720 10.973 10.989 10.915

al_0680 10.587 10.649 10.550


Location Model node




Alconbury Brook at

channel re-alignment

location

al_0760ds 11.312 0.000 11.288 0.000 11.252 0.000

al_0720 10.973 0.000 10.989 0.000 10.915 0.000

al_0680 10.587 0.000 10.649 0.000 10.550 0.000


Figure 5: Maximum Stage for Ellington Brook in ISIS 1D model – 4% AEP


Figure 6: Maximum Stage for Ellington Brook in ISIS 1D model – 1% AEP


Figure 7: Maximum Stage for Ellington Brook in ISIS 1D model – 1% AEP + Climate Change


5.5 2D Water Levels

The area surrounding the proposed road link has been interrogated in order to compare the effects of the proposed scheme. The location of the points where water levels have been extracted is shown in Figure 8.

Figure 8: Location of the Interrogation Points

For each point water levels have been extracted for the 4%AEP, 1%AEP and 1%AEP + CC flood events. Water levels extracted for all flood events are presented in the following tables:


4%AEP Flood Event

Point ID Baseline Water Level (mAOD)

Design Water Level (mAOD)

Change in Water Level (m)

1 13.258 13.259 0.001

2 13.249 13.250 0.001

3 12.964 13.270 0.306

4 13.060 13.086 0.026

5 12.947 12.981 0.034

6 12.909 12.910 0.001

7 12.963 12.960 -0.003

1%AEP Flood Event




1 13.302 13.304 0.002

2 13.287 13.288 0.001

3 13.071 13.306 0.236

4 13.091 13.144 0.053

5 13.032 13.052 0.020

6 13.010 13.008 -0.002

7 13.069 13.073 0.004

1%AEP + CC Flood Event




1 13.334 13.338 0.004

2 13.312 13.315 0.003

3 13.133 13.357 0.224

4 13.143 13.198 0.055

5 13.105 13.118 0.013

6 13.081 13.082 0.002

7 13.130 13.140 0.010

Flood Extents The flood extents for the Existing Model and the Scheme Model for each flood event are presented in Figures 9-11.


Figure 9: Flood extent for the Existing Model (blue) and the Scheme model (green) for the 4%AEP flood event.


Figure 10: Flood extent for the Existing Model (blue) and the Scheme model (green) for the 1%AEP flood event.


Figure 11: Flood extent for the Existing Model (blue) and the Scheme model (green) for the 1%AEP +CC flood event.


Effect of proposed Structures

The modelled scenario with the proposed scheme shows minor increases of in-channel water levels immediately upstream of the proposed scheme, in the order of 50mm for the 1%AEP event. There is little to no change in water level seen from approximately 800m upstream of the structure or for in channel water levels downstream of the structure (see Section 13.2).

The 2D scheme model results also show a general increase of water levels for the area surrounding the proposed road location. Little to no impact is observed on water levels beyond about 150 metres from the proposed road, and no change is present throughout most of the modelled area. The flood extent changes slightly upstream of the proposed structure, with a small number of additional cells wetted in the “with scheme” scenario.

A significant level change due to the scheme is observed on the right bank immediately upstream of the link road crossing. Here a small area (1000m

2)

adjacent to the A14 and the link road embankments sees significant interruption to the existing flood flow path. This interruption causes an increase in maximum level of approximately 240mm for the 1%AEP event (see inspection point 3 in Section 5.3).

The scheme does not cause any effect on water levels at the A1 crossing of Ellington Brook because the existing culvert extent is significantly greater than the full width of the proposed highway widening. Therefore no modification to the culvert will be required, and the design and existing simulation results in this location are seen to be almost identical

On Alconbury Brook where the watercourse runs next to the A1 highway widening, as stated previously in section 4.1, the extent of the topographic modification is too small to be represented in the hydraulic model. As such Design and existing simulation results in this location are identical.

6. Key model assumption and limitations

The supplied hydraulic models were assumed to be fit for purpose and no detailed review was undertaken for the purpose of Flood Risk Assessment of the A14 scheme.

A bridge unit of type USBPR1978 has been used assuming this is fit for purpose for the type of proposed bridge in the specifications (RC with abutment and wing walls). This bridge unit was accommodated using the cross section E1.2 and using the clearance level and length of the deck provided by the design team.

No calibration or sensitivity testing simulations were carried out as part of this hydraulic modelling exercise.

fluvial hydraulic modelling report 1. basic model information · hydraulic model summary report –...

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