The River Irwell Case Study

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The River Irwell is made up of 13 catchments in the North West region that drain naturally into the single river system (EA, 2008a). The catchment covers an estimated 700 square kilometres with over two million people within its vicinity (ref). Two of the thirteen catchments are responsible for the contributing effect of river flooding in Radcliffe along the River Irwell. The little town of Radcliffe located south-west of Bury is made up of three wards; Radcliffe North, East and West. It derived the name Red-cliff as a bank on the river Irwell filled with stones and it is famous for its many medieval buildings. Its resident population is an estimated 33,149 which represents about 18% of the Bury population as at 2007 mid-year population estimates. The study area in focus is a 10.06km section of the river Irwell starting from the Bury grounds catchment and flowing down stream towards Radcliffe West, with an inflow from the river Roch catchment at the Blackford Bridge. The figure 3.2 below is the FEH CD-ROM geographical interface developed by the Centre for Ecology and Hydrology (CEH) showing the Bury ground and the Blackford Bridge catchment descriptors; this Flood Estimation Handbook (FEH) provides guidance on rainfall and river flood frequency estimation throughout the UK with a latest version printed in 2008.

Figure 3.1: Map showing the Radcliffe area in Bury

Source; Bury Metropolitan Council (2007)

Figure 3.2: Screenshot from the FEH CD-ROM 3 showing the catchment descriptors for the River Irwell at Bury,

Source: Wallingford Hydrosolutions Ltd (2009)

3.2 Data Available

For the 10.06 km reach area of study along the river Irwell, geo-referenced cross-sections closest to the catchment descriptors were made available by the EA (2010). The data was however not complete as, there was still a large portions of the river unmarked by the surveyed cross sections.

A LiDAR 10 meter resolution DTM with (cell size X,Y) and a pixel depth of 32 bit was downloaded from the LANDMAP Kaia website in an ASCII format (recognisable by the software, such as ArcGIS and MapInfo), with coordinates matching the study area with reference to the British National Grid.

A high scale Ordinance Survey (OS) raster map with a scale of 1:10 000 was made available by download form the Ordinance Survey website. Physical features of this large scale up-to-date map includes; buildings, rivers, road or rail and other infrastructure around the environs of Radcliffe, Bury.

3.3 The Model Used

The evaluation of this study is undertaken by using the available data to support modelling of the Major flood event which occurred in January-February 1995 and 2008 along the river Irwell and the modelling package in use, is the ISIS 3.3v a software package developed by Halcrow Group used for river modelling purposes to showcase flood forecasting, flood risk mapping, as well as assessments. However, the ISIS 3.3v produced late in 2009 had some upgrades form the former, as the new interface features included ISIS Mapper which enables the creation of hydraulic models built from a network of existing geo-referenced data sets for better analysis of results and visualising of 1D or 2D modelling outputs.

The ISIS model makes use of one-dimension numerical solution for producing steady and un-steady simulation of flood propagation results along the modelled river channel. The river in this case is represented using surveyed cross-sections which are then linked to floodplain storage cells (reservoirs) with the use of spill units which compute the exchange of water between the river flow and the reservoirs.

In the ISIS Mapper interface, all inputted model units were built up using GIS processing to extract data from the digital terrain model (DTM) provided for this study so as to match the model with existing data relating to the geographical surface area, as "DTM are deduced mainly from the observation of the terrain surfaces which represent the bare earth at some level of detail" (Karel et. al, 2006).

Figure 3.3: digitised River Irwell at the study area together with reservoirs and spill units, in MapInfo

3.4 Model Building

First, the study section of the river Irwell (10.06 km long) was digitised in MapInfo, together with the spill units and reservoirs which were digitised along side the river, mainly by observation of existing reservoirs on the Survey Map provided, after which the outcome was imported into the ISIS mapper for analysing. This is to support the concept of building a hydraulic model, were the various data used relates to the physical data derived from the O.S map (Wicks et al, 2004). However, in the main ISIS interface node labels are being assembled to suit the model, as these included the model extents starting with the a flow-time boundary (QTBDY) IRWE05_2618 denoting the average flow of the river as 15m3/s and a time interval of 0 hrs - 27 hrs for runtime and connected by a junction to an upstream inflow boundary (REFHBDY) IRWE05_2618f which contains gauged information of Bury Ground catchment descriptor, after which a series of the surveyed cross-sections were added as node points to the structure. The table () below shows the cross-sections data provided by the EA (2010) containing information on 3 different reach lengths that form the river Irwell study area.

UNIT LABEL

 

IRWE05_2618

River Section

IRWE05_2162

River Section

IRWE05_2081

River Section

IRWE05_2043

River Section

IRWE05_1960

River Section

IRWE04_3031

River Section

IRWE04_2875

River Section

IRWE04_2688

River Section

IRWE04_2536

River Section

IRWE04_2192

River Section

IRWE04_1979

River Section

IRWE04_1792

River Section

IRWE04_1698

River Section

IRWE04_1621

River Section

IRWE04_1503

River Section

IRWE04_1457

River Section

IRWE04_1155

River Section

IRWE04_0926

River Section

IRWE04_0753

River Section

IRWE04_0559

River Section

IRWE04_0436

River Section

IRWE04_0312

River Section

IRWE04_0232

River Section

IRWE04_0146

River Section

IRWE04_0001

River Section

IRWE03_6022

River Section

IRWE03_5862

River Section

Table 3.1: showing River Sections

Along the study river, the first series of cross-section data (IRWE05_2618 - IRWE05_1960) had a long gap of () km before the next river section data IRWE04_3031. However, this prompted the need to fill the gaps with cross-section information using a tool within the ISIS Mapper environment, there by importing the created nodal information in the ISIS environment to the ISIS Mapper environment for better visual analysis. The downloaded 10m DTM was uploaded into ISIS Mapper, together with the digitised river shapefile and were both matched together so the river information (depth level) was picked form the contour of the DTM after which the insert cross-section tool was used to add cross-sections in the river gaps. The specific distance between the new set cross-sections added were specified in the tool box and this covered all areas of the 10.06 km river length which was digitised in MapInfo for the purpose of the study. The new river Sections added to the study river are shown in table ().

River Label

 

IM_0092

River Section

IM_0087

River Section

IM_0083

River Section

IM_0078

River Section

IM_0077

River Section

IM_0074

River Section

IM_0071

River Section

River Label

 

IM_0068

River Section

IM_0094

River Section

IM_0036

River Section

IM_0040

River Section

IM_0043

River Section

IM_0044

River Section

IM_0046

River Section

Table 3.2: showing newly inserted River Sections

After the new set of river sections were added, it was observed that the gaps between each of the new cross-sections were still greater than that of the provided river sections. This however, was addressed by providing interpolates to link the gaps between the various cross-sections which was done with the use of an interpolator tool produced by Halcrow Group. This ISIS Interpolator version 3.10 was used to identify the whole length of the Study River and then set the maximum distance to 150 m between interpolates and to the next river section. This produced the following results seen in table ().

Unit Label

 

IRWE_2618_1

Interpolates

IM_0092_1

Interpolates

IRWE_1960_1

Interpolates

IRWE_1960_2

Interpolates

IRWE_1960_3

Interpolates

IM_0087_1

Interpolates

IM_0087_2

Interpolates

IM_0087_3

Interpolates

IM_0087_4

Interpolates

IM_0083_1

Interpolates

IM_0083_2

Interpolates

IM_0083_3

Interpolates

IM_0083_4

Interpolates

IM_0078_1

Interpolates

IM_0077_1

Interpolates

IM_0077_2

Interpolates

IM_0077_3

Interpolates

IM_0074_1

Interpolates

IM_0074_2

Interpolates

IM_0074_3

Interpolates

IM_0071_1

Interpolates

IM_0071_2

Interpolates

IM_0071_3

Interpolates

IM_0068_1

Interpolates

IM_0068_2

Interpolates

IRWE_3031_1

Interpolates

IRWE_2875_1

Interpolates

IRWE_2688_1

Interpolates

IRWE_2536_1

Interpolates

IM_0094_1

Interpolates

IRWE_2192_1

Interpolates

IRWE_1979_1

Interpolates

IRWE_1457_1

Interpolates

IRWE_1457_2

Interpolates

IRWE_1155_1

Interpolates

IRWE_0926_1

Interpolates

IRWE_0753_1

Interpolates

IRWE_6022_1

Interpolates

IRWE_5862_1

Interpolates

IM_0036_1

Interpolates

IM_0036_2

Interpolates

IM_0036_3

Interpolates

IM_0040_1

Interpolates

IM_0040_2

Interpolates

IM_0040_3

Interpolates

IM_0043_1

Interpolates

IM_0044_1

Interpolates

IM_0044_2

Interpolates

IM_0044_3

Interpolates

Table 3.3: showing cross-section interpolates

3.4.1 Reservoirs and Spill units

These are created to represent the floodplains or channel interaction used in areas where the floodplain slopes upwards away from the river channel without any form of embankments. These reservoir units, just like the river shape file are matched against the DTM to extract Elevation and Area relationships for each of the reservoir units which are connected the river channel by spill units, which in this case are used to allow water spill over into the reservoir (floodplain) once it is out of banks. This gives a graphical presentation the flow route between the river channel and the reservoirs, as the creation of each reservoir was with respect to what was observed on ground through the 1:10 000 OS street map and Google Earth with the aim of eliminating areas with existing flood defences.

Unit Label

 

IM_0083LD

Reservoir

IM_0071_3LS

Spill

IM_0071_2LS

Spill

IM_0071_1LS

Spill

IM_0071LS

Spill

IM_0074_3LS

Spill

IM_0074_2LS

Spill

IM_0074_1LS

Spill

IM_0074LS

Spill

IM_0077_3LS

Spill

IM_0077_2LS

Spill

IM_0077_1LS

Spill

IM_0077LS

Spill

IM_0078_1LS

Spill

IM_0078LS

Spill

IM_0083_4LS

Spill

IM_0083_3LS

Spill

IM_0083_2LS

Spill

IM_0083_1LS

Spill

IM_0083LS

Spill

IM_0077RD

Reservoir

IR2688_1RS

Spill

IM_0071RS

Spill

IM_0074_3RS

Spill

IM_0074_2RS

Spill

IM_0074_1RS

Spill

IM_0074RS

Spill

IM_0077_3RS

Spill

IM_0077_2RS

Spill

IM_0077_1RS

Spill

IM_0077RS

Spill

IM_0071_1RD

Reservoir

IR04_2688RS

Spill

IR2875_1RS

Spill

IR04_2875RS

Spill

IR3031_1RS

Spill

IR04_3031RS

Spill

IM_0068_2RS

Spill

IM_0068_1RS

Spill

IM_0068RS

Spill

IM_0071_3RS

Spill

IM_0071_2RS

Spill

IM_0071_1RS

Spill

IR04_3031LD

Reservoir

IR1979_1LS

Spill

IR04_1979LS

Spill

IR2192_1LS

Spill

IR04_2192LS

Spill

IM_0094_1LS

Spill

IM_0094LS

Spill

IR2536_1LS

Spill

IR04_2536LS

Spill

IR2688_1LS

Spill

IR04_2688LS

Spill

IR2875_1LS

Spill

IR04_2875LS

Spill

IR04_3031LS

Spill

IR3031_1LS

Spill

IM_0094RD

Reservoir

IR04_0232RS

Spill

IR04_0312RS

Spill

IR04_0436RS

Spill

IR04_0559RS

Spill

IR0753_1RS

Spill

IR04_0753RS

Spill

IR0926_1RS

Spill

IR04_0926RS

Spill

IR1155_1RS

Spill

IR04_1155RS

Spill

IR1457_2RS

Spill

IR1457_1RS

Spill

IR04_1457RS

Spill

IR04_1503RS

Spill

IR04_1621RS

Spill

IR04_1698RS

Spill

IR04_1792RS

Spill

IR1979_1RS

Spill

IR04_1979RS

Spill

IR2192_1RS

Spill

IR04_2192RS

Spill

IM_0094_1RS

Spill

IM_0094RS

Spill

IR04_0559LD

Reservoir

IR6022_1LS

Spill

IR03_6022LS

Spill

IR04_0001LS

Spill

IR04_0146LS

Spill

IR04_0232LS

Spill

IR04_0312LS

Spill

IR04_0436LS

Spill

IR04_0559LS

Spill

IR03_5862LD

Reservoir

IM_0036_3LS

Spill

IM_0036_2LS

Spill

IM_0036_1LS

Spill

IM_0036LS

Spill

IR5862_1LS

Spill

IR03_5862LS

Spill

IM_0036RD

Reservoir

IM_0040_3RS

Spill

IM_0040_2RS

Spill

IM_0040_1RS

Spill

IM_0040RS

Spill

IM_0036_3RS

Spill

IM_0036_2RS

Spill

IM_0036_1RS

Spill

IM_0036RS

Spill

Table 3.4: showing entire Reservoir and Spill units

With the assembling of all the river sections, spill units and reservoirs in alignment with the river line in ISIS Mapper, all the information gathered with statistical analysis derived from the DTM was then saved as an ied.file and imported into the ISIS environment for the purpose of populating the node table to ensure the successful run of the model. The nodal inputs begin wit a flow-time boundary (QTBDY ) IRWE05_2618 of 15 m3/s connected to a catchment descriptor (REFHBDY ) IRWE05_2618f from the Bury ground gauge station and joined to a downstream river-section IRWE05_2618d with the use of a junction (), to indicate the start-up of the study river flowing downward stream and a second catchment descriptor (REFHBDY ) IRWE04_1698f signifying an in-flow from the Blackford bridge gauge station was connected to the river section IRWE04_1698d with a junction () and still flowing downstream of the river. 'The nodal points in the ISIS environment are usually connected by junction nodes (indicating all connecting nodes) or by having similar node labels'. The following river sections (both the sections provided by the EA together with the newly created river sections) together with all the Interpolates, Reservoirs and Spill units were all created with node label points to suit their respective functions in the ISIS interface. The following were represented in the ISIS environment as below;

UNIT LABEL

UNIT DIAGRAM

River Section

 

Interpolate

 

Reservoir

 

Spill

Table 3.5: showing unit labels that make up the model

From the information on bridges and weirs provided by the EA (2010) and according to visual observation from the OS street view map, as well as Google earth, the following information on each existing structure was gathered so as to fit the purpose of the model build up which was done in the ISIS environment so as to link the affected river sections to the available structures.

3.4.2 Bridges

These bridge structures are being modelled as bridge units which is represented by a bridge arch unit node label () to represent the loss of water across the structure with the input of spill units in parallel also to present the flow of water over and around the structure once it becomes surcharged. The EA provided surveyed information on two (2) bridges while the third (3rd) bridge information was worked out (as it was identified as an existing feature through Google Earth) using information from the river-sections and duplicating the height of a similar bridge with already provided information and there were three (3) bridge units created.

UNIT LABEL

 

IRW05_2081bu

Bridge

IRW05_2081su

Spill

IRW03_5862bu

Bridge

IRW03_5862su

Spill

IM_0036bu

Bridge

IM_0036su

Spill

Table 3.6: showing Bridge unit and Spills

3.4.3 Weirs

These are provided for various fundamental reasons and in most cases for channel stabilisation (Rickard et al, 2003). The weir located along the river channel in this study, is represented by a round-nosed broad crested weir () from the label nodes. This represents the extent and magnitude of the actual weir along the study area. Information on the Weir was not provided therefore, it was identified with the help of Google earth. The height and width of the weir was determined through the help of Arc tools in the ArcGIS environment. The unit is displayed as IM_0077wu.

After all the node units were inserted into the ISIS environment in the order of the stream flow (from up-stream to down-stream), the last river section ended with a normal/critical depth boundary node () having similar labels (IM_0046) with the cross-section to mark the end of the run model. This enabled the successful run of an unsteady simulation in with an 'adaptive timestep' strategy of initial timestep of 5s and a save interval of 300s (seconds) running from a start time of 0hrs to a finish time of 27hrs (hours). Whilst the run is in progress, ISIS produces graphics which follow the simulation and logs any error or warnings into a diagnostic file.

Each of the in-flow catchment descriptors (REFHBDY) is set to a 5yr (rear) flood return period to produce output results in hydrographs for analysing and the end run of the model produces an output 'Animated Simulation' to show the extent of the river spill in a flood event occurrence and also output hydrographs can be used to analyse the flow of the river after the model is run.

The geographical location of the model nodes which is integrated within the model produces a schematic diagram. This schematic produces is uniform scale georeferenced map, which enables visual graphic viewing according to location of cross-sections and also facilitates future updating of the model. This enable the model outputs to directly link into spatial tools for analysis of results. This is viewed in the ISIS environment by selecting the GIS visualiser ().

Figure 3.4: shows the produced schematic diagram of the study area along with various parameters used in setting up the model.

3.5 Data Limitations

Due to the unavailability of most actual surveyed cross-sections, together with lack of specific data on the bridges and weirs which could not be taken into consideration during the model building posed a slight limitation to the scope of the study. The low resolution DTM also had some limitations as a much lower resolution (i.e. 2.5m or 1m) would have produced a better analysis of the model creation. However, the model's predictive ability was not affected in anyway and it produced accurate and similar information when compared to the output results produced by the EA.

3.6 Model testing

1:5yr design flow Vs. January, 2008 simulated flow.

Figure 3.5: Hydrographs of Observed vs. Modelled/Simulated flow for the Blackford bridge catchment

There were high peak flows recorded for Bury ground and Blackford bridge catchment in the month of January 1995 and 2008 respectively (EA, 2010). The 15 minutes flow data was provided by the EA and used to compare the peak flow for a 1:5 year return period in the design model. The output results were transported to Microsoft Excel spreadsheet and a graph produced for analyses. The observed river flow at the Blackford Bridge for January, 2008 was maintained at 119m3/s (as recorded) and the 1:5 year design flow produced 126m3/s. This statistical approach combines index flood with growth curve from observed data. Thus, the increase observed form the design flow depicts what the model is designed to propagate.

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