hydrology - flooding in ribchester

7
Catchment Hydrology Practical 3 Report Cobain Schofield 1 | Page Assessing the impact of flooding on buildings and agricultural land in Ribchester, Lancashire Assessing the impacts of flooding on existing buildings and agricultural land There are currently a total of 1080 buildings and 1614 plots of land within the 7.5km 2 study area of the Ribble Valley, Lancashire. Of the 1614 plots of land there are 31 which are deemed to be arable, which take up 0.97km 2 of the 7.5km 2 study area. Flood data for the Ribble River was superimposed onto the study area to assess the impacts of floods on existing buildings and agricultural land. ArcGIS was used to calculate the number buildings and agricultural plots that would be impacted by floods of varying intensity. Figure 1 shows how floods with different rates of flow will impact on the area. Figure 1 A graph showing how floods of different flow rates will impact buildings and land within the study area. Raw data from this graph is provided in Appendix 1. After a flood with a flow rate of 200 cumecs there will be 53 buildings and 267 plots of land affected by the flood. The plots of land affected by the flood cover 3.1km 2 of the study area, but only 42% (1.3km 2 ) of this land is actually underwater during a 200 cumec flood event (Appendix 1). The map in Figure 2 shows the extent of flooding in this area. The affected buildings make up less than 5% of those within the study area, which suggests that the impacts of flooding on buildings in this area are not great. However, of the land affected by flooding there are 8 arable plots with a combined area of 0.42km 2 . This means that almost half of all arable land in the study area is going to be affected to some degree by flooding during a 200 cumec flood. It is therefore reasonable to assume that impacts will be greater to farmers & land owners than to building owners, particularly if the land is being used for growing crops or grazing. It is also beyond reasonable doubt to assume that a proportion of the flooded buildings will belong to the same owners as the flooded land, given that there are 6 flooded properties within the flooded agricultural land plots alone, thus increasing the impact for individual land owners. 0 10 20 30 40 50 60 0 100 200 300 400 500 600 0 50 100 150 200 Cumulative Number of Flooded Buildings Percentage Change of Water Area (relative to base flow) & Cumulative Number of Flooded Fields Rate of Flow (cumecs) Flooded fields Percentage Change of Water Area Flooded Houses

Upload: cobain-schofield

Post on 12-Feb-2017

338 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

1 | Page

Assessing the impact of flooding on buildings and agricultural land in Ribchester, Lancashire

Assessing the impacts of flooding on existing buildings and agricultural land

There are currently a total of 1080 buildings and 1614 plots of land within the 7.5km2 study area of the Ribble Valley, Lancashire. Of the 1614 plots of land there are 31 which are deemed to be arable, which take up 0.97km2 of the 7.5km2 study area.

Flood data for the Ribble River was superimposed onto the study area to assess the impacts of floods on existing buildings and agricultural land. ArcGIS was used to calculate the number buildings and agricultural plots that would be impacted by floods of varying intensity. Figure 1 shows how floods with different rates of flow will impact on the area.

Figure 1 – A graph showing how floods of different flow rates will impact buildings and land within the study area. Raw data from this graph is provided in Appendix 1.

After a flood with a flow rate of 200 cumecs there will be 53 buildings and 267 plots of land affected by the flood. The plots of land affected by the flood cover 3.1km2 of the study area, but only 42% (1.3km2) of this land is actually underwater during a 200 cumec flood event (Appendix 1). The map in Figure 2 shows the extent of flooding in this area.

The affected buildings make up less than 5% of those within the study area, which suggests that the impacts of flooding on buildings in this area are not great. However, of the land affected by flooding there are 8 arable plots with a combined area of 0.42km2. This means that almost half of all arable land in the study area is going to be affected to some degree by flooding during a 200 cumec flood. It is therefore reasonable to assume that impacts will be greater to farmers & land owners than to building owners, particularly if the land is being used for growing crops or grazing. It is also beyond reasonable doubt to assume that a proportion of the flooded buildings will belong to the same owners as the flooded land, given that there are 6 flooded properties within the flooded agricultural land plots alone, thus increasing the impact for individual land owners.

0

10

20

30

40

50

60

0

100

200

300

400

500

600

0 50 100 150 200

Cu

mu

lati

ve N

um

ber

of

Flo

od

ed

Bu

ild

ing

s

Perc

en

tag

e C

han

ge o

f W

ate

r A

rea

(rela

tiv

e t

o b

ase f

low

) &

Cu

mu

lati

ve N

um

ber

of

Flo

od

ed

Fie

lds

Rate of Flow (cumecs)

Flooded fields

Percentage Change of Water Area

Flooded Houses

Page 2: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

2 | Page

Figure 2 – A map showing the extent of flooding on the study area. Flooded land including flooded arable pasture is highlighted in red and orange respectively, and affected buildings are coloured yellow.

Impacts of flooding on proposed housing development

Is the proposed location suitable?

The proposed housing development consisting of 34 properties is to be located just east of Ribchester. The development plans have been imported into ArcGIS on their proposed location in order to assess the suitability of their location. The Map in Figure 3 shows the development.

Figure 3 – Proposed housing development location (Location A on Figure 2)

Figure 3 shows that in this particular location at least 17 of the 34 plots will experience some degree of flooding at 200 cumecs. The development also spans 2 fields, of which 1 is arable. The proposals mean that 1/3 of the arable field would be lost to the development.

It is therefore possible to conclude that the proposed location is not suitable for this development.

Identifying alternative development sites

In order to identify new potential development sites parameters must be set so as to exclude unsuitable locations.

Legend

Flooded Building

Ribble Flooding

10 cumecs

60 cumecs

100 cumecs

160 cumecs

200 cumecs

Arable Pasture Flooded

Non-flooded Fields (at 200cumecs)

Flooded Field (at upto 200cumecs)

Road

Building

±

0 0.3 0.6 0.9 1.20.15Kilometers

Legend

Building

Road

Proposed Plot

Ribble Flooding

10 cumecs

60 cumecs

100 cumecs

160 cumecs

200 cumecs

Arable Pasture Flooded

Non-flooded Fields (at 200cumecs)

Flooded Field (at upto 200cumecs)

±0 100 200 300 40050

Meters

A

B

C D

Page 3: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

3 | Page

For the purpose of this report, the following parameters have been chosen based on information set out in the project specification.

Table 1 – parameters for selecting suitable sites for the proposed housing development

Centre of Ribchester Location D on Figure 2

Maximum Distance from Centre of Ribchester 0.8km

Minimum allowed flood 201 cumecs

Land proximity to a road < 5m

Allowed land use Improved pasture, Unimproved pasture

Preferred land use Unimproved pasture

Using this list of parameters it was possible to disregard large areas of unsuitable land and focus only on feasible plots. The map in Figure 4 shows the output from ArcGIS after the considerations in Table 1 were made.

Figure 4 – A map showing land categorisation based on the parameters outlined in Table 1. Only plots within 0.8km of the centre of Ribchester have been considered. Plots highlighted in bright yellow show the most suitable sites for development, with B and C marking the two alternative development sites.

Analysis of the new proposed development sites

Alternative Site B – South-West Ribchester

This site was selected because it is comfortably within the 0.8km radius of Ribchester and it is situated on a road which directly links into the village. The site is not at risk of flooding based on the parameters in Table 1, however the 3D visualisation in Figure 5 does show that the site is still close to the river and is only elevated slightly above land affected by a 200 cumec flood. This suggests that flooding might be likely in future should the flow rate exceed 200 cumecs. Site B also offers views similar to those offered by Site A.

Alternative Site C – West-North-West Ribchester

This site was selected because it has close proximity to Ribchester centre and is on the main road leading into the village. The site is not at risk of flooding up to 200 cumecs, and based on evidence in Figure 5, the

Legend

Building

Proposed Plot

Road

Ribble Flooding

10 cumecs

60 cumecs

100 cumecs

160 cumecs

200 cumecs

Suitable Land

Arable Land (no flooding)

All Dry Land (at 200cumecs)

±

0 0.2 0.4 0.6 0.80.1Kilometers

C

B

A

Page 4: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

4 | Page

raised altitude of the hill slope suggests that it is not likely to flood unless there is a considerable increase in flow rate from 200 cumecs, although exaggeration of the 3D projection must be considered when comparing elevation between sites. Site C is not likely to offer the same views as sites A or B because it is located further from the river with buildings blocking most of the line of site to the river.

Figure 5 – A 3D view of the 3 development sites surrounding Ribchester. The 3D projection in this Figure is exaggerated by a factor of 2.5.

Most Suitable Alternative Site

Alternative Site C has been selected as the most suitable alternative site to Site A in Figures 2,4 and 5. This is based on two main considerations:

Existing land use of proposed site – According to the Land Use Map in Appendix 2, the land at Site C is currently classified as ‘Unimproved pasture’ meaning that it has little value to agriculture owing to poor soil quality. Site B was situated over land classified as ‘Improved pasture’ which has a greater value and useful nature than unimproved pasture and could be converted to arable land in future.

Future sustainability of the development – although Site B was situated on land deemed as ‘safe’ from flooding, based on the parameters in Table 1, considerations were made for future flood events having the potential to yield a higher flow rate, thus potentially jeopardising the development. Site C is situated at a higher elevation and a greater distance from the river, thus is at lower risk of flooding in any future flood event.

It is however important to consider that this conclusion has been drawn based only on the limited data made available for this section of the Ribble Valley. For example: it is not known whether flooding from the streams flowing around Ribchester will have an impact on the alternative developments which are both situated within 100m of a stream. The limited number of sites detailed in Figure 4 might be impacted by lack of land use data. In order to create Figure 4, arable, improved and unimproved pasture had to be manually allocated, leaving the interpretation of the land type open to error, thus potentially impacting the viability of any alternative site proposal if the land use has been wrongly determined.

C

B

A

NORTH

Page 5: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

5 | Page

Appendices

Appendix 1 – Dataset for flooding, fields, land use and buildings

Appendix 2 – Land use map

Page 6: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

6 | Page

Appendix 1

Tables showing data relating to the area of fields affected by flooding, changes in the area of flood water, and the area of different types of field.

Table A – areas of fields affected by different flow rates

Field Area (m2) Field Area (km2) % Change

All Fields Area 7853482 7.9 0

10 cumecs Fields 1409776 1.4 18

60 cumecs Fields 1692525 1.7 22

100 cumecs Fields 2018086 2.0 26

160 cumecs Fields 2684135 2.7 34

200 cumecs Fields 3103024 3.1 40

>200c Fields 4750458 4.8 n/a

Table B – area of water at different flow rates

Water area (m2) Field Area (km2) % Change

OS Base Flow 347300 0.3 0

10 cumecs Flood 450000 0.5 30

60 cumecs Flood 566100 0.6 63

100 cumecs Flood 783300 0.8 126

160 cumecs Flood 1423500 1.4 310

200 cumecs Flood 1803100 1.8 419

Percentage of Affected Fields underwater at 200 cumecs 41.9 ((3.1*100) / 1.3)

Total flooded area of affected fields (km2) 1.3 (3.1 - 1.8)

No. of flooded fields 267

No of non-flooded fields 1347

Area of flooded arable field (km2) 0.4

Area of all arable land (km2)

Total Buildings

1.0

1080

Percentage of buildings affected by 200 cumec flow 4.9 ((53*100)/1080)

Flood (cumecs) No of flooded houses (cumulative) No of flooded fields (cumulative)

0 0 0

10 2 69

60 4 91

100 7 120

160 24 204

200 53 267

Page 7: Hydrology - Flooding in Ribchester

Catchment Hydrology – Practical 3 Report Cobain Schofield

7 | Page

Appendix 2

Land use map of Ribchester used to justify the suitability of alternative development locations

Legend

Building

Proposed Plot

Road

Arable Pasture

Improved Pasture

Unimproved Pasture

Ribble Flooding

10 cumecs

60 cumecs

100 cumecs

160 cumecs

200 cumecs

±

0 0.15 0.3 0.45 0.60.075Kilometers

C

B

A