Reading Borough Council - GeoSmart Information · The Environment Agency issues warnings of anticipated river flooding within this area, and due to the relatively long catchment response

Level 2 SFRA Site RC3c – Forbury Retail Park July 2008

1

Reading Borough Council Level 2 Strategic Flood Risk Assessment

RC3c – Forbury Retail Park Flood risk map for site

1

Zone 2 Medium Probability

Climate Change

Zone 3a High Probability

Zone 3b Functional Floodplain

RC4i RCAAP Development Sites

Elevation map for site1

43 - 44 mAOD

44 - 45 mAOD

45 - 46 mAOD

46 - 47 mAOD

47 - 48 mAOD

48 - 49 mAOD

49 - 50 mAOD

> 50 mAOD

< 36mAOD

36 - 37mAOD

37 - 38mAOD

38 - 39 mAOD

39 - 40 mAOD

40 - 41 mAOD

41 - 42 mAOD

42 - 43 mAOD

Description of the Flood Risk Flood Depth Site RC3c is situated in an area that is largely situated above the 0.1% (1 in 1000) design flood levels. A small proportion of the site falls within Zone 2 Medium Probability, affected by flooding from the River Kennet in the 0.1% (1 in 1000) flood event.

Ground levels in the site vary from 37mAOD to 41.5mAOD.

Speed of Floodwaters The River Kennet forms the southern boundary of the site. Whilst the anticipated frequency of flooding is low (i.e. no greater than a 0.1% chance of flooding in any year), when flooding does occur, the floodwaters flowing through the site will be relatively fast flowing.

1This map is based upon the Ordnance Survey Material with the permission of Ordnance Survey on behalf of the Controller Her Majesty’s

Stationery Office © Crown copyright. Unauthorised reproduction infringes Crown copyright and may lead to prosecution or civil proceedings (license number 100019672).


2

Period of Inundation The River Kennet catchment is large, and response times (i.e. the period between which rainfall is observed in the upper catchment, and river levels rise downstream) can be relatively long. For this reason, water levels in the river will rise and fall slowly, and flood levels in the river will typically remain high for an extended period. Consequently, once flooded, the site will be inundated for a period of up to a few days.

Flooding of the River Kennet at this location is typically a result of prolonged rainfall over the catchment, and/or flooding from the River Thames (i.e. raising flood levels in the lower reaches of the Kennet). The Environment Agency issues warnings of anticipated river flooding within this area, and due to the relatively long catchment response times, considerable forewarning of a pending flood event can generally be provided. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.

Flood Defences Site RC3c is not offered any form of protection from formal raised defences. The railway embankment to the north of the site however does provide a degree of informal protection against flooding from the River Thames in the 0.1% (1 in 1000) event. Whilst the likelihood of failure is very low, this should be considered further as a potential source of flood risk in the detailed site based Flood Risk Assessment.

Localised Drainage Issues There are no recorded incidents of localised flooding within Site RC3c held by either the Environment Agency or the Council.

Overview of Flood Risk (PPS25) A summary of flood risk within the site is provided below:

Approximately one quarter of the site is situated within Zone 2 Medium Probability, with a 0.1% (1 in 1000) annual chance of flooding. The remainder of the site is situated in Zone 1 Low Probability; When flooding does occur he speed of flow within the site will be relatively fast; It is likely that affected areas of the site will be inundated for a prolonged period, exceeding a few days.

In summary, it is considered entirely feasible that site RC3c can be developed safely to mitigate the potential risks posed by flooding from the River Kennet. It is important that the design process considers the risk of flooding from the earliest conceptual stage, encompassing measures that will ensure the safety of tenants during a flooding event. A number of important design recommendations are set out below.


3

Planning Recommendations Spatial Planning A small proportion of Site RC3c is situated within Zone 2 Medium Probability, affected by flooding from the River Kennet in the 0.1% (1 in 1000) design event. The remainder of the site falls in Zone 1 Low Probability.

The Council has adopted a sequential approach to the allocation of sites for future regeneration within the Borough, establishing a strong planning argument for the development of the ‘Forbury Retail Park’ site for mixed use purposes (March 2008). Given the relatively low risk of flooding within the site, the proposed land use meets the requirements of PPS25. It is important to ensure however that the site can be developed safely, without increasing the risk of flooding elsewhere.

A review of flood risk within the site has been carried out, and it is considered feasible to design the site in such a way that it remains safe throughout the lifetime of development. It is essential that the following design recommendations are entrenched into the design process from the conceptual stage. A detailed site-based Flood Risk Assessment will be required as an integral part of the planning application stage (to be carried out in accordance with Section 6.6.1 of the Level 1 SFRA).

Development Control 1. All ‘more vulnerable’ uses should be steered towards areas within the site that are at lowest

risk. If at all possible, residential uses should be restricted to those areas of the site that fall within Zone 1 Low Probability, situated away from the River Kennet;

2. Floor levels within the site should be situated a minimum of 300mm above the 1% (1 in 100) peak design flood level, including an allowance for climate change calculated assuming a 100 year lifetime for residential development;

3. All buildings within the site should adopt resilient design techniques to minimise the damage and disruption sustained by businesses and/or residents following a flooding event. Further guidance can be found in Flood Performance of New Buildings (Flood Resilient Construction), CLG (2007);

4. Sustainable drainage systems (SUDS) must be incorporated into the site design to ensure that runoff from the site does not exceed greenfield runoff rates. It is important to ensure that SUDS are designed with due consideration to soil and groundwater conditions. Infiltration techniques should be sought wherever possible, however are likely to be unsuitable in areas of shallow groundwater and/or impermeable soils. Further guidance is provided in Section 6.6.3 of the Level 1 SFRA. Buildings and landscaping should be oriented within the site to avoid blocking overland flow routes;

5. Basement areas within flood affected areas of the site should be watertight, and the entrance point is situated above the 1% (1 in 100) design flood level, including climate change.

Level 2 SFRA Site RC3d – Kenovan Drive & Forbury Business Park July 2008

1


RC3d – Kenavon Drive & Forbury Business Park Flood risk map for site

1


Climate Change





43 - 44 mAOD

44 - 45 mAOD

45 - 46 mAOD

46 - 47 mAOD

47 - 48 mAOD

48 - 49 mAOD

49 - 50 mAOD

> 50 mAOD

< 36mAOD

36 - 37mAOD

37 - 38mAOD

38 - 39 mAOD

39 - 40 mAOD

40 - 41 mAOD

41 - 42 mAOD

42 - 43 mAOD

Description of the Flood Risk Flood Depth

Site RC3d is affected by flooding in the 1% (1 in 100) flood event from the River Thames to the north, and the River Kennet to the south. In the 0.1% (1 in 1000) event, the site will be fully inundated from the River Kennet.

Flooding occurs as a result of floodwaters from the River Thames and River Kennet spilling into low lying areas within the northern and southern extremities of the site. Detailed modelling of the rivers has been carried out on behalf of the Environment Agency (2007), providing the basis for this Level 2 Assessment.

Ground levels in the site vary from 35.5mAOD to 39mAOD, and the vast majority of the site is between 37 and 38.5mAOD. The maximum flood depth in the 1% (1 in 100) design event is approximately 1400mm along the northern boundary of the site (i.e. at risk from the River Thames). Along the southern boundary of the site, the maximum flood depth in the 1% (1 in 100) event from the River Kennet is approximately 900mm. These depths may increase by up to approximately 100mm as a result of climate change.




2

Speed of Floodwaters To the south of the site, the source of flooding is from the adjacent River Kennet. As levels in the River Kennet rise, water will flow through swiftly through the site, inundating areas to the south of Gas Works Road.

To the north of the site, the source of flooding is from the River Thames which is some distance away. As levels in the River Thames rise, water will flow through the site as lower lying areas are progressively inundated. The speed of floodwaters within the site will be slow as water spills into, and out of, the Thames floodplain.

Period of Inundation Both the River Thames and River Kennet catchments are large, and response times (i.e. the period between which rainfall is observed in the upper catchment, and river levels rise downstream) can be relatively long. For this reason, water levels in the river will rise and fall slowly, and flood levels in the river will typically remain high for an extended period. Consequently, once flooded, the site will be inundated for a period of up to a few days.

The Environment Agency issues warnings of anticipated river flooding within this area, and due to the relatively long catchment response times, considerable forewarning of a pending flood event can generally be provided. This enables the Council, emergency services, residents and businesses to prepare in an endeavour to minimise property damage and risk to life.

Flood Defences Site RC3d is not offered any form of protection from formal raised defences. The railway embankment to the north of the site however does provide a degree of informal protection against flooding from the River Thames in the 0.1% (1 in 1000) event. Whilst the likelihood of failure is very low, this should be considered further as a potential source of flood risk in the detailed site based Flood Risk Assessment.

Localised Drainage Issues There are no recorded incidents of localised flooding within Site RC3d held by either the Environment Agency or the Council.


A relatively small proportion of the site is situated within Zone 3a High Probability, with a 1% (1 in 100) annual chance of flooding at the current time. The remainder of the site is situated within Zone 2 Medium Probability, with a 0.1% (1 in 1000) annual chance of flooding;Maximum indicative flood depths within the site are up to 1.4m in the 1% (1 in 100) design event in areas immediately adjoining the river corriodor(s); The speed of flow within the site will be relatively fast in areas within the south of the site; It is likely that affected areas of the site will be inundated for a prolonged period, exceeding a few days.

The maximum depth of flooding within the site is relatively deep in the 1% (1 in 100) design event, and may pose a potential risk to some. The speed of floodwaters through the site is generally low however in areas subject to deep floodwater. The Environment Agency can generally provide substantial forewarning of a pending flood event, allowing tenants to prepare and evacuate the site safely before floodwaters arrive. This is reliant upon people being aware of the potential risks that flooding poses to the site, and actively seeking (and adhering to) issued flood warnings.

In summary, it is considered entirely feasible that site RC3d can be developed safely to mitigate the potential risks posed by flooding from the River Kennet and the River Thames. It is important that the design process considers the risk of flooding from the earliest conceptual stage, encompassing measures that will ensure the safety of tenants during a flooding event. A number of important design recommendations are set out below.


3

Planning Recommendations Spatial Planning A small proportion of Site RC3d is situated within Zone 3a High Probability, affected by flooding from the River Thames and the River Kennet in the 1% (1 in 100) design event. The remainder of the site falls in Zone 2 Medium Probability, affected by flooding from the River Kennet in the 0.1% (1 in 1000) design event.

The Council has adopted a sequential approach to the allocation of sites for future regeneration within the Borough, establishing a strong planning argument for the development of the ‘Kenovan Drive and Forbury Business Park’ site for residential purposes (March 2008). To meet the requirements of PPS25, it is necessary to demonstrate that the site can be developed safely, without increasing the risk of flooding elsewhere, and will therefore satisfy the Exception Test.


Development Control 1. If at all possible, residential buildings should be restricted to those areas of the site that fall

within Zone 2 Medium Probability, arranging the site layout so that (for example) carparking and landscaping features are situated within areas that are at risk of flooding;

2. Floor levels within the site should be situated a minimum of 300mm above the 1% (1 in 100) peak design flood level, including an allowance for climate change calculated assuming a 100 year lifetime for residential development;



5. It is essential that tenants within the site are made aware of the potential risks of flooding, and are actively encouraged to sign up to the Environment Agency’s flood warning service. A safe evacuation route should be established, in accordance with Environment Agency guidance provided within the Level 1 SFRA;

6. Basements should not be used for habitable purposes within Zone 3a High Probability. It is essential to ensure that all basement areas within flood affected areas of the site are watertight, and the entrance point is situated above the 1% (1 in 100) design flood level, including climate change.


RC3e – 42 Kenavon Drive Flood risk map for site

1


Climate Change





43 - 44 mAOD

44 - 45 mAOD

45 - 46 mAOD

46 - 47 mAOD

47 - 48 mAOD

48 - 49 mAOD

49 - 50 mAOD

> 50 mAOD

< 36mAOD

36 - 37mAOD

37 - 38mAOD

38 - 39 mAOD

39 - 40 mAOD

40 - 41 mAOD

41 - 42 mAOD

42 - 43 mAOD

Description of the Flood Risk Flood Depth Site RC3e is situated in an area that has a 0.1% (1 in 1000) annual chance of flooding from the River Kennet. Detailed modelling of the River Kennet system has been carried out on behalf of the Environment Agency (2007), providing the basis for this Level 2 Assessment.

Ground levels in the site vary from 37.5mAOD to 40mAOD.

Speed of Floodwaters The source of flooding is from the River Kennet forming the southern boundary of the site. As levels in the River Kennet rise, water will flow through the site as lower lying areas are progressively inundated. Given the relatively close proximity of the site to the main river corridor, it is anticipated that floodwaters will flow through the site relatively swiftly, albeit at shallow depth.





Flood Defences Site RC3e is not offered any form of protection from formal raised defences. The railway embankment to the north of the site however does provide a degree of informal protection against flooding from the River Thames in the 0.1% (1 in 1000) event. Whilst the likelihood of failure is very low, this should be considered further as a potential source of flood risk in the detailed site based Flood Risk Assessment.

Localised Drainage Issues There are no recorded incidents of localised flooding within Site RC3e held by either the Environment Agency or the Council.


The site is situated wholly within Zone 2 Medium Probability, with a 0.1% (1 in 1000) annual chance of flooding; Once flooding occurs, the speed of flow within the site will be relatively fast, albeit at a shallow depth; It is likely that affected areas of the site will be inundated for a prolonged period, exceeding a few days.

In summary, it is considered entirely feasible that site RC3e can be developed safely to mitigate the potential risks posed by flooding from the River Kennet. It is important that the design process considers the risk of flooding from the earliest conceptual stage, encompassing measures that will ensure the safety of tenants during a flooding event. A number of important design recommendations are set out below.

Planning Recommendations Spatial Planning Site RC3e is situated entirely within Zone 2 Medium Probability, affected by flooding from the River Kennet in the 0.1% (1 in 1000) design event.

The Council has adopted a sequential approach to the allocation of sites for future regeneration within the Borough, establishing a strong planning argument for the development of the ’42 Kenavon Drive’ site for residential purposes (March 2008). Given the relatively low risk of flooding within the site, the proposed land use meets the requirements of PPS25. It is important to ensure however that the site can be developed safely, without increasing the risk of flooding elsewhere. It is recognised that planning permission has been provided for residential development within this site.

A review of flood risk within the site has been carried out, and it is considered feasible to design the site in such a way that it remains safe throughout the lifetime of development. It is recommended that the following design criteria are adopted to influence the site layout and building design within the site.

Development Control 1. Floor levels within the site should be situated a minimum of 300mm above the 1% (1 in 100)

peak design flood level, including an allowance for climate change calculated assuming a 100 year lifetime for residential development;



4. Basement areas within flood affected areas of the site should be watertight, and the entrance point is situated above the 1% (1 in 100) design flood level, including climate change.

Level 2 SFRA Site RC3f – Gas Holder July 2008

1


RC3f – Gas Holder Flood risk map for site

1


Climate Change





43 - 44 mAOD

44 - 45 mAOD

45 - 46 mAOD

46 - 47 mAOD

47 - 48 mAOD

48 - 49 mAOD

49 - 50 mAOD

> 50 mAOD

< 36mAOD

36 - 37mAOD

37 - 38mAOD

38 - 39 mAOD

39 - 40 mAOD

40 - 41 mAOD

41 - 42 mAOD

42 - 43 mAOD

Description of the Flood Risk Flood Depth Site RC3f is situated in an area that predominantly has a 0.1% (1 in 1000) annual chance of flooding from the River Kennet. It is expected that the risk of flooding from the River Kennet will increase over time as a result of climate change, and in approximately 100 years the site will be situated within the 1% (1 in 100) design event flood envelope.

Flooding occurs as a result of floodwaters from the River Kennet spilling into low lying areas situated to the north of the river. Detailed modelling of the River Kennet system has been carried out on behalf of the Environment Agency (2007), providing the basis for this Level 2 Assessment.

Ground levels in the site vary from 36.5mAOD to 38mAOD. The maximum flood depth in the 1% (1 in 100) design event is approximately 300mm along the southern boundary of the site. This may increase by up to approximately 100mm as a result of climate change.

Speed of Floodwaters The source of flooding is from the River Kennet forming the southern boundary of the site. As levels in the River Kennet rise, water will flow through the site as lower lying areas are progressively inundated. Given the close proximity of the site to the main river corridor, it is anticipated that floodwaters will flow through the site relatively swiftly.




2



Flood Defences

Site RC3f is not offered any form of protection from raised (formal or informal) defences.

Localised Drainage Issues There are no recorded incidents of localised flooding within Site RC3f held by either the Environment Agency or the Council. It is worth noting however that the school on the opposite side of the River Kennet flooded in February 2002. The site lies on lower ground than the school, and it follows therefore that the site may also have flooded.


Approximately one quarter of the site is situated within Zone 3a High Probability, with a 1% (1 in 100) annual chance of flooding at the current time. The remainder of the site is situated within Zone 2 Medium Probability, with a 0.1% (1 in 1000) annual chance of flooding;The extent of flooding within the site in the 1% (1 in 100) design event will increase substantially as a result of climate change; Maximum indicative flood depths within the site are approximately 0.3m in the 1% (1 in 100) design event; The speed of flow within the site will be relatively fast; It is likely that affected areas of the site will be inundated for a prolonged period, exceeding a few days.

In summary, it is considered entirely feasible that site RC3f can be developed safely to mitigate the potential risks posed by flooding from the River Kennet. It is important that the design process considers the risk of flooding from the earliest conceptual stage, encompassing measures that will ensure the safety of tenants during a flooding event. A number of important design recommendations are set out below.


3

Planning Recommendations Spatial Planning A small proportion of Site RC3f is situated within Zone 3a High Probability, affected by flooding from the River Kennet in the 1% (1 in 100) design event. The remainder of the site falls in Zone 2 Medium Probability, affected by flooding from the River Kennet in the 0.1% (1 in 1000) design event.

The Council has adopted a sequential approach to the allocation of sites for future regeneration within the Borough, establishing a strong planning argument for the development of the ‘Gas Holder’ site for residential purposes (March 2008). To meet the requirements of PPS25, it is necessary to demonstrate that the site can be developed safely, without increasing the risk of flooding elsewhere, and will therefore satisfy the Exception Test.


Development Control 1. Floor levels within the site should be situated a minimum of 300mm above the 1% (1 in 100)

peak design flood level, including an allowance for climate change calculated assuming a 100 year lifetime for residential development;



4. It is essential that tenants within the site are made aware of the potential risks of flooding, and are actively encouraged to sign up to the Environment Agency’s flood warning service. A safe evacuation route should be established, in accordance with Environment Agency guidance provided within the Level 1 SFRA;

5. Basements should not be used for habitable purposes within the site. It is essential to ensure that all basement areas within flood affected areas of the site are watertight, and the entrance point is situated above the 1% (1 in 100) design flood level, including climate change.

Documents

Reading Borough Council - GeoSmart Information · The Environment Agency issues warnings of anticipated river flooding within this area, and due to the relatively long catchment response