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Cwmaman Flood Alleviation Scheme

Rhondda Cynon Taf Council has published a statutory notice signalling its intention to start land drainage improvement works as part of a wider Flood Alleviation Scheme for Cwmaman.

Parts of Cwmaman are considered high risk in the Council’s Flood Risk Management Plan and have previously been adversely affected following heavy rainfall. The statutory notice outlines the Council’s plan to reduce the risk of flooding in the area.

The planned flood alleviation works provide further good news for the local area which is also benefiting from a new £7.2m community primary school from September 2018, with Glynhafod Junior and Cwmaman Infant schools coming together at a state-of-the-art site on Glanaman Road.

The wider development includes a Multi Use Games Area, staff car park, service yard, grass pitches and a variety of external pupil spaces.

Leader of Rhondda Cynon Taf Council, Councillor Andrew Morgan, said: “The statutory notice for the Cwmaman Flood Alleviation Scheme is the first stage in the Council’s delivery of this potential project, to alleviate the wider flood risk for an area considered a high risk.

“The planned flood alleviation works demonstrate our commitment to investing in the right areas to improve our communities.

“The project is further positive news for the local community as good progress continues to be made on site of the new community primary school.”

Below taken from RCTC Planing Application for the New Cwmaman School

 

Para. 11.1.11 (open space) states that formal and informal open green spaces, including parks with significant recreational or amenity value, should be protected from development. (Well the paddling pool was not protected was it)

Para. 13.3.2  (flood risk) states that Local Planning Authorities should recognise that it will be inappropriate to locate certain types of development including schools within areas defined as being of high flood hazard. 

 

TAN 15 (Development and Flood Risk)

... defines areas of floodplain without significant flood defences as Zone C2, where only less vulnerable development should be considered, subject to application of the justification test, including acceptability of consequences (4.2). Schools are included in the definition of highly vulnerable development (5.1).

Highly vulnerable development in Zone C2 should not be permitted (6.2). Less vulnerable development in Zone C2 is only justified if it is part of a regeneration initiative or is needed to sustain a settlement, or it relates to key employment objectives, provided the site is previously developed by the PPW definition and the consequences of flooding are acceptable (6.2).

Previously developed land is defined by PPW in figure 4.4, and excludes “previously developed land subsequently put to an amenity use”. 

A timetable to show phasing of construction activities to avoid periods of the year when sensitive wildlife could be harmed (such as nesting bird season, hibernating and breeding reptiles, etc. 

Flood Risk - Policy Context

Schools are defined in Technical Advice Note 15 (Development and Flood Risk) as “highly vulnerable development” for flood risk purposes. Part of the application site is located within a C2 Flood Zone (undefended flood plain) as defined by Natural Resources Wales Development Advice (DAM) Maps. Not all of the site is covered by this designation therefore, there is no requirement to refer the application to Welsh Government under Circular 07/12 – The Town and Country Planning (Notification) (Wales) Direction 2012.

The applicant was made aware of this designation in early pre-application discussions and has been working closely with Natural Resources Wales prior to and during the application to ensure that they have all of the information required to make an informed recommendation in respect of the proposal. Accordingly, the application was submitted with a Flood Consequences Assessment (FCA). The applicant’s approach to siting highly vulnerable development in a C2 Flood Zone is to raise the site for the school building above the level of the floodplain and to mitigate the resultant loss of floodplain by providing flood water storage areas.

Technical Advice Note (TAN) 15 (Development and Flood Risk) generally discourages the provision of highly vulnerable development in undefended areas that are prone to flooding (C2 Zones). However, it is also advises that “where a site falls partially within Zone C it will be a matter for the planning authority to judge whether to apply Section 6, though it is probable that an assessment in accordance with Section 7 and Appendix 1 will be required.”

In this instance much but not all of the site falls within the C2 floodplain and as such it is at the discretion of the Council as the Local Planning Authority, to decide how much weight to apply to Section 6 of the TAN in determining this planning application. Committee is advised that, whilst in simple terms the proposals do not comply with Section 6 of TAN 15, taking into account the proposal to raise the site for the school above the extreme flood levels and to provide compensatory flood water storage, it is considered appropriate in this case to place lesser weight on the policy of directing highly vulnerable development away from the C2 Flood Zone.

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Development Control Committee Agenda - 2nd March 2017

Subject to Natural Resources Wales offering no objection to the modelling and recommendations of the FCA prepared in support of the current proposals and appropriate conditioning being attached to any planning permission to ensure the provision of the flood alleviation measures and demonstrable emergency evacuation procedures as appropriate are in place, it is considered reasonable to conclude that no policy objection should be raised to the current proposals.

Part of the justification for development in this location is the overriding need for the school, in line with Planning Policy Wales sustainability objectives (section 4.4; para. 4.6.2) and LDP policy CS 1 for building strong, sustainable communities. The Design and Access Statement (DAS) submitted with the application only addresses the educational need for the school in a high level way; however the processes and procedures involved in the delivery of a new school (that the Council’s Education Directorate would have done prior to the planning application) will have clearly identified such a need.

Similarly, the other justification is that the location of the school is, by necessity, limited to its catchment area. Evidence of a lack of suitable and available alternative sites would also have been an issue in the consultation process undertaken by the Local Education Authority and it is apparent, from discussions with colleagues in the Education Directorate, that the proposed site is indeed the only viable education site that could meet the criteria set down by the Welsh Government and the needs of the local community. No suitable and available sites outside the C2 zone were found.

Therefore provided the flood risk issue can be adequately addressed, there would be no objection under policy AW 2. 

Details of invasive plant control measures 

FCA Assessment

This section summarises the findings of a Level 3 Flood Consequences Assessment (FCA) undertaken by Waterman Infrastructure & Environmental Ltd. on behalf of Rhondda Cynon Taf County Borough Council for a proposed primary school on land adjacent to Glanaman Road, Cwmaman.

The FCA was undertaken in line with Planning Policy Wales and Technical Advice Note 15: Development and Flood Risk (TAN15). The aim of the FCA was to demonstrate that the site can be redeveloped safely, without exposing the new development to an unacceptable degree of flood risk or increasing the flood risk to third parties. The objectives of the study were to:

  • Identify potential sources of flooding and assess the risks they pose to the site;

  • Consider the effect of the predicted impacts of climate change on future flood risk to the site;

  • Assess the impact of the proposed development on flood risk to third parties; and

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Development Control Committee Agenda - 2nd March 2017

• Recommend appropriate flood risk mitigation measures.

The requirement for a Level 3 FCA arose from the outcomes of a discussion with Natural Resources Wales (NRW), who requested a hydrological and hydraulic modelling exercise to be undertaken assessing the flood risk posed by the Aman Fach and Aman Fawr watercourses.

On Site
The site is located off Glanaman Road in Cwmaman and is currently used as an amenity area for the local community. The Aman Culvert runs beneath the site and follows the approximate path of the historic watercourse linking the Nant Aman Fach and Nant Aman Fawr with the downstream reaches of the Aman River.

The course of the Aman River was progressively culverted to the east of the site during the development of Fforchaman Colliery. Since the discontinuation of work at the colliery in the 1960’s the surrounding land was landscaped to form the common ground that exists at the site today. Ground levels at the site have changed dramatically since the closure and landscaping of the site, with colliery spoil in the 1970’s resulted in levels rising by up to 6m.

As a result, the River Aman which used to run openly through the site is now culverted over a significant length which includes a number of sharp bends and multiple inflows from the surrounding land to manage surface water discharges back into the watercourse downstream at Llanwonno Road.

The historical context of the River Aman demonstrates the fact that historically the floodplain would not have been defined by the capacity of a wider culvert network, instead being defined by the capacity of the River’s natural floodplain. As the levels have changed and flow constrictions added, the mechanisms of flooding too have changed.

Despite the significant changes made in this area, only one known record of flooding in Cwmaman was recorded in 1986. Evidence indicates that the culvert upstream of the Aman Culvert on the Nant Aman Fawr was deliberately vandalised and blocked causing widespread flooding throughout the western terrace on Glanaman Road with significant depths of flooding reported. No details on this event are available and it proved difficult to use this information in the hydrological / hydraulic modelling exercise.

The scheme involves the re-development of the site to include the provision of a new primary school and associated sports pitches, recreational areas and access roads. In addition, the proposals include for the straightening of a section of the Aman Culvert which runs beneath the plot of the proposed school and will remove two significant bends that currently exist along the alignment.

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Development Control Committee Agenda - 2nd March 2017

The majority of the site lies within Zone C2 of the NRW Development Advice Map (DAM). Zone C2 is defined by the 0.1% (1000 year) Annual Exceedance Probability (AEP) Fluvial, Tidal, or combined event that does not benefit from flood defences.

The risk reported appears to be solely posed by the Nant Aman Fach which is inconsistent with the site’s flood history, which suggests that the Nant Aman Fawr has caused flooding historically due to a blockage. As a result this would appear to call into question the accuracy of the hydraulic model and previous studies in this area have confirmed that the mapping is based on national generalised modelling, not accounting for local characteristics of the watercourse including hydrological data and hydraulic structures.

As such, more detailed information was deemed a necessity to properly understand the flood risk posed at the site and detailed hydraulic modelling was undertaken to this end.

The major source of flood risk at the site appears to be fluvial flooding as a result of the Nant Aman Fach and Nant Aman Fawr watercourses. The Nant Aman Fach rises on the southern slope of the Cefnrhos-Gwawr and flows in a south-easterly direction towards Cwmaman. The headwaters of the Nant Aman Fawr lie within St. Gwynno Forest close to the A4233 and flow in an eastward direction towards Cwmaman.

The two watercourses converge at a point south of the Glanaman Road Bridge to form the Aman River which is culverted for approximately 700m flowing beneath the site before reaching its outfall under Llanwonno Road.

It is reported that the flood risk mapping of this area is unlikely to have considered the interaction between the Aman Fawr and Aman Fach and is highly unlikely to have considered in detail the impacts of the culverted section of the River Aman and the impacts of the Aman Fawr culvert / Glanaman Road Bridge.

Each of these characteristics has significant impacts on flood risk in Cwmaman and was considered in detail as part of the hydraulic modelling study.

Waterman carried out a detailed hydraulic model build in line with the requirements of NRW and to inform the FCA. This required a detailed review of existing data and, where required, collection of new data for the construction of the hydraulic model including LiDAR Data, river channel surveys, culvert details, OS Mastermap and site topographic surveys.

One dimensional and two dimensional ground models and structures were developed accounting for vegetation and cultural features and ground, channel, flood plain and culvert roughness factors. Hydrological assessments explored the statistical and flood hydrographs to define flows at key locations in order to estimate peak flows.

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Development Control Committee Agenda - 2nd March 2017

A complete set of mapped results were derived from the baseline model summarising flood depths in a 1 in 100 year plus climate change and 1 in 1000 year event.

As the results show, there is a clear fluvial flood risk posed to a large area of Cwmaman including the undeveloped site, with depths above 0.6m occurring during a 1 in 100 year event plus climate change. The proposed development therefore required a consideration of these flood depths in setting the finished floor levels of the school and mitigation measures have been rigorously assessed to offset any impacts to third party land and property that result from these alterations.

Design measures have been implemented to ensure that finished floor levels within the school and grounds are sufficiently managing flood risk, providing a safe environment for its proposed use. This exercise has included a consideration of third party impacts posed by any changes to ground levels and any mitigation measures required to counter these impacts.

In the early stages of the proposed model development, a series of simple design guidelines were set out by the project team ensuring that TAN15 was followed and design criteria set out by RCT was satisfied. The following provides a summary of these principles:

  • The main school site (school building, car park and surrounding grounds) should be flood free in a 1 in 100 years plus climate change event as per TAN 15.

  • The applicant (RCT) requested that the school building is dry during a 1 in 1000 year event which significantly exceeds the already stringent requirements of TAN 15.

    The proposed solution controls the direction and rate of flows on the floodplain to ensure that the school remained flood free without increasing flood risk elsewhere. This has been achieved through application of a series of earth bunds, flood channels and storage areas.

    An earth bund that runs along the Cwmaman FC football ground to the west of the Site is highly effective in reducing flood depths along Glanaman Road. The bund separates the two sources of flooding from the Aman Culvert and Glanaman Road Bridge and in doing so; flows that overtop the Aman Culvert are diverted away from the residential properties along Glanaman Road and instead flow to the east.

    The low level playing fields and MUGA to the south and west of the school plateau join with the flood storage area to the east of the school. A level access has been created between Glanaman Road and the playing fields to effectively create a new flow route which intercepts flood water as it makes its way down Glanaman Road and directs it around the school to the south and finally eastwards.

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Development Control Committee Agenda - 2nd March 2017

Flood waters enter the flood storage area to the east of the school via a series of diversion channels present to the north and south of the school which ensure the quick transfer of flows around the school. The diversion to the south is a shallow flood relief channel that follows the western and southern perimeter of the site.

The north diversion is a culvert built beneath the school’s access road. The result of these channels is that water is efficiently diverted around the school minimising the potential for flood waters to spill back onto Glanaman Road.

The flood storage area to the east of the school has not been significantly changed in terms of ground levels. Once in the storage area, flood water is held back and attenuated by the presence of an earth bund to the east of the site. Overspill weirs control the release of flow back onto its natural course along the valley ensuring even distributions of flows are achieved to reduce existing flooding levels for the Cwrt Alun Lewis Sheltered Housing and residences to the east.

It is important to note that the proposals allow for a drainage route from the flood storage area back into the Aman Culvert via an outlet feature in the south east corner of the site.

These measures are implemented with the intention of moving flow around the school and keeping the most vulnerable areas of the site - the school, play grounds, car park and access road - dry in even the most extreme flooding events. These levels tie in the school access road and car park to existing ground levels on Glanaman Road ensuring a safe and dry exit from the site during extreme flooding events.

The on-site impacts of the mitigation measures are very positive. All of the design criteria agreed have been met and a good degree of freeboard to the school has been achieved. The site is not flooded in a 1 in 100 year event (plus climate change event) and areas to the east on Glanaman Road are not flooded meaning there is a clear means of vehicular egress during this event.

Flood depths in the 1 in 1000 year event are not significant and are shallow and of little danger. The maximum water level during a 1 in 1000 year event means that the School itself remains dry as 150mm of freeboard is still provided.

As a result the FCA concludes, the proposed scheme is considered to be compliant with TAN15 and the development can be considered safe.

Off Site
The mitigation measures employed for this development have been derived with a strong emphasis on delivering betterment to the wider community by reducing baseline flooding levels wherever possible and minimising third party effects. The FCA concludes that this has been achieved.

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Development Control Committee Agenda - 2nd March 2017

Results from the 1 in 100 year (plus climate change) event demonstrate that resulting from the proposals there are widespread benefits to the surrounding community in Cwmaman. Most notably, properties along Glanaman Road are shown to have flood depths either substantially reduced or removed completely to the houses facing the site. Furthermore, flood depths are reduced across the Cwrt Alun Lewis Sheltered Housing Development and along most of the residences to the east of the site.

Minor increases in flood levels to the area of out-buildings to the rear of the gardens of properties in Aman Street of approximately 22mm and 85mm are possible in the Q100 plus climate change and Q1000 events respectively which when compared to the betterment in reduction of flooding levels to the Aman Street properties themselves is considered by the FCA to be a reasonable compromise.

A 1 in 1000 year event has proved the most significant return period with regards to increases in flood depths. Again, it is noted that all significant areas are shown to be unaffected by the development with many areas still seeing some improvements. The areas shown to be negatively impacted are those same locations in which garden sheds and storage containers are located and are of lesser consequence then the residences themselves.

As part of the development proposals the plans include an engineering solution for straightening a section of the Aman Culvert. This is not being considered as a mitigation measure but the impacts of the culvert straightening were investigated to ensure that downstream velocities were not impacted.

Very little difference was noted in the FCA between the peak velocities experienced within the model, both under normal in bank conditions and during more extreme events. As a result, it is considered that the straightening of the culvert will not significantly increase downstream velocities or pose an additional risk to the properties and land downstream of the Aman Culvert.

Flood Risk - NRW Response

NRW have advised that they have previously provided pre-application advice stating that further modelling work was required to demonstrate that the risks and consequences of flooding can be managed to an acceptable level in accordance with TAN15.

They have concluded that this modelling work has now been completed and they are satisfied that it was fit to inform the Flood Consequences Assessment (FCA).

Notwithstanding this, NRW express significant concerns with the proposed development as submitted. They recommend that the LPA should only grant planning permission if the scheme can demonstrate that the proposed development

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Development Control Committee Agenda - 2nd March 2017

will not cause an increase in flooding elsewhere (third party land) in accordance with TAN15. NRW have advised that they would object if the scheme does not meet this requirement.

Flood Risk

The application site lies almost entirely within Zone C2, as defined by the Development Advice Map (DAM) referred to under Technical Advice Note 15: Development and Flood Risk (July 2004).

NRWs Flood Map information, which is updated on a quarterly basis, confirms the site to be within the 1% (1 in 100 year) and 0.1% (1 in 1000 year) annual probability fluvial flood outlines of the River Cwm Aman, which is a designated main river.

NRW refer to Section 6 of TAN15 and the Chief Planning Officer letter from Welsh Government, dated 9th January 2014, which affirms that highly vulnerable development should not be permitted in Zone C2 (paragraph 6.2 of TAN15).

Notwithstanding this policy advice, NRW have reviewed the hydraulic model which has been developed to support the findings of the Flood Consequences Assessment (produced by Waterman Infrastructure and Environment Ltd, dated February 2017) in order to provide advice on the acceptability of the consequences of flooding.

NRWs detailed advice in relation to flood risk is set out below:

As outlined above proposed development is located within the 1% (1 in 100 year) and 0.1% (1 in 1000 year) flood outlines. Therefore a number of mitigations measure are proposed to manage flood risk at the site.

NRW note that the school building will be raised to a level of 189.75m AOD and the school grounds (area immediately adjacent to the school buildings, as shown in figure 10 of the FCA) will range in levels between 189.45m AOD and 189.60m AOD.

The car park and access road will range in levels between 188.81m AOD and 189.59m AOD. Therefore, the school building, school grounds and access/car park, as described in Figure 10 of the FCA, are predicted to be flood free during the 1% (1 in 100 year) event with an allowance for climate change (25%) and is therefore considered to be in line with the guidance set out in A1.14 of TAN 15.

The school building is also predicted to be flood free during the 0.1% (1 in 1000 year) flood event. However the car park, which is set at 189.45m AOD, is predicted to flood to a maximum depth of 300mm with a maximum velocity of 0.45 m/s. This predicted depth is in line with the guidance set out in A1.15 of TAN 15, however the maximum velocity exceeds the indicative guidance. The access road to the school is predicted to flood to a depth of 600mm with a maximum velocity of 0.45 m/s during this event. This predicted depth is in line with the guidance set out in A1.15 of TAN 15, however the maximum velocity exceeds the indicative guidance

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Development Control Committee Agenda - 2nd March 2017

Further flood mitigation measures

In order to offset the above ground raising a complex set of mitigation measures are proposed. These mitigation measures include a series of earth bunds, flood channels and storage areas. These are described below, although Committee is advised that this is not exhaustive. Section 5 of the FCA describes the mitigation measures in full.

The playing fields on the western and southern side of the raised school will be lowered to a level of 188.0m AOD. The lowered playing fields create a new flow route around the school and are designed to flood. These areas (which include MUGAs (Multi Use Games Area) and grass pitches) will act as flood storage and conveyance areas. They are predicted to flood during the 1% (1 in 100 year) and 0.1% (1 in 1000 year) flood events to depths of 1-2m with a maximum velocity of 1.00m/s. This equates to a hazard rating of “Danger” for all (including the emergency services).

Water will enter the flood storage area on the eastern area from the new flood flow route (as described in the above point) via a series of diversion channels to ensure the transfer of flows around the school. These will be made up of relief channels as well as new culverts.

Once the water is within the flood storage area, a significant earth bund around the eastern perimeter will keep the flood water held within the storage area. It is noted that a ‘play area’ is proposed within the flood storage area. They highlight that as a part of the mitigation measures this area is specifically intended for the purpose of storing flood water and is therefore predicted to flood during the 1% (1 in 100 year) and 0.1% (1 in 1000 year) flood events.

An earth bund is proposed that runs along the football ground on the western side of the site. The bund will be set to height of between 189.7m AOD and 190.3m AOD. They suggest that the most western part of this bund does not appear to fall within the red line boundary of the school development.

There will be an earth bund around the Vodafone Communications tower. The bund will stand at 1.0m and therefore is predicted to keep this infrastructure flood free during and up to the 0.1% (1 in 1000 year) flood event. However, this bund is also considered to be outside of the red line boundary of the school development.

Impact of the development on flooding elsewhere

As a result of these mitigation measures and due to the complexity of flooding in the area, there is an increase in flood risk elsewhere. These are located to the rear of Aman Street within the garden areas of residential properties, including sheds and storage containers.

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Development Control Committee Agenda - 2nd March 2017

The increase in flooding is shown to be to be 22mm during the predicted 1% (1 in 100 year) event and a maximum of 220mm during the predicted 0.1% (1 in 1000 year) flood event. NRW note that these properties are already predicted to flood during these events. However, this is not in line with guidance set out in Section 9 of TAN 15 which states that there should be no increase in flooding elsewhere.

Areas of new flooding were also identified (i.e. areas that were not predicted to flood previously, in Figure 14 of the FCA). NRW understand that this area falls within land that is within RCT ownership and that no new properties are now predicted to flood as a result of this development.

Notwithstanding this advice, they advise that if the Council are minded to grant permission, the development would only be acceptable if the measures, as detailed in the Flood Consequences Assessment, are implemented and secured by way of conditions on any planning permission.

However, as described above, NRW note that two flood defence bunds are outside of the red line boundary of the school development. NRW consider that these will need to be secured through appropriate mechanisms (such as a S106 agreement - but Committee is advised that these are located on Council owned land therefore this type of mechanism is not necessary.

NRW advise that the following conditions are necessary if the Council intend to grant planning consent:

The development permitted by this planning permission shall only be carried out in accordance with the approved Flood Consequences Assessment (FCA) submitted by Waterman dated February 2017 and drawings produced by Bradley Associates Job no 152307, drawings 02 - 17, and the following mitigation measures detailed within the FCA:

  • Provision of the mitigation measures as described on pages 15 &16

  • The finished floor levels of the school building must be set to 189.75m

    AOD.

  • The finished floor levels of the school grounds must be set to 189.45 to 189.60m AOD

  • The finished level of the access and car park must be set to 188.82 – 189.59m AOD

  • The finished level of the playing fields must be set to 188.00m AOD Reason: To prevent and reduce the risk of flooding.

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Development Control Committee Agenda - 2nd March 2017

NRW are particularly concerned over the complexity of the proposed mitigation measures and the FCA has not provided any information on how the mitigation measures will be maintained to adequate standards throughout the lifetime of development. Therefore NRW advise that the following measures must be secured with suitably worded conditions if the Council want to grant permission.

Prior to the first beneficial occupation of the development hereby permitted, a scheme shall be submitted to, and approved in writing by, the local planning authority detailing measure for the long term maintenance (for the lifetime of the development) of the completed flood mitigation measures. Maintenance must be carried out in strict accordance with the measures detailed in the approved scheme

Reason: To ensure the maintenance of the mitigation measures.

All flood storage areas (as defined by the FCA) shall remain free from built development for the lifetime of the development; this includes the flood storage area to the eastern side of the school as well as the flood flow routes around to the south and west of the school.

Reason: To ensure the effective storage of flood waters.

NRW advise that, as it is for the Authority to determine whether the risks and consequences of flooding can be managed in accordance with TAN15, they recommend that the applicant considers consulting other professional advisors on the acceptability of the developer’s proposals, on matters that NRW cannot advise on such as emergency plans, procedures and measures to address structural damage that may result from flooding. NRW refer to the above information and the FCA to aid these considerations. They also advise that they do not normally comment on or approve the adequacy of flood emergency response and procedures accompanying development proposals, as they do not carry out these roles during a flood. Their involvement during a flood emergency would be limited to delivering flood warnings to occupants/users. In light of these comments, the following condition is also proposed.

Within six months prior to the first beneficial occupation / use of any part of the development, a scheme shall be submitted to, and approved in writing by, the local planning authority detailing a strategy for the provision of a Flood Management Plan.

The strategy shall set out a timetable for the provision of a phased Flood Management Plan based on key milestones to be completed as part of the development. The strategy shall be implemented in accordance with the approved scheme.

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Development Control Committee Agenda - 2nd March 2017

The Flood Management Plan(s) shall be prepared in consultation with Rhondda Cynon Taf County Borough Council’s Emergency Planning Team and the Emergency Services. The Plan should, as a minimum, include:

  • Details to confirm that occupiers of the development are signed up to the Environment Agency flood warning service and are aware of the flood consequences.

  • Arrangements for vacating those parts of the site shown to be at highest risks of flooding where the risks to people and property could be significant on receipt of a flood warning.

  • Arrangements for evacuating the site if necessary.

  • Arrangements for post event clear up and recovery which should also include inspection of any infrastructure that may have been damaged during the flood to ensure that it is safe before the site is re-occupied.

    The Flood Management Plan shall be implemented in accordance with any approval and prior to the first beneficial occupation of the school.

    Reason: In the interests of public safety having regard to the location of the site within a C2 Flood Zone.

    Flood Risk Activity Permit

    NRW advise that the straightening of the Aman culvert will require a Flood Risk Activity Permit.

    Flood Risk Conclusions

    The concern of NRW is a significant material consideration in the determination of this application and Committee will need to decide whether these risks are acceptable having regard to the specific advice of the Council’s flood risk consultants and all other relevant consultees.

    Waterman have carried out a detailed hydraulic assessment of the Aman Fach, Aman Fawr and Aman River including detailed modelling of the Aman Culvert which, when referenced to current Natural Resources Wales guidelines and Welsh Government climate change directives, suggests that flooding within Cwmaman could occur with greater frequency in the future.

    The potential causes for flooding are a result of insufficient capacity within the culvert networks that direct flows from the Aman Fawr and Aman Fach downstream to the River Aman. Specifically, the Aman Culvert is shown to be a flood restriction which could cause water to back up and overtop the banks at Glanaman Road Bridge and at the headwalls of the culvert itself in more extreme events.

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Development Control Committee Agenda - 2nd March 2017

Flood management principles including raising finished floor levels have been agreed with NRW and applied to the proposed school development site and the provision of mitigation measures to protect third parties from any detrimental impacts of the development have been adopted.

The proposed mitigation measures have been shown to be effective in all return periods from 1 in 5 year to 1 in 1000 year flood events. The main school site is shown to be flood free in all events with a clear vehicular egress route in all events up to a 1 in 100 year plus climate change event.

Significant reduction and elimination of flooding levels to third party properties is shown to result from the proposals with minor increase in flooding levels limited to the lower garden areas and out buildings of properties to the east of the site in Aman Street where the residential buildings themselves will actually benefit from a reduction in flooding levels.. While this requires a balanced judgement that strict adherence to the over-arching policy context does not readily offer as no distinction is made between the flooding of properties and the flooding of curtilage, there would seem to be a tangible benefit to the owners of these few affected properties in a flooding scenario than if the development did not proceed.

Sensitivity analysis has shown that changes to the model parameters do not significantly affect flood risk at the school or its grounds.

Blockage analysis has shown that the school building is capable of remaining flood free in even the most extreme blockage scenario of the Aman Culvert during a 1 in 1000 year flood event and whilst flooding of the school grounds is noted within some of the scenarios, depths do not exceed the thresholds set out in TAN15.

In respect of the potential to exceed recommended velocities (for flood water) it has been advised that In a 1 in 100 year (plus climate change) flood event, the escape routes to safety from the school via the play grounds , car park and access road are safe and remain dry and flood free.

In a 1 in 1000 year extreme flood event it is almost certain the school would have been evacuated along with all of the residents of Cwmaman. However even in the case of somebody been still in the building they are in a place of safety which remains flood free and the rescue services could still reach them as the water level of 300mm is 50% of the depth of 600mm which is the maximum limit acceptable to NRW, therefore the velocity of 0.45 m/s, which is limited to a small area of the access road becomes much less significant.

As a result, the FCA concludes that the site is capable of being developed safely having regard to the consequences associated with flooding when compared with an event if the development did not proceed. Committee is advised that this overall betterment is a material consideration that weighs in favour of the application.

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(At the time of writing the report, the applicant was working closely with its flood risk consultants to see if any additional mitigation could be incorporated into the scheme which would result in no additional detriment. An update will be provided at Committee on any progress that has been made).

Drainage

As part of the application process, the Council’s Drainage Section and Dwr Cymru / Welsh Water were consulted in respect of the drainage arrangements for the development.

DC/WW are satisfied that the discharges from this site can be accommodated at the Cynon Waste Water Treatment Works and subject to the following conditions, are satisfied with the proposal (and the development therefore complies with Policy AW6 and AW10 of the Rhondda Cynon Taf Local Development Plan)

Only foul water from the development site shall be allowed discharge to the public sewerage system and this discharge shall be made between manhole reference number SS99997505 and ST00990402 as indicated on the extract of the Sewerage Network Plan attached to this decision notice.

Reason: To prevent hydraulic overloading of the public sewerage system, to protect the health and safety of existing residents and ensure no pollution of or detriment to the environment (and to comply with Policy AW10 of the Rhondda Cynon Taf Local Development Plan).

The proposed development site is crossed by a public sewer with the approximate position being marked on the attached Statutory Public Sewer Record. The position shall be accurately located marked out on site before works commence and no operational development shall be carried out within 3 metres either side of the centreline of the public sewer.

Reason: To protect the integrity of the public sewer and avoid damage there to protect the health and safety of existing residents and ensure no pollution of or detriment to the environment (and to comply with Policy AW10 of the Rhondda Cynon Taf Local Development Plan).

The approved use shall not commence until an adequate grease trap has been fitted in accordance with details that have been submitted to and approved in writing by the local planning authority. Thereafter the grease trap shall be maintained so as to prevent grease entering the public sewerage system.

Reason: To protect the integrity of the public sewage system and ensure the free flow of sewage (and to comply with Policy AW10 of the Rhondda Cynon Taf Local Development Plan).

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Development Control Committee Agenda - 2nd March 2017

The Council’s Drainage Section has offered no objection in regards to flood risk management/surface water drainage as the submitted drainage strategy provides enough detail in principal to adequately manage the surface water on site.

It is suggested however that the applicant provides further detailed information in relation to the surface water drainage in accordance with their document ‘Planning Requirements Relating to Flood Risk Management’ document to ensure the surface water is managed appropriately in line with the proposed drainage strategy.

No development shall take place until such time as a timescale for the submission of site specific surface water drainage details to serve the built development (in accordance with ‘Planning Requirements Relating to Flood Risk Management’) is submitted to, and approved in writing by, the local planning authority. The drainage details shall be provided in accordance with the timescales approved and completed prior to the first beneficial occupation of the development.

Reason: In the interests of the amenities of the area and to comply with Policy AW10 of the Rhondda Cynon Taf Local Development Plan.