Wood Farm Cottages, Sykes LaneSaxilby
Flood Risk Assessment
Prepared By: C. Mason C.Eng. M.I.C.E
October 2015
Contents
Page No.
Section
1. Introduction
2
2. Development Description
2
3. Flood Hazard
3
4. Probability
4
5. Climate Change
5
6. Detailed Development Proposals
5
7. Residual Risks
6
8. Off-site Impacts
7
9. Flood Risk Management Measures
7
10. Conclusions
7
Appendix A – Calculations
Appendix B – Emergency Response Plan
Figures
1. Location Plan
2. Existing Site Plan
3. Proposed Site Plan
1
1.0 Introduction
1.1 The owners of Wood Farm Cottages, off Sykes Lane, Saxilby wish to convert the two
existing cottages into a single dwelling, plus a “Granny Annexe”.
1.2 Planning permission for the re-development is being sought, and Mr J Chapman is
submitting plans on behalf of the owners. He is also collating technical reports which
are needed in support of the application. This Flood Risk Assessment (FRA) has
been prepared by Clive Mason C.Eng. M.I.C.E, in accordance with the National
Planning Policy Framework (NPPF), and the accompanying Technical Guidance
document.
1.3 The assessment takes into account information from the West Lindsey District
Councils Strategic Flood Risk Assessment, (SFRA), from the Environment Agency’s
Flood Maps, from the River Trent Catchment Flood Management Plan (CFMP), from
Upper Witham Internal Drainage Board (UWIDB).
1.4 The format of the document is based on the standard Pro-Forma set out in Appendix
C of the previous practice guide to Planning Policy Statement 25 (PPS25 – now
superseded by the NPPF).
2.0 Development Description
2.1 The site location is shown on Figure 1 – Location Plan.The site currently contains 2
semi-detached cottages on the south side of a track which leads off the west side of
Sykes Lane. The cottage on the eastern side of the building is currently occupied,
whilst the one on the western side is empty, and becoming derelict (rain water pipes
are broken, there is some damage to external walls, and the garden area is
overgrown).
2.2 The cottages are relatively isolated and surrounded by agricultural land. The site is
shown as being in Flood Zone 2 on the Environment Agency’s Flood Map and on
WLDC’s Strategic Flood Risk Assessment (SFRA) map.
2.3 In Table 2 of the NPPF residential uses are classified as being “more vulnerable.”
This usage is acceptable in Flood Zone 2, without application of the Exception Test.
The proposed conversion will not change the vulnerability classifications.
2.4 When development is contemplated, the NPPF requires a Sequential Approach to be
adopted which aims to locate developments in areas of least flood risk. However,
where a site is to be re-developed the NPPF recognises that consideration of other
sites may not be appropriate as alternatives may not be available to the site owner.
Also, any potential improvements (i.e. reduction of flood risk on the site) may depend
on the re-development proceeding. These considerations apply in this case, hence
the Sequential Test is not applicable.
2.5 The conversion of the 2 cottages will result in a small increase in the total roof area,
which will give a marginal increase in the rate of run-off of surface water. More
detailed consideration of drainage is given in Section 6 and Appendix A.
2
3.0 Flood Hazard
3.1 The potential sources of flooding in the vicinity of the site have been identified as:




The Foss Dyke Canal
The River Trent (Fluvial and Tidal)
The arterial drainage system
Ground water flooding
On site drainage systems
3.2 The Foss Dyke canal is approximately 1.0km to the south of the re-development site.
It connects the River Witham (from Brayford Pool), to the River Trent (at Torksey). It
is in hydraulic continuity with the River Witham. If the banks on the north side of the
canal were overtopped, flows could be towards the site. However, the estimated 1 in
100 year (plus climate change) water level in the River Witham is 5.8m AoD. Ground
levels in some areas near the site are above 6.0m AoD. Hence, it is very unlikely that
fluvial flows from the canal would reach the site. Levels at the junction of the track
and Sykes Lane are around 7.2m AoD, and near Wood Farm, contours reach
10.00m AoD.
3.3 The River Trent is approximately 5.5 kilometre to the west of the site, where the small
village of Laughterton is protected from flooding by a system of flood defence banks.
There are “primary” banks alongside the river, and a “secondary” line of defence
further eastwards. At this point the River Trent is regarded as being “tidal” but some
Environment Agency staff are of the opinion that tidal effects are not very significant
beyond the road bridge at Gainsborough. The flood banks provide a good standard
of defence, but overtopping of the primary defences does occur. Earth banks on the
west side of the A1133 road provide protection for areas to the east. Overtopping of
the defences would be unlikely to reach the site, because of relative levels and
distances. Potential impacts of breaches are considered in Section 7 – Residual
Risks.
3.4 The arterial drainage system in the area is operated and maintained by Upper
Witham Internal Drainage Board (UWIDB). The relevant drain receiving water from
the area around the cottages is the Hardwick Drain which is approximately 90m to
the east of the cottages. Water in the Hardwick Drain can flow southwards by gravity
to discharge into the Foss Dyke Canal. However, it can also flow northwards to
connect with the Sykes Branch Drain which is then pumped into the Foss Dyke
Canal at Torksey Syke Pumping Station. The flexibility in this arterial drainage
system helps to avoid flooding of land in the vicinity of the cottages.
3.5 No evidence has been found during this assessment of flooding on the site, as a
result of groundwater. The WLDC SFRA does not identify the location as being
prone to this type of event. However, some instances of “water logging” of sub-soil in
the area have been noted by staff working for the Upper Witham IDB.
3.6 The on-site system will include rain-water harvesting, overflow pipes and if possible,
a soakaway system. Blockages in piped systems could cause temporary, shallow,
localised flooding. This could also be caused by high intensity, localised rain storms
which exceed the capacity of the on-site systems. However, “exceedance” flows and
flood water arising from on-site systems blockages will flow eastwards towards the
3
Hardwick Drain and should not cause dangerous conditions. These types of events
on small sites are more of a nuisance rather than significant flood events causing
damage.
4.0 Probability
4.1 The annual probability of the site being flooded from the Foss Dyke Canal (fluvial
sources) is less than 1% i.e. 1 in 100 years. The EAs National Assessment Map
(NeFRA) indicates that the annual probability is LOW (i.e between 0.1% and 1%
annual probability). This is allowing for the presence of flood defences.
4.2 Without flood defences, the probability of the site flooding from the River Trent would
be more than 0.5% (i.e. less than 1 in 200 years). However, the presence of the
defences result in the probability being classified as “LOW” on the EAs National
Assessment Map (NaFRA). This indicates that the annual probability of flooding is
less than 0.5%, but more than 0.1% (i.e. it is between 1 in 200 and 1 in 1000 years).
Hence, it is unlikely to flood except in extreme circumstances.
4.3 Standards of protection provided by arterial drainage systems, to agricultural land are
typically 1 in 10 to 1 in 20 years and in urban areas 1 in 50 years. However, as
identified in Section 3.6, the probability of flooding from the arterial drains is low.
4.4 Currently, the surface water run-off from the site does not connect directly to any
drainage system. It soaks into the ground. Levels are such that any surface water
flows would be to the east (towards the Hardwick Drain) via a drain on the western
boundary). Calculations in Appendix A show that even after development surface
water run-off rates and volumes will be very low, and they will be dealt with “on-site”
by interception and infiltration.
5.0 Climate Change
5.1 Climate change will increase the water levels in the tidal River Witham/Foss Dyke
Canal System, and it is likely to increase the intensity and frequency of rainfall
events.
5.2 In the Technical Guide to the NPPF, Section 11 contains information to enable the
future impacts of climate change to be taken into account. Table 4 gives allowances
for rates of relative sea level rise, and Table 5 gives sensitivity ranges for wave
height and wind speed. It also gives precautionary sensitivity ranges for peak rainfall
intensity and peak river flows.
The relevant factors from the tables are given below:Table 4 – Sea Level Risk – East of England
Years
1990-2025
Sea Level Rise 4.0
(mm/yr)
2025-2055
8.5
4
2055-2085
12.0
2085-2115
15.0
Table 5 – Precautionary Sensitivity Ranges
Peak
+5%
Rainfall
Intensity
Peak River
+10%
Flows
Wave
Height
Wind Speed
+10%
+20%
+30%
+20%
+5%
+10%
+5%
+10%
5.3 From table 4 it can be calculated that in the next 102 years, sea level rise could
amount to 1.113m. From 1990 to 2115, the rise could be 1.205m. The degree of
accuracy in these predictions (particularly in the later years) is currently the subject
of much debate, with professional/expert opinions differing considerably. Obviously,
if large increases did occur, then the level of existing defences may have to be
raised again. Currently there is a reasonable degree of freeboard for the next 30
years or so, and the Environment Agency will keep the situation under review, by
regular monitoring. Adaptive policies will be selected, as necessary, to retain similar
standards of protection to those currently enjoyed (as defined in the River Trent
CFMP).
5.4 Marginal increases in surface water run-off from the site will occur in the future as a
result of the possible 30% increase in peak rainfall intensity. However, the rates and
volumes are not significant in terms of increasing flood risk to the property or
surrounding areas. They can be easily mitigated in the design of the on-site drainage
systems (see Drainage Calculations in Appendix A).
6.0 Detailed Development Proposals
6.1 The detailed proposals are shown on Figure 5 –Proposed Site Layout.
6.2 The footprint of the cottages will be changed, and the conversion will result in an
increase in impermeable areas of 13m2 (i.e. from 179m2 to 192m2 – see Appendix A
–Calculations). Floor levels will be similar to the existing ones, but, if possible they
will be at least 150mm above surrounding ground levels, to avoid internal flooding
from localised, high intensity rainfall.
6.3 Consideration should be given to the possibility of including some degree of flood
resilience in the converted premises. Specific items should be agreed between the
owner, the architect and the Councils Building Inspectors. Potential items (such as
keeping electrical sockets well above floor level, having solid floor construction,
using appropriate wall plastering specifications, etc.) are identified in 2 publications :see Section 9 – Flood Risk Management Measures.
6.4 Existing outbuildings will be demolished, and a new garage constructed near the
south-west corner of the site. Drains and accesses will be paved with permeable
paving and gravel i.e. all in permeable materials.
5
6.5 Currently the roof water from the cottages appears to go to soakaways (although
some may be connected to the existing septic tank). When the conversion takes
place, all roof water from rainfall events up to 5mm will be intercepted by some form
of rain water harvesting (probably rain-water buts) and overflow will be directed to
soakaways. Even if the infiltration rates are slow, there is sufficient space on site to
construct soakaways in accordance with the requirements of BRE Digest 365, and
the Building Regulations. Note. If it was decided that soakaways should not be used,
flows could be balanced to required discharge rates and connected into the existing
ditch/dyke which runs along the southern boundary.
6.6 Foul sewage will be treated in a Klargester (or similar) treatment plant, with
discharge of effluent to the ditch/dyke on the southern boundary. This will be an
improvement to the existing septic tank installation. Access to the plant will be
available from the driveway to the garage and sample points will be built, before and
after the flow through the plant to enable monitoring of effluent standards.
6.7 The conversion will not have any adverse impact on adjoining land, and it will
improve the environment within the site boundary. It will also reduce the potential
impact of flooding by including flood resilience in the design.
7.0 Residual Risks
7.1 Residual risks of flooding can occur as a result of extreme rainfall events, from
failures in flood defence systems, and blockages in drainage systems. The latter can
be minimised by regular inspections and cleaning, and their impacts are usually very
localised and of relatively short duration. Resultant flooding is usually shallow.
7.2 Extreme events which could affect this location are storms with very intense rainfall,
or combined tidal/storm events which create water levels in the River Trent which
exceed the flood defence levels. Overtopping events in both the fluvial systems
(Foss Dyke and the arterial drainage system) and the tidal River Trent would be
unlikely to effect the site, because of the distances and levels involved. The systems
have appropriate standards of defence, at present, and the Catchment Flood
Management Plans (River Witham and River Trent) indicate that standards will be
retained in the future, allowing for the impacts of climate change.
7.3 If the tidal flood defences were breached, then the impacts would depend on the
location of the breach, the width of the breach, and the time taken to repair or seal it.
The small scale of this development does not justify the costs involved in carrying
out a full hydro-dynamic modelling exercise to establish the hazard rating at the site.
However, some basic calculations have been carried out to estimate the potential
Hazard Rating at the site and there are given in Appendix A – Calculations. With
shallow depth of water in the area the Hazard Rating would probably be “LOW
HAZARD” and safe access/egress would be available via the track to Sykes Lane
(where ground levels reach between 6.0 and 7.0m AOD).
7.4
Overall, the residual risks of flooding affecting this site, significantly, are small.
Issues arising from pump failure, blockages, overtopping of drains and flood
defences are negligible. There would have to be very extensive, long lengths of
breaching in the River Trent defences before impacts become significant (Note. This
6
occurred in 1795 when defence standards were much lower. Earth embankments
now provide protection along the west side of the A1133 road).
8.0 Off-site Impacts
8.1 No adverse off-site impacts will be created as a result of this conversion.
9.0 Flood Risk Management Measures
9.1 Section 18 of the Technical Guide to the NPPF gives guidance on managing residual
flood risk. However, in this instance there is no significant danger arising from the
various sources of “residual risk”. However, the prediction of flood levels is not a
precise science, as there are many variables and uncertainties involved. Hence,
even with a relatively safe site, in this location, it would be sensible to adopt a few
precautionary measures: Floor levels – should be at least 150mm above surrounding ground levels (to
avoid properties being affected by rainwater from high intensity, local storms).
 If possible the property should be connected to the EAs Flood Warning
System “Flood Warnings Direct”. This will enable warnings to be received at
various stages i.e. Flood Alert, Flood Warning and Severe Flood Warning.
 An outline Emergency Response Plan should be used – as necessary (See
Appendix B).
 Consideration should be given to the inclusion of appropriate flood resilience
measures in the construction of the building e.g. keeping electrical sockets
high using water resistant materials for floors, use of cement/lime render for
walls etc. (exact list to be determined by Architect/owner in consultation with
LA officers.) Two relevant publications are :
 Homeowners Guide to Flood Resilience – A Living Document, and
 Improving the Flood Performance of New Buildings, Flood Resilient
Construction, published by the Department of Communities and Local
Government.
9.2 The basic flood risk management items outlined above will – help to avoid the impact
of localised floods, increase awareness of potential flooding, improve preparedness
for extreme events, and limit any potential damage to property. Upstairs rooms will
provide a safe haven in the event of extreme floods.
10.0 Conclusions
10.1 As a result of this assessment the following conclusions have been reached:


The site is in Flood Zone 2 i.e. without defences the annual probability of
flooding would be between 1% and 0.1% (from rivers and the sea)
Residential development is acceptable in FZ2, without application of the
Exception Test
The conversion of the cottages will not change the vulnerability classification
(i.e. more vulnerable)
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







10.2
The actual flood risk, taking account of existing flood defences is LOW
(between 0.5% and 1 %)
The site is very unlikely to be affected by overtopping of fluvial or tidal
defences
Arterial systems in the locality provide a good standard of land drainage and
flood defence. Local systems have a degree of flexibility in the way in which
they operate. This gives added security.
The site is unlikely to be flooded from underground water sources, but some
water logging of the ground may occur when the water table is high.
The current surface water run-off infiltrates into the sub-soil. This will still
apply after development (Interception and infiltration by soakaways will be
used).
There are no residual risks which will have significant impact on the site.
However, precautionary measures will be taken to avoid localised flooding,
increase awareness and preparedness and limit potential property damage
(See Section 9.1).
Soakaways will be designed in accordance with BRE Digest 365, and in
compliance with the requirements of the building regulations. If soakaways
are not used, flows will be attenuated and discharged to the ditch on the
southern boundary of the site. Hence, a sustainable surface water system is
possible
Even in extreme events, hazards around the site will be “low” and safe
access/egress routes will be available via the track to Sykes Lane.
The overall conclusion is that this conversion will improve the environment within
the site, it will reduce the potential impact of flooding and provide an opportunity
to increase awareness and preparedness for extreme flood events. The actual
probability of flooding from all sources is LOW.
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Appendix A
Calculations
A1. Rainwater Harvesting
Volume of roof water, from converted property, in a 5mm rainfall
= 134 x 5 = 0.67m3
1000
Suggested total tank capacity = 0.75m3
A2. Areas
Before Conversion
Impermeable surfaces 179m2
Permeable surfaces 158m2
Garden/lawns 903m2
Area Conversion
Impermeable surfaces 192m2
Permeable surfaces 200m2
Gardens/lawns 848m2
A3. Surface Water run-off
Before conversion, for 20mm of rainfall in 1 hour, run off
= 179 x 20 x 1000 = 0.99l/s
1000 3600
After conversion, for a similar event, run off = 192 x 20 x 1000 = 1.07l/s
1000 3600
To balance to the original run-off, the volume required = (1.07 – 0.99) x 3.6 = 0.29m3
For a 1 in 100 year, 1 hour storm using a growth factor of 3.56, the volume required
to balance = 1.03m3 (long term = 1.03 – 0.29 = 0.74m3)
The extra volume produced as a result of climate change in a 1 in 100 year, 6 hour
storm = (192 – 179) x (150 – 115) = 13 x 35 = 0.45m3
1000
1000
Hence, total volume required to balance to pre-conversion = Annual 0.29
Long term 0.74
Climate change 0.45
1.48 – say 1.5m3
This volume could easily be achieved in MHs and pipework.
A4. Soakaways
There is ample space on the site for trench soakaways. Historically soakaway design
has been related to 1 in 10 year events. Currently, consideration should be given to
1 in 30 and 1 in 100 year events (the latter, allowing for 6 hours, and 30% increase in
rainfall intensity for climate change impacts. When this assessment was prepared,
the infiltration rate on the site was not known. However, if a 10 year event is
considered of 2 hour duration, then using a Growth Factor and M5 rainfall figures
from Table 1 in BRE Digest 365, the rainfall amount is 24.8mm. If a soakaway size of
2.0m by 2.0m, with an effective depth of 1.0m, the permeable area = 12m 2. To clear
the water in 12 hours, the required infiltration rate would be:-
9
IR = volume
Area x time
= 134 x 0.025
12 x 12 x 3600
= 3.35
518,400
=6.4 x 10-6
If half of the water was to be cleared, an IR of 3.2 x 10-6 would be required. IR rates
around these values may well be available.
A5. Residual Risks
A5.1 Overtopping from the River Trent
Bank level for 1 in 100 – 7.75m
Water level for 1 in 200 – 8.52m
0.77m
Consider 200m of bank overtopping and take tidal fluctuation as 1m/hr
Time of overtopping = 92.4 minutes (2 x 0.77 x 60)
Peak discharge rate Qp = 1.7bh 3/2
= 1.7 200 (0.77) 3/2
= 340 0.675
= 229.7 m3/s say 230m3/s
Average discharge = 115m3/s
Total volumes = 92.4 x 60 x 115m3
= 637,560m3
Available floodplain area to reach the site = 26km2 (i.e. 26,000,000m2)
Average depth = 637,560 = 0.025m
26,000,000
This is very shallow and flood water would probably not reach the site. It would tend
to pond, to greater depths in lower areas. If some water did reach the site it would be
very LOW hazard.
A5.2 Breaching of the River Trent Defences
Assume 50m breach – 2.0m deep
Q = 1.7bh 3/2
= 85.23/2
= 240m3 /s – peak discharge rate
Average discharge rate – say 120m3 /s
Length of time 1st “tide”
2 hours = 7200 secs
7,200 x 120 = 864,000m3
If available flood plain to reach the site = 26km2 = (i.e. 26,000,000m2) then
Average depth = 864,000 = 0.033m
26,000,000
If the next discharge = 4 hours = 14,200 secs = 1,728,000m 3
If area of flood plain remains the same, average depth = 0.099m – say 0.1m
Assuming the average discharge rate is maintained with a depth of 0.1m and a flood
wave width of 10,050m the velocity (v)
10
=Q
A
= 120
10,050 x 0.1
= 0.12m/s
The Hazard Rating (HR)
= (V + 0.5) d + DF where DF = Debris Factor
= (0.12 +0.5) 0.1 + 0.5
= 0.56 i.e Danger for Some
The situation is unlikely to arise, but there is an alternative route for flood waters
from the River Trent via Torksey and the Foss Dyke Canal. However, Torksey is
further from the site and although it may be an easier hydraulic route for flood water,
the hazard rating is unlikely to exceed the one calculated assuming a significant
breach near Laughterton.
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Appendix B
Emergency Flood Response Plan
B1. The ground floor level of the conversion will probably be above the water levels in
flood envelopes created by overtopping or breaching in the fluvial or tidal main river
systems.
B2. However, it is not possible to predict flood levels with a high degree of certainty,
because there are so many variables, and climatic conditions can vary significantly
from forecast values. Because of this uncertainty an emergency response “action
plan” is a sensible precautionary measure. This needs to relate to the EA’s Floodline
Warnings Direct Service, and the following is suggested:1. If possible register to receive warnings – telephone Floodline on 0845 988
1188. Currently the site is shown to be outside the area where the EA issue
warnings. However, these areas are constantly being revised and an
application to register may be favourably received.
2. Be aware of, and make use of the 3 day Flood Risk Forecast which is
available on the EA’s website. The forecast indicates which parts of the
country (i.e. counties), have High, Medium, Low or Very Low risk levels of
flooding.
3. Respond to Flood Warnings received:Flood Warning Status
Actions
Flood Alert (flooding is possible)
1. Be prepared at act on flood plan.
2. Check essential items are available
(i.e. food, water, medical kit, safe storage
facilities, warm clothes etc.).
3. Check river water levels, and flood
forecast on EA website.
Flood Warning (flooding is expected –
take immediate action)
1. Move assets/valuables to safe location
e.g. electrical appliances, family
documents, passports, insurance policies
etc.
2. If safe to do so, turn off electricity and
gas.
3. Keep informed by using local radio
and other sources.
4. If required use sandbags to restrict
water entry into building.
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Severe Flood Warning (when severe
flooding is expected which poses
significant threat to life)
1. Stay in a safe place (possibly in an
upstairs room) – but check means of
escape.
2. Avoid movements in deep flood water.
3. Co-operate with emergency services
and be ready to evacuate if necessary.
4. If in immediate danger, call 999.
5. If evacuation is not necessary, stay in
the property until flood water has
subsided from around the property.
Warnings no longer in force
1. Inspect around property – but be
careful, as conditions may still be
hazardous.
2. If applicable, ring your insurance
company as soon as possible.
3. Start clean-up operations and resume
normal activities.
4. Keep informed of current forecasts.
B3. Under the Civil Contingencies Act 2004, West Lindsey District Council and
Lincolnshire County Council are both classed at “Category 1” responders, and both
have responsibilities for risk assessment, emergency planning and warning/informing
communities at times of emergencies. The multi-agency Local Resilience Forum
(LRF) is the strategic level group for Lincolnshire, and it would co-ordinate
emergency management in the event of a major emergency.
B4. General advice, at times of severe floods is to “stay indoors” and tune in to local TV
and radio stations to find out more about what is happening. The EA Floodline
number in 0845 988 1188 and information can be obtained on current flood status
and relevant river levels.
B5. Emergency resources (i.e. food, water, medical kit etc.) should always be available
to enable a period of residence, if this proves to be necessary. This is unlikely, as
safe access/exit routes will usually be available from the properties.
B6. Particular care should be taken to ensure that residents in the Annex are kept aware
of and are prepared for possible extreme events (e.g. moving to upstairs rooms if
necessary).
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Figures
1. Location Plan
2. Existing Site Plan
3.Proposed Site Plan
14