19
P/2014/00830 Received 27/06/2014
P/2014/00830Received 27/06/2014
1
FLOOD RISK ASSESSMENT
PROPOSED SOLAR FARM
ROLLESTON PARK, TUTBURY,
STAFFORDSHIRE.
Contents: -
1. Introduction.
2. Site details and proposed development.
3. Flooding risks.
4. Surface water drainage of the proposal site.
5. Conclusions
Appendices: -
A. Location Plan and Site Layout Plans. B. Environment Agency Indicative Flood Risk Map.
C. British Geological Survey Map extract.
D. Hydrological Data
E. Infiltration estimation results.
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1.0 INTRODUCTION
1.1 This flood risk assessment has been prepared in accordance with the
recommendations contained in the National Planning Policy Framework
(NPPF) and in accordance with the East Staffordshire Borough Council Policy
requiring Flood Risk Assessments to accompany planning submissions.
2.0 SITE DETAILS AND PROPOSED DEVELOPMENT
2.1 The site is located on arable farmland addressed Rolleston Park, Tutbury,
Staffordshire. The site centroid is at National Grid Reference SK 21585
27445. Refer to Appendix A for general location and site layout plans.
2.2 The proposed development site is a generally flat plateau with steepening
gradients at the southern edge and is arable farmland. No watercourses exist
within the site curtilage.
2.3 The proposed development is to provide a Solar Farm of nominally 18m2
impermeable surfaces which will require formal drainage. The surfaces
requiring formal drainage systems are presented by the roof drainage serving
the adjacent Control Room (9m2) and the Grid Connection Cabinet (9m2).
2.4 The proposed development is to be formed by solar panels distributed evenly
over the majority of the site area. The distribution of the panels, which are not
formally drained, will not significantly concentrate rainfalls within the dense,
relatively rough grass field surfaces and the land drainage status quo will not
be measurably modified.
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3.0 FLUVIAL FLOODING RISKS
3.1 The proposed development site is not shown by the Environment Agency
Indicative Flood Maps to be within the 1:100 or 1:1000 year return period
extent of fluvial flooding concern. Refer to Appendix B for Environment
Agency Indicative Flood Risk Map.
3.2 The site area has no recorded history of flooding.
4.0 SURFACE WATER DRAINAGE OF THE PROPOSAL SITE.
4.1 It is intended to provide formal drainage for impermeable roof areas by
directing roof waters, via piped systems to shallow trench soakaways. It is
intended that the dispersed solar panels will not be formally drained and that
waters from each individual panel will fall to the grassed surface. The panels
will be sited in rows generally across the north to south slope of the site. The
site slopes nominally at an average 1:500 gradient and at this gradient it is
improbable that the runoff from each solar panel could coalesce on the
underlying grassland surface to become a meaningful surface flow before
infiltration. It is expected that the virtually un-trafficked grassland surface will
remain sufficiently permeable and to maintain the land drainage status quo.
4.2 The British Geological Survey describes the general soils of the area as loamy
and clayey soils with slightly impeded drainage overlying Fluvio Glacial
Gravels. Refer to Appendix C for map extract.
4.3 The soil classification is given as WRAP type 4 slightly permeable type soils.
Refer to Appendix D for Hydrological Data.
4
4.4 Although the NPPF mentions a preference for infiltration systems it does not
refer to a design methodology for infiltration systems other than that required
by Building Control (BRE Digest 365). Also, it is announced that National
Standards for SuDS will not take place until late 2014. Accordingly, the use of
trench soakaways has been considered using the BRE Digest 365 ‘Soakaway
design’
4.5 The nature of the site sub-soils is slightly impermeable. Although the soil type
appears to be relatively free draining data from infiltration testing on similar
soils suggest that an infiltration factor of 0.0000028m/s) has been taken to
critically represent the potential infiltration rate. CIRIA Report 156 ‘Infiltration
drainage – Manual of good practice’ also defines 0.010m/hr as appropriate for
the prevailing soil types. This rate should be confirmed by infiltration testing at
the site.
4.6 The Sub-station house has an area of 9m2 impermeable surface whilst the Grid
Connection Cabinet has an impermeable surface area of 9m2. The transformers
and inverter housings are proposed to be widely spaced and are so small as to
not require formal drainage systems. The Connection Cabinet and Control
Room are adjacent and are calculated a single area of 18m2.
4.7 Using the indicated data the soakage trench requirements for the Connection
Cabinet and Control Room have been calculated using the BRE Digest 365
methodology. The resultant soakaway trench length is 3.60m. The trench is to
be formed 0.75m wide by 1.00m depth below invert with a total depth of
1.45m. Refer to Appendix E for soakaway calculation sheets.
5.0 CONCLUSIONS
5.1 The proposal, access and egress routes are not zoned to be within any fluvial or
tidal flood risk areas.
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5.2 Surface waters discharging to infiltration systems will not alter the land
drainage status quo.
5.3 The site area is shown to be formed of slightly impermeable surface and
shallow sub-soils. If final impermeability testing indicates a requirement to
vary the critically predicted trench soakaway sizes then the sizes can be easily
modified without effect to the proposed layout. Given the soil types, any
variation in soakaway proportions will be linear only.
5.4 This Flood Risk Assessment concludes that the proposed development, utilising
surface water drainage systems to infiltration systems (soakaways) and widely
dispersed runoffs from the solar panels, as described above, cannot be
reasonably defined as being at, or may cause, harmful flood risk from 1:100
year return period rainfall events, including climate change allowances, within
the parameters given by the NPPF and the associated Technical Guidance.
…………………………………A D Rich. for A E Designs Ltd. Ref: 1333 FRA
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THE SITE
THE SITE
MasterDrainHY 8.66
A E DesignsLtd.
Consulting Civil Engineers20 Short Rd, Stretham, Ely, CB6 3LS
Tel: 01353 649002
e-mail: [email protected]
Job No.
Sheet no.
Date
Project
Title
By Checked Reviewed Rolleston Park
Hydrological data for Tutbury
1333
27/05/14
1
AR
Data:-
FSR valuesLocation = Tutbury Grid reference = SK2127
M5-60 (mm) = 19.0 SAAR (mm/yr) = 700
r = 0.40 Soil = 0.45
Long reference = 421327 Area = England and Wales
Hydrological area = 4 Hydrological zone = 1
Soil classification for WRAP type 4Clayey, or loamy over clayey soils with an impermeable layer at shallow depth.
FEH values
No FEH data available in this file.
MasterDrainHY 8.66
A E DesignsLtd.
Consulting Civil Engineers20 Short Rd, Stretham, Ely, CB6 3LS
Tel: 01353 649002
e-mail: [email protected]
Job No.
Sheet no.
Date
Project
Title
By Checked Reviewed Rolleston Park
Hydrological data for Tutbury
1333
27/05/14
2
AR
<- Soil
M5-60 ->
<- SAAR
r x 100 ->
MasterDrainSW 13.16
A E DesignsLtd.
Consulting Civil Engineers20 Short Rd, Stretham, Ely, CB6 3LSTel: 01353 649002
e-mail: [email protected]
Job No.
Sheet no.
Date
Project
Title
By Checked Reviewed Rolleston Park
BRE365 Trench calculations for Tutbury
1333
28/05/14
1
AR
Rectangular pit design data:-Pit length = 3 m Pit width = .75 mDepth below invert = 1 m Percentage voids = 42.0%Imperm. area = 18 m² Infilt. factor = 0.0000028 m/sReturn period = 100 yrs
Calculations :-Surface area of soakaway to 50% storage depth (not inc. base):-
as50 = 2 x (length + width) x depth/2 = 3.8 m²
Outflow factor : O = as50 x Infiltration rate = 0.0000104 m/s
Soakaway storage volume : Sactual = length x width x depth x %voids/100 = 0.9 m³
Duration Rainfall Inflow Depth Outflow Storage
mm/hr m³ (hmax) m m³ m³
5 mins 155.2 0.2 0.00 0.20.21
10 mins 115.9 0.3 0.01 0.30.32
15 mins 93.3 0.4 0.01 0.40.42
30 mins 61.3 0.6 0.02 0.50.53
1 hrs 38.4 0.7 0.04 0.70.74
2 hrs 23.3 0.8 0.07 0.80.85
4 hrs 13.7 1.0 0.15 0.80.85
6 hrs 10.0 1.1 0.22 0.90.95
10 hrs 6.7 1.2 0.37 0.80.85
24 hrs 3.3 1.4 0.90 0.50.53
Actual volume : Sactual = 0.9 m³
Required volume : Sreqd. = 0.9 m³
Soakaway volume storage OK.
Minimum required as50 : 3.57 m²
Actual as50 : 3.75 m²
Minimum depth required: 0.95 m
Time to maximum 6 hrs
Emptying time to 50% volume = ts50 = Sreqd x 0.5 / (as50 x Infiltration rate) = 12:00 (hr:min))
Soakaway emptying time is OK.
MasterDrainSW 13.16
A E DesignsLtd.
Consulting Civil Engineers20 Short Rd, Stretham, Ely, CB6 3LSTel: 01353 649002
e-mail: [email protected]
Job No.
Sheet no.
Date
Project
Title
By Checked Reviewed Rolleston Park
BRE365 Trench calculations for Tutbury
1333
28/05/14
2
AR
Location hydrological data (FSR):-Location = Tutbury Grid reference = SK2127M5-60 (mm) = 19 r = 0.40Soil index = 0.45 SAAR (mm/yr) = 700WRAP = 4 Area = England and Wales
Soil classification for WRAP type 4Clayey, or loamy over clayey soils with an impermeable layer at shallow depth.
N.B. The rainfall rates are calculated using the location specific values above in accordance with the Wallingford procedure.
