the common car park drainage strategy and water quality

The Common Car Park Drainage Strategy and Water Quality Management Report

Prepared for

Bourne Parking Limited BHF-AKSW-XX-XX-RP-C-0001 August 2019 Job №: S188133

Contents 1.0 Introduction .................................................................................................................................................... 1

2.0 Development Site Details .............................................................................................................................. 1

3.0 Site Drainage Strategy .................................................................................................................................. 2

4.0 Water Quality Management ........................................................................................................................... 4

Figures Figure 1 – Ordnance Survey Mapping – Source: Bing Maps

Appendices Appendix A

Appendix B

Appendix C

Appendix D

Appendix E

Appendix F

Survey Maps and Historic Information

Existing Calculations

Proposed Site Plans

Proposed Calculations

Surface Water Drainage Pro-Forma

SuDS Maintenance Schedule

Z e

Revision Amendments Prepared By Checked Date

P01 Preliminary issue DH GT 16.08.19

S188133, The Common Carpark, Hatfield Drainage Strategy and Water Quality Management Report

1

1.0 Introduction

1.1 AKS Ward have been commissioned by Bourne Parking Limited to undertake a Drainage Strategy and Water Quality Management report to support the development of a multi-story car park on the site of existing surface level car park.

2.0 Development Site Details

2.1 The site is located at grid reference TL 22470 08721 and is bounded by a one-way street ‘The

Common’ to the north, Robin Hood Lane (Parking Ingress) to the east and a delivery access road to the south which forms a junction to The common to the east and acts as site egress . The site lies in Flood Zone 1. The total site area is 0.541 Ha which is served by surface water drainage.

2.2 This document is to be read in conjunction with the WHBC, The Common Carpark Flood Risk Assessment produced by Conisbee Structural & Civil Engineers, dated 26 January 2019. The plans of the development are contained within Appendix C.

Figure 1 – Ordnance Survey Mapping – Source: Bing Maps

The Common Carpark


2

3.0 Site Drainage Strategy

3.1 Existing Surface Water The existing car park surface water is collected by gullies located around the car park. These gullies connect to an existing below ground drainage system prior to discharging into the Thames Water public surface water sewer located in the pavement.

Gullies draining the access road to the south connect to the car park drainage system prior to discharging into the Thames sewer network. All existing drainage passes through a bypass separator.

Site investigation indicates that made ground was encountered in all exploratory holes to a maximum depth of 2.45 mbgl and generally comprised of firm brown gravelly clay. Below the made ground is a Kesgrave Catchment Subgroup. Infiltration as means of disposal is not feasible on site due to made ground and impermeable subsoils below the made ground.

See Conisbee Flood Risk Assessment for full details. MicroDrainage calculations have been carried out taking into consideration the contributing areas within the development boundary. Any contributing areas upstream of the site have not been included. The existing flow rates have been calculated as follows:

• 1 year return period: 2.8 l/s

• 30 years return period: 5.7 l/s

• 100 years return period: 6.8 l/s QBar calculations provided by MicroDrainage can be found in Appendix B.

Topographical survey and site investigation are included in Appendix A.

3.2 Proposed Surface Water The proposed surface system will be designed to agree with the National Standards for Sustainable Drainage.

Refer to drainage drawings and Microdrainage calculations in Appendix C & D. Surface Water Pro-Forma has been completed with a copy contained in Appendix E to ensure that the design is in accordance with the current SuDS requirements.

The proposed surface water infrastructure will be restricted to 2.8 litres/sec (existing 1 year return period) prior to its connection onto the existing Thames Water surface water sewer.

In line with the Lead Local Flood Authority (LLFA) guidance it has been proposed to discharge at 2.8 l/s, 1 in 1 Greenfield runoff rate for all rainfall events up to and including the 1 in 100 years event inclusive of a 40% climate change allowance.

See Conisbee Flood Risk Assessment for full details.

Surface water drainage from the from the existing delivery access road to the south (does not form part of the site) is to be diverted away from car park cellular tank and connected downstream of the HydroBrake flow control device and upstream of the bypass separator, the existing drainage has a discharge rate of 7.8 l/s.


3

Existing discharge rate offsite has been calculated as 47 l/s, giving a significant reduction. Refer to existing drainage Microdrainage calculations in Appendix B.

3.3 Runoff Destination

Soil samples and infiltration tests provided by Geotechnical & Environmental Associates show poor ground conditions confirming infiltration as being an unsuitable method of disposal. Surface water drainage from the new carpark and paved areas will fall under gravity and be attenuated using a cellular attenuation system connected to a flow control device limited to 2.8 l/s prior to discharging under a gravity connection into the Thames Water surface water system.

3.4 Peak flow control Flow rates off site will be restricted using a Hydrobrake flow control device downstream of the cellular attenuation tank to a maximum flow rate of 2.8 litres/sec prior to connection to the public sewer. Discharged flow from the site will not exceed the current QBAR flow rate (1 in 1 year)

Water quality for the new car park will be utilised on site by means of a bypass separator.

3.5 Volume control A 136.80m³ cellular attenuation tank will be keep additional discharge volume on site. A new flow control on the new surface water drainage ensures no increase in offsite flow will occur therefore no increase in flood risk downstream. The runoff volume discharged for 6-hour rainfall event from the site without attenuation:

• 1-year return period: 51.231m³

• 2-year return period: 62.498m³

• 30 years return period: 113.966m³

• 100 years return period: 149.152m³

• 100 years + 40% climate change return period: 207.484m3

3.6 Flood risk within the development The system will be designed in accordance with CIRIA SuDS manual with no flooding up to and including the 100 year + 40% climate change critical storm event.

3.7 Exceedance Events In storm events exceeding the designed storm events above the 100 year + climate change the flow of water would run towards the east of the development and be retained on site.

3.8 Structural integrity and construction Surface system will be designed and constructed using approved materials in line with Building Regulation’s and current British Standards appropriate for the location and proposed use.

3.9 Maintenance and operation The drainage system will be CCTV surveyed on completion to ensure that the system is fully operational and maintenance schedules provided in the O&M manual for the owner to maintain the drainage pipework and ancillary products. Maintenance schedules have been provided in Appendix F for the SuDS. The owner of the site will be responsible for maintaining the SuDS on site.


4

3.10 Existing and Proposed Foul Water. There is no requirement for Foul Water Drainage on site.

4.0 Water Quality Management The surface system will be designed in order to not affect the water quality of the receiving watercourse.

A Class 1 bypass separator prior to connection to the existing public sewer will be utilised to ensure water quality discharge is appropriate.

CIRIA SuDS Manual 2015 Chapter 26 assigns pollution hazard indices for different land use types and SuDS mitigation index for every SuDS component depending on where the discharge is, surface or ground water. Catchpit manholes & will be installed upstream and downstream of the attenuation tank providing additional treatment for the surface water drained from the impermeable areas which will improve the water quality further.

Appendix A

Survey Maps and Historic Information

Thames Water Utilities Ltd, Property Searches, PO Box 3189, Slough SL1 4W, DX 151280 Slough 13

T 0845 070 9148 E [email protected] I www.thameswater-propertysearches.co.uk

Page 9 of 26

CommercialDW Drainage and Water Enquiry Sewer Map- CDWS/CDWS Standard/2018_3732825

The width of the displayed area is 200m

The position of the apparatus shown on this plan is given without obligation and warranty, and the accuracy cannot be guaranteed. Service pipes are not shown but their presence should be anticipated. No liability of any kind whatsoever is accepted by Thames Water for any error or omission. The actual position of mains and services must be verified and established on site before any works are undertaken. Based on the Ordnance Survey Map with the Sanction of the controller of H.M. Stationery Office, License no. 100019345 Crown Copyright Reserved.

225225

300

100 150 225

150

225

225

300225

225

225

225

225

375

225100

150

U

%

%

%

%

%%

%%

%

%

%

%

%

%%

%

%

471A

4804

4701

4706

570

02

371A

381A

3801

381B

371B

3701

3702

4604

4704

4703

4705

4702

4605

3601

3602

3603

4603

=

==!

=!

=!

=

=!

!

!

=

!

!

!

!

!

!!

!

!

!

HATFIELD

22

Bank

to

60

31

39

65

80

44

7473

72

66

78

40

86

62

Court 82.3m

Car Park

White Lion HouseSylvia Adam

s House

THE COMMON

RO

BIN

HO

OD

LAN

E

1 to 9

Alfred

1 to 16

88 to 96

41 to 45

46 to 48

Superstore

House

(Health Centre)

Bank

Bank

Bank

House

47

54

5250

Lothair

Aragon

Broomfield

El Sub Sta

St Martins

Lister House

4a

29

d House

1 to 16

Court

House



Page 10 of 26

NB. Levels quoted in metres Ordnance Newlyn Datum. The value -9999.00 indicates no survey information is available.

Manhole Reference Manhole Cover Level Manhole Invert Level

4703 4705 4704 4706 4804 4603 4604 4605 4702 4701 471A 5609 3701 3702 371A 371B 3801 381A 381B 3802 3603 3602 3601

82.06 81.96 81.98 81.69 81.7 81.73 81.72 82.22 81.95 81.97 n/a n/a 82.46 82.54 n/a n/a 81.46 n/a n/a 81.48 82.53 82.43 82.44

78.81 79.28 78.4 79.28 78.22 78.62 77.92 79.95 80.61 80.59 n/a n/a 79.21 78.89 n/a n/a 79.9 n/a n/a 80.2 79.21 78.36 78.05

The position of the apparatus shown on this plan is given without obligation and warranty, and the accuracy cannot be guaranteed. Service pipes are not shown but their presence should be anticipated. No liability of any kind whatsoever is accepted by Thames Water for any error or omission. The actual position of mains and services must be verified and established on site before any works are undertaken.



Page 12 of 26

.

Appendix B

Existing Calculations

Conisbee Page 11-5 Offord StreetIslingtonLondon N1 1DHDate 25/02/2019 17:57 Designed by tapiwa.gavazaFile Checked byInnovyze Source Control 2017.1.2

ICP SUDS Mean Annual Flood

©1982-2017 XP Solutions

Input

Return Period (years) 100 Soil 0.300Area (ha) 0.541 Urban 0.750SAAR (mm) 688 Region Number Region 5

Results l/s

QBAR Rural 1.0QBAR Urban 3.2

Q100 years 6.8

Q1 year 2.8Q30 years 5.7Q100 years 6.8

AKSWard Page 1

Seacourt Tower

West Way

Oxford

Date 16/08/2019 12:13 Designed by daniel.harwood

File EXISTING_P01.MDX Checked by

Micro Drainage Network 2018.1.1

Time Area Diagram for Existing

©1982-2018 Innovyze

Time

(mins)

Area

(ha)

Time

(mins)

Area

(ha)

0-4 0.320 4-8 0.094

Total Area Contributing (ha) = 0.414

Total Pipe Volume (m³) = 8.821

Simulation Criteria for Existing

Volumetric Runoff Coeff 0.750 Additional Flow - % of Total Flow 0.000

Areal Reduction Factor 1.000 MADD Factor * 10m³/ha Storage 2.000

Hot Start (mins) 0 Inlet Coeffiecient 0.800

Hot Start Level (mm) 0 Flow per Person per Day (l/per/day) 0.000

Manhole Headloss Coeff (Global) 0.500 Run Time (mins) 5

Foul Sewage per hectare (l/s) 0.000 Output Interval (mins) 1

Number of Input Hydrographs 0 Number of Storage Structures 0

Number of Online Controls 0 Number of Time/Area Diagrams 0

Number of Offline Controls 0 Number of Real Time Controls 0

Synthetic Rainfall Details

Rainfall Model FSR Profile Type Summer

Return Period (years) 30 Cv (Summer) 0.750

Region England and Wales Cv (Winter) 0.840

M5-60 (mm) 20.000 Storm Duration (mins) 30

Ratio R 0.438

AKSWard Page 2

Seacourt Tower

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Oxford




1 year Return Period Summary of Critical Results by Maximum Level (Rank 1)

for Existing


Simulation Criteria

Areal Reduction Factor 1.000 Additional Flow - % of Total Flow 0.000

Hot Start (mins) 0 MADD Factor * 10m³/ha Storage 2.000

Hot Start Level (mm) 0 Inlet Coeffiecient 0.800

Manhole Headloss Coeff (Global) 0.500 Flow per Person per Day (l/per/day) 0.000

Foul Sewage per hectare (l/s) 0.000





Rainfall Model FSR Ratio R 0.438

Region England and Wales Cv (Summer) 0.750

M5-60 (mm) 20.000 Cv (Winter) 0.840

Margin for Flood Risk Warning (mm) 300.0 DVD Status OFF

Analysis Timestep Fine Inertia Status OFF

DTS Status ON

Profile(s) Summer and Winter

Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080

Return Period(s) (years) 1, 30, 100

Climate Change (%) 0, 0, 0

PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)

E1.000 ESW02 15 Winter 1 +0% 30/15 Summer 80.546

E2.000 ESW06 15 Winter 1 +0% 30/15 Winter 81.250







E1.002 EBypass 15 Winter 1 +0% 30/15 Summer 80.018

E1.003 EMH3701 15 Winter 1 +0% 79.383

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

E1.000 ESW02 -0.169 0.000 0.13 6.1 OK

E2.000 ESW06 -0.110 0.000 0.16 3.1 OK

E2.001 ESW07 -0.091 0.000 0.32 5.2 OK

AKSWard Page 3

Seacourt Tower

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for Existing


E2.002 ESW08 -0.087 0.000 0.37 8.3 OK

E2.003 ESW04 -0.177 0.000 0.10 8.3 OK

E3.000 ESW05 -0.175 0.000 0.11 5.6 OK

E2.004 ESW03 -0.101 0.000 0.54 25.5 OK

E1.001 ESW01 0.071 0.000 1.61 48.1 SURCHARGED

E1.002 EBypass -0.097 0.000 0.62 49.8 OK

E1.003 EMH3701 -0.277 0.000 0.32 49.7 OK

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

AKSWard Page 4

Seacourt Tower

West Way

Oxford





for Existing


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080



PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)










E1.003 EMH3701 15 Winter 30 +0% 79.472

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

E1.000 ESW02 0.407 0.000 0.29 13.2 SURCHARGED

E2.000 ESW06 0.061 0.000 0.39 7.7 SURCHARGED

E2.001 ESW07 0.210 0.000 0.80 12.8 SURCHARGED

AKSWard Page 5

Seacourt Tower

West Way

Oxford





for Existing


E2.002 ESW08 0.304 0.000 0.93 21.0 SURCHARGED

E2.003 ESW04 0.318 0.000 0.29 23.7 SURCHARGED

E3.000 ESW05 0.352 0.000 0.23 11.6 SURCHARGED

E2.004 ESW03 0.891 0.000 1.07 50.7 SURCHARGED

E1.001 ESW01 0.916 0.000 3.22 96.1 SURCHARGED

E1.002 EBypass 0.421 0.000 1.24 100.5 SURCHARGED

E1.003 EMH3701 -0.188 0.000 0.64 100.5 OK

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

AKSWard Page 6

Seacourt Tower

West Way

Oxford




100 year Return Period Summary of Critical Results by Maximum Level (Rank

1) for Existing


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080



PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)










E1.003 EMH3701 15 Winter 100 +0% 79.500

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

E1.000 ESW02 0.991 0.000 0.35 16.0 FLOOD RISK

E2.000 ESW06 0.707 0.000 0.42 8.3 FLOOD RISK

E2.001 ESW07 0.856 0.000 0.98 15.9 FLOOD RISK

AKSWard Page 7

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1) for Existing


E2.002 ESW08 0.952 0.000 1.08 24.3 FLOOD RISK

E2.003 ESW04 0.953 0.000 0.36 29.0 FLOOD RISK

E3.000 ESW05 0.988 0.000 0.27 13.7 FLOOD RISK

E2.004 ESW03 1.516 0.000 1.23 58.3 SURCHARGED

E1.001 ESW01 1.491 0.000 3.74 111.6 SURCHARGED

E1.002 EBypass 0.809 0.000 1.44 116.6 SURCHARGED

E1.003 EMH3701 -0.160 0.000 0.74 116.5 OK

PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

.

Appendix C

Proposed Site Plans

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex S

W

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex S

W

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

Ex SW

D

o

w

n

U

P

I

N

O

U

T

L

I

F

T

R

I

S

E

R

F

F

L

8

2

.

6

8

5

V

B

V

B

V

B

V

B

S

t

a

i

r

2

E

X

I

T

U

P

F

a

l

l

1

:

6

0

F

a

l

l

1

:

6

0

F

a

l

l

1

:

6

0

F

a

l

l

1

:

6

0

P

P

P

P

R

i

s

e

1

:

1

0

R

i

s

e

1

:

1

0

P

P

P

P

E

x

i

s

t

i

n

g

k

e

r

b

l

i

n

e

ST01

ST03

ST06

82.59

82.56

82.53

82.51

82.48

82.43

82.38

82.42

82.36

82.35

82.32

82.30

82.29

82.27

82.20

82.15

82.13

82.06

82.08

82.11

82.12

82.17

82.17

82.22

82.23

82.28

82.30

82.33

82.36

82.37

82.39

82.42

82.50

82.54

82.52

82.50

82.35

82.36

82.34

82.23

82.20

81.86

81.90

81.92

81.98

81.99

82.00

82.01

82.04

82.07

82.09

SI

82.09

82.11

82.12

82.16

82.18

SI

82.17

82.26

82.34

82.35

82.33

82.33

82.30

82.28

82.24

82.23

82.21

82.20

82.19

82.16

82.13

82.12

82.11

82.07

82.20

82.21

82.24

82.27

82.29

82.31

82.33

82.37

82.40

82.44

82.42

82.38

82.39

82.34

82.32

82.31

82.27

82.25

82.23

82.21

82.19

82.17

82.13

82.14

82.13

82.10

SI

82.02

82.03

82.06

82.07

82.09

82.11

82.11

82.12

82.17

82.17

82.17

82.20

BO

BO

BO

SI

SI

SI

SI

SI

SI

SI

82.17

82.19

82.20

82.20

82.23

82.26

82.24

82.28

82.26

82.31

82.32

82.35

82.36

82.38

BO

BO

BO

82.30

82.31

82.30

82.25

82.23

82.20

82.12

81.99

82.05

82.11

82.14

82.17

82.21

82.17

82.16

82.27

82.33

82.37

82.42

82.42

82.44

82.48

82.41

82.35

82.33

82.33

82.27

82.22

82.19

82.32

82.37

82.48

82.47

82.50

82.50

82.56

82.56

82.54

82.54

82.53

82.50

82.42

82.31

82.27

82.27

82.22

82.28

82.25

82.23

82.12

82.04

82.16

82.12

82.17

82.18

82.20

82.27

82.38

82.31

82.10

82.21

82.32

82.38

82.44

82.51

82.53

82.47

82.43

82.39

82.31

82.26

82.33

82.35

82.33

82.19

82.22

8

2

.

2

6

82.2

8

82.28

82.37

82.35

82.37

82.41

82.43

82.47

82.47

82.44

82.39

82.33

82.27

82.22

82.16

82.10

82.11

82.03

82.03

82.00

82.06

82.07

82.25

82.25

82.26

82.33

82.38

82.30

82.42

82.48

82.54

82.55

82.57

82.55

82.52

82.53

82.46

82.32

82.29

82.25

82.17

FH

SV

82.30

82.36

82.40

82.36

82.31

GV

GV

GV

82.33

SI

82.79

82.81

82.84

LP

82.49

82.69

82.64

82.49

82.46

82.49

82.13

82.42

82.49

82.65

82.54

82.56

82.59

82.61

82.63

82.66

82.66

82.72

82.72

82.71

82.73

82.82

82.72

82.68

82.62

82.63

82.59

82.68

8

2

.

5

3

82.50

8

2

.

4

5

8

2

.

4

3

8

2

.

4

0

82.34

82.29

82.41

82.48

82.50

82.53

82.57

82.57

C

L

8

2

.

6

1

M

H

82.59

82.65

82.71

82.75

82.78

82.80

ST04

E

L

8

7

.

9

4

E

L

8

7

.

9

1

E

L

8

7

.

7

4

E

L

8

7

.

7

7

E

L

8

7

.

7

2

R

L

8

9

.

4

8

R

L

8

9

.

4

9

R

L

8

9

.

9

9

E

L

9

4

.

2

6

E

L

9

5

.

0

9

E

L

9

2

.

9

5

81.96

81.98

82.00

8

2

.

0

6

82.04

82.10

82.09

82.08

82.10

82.10

82.08

82.18

82.15

8

2

.

1

4

82.14

82.17

82.21

82.05

82.10

82.07

82.06

82.00

82.01

82.11

82.14

82.11

82.15

82.16

82.19

82.11

82.03

82.07

82.10

82.23

82.17

82.15

82.24

82.29

82.37

SI

SI

82.01

82.04

82.03

82.13

82.13

82.15

82.07

82.08

82.02

82.04

82.08

82.10

82.11

82.07

82.00

81.95

81.90

82.12

82.14

82.10

82.06

82.09

82.06

82.06

82.18

82.12

8

2

.

1

4

82.10

82.13

SI

TC

82.12

82.19

TC

82.18

82.17

82.17

82.11

82.08

82.09

82.05

82.04

82.02

82.02

81.99

GV

TC

81.95

82.01

82.09

82.13

82.15

82.17

82.16

82.16

82.15

E

L

8

6

.

3

9

E

L

8

5

.

5

5

E

L

8

5

.

5

6

LP

81.83

81.88

81.97

82.07

8

2

.

0

6

8

1

.

9

9

8

1

.

9

3

8

1

.

8

8

81.84

81.92

82.28

82.28

82.36

82.38

82.38

82.38

82.32

82.32

82.52

82.46

82.39

82.45

82.52

82.57

82.54

82.77

82.59

BH

8

2

.

4

4

82.46

82.39

82.38

82.34

82.35

82.39

82.43

82.27

82.45

LP

82.38

82.33

82.36

82.33

82.36

82.36

82.37

8

2

.

2

2

82.22

82.45

82.46

82.50

82.50

82.41

82.35

8

2

.

4

5

WB

82.02

82.01

82.07

82.07

ER

82.27

82.35

82.38

82.37

81.96

81.93

81.92

81.92

ER

BO

BO

BO

82.29

82.34

82.26

82.24

82.17

82.15

82.01

82.06

82.11

82.20

82.27

82.16

81.99

82.21

82.05

82.02

82.08

82.11

82.14

82.13

82.12

82.10

82.09

82.14

CL 82.14

82.14

82.14

TC

LP

GV

82.08

82.09

82.17

82.10

81.99

81.97

8

2

.

0

0

8

2

.

0

4

82.13

EL 90.25

81.95

81.94

81.90

81.94

81.91

81.95

82.02

8

2

.

0

3

8

2

.

0

4

82.01

82.00

82.02

82.04

82.07

GV

TC

GV

GV

TC

LP

DP

82.10

82.08

82.08

DP

82.10

DP

82.09

DP

82.11

DP

82.09

DP

82.07

82.03

81.89

81.95

81.98

82.01

82.06

82.06

82.11

82.16

GV

81.96

82.05

82.16

82.09

82.08

81.88

8

1

.

9

7

C

L

8

1

.

9

8

I

C

CL 82.16

IC

82.14

82.16

82.23

82.19

81.99

82.02

82.24

82.25

82.07

82.06

82.07

82.05

82.02

BO

BO

BO

BO

BO

BO

BO

BO

BO

BO

BO

82.24

82.20

82.17

82.15

82.18

82.15

82.07

82.08

8

2

.

1

7

82.11

82.04

81.85

81.98

82.12

8

2

.

1

0

81.95

82.16

E

L

8

6

.

8

4

R

L

8

8

.

9

4

E

L

9

3

.

0

4

E

L

9

1

.

3

4

E

L

9

3

.

0

9

E

L

9

0

.

2

8

E

L

9

0

.

3

1

E

L

8

5

.

0

6

E

L

8

7

.

5

8

E

L

8

8

.

1

9

E

L

8

5

.

0

6

E

L

8

6

.

8

9

E

L

9

5

.

0

9

E

L

9

7

.

3

0

E

L

8

6

.

9

0

82.14

82.11

82.11

82.09

82.08

82.08

82.02

82.17

82.17

8

2

.

1

6

82.21

8

2

.

2

3

82.23

82.16

82.19

82.21

82.12

82.20

82.16

82.19

82.18

SI

82.19

82.08

82.08

82.11

82.03

82.04

82.07

82.13

82.10

82.11

82.04

82.09

81.92

82.11

82.05

82.06

82.01

82.12

82.13

82.14

82.03

82.00

81.98

82.01

O

N

L

Y

A

H

E

A

D

82.41

82.43

82.43

82.44

82.30

82.27

82.30

O

N

L

Y

A

H

E

A

D

82.34

TC

MH

A

/

C

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n

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R

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R

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L

9

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3

2

E

L

8

4

.

1

1

82.12

E

L

9

0

.

2

5

82.47

82.26

MH SW03

82.17

MH SW03

82.30

MH SW01

82.03

82.73

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Ex. SW03

Ex. SW05

Ex. SW02

Ex. SW01

Ex. SW04

Ex. SW08

Ex. SW07

Ex. SW06

MH 3701

Divert existing SW sewer - connectto existing manhole SW03

225Ø 1:100

225Ø 1:150

Clereflo bypass separator:Model CNSB6s/21IL (In) = 79.83IL (Out) = 79.78

Contractor to confirm Invert Level of existing SW sewerprior to commencement of any works, level showninterpolated from available information

Flow control Chamber - 2.8l/s

S07

SW

SW

SW

SW

SW

SW

SW

SW

SW

S08

S09

S06

S02

SW

S01

SW

S03

SW

S05

SW

SW

SW

SW

SW

SW

S04

SW

SW

SW

SWSW

SW

SW

SW

SW

SW

SW

SW

Cellular Attenuation system24m x 7.5m x 0.8m Deep720 Crates with 136.80m3 volume1:240 Fall along length of tank

Top of tank 80.95mBase of tank 80.15m

SW

SW

SW

SW

Top of tank 80.85mBase of tank 80.05m

225Ø

225Ø 1:100

225Ø 1:10

2 x 150Ø

225Ø 1:100

225Ø 1:6.7

225Ø 1:150

225Ø1:100

225Ø 1:1

00

2 x 150Ø

1600*

1600*

1600*

1600*

* Denotes extent of 6F2 - See 9252

450Ø Public Surface Water Sewer

Duct for alarm & data fromseparator to column

SB

SB

B9252

A9252

SB

SB

SWSW

Existing bypass separatorto be removed

SW

SW

IL = 79.42

P01 Preliminary Issue DH GT 21.05.19

PRELIMINARY S2

Drainage Layout

GT 21.05.19

1:200

XX XX C 9201 P01

Seacourt Tower

West Way

Oxford

OX2 0JJ

Tel: 01865 240071

Fax: 01865 248006

e-mail: [email protected]

web: www.aksward.com

AmendmentRev. DateChkd

GENERAL NOTES

Scales at A1 Project No.

Originator Level Type Drg No. Rev.

Reviewed Scheme

Reviewed Final

Date

Date

Title

Project

Client

Drg. Status

Drn

This drawing is the property of AKSWard Limited. The drawing is issuedon the condition that it is not copied, reproduced, retained or disclosedto any unauthorised person, either wholly or in part without the writtenconsent of AKSWard Limited.Do NOT scale from this drawing. AKSWard takes no responsibility forerrors during photographic reproduction or printing. Any discrepancy'sare to be reported to the engineer immediately.

Project Ref. Zone Role

- - - - - _-DR

Suitability

AKSW

Southampton

Oxford

London

Hitchin

Birmingham

Bourne Parking Limited

The Common Carpark

Hatfield

S188133

BHF

IDENTIFIES RISKS DURING THECONSTRUCTION PROCESS ON THEDRAWINGS:

NOTE: The list below and notes on the drawing identifyrisks which are deemed to be unusual, abnormal, residualor unexpected to a competent contractor carrying out theworks. These notes relate to risks which we have beenunable to design out.

1. All setting out to be in accordance with the Architects drawings. Any

discrepancies between the Engineers and the Architects drawings to

be referred to the Architect before proceeding. Dimensions must not

be scaled.

2. All drainage to be installed in accordance with relevant Building

Regulations documents and Current Sewers for Adoption where

applicable.

3. Connections to Public sewers to be agreed and inspected by Water

Authority.

4. Invert level, size and cover levels to existing manholes and sewers to

be checked prior to any construction. Any discrepancies to be

reported immediately.

5. Invert to base of soil stack bends to be 450mm below lowest branch

connection for up to 3 storeys buildings. For buildings up to 5 storeys

the invert to base of soil stack bends should be not less than 750mm.

6. All RWP and Foul Water drain point setting out is to be confirmed by

Architect.

7. All RWP & SVP sizes & setting out by Architect / M&E Engineer. All

below ground connections to match above ground outlet size, Min

100/110mm diameter.

8. Foul drains to project 100mm above finished floor level.

9. All internal Manholes and Inspection Chambers to have double sealed

recessed covers to suit floor finishes by Architect.

10. All external covers in footpaths and roads in non tarmac areas to have

recessed trays to suit the paving material.

11. Refer to drainage specification for pipe materials.

12. All pipework to be 100/110Ø UNO. Refer to note 7 connection sizes.

13. All foul and surface water drainage stacks to have above ground

rodding access, refer to above ground drainage layout by others.

14. This drawing has been produced in colour and should be reproduced

in colour for clarity.

15. A CCTV Survey and report in WINCAN format for all new drainage

will be required before the "As Built" drawings will be issued.

Key

Storm Concrete Manhole

SW

Ex FW

Ex SW

Ex FW

Ex SW

Cellular Attenuation system

Sump BoxSB

New Surface Water Sewer

New Linear Drainage System

Existing Drainage

Existing Manholes

Existing Foul Sewer

Existing Surface Water Sewer

Existing Sewers to be abandoned and

grouted up either end

Appendix D

Proposed Calculations

AKSWard Page 1

Seacourt Tower MSCP Hatfield

West Way Hertfordshire

Oxford

Date 21/05/2019 Designed by DH

File PROPOSED_P02.MDX Checked by GT


STORM SEWER DESIGN by the Modified Rational Method

Design Criteria for Storm


Pipe Sizes STANDARD Manhole Sizes STANDARD

FSR Rainfall Model - England and Wales

Return Period (years) 30 PIMP (%) 100

M5-60 (mm) 20.100 Add Flow / Climate Change (%) 0

Ratio R 0.446 Minimum Backdrop Height (m) 0.200

Maximum Rainfall (mm/hr) 50 Maximum Backdrop Height (m) 1.500

Maximum Time of Concentration (mins) 30 Min Design Depth for Optimisation (m) 1.000

Foul Sewage (l/s/ha) 0.000 Min Vel for Auto Design only (m/s) 1.00

Volumetric Runoff Coeff. 0.750 Min Slope for Optimisation (1:X) 150

Designed with Level Soffits

Time Area Diagram for Storm

Time

(mins)

Area

(ha)

Time

(mins)

Area

(ha)

0-4 0.234 4-8 0.062



Simulation Criteria for Storm















Ratio R 0.446

AKSWard Page 2



Oxford




Online Controls for Storm


Hydro-Brake® Optimum Manhole: S06, DS/PN: S1.006, Volume (m³): 5.8

Unit Reference MD-SHE-0079-2800-1020-2800

Design Head (m) 1.020

Design Flow (l/s) 2.8

Flush-Flo™ Calculated

Objective Minimise upstream storage

Application Surface

Sump Available Yes

Diameter (mm) 79

Invert Level (m) 79.929

Minimum Outlet Pipe Diameter (mm) 100

Suggested Manhole Diameter (mm) 1200

Control Points Head (m) Flow (l/s)

Design Point (Calculated) 1.020 2.8

Flush-Flo™ 0.306 2.8

Kick-Flo® 0.635 2.3

Mean Flow over Head Range - 2.5

The hydrological calculations have been based on the Head/Discharge relationship for the

Hydro-Brake® Optimum as specified. Should another type of control device other than a

Hydro-Brake Optimum® be utilised then these storage routing calculations will be

invalidated

Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s)

0.100 2.3 1.200 3.0 3.000 4.6 7.000 6.9

0.200 2.7 1.400 3.2 3.500 5.0 7.500 7.1

0.300 2.8 1.600 3.4 4.000 5.3 8.000 7.3

0.400 2.8 1.800 3.6 4.500 5.6 8.500 7.5

0.500 2.7 2.000 3.8 5.000 5.9 9.000 7.7

0.600 2.4 2.200 4.0 5.500 6.1 9.500 7.9

0.800 2.5 2.400 4.2 6.000 6.4

1.000 2.8 2.600 4.3 6.500 6.6

AKSWard Page 3



Oxford




Storage Structures for Storm


Cellular Storage Manhole: S06, DS/PN: S1.006

Invert Level (m) 80.150 Safety Factor 2.0

Infiltration Coefficient Base (m/hr) 0.00000 Porosity 0.95

Infiltration Coefficient Side (m/hr) 0.00000

Depth (m) Area (m²) Inf. Area (m²) Depth (m) Area (m²) Inf. Area (m²)

0.000 177.2 187.5 0.801 0.0 239.5

0.800 177.2 239.5

AKSWard Page 4



Oxford





for Storm


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080



PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)

S1.000 S01 15 Winter 1 +0% 100/15 Summer 81.184

S1.001 Sj1 15 Winter 1 +0% 81.165

S1.002 Sj2 15 Winter 1 +0% 81.123

S1.003 Sj3 15 Winter 1 +0% 81.084

S1.004 S02 15 Winter 1 +0% 100/120 Winter 81.012

S1.005 S03 15 Winter 1 +0% 100/15 Summer 80.381

S2.000 S04 15 Winter 1 +0% 100/15 Summer 81.202

S2.001 Sj4 15 Winter 1 +0% 81.162

S2.002 Sj5 15 Winter 1 +0% 81.120

S2.003 Sj6 15 Winter 1 +0% 81.085

S2.004 S05 15 Winter 1 +0% 100/120 Winter 81.010

S1.006 S06 60 Winter 1 +0% 1/15 Summer 80.271

S1.007 S08 15 Summer 1 +0% 79.924

S1.008 Sj7 240 Winter 1 +0% 79.826

S1.009 S09 15 Winter 1 +0% 79.770

S1.010 Sj8 30 Winter 1 +0% 79.550

AKSWard Page 5



Oxford





for Storm


PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

S1.000 S01 -0.161 0.000 0.18 5.2 OK

S1.001 Sj1 -0.153 0.000 0.22 6.8 OK*

S1.002 Sj2 -0.147 0.000 0.26 8.5 OK*

S1.003 Sj3 -0.133 0.000 0.35 10.6 OK*

S1.004 S02 -0.167 0.000 0.15 16.0 OK

S1.005 S03 -0.218 0.000 0.17 15.7 OK

S2.000 S04 -0.143 0.000 0.28 8.8 OK

S2.001 Sj4 -0.133 0.000 0.34 11.1 OK*

S2.002 Sj5 -0.122 0.000 0.43 13.4 OK*

S2.003 Sj6 -0.107 0.000 0.54 16.1 OK*

S2.004 S05 -0.166 0.000 0.16 22.1 OK

S1.006 S06 0.117 0.000 0.09 2.8 SURCHARGED

S1.007 S08 -0.189 0.000 0.06 2.8 OK

S1.008 Sj7 -0.179 0.000 0.09 2.8 OK*

S1.009 S09 -0.196 0.000 0.04 2.8 OK

S1.010 Sj8 -0.179 0.000 0.09 2.8 OK*

AKSWard Page 6



Oxford





for Storm


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080



PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)

S1.000 S01 15 Winter 30 +0% 100/15 Summer 81.225

S1.001 Sj1 15 Winter 30 +0% 81.216

S1.002 Sj2 15 Winter 30 +0% 81.186

S1.003 Sj3 15 Winter 30 +0% 81.169

S1.004 S02 15 Winter 30 +0% 100/120 Winter 81.057

S1.005 S03 120 Winter 30 +0% 100/15 Summer 80.555

S2.000 S04 15 Winter 30 +0% 100/15 Summer 81.337

S2.001 Sj4 15 Winter 30 +0% 81.295

S2.002 Sj5 15 Winter 30 +0% 81.242

S2.003 Sj6 30 Winter 30 +0% 81.192

S2.004 S05 15 Winter 30 +0% 100/120 Winter 81.051

S1.006 S06 120 Winter 30 +0% 1/15 Summer 80.553

S1.007 S08 960 Winter 30 +0% 79.924

S1.008 Sj7 15 Winter 30 +0% 79.826

S1.009 S09 30 Summer 30 +0% 79.770

S1.010 Sj8 15 Summer 30 +0% 79.550

AKSWard Page 7



Oxford





for Storm


PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

S1.000 S01 -0.120 0.000 0.43 12.9 OK

S1.001 Sj1 -0.103 0.000 0.56 17.5 OK*

S1.002 Sj2 -0.084 0.000 0.69 22.2 OK*

S1.003 Sj3 -0.048 0.000 0.95 28.4 OK*

S1.004 S02 -0.122 0.000 0.42 45.0 OK

S1.005 S03 -0.044 0.000 0.14 13.1 OK

S2.000 S04 -0.008 0.000 0.66 20.8 OK

S2.001 Sj4 0.000 0.000 0.84 27.1 SURCHARGED*

S2.002 Sj5 0.000 0.000 1.05 33.1 SURCHARGED*

S2.003 Sj6 0.000 0.000 1.02 30.5 SURCHARGED*

S2.004 S05 -0.124 0.000 0.40 56.9 OK

S1.006 S06 0.399 0.000 0.09 2.8 SURCHARGED

S1.007 S08 -0.189 0.000 0.06 2.8 OK

S1.008 Sj7 -0.179 0.000 0.09 2.8 OK*

S1.009 S09 -0.196 0.000 0.04 2.8 OK

S1.010 Sj8 -0.179 0.000 0.09 2.8 OK*

AKSWard Page 8



Oxford





1) for Storm


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080



PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)

S1.000 S01 240 Winter 100 +40% 100/15 Summer 81.614

S1.001 Sj1 180 Winter 100 +40% 81.319

S1.002 Sj2 240 Winter 100 +40% 81.270

S1.003 Sj3 240 Winter 100 +40% 81.217

S1.004 S02 240 Winter 100 +40% 100/120 Winter 81.612

S1.005 S03 240 Winter 100 +40% 100/15 Summer 81.611

S2.000 S04 15 Winter 100 +40% 100/15 Summer 81.790

S2.001 Sj4 240 Winter 100 +40% 81.295

S2.002 Sj5 240 Winter 100 +40% 81.242

S2.003 Sj6 240 Winter 100 +40% 81.192

S2.004 S05 240 Winter 100 +40% 100/120 Winter 81.611

S1.006 S06 240 Winter 100 +40% 1/15 Summer 81.609

S1.007 S08 240 Winter 100 +40% 79.929

S1.008 Sj7 240 Winter 100 +40% 79.831

S1.009 S09 240 Winter 100 +40% 79.774

S1.010 Sj8 240 Winter 100 +40% 79.555

AKSWard Page 9



Oxford





1) for Storm


PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

S1.000 S01 0.269 0.000 0.14 4.2 SURCHARGED

S1.001 Sj1 0.000 0.000 0.22 7.0 SURCHARGED*

S1.002 Sj2 0.000 0.000 0.22 7.1 SURCHARGED*

S1.003 Sj3 0.000 0.000 0.30 9.1 SURCHARGED*

S1.004 S02 0.434 0.000 0.13 14.2 SURCHARGED

S1.005 S03 1.012 0.000 0.15 13.9 SURCHARGED

S2.000 S04 0.445 0.000 1.17 37.0 SURCHARGED

S2.001 Sj4 0.000 0.000 0.28 9.1 SURCHARGED*

S2.002 Sj5 0.000 0.000 0.36 11.2 SURCHARGED*

S2.003 Sj6 0.000 0.000 0.46 13.7 SURCHARGED*

S2.004 S05 0.435 0.000 0.14 19.4 SURCHARGED

S1.006 S06 1.455 0.000 0.12 3.5 SURCHARGED

S1.007 S08 -0.184 0.000 0.08 3.5 OK

S1.008 Sj7 -0.174 0.000 0.12 3.5 OK*

S1.009 S09 -0.192 0.000 0.05 3.5 OK

S1.010 Sj8 -0.174 0.000 0.12 3.5 OK*

AKSWard Page 1



Oxford


File PROPOSED_P02 - (100 YEA... Checked by GT


STORM SEWER DESIGN by the Modified Rational Method

Design Criteria for Storm


Pipe Sizes STANDARD Manhole Sizes STANDARD

FSR Rainfall Model - England and Wales

Return Period (years) 30 PIMP (%) 100

M5-60 (mm) 20.100 Add Flow / Climate Change (%) 0

Ratio R 0.446 Minimum Backdrop Height (m) 0.200

Maximum Rainfall (mm/hr) 50 Maximum Backdrop Height (m) 1.500

Maximum Time of Concentration (mins) 30 Min Design Depth for Optimisation (m) 1.000

Foul Sewage (l/s/ha) 0.000 Min Vel for Auto Design only (m/s) 1.00

Volumetric Runoff Coeff. 0.750 Min Slope for Optimisation (1:X) 150

Designed with Level Soffits

Time Area Diagram for Storm

Time

(mins)

Area

(ha)

Time

(mins)

Area

(ha)

0-4 0.234 4-8 0.062



Simulation Criteria for Storm















Ratio R 0.446

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Online Controls for Storm


Hydro-Brake® Optimum Manhole: S06, DS/PN: S1.006, Volume (m³): 5.8

Unit Reference MD-SHE-0079-2800-1020-2800

Design Head (m) 1.020

Design Flow (l/s) 2.8

Flush-Flo™ Calculated

Objective Minimise upstream storage

Application Surface

Sump Available Yes

Diameter (mm) 79

Invert Level (m) 79.929

Minimum Outlet Pipe Diameter (mm) 100

Suggested Manhole Diameter (mm) 1200

Control Points Head (m) Flow (l/s)

Design Point (Calculated) 1.020 2.8

Flush-Flo™ 0.306 2.8

Kick-Flo® 0.635 2.3

Mean Flow over Head Range - 2.5

The hydrological calculations have been based on the Head/Discharge relationship for the

Hydro-Brake® Optimum as specified. Should another type of control device other than a

Hydro-Brake Optimum® be utilised then these storage routing calculations will be

invalidated

Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s) Depth (m) Flow (l/s)

0.100 2.3 1.200 3.0 3.000 4.6 7.000 6.9

0.200 2.7 1.400 3.2 3.500 5.0 7.500 7.1

0.300 2.8 1.600 3.4 4.000 5.3 8.000 7.3

0.400 2.8 1.800 3.6 4.500 5.6 8.500 7.5

0.500 2.7 2.000 3.8 5.000 5.9 9.000 7.7

0.600 2.4 2.200 4.0 5.500 6.1 9.500 7.9

0.800 2.5 2.400 4.2 6.000 6.4

1.000 2.8 2.600 4.3 6.500 6.6

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Storage Structures for Storm


Cellular Storage Manhole: S06, DS/PN: S1.006

Invert Level (m) 80.150 Safety Factor 2.0

Infiltration Coefficient Base (m/hr) 0.00000 Porosity 0.95

Infiltration Coefficient Side (m/hr) 0.00000

Depth (m) Area (m²) Inf. Area (m²) Depth (m) Area (m²) Inf. Area (m²)

0.000 177.2 187.5 0.801 0.0 239.5

0.800 177.2 239.5

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1) for Storm


Simulation Criteria












M5-60 (mm) 20.000 Cv (Winter) 0.840



DTS Status ON


Duration(s) (mins) 15, 30, 60, 120, 180, 240, 360, 480, 600,

720, 960, 1440, 2160, 2880, 4320, 5760,

7200, 8640, 10080

Return Period(s) (years) 100

Climate Change (%) 0

PN

US/MH

Name Storm

Return

Period

Climate

Change

First (X)

Surcharge

First (Y)

Flood

First (Z)

Overflow

Overflow

Act.

Water

Level

(m)

S1.000 S01 15 Winter 100 +0% 81.276

S1.001 Sj1 15 Winter 100 +0% 81.268

S1.002 Sj2 15 Winter 100 +0% 81.255

S1.003 Sj3 15 Winter 100 +0% 81.217

S1.004 S02 15 Winter 100 +0% 81.073

S1.005 S03 180 Winter 100 +0% 100/60 Summer 80.733

S2.000 S04 15 Winter 100 +0% 100/15 Summer 81.484

S2.001 Sj4 30 Winter 100 +0% 81.295

S2.002 Sj5 30 Winter 100 +0% 81.242

S2.003 Sj6 30 Winter 100 +0% 81.192

S2.004 S05 15 Winter 100 +0% 81.068

S1.006 S06 180 Winter 100 +0% 100/15 Summer 80.732

S1.007 S08 600 Winter 100 +0% 79.924

S1.008 Sj7 480 Summer 100 +0% 79.826

S1.009 S09 15 Summer 100 +0% 79.770

S1.010 Sj8 30 Summer 100 +0% 79.550

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1) for Storm


PN

US/MH

Name

Surcharged

Depth

(m)

Flooded

Volume

(m³)

Flow /

Cap.

Overflow

(l/s)

Pipe

Flow

(l/s) Status

Level

Exceeded

S1.000 S01 -0.069 0.000 0.55 16.5 OK

S1.001 Sj1 -0.051 0.000 0.71 22.3 OK*

S1.002 Sj2 -0.014 0.000 0.88 28.4 OK*

S1.003 Sj3 0.000 0.000 1.21 36.3 SURCHARGED*

S1.004 S02 -0.105 0.000 0.52 56.3 OK

S1.005 S03 0.134 0.000 0.13 12.5 SURCHARGED

S2.000 S04 0.139 0.000 0.85 27.0 SURCHARGED

S2.001 Sj4 0.000 0.000 0.85 27.6 SURCHARGED*

S2.002 Sj5 0.000 0.000 1.08 33.9 SURCHARGED*

S2.003 Sj6 0.000 0.000 1.39 41.5 SURCHARGED*

S2.004 S05 -0.108 0.000 0.52 73.7 OK

S1.006 S06 0.578 0.000 0.09 2.8 SURCHARGED

S1.007 S08 -0.189 0.000 0.06 2.8 OK

S1.008 Sj7 -0.179 0.000 0.09 2.8 OK*

S1.009 S09 -0.196 0.000 0.04 2.8 OK

S1.010 Sj8 -0.179 0.000 0.09 2.8 OK*

Appendix E

Surface Water Drainage Pro-Forma

Surface Water Assessment Pro-forma

Surface Water Drainage Pro-forma for new developments

This pro-forma accompanies our “Surface Water Drainage; Local Guidance for Planning Applications” note. It is expected that applicants/developers should complete and submit the pro-forma to present a summary of the surface water drainage strategy for the site and demonstrate compliance with the National Planning Policy Guidance and Non-Statutory Technical Standards. The pro-forma will then be used to support the LPA in making a decision on the suitability of the proposal and, if the LPA is minded to find the completed pro-forma acceptable, then it may be used as an evidence base for a relevant surface water condition to be appended to the decision notice, stating that the developments drainage proposal will be constructed in accordance with the details set out in the relevant pro-forma. It must however be noted that this pro-forma submitted alone, will not be considered a suitable surface water drainage strategy. It should be clearly referenced within the pro-forma where in the other submission documents the details provided are taken from. The pro-forma is supported by the Defra/EA guidance on Rainfall Runoff Management. and uses the storage calculator on www.UKsuds.com. The pro-forma should be considered alongside other supporting SuDS Guidance, but focuses on ensuring flood risk is not made worse elsewhere. This proforma is based upon current industry standard practice. 1. Site Details

Site The Common Carpark, Hatfield

Address & post code or LPA reference 22 The Common, Hatfield, AL0 0LP

Grid reference TL 22470 08721

Is the existing site developed or Greenfield? Developed (Existing Carpark)

Total Site Area 0.541 Hectares

Total Site Area served by drainage system (excluding open space) (Ha)*

0.541 Hectares

Pre-application sought? (Ref)

* The Greenfield runoff off rate from the development which is to be used for assessing the requirements for limiting discharge flow rates and attenuation storage from a site should be calculated for the area that forms the drainage network for the site whatever size of site and type of drainage technique. Please refer to the Rainfall Runoff Management document or CIRIA manual for detail on this.


2. Impermeable Area

Existing Proposed Difference (Proposed-Existing)

Notes for developers

Impermeable area (ha) 0.541 Hectares

0.541 Hectares

N/A If proposed > existing, then runoff rates and volumes will be increasing. Section 6 must be filled in. If proposed ≤ existing, then section 6 can be skipped & section 7 filled in.

Drainage Method (infiltration/sewer/watercourse)

Sewer Sewer N/A If different from the existing, please fill in section 3. If existing drainage is by infiltration and the proposed is not, discharge volumes may increase. Fill in section 6.

3. Proposing to Discharge Surface Water via

Yes No Justification and Evidence that this is possible


Infiltration X Soakage tests will need to be provided and results included in drainage strategy. Section 7 (infiltration) must be filled in if infiltration is proposed.

To watercourse X If infiltration is not possible - is there a watercourse nearby? Have the EA or IDB provided input where necessary?

To surface water sewer X This should be a last resort. If required, has sewer provider confirmed that sufficient capacity exists for this connection? Has an appropriate connection detail been agreed?

Combination of above X e.g. part infiltration, part discharge to sewer or watercourse. Provide evidence as above.


4. Peak Discharge Rates – This is the maximum flow rate at which storm water runoff leaves the site during a particular storm event.

Existing Rates (l/s)

Proposed Rates (l/s)

Difference (l/s) (Proposed- Existing)


Greenfield QBAR 3.2 l/s N/A N/A QBAR is approx. 1 in 2 storm event. Provide this if Section 7 (QBAR) is proposed.

1 in 1 2.8 /s 2.8 l/s N/A Proposed discharge rates (with mitigation) should be no greater than existing rates for all corresponding storm events. E.g. discharging all flow from site at the existing 1 in 100 event increases flood risk during smaller events.

1 in 30 5.7 l/s 2.8 l/s -2.9 l/s

1in 100 6.8 l/s 2.8 l/s -4.0 l/s

1 in 100 plus climate change

N/A 2.8 l/s -4.0 l/s To mitigate for climate change the proposed 1 in 100 +CC must be no greater than the existing 1 in 100 runoff rate. If not, flood risk increases under climate change. - Where lifetime of development is 100 years (residential) 30% should

be added to the peak rainfall intensity. - Where lifetime of development is 60 years (residential) 20% should

be added to the peak rainfall intensity.

5. Calculate additional volumes for storage –The total volume of water leaving the development site. New hard surfaces potentially restrict the amount of storm water that can go to the ground, so this needs to be controlled so not to make flood risk worse to properties downstream.

Existing Volume (m3)

Proposed Volume (m3) Without mitigation

Difference (m3) (Proposed-Existing)


1 in 1 51.231m3 51.231m3 - Proposed discharge volumes (without mitigation) should be no greater than existing volumes for all corresponding storm events. Any increase in volume increases flood risk elsewhere. Where volumes are increased section 6 must be filled in.

1 in 30 113.966m3 113.966m3 -

1in 100 149.152m3 149.152m3 -

1 in 100 plus climate change

N/A 207.484m3 58.332m3 To mitigate for climate change the volume discharge from site must be no greater than the existing 1 in 100 storm event. If not, flood risk will increase under climate change.


6. Calculate attenuation storage – Attenuation storage is provided to enable the rate of runoff from the site into the receiving watercourse to be limited to an acceptable rate to protect against erosion and flooding downstream. The attenuation storage volume is a function of the degree of development relative to the greenfield discharge rate.


What Storage Attenuation volume (Flow rate control) is required to retain rates as existing (m3) Where is the storage to be accommodated on site?

All hardstanding areas will fall under gravity and be attenuated in a 136.80m³ cellular attenuation system. Offsite flow will be restricted to 2.8 l/s prior to passing through a bypass separator.

Volume of water to attenuate on site if discharging at existing rates. Can’t be used where discharge volumes are increasing

7. How is Storm Water stored on site? Storage is required for the additional volume from site but also for holding back water to slow down the rate from the site. This is known as attenuation storage and long term storage. The idea is that the additional volume does not get into the watercourses, or if it does it is at an exceptionally low rate. You can either infiltrate the stored water back to ground, or if this isn’t possible hold it back with on site storage. Firstly, can infiltration work on site?


Infiltration

State the Site’s Geology and known Source Protection Zones (SPZ)

None - Infiltration rates are highly variable, soakage tests should be comprehensive.

- Avoid infiltrating in made ground. - Refer to Environment Agency website to identify and

source protection zones (SPZ)

Infiltration Rate (m/s)? Infiltration rates should be no lower than 1x10 -6 m/s.

State the distance between a proposed infiltration device base and the ground water (GW) level

No infiltration device proposed Need 1m (min) between the base of the infiltration device & the water table to protect Groundwater quality & ensure GW doesn’t enter infiltration devices. Avoid infiltration where this isn’t possible.

Were infiltration rates obtained by desk study or infiltration test?

Boreholes were undertaken, deep made ground encountered

Infiltration rates can be estimated from desk studies at most stages of the planning system if a back up attenuation scheme is provided.


Is the site contaminated? If yes, consider advice from others on whether infiltration can happen.

-- Water should not be infiltrated through land that is contaminated. The Environment Agency may provide bespoke advice in planning consultations for contaminated sites that should be considered.

In light of the above, is infiltration feasible?

Yes/No? If the answer is No, please identify how the storm water will be stored prior to release

NO. water attenuated on site by means of cellular attenuation tank.

If infiltration is not feasible how will the additional volume be stored? The applicant should consider the following options in the next section.

Storage requirements The developer must confirm one of the two methods for dealing with the amount of water that needs to be stored on site. Option 1 Simple – Store both the additional volume and attenuation volume in order to make a final discharge from site at QBAR (Mean annual flow rate). This is preferred if no infiltration can be made on site. This very simply satisfies the runoff rates and volume criteria. Option 2 Complex – If some of the additional volume of water can be infiltrated back into the ground, the remainder can be discharged at a very low rate of 2 l/sec/hectare. A combined storage calculation using the partial permissible rate of 2 l/sec/hectare and the attenuation rate used to slow the runoff from site.


Please confirm what option has been chosen and how much storage is required on site.

Simple – A new 136.80m³ cellular attenuation system will be installed. A bypass separator will be installed.

The developer at this stage should have an idea of the site characteristics and be able to explain what the storage requirements are on site and how it will be achieved.

8. Please confirm



1. Which Drainage Systems measures have been used? Provide an overview of the SuDS design scheme used? - Is the runoff managed at, or close to, the surface

wherever possible. - Where the system serves more than one property, is

public space used and integrated with the drainage system in an appropriate and beneficial way?

Bypass seperator SUDS can be adapted for most situations even where infiltration isn’t feasible e.g. impermeable liners beneath some SUDS devices allows treatment but not infiltration. See CIRIA SUDS Manual C697.

2. Functionality Are the design features sufficiently durable to ensure structural integrity over the system design life (residential 100 years and commercial 60 years), with reasonable maintenance requirements?

Yes

Are all parts of the SuDS system outside any areas of flood risk?

Yes If not, provide justification and evidence that performance will not be adversely affected.

Has runoff and flooding from all sources (both on and off site) been considered and taken into account in the design?

Yes

Has residual risk been addressed? Yes refer to Drainage strategy • Does the drainage system contain the 1 in 30 storm event without any flooding (include description of how any exceedance of surface water systems will be routed exceptional rain fall away from property)?

• Are 1 in 100 year flows contained or stored on-site within safe exceedance storage areas and flow paths?

• Is any flooding between 1 in 30 and 100 +CC storm events safely contained on site, without causing property flooding or a hazard to site users?

• Has it been ensured that there is no flooding from the system to downstream property or access routes for the 100 year + climate change event?

How are rates being restricted (hydro brakes etc.)? Hydrobake. Final discharge rate will be restricted to 2.8 l/s.

- Hydrobrakes to be used where rates are between 2l/s to 5l/s.

- Orifices not to be used below 5l/s as the pipes may block.

- Pipes with flows < 2l/s are prone to blockage.

3. Please confirm the owners/adopters of the entire drainage systems throughout the development. Please list all the owners.

Hatfield County Council If these are multiple owners then a drawing illustrating exactly what features will be within each owner’s remit must be submitted with this Proforma.


How is the entire drainage system to be maintained? An acceptable maintenance plan, clearly defining the operating and maintenance requirements of the drainage system will need to be submitted and approved.

The drainage drawings and schedules will form part of the O&M manual along with a post completion CCTV survey to ensure the system if fully operational at handover.

If the features are to be maintained directly by the owners as stated in answer to the above question please answer yes to this question and submit the relevant maintenance schedule for each feature. If it is to be maintained by others than those above, please give details of each feature and the maintenance schedule. Clear details of the maintenance proposals of all element of the proposed drainage system must be provided. Poorly maintained drainage can lead to increased flooding problems in the future.

9. Evidence Please identify where the details quoted in the sections above were taken from. i.e. Plans, reports etc. Please also provide relevant drawings that need to accompany your pro-forma, in particular exceedance routes, ownership and location of SuDS (maintenance access strips etc.)

Pro-forma Section Document reference where details quoted above are taken from Page Number

Section 2 Drainage Strategy 1, Appendix A &C Section 3 Drainage Strategy Appendix A & C Section 4 Drainage Strategy Appendix B &D Section 5 Drainage Strategy Appendix B&D Section 6 Drainage Strategy Appendix C Section 7 Drainage Strategy Appendix A & C

The above form is completed using factual information and evidence from the Surface Water Drainage Strategy, Flood Risk Assessment and site

plans and can be used as a summary of the surface water drainage strategy on this site, clearly showing that the proposed surface water rate and

volume will not be increasing as a result of the development. Where an increase in rate or volume is shown the appropriate sections of the pro-forma

have been completed setting out how the additional rate/volume is being dealt with, to ensure no increased flood risk on or off site.


Where the pro-forma is found to be acceptable to the Local Planning Authority then the surface water drainage system design must be built in

accordance with the details provided here.

Form completed by: Graham Taylor

Qualification of person responsible for signing of this pro-forma: IEng MICE

Company: AKSWard

On behalf of (Client’s details): Bourne Parking Limited

Date 16.08.19

the common car park drainage strategy and water quality

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