Cumulative Flood Risk North Petherton Vr.1.6A Aug 2026

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Critical Assessment: The Cumulative Flood Risk of North Petherton
Development on the Somerset Levels

Executive Summary
The Core Issue: Unacceptable Cumulative Flood Risk
This report finds that the aggregate flood risk from 1,330 new residential units in North Petherton
has been fundamentally underestimated. The proposals, including sites at Park Lane, Gateway,
Stafflands Farm, Daws Lane, and West of Shovel Lane, are being assessed in isolation, ignoring their
collective impact on the vulnerable Somerset Levels catchment, which is already severely
constrained, as evidenced by the 2014 flood events.

© Vr 1.6A August 2026

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Contents
Executive Summary ...................................................................................................................... 1
1. Introduction: The Critical Failure to Address Cumulative Flood Risk............................................ 4
2. Exacerbated Flood Risk to Moorland and the Somerset Levels ................................................... 4
The Mechanism of Increased Risk ................................................................................................. 5
3. Inadequate Pumping and Drainage Capacity ............................................................................. 5
4. The Flawed Reliance on Attenuation and SuDS .......................................................................... 6
5. Reduced Waterway Maintenance and Legislative Failures .......................................................... 7
6. NPPF Policy Weight: A National Imperative ............................................................................... 8
7. Sedgemoor Local Plan: Specific Local Protection ........................................................................ 9
Summary of Policy Application ................................................................................................... 10
Appendix X ................................................................................................................................ 11
Estimated Runoff from Individual Applications (Piece-meal) & References for Assumption
Calculations ............................................................................................................................... 11
Site-by-site worked calculations ................................................................................................. 12
Estimated piece-meal runoff (method and results) ...................................................................... 15
Recommended Reference Sources for Assumptions .................................................................... 16
Reference Basis for Assumptions ................................................................................................ 17
Version Control .......................................................................................................................... 18
Addendum to the Cumulative Flood Risk Assessment of North Petherton .................................... 19
Addendum No 1 to the Cumulative Flood Risk Assessment of North Petherton (December 2025) . 20
Implications of Recent Flood Events for Ongoing Development Proposals ...................................... 20
1. Purpose of this Addendum ........................................................................................................... 20
2. Relationship to the December 2025 Assessment ......................................................................... 20
3. Observed Flood Events Since Publication ..................................................................................... 20
4. Implications for Flood Risk Assessments (FRAs) ........................................................................... 20
5. Implications for SuDS and Attenuation-Based Mitigation ............................................................. 21
6. Cumulative Impact: From Risk to Demonstrated Constraint ........................................................ 21
7. Planning Implications .................................................................................................................... 21
8. Conclusion and Recommendation ................................................................................................ 21
Addendum No. 2 (Version 1.4 – April 2026) Cumulative Flood Risk Assessment of North Petherton
Forensic Review of System Resilience and Recent Flood Evidence ................................................ 22
Contents .................................................................................................................................... 22
1. Purpose of Addendum ............................................................................................................ 23
2. Status of Original Report ........................................................................................................ 23
3. New Evidence Considered....................................................................................................... 23
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4. Review of System Resilience Claims ........................................................................................ 23
5. Implications for Hydraulic Capacity ......................................................................................... 24
6. Implications for Flood Risk Assessments (FRAs) ....................................................................... 24
7. Implications for SuDS and Attenuation .................................................................................... 24
8. Cumulative Impact – Confirmed Constraint ............................................................................. 24
9. Planning and Policy Implications ............................................................................................. 25
10. Updated Conclusion ............................................................................................................. 25

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1. Introduction: The Critical Failure to Address Cumulative Flood Risk
This report highlights a critical and unacceptable failure to fully address the cumulative flood risk
posed by the current portfolio of major housing applications in North Petherton.
The proposed developments—totalling 1,330 new dwellings—are being assessed in isolation,
overlooking the aggregate hydraulic burden placed on the already marginal drainage capacity of the
receiving environment. This approach contravenes national and local planning policy aimed at
protecting vulnerable communities.
Development Scale and Geographic Context
The collective impact from the following key applications, currently in various planning stages,
demands an immediate, unified reassessment:
Application Reference Site Name

Proposed Dwellings Notes

37/25/00044

Park Lane

64

37/22/00125

Vistry Gateway

150

37/25/00055

Gateway

15

37/25/00042

Gateway

346

37/25/00023

Stafflands Farm

190

37/25/00018

Daws Lane

175

37/25/00088

Dancing Hill

140

Proposal

West of Shovel
Lane

250

SUB-TOTAL

(New figure as of August 25)

Up to 250 dwellings

1,330

2. Exacerbated Flood Risk to Moorland and the Somerset Levels
The proposed development of 1,330 new homes in North Petherton, which sits close to the
catchment area of the Somerset Levels and Moors, presents an undeniable increase in flood risk to
communities downstream, particularly Moorland.

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The Mechanism of Increased Risk
•

Increased Impermeable Surfaces: New development, by its nature, replaces porous land
(fields, open ground) with impermeable surfaces (roofs, roads, driveways). This dramatically
accelerates and increases surface water runoff volume.

•

Catchment Impact: North Petherton is situated on the edge of the catchment area that
drains into the low-lying Somerset Levels, including the Parrett and Tone River systems. A
sudden increase in surface water runoff from this scale of development may place an
immediate and additional hydraulic burden on the main rivers and, crucially, the surrounding
moors used as designated flood storage areas (e.g., Northmoor, Saltmoor, Currymoor,
Fordgate).

•

Direct Impact on Moorland: Communities like Moorland, which are geographically low-lying
and historically prone to inundation, function as a spillover point when the river systems are
at capacity. The cumulative effect of 1,330 new homes discharging surface water, even at
'greenfield' runoff rates (the ideal scenario, which is often not met in practice), may
incrementally increase the frequency, duration, and depth of flooding in these already
vulnerable areas. The tragic experiences of 2014, where Moorland was severely impacted,
underscore the catastrophic consequences of exceeding the system's storage and
conveyance capacity.

3. Inadequate Pumping and Drainage Capacity
The current capacity for managing water on the Somerset Levels, particularly the system of pumping
stations and drainage networks, is not sufficient to manage the existing flood risk, let alone the
added pressure from significant new development.
•

Existing Capacity Limitations: The pumping system is designed to manage water within the
context of the historical environment. However, the 2014 floods and subsequent near-miss
events have repeatedly demonstrated that the rivers (e.g., River Tone, River Parrett) can
reach levels where pumping stations cannot operate effectively because the receiving
watercourse is already too high, a concept known as being "tide-locked" or "river-locked."

•

Reliance on Temporary Measures: Although investments have been made since 2014,
reports often highlight the need to deploy temporary mobile pumps and take pre-emptive
action when intense rainfall is forecast. This indicates that the permanent, built-in capacity is
already marginal. Adding the runoff from 1,330 homes may reduce the operational window
for existing pumps and accelerate the trigger points for deploying temporary measures,
increasing the long-term operational cost and risk.

•

Climate Change and Future Storm Events: The current capacity is insufficient when factoring
in the increased frequency and intensity of rainfall events predicted under climate change
allowances (which mandate a 20-40% increase in design rainfall depths). The development
proposals fail to provide a robust demonstration that the capacity exists to manage the
worst-case, climate-adjusted scenario without detriment to downstream residents.

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4. The Flawed Reliance on Attenuation and SuDS
The primary mitigation strategy for new developments, as often promoted under the National
Planning Policy Framework (NPPF), involves the use of attenuation ponds/tanks and Sustainable
Drainage Systems (SuDS) to manage surface water runoff. However, for a flood-sensitive location like
North Petherton and the Somerset Levels, this approach carries significant inherent risks and
potential for failure, leading to a repeat of the 2014 situation.
Mitigation
Strategy

Risk of Failure & Consequence

Attenuation
Ponds/Tanks

These features temporarily store water before slow release. In a prolonged
rainfall event, such as in 2014, these structures may fill up and lose all capacity
before the storm event is over, resulting in uncontrolled discharge and
compounding the flood peak. Their effectiveness is critically dependent on the
downstream drainage network having sufficient capacity to accept the
attenuated flow, which, as established, is already questionable.

While designed to mimic natural processes (infiltration, slow conveyance), a
review of SuDS in Somerset found that many schemes favour underground
Sustainable
storage/pipe-to-pond solutions over truly multi-functional source control
Drainage Systems
features (e.g., rain gardens, permeable paving). Furthermore, poor construction
(SuDS)
site management and lack of ongoing maintenance can lead to blockages and
system failure at a critical time, rendering them ineffective.

Groundwater
Interaction

The Somerset Levels have a naturally high-water table. Attenuation features, if
not correctly lined or managed, risk being filled with rising groundwater, which
significantly reduces their capacity to store surface runoff from rainfall,
increasing the flood risk to the development itself and the wider area.

Conclusion: The design and maintenance limitations of these mitigation measures mean they offer
false security. Relying on them for 1,330 homes introduces an unacceptable systemic risk that, in an
extreme event, may inevitably contribute to widespread flooding across the Levels, directly
impacting Moorland.

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5. Reduced Waterway Maintenance and Legislative Failures
Compounding the capacity and development issues is the observable trend of reduced maintenance
work on the network of rivers, rhynes, and drainage ditches in the area, a vital component of the
Somerset Levels flood defence strategy.
•

Dredging and Desilting: The effectiveness of the river system to convey water rapidly is
dependent on continuous maintenance, notably dredging and desilting. A lack of consistent,
timely maintenance leads to reduced channel capacity, causing water to overtop banks
sooner and flow onto the floodplains (moors) unnecessarily or prematurely.

•

Rhyne and Ditch Clearance: The smaller drainage networks (rhynes and ditches), managed
by Internal Drainage Boards (IDBs), are critical for local water management. If maintenance is
neglected, these waterways become choked with vegetation and silt, impeding the flow of
water off the moors and into the main rivers, thereby prolonging the duration of flooding in
communities like Moorland.

•

Sedgemoor and NPPF Policy Conflict: While Sedgemoor's planning policy (and the
overarching NPPF) requires developers to manage flood risk and promote SuDS, the sheer
volume of development, coupled with the systemic underinvestment in core downstream
infrastructure (pumping and channel maintenance), creates an untenable conflict. Under the
August 2026 NPPF, the Sequential and Exception Tests remain relevant where applicable,
while Policy F7 directly requires development not to increase flood risk elsewhere. The
cumulative downstream impact on highly vulnerable receptors like the Somerset Levels
should therefore be robustly addressed.

The current trajectory of high-volume development without a demonstrably robust, fully funded,
and future-proofed flood defence and maintenance strategy may inevitably place an overwhelming
and unacceptable burden on the residents of Moorland and the wider Somerset Levels. Planning
permission for these 1,330 dwellings may need to be reconsidered until an independently verified,
holistic, and capacity-enhancing solution for the entire catchment is delivered.

The core arguments in the report are strongly supported by key policies in the National Planning
Policy Framework (NPPF) and the Sedgemoor Local Plan 2011-2032, particularly Policy D1 on flood
risk and surface water management, in relation to the Sequential Test, cumulative risk, and the longterm operation and maintenance of Sustainable Drainage Systems (SuDS).

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6. NPPF Policy Weight: A National Imperative
The NPPF provides the overarching national policy framework for flood-risk decision-making,
including the requirement that development should be safe for its lifetime and should not increase
flood risk elsewhere.
NPPF Policy F5 (and F6 where relevant): Sequential and Exception Tests
•

Policy Link: Policy F5 sets the Sequential Test, steering development to areas with the lowest
flood risk from any source. Policy F6 sets the Exception Test where required in Flood Zones 2,
3a and 3b.

•

Report Weight: The proposed 1,330 homes may need to satisfy Policy F5 where the
Sequential Test applies; F5 also specifies circumstances where the test is not required. The
report's downstream concerns remain directly relevant to Policy F7, which requires
development not to increase flood risk elsewhere, and to the wider assessment of
cumulative risk.

NPPF Policy F7: Ensuring Development is Safe from Flooding
•

Policy Link: Policy F7 requires development not to present flood risk to occupiers, users or
visitors and not to increase flood risk elsewhere. Where a location is known to be at risk now
or in the future, F7 sets specific safeguards.

•

Report Weight: This directly engages the report's central concern: whether cumulative
runoff from 1,330 homes could increase flood risk to vulnerable downstream communities
such as Moorland and the Levels. Policy F7 makes the no increase elsewhere requirement an
explicit decision-making test.

NPPF Policy F8 (and PPG): Sustainable Drainage Systems and Maintenance
•

Policy Link: NPPF Policy F8 requires SuDS where development could affect drainage, to
control flow rates and reduce runoff volumes. F8(2)(b) requires maintenance arrangements
for acceptable operation over the development’s anticipated lifetime. Planning Practice
Guidance (Flood Risk and Coastal Change) clarifies that the Sequential Test must not rely on
mitigation measures requiring ongoing active maintenance to make a site safe; such
measures may be considered at later stages but cannot be used to justify site selection.

•

Report Weight: The report's concerns about long-term SuDS performance (including silting,
blockage and poor maintenance) gain weight from F8(2)(b), which requires maintenance
arrangements to ensure acceptable operation for the anticipated lifetime of the
development—a key vulnerability highlighted by the 2014 floods and regional waterway
conditions.

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7. Sedgemoor Local Plan: Specific Local Protection
The Sedgemoor Local Plan reinforces national policy with a clear local framework for flood risk and
surface-water management, directly relevant to North Petherton as a Tier 1 rural settlement within
the former Sedgemoor area.
Sedgemoor Local Plan Policy D1: Flood Risk and Surface Water Management
•

Policy Link: Policy D1 requires site-specific Flood Risk Assessments to demonstrate that
development will be safe over its lifetime and not increase flood risk elsewhere. It also
requires regard to relevant sources of flooding identified through the Strategic Flood Risk
Assessment and more recent mapping from the Environment Agency and other flood-risk
management bodies.

•

Report Weight: Policy D1 directly supports the report's cumulative-risk concern because it
requires the wider-area implications of surface-water drainage to be considered and seeks an
overall reduction in flood risk, with betterment particularly sought where known flooding
issues exist. The report's concern about potential increased risk to Moorland therefore
directly engages Policy D1.

Sedgemoor Local Plan Policy D1: Surface Water Drainage and Sustainable Drainage Systems (SuDS)
•

Policy Link: Policy D1 states that proposals should seek to reduce flood risk overall through
green infrastructure and SuDS; betterment is sought particularly where there are known
flooding issues. Where development increases the rate of surface-water drainage, the
implications for the wider area should be considered. SuDS are expected for all major
developments, and proposals must include clear arrangements for ongoing operation and
maintenance. Supporting paragraph 7.10 states that development should be run-off neutral
as a minimum wherever possible.

•

Report Weight: The report's concern is therefore not whether SuDS are required, but
whether the cumulative development can demonstrably satisfy Policy D1's wider-area, noincrease-elsewhere and maintenance requirements in a constrained receiving system.
Achieving controlled or nominally greenfield/run-off-neutral discharge at individual sites
does not, by itself, resolve the report's cumulative downstream-capacity concern.

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Summary of Policy Application
The report's claims are therefore a direct test of compliance with national and local policy:
•

Cumulative Risk: The 1,330 dwellings engage NPPF Policy F7 and Sedgemoor Local Plan
Policy D1 because both require development not to increase flood risk elsewhere, including
at Moorland.

•

Pumping Capacity: Limited pump capacity is material to the NPPF F7 / SLP D1 safe-lifetime
and no-increase-elsewhere tests.

•

SuDS Reliability: SuDS maintenance or groundwater-interaction risks engage NPPF Policy F8
and Sedgemoor Local Plan Policy D1, which require runoff control and ongoing operation
and maintenance.

Appendixes - Next page

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Appendix X
Estimated Runoff from Individual Applications (Piece-meal) &
References for Assumption Calculations
Purpose and scope (short):
The table below gives order-of-magnitude estimates of additional runoff generated by each planning
application considered in this report. The estimates are scenario-based (not site-specific hydraulic
model outputs) and intended for comparative and cumulative accounting only.
Assumptions (conservative/default):
1. Assumed impermeable area per dwelling = 200 m² (roofs + hardstanding/drive).
2. Runoff coefficient (impermeable surfaces) = C = 0.90.
3. Annual rainfall = 800 mm = 0.8 m (typical Somerset order).
4. Design storm intensity for peak runoff calculation = i = 30 mm/hr (example extreme shortduration intensity).
5. Peak runoff estimated using the Rational method:
Q=C×i (mm/hr)1000×A (m2)3600(m3 ⁣/s)Q = C \times \frac{i~(\text{mm/hr})}{1000} \times
\frac{A~(\text{m}^2)}{3600} \quad\text{(m}^3\!/s)Q=C×1000i (mm/hr)×3600A (m2)(m3/s)
then convert to litres per second (l/s) by multiplying m³/s × 1,000.
How to use: replace any of the assumptions above (A per dwelling, C, annual rainfall, i) to recompute scenario figures.

Calculations (step-by-step) and results
Notes on the arithmetic convention used: every multiplication and division below is shown step-bystep (digit by digit) to avoid arithmetic mistakes.
Common intermediate arithmetic
•

Assumed impermeable area per dwelling = 200 m².

•

To compute total impermeable area for a site: Total area (m²) = Dwellings × 200.

•

To compute annual runoff volume (m³/year): Annual vol = Total area × Annual rain (m) × C.

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•

To compute peak Q (m³/s) for the design storm: Q(m³/s) = C × (i/1000) × Total area / 3600.
Convert to l/s by Q(l/s) = Q(m³/s) × 1000.

Site-by-site worked calculations
1. 37/25/00044 — Park Lane — 64 dwellings
•

Total impermeable area: 64 × 200 = 12,800 m².

•

Annual runoff (m³/yr):

•

o

12,800 × 0.8 = 10,240 (this is area × annual rain)

o

10,240 × 0.9 = 9,216 m³/yr (apply C).

Peak runoff (design storm 30 mm/hr):
o

i/1000 = 30 / 1000 = 0.03 m/hr

o

Area × 0.03 = 12,800 × 0.03 = 384 m³/hr

o

384 / 3600 = 0.1066666667 m³/s

o

0.1066666667 × 0.9 = 0.096 m³/s (apply C)

o

0.096 m³/s × 1000 = 96 l/s

2. 37/25/00055 — Gateway — 15 dwellings
•

Total impermeable area: 15 × 200 = 3,000 m².

•

Annual runoff:

•

o

3,000 × 0.8 = 2,400

o

2,400 × 0.9 = 2,160 m³/yr.

Peak runoff:
o

3,000 × 0.03 = 90 m³/hr

o

90 / 3600 = 0.025 m³/s

o

0.025 × 0.9 = 0.0225 m³/s

o

0.0225 × 1000 = 22.5 l/s

3. 37/25/00042 — Gateway — 346 dwellings
•

Total impermeable area: 346 × 200 = 69,200 m².

•

Annual runoff:

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•

o

69,200 × 0.8 = 55,360

o

55,360 × 0.9 = 49,824 m³/yr.

Peak runoff:
o

69,200 × 0.03 = 2,076 m³/hr

o

2,076 / 3600 = 0.5766666667 m³/s

o

0.5766666667 × 0.9 = 0.519 m³/s
0.519 × 1000 = 519 l/s

4. 37/22/00126 — Vistry Gateway — 150 dwellings
•

Total impermeable area: 150 × 200 = 30,000 m².

•

Annual runoff:

•

o

30,000 × 0.8 = 24,000

o

24,000 × 0.9 = 21,600 m³/yr.

Peak runoff:
o

30,000 × 0.03 = 900 m³/hr

o

900 / 3600 = 0.25 m³/s

o

0.25 × 0.9 = 0.225 m³/s

o

0.225 × 1000 = 225 l/s

5. 37/25/00023 — Stafflands Farm — 190 dwellings
•

Total impermeable area: 190 × 200 = 38,000 m².

•

Annual runoff:

•

o

38,000 × 0.8 = 33,600

o

33,600 × 0.9 = 27,360 m³/yr.

Peak runoff:
o

38,000 × 0.03 = 1,260 m³/hr

o

1,260 / 3600 = 0.35 m³/s

o

0.35 × 0.9 = 0.285 m³/s

o

0.285 × 1000 = 285 l/s

6. 37/25/00018 — Daws Lane — 175 dwellings
•

Total impermeable area: 175 × 200 = 35,000 m².

•

Annual runoff:
o

35,000 × 0.8 = 28,000
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o
•

28,000 × 0.9 = 25,200 m³/yr.

Peak runoff:
o

35,000 × 0.03 = 1,050 m³/hr

o

1,050 / 3600 = 0.2916666667 m³/s

o

0.2916666667 × 0.9 = 0.2625 m³/s

o

0.2625 × 1000 = 262.5 l/s

•

37/25/00088 — Dancing Hill — 140 dwellings
Total impermeable area: 140 × 200 = 28,000 m².

•

Annual runoff:
o
o

•

•
•

Peak runoff:
o i/1000 = 30 / 1000 = 0.03 m/hr
o Area × 0.03 = 28,000 × 0.03 = 840 m³/hr
o 840 / 3600 = 0.2333333333 m³/s
o 0.2333333333 × 0.9 = 0.21 m³/s (apply C)
o 0.21 m³/s × 1000 = 210 l/s
Proposed development — West of Shovel Lane — up to 250 dwellings
Total impermeable area: 250 × 200 = 50,000 m².
Annual runoff:
o
o

•

28,000 × 0.8 = 22,400 (this is area × annual rain)
22,400 × 0.9 = 20,160 m³/yr (apply C).

50,000 × 0.8 = 40,000 (this is area × annual rain)
40,000 × 0.9 = 36,000 m³/yr (apply C).

Peak runoff:
o
o
o
o
o

i/1000 = 30 / 1000 = 0.03 m/hr
Area × 0.03 = 50,000 × 0.03 = 1,500 m³/hr
1,500 / 3600 = 0.4166666667 m³/s
0.4166666667 × 0.9 = 0.375 m³/s (apply C)
0.375 m³/s × 1000 = 375 l/s

Aggregated totals (for the eight sites / 1,330 dwellings)
•

Total dwellings = 64 + 15 + 346 + 150 + 190 + 175 + 140 + 250 = 1,330 dwellings.

•

Total impermeable area = 12,800 + 3,000 + 69,200 + 30,000 + 38,000 + 35,000 + 28,000 +
50,000 = 266,000 m².

•

Total annual runoff (sum of site annual vols) = 9,216 + 2,160 + 49,824 + 21,600 + 27,360 +
25,200 + 20,160 + 36,000 = 191,520 m³/yr.
o

(Equivalently: 266,000 × 0.8 = 212,800 then 212,800 × 0.9 = 191,520 m³/yr.)

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•

Total design-storm peak (Rational sum) = sum of site Q (m³/s) = 0.096 + 0.0225 + 0.519 +
0.225 + 0.285 + 0.2625 + 0.21 + 0.375 = 1.995 m³/s = 1,995 l/s

Estimated piece-meal runoff (method and results)
A simple scenario calculation was carried out to estimate the additional impermeable area and
resulting runoff attributable to each planning application considered in this report. Assumptions
used: impermeable area per dwelling = 200 m²; runoff coefficient C = 0.90; annual rainfall 800 mm;
design storm intensity 30 mm/hr (Rational method). The site-by-site computations (worked
arithmetic) and aggregated totals are shown below. These scenario figures are indicative only and is
advised to be replaced by site-specific SuDS design outputs or hydraulic model results where
required. (See Appendix X table and worked calculations.)

Application
Ref

Site

Impermeable
Dwellings area per
dwelling (m²)

Total
impermeable
area (m²)

Estimated
annual
runoff
(m³/yr)

Peak Q
(design
storm 30
mm/hr)
(l/s)

37/25/00044 Park Lane

64

200

12,800

9,216

96

37/25/00055 Gateway

15

200

3,000

2,160

22.5

37/25/00042 Gateway

346

200

69,200

49,824

519

37/22/00125

Vistry
Gateway

150

200

30,000

21,600

225

37/25/00023

Stafflands
Farm

190

200

38,000

27,360

285

37/25/00018 Daws Lane

175

200

35,000

25,200

262.5

37/25/00088

Dancing
Hill

140

200

28,000

20,160

210

Proposal

West of
Shovel
Lane

250

200

50,000

36,000

375

266,000

191,520

1,995

TOTAL

1,330

Caveats and recommended next steps (brief)
•

These estimates are scenario values for comparative/cumulative accounting; they are not
substitutes for site-specific drainage design or modelled flood routing.
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•

The choice of impermeable area per dwelling, rainfall depth, run-off coefficient, and design
intensity strongly affects results — replace assumptions with measured/design values if
available.

•

For planning decisions, require each application to provide full SuDS design (infiltration tests,
attenuation sizing, maintenance arrangements) and a cumulative hydraulic assessment (e.g.,
catchment model or linked storage/routeing) that accounts for downstream capacity
constraints and climate change allowances.

Recommended Reference Sources for Assumptions
Assumption

Reference Source

Citation (suggested format) Notes

Impermeable
area per
dwelling (≈150–
250 m² typical)

CIRIA C753 The SuDS
CIRIA gives
“Typical residential plot
Manual (2015), Section
representative roof +
impermeable area 150–250
24.2; DEFRA/EA “Rainfall
paved area ratios; the
m² per dwelling, depending
runoff management for
DEFRA R&D report
on plot density (CIRIA,
developments” (R&D Report
corroborates the
2015).”
W5-074/A/TR/1, 2005)
range.

Runoff
coefficient (C =
0.9 for
impermeable
surfaces)

CIRIA C753 (Table 24.2), BS
EN 16941-1:2018 (On-site
rainwater harvesting
systems – Design,
installation and
maintenance), and EA
Greenfield runoff guidance

“Impermeable surface
Commonly adopted
runoff coefficients typically for roofs, paving,
0.85–0.95 (CIRIA, 2015).”
tarmac, and concrete.

Environment Agency Flood
Annual rainfall Estimation Handbook
800 mm
(FEH13), Vol. 2: Rainfall
(Somerset order- frequency estimation; Met
of-magnitude)
Office UK Climate Averages
(1991–2020)

“Average annual rainfall for
Bridgwater/North
FEH grid (SAAR) data
Petherton catchment ≈
gives ~820 mm for
780–850 mm (Met Office, local catchment.
2023).”

FEH13, EA/DEFRA R&D
Design storm
Report W5-074/A/TR/1
intensity (30 mm
(2005), Table 6.1; CIRIA
h⁻¹)
C753 (2015)

“Short-duration design
rainfall intensities (10–60
30 mm h⁻¹ represents
min) typically 20–40 mm h⁻¹ a moderate-high
for south-west England
short-duration event.
(FEH13; DEFRA/EA, 2005).”

“Peak runoff estimated
Sewers for Adoption 7th
using Rational Method (Q = Standard approach
Rational Method Edition (WRc, 2012); CIRIA
C i A) in accordance with
for small catchments
(Q = C i A)
C753 §24.3.1; EA/DEFRA
WRc (2012) and CIRIA
< 200 ha.
R&D Report W5-074/A/TR/1
(2015).”

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Reference Basis for Assumptions
The input parameters used in the above scenario calculations are derived from recognised UK
guidance:
•

CIRIA C753 (2015) The SuDS Manual — Table 24.2 (impermeable area, runoff coefficients,
design intensities).

•

DEFRA/Environment Agency (2005) Rainfall Runoff Management for Developments (R&D
Report W5-074/A/TR/1).

•

Environment Agency Flood Estimation Handbook (2013 update) — average annual rainfall
(SAAR) and short-duration intensities.

•

Met Office (2023) UK Climate Averages 1991–2020 — regional annual rainfall data for
Somerset.

•

WRc (2012) Sewers for Adoption, 7th Ed. — Rational method formulation for peak discharge
estimation.
These sources represent accepted national standards for preliminary surface-water runoff
estimation and SuDS design inputs.

Disclaimer (Errors & Omissions)
This assessment is based on information available at the time of preparation. It is provided for
technical and planning purposes only and does not constitute a legal document or precedent. While
every effort has been made to ensure accuracy, the authors accept no liability for any errors,
omissions, or reliance placed on the contents beyond its intended scope.

Clarification on peak flow calculations:
Any summed peak runoff figures presented in this document are retained for illustrative and
scenario-based purposes only. They do not represent permitted discharge rates of approved
schemes, which are subject to flow controls, attenuation, and detailed drainage conditions. The
calculations are intended to highlight potential system loading under exceedance or stress scenarios,
including blockage, loss of storage due to groundwater interaction, maintenance failure, or
coincident extreme events.
Information limitations:
This document was prepared using information reasonably available at the time of drafting. Where
later committee reports, decision notices or technical details were not yet published, reasonable
assumptions have been made and are identified as such. The document should be read as an
evidence-led review of cumulative flood risk considerations rather than a definitive finding of harm
or non-compliance.
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Version Control
Version Date

Description

1.3

December 2025

Updated and expanded cumulative assessment, runoff calculations, policy
analysis, and appendices

1.3

January
2026

Addendum created to be read in conjunction with main report

1.3

February 2026 NPPF Polices Updated

1.4

April 2026

additional information from the Parret Catchment Group and the ada
Gazette, Spring 2026, which includes the IDB article. I have brought together
the findings of these two items and added an Addendum No2 to the main
report.

1.5

July 2026

Added West of Shovel Lane proposal (up to 250 dwellings) and updated
cumulative totals and runoff calculations.

1.6A

August 2026

Policy-reference update: superseded NPPF paragraphs replaced by August
2026 NPPF F5-F8; superseded Core Strategy / legacy CP5 and SU1 references
replaced by adopted Sedgemoor Local Plan 2011-2032 Policy D1. North
Petherton terminology updated to Tier 1 rural settlement. No runoff
calculations, evidence or substantive conclusions changed.

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Addendum to the Cumulative Flood Risk Assessment
of North Petherton
Implications of Recent Flood Events for Ongoing Development Proposals
Status:
Formal Technical Addendum (Statutory Consultee)
Related Document:
Cumulative Flood Risk Assessment of North Petherton
Version 1.3 – December 2025
Date:
27/01/2026

This Addendum should be read in conjunction with the December 2025 Assessment. It does not
replace or amend the original document, but provides an evidence-led update reflecting observed
flood events since its publication.

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Addendum No 1 to the Cumulative Flood Risk Assessment
of North Petherton (December 2025)
Implications of Recent Flood Events for Ongoing Development Proposals
1. Purpose of this Addendum
•
•
•
•
•

Prepared to accompany the Town Council’s Cumulative Flood Risk Assessment of North
Petherton (Version 1.3, December 2025).
Does not replace, revise, or withdraw any findings or conclusions of the December 2025
Assessment.
Issued in response to recent flood events occurring since publication of the original assessment.
Recent flood events constitute new and material planning evidence.
Purpose is to assess implications of observed flood events for Flood Risk Assessments (FRAs),
Sustainable Drainage Systems (SuDS), and cumulative impacts.

2. Relationship to the December 2025 Assessment
•
•
•
•
•

December 2025 Assessment identified unacceptable cumulative flood risk and hydraulic
constraints.
Addendum reaffirms all conclusions of the December 2025 Assessment.
No new modelling or recalculations introduced.
Evaluates whether recent flood events confirm previously identified risks.
Original assessment should be read alongside this Addendum.

3. Observed Flood Events Since Publication
•
•
•
•
•

Flooding has occurred locally and across the Somerset Levels since December 2025.
Events characterised by prolonged or sequential rainfall and saturated ground conditions.
Elevated river and drainage levels limited surface water discharge.
Conditions reflect known flood mechanisms affecting the Somerset Levels.
Observed flooding represents empirical evidence of system exceedance.

4. Implications for Flood Risk Assessments (FRAs)
•
•
•

Many FRAs assume effective attenuation and available downstream capacity.
Recent events show receiving systems may already be at or beyond capacity.
Controlled discharge may be ineffective during high river levels.
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•
•

Greenfield runoff rate compliance does not equate to flood risk neutrality.
FRAs relying primarily on rate control should be treated with caution.

5. Implications for SuDS and Attenuation-Based Mitigation
•
•
•
•
•

Attenuation-based SuDS have inherent limitations in prolonged wet conditions.
Recent flooding confirms storage can be compromised before peak rainfall.
Groundwater levels may reduce available attenuation capacity.
Submerged outfalls restrict discharge effectiveness.
Reliance on SuDS in isolation does not provide robust mitigation.

6. Cumulative Impact: From Risk to Demonstrated Constraint
•
•
•
•
•

Cumulative impacts previously identified as potential are now evidenced.
Receiving systems are already under significant stress.
Limited residual capacity exists during wet periods.
Minimal tolerance remains for additional managed discharge.
Cumulative effects are a material planning consideration.

7. Planning Implications
•
•
•
•
•

Planning policy requires development to be safe for its lifetime.
Development must not increase flood risk elsewhere.
Recent flood events are a material consideration.
Site-by-site assessment alone is insufficient.
Precautionary approach is justified in flood-sensitive locations.

8. Conclusion and Recommendation
•
•
•
•
•

Addendum reinforces findings of the December 2025 Assessment.
Observed flood events validate identified flood risk mechanisms.
Receiving environment is operating at or near capacity.
Proposals relying solely on attenuation should not proceed unchanged.
Further development must demonstrate clear flood risk betterment.

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Addendum No. 2 (Version 1.4 – April 2026)
Cumulative Flood Risk Assessment of North Petherton
Forensic Review of System Resilience and Recent Flood
Evidence
This Addendum should be read in conjunction with the December 2025 Assessment. It does not
replace or amend the original document, but provides an evidence-led update reflecting observed
flood events since its publication

Contents
1. Purpose of Addendum
2. Status of Original Report
3. New Evidence Considered
4. Review of System Resilience Claims
5. Implications for Hydraulic Capacity
6. Implications for Flood Risk Assessments (FRAs)
7. Implications for SuDS and Attenuation
8. Cumulative Impact – Confirmed Constraint
9. Planning and Policy Implications
10. Updated Conclusion

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1. Purpose of Addendum
This Addendum (No.2, Version 1.4) should be read alongside the original Cumulative Flood Risk
Assessment of North Petherton (Version 1.3, December 2025) and subsequent Addendum (January
2026).
The purpose of this document is to incorporate further evidence arising from:
- Environment Agency flood event reporting (2026)
- Observed flood conditions during recent events
- A forensic review of system performance under stress conditions
This document does not replace previous findings, but strengthens and clarifies them in light of new
evidence.

2. Status of Original Report
All conclusions of the December 2025 Assessment and January 2026 Addendum remain valid and are
reinforced by this Addendum.
In particular, the following findings are upheld:
- The cumulative impact of 1,330 dwellings represents a material increase in hydraulic loading
- The receiving system is constrained and sensitive to incremental change
- Site-by-site assessment is insufficient to determine overall flood risk

3. New Evidence Considered
This Addendum considers the following additional evidence:
1. Recent flood events (2025–2026), characterised by prolonged rainfall and saturated ground
conditions
2. Environment Agency operational reporting indicating:
- reliance on temporary pumping
- constraints imposed by river levels
- system dependency on operational intervention
This evidence provides empirical validation of previously identified risks.

4. Review of System Resilience Claims
Recent material indicates improved operational response to flood events. However, this reflects
enhanced management rather than increased system capacity.
The system remains dependent on:
- favourable downstream conditions
- active pump deployment
- timely intervention
These characteristics indicate conditional performance rather than inherent resilience.
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5. Implications for Hydraulic Capacity
The introduction of additional runoff from cumulative development must be considered in the
context of a system already operating near capacity.
Observed conditions demonstrate:
- limited ability to convey additional flow during peak events
- reduced effectiveness of pumping under high river levels
- constrained discharge pathways
This indicates minimal residual capacity within the system.

6. Implications for Flood Risk Assessments (FRAs)
Many Flood Risk Assessments assume that downstream systems are capable of receiving controlled
discharge.
Recent evidence challenges this assumption, demonstrating that:
- discharge may not be possible during critical periods
- compliance with greenfield runoff rates does not ensure flood neutrality
FRA methodologies should therefore be reviewed in the context of cumulative system constraints.

7. Implications for SuDS and Attenuation
Attenuation-based mitigation strategies are dependent on available storage and discharge
conditions.
Observed flood events confirm that:
- storage systems may be compromised during prolonged rainfall
- groundwater levels may reduce available capacity
- discharge points may be submerged
This limits the reliability of SuDS as a sole mitigation mechanism.

8. Cumulative Impact – Confirmed Constraint
The cumulative impact identified in the original report has now transitioned from a theoretical risk to
a demonstrated constraint.
The system has shown:
- limited tolerance for additional inflow
- sensitivity to prolonged wet conditions
- reliance on active intervention to prevent exceedance
This confirms that cumulative effects are a material planning consideration.

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9. Planning and Policy Implications
Planning policy requires that development must not increase flood risk elsewhere and must be safe
for its lifetime.
In the context of demonstrated system constraints:
- site-specific compliance is insufficient
- cumulative impacts must be considered
- reliance on mitigation measures must be critically assessed
The precautionary principle is therefore applicable.

10. Updated Conclusion
This Addendum confirms that the receiving environment is operating at or near its functional limits
under stress conditions.
The cumulative development proposals introduce additional hydraulic loading into a constrained
system, with limited evidence of available capacity.
The findings of the original assessment are therefore reinforced, and the need for a comprehensive,
catchment-wide evaluation of flood risk is substantiated.

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