5.49 Policy 5 – Flooding and Drainage
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All new build must have a sustainable drainage system (SuDS) installed to dispose of rainwater
into the ground rather than the sewer. (The Floods and Water Management Act 2010
establishes a Sustainable Drainage Systems Approving Body the County Council. This body must
approve drainage systems in new developments and re-developments before construction
begins.) Rainwater harvesting will be encouraged.
Where possible all new build should have on-site sewage treatment plants to avoid increasing
the load on the present system. Such arrangements must ensure that additional net flow
induced must be capable of being managed in balancing arrangements so as not to overwhelm
existing land drains, ditches, culverts and sluices en route to the harbour via either the East
Ditch system or the Central Watercourse system.
Villages outside Bosham parish who use the Harts Farm Waste Water Treatment Works
(WwTW) should make similar provision to minimise any further load on the plant and in
particular the pumping station at Stumps End.
Southern Water, the agency responsible for the local system, must be lobbied to produce plans
to upgrade the capacity of the Stumps End pumping station and pipework leading to the Harts
Farm plant, so as to reduce the amount of untreated sewage being regularly discharged into the
northern part of the Harbour.
Improvements planned to the sluices at the Sailing Club end of the Bosham Stream must be
monitored to ensure they deliver the expected relief flow. The Environment Agency (EA)
remains responsible for monitoring the high and low flow of Bosham Stream and shall regulate
the Penstock accordingly.
The flow from the East Ditch and Stumps End into the harbour is still erratic at times of very
high tides and improvements to this must continue to be sought by the EA and the Parish
Council.
Justification and Evidence
Other supporting policies
The National Planning Policy Planning Framework (NPPF)
states, “inappropriate development in areas at risk of flooding should be avoided by directing
development away from areas at highest risk, but where development is necessary, making it safe
without increasing flood risk elsewhere”. (section 100)
It further states inter alia “When determining planning applications, local planning authorities should
ensure flood risk is not increased elsewhere”. (section 103)
The NPPF requires that “Local Plans should apply a sequential, risk-based approach to the location of
development to avoid where possible flood risk to people and property and manage any residual risk,
taking account of the impacts of climate change, by:
● applying the Sequential Test;
● if necessary, applying the Exception Test;
● safeguarding land from development that is required for current and future flood management;
● using opportunities offered by new development to reduce the causes and impacts of flooding;
and
● where climate change is expected to increase flood risk so that some existing development may
not be sustainable in the long-term, seeking opportunities to facilitate the relocation of
development, including housing, to more sustainable locations”. (section 100)
It further states that “the aim of the Sequential Test is to steer new development to areas with the
lowest probability of flooding. Development should not be allocated or permitted if there are
reasonably available sites appropriate for the proposed development in areas with a lower
probability of flooding”. (Section 101)
The exception test applies “if, following application of the Sequential Test, it is not possible,
consistent with wider sustainability objectives, for the development to be located in zones with a
lower probability of flooding, the Exception Test can be applied if appropriate. For the Exception Test
to be passed:
● it must be demonstrated that the development provides wider sustainability benefits to the
community that outweigh flood risk, informed by a Strategic Flood Risk Assessment where one has
been prepared; and
● a site-specific flood risk assessment must demonstrate that the development will be safe for its
lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and,
where possible, will reduce flood risk overall.
Both elements of the test will have to be passed for development to be allocated or permitted.”
The National Planning Policy Planning Framework Technical Guidance (NPPFTG) provides further
clarification. It states “areas at risk of flooding” means land within Flood Zones 2 and 3; or land within
Flood Zone 1 which has critical drainage problems and which has been notified to the local planning
authority by the Environment Agency”; (paragraph 2)
It defines “flood risk” as “... risk from all sources of flooding - including from rivers and the sea,
directly from rainfall on the ground surface and rising groundwater, overwhelmed sewers and
drainage systems, and from reservoirs, canals and lakes and other artificial sources”. (paragraph 2)
The degree of flood risk is set out as a series of classifications of “Flood zones” and these “These flood zones refer
to the probability of river and sea flooding, ignoring the presence of defences”.
Even for the lowest classification of flood risk, the technical guidance requires that
“For development proposals on sites comprising one hectare or above the vulnerability to flooding
from other sources as well as from river and sea flooding, and the potential to increase flood risk
elsewhere through the addition of hard surfaces and the effect of the new development on surface
water run-off, should be incorporated in a flood risk assessment “ (Table 1, Zone 1 - low probability)
On all levels of flood risk, the technical guidance requires “developers and local authorities should
seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout
and form of the development, and the appropriate application of sustainable drainage systems”.
(SUDS). It is further explained that “sustainable drainage systems cover the whole range of
sustainable approaches to surface drainage management. They are designed to control surface water
run off close to where it falls and mimic natural drainage as closely as possible.” (Table 1,)
The highest level of flood risk is scheduled as Zone 3b, “the functional floodplain”. It is defined as a
“zone comprises land where water has to flow or be stored in times of flood”. It goes on describe it
probabilistically as “land which would flood with an annual probability of 1 in 20 (5%) or greater in any
year, or is designed to flood in an extreme (0.1%) flood, should provide a starting point for
consideration and discussions to identify the functional floodplain” (Table 1)
Zone 3a is defined as an area with a “high probability” of flooding, more precisley, “this zone
comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%), or a
1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.”
Flood risk assessment requirements for this area are that “all development proposals in this zone
should be accompanied by a flood risk assessment.“. Since many of the potential development sites
within the parish fall within this zone, the following policy aims will apply.
In this zone, developers and local authorities should seek opportunities to:
• reduce the overall level of flood risk in the area through the layout and form of the development
and the appropriate application of sustainable drainage systems;
• relocate existing development to land in zones with a lower probability of flooding; and
• create space for flooding to occur by restoring functional floodplain and flood flow pathways and
by identifying, allocating and safeguarding open space for flood storage
For further details, please see NPPF and NPPFTG
The West Sussex Strategic Flood Risk Assessment states (January 2010) Volume II - Technical Report
(SFRA)
With reference to the occurrence of flooding from the land, the report identifies an area covering the
whole parish and beyond as prone to this type of flooding and seven distinct locations within the
parish as being prone to surface water flooding (Map L).
(Paragraph 5.71)with reference to tidal flooding “ In Chichester Harbour, the majority of Thorney
Island, and low lying areas of Chidham, Bosham, West Itchenor and West Wittering lie within Flood
Zone 3a”
In relation to groundwater –induced flooding the SRFRA states
“Soils on the Manhood Peninsula are seasonally waterlogged and clay-rich. The River Ems and Bosham
Stream in the west of the peninsula run through this relatively impermeable coastal plain, however
they have a high winter baseflow component as the headwaters are fed by chalk springs in the south
of the South Downs. Prolonged wet winter periods lead to high groundwater levels that result in
saturated ground conditions and extensive surface water in the upper catchments. This leads to an
immediate response to additional rainfall and high flow velocities due to the steep stream gradients at
the foot of the Downs. Groundwater processes are an important contributor to flooding in these
areas.“ (Paragraph 7.23)
In addition, most of the land in Bosham to the south of the A259 is described as having a high risk of groundwater
flooding and furthermore is in the groundwater emergence zone. (Map G)
Position Statement on Wastewater and Delivering Development in the Local Plan January 2014
History of Bosham Parish Foul Drains System
5.50
Many historical developments have led to a situation where parts of the parish, not
necessarily just in the south, have become more vulnerable to flood episodes. Among these may be
counted the following: Drainage management lapses leading to flooding from rainwater run-off
 Riparian owner obligations being ignored
 Culverting of open ditches
 Blocked or inadequate culverts
 Road drains not being kept clear
 Hedgerow depletion
 Farmers infilling withy ponds and osier beds
 Housing developments in the 60s and 70s
 Increased areas of hardstanding without SUDS arrangements
5.51
The sewer and storm water drainage system dates back to the early 1930s. It is a complex
arrangement of sewers relying upon six outlying pumping stations all feeding to a main station at
Stumps Lane which pumps the entire system uphill to the Harts Farm Waste water treatment plant on
the Bosham Peninsula. Treated sewage is then discharged into Chichester Harbour at the junction of
Itchenor Reach and Bosham Channel. The WSCC SRFRA has six recorded locations in the parish where
flooding has occurred from sewers. Residents know of but may not have fully reported other incidents
(Map S)
5.52
Moreover, when groundwater conditions are high following either intense or prolonged
periods of rain, water leaks into the sewer system and flows can exceed capacity making it impossible
to treat it fully. On these occasions, partially treated sewage is pumped into Chichester Harbour
through screens which only remove solid matter. The Harts Farm WWTW has been upgraded so that
all sewage now receives Ultra Violet and bacteriological treatment and nutrient stripping before it is
discharged to the harbour. However, Southern Water say that there are no plans to install UV
disinfection on storm discharges from the combined sewage/storm water outfalls (CSOs) as the
Environment Agency have not deemed this necessary in their National Environment programme 2015
– 2020.
5.53
The sewer system deals with the output of houses in Bosham Parish, some 1300, as well as
the output from the villages of West Ashling and Funtington. There have been no changes to the
capacity of Harts Farm since it was built although the main pumping station at Stumps Lane has had
more powerful pumps installed in 2006 as well as enlarged storage tanks for emergency flow.
5.54
Regular discharges/surcharges from the main sewer at Stumps Lane take place at times of
heavy rainfall as well as discharges from manholes throughout the village leaving raw untreated
sewage to be washed into the harbour. Southern Water, who manage the foul drains system,
maintain that their system is capable of accepting further house building and a figure of 400 extra
houses has been used in discussion with the District council. This figure is based upon ‘dry flow’, i.e.
no rainfall water from the roofs of houses entering the foul drain system is allowed for in the
calculation. This is clearly illogical and it is clear to the residents who live close to the harbour that the
foul drains system is already at full capacity and in times of heavy rain is overloaded.
History of Flooding from North and South
5.55
Bosham, situated on the plains below the South Downs and with a southern boundary on the
tidal harbour has always been at risk of flooding by heavy rains and from the sea by tidal surges.
Flooding from the north (rain water) has been an increasing problem as house building increased in
the years after World War II and as a result of major road building in the 1970s. These activities all
required the use of green field land, thus increasing the amount of hard standing which increased the
run-off of surface water which had previously soaked away and moved slowly through the substrate.
Incidents of serious surface water flooding resulted in a group of concerned residents all with civil
engineering backgrounds making a detailed survey in the early part of the new century which has
achieved a much improved programmed of ditch and rife clearing to ensure run-off. This has been
aided by the EA agreeing that Bosham has 3 ‘Main Rivers’ which deal with water coming from the
North. Designation as ‘Main River’ allows public funding for maintenance. These are the Bosham
Stream, the central watercourse and the East Ditch system. This latter system terminates in a ‘lagoon’
as flooding regularly occurs in the vicinity at times of heavy rainfall. Discussions are ongoing with the
EA to map the local topography and initial results suggest the creation of another ‘lagoon’ or holding
pond at the junction of East Ditch and the north ditch draining the Walton Farm area.
5.56
Flooding from the sea has been part of Bosham’s history since the earliest times. Expert
opinion varies on the amount of sea level rise through to 2050. The Harbour Conservancy work on a
figure of 18.4cm whilst the meteorologist Storm Dunlop considers it to be less at 14.8cm. Although
tidal and wind surge incidents appear to have occurred more frequently, possibly as a result of climate
change, there has been no noticeable change in tidal predictions over the last 10 years. Houses along
the High Street all have raised cills to their front doors to resist the extra high spring tides. Still parts
of the village are susceptible to flooding and many properties at the lowest level of the village have
invested in flood barriers which are installed at exceptional high tide and when gales are forecast.
Further along the Shore Road the sea wall is in poor state of repair and a programme of repairs is
needed to maintain this important defence. At the eastern end of Shore Road, an extension of the sea
wall has been completed partially funded by residents and was built by the County Council.
5.57 Effectively parts of the Bosham Parish should be considered liable to flooding under the
provisions of para 15 of the NPPFTG. This requires under the general topic of ‘Taking climate change
into account’ and states
“Flooding in estuaries may result from the combined effects of high river flows and high sea surges.
When taking account of impacts of climate change in flood risk assessments covering tidal estuaries, it
will be necessary for the allowances for sea level rise in table 4 and the allowances for peak flow,
wave height and wind speed in table 5 to be combined”. These can be found at
Plans showing the Village Sewage Systems and Harts Farm Waste Water Catchment Area
Diagrammatic representation of Bosham known flood locations, fluvial flows and field drainage,
including the Colner stream (course of ancient river), Bosham Mill-Stream, the central waterway and
the East Ditch systems. The last three are considered by the EA to be ‘main rivers’ and as such
enjoy the benefit of EA management.
The diagram overlays the 2014 SHLAA map
Diagrammatic representation of Bosham known flood locations, the three major drainage
catchments and the approximate orientation of the watersheds.
The diagram overlays the 2014 SHLAA map
Photograph showing storm-driven high tide overtopping the Trippet Wall at the mouth to
the Central Watercourse.