Beachlands Comprehensive Flood Management Plan
Beachlands Catchment Comprehensive Flood Mangement Plan, Babbage Consultants, 1993 (June).
Description | Management Study | Management Plan | Proposed Works
Catchment Description
Beachlands Catchment is an isolated coastal urban development within the rural area of Manukau City. The catchment is predominantly zoned Residential R5 with a total area of approximately 200 hectares.
The urban area is divided into two main catchments, draining
east and west respectively from Wakelin Road which follows
a low north-south ridge across the middle of the area. The
eastern catchment called Shelly Bay Road Catchment is drained
by an open water course piped under Bell and Shelly Bay Roads
and discharging to the sea just outside the eastern limits
of the urban area The western catchment called Cherrie Road
catchment drains to an open water course discharging to the
sea at the western limit of the area, to the north of the
Beachlands marina.
The northern limit of these two main catchments is roughly
along First View Ave and there are several smaller catchments
to the north of this which drain directly to the harbour via
outfalls down the cliffs.
Catchment Background
Beachlands is a long established residential area which has developed from a beach holiday destination with many bach and temporary or lower quality houses being constructed over the years to the point where the existing urban area is fairly fully developed in single family housing with a small commercial area in Wakelin Road and a small partly developed light industrial area south of Third View Ave at Bell Road. Beachlands was subdivided during the 1920s into 870 1012m2 lots with a uniform grid layout. The 1945 census recorded a permanent population of 90. The 1991 Provisional Census recorded a population of 2242. Development over the years has continued without regard to adequate stormwater drainage systems, and sanitary sewage disposal has been by way of septic tanks.
In recent years ground soakage has been a problem which has been exacerbated by the lack of adequate stormwater a;sposal systems. Beachlands generally has a high water table due to the relatively flat contours and low soil permeability and during the winter months the ground tends to reach high levels of saturation. This prevents the septic tank soakage systems from functioning adequately. This has the potential of becoming a health hazard as polluted water is finding its way into the roadside water table drains and open water courses. This is evidenced by the smell and algae growth which is evident in many locations particularly at Beachlands Road in the vicinity of the reserve, and has also been reported by local residents.
Stormwater surface flooding has been reported in various areas within the catchment. This is generally due to the lack of suitable reticulation to remove water from lower lying areas and constitutes a nuisance more than a risk to property or buildings. There is as a consequence a restriction on new development and infill housing in Beachlands until these problems can be resolved.
Design of a sewer system is now being carried out and sewer connections should be available for all lots within a few years.
FUTURE STORMWATER RETICULATION
As part of the Study a conceptual piped stormwater system was designed to connect cesspits in all streets assuming full kerb and channel and to provide drainage connections to all house lots. The system assumes existing open channels in the lower reaches of the catchment remain where these are in undeveloped blocks, but assumes that all areas at present zoned residential are fully developed for the estimation of percentages of impervious areas for calculation of runoff. It is also assumed that with a future reticulated water supply all roof water will also go to the stormwater system. The system incorporates the existing pipework and culverts except where existing pipes are substantially undersized for future developed conditions.
Allowance has been made for the most probable development, all presently zoned residential lots developed with single unit housing and 50% of lots over 1000m2 subdivided and developed with infill housing. Note: The current 1989 District Scheme has a minimum subdivisional area requirement of 1500m2 for Zone 5.
Most of the existing pipework has been found to have adequate capacity, but the particular problems have been identified as summarised in section 4 of this Plan.
Management Study
This Catchment Management Plan is based on the findings of a Catchment Management Study carried out jointly between Manukau City Council Technical Services and Babbage Consultants Ltd. A brief summary of the analysis procedures used in the Study is included below:
Hydrological and Hydraulic Parameters
Modelling Technique
The catchment stormwater piping network and overland flow
system was analysed using the Ilsax computer modelling technique:
Treatment of Rainfall
The Ilsax method applies a set of rainfall precipitation data
to the hydraulic model.
The Auckland City Council 'Model Rainstorms for Stormwater
Runoff Analysis' was used as the basis for developing hyetographs.
As these storms are based on Albert Park records, multiplication
factors of around 0.9 were incorporated to attempt to represent
likely rainfall in Manukau City. A series of model rainfall
data for various durations and for summer and winter were
applied to the catchment model.
Treatment of Losses
Allowances were made for the various loss factors considering
surface depression storage, soil infiltration and initial
soil wetness.
Catchment Makeup
Each subcatchment of the area was assigned an estimated ratio
of impervious area. Allowance was made on the assumption that
50% of residential lots over 1000m2 would be developed with
future infill housing, being the assessed future most probable
development of the catchment with infill housing as may be
permitted by proposed revisions to the District Scheme Plan
(1989). It is assumed that roof water will also go to the
stormwater system.
Pipe Network
<< Refer to previous page for stormwater reticulation
4.0 Summary of Drainage Problems
For the 20% AEP event which may be considered to be the design storm for the piped network there are a number of existing pipe reaches in which flow capacities of the pipes will be exceeded. (Refer to Primary System 20% AEP Performance in the Study Report, section 2.6).
It has been conservatively assumed that all properties will contribute directly to the assumed future pipe network. In actual fact a number of properties may remain with soakage pits rather than piped connections.
Specific Drainage Problems
From the analysis carried out the following major problems have been identified:
- Pippins Culvert at Cherrie Road
This culvert has insufficient capacity to pass the expected flows in a 1 % AEP event. The overland flow would result in a habitable floor level flooding (at number 27), and Cherrie Road becoming impassable. The storage upstream of the culvert tends to be effective only in a 20% AEP event. Concern has been expressed by local residents regarding frequent flooding and the associated health and erosion problems.
- Sunkist Bay Road Culvert at Fourth View Road
The overland flow expected in a 1 % AEP event will result in a habitable basement floor at number 8 Sunkist Bay Road being flooded. This can be attributed to the floor level being constructed close to grade, and the fact that three minor overland flowpaths combine into one significant overland flowpath immediately upstream.
- Shelly Bay Road Culvert
The house at number 48/50 immediately upstream of the 900 diameter culvert south of Third View Ave has its floor level below that of the road level. This would result in its habitable floor level becoming flooded if the culvert became blocked or in an extreme event once development has occurred in the upstream undeveloped block of land. The frequency of such flooding would be increased if the storage capacity of the open water courses were to be reduced.
- 24-26 Shelly Bay Road
The developed basements of these houses are liable to be flooded due to overland flow being impounded by the roadway.Ealing Crescent Subsurface Water
At the west side of Ealing Crescent between Bell Road and Pohutukawa Road there are properties threated by cliff erosion and slips. To reduce any ingress of water from the present roadside channels to the underlying strata there is a proposal for subsoil cutoff drains to be constructed in this area.
- Local flooding in low lying areas
The area has generally moderate slopes with some areas flat and relatively poorly drained. All streets are sealed but only a few have kerb and channel and a piped system to collect stormwater from the cesspits. All the remaining streets are drained by deepish open channels each side of the relatively narrow sealed carriageway with local culverts under intersections and vehicle accessway. Stormwater is piped from low points in streets by short local pipe system to natural waterways or to the sea.
Except for a few areas no piped reticulation is provided to lots for collection of roof water, etc. The existing stormwater drainage is therefore below normal urban standards and there are problems with minor flooding in flatter areas, sitting of drains under roads and stagnant water lying in roadside open drains.
- View Bay
The existing local piped drains are too small to be incorporated in a future piped system for the fully developed catchment except as secondary drainage. However, the existing 600 diameter culvert across Pohutukawa Road and the pipeline to the existing outfall have sufficient capacity to pass the 20% AEP storm for the future developed situation.
The non-habitable basement of house 5A is expected to flood from the overland flow.
- Hawke Catchment
The existing 360 culvert crossing Hawke Cres and the downstream 460 diameter pipe would be overloaded with future development, but can be bypassed with future reticulation.
- Puriri Catchment
The present outfall for road drainage is by way of an open channel which discharges through a pipe over the cliff. This outfall location is also not suitable for draining clifftop properties west of Puriri Road.
- Sunkist Catchment
The existing 460 pipeline from First View Ave to the outfall traverses an old slip area behind the beach.
- Unserviced Areas
There are a number of problems of local nuisance due to the lack of piped house connections and the lack of kerbs and channels to control road drainage in some areas.
These problems will be reduced when future reticulation and kerb and channel works are carried out.
- Cliffside Properties
Generally all cliffside properties in Beachlands are under threat of erosion and landslip. Control of surface water, and subsurface water is therefore important in such areas.
Management Plan Key Features
The following items are the key features of the Management Plan.
- Primary System Upgrading - installation of additional piping to provide full reticulation as required to suit existing and future development.
- Secondary System Modifications & Protection - of overland flowpath for flows not conveyed by pipe network.
- Building Controls - restrictions imposed to minimise risk of flooding hazards.
- Environmental Protection
- reduction of detrimental effects of stormwater discharge
- upgrade selected outfalls.
These are shown schematically on Figure 2>>
Particular Management Principles
- Progressively install additional piping to upgrade the reticulation .to MCC urban standards.
- Progressively construct kerb and channels to all streets to MCC urban standards.
- Protect all overland flowpaths so they can operate when required so as not to flood habitable floors.
- Impose building controls to protect floor levels of future developments.
Proposed Works
Primary System Upgrading
Cherrle Road Culvert
A new parallel pipe system is proposed including the road culvert and the next downstream reach. The sizes indicated are for 1 % AEP flows without storage upstream of the culvert and allows for possible future piping of the upstream open channel. The design flows are 7.7 m3/sec and 11.1 m3/sec respectively.
Sunkist Bay Road
Construct kerb and channel with cesspits, on west side of Sunkist Bay Road between Third View Avenue and Beachlands Road. In conjunction with future reticulation which will divert flows away from this location, the above measures will reduce the overland flow passing across the road and through property number 8 where basement flooding occurs. Refer also to 8.2.5.
Property Connections
Progressively upgrade the reticulation to service all properties
as determined by development pressure and availability of
funds.
Street Drainage
Progressively construct street kerbs, channels and drainage.
The priority of this will be as determined by the Roading
Section of Council, but note that the western side of Ealing
Crescent should have a high priority on account of the surface
flows and ground percolation of stormwater being a possible
contribution to the nearby cliff erosion and slip risk.
Secondary System Modification and Protection
Overland Flowpaths General
The Flood Hazard Map indicates the major overland flowpaths within the catchment. It needs to be recognised that stormwater will flow generally along these paths whenever the piped system is unable to convey the runoff. Future building development including fences and landscaping should be controlled to prevent blocking of fiowpaths. Floor levels should be established to ensure 0.500m minimum freeboard from water levels which can be estimated from the given or specifically calculated flow rates.
Forming of Overland Flowpaths with New Pipe Installation
Where new pipes are installed along overland flowpaths an
improved overland flowpath should be formed as a depression
in the ground along the pipe route where practical as part
of the reinstatement works.
Kerbs & Channels
Where kerbs and channels are constructed across overland flowpaths,
kerbs should be set at a level to maximise the capture of
water by road cesspits and thereby reduce the overland flow
in locations where it causes problems in adjacent downstream
properties.
Driveway crossings in such locations should be constructed
in such a manner to minimise the overflows of water from the
road to private properties.
Ealing Crescent
A project is in hand to install subsoil cutoff drains alongside
the seaward side of the carriageway of Ealing Crescent to
intercept groundwater flow which may be contributing to the
local cliff erosion problems.
Sunkist Bay Road
Form overland flowpath between properties number 6 and 8.
48/50 Shelly Bay Road
It is recommendea for consideration that the house on property 48/50 should be raised approximately 1 metre in level so that the floor is at least 0.5 metres above the crest of the road at its low point adjacent to the property. This should be done prior to any significant development of the open land southwest of this property. It is also recommended that blockage protection is installed.
24-26 Shelly Bay Road
The basements of these houses should be provided with some protection against flooding by the formation of a stop-banked overland flowpath designed to contain and direct the overland flow away from the basements. Note that as the basements are below road level the potential of flooding will still remain.
Building Controls
Floor Levels In the Vicinity of Overland Flowpaths
Locations of proposed buildings and levels of floors should
take into account overland flow. In particular note the flowpaths
shown on Figure 12.
Buildings should not obstruct the overland flows and floor levels of buildings should have adequate freeboard (0.5m minimum) above potential flood ponding or flow levels which can be estimated from the flow rates. New buildings may be located in a flowpath provided any impeding of overland flow is not detrimental to other properties and provided care is taken to divert overland flow away from the building to prevent possible erosion danger to building foundations and to avoid excessive wetness under the building.
Where the flowpath is intercepted by roadways the local floor levels upstream of the roadway should also be a minimum of 1.0m above the roadway level.
The above recommendations should be confirmed at individual
locations and can be modified where calculations can be provided
to determine more accurate potential flood levels. A freeboard
of 0.500 metres should be allowed in addition to estimated
flood water levels.
8.3.2 Private Foul Water Drains
Gully traps on private foul water drains should be at such
a level to exclude the entry of stormwater, but kept below
habitable floor level.
Cllff-top Properties
Proposals for development of cliff-top properties should have
adequate means of disposal of stormwater without the use of
existing or new private outfalls over the cliff face unless
these can be shown to be completely acceptable. Such outfalls
will be required to have specific discharge permits.
Proposed Developments
Development proposals in areas where there is no existing
piped stormwater system need not necessarily be disallowed
prior to the proposed full reticulation works being carried
out. However it will need to be demonstrated that such developments
will not significantly cause or increase flooding or adverse
effects to adjacent or downstream properties. At the time
of such development it may be prudent to install local stormwater
piping which will eventually be connected to the proposed
future reticulation system.
Property Information Register
Refer to Appendix for list of properties which should be identified
on the P.LR.
Environmental Protection
Refer to general principles of coastal protection contained in Part 21 of the 2nd Reviewed District Scheme 1993 and the Coastal Management Plan - Manukau City Council - 1991.
Soakage Pits
As development proceeds and as stormwater connections can
be made from properties to the proposed future stormwater
piping system the use of soakage pits should be generally
discouraged in this catchment where the seepage water is likely
to find its way to the coastal cliffs. Such seepage is part
of the source of cliff erosion problems. For a similar reason
the use of abandoned deep bore foul water seepage pits should
not be used for stormwater disposal.
Precautions to be observed In Drain Construction
Works
Certain areas of the catchment are underlain by organic subsoils
which could be the source of consolidation settlement problems
if dewatering occurs as the result of drain trenching. A further
consideration in the region of the shoreline cliffs and slopes
is the risk of stormwater ingress through trench backfill
resulting in seepage at the cliffline and possibly increased
slope instability. A Geotechnical Investigation Report has
been prepared by Fraser Thomas Partners September 1992 for
the Beachlands Sewerage Reticulation Scheme. Conclusions and
recommendations of this report are appended to. the stormwater
study together with 'Beachlands Stormwater Reticulation -
Geotechnical Issues'. Extracts from this report are appended
to this Plan.
Preservation & Maintenance of Open Channels
Natural open channels and detention areas within the catchment
should generally be protected to remain in their natural state
as far as practical.
Culvert Blockage Protection
The main culvert at Shelly Bay Road adjacent to property number
50 should have a debris screen installed to reduce the potential
of blocking and subse.
Gross Pollutant Traps
A gross pollutant trap should be installed near the outfall
from each of the two major subcatchments, ie. near outfall
A from Cherrie Road catchment and outfall O from Shelly Bay
Road catchment.
The Cherrie Road gross pollutant trap could be located immediately downstream of the proposed sanitary sewer pumping station to be located behind property number 27 Cherrie Road.
Stormwater Outfalls
This catchment has a number of outfalls consisting of open
pipes discharging over the top of the shoreline cliffs. Prior
to any construction of new piped reticulation systems, the
particular outfalls should be inspected and approved for use
if suitable, or modified, rebuilt or relocated as appropriate
to comply with acceptable erosion protection and visual standards.
The existing outfalls have been inspected and a summary report
is included as an appendix to the Study Report.
It is proposed that several of the outfalls should receive some rebuilding or maintenance attention.
These are designated as outfalls B, C, E,and N, the locations of which are indicated on figure 2.
Proposed works are as follows:
Outfall B (off Puriri Road) : Construct concrete/rock erosion protection lining to open channel between end of pipe and cliff.
Outfall C (off Hawke Cres.) : Place rocks set in concrete around existing lower concrete structure to arrest erosion and improve appearance. Existing concrete deflectors are somewhat ugly, but as the location is not one of high visibility, it is proposed that they be left.
Outfall E (off Cherrie Road) : The protruding pipe should be cut back and grouted rock protection constructed down to the beach to restrict erosion.
Outfall N (Shelly Bay) : Remove unsightly section of existing concrete pipe and concrete saddle from the beach in front of the timber wall.
New Private Outfalls
Should individual properties require future drainage upgrades
it is strongly recommended to connect to existing outfalls.
On the other hand it is not considered necessary to absolutely
restrict the construction of new beach-front or cliff outfalls.
However these will require specific discharge and coastal
permits. In some cases it may in fact be preferable to use
new outfalls rather than to construct drains across the cliff
tops or across steeply sloping land.
Abatement Measures
The Study Report, provides a conceptual stormwater system to provide connections to all properties.
However, note that stormwater source abatement measures should be introduced where practical. Refer to sections 8.6 and 8.7 of this Management Plan.
Cesspit Cleaning and Street Sweeping
A maintenance programme which includes annual cleaning and
sweeping should be maintained. It should be noted that cesspit
cleaning and street sweeping not only removes obvious visual
pollution, but also removes other environmental pollutants
which would otherwise be discharged in the stormwater and
contaminate the receiving waters.
As the receiving waters are open with good flushing compared
to an estuary, further treatment of stormwater discharge is
not considered to be necessary for this catchment.
Collection and Disposal of Debris
There needs to be a programme for collection of debris from
debris screens, wetlands, cesspit cleaning and street sweeping..
Disposal of debris and contaminated sifts should be in accordance
with Auckland Regional Council guidelines. This will generally
mean transfer to a controlled landfill site.
Allowance for Future Development
The proposed stormwater system upgrading (both primary and secondary) have made allowance for the most probable level of infill development up to the year 2011, on the same basis as that used for the sewerage scheme development (Refer 2.3). Note that the 'Beachlands/Maraetai Planning Study' prepared for the Council by Colleen Crampton July 1992 proposes a lower density of allowable development.
When future developments are proposed for properties which are presently unserviced by direct stormwater connections the following matters should be considered:
- As a first preference, source control measures should be utilized to eliminate or minimise the increase in stormwater runoff from new developments. This will reduce the impact of new developments on stormwater flows in downstream regions of the catchment. Refer to section 8.7 for possible options.
- Installation of new piped connection to nearest public drain. This may include installation of branch drains and consequent downstream system improvements as indicated on the conceptual reticulation system shown in the Study Report.
Control of Stormwater Runoff From New Developments
Restrictions or source control measures shall be utilised to eliminate or minimise the increase in stormwater runoff from new developments.
Consideration should include the following controls:
- Prevent development in likely problem areas
- Require some form of runoff control - detention tanks or ponds
- Discourage the over-use of impervious ground cover
- Paved areas should be kerbed and shaped to direct all runoff into cesspits and then into the piped or soakage system
- Impose a levy to fund future drainage improvements, and possibly delay development until such improvements can be carried out.
Stormwater disposal by site soakage only. This will only be possible where ground conditions allow soakage and where suitable engineering designs are carried out. Seepage overflows must also be considered. Note 8.4.1 for recommended restrictions on use of soakage disposal in this catchment.