Beachlands Comprehensive Flood Management Study

Beachlands Catchment Comprehensive Flood Management Study, Babbage Consultants, 1993 (June).

Catchment Description

Location

Beachlands is an isolated area of Coastal urban development within the rural area of Manukau City. The area considered in this study is that area predominantly zoned R5 on Planning Map No. 28 and with a total catchment area of approximately 200 ha. Its location within the Auckland region is shown on drawing 11923/BCH-1.

Drawing 11923/BCH-1 defines catchment location>>

The boundaries of the residential area generally follow stormwater divides and all stormwater in the urban area is generated within the area. Part of Karaka Road and lots to the south of this road drain to the neighbouring rural catchment and have not been included in the study.

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 which we have 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.

Drawing 11923/BCH-2 defines these catchment boundaries in detail>>

Topography and Existing Drainage

The area has generally moderate slopes with some areas flat and relatively poorly drained. The coastline to the north and west consists of steep cliffs averaging about 12m high, To the east and south the area abuts on rolling farmland.

All streets are sealed but only the east end of Beachlands Road to Wakelin Road, Wakelin Road itself between Ealing Crescent and Fourth View Ave and short lengths of Ealing Crescent and Third View Ave have kerb and channel and a piped system to collect stormwater from 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 accessways. Stormwater is piped from low points in streets by short local pipe systems to natural waterways or to the sea.

Except in a few local areas no piped reticulation is provided to lots for collection of roof water etc. However there is a piped system serving lots in the block bounded by Wakelin, Beachlands and Sunkist Bay Roads and Fourth Ave, and also for the industrial lots south of Third View Ave between Wakelin and Bell Roads. There are also lot drainage systems serving 3 small recent back lot subdivisions in the Shelly Bay Road catchment and some local drainage to lots in other areas where flooding has been a problem.

The existing stormwater drainage is therefore below normal urban standards and there are problems with minor flooding in flatter areas, sifting of drains under roads and stagnant water lying in roadside open drains. The reduced roadway width required by the open channel drainage system also causes problems in vehicles parking and/or passing. With the construction of a public sewer system it will be possible to allow more intensive development by subdivision of lots for infill housing and the need for an improved drainage system will increase.

There are problems on many clifftop lots with weathering and erosion of the top of cliffs. This is a natural and unavoidable occurrence but the process is considerably accelerated when roof water is discharged over the cliff edge or where roof water is taken to soakholes near the clifftop. Discharge of roof water down the cliff in yellow plastic pipes reduces erosion but is environmentally undesirable, and such outfalls should be collected in a piped system where possible.

The existing drainage reticulation is shown in plans 11923/BCH-3,4,5

Geology & Soils

The geology of the Beachlands Catchment area under study is shown on the New Zealand Geological Industrial Map Series - Sheet N42/6 Howick (1:25,000). The urban area under study is shown to have alluvium deposits from the Tauranga' Group's overlying interbedded sandstone and siftstone of the Waitemata Group. The alluvia! deposits are described as being clay and silt with some peat, firm and deeply weathered, derived from coastal and river terraces 12 to 20m above mean sea level.

From our experience in this area the alluvium (a₄) comprises light orange to grey soft to very stiff clays and silts. Soakage testing for effluent disposal generally gives low permeabilities in these deposits. The Waitemata Group sandstones and siltstones at shallow depth are hard and moderately weathered. These form the undulating hills and vertical sea cliffs found within this catchment. Joints in the Waitemata Formation are generally moderately to widely spaced and tightly closed. These sediments typically act as an impervious layer to percolating groundwater, with springs and streams forming at or near this horizon.

Surficial deposits in the Beachlands Catchment are generally alluvium from flwiatile and coastal environments. There are the occasional scans or lenses of silty sands which are considered to have moderate horizontal permeability. Vertical percolation in this catchment would be negligible.

Existing Development

The existing urban area is at present fairly fully developed in single family housing with a small commercial area in Wakelin Road and a small partly developed Industrial area south of Third View Ave at Bell Road. Most of the area is subdivided into 1011 mz lots.

There are three areas at the rear of front lots and at present zoned residential 5 which have not yet been subdivided into residential lots. These are areas east of Shelley Bay Road, in the area bounded by Bell Road, Shelley Bay Road, Third View Ave and Beachlands Road, and an area south of Second View Ave between Puriri and Cherrie Roads. These unsubdivided areas total approximately 20 Ha.

No reticulation for sewer or water supply is at present provided. Design of a sewer system is now being carried out and sewer connection should be available for all lots within a few years.

Development Potential

The Beachlands Catchments are substantially urbanised with land use as described in drawing 11923/BCH-5A overleaf. The land use comprises:

predominantly residential 5 zone (70%) dispersed throughout the Beachlands area. Residential 5 is defined as unserviced settlements in the rural part of the city, where no reticulation of sewage or water supply is provided.
a small pocket of industrial 1 zone which is defined as light industrial
small commercial areas (predominantly centred around Wakelin Road) and the designated area of Beachlands Primary School.
various areas of public space designated Reserve 1, 2, or 4, and the public streets.

The Manukau City Council population projections for the Beachlands area to the year 2011 assume that all lots under 1000m2 are occupied by single unit dwellings. Lots over 1000m² will be able to be developed with a minimum lot size of 500m² and it is assumed that 15% of these will be developed to their full potential.

For the assessment of future stormwater drainage flows this study therefore assumes that all unsubdivided areas at present zoned residential are developed, that 15% of all lots are developed for infill housing, that all streets are upgraded with kerb and channel drainage to normal subdivision standards, and that generally all lots will have a stormwater drainage connection for roof and yard water.

Background

General History of Beachlands Catchment

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. Development has continued over the years without regard to adequate stormwater drainage systems, and sanitary sewage disposal has been by way of septic tanks.

In recent years soakage has been a problem which has been exacerbatedby the lack of adequate stormwater disposal systems. Beachlands generally has a high water table due to the relatively flat contours and low permeability over the area and during the winter months the ground tends to reach high levels of saturation. This prevents the septic tank systems from functioning adequately as soakage from the septic tank drainage lines is impeded, This has the potential of becoming a health hazard as polluted water is finding its way into the roadside water table drains. 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.

There is as a consequence a restriction on new development and infill housing in Beachlands until these problems can be resolved.

Flooding History

Flooding has been reported in various areas within the catchments. This is generally due to the lack ofsuitable reticulation to remove water from lower lying areas and constitutes a nuisance more than risk to property. The main areas of concern are at the Cherrie Road catchment at the Pippins Culvert and in the Shelly Bay Catchment at the two culverts passing under Shelly Bay Road.

Analysis Summary

Critical Storm Season & Duration
From the catchment analysis it has been found that the storms producing the highest flow rates and highest storm flood water levels are as follows:

1% AEP event:	summer
20% AEP event:	winter

The reasons leading to these results are that the 1% AEP summer rainfall intensities are approximately 45% higher than those for winter and these high intensities produce higher runoff despite the greater losses which occur in the drier summer ground conditions. However for the 20% AEP events, although the summer rainfall is still significantly higher than winter (36%), the saturated winter ground conditions have a greater proportional effect on runoff and the winter season rainfalls produce the higher runoffs for this 20% AEP event.

For comparison, the results at the outfall of the Cherrie Road Catchment are as follows, assuming detention above Pippins Culvert:

		Peak Flow (m³/s)	Runoff Volume (m³/s)	Rainburst Duration (min)
1% AEP	Summer	16.24	28600	60
1% AEP	Winter	12.50	25600	60
20%AEP	Summer	4.30	9120	60
20%AEP	Winter	5.20	8140	30

Without detention above Pippins Culvert and with the culvert upgraded the results are as follows:

1% AEP	Summer	17.20	28700	60
1% AEP	Winter	13.33	25700	60
20%AEP	Summer	6.50	8930	60
20%AEP	Winter	7.86	7960	30

The 'critical storms'. i.e, events which result in maximum flow rates, are generally of duration approximately 60 minutes for most areas within the catchment. At the upper ends of reaches, peak flows will occur for shorter duration, typically 30 minutes.

Summary of Drainage Problems
It has been conservatively assumed that all properties contribute to the runoff. In actual fact a number of properties may continue to use soakage pits rather than piped connections, and local losses may be higher than assumed for the overall catchments. On the other hand, some surcharging of pipes at down pipe connections or at cesspits and manholes will increase the flow capacities of the pipes. Against this there are likely blockages particularly at cesspits which will restrict inflow to the piped system, It is difficult to take account of these factors in any precise manner, but for any particular location or indicated problem area these points may need to be addressed.

Primary System 20% AEP Performance

the existing pipes that are stressed are shown on plans 11923/BCH-12>>
11923/BCH-13>>
11923/BCH-14>>

The extent to which they are stressed is indicated by a percentage figure eg 60% means that the pipe is 60% overstressed compared to the design flow. The stress indicated is that which would occur in existing pipes when upstream piping is upgraded as shown, but without the installation of parallel or bypass pipes as shown. The final complete networks on these plans will function adequately for the 20% AEP design condition.

Specific Drainage Problems
From the analysis various problems have been identified as referred to in the following section s 2.4.3 to 2.4.10 (not included in this summary).

Management Options

General Management Options

Management options consider various alternatives that may be appropriate to remove or alleviate existing problems or prevent or reduce the effects of flooding due to future property developments. Options categorise into Structural Options, which involve the construction or modification of existing drainage works, and Planning Options, which involve legislated or recommended restrictions, controls, precautions, etc., to minimise or avoid future problems.

Structural Options

Possible physical modifications to the drainage system include the following:

a) do nothing: allow existing problems to remain with existing nuisance potential, dissatisfied local residents, insurance claims, etc.

b) reduce inflow:
construct soakage pits instead of or in series with existing direct connections

c) reduce flow peaks:
construct detention tanks or ponds

d) facilitate entry of stormwater into piped system: create conditions to surcharge inlets

e) increase capacity of piped system:

install larger pipes
install parallel pipes
streamline or widen natural open channels
provide piped diversions to bypass flows

f) remove flow obstructions:

improve entry into cesspits
streamline culvert entrances
debris protection to culvert entrances

g) increase capacity of overland (secondary) flowpaths:

widen or streamline flowpaths
remove obstructions to flowpaths
provide new overland paths

h)provide pumping facilities

i) construct stop banks

j) purchase properties:

remove flood prone dwellings
raise level of flood prone dwellings
raise ground level of access driveways or yards in flooding properties

k) maintenance:
regular removal of siltation in street channels, cesspits and culverts, open channel debris cleaning, vegetation control, siftation control.

Planning Options
Possible planning options to reduce the consequence of flooding include the following:

Land use restrictions in flood-prone locations
prevention of new building construction or additions
requirements for elevated floors
recommendations for property modifications
imposition of flood criteria on new developments
preserving of flood attenuation features such as open streams overland flowpaths,
flood plains and detention ponds in public reserves and also in private properties.

Economic Appraisal
The benefits and costs associated with management options for rehabilitation of the existing stormwater system arising from infill housing or other means of 'urban consolidation' cannot be treated in isolation to the total impact of 'urban consolidation' on the community and its total economic feasibility.

Urban consolidation may be defined as

'achieving a greater proportion of future urban growth within established areas'

The arguments for urban consolidation may be grouped within three broad themes

Economic & Efficiency
Social Justice
Sustainability

Economic and Efficiency Arguments

substantial economic and financial benefits, in terms of capital cost savings on infrastructure and travel cost savings for the individual, will be achieved.
better use will be made of existing infrastructure
better use will be made of existing urban land
there will be a reduction in the servicing needs of the new development on the fringe.

Social Justice Arguments

greater housing choice and affordability
better access to public transport, employment, retail, community and recreational facilities
enhancement of the social mix of an area
revitalisation of run down areas previously vacant, under-utilised, or derelict.

Sustainability

reduction of the pressure to continue the energy wasting urban sprawl and which will result in a more environmentally sound form of urban development.

Together with the above propositions which support the arguments for urban consolidation, concerns and issues exist. It is of interest to note that in a study carried out for the City of Melbourne in 1987 the estimated average economic and financial benefit to the community was A$41,640 per household where a dwelling was constructed in the Municipality of Melbourne (or in the inner ring suburbs) in preference to one built in the growth corridor fringe areas. The estimate was developed from calculations of the reduced travel to work costs for the individuals and both the capital and recurrent cost savings to the Responsible Authority for providing water, sewerage, roads, electricity, education and community services. The underlying assumption was that the existing infrastructure is not being used to its capacity and therefore could accommodate increased patronage and usage.

It is outside the scope of this report to consider impacts from urban consolidation other than those relating to the stormwater infrastructure. Only the cost associated with implementing management options have been identified and recommendations given with reference to the capital cost.

Analysis

Beachlands General

The objective of the Beachlands stormwater study. apart from identifying specific drainage problems, is to provide the basic design information for the future extension of the existing incomplete system to a full stormwater drainage reticulation system to M.C.C. standards including the envisaged future development of the urban area.

The timing of the actual construction of the stormwater reticulation will depend on many factors, such as:

the programme of construction of kerb and channel in existing streets and the consequent elimination of open drain water tables.
the subdivision of residential land at present undeveloped.
the proposed construction of the sanitary sewerage system will possibly make it expedient and economic to provide stormwater reticulation to lots at the same time in some areas.

The construction of the stormwater drainage extensions may therefore be rather piecemeal, but the basic design information provided by the study will enable any part of the system to be constructed in accordance with the overall design plan.

The drawings and estimates show some of the existing open drains, in particular those in the lower Cherrie Road and the lower Shelly Road catchments, remaining as open drains. If it is required that any of these drains be piped the design information will enable appropriate pipe sizes to be selected to suit the overall design plan.

Many of the concerns about stormwater drainage expressed by residents relate to the present absence of kerb and channels and piped drainage in most streets at Beachlands. The deep open channels restrict road widths leaving insufficient parking or passing space for cars. Regular maintenance of channels is required to prevent problems occurring with standing water, poor control of stormwater runoff onto sections and silt runoff.

The extension of the stormwater drainage system should therefore occur in conjunction with a general street upgrading programme. However there are several areas where piped drainage connections for roofwater should be provided to lots where flat sections and poor soakage cause flooding problems. Piped drainage connections are also desirable for many cliff top sections to reduce cliff erosion problems.

Cherrie Road Catchment

This catchment is almost fully subdivided for housing except for the bottom of the catchment below Cherrie Road where development will have little effect on the stormwater drainage system of the rest of the catchment.

Extension and upgrading of the existing stormwater drainage will be dependent on road upgrading and the programme for sanitary sewer reticulation and new housing development.

Several stressed sections of existing stormwater pipeline have been identified and it is recommended that these be relieved by by-passing some upstream flows to new pipelines in the upgraded system. Generally the overflows from existing stressed pipes do not cause sufficiently serious problems to require urgent action and planning of stormwater system extensions in conjunction with new street kerb and channel construction and new housing development will overcome problems in a reasonable time.

Pippins culvert in Cherrie Road should be programmed for upgrading in the near future. The existing culvert in conjunction with detention storage upstream of the culvert entrance, will pass the 20% AEP storm flow but partial blockage of the culvert is difficult to prevent and additional capacity is desirable to avoid overflow across the road. The culvert would then have sufficient capacity to pass the 20% AEP flow without the need for upstream detention.

The existing 900 pipe across Sunkist Bay Road has capacity to carry the 20% AEP flow, but overflow in lower frequency storms could cause property damage. It is considered that with the proposed extensions to the stormwater system which will divert some of the present flow from the Sunkist Bay Road culvert the frequency of overflow will reduce so that risk of damage from overflow will be acceptable.

Shelly Bay Road Catchment

This catchment contains most of the areas in Beachlands not yet fully subdivided for housing, and extensions and upgrading of the stormwater system will be depended on further development as well as the programme for roading improvement and for sanitary sewer reticulation. Several existing pipelines have been identified as being stressed or becoming stressed as development in the catchment occurs.

In addition the possible requirement for upgrading the two main culverts across Shelly Bay Road should be included in planning for future development in the catchment so that the cost of culvert upgrading can be recovered from developers.

A schedule of costs based on the completed stormwater reticulation system for the existing developed areas as shown on Plan Nos BCH-7 and BCH-5.

View Bay Catchment

Upgrading of the storm, water system in this catchment will be dependent on factors such as housing development. road upgrading and the progress of the proposed sanitary sewer reticulation.

Several lines have been identified as stressed and should be upgraded at the time of extending the stormwater system; H3 and H4 on plan BCH-11 as the upstream sections are reticulated, and HB3 as the drainage is extended into the eastern end of First View Avenue.

A schedule of costs based on a completed reticulation system as shown on plan BCH-8.

Hawke Crescent Catchment

Upgrading of the stormwater system in this "atchment will be dependent on road upgrading and the progress of the proposed sanitary sewer reticulation.

Several lines have been identified as stressed and should be upgraded at the time of extending the stormwater system.

Within this catchment, priority should be given to upgrading the lines from Hawke Crescent to the outfall to ensure there is sufficient capacity for upstream development as it occurs.

Reticulation to new cesspits on Hawke Crescent will help to control the amount of surface water which discharges across the cliff edge. As this is a sensitive area in terms of erosion, any measures which minimise this occurrence should be implemented.
A schedule of costs based on a completed reticulation system as shown on plan BCH-8 is included overleaf.

Puriri Catchment

Upgrading of the stormwater system in this catchment will be dependent on road upgrading and the progress of the proposed sanitary sewer reticulation.

There is minimal drainage reticulation within the catchment, with discharge over the cliff causing erosion.

A new outfall should be constructed at reach E4, plan BCH-9 with lines E3, EB1, EB2 to collect water from Puriri Road as first priority. This new outfall will also allow cliff top properties on the westside of Puriri Road to be connected to the stormwater system.
Reticulation into Puriri Road and Second View Avenue should occur at the time of road upgrading.

The line along the cliff top to reticulate residential lots will reduce surface runoff from discharging over the cliff edge.

Wakelin Catchment

Extension to the stormwater system in this catchment can be programmed to the requirement for drainage of house lots and to fit in with progress of the proposed sewer reticulation. Street upgrading and kerbs and channelling in First View Ave will also require stormwater extensions.

A schedule of costs for the completed stormwater extension as shown on Plan BCH-8.

Pohutukawa Catchment

Upgrading of stormwater drainage in this catchment will be dependent on housing development in Pohutukawa Road and First View Ave and on street upgrading in Pohutukawa and Bell Roads. The proposed new outfall line from Pohutukawa Road will be required at a reasonably early stage to allow development to proceed without overstressing the existing outfall line.

A schedule of costs for the completed stormwater extension as shown on Plan BCH-8.

Ealing Catchment

Upgrading of the stormwater system in this catchment is a straightforward extension to provide drainage connections to house lots and will be dependent on progress with sewer reticulation and on housing development. A schedule of costs for the completed stormwater extension work as shown on Plan BCH-8.

Sunkist Bay Catchment

Upgrading of the stormwater system in this catchment will be dependent on progress with sewer reticulation and the need to provide a connection for overflow of the proposed sewer pumping station in Sunkist Bay.

A schedule of costs for extensions to the stormwater system as shown on Plan BCH-8.

However before any decision on the route for the outfall pipeline is made there should be an engineering investigation of the stability of the slopes above Sunkist Bay so that proper provision for movement can be made in laying pipelines, or alternative pipeline routes can be chosen to avoid unstable ground.

Flood Hazard Maps

These maps show the major overland flowpaths within the Catchment. Minor overland flowpaths have not been shown. The expected flowrate (litres/sec) of the flowpath is shown for a 1 % AEP event. The location of the overland flowpath has been derived from existing contour information, and exact location will need to be field inspected.

No attempt has been made to estimate the width of the flowpath as this will vary with the local contour and any constructed structures within the indicated flowpath.

Areas of ponding have been indicated.

Effects of New Developments on Overland Flowpaths

New developments should not be allowed to impede overland flow. Furthermore in some instances it may be advisable to require the improvement or formation of overland flowpaths as a condition of approval for development.

The Flood Hazard Maps indicate the general major locations where overland flowpaths should be considered. For any development, the local flowpath will need to be more accurately determined from a site inspection perhaps requiring some survey measurements. If the channel cross sections and slopes are determined at positions critical to the particular development then estimates of likely flood water levels can be calculated using the flow rates given. Habitable floor levels of new buildings should then be established at suitable clearances above these estimated flood levels. Any existing downstream features which may impound flood waters and cause ponding should also be taken into account.

Water Quality

Sedimentation and Pollution Control
Background

In recent years, research has shown that urbanisation can have adverse consequences on streams and receiving waters, including an increase in flooding, stream bank erosion and pollutant export. Historically, management efforts have primarily concentrated on reducing the risk of downstream flooding.

Recently some countries have developed a series of practices that could remove urban pollutants.

Most of these involve extra detention, retention or infiltration of urban stormwater to enhance pollutant removal and to provide additional stormwater management. Initial tests performed in the USA have demonstrated that these could serve a dual purposes; controlling pollution and providing effective stormwater management. A practical manual for planning and designing stormwater quality control devices has been produced by the Department of Environmental Programs in Washington DC USA. The Auckland Regional Council is in the process of developing guidelines suitable for application in the Auckland region.

Impact of Urban Runoff

In areas under development, construction sites can become the source of silt pollution to waterways. In a developed catchment, pollutants accumulate rapidly on impervious surfaces and are easily washed off. The primary source of most pollutants is from the atmosphere. Once deposited up to 90% of the atmospheric pollutants deposited on impervious surfaces are subsequently delivered to receiving waters.

The various surfaces of the urban environment are also an important source of many pollutants. Trace metals, for example, are a common component of many urban surfaces, such as flashing and other roofing materials, galvanised pipes, paints, wood preservatives, brake linings and tyres. Over time these surfaces corrode, flake, decay, dissolve or leach out enabling the metals to wash away in runoff. This process can be exacerbated by the acidity of rainfall caused by atmospheric pollution.

The net effect of urbanisation is to increase pollutant export by at least an order of magnitude over pre-development levels. The impact of this higher export is felt not only on adjacent streams, but also on downstream receiving waters such as lakes, streams and estuaries.

The Treatment Techniques
The Purpose of a Treatment System

A treatment system is intended to remove as much as possible of the pollutants that are exported from a catchment and to reduce the peak flow rates that cause erosion.

The pollutant removal capability of a treatment system is primarily governed by the following inter-related factors.

the removal mechanism used
the fraction of the annual runoff volume that is effectively treated
the nature of the urban pollutant being removed

Treatment Methods

There are three broadly based options available

Wet ponds - which have a moderate to high capability of removing most urban pollutants depending upon how large the volume of the permanent pool is in relation to the runoff produced from the catchment. Wet ponds utilise both settling and biological uptake, and are capable of removing both particulate and soluble pollutants. In addition to increasing the volumes of the permanent pool, wet pond removal rates can be enhanced by establishing marshes around the perimeter and by adjusting the geometry of the pond.
Extended Detention Ponds - rely primarily on settling to remove pollutants. Depending on how much and how long runoff is detained, it is possible to achieve moderate or high removal rates for particulate pollutants that are relatively easy to settle. However, removal rates for most soluble pollutants are quite low, although it is possible to enhance rates by incorporating biological removal mechanisms into the design of the pond, for instance by establishing a shallow marsh in the bottom stage of a dry extended detention pond.
Infiltration Practices - which filter runoff through the soil layer have a moderate to high removal capability for both particulate and soluble urban pollutants, depending upon how much of the runoff volume is effectively exfiltrated through the soil layer.

Effectiveness

Current guidelines have the objective of removing approximately 75% of pollution levels that would otherwise be exported from the catchment. In developed urban areas, quality improvement measures are considered to be acceptable to a lower level taking account of the practical difficulties of installing pollution control systems in urban areas.