Maraetai Catchment

Maraetai Comprehensive Flood Management Plan-
Prepared by Manukau Consultants in conjunction with Babbage Consultants Ltd, June 1993

Background | Management | Proposed Works | Environment | Abatement

Background

Catchment Description

Maraetai is an isolated coastal urban development within the rural area of Manukau City. The catchment is predominantly zoned residential R5 with an area of approximately 210 hectares. The catchment is divided into six major subcatchments as follows:

• Maraetai Shcool Road Catchment - discharging to the sea at the eastern end of the Omana Regional Park.
• Omana Beach Catchment - a series of subcatchments north of Maraetai Drive discharing at several outfalls along Omana beach.
• Te Pene, Maraetai and Rewa Road Catchments - discharging to the sea at the western end of Maraetai Beach
• Maraetai Beach Catchments - a series of sub catchments discharging at several outfalls along Maraetai Beach

Figure 1. Maraetai Catchment Layout>>

Catchment Background

Maraetai originated as a beach holiday settlement but has now developed a generally suburban character with predominantly good quality single family homes of permanent residents.

The land was subdivided in stages from the late 1920s with 300 lots created in the first ten years. The 1945 census recorded a permanent population of 130. Most development had occurred since the 1950s. The 1991 Provisional Census recorded a population of 1249.

Approximately 55% of the area zoned residential is presently built on.

Future Stormwater Reticulation

As part of the Study a conceptual piped storwmater system was designed to provide piped connections to local upper areas of the main catchment and to similar smaller catchments.

Most existing streets are seale dwith kerbs and channels and have piped drainage from cesspits mainly in short runs to outfalls to the sea or to watercourses. In both main catchments the lower primary drainage lines are open channels, except for local piped culverts under roads.

Generally therefore prelimnary design for future stormwater drainage has been confined to local areas of upper catchments and to the smaller catchments and to checking the effects of future development on the existing road culverts.

At present few lots have piped connections for drainage of roof and yard water. Where lots slope to a street frontage sufrace flows to the street or roofwater connections to the street channel are unlikely to cause problems. Where lots drain to existing residential land it is considered that a piped system for collection of roof an

Management Study

The 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.

Summary of Drainage Problems

Most of the present piped stormwater drainage is in similar pipes up to 300 diameter in upper reaches fo catchments while the lower reaches of the primary system are open channels with culvert under roads. Nearly all existing pipes have adequate capacity for the 20% AEP design storm flows, and where pipes are undersize, the resulting overflows are not likely to cause any serious problems.

Specific Drainage Problems

From the analysis carried out the following major problems have been identified:

• Maraetai Drive Culvert near Rewa Road
  The existing twin 1500 dia culverts under Maraetai Drive to the Maraetai Beach Car park are undersized for 20% AEP flows and the road is likely to flood to 1500mm above the crown for a short period. For the 1% AEP event flows will overtop the road for about 30 minutes to a depth of up to approximately 750mm.
• Rewa Road
  A short length of shallow open channel in lot No.122 north of the end of Rewa Road does not allow the downstream pipe (900dia) to flow full before hte overland flow occurs.
  
  Northwest of the bend at the end of Rewa Road there is a short length of open channel and a branch channel upstream of 1350 dia pipe. These channels do not allow the 1350 dia pipe to flow full before overland flow occurs.

Both of these problems could be reduced by completing the piping as part of the future reticulation works.

• Unserviced Areas
  There are a number of problems of local flooding 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 adn channel works are carried out.

Management Plan Key Features

The following 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 and Protection - of overland flow path for flows not conveyed by pipe network.
• Building controls - restrictions imposed to minimise risk of flooing hazards.
• Environmental Protection - reduction of detrimental effects of stormwater discharge and upgrade selected outfalls

Particular Management Principles

• Progressively install additonal piping to upgrade the reticulation to MCC urban standards.
• Progressively construct kerb and channels to all streets to MCC urbans 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
Maraetai Drive Near Rewa Road

The road has a low point near the culvert adjacent to the Maraetia Beach Reserve Car Park. There is a risk of flooding to 1500mm above the road crown for 20% AEP flows. At 15 AEP the road overflow is approximately 12m³/sec which would make the road impassable for about 30 minutes.

Figure 5. Maraetai Drive/Rewa Road Flooding>>

If in the future it is decided to raise the road to alleviate this flooding, a high level overflow culvert could be incorporated under the raised road. This culvert together with the existing twin 1500 diameter culverts should be designed to apss the 1% AEP flow without impounding the flood waters south of the road embankment to higher than say RL 4m which is approximately the existing situation. This would prevent additional flooding threat to upstream properties.

It is anticipated the road would need to be raised to about 1 metre above the adjacent downstream levels in the reserve. The overland flows for the west catchments could remain on the respective north and souht sides of Maraetai Drive without flooding the road or requiring any piping upgrades. Raising of the road is considered to be the best long term option for solving the road flooding.

A considerably lower cost alternative proposal would be to cut down the ground levels over a small area of the reserve to reduce the depth of flooding which could occur across the Maraetai Drive. However, it is recommended that the above option of raising Maraetai Drive should be further considered prior to any works being carried out.

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 the Council.

Secondary System Modification and Protection

Overland Flow Paths General

It needs to be recongised 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 flowpaths. Floor levels should be established to ensure 0.500m minimum freeboard from water levels which can be estimated from teh given flow rates or specifically calculated.

Figure 9. Maraetai Catchment Flood Hazard Map>>

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 of works.

Kerbs and 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 overflow of water from the road to private properties.

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 9 and the Flood Hazard Map. 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. Refer also to 8.4.3.

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.

Maraetai Road / Rewa Road Flooding

Refer to figure 5 which shows the extent of flooding upstream of Maraetai Drive due to the embankment formed by the level of the Maraetai Beach carpark (approximately RL 3.5). Habitable floors in the location shown should not be lower than RL 4.5 which is the approximately maximum flood water level plus a freeboard of 0.500 m.

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 future reticulation system.

Environment 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.

Slope Instability

At some locations the steep slopes within this catchment are subject to the risk of slope instability. A Geotechnical Investigation Report has been prepared by Fraser Thomas Partners January and September 1992 for the Sewerage Reticulation Scheme. Conclusions and recommendations of this report are appended to the stormwater study together with "Maraetai Stormwater Reticulation Geotechnical Issues". Extracts from the latter are appended to this Plan.

Precautions to be observed in Drain Construction Works

Slope instability as mentioned above can be adversely affected by infiltration of surface water into uncontrolled pipe trench backfill. The-above-mentioned report contains recommendations to guard against this danger. Refer to appendix.

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.

In line with the general objectives and policies set out in Section 6.1 of the Second Reviewed District Scheme, this Comprehensive Flood Management Plan recommends requirements to maintain and preserve significant gully and stream areas to enhance water quality and minimise the impact of urban development.

In order to protect the existing natural gully areas, development should not be permitted in these areas in a manner that would adversely affect the natural overland flow paths and storage characteristics which reduce the impact of urban development on the land drainage system.

The open gully systems should be maintained as close to natural condition as practical.

In particular, no building development, filling or other works should be permitted within that part of the existing natural stream valleys where water will flow in the predicted 1 % AEP storm event.

In steep regions there exists potential for erosion. To provide acceptable control against such scour it should be the Council's policy to undertake and encourage private property owners to undertake protective planting as outlined below. In this regard the Council could make available, free of cost, to property owners plant material to achieve these objectives. In doing so the property owners involved for part, would be expected to plant, maintain and protect the gully areas in their natural state.

To encourage the re-vegetation of the gully areas the Council should advise all property owners concerned of the above policy and seek cooperation in maintaining the open waterways.

Debris Screen

The main culverts under Maraetai Drive at Omana Beach Rd and at Maraetai Beach Reserve near Rewa Rd should have debris screens installed to reduce the risk of blocking and subsequent flooding upstream and over the road. Regular maintenance will be required particularly after major storms.

Refer to figure 6>>
Refer to figure 7>>

Gross Pollutant Traps

A gross pollutant trap should be installed near the culvert at Maraetai Drive near Rewa Road.

Refer to figure 7>>

Stormwater Outfalls

This catchment has a number of outfalls consisting of pipes or open channels discharging at beaches. 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.

It is proposed that several of the outfalls should receive some rebuilding or maintenance attention. Refer to figure 8. These are designated as outfalls G,S,M,H and P, the locations of which are indicated on figure 2. Proposed works are as follows:

Outfall G (Omana Esplanade) : Remove existing 900 dia pipe from beach and terminate with an outflow manhole in the grass reserve. Connect the manhole with a low-flow (approx 20% AEP) connection to the emergency overflow pipe from the proposed sewer pump station. (upgrade the overflow pipe from 300 to 450 dia.) A specific discharge permit has been sought for this.

Outfall S (Maraetai Beach) : Reduce the visual impact of the existing 600 dia pipe exposed on the lower half of the beach by constructing a gently sloping rock-faced encasement over the pipe.Place a marker pole at the end of the pipe.

Outfall M (On east side of Te Pene point between Omana & Maraetai) : Re-shape existing open channel across grass reserve to allow mowing maintenance.

Outfall H (Omana Esplanade) : Re-shape existing open channel across grass reserve to allow mowing maintenance.

Outfall P (Maraetai Beach Reserve) : Cut back exposed end of 900 dia pipe to finish flush with rock-faced embankment and tidy up with mortar or stone facing.

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.

Refer to figure 4>>

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.
Because 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 silts 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 as the basis for the sewerage scheme development. Note that the "Beachlands/Maraetai Planning Study" prepared for the Council by Colleen Crampton July 1992 proposes a lower density of allowable developments.

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.
• Installation of new piped connection to nearest public drain. This may include installation of branch drains and ponsequent 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:

1. Prevent development in likely problem areas
2. Require some form of runoff control - detention tanks or ponds
3. Discourage the over-use of impervious ground cover
4. Paved areas should be kerbed and shaped to direct all runoff into cesspits and then into the piped or soakage system
5. Impose a levy to fund future drainage improvements, and possibly delay development
   until such improvements can be carried out.
6. 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.