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TOOLBOXSite Design and Prevention of Stormwater Effects

April 2010

THE COUNTRYSIDE LIVING

TOOLBOX:

A GUIDE FOR THE MANAGEMENT OF STORMWATER DISCHARGES IN COUNTRYSIDE

LIVING AREAS IN THE AUCKLAND REGION

April 2010 There are 4 publications in this series The Countryside Living Toolbox: Background The Countryside Living Toolbox: Site Design and Prevention of Stormwater Effects The Countryside Living Toolbox: Stormwater Management Device Design Details The Countryside Living Toolbox: Water Supply Public Health Guidelines and Wastewater Management Considerations Acknowledgement: This Toolbox is Version 4.0 of several original documents done by and on behalf of the Rodney District Council and the Waitakere City Council over the past eight years. It has borrowed from the earlier versions where changes were not needed and it supercedes those documents. Permission was given by Rodney District Council and Waitakere City Council to use information from the earlier documents where use of that information was appropriate.

ISBN 978 – 1 – 877540 – 64 – 6

Documents in the Series The Countryside Living Toolbox is divided into 4 publications. Countryside Living Toolbox: Background and Application – This section defines the applicability of the Toolbox; provides background information on stormwater effects in rural areas; details the regulatory context of this guideline; describes the key stormwater design objectives and approaches; and summarises the different techniques available for use. Countryside Living Toolbox: Site Design – This section provides information on how site design can affect the volume and rate of stormwater which is discharged as a result of development. This section of the Toolbox will assist developers to “avoid” or “prevent” effects. Countryside Living Toolbox: Stormwater Management Device Design Details – This section provides design information for structural stormwater practices. Ponds, wetlands, filter strips, swales, rain gardens, infiltration trenches and rain tanks are discussed. This section of the Toolbox will assist developers to “mitigate” effects. Countryside Living Toolbox: Water Supply and Wastewater Management Considerations – This section of the Toolbox briefly discusses requirements relating to both potable and non-potable water supply. It also provides an overview of the design features and maintenance considerations associated with on-site wastewater treatment and disposal systems.

Disclaimers Waitakere District Council In situations where there are differences to the earlier versions and where they have been relied on or embodied into planning documents such as Structure Plans or Resource Consents then the requirements of the earlier versions shall take precedence over Version 4.0. Rodney District Council Infiltration in Rodney District Council Rodney District Council has significant areas of countryside where soil stability is strongly dependent on and particularly sensitive to changes in moisture content and the hydrological cycle in general. for that reason infiltration as a means of stormwater management is not seen as a viable management tool. Water Supply for re-use This section is not applied in Rodney District Council. RDC has its own provisions for re-use. Where a particular re-use application is required RDC can make available a protocol for calculating storage v consumption requirements to estimate tankage against supply. Papakura District Council The guideline provides for a number of methods and tools to mitigate the effects of storm water run-off from countryside living areas but the acceptance of any particular method and tool will depend with the respective TLA (PDC) . The extent and sharing of responsibility by TLA, property owners and developers to ensure continued performance from these methods and tools is not covered in this guideline and will depend on the consenting and approval processes of respective TLA. © 2008 Auckland Regional Council This publication is provided strictly subject to Auckland Regional Council's (ARC) copyright and other intellectual property rights (if any) in the publication. Users of the publication may only access, reproduce and use the publication, in a secure digital medium or hard copy, for responsible genuine non-commercial purposes relating to personal, public service or educational purposes, provided that the publication is only ever accurately reproduced and proper attribution of its source, publication date and authorship is attached to any use or reproduction. This publication must not be used in any way for any commercial purpose without the prior written consent of ARC. ARC does not give any warranty whatsoever, including without limitation, as to the availability, accuracy, completeness, currency or reliability of the information or data (including third party data) made available via the publication and expressly disclaim (to the maximum extent permitted in law) all liability for any damage or loss resulting from your use of, or reliance on the publication or the information and data provided via the publication. The publication and information and data contained within it are provided on an "as is" basis.

Countryside Living Toolbox - Site Design and Prevention of Stormwater Effects Page - i

Countryside Living Toolbox: Site Design and Prevention of Stormwater Effects

Table of Contents 1. THE FUNDAMENTALS OF SITE DESIGN ......................................................... 1

1.1 General Principles...................................................................................... 1 1.2 Preliminary Site Design – Collecting Information ........................................ 1 1.3 Source, Flowpaths and Receiving Environments ....................................... 2

1.3.1 Source ................................................................................................... 2 1.3.2 Pathway ................................................................................................. 2 1.3.3 Receiving environment ........................................................................... 5

1.4 Water and Wastewater Considerations ...................................................... 6 1.4.1 Water Considerations ............................................................................. 7 1.4.2 Wastewater Considerations ................................................................... 7

2. PREVENTING STORMWATER EFFECTS THROUGH SITE DESIGN (LOW

IMPACT DESIGN) .............................................................................................. 8 2.1 Stormwater Sensitive Site Design/ Low Impact Design .............................. 8 2.2 Reducing Site Disturbances ....................................................................... 8 2.3 Reducing Impervious Surfaces .................................................................. 9

2.3.1 Roads .................................................................................................... 9 2.3.2 Kerbing ................................................................................................ 10 2.3.3 Parking and Streetscape ...................................................................... 11

2.4 Alternative Lot Configuration/ Clustering .................................................. 12 2.5 Natural Areas and Riparian Margins ........................................................ 14

2.5.1 Riparian Margins .................................................................................. 14 2.5.2 Bush Replanting ................................................................................... 14

2.6 Biofiltration ............................................................................................... 15 2.7 Source Control ......................................................................................... 15

2.7.1 Contaminants ....................................................................................... 15 2.7.2 Volume of Stormwater Runoff .............................................................. 16

2.8 Summary ................................................................................................. 16 3. SEDIMENT AND EROSION CONTROL ........................................................... 18

3.1 Background.............................................................................................. 18 3.2 The Ten Commandments ........................................................................ 18 3.3 Small Site Sediment and Erosion Control ................................................ 19

3.3.1 Haybales .............................................................................................. 19 3.3.2 Silt Fences ........................................................................................... 20 3.3.3 Earth Bunds ......................................................................................... 22 3.3.4 Stabilisation ......................................................................................... 22

3.4 Compiling a Sediment and Erosion Control Plan ...................................... 23 3.5 Installation of Sediment Control Devices and Monitoring.......................... 25

4 CONSENTING AND SITE DESIGN PROCEDURE .......................................... 27 5 REFERNCES ................................................................................................... 29

Countryside Living Toolbox - Site Design and Prevention of Stormwater Effects Page- 1

1. THE FUNDAMENTALS OF SITE DESIGN

1.1 General Principles Stormwater management needs to be considered during the earliest stages of site design. The way a site or subdivision is laid out can directly affect the volume and quality of stormwater which is discharged from the site. There are a number of general principles which apply to site design and preventing stormwater effects (URS, 2005):

• Changes to the hydrological regime should be minimised (in other words stormwater discharges should mimic, to the extent possible, the natural drainage process of an area);

• Piped stormwater discharges should be set back from streams and incorporate erosion protection to prevent localised scour and erosion;

• Diffuse discharges are preferred over point source discharges;

• Site disturbance should be minimised and natural features/ vegetation protected;

• Sediment control should be utilised during the construction phase;

• Impervious areas should be limited to the greatest extent possible;

• Source control should be used where possible; and

• Appropriate stormwater management practices should be used to attenuate flows and provide water quality treatment.

Effective stormwater management includes the implementation of both structural controls (i.e. stormwater devices) and non-structural controls (such as site design). Part B of the Toolbox focuses on providing guidance on meeting the general principles described above through site design techniques.

1.2 Preliminary Site Design – Collecting Information Before beginning the design of a rural subdivision there are a few steps which need to be followed (URS, 2005). These primarily relate to investigating overland flow paths and flood levels on site. It is very important to consider the location of any new development in relation to existing overland flow paths. Blockage of these flow paths (by new buildings or planting) can cause obstructions and stormwater to be diverted to neigbouring properties. This is often the most common cause of localised flooding.

1. Obtain the relevant drainage plans from Council for the site. 2. Before progressing with any new development talk to Council about

overland flow paths to check if any run through the property and try and observe directly where water flows on the site when it rains.


3. Based on site topography the ultimate discharge point for stormwater from

the site should be determined. Examples of discharge points include streams, road drains, ground soakage, the coast.

4. If a stream runs through the property check if the Council can provide an

estimate of the 100 year ARI flood level.

5. Floor levels should be a minimum of 500mm above the 100 year ARI flood level.

6. Mark off the extent of the 100 year ARI floodplain on a plan. No filling is

allowed in this area.

7. The floodplain area should be set aside as a riparian margin/ buffer strip.

1.3 Source, Flowpaths and Receiving Environments Once the relevant information regarding a site has been obtained from council and the overland flow paths identified, the flows can be analysed and mapped. This process depends on three levels of consideration:

• Source,

• Pathway, and

• Receiving environment

1.3.1 Source In this context, source essentially means the impervious areas created by development itself, specifically the roofs, driveways, local roads and parking areas. For rural residential development, roofs will generally go to water tanks, driveways should disperse downslope and local roading will have swales or filter strips. Swales and filter strips function as pathways for the stormwater to travel from the source to the receiving environment. The main issues related to source are the level of imperviousness and the potential contaminant load coming from those surfaces. Reducing the source therefore reduces the effects of impervious surfaces. One of the aims of Part B is to recommend measures which will reduce and disconnect impervious areas (see Section 2.2).

1.3.2 Pathway The pathway is the route taken by stormwater runoff from the source to the receiving environment. The pathway can include:

• Overland dispersed flow across vegetation

• Flow via a vegetated swale

• Surface flow in a ditch

• Flow in a concrete channel or reticulation system


The pathway is important from two different contexts:

• a means of delivery of stormwater runoff to a receiving environment, and

• the pathway itself. As a means of delivery, the pathway can, depending on its composition, convey water very quickly to a receiving system or, by following natural drainage paths, deliver the stormwater to the receiving system at a more natural rate, resulting in fewer adverse impacts. The following two tables provide information on stormwater conveyance. The first table, Table B1 (McCuen, 1998) provides information on Mannings roughness coefficients, which in turn relate to flow velocities. The higher the Mannings coefficient, the lower the velocity.

Table B1 Recommended Design Values of Manning Roughness Coefficients

Channel condition Mannings n range

Unlined open channels Earth, uniform section

Clean, after weathering 0.018-0.02

In graveled soil, uniform section, clean 0.022-0.025 Earth, fairly uniform section

No vegetation 0.022-0.025 Sides clean, cobble bottom 0.030-0.040 Rock

Smooth and uniform 0.035-0.040

Jagged and irregular 0.040-0.045 Channels not maintained, weeds and brush uncut

Dense weeds, high as flow depth 0.08-0.12

Clean bottom, brush on sides 0.05-0.08 Roadside channels and swales with maintained vegetation

Depth of flow up to 210 mm

Good stand, any grass

Length about 300 mm 0.09-0.18

Length about 600 mm 0.15-0.30 Depth of flow 210mm-450mm

Good stand, any grass

Length about 300 mm 0.07-0.12

Length about 600 mm 0.10-0.20 Natural stream channels Minor streams

Fairly regular section

Some grass and weeds, little or no brush 0.030-0.035

Some weed, heavy brush on banks 0.05-0.07

Irregular section, with pools, slight channel meander; increase two above values by

0.01-0.02

Mountain streams, no vegetation in channel, banks usually steep

Bottom of gravel, cobbles and a few boulders 0.04-0.05

Bottom of cobbles, with large boulders 0.05-0.07

Table B2 (ARC, 2003) relates the type of channel material with permissible velocities to ensure that channel erosion is kept to a minimum.


Table B2 Maximum Permissible Velocities for Unlined Channels

Material Mean Velocity (m/sec)

Fine sand, colloidal 0.4

Sandy loam, noncolloidal 0.5

Silt loam, noncolloidal 0.6

Alluvial silts, noncolloidal 0.6

Ordinary firm loam 0.8

Volcanic ash 0.8

Stiff clay, very colloidal 1.1

Alluvial silts, colloidal 1.1

Shales and hardpans 1.8

Fine gravel 0.8

Graded loam to cobbles, noncolloidal 1.1

Graded silts to cobbles, colloidal 1.2

Coarse gravel, noncolloidal 1.2

Cobbles and shingles 1.5

The combination of the two tables helps to answer questions related to the following issues:

• Is the receiving pathway adequate in terms of stability for stormwater drainage to enter it with minimal impact, and

• Would the pathway provide mitigation for stormwater runoff quantity and quality?

The answer to the first question can be resolved through consideration of the pathways for runoff from a specific development. The answer to the second one is slightly more complicated and requires some discussion. As adverse effects of stormwater are cumulative, a key question relating to pathways will relate to the following:

• The erosional potential of the pathway, and

• The expected contaminant load The Erosional Potential of the Pathway If the stormwater discharge goes to a high-energy receiving environment or natural features of the pathway provide stormwater treatment and water quality concerns are not an issue, the stability of the pathway will determine whether stormwater management would be recommended for a project. If the pathway is a stable system and can accommodate the additional flow of a new project then stormwater management for pathway protection is not required. The Expected Contaminant Load A key element with respect to water quality treatment will depend on the pathway. If the pathway is a densely vegetated system that can provide water quality treatment, that treatment may substitute for downstream stormwater quality treatment on a given project. Drainage systems in rural areas that are not kerbed and rely on passage of stormwater through swales or filter strips will have water quality treatment benefits that may substitute for more formal treatment systems. Having organic matter and vegetation can provide significant water quality benefits and the potential value of using those systems should be incorporated into design and operation.


1.3.3 Receiving environment

Whether a particular receiving environment is at risk is determined by a consideration of the overall risk from the source-pathway-receptor combination. The magnitude of the risk to the receiving environment depends on the ‘strength’ of the source term, the connectivity of the pathway, and the sensitivity of the receiving environment (Gardiner, L., Armstrong, B., 2007). Types of receiving environments include:

• Streams

• Ground

• Estuaries

• Harbours

• Open coasts

• Lakes

Stormwater issues related to receiving environment are detailed in Table B3.

Table B3 Receiving Environments and Stormwater Issues

Receiving system Flooding issues Stream erosion issues

Water Quality

Streams May be a priority depending on location within a catchment

High priority if the receiving stream is a natural, earth channel

High priority

Ground Not an issue depending on overflow and groundwater levels

Not an issue High priority

Estuaries Not an issue Not an issue High priority

Harbours Not an issue Not an issue Moderate priority

Open Coast Not an issue Not an issue Low priority

Lakes Not an issue Not an issue High priority

In addition, a key issue relating to all receiving environments is the provision of outfall erosion in order to ensure scour at the point of discharge is minimised. The following Table B4 details stormwater recommendations for the individual receiving systems.

Table B4 Stormwater Management Recommendations for Receiving Systems

Receiving system Peak Control Recommended

Stream erosion control

recommended

Water Quality Treatment

Recommended

Streams Depending on location in catchment

Yes, meeting 34.5 mm rainfall release over 24 hours

Yes, depending on rural/urban and pathway

Ground No No Yes

Estuaries No No Yes depending on pathway buffering potential

Harbours No No Yes depending on flushing potential, pathway buffering potential and rural/urban context

Open Coast No No Generally not

Lakes No No Yes depending on pathway buffering potential


An example of the consideration of source, pathway and receiving environment is shown in Figure B1 (adapted from Figure 5-3 in Version 3.0, Countryside Living Methods, 2005).

Consideration of source, pathway and receiving environment will therefore assist in determining the stormwater management design objectives.

1.4 Water and Wastewater Considerations Much of the site/ background information which is needed to determine a suitable stormwater management plan for the site is also needed to determine water supply needs and whether or not on-site wastewater disposal is appropriate. The water and wastewater design issues should also be considered early in the site design phase.

Figure B1 Site Plan Showing Source, Pathway and Receiving Environment


More detail regarding water and wastewater issues is provided in Part D of the Toolbox, however, the key considerations relating to site design are provided below.

1.4.1 Water Considerations

Sources of potable water need to be determined by checking whether there is a network or groundwater supply. If no potable water is available then rain tanks can be used to provide for the potable and non-potable water supply (URS, 2005). The sizing of rain tanks for water supply, as well as treatment requirements is discussed in more detail in Part C. Also, Part D contains information relating to water supply public health guidelines for rainwater tanks.

1.4.2 Wastewater Considerations

The implications of an on-site wastewater treatment and disposal system can be quite significant, especially on small lots, and it needs to be considered in conjunction with the stormwater design. The main reason for this is because it is not appropriate for stormwater to flow through a wastewater disposal field. The type of treatment system and the size and location of the disposal field will be determined by factors such as soil conditions, dwelling occupancy, landuse and environmental constraints of the site. The land requirement of a wastewater disposal field is normally around 1000m3 for an average, three bedroom house on clay soils. This includes a reserve (back-up) area which may be equal in size to the primary disposal field. At the preliminary design stage comprehensive site surface and subsurface investigations will be needed to determine if on-site wastewater treatment and disposal is appropriate for the site. This work needs to be undertaken by an appropriately qualified engineer and requires the identification of:

1. Potential disposal areas; 2. Site characteristics (rainfall, vegetation cover, surface water drainage);

3. Site constraints (slope shape, slope angle, aspect);

4. Environmental constraints (ground water; surface water);

5. Type of soil under the disposal area; and

6. Separation between disposal fields and dwellings and/ or stormwater

disposal locations and/ or watercourses. Once the site assessment has been undertaken, design of various aspects of the wastewater treatment and disposal system can be carried out (URS, 2005). The ARC’s TP58 provides design guidance for on-site wastewater disposal and a summary of the relevant information is provided in Part D of the Toolbox.


2. PREVENTING STORMWATER EFFECTS THROUGH SITE DESIGN (LOW IMPACT DESIGN)

2.1 Stormwater Sensitive Site Design/ Low Impact Design Stormwater sensitive site design or low impact design (LID) is design approach for site development that protects and incorporates natural site features into stormwater management (ARC, 2000). It reinforces the importance of considering stormwater management at the earliest stages of site design and promotes the use of techniques that will reduce the volume, rate and level of contaminants discharged to the receiving environment. Whilst it will still be necessary to utilize the structural stormwater devices presented in Part C, LID processes will assist in reducing the sizing requirements, maintenance obligations and potentially the cost of subdivision. The key site design components of LID are:

• Reducing site disturbances;

• Reducing impervious surfaces;

• Alternative lot configuration/ clustering;

• Creating natural areas and using vegetation,

• Constructing biofiltration practices; and

• Source control. Each of these design components are discussed below (ARC, 2000). For more detail, please refer to the ARC’s Technical Publication 124: Low Impact Design Manual for the Auckland Region (2000).

2.2 Reducing Site Disturbances Minimising or reducing site disturbances is an approach to site development whereby clearance of vegetation and earthworking of soils is limited as much as practicably possible across the site. More specifically, disturbance should only occur within a prescribed distance from proposed structures and new impervious areas. The reason for limiting disturbance is because many sites have existing resources (such as native bush, trees, wetlands, etc) which have a number of environmental benefits, including stormwater management and soil retention capabilities. From a construction perspective, limiting earthwork areas will reduce sediment yields. Furthermore, newly earthworked areas are often highly compacted thereby reducing the storage/ soakage capacity of soils and promoting stormwater runoff rather than infiltration. The objective of minimum site disturbance is to maximize existing site resources and minimize sediment generation and the creation of an artificial landscape. There are 3 steps which should be followed to implement this concept (Figure B2):

1. Establish a ‘limit of disturbance’ (LOD) zone based on maximum required disturbance lengths and distances. This ‘zone of disturbance’ needs to take into account slope, soils, type of building proposed, surrounding landuses,


etc in order to ensure it is realistically achievable from a construction standpoint.

2. Decide on the LOD early in the design phase as it may influence the lot

layout/ configuration and mark it on the relevant design plans.

3. Prior to construction ensure that the LOD is staked out on the ground so that the contractor is aware of any ‘no-go’ areas.

Figure B2 An example of reducing site disturbance by creating combined

driveways (ARC, 2000)

2.3 Reducing Impervious Surfaces One of the best ways to reduce the volume of stormwater discharged is to reduce or limit the amount of new impervious area. Incremental increases in impervious area on a site or catchment-wide basis leads to increased stormwater generation as well as a deterioration in the quality of water discharged. Streetscape or planning of street systems provide us with the greatest opportunity for seeking reductions in impervious area (together with clustering development which is discussed in Section 2.4). Residential subdivisions can limit imperviousness by reducing width of roadways or roadway design to reduce total length needed to service individual properties. Commercial subdivisions can reduce imperviousness by using alternative materials (such as permeable paving) for parking areas. Kerbing of roads has a profound effect on concentrating stormwater flows and can be reduced or eliminated in some rural areas.

2.3.1 Roads

There are numerous pressures on road reserves to accommodate services, footpaths, cycleways, etc., and each local council has differing requirements relating to minimum road widths. However, wider roads are expensive to construct and use valuable land for open space, residential or commercial purposes.


The design of road widths should be directly related to expected traffic volumes. Reduction of road widths offers considerable potential for the reduction of stormwater volumes. Road lengths are also an important issue which should be considered during the site design phase. Impervious areas on accessways or driveways can be reduced by:

• Using a dual strip driveway with a grassed median strip;

• Providing shared accessways; and

• Locating the house closer to the street thereby reducing driveway length.

Another technique that can be used to minimize the effect of impervious area is to “break up” road/ hardstand areas through the use of biofiltration devices. For example, swales can be used along the side of roads or as a median strip to create “unconnected” impervious areas.

2.3.2 Kerbing

Kerbing has a profound effect on stormwater flows by concentrating flows along the kerb channel and necessitating the need for a piped reticulation system. These reticulated systems concentrate stormwater flows, thereby causing erosion and increased peak flows at the discharge point. In many instances in rural areas kerbing is not necessary. It is not the intention to advocate the elimination of kerbing, but to allow flexibility for other options where they may be viable. For example, concrete edging can be used to ensure the integrity of the road surface and kerb cuts can be used to prevent people driving onto grassed areas. Both of these concepts will allow stormwater to flow diffusely into rain gardens, swales or filter strips.

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2.3.3 Parking and Streetscape

There are some low impact approaches to parking areas which will help to reduce the amount of impervious area within the road corridor. In general in rural areas where space is not a constraint, parking areas can be off-set from the main roadway and be constructed of alternative materials such as gobi blocks or permeable pavers. These devices are discussed in more detail in Part C. Streetscape design can also have a considerable impact on stormwater flows:

• design footpaths on one side of the road only;

• use rain gardens or swales as median strips to break up impervious surfaces;

• on slopes greater than 5% use check dams to reduce flow velocities;

• use a variety of plants to aid interception of rain drops and increase evapotranspiration; and

• use permeable paving for parking areas where appropriate.

Figure B3 illustrates some different streetscape, roading and parking concepts. Engineering designs of the vehicle crossings are provided in Part C.


Figure B3 Examples of minimising impervious areas through road design

2.4 Alternative Lot Configuration/ Clustering It is during the earliest stages of site design, when a developer is still deciding on the site layout, which provides the maximum potential for stormwater effects to be minimised. Not only does the site layout affect the roading layout as discussed in Section 2.3, but also the lot configuration. Clustering of lots allows for development to be concentrated in certain areas and this has positive benefits in the form of:

• protecting environmentally sensitive areas of the site;

• minimising disturbance;

• minimising impervious areas;

• increasing open space areas for passive and active recreation; and

• reducing costs (both during the construction and long term maintenance phases).

Clustering does not seek to increase or decrease the number of available lots within a site, but rather to use an alternative design to meet district planning densities (Figure B4).

Source: Breathing Spaces (ARC, 2006)


Figure B4 Conventional (A) versus low impact (B) site development

A

B


2.5 Natural Areas and Riparian Margins The use of existing areas of vegetation and enhancement of those areas through further bush planting is integral to the low impact design approach in rural areas. Not only can additional bush areas offset increases in impervious surfaces, but redirection of stormwater from pathways or driveways onto vegetated areas provides treatment benefits and assists in ensuring stormwater discharges remain diffuse.

2.5.1 Riparian Margins

Riparian margins should be applied to all streams. The ARC recommends that margins should be a minimum of 10m in width on either side of the stream in order to ensure that the plants within the strip are self sustaining and not overwhelmed by the encroachment of undesirable species (ARC, 2001). Riparian margins serve a number of purposes, namely they:

• protect and enhance natural habitat and ecological functioning of streams;

• provide a buffer and the vegetation stabilizes stream banks thereby protecting the stream from the erosive effects of stormwater flows;

• filter stormwater runoff and reduce contaminants; and

• provide shading to streams to reduce thermal effects.

The key steps which should be followed to create a riparian margin include:

• Based on topography, stream dynamics and site design, delineate a riparian corridor on the design plans (remember that it should be a minimum of 10m on either side of the stream).

• Identify the different “plant zones”: this should be based on slope; stream low flow levels; flood levels.

• Select native plants for the different “plant zones” based on their ability to endure the site conditions and compile a planting plan.

• Clear the riparian margin of any undesirable species and start planting.

• If livestock will still be grazing in the vicinity of riparian margins then it is recommended that the margins are fenced to restrict stock access.

• Active maintenance (in the form of weeding; replanting) is required for the first few years until the riparian margin is well established.

A riparian planting guide (ARC, undated(a)) is provided in Appendix A.

2.5.2 Bush Replanting

On properties that have significant areas of pasture the establishment of native bush is able to offset increases in impervious surfaces. A relationship can be established between the level of imperviousness and the amount of new bush required to offset these areas so that it fully or partially eliminates increases in stormwater runoff volumes and peak flows. Bush replanting has a number of benefits:


• it is one of the few stormwater practices that can actually reduce the volume of water discharged;

• it provides ecological, landscape and visual amenity benefits; and

• it has reduced long term maintenance obligations in relation to other stormwater practices.

The sizing requirement for areas of bush is discussed in greater detail in Part C. In summary, to maximize the use of natural areas, as part of the site design process:

• delineate natural areas that require protection;

• delineate natural areas that require bush replanting; and

• delineate riparian margins/ buffer strips.

2.6 Biofiltration The use of biofiltration devices such as rain gardens, filter strips, infiltration and swales can provide significant water quality benefits in addition to reducing the volume of stormwater runoff. Biofiltration devices primarily use filtering of contaminants in stormwater runoff and infiltration of stormwater ground discharges to provide treatment. Discharges to ground, as well as increased evapotranspiration from the vegetation assist in reducing stormwater runoff volumes. The design of these devices is discussed in detail in Part C.

2.7 Source Control Source Control is a stormwater management approach that aims to prevent stormwater pollution at the point at which the pollutant is entrained in runoff and to minimse the volume and rate of stormwater runoff. Two key elements source control are:

• Source control includes both water quality and water quantity.

• Source control is about prevention, not mitigation, and therefore does not include traditional stormwater treatment.

2.7.1 Contaminants

Contaminants which are derived from countryside living subdivisions are most likely to include sediment from earthworking activities and metals from building materials and road runoff. Sediment Source control of sediment can be achieved by minimizing the area disturbed (as discussed in Section 2.2) and undertaking the erosion control techniques discussed in Section 3 below.


Metals By using only inert building materials (i.e. no exposed copper or zinc roofs), soluble metals discharged from roofs and walls can be prevented. Given that copper can be toxic to human health, special care should be taken to ensure that copper roofs are not used when rain tanks are the main water supply source.

2.7.2 Volume of Stormwater Runoff

Prevention/ source control of increases in the volume of stormwater runoff can be achieved by minimising impervious areas (as discussed in Section 2.3). In addition, the effect of using roof generated runoff for water supply purposes means that one can eliminate roof impervious surfaces as a contributor to stormwater runoff. Rain tanks for stormwater attenuation, as well as water reuse are discussed in Part C.

2.8 Summary Section 2 of Part B has provided a summary of site design tools which will assist in preventing the effects of stormwater discharges on the receiving environment. These tools are part of a philosophy towards site design known as low impact design or stormwater sensitive site design. Figure B5 shows an example of a conceptual site layout for a countryside living subdivision which uses these techniques.


Figure B5 Conceptual site layout of the Regis Park Subdivision in Flat Bush. The subdivision is a good example of many of the low impact design techniques discussed in Section 2.


3. SEDIMENT AND EROSION CONTROL

3.1 Background All land disturbing activities (or earthworks) are required to encompass erosion and sediment control in order to minimise the amount of sediment discharged during the construction phase. The current standard for sediment and erosion control design is the ARC’s “Technical Publication 90: Erosion and Sediment Control: Guidelines for Land Disturbing Activities“(TP90) (ARC, 2007). Information in this Section is taken from TP90 and the associated fact sheets produced through the ARC’s sediment control education programme. The full version of TP90 can be downloaded from the ARC’s website: http://www.arc.govt.nz. Sediment and erosion management needs to be thought of as two separate components, sediment control and erosion control. Sediment control involves utilising devices to limit and treat the amount of sediment that is discharged from an earthworked area. Examples of devices include hay bales; silt fences; decanting earth bunds; sediment ponds; etc. Erosion control involves limiting the process by which sediment becomes displaced (i.e. erosion) by techniques such as limiting the exposed area; earthwork along contours rather than across them; stabilising exposed surfaces with mulch or matting to reduce erodibility; using diversion or contour drains to limit the amount of water flowing over an exposed surface, etc.

3.2 The Ten Commandments There are ten commandments of sediment and erosion control which the Region has adopted and which should always be considered during the construction phase. Many of these ‘commandments’ are integrally linked to the site design / LID techniques discussed above, but it is worth highlighting them individually.

I. Minimise Disturbance: - the smaller the area to be disturbed, the less sediment that will be generated. Unexposed soil can’t erode!

II. Stage Construction: - the larger the area of exposed earth the higher the

risk of erosion and sediment discharges. Reduce the amount of exposed areas into manageable portions.

III. Protect Steep Slopes: - the steeper the slope the higher the velocity of

water and the higher the likelihood of erosion. Either keep away from earthworking steep slopes or earthwork along contours and provide good stabilization.

IV. Protect Watercourses: - streams are sensitive life supporting aquatic

systems which need to be protected. Try and avoid working within 10m of any watercourse.

V. Stablise Exposed Areas Rapidly: - the more quickly bare soil areas are

stablised using straw mulch, matting or vegetation the less the potential for erosion.


VI. Install Perimeter Controls: - clean water from upstream areas should be diverted around the site so that it is not contaminated by the earthworking activity.

VII. Employ Detention Devices: - use sediment control devices such as

sediment ponds and silt fences to trap sediment.

VIII. Attend Local Sediment and Erosion Control Courses: - the ARC runs courses for contractors and designers.

IX. Create a Sediment and Erosion Control Plan: - creating a plan showing all

the different techniques helps with implementation. Make sure that the plan evolves as the site develops.

X. Assess and Adjust: - inspect, monitor and maintain devices to ensure they

are functioning correctly.

3.3 Small Site Sediment and Erosion Control There are a number of different sediment and erosion control techniques which can be used to mitigate the effects of sediment discharges and these are all described in detail in TP90. Hay bales, silt fences and earth bunds are probably the key devices that would be used on small-scale rural developments and these are discussed in more detail below. Prior to using these devices, perimeter controls should be installed in order to divert any cleanwater from upstream areas away from the site. The drain should be stablised using a geofabric material and regularly monitored and maintained.

3.3.1 Haybales

Hay bales are temporary barriers which are used to intercept and direct surface run-off from small areas. They do not actually filter sediment, so should always be used in conjunction with other devices. They can also be used to protect cesspits from being overwhelmed with construction sediment. They should not be used for longer than a month at a time. Installation

o Work out where the haybales need to be located on the site (usually they are placed at the lower end of the work site).

o Place haybale barriers length-wise along the contours with bales in a row and

the ends tightly abutting each other.

o Dig each bale into the ground 100mm and do not place the bales more than 1 bale high.


o Secure the bales in place by using two stakes driven through the bale 300 – 400mm into the ground. Drive the first stake towards the previously laid bale at an angle to force the bales together. The stakes should be flush with the top of the bale.

Figure B6 illustrates the key features of using hay bales for sediment control and how they should be installed.

Figure B6 Installation of Haybales (ARC, undated(b)).

Maintenance

o Inspect the hay bales frequently and undertake maintenance as necessary.

o Once you have completed clearing the 50m stretch of stream and you have stabilized and reseeded it, remove the hay bale and stabilize the trench where the bale was located, and move onto the next 50m stretch.

Do not use hay bales for slopes exceeding 20%. An alternative to using hay bales to protect cesspits is the use of “sediment sausages”. The sausages are placed around the cesspit to filter sediment laden runoff and protect newly created cesspits or stormwater devices.

3.3.2 Silt Fences

A silt fence is a temporary barrier of woven geotextile fabric used to intercept runoff, reduce its velocity and impound sediment laden runoff from small areas of disturbed soil. Its purpose is to hold up flows so that sediment can be deposited at the foot of the silt fence. Installation

o Work out where the silt fences need to be located on the site (usually they are


placed at the lower end of the work site).

o The top of the silt fence should be at least 600mm above ground level. o Place supporting posts (or waratahs) along the silt fence no more than 2m

apart. Ensure the supporting posts are embedded at lest 400mm into the ground.

o Join lengths of silt fence by doubling over the fabric ends around a wooden

post or batten, or by stapling the fabric ends to a batten and butting the two battens together.

o Install silt fence wings at either end up the slope to prevent water going

around the fence.

o Make sure the silt fence is embedded into the ground by at least 200mm. Figure B7 illustrates the key features of a silt fence and how it should be installed. Maintenance

o Inspect silt fences on a regular basis and make any repairs necessary.

o If sediment reaches 50% of the silt fence height then this sediment should be removed and disposed of accordingly.

Figure B7 Installation of Silt Fences (ARC, undated(b)).


3.3.3 Earth Bunds

An earth bund is a ridge or berm of compacted soil (including topsoil) which has been constructed to impound or pond runoff so that suspended sediments can settle before the runoff is discharged. They can be constructed across slopes and near the edges of the site. They should be kept in place until the disturbed area is permanently stabilised. Installation

o Work out where the earth bunds need to be located on the site. o Using a digger, build the earth bund

and place a 160mm non-perforated pipe through the bund.

o Attach a perforated pipe to the inlet

and ensure that the top opening of the perforated pipe is 100mm lower than the stabilised spillway.

o Stablise the spillway and outlet using

a geotextile fabric.

o Ensure the top of the bund is at least 250mm higher than the invert of the perforated pipe inlet.

o Make sure the contributing catchment area is no greater than 3000m2.

Figure B8 illustrates the key features of an earth bund and how it should be installed. Maintenance

o Inspect earth bunds on a regular basis and after major storms to check for accumulated sediment.

o Check the outlet for any signs of scour or erosion.

o Make repairs as necessary.

3.3.4 Stabilisation

As soon as the earthworks are complete it is very important that any remaining exposed areas are stabilised. This can be done by:

o placing a degradable geosythentic

erosion mat along any diversion channels, swales, stream banks and reseeding the area (either using hydroseed or by hand);

o placing straw mulch over the exposed

areas; or

o placing metal over the exposed areas.


Figure B8 Installation of an Earth Bund (ARC, undated(b)).

3.4 Compiling a Sediment and Erosion Control Plan An indicative “Sediment and Erosion Control Plan” should be drawn up to assist the developer/ landowner in implementing good sediment and erosion control. Figures B9 and B10 are examples of what they might look like. There are a few key steps that the designer should go through:

1. Based on the site topography identify any overland flow paths and upstream areas which discharge onto the site.

2. Delineate the site boundaries and show clean water diversion drains to

drain off any ‘clean’ upstream water.

3. Work out the total site area and decide whether the site needs to be split into discrete catchments to provide for optimal treatment.

4. Show areas on the plan which will be cut (i.e. lowered) or filled during the

earthwork activities.

5. Show any stockpile areas on the plan.

6. Show any proposed staging on the plan.

7. Decide on the best sediment control devices to use and indicate them on the plan.

8. Indicate the proposed stabilisation method on the plan.


Figure B9 Example of an individual lot Erosion and Sediment Control Plan (amended from Brisbane City Council, undated).

Silt Fence


Figure B10 Example of a countryside living subdivision erosion and sediment control plan for the roadway areas (adapted from Figure 5-3 in Version 3.0, Countryside Living Methods, 2005).

3.5 Installation of Sediment Control Devices and Monitoring The timing, installation and monitoring of sediment control measures has a major influence on the effectiveness of the devices to mitigate the effect of sediment laden runoff to the receiving environment. Timing and Installation Sequence (Brisbane City, undated)

1. Establish a single, stablised entry and exit point to the site. 2. Install the sediment control devices on the lower side of the site. 3. Divert up-slope clean water around the work site using appropriately lined

diversion channels. 4. Clear vegetation from only those areas which need to be earthworked. 5. Stockpile topsoil within the sediment controlled area.

Proposed Accessway Alignment


6. Install remaining sediment control devices as per the erosion and sediment control plan.

7. Install on-site waste receptacles (eg mini-skips, bins, etc). These should be covered to prevent waste being removed by wind.

8. If refueling on site, ensure all refueling areas are kept away from streams. 9. Commence building activities. 10. Maintain all control measures in good working order. 11. Revegetate and stabilise the site. 12. Remove remaining sediment control devices.

Maintenance and Monitoring of Devices (Stormwater Manager’s Resource Centre, 2008) Sediment control devices such as silt fences or earth bunds will only function effectively if they are installed correctly and maintained throughout the duration of the earthworks. There are also some simple techniques which can be used to further prevent sediment discharges include:

1. Inspect controls at least once a week and after any rainfall events. 2. If problems are found, make repairs immediately. 3. At the end of each day sweep or scrape away any soil tracked onto

roadways (do not wash it down the stormwater drain). 4. If rain is predicted then cover topsoil and other stockpiles to prevent

erosion. 5. Dispose of any accumulated sediment by either spreading over the

earthworked area or at an approved disposal site.


4 CONSENTING AND SITE DESIGN PROCEDURE As discussed earlier, the main consideration for stormwater management in countryside living areas is limiting changes to site pre-development hydrology. This may not be the case for commercial or agricultural related land uses. These projects may have higher levels of site imperviousness and greater contaminant discharge potential than rural residential land uses. Some of these projects may require consent from the Auckland Regional Council but others may be permitted activities due to limited impervious surfaces. Most will require either landuse or building consent from the local council. A series of checklists have been provided in Appendix B. The purpose of these checklists is to assist designers and landowners with understanding the information requirements and thought processes needed to undertake responsible stormwater management. In addition, the checklists also support the information requirements for the stormwater and earthwork aspects of building and subdivision consents. A flow chart is provided (Figure B11) which expresses the logical progression of the site design process integrated with stormwater practice selection.


Step 4: Use the tables in Part A of the Toolbox to determine site constraints and the relevant stormwater management practices for the development.

Catchment Size: Table A1

Slope: Table A2

Soils: Table A3

Step 3: Undertake a site analysis to identify natural features/ vegetation on the site worth preserving.

Wetlands; riparian cover; existing bush,

archaeological sites; steep slopes; flood plains & other

natural features.

Step 5: Design the development/ subdivision around the sensitive areas; site constraints. The site design should consider “preventative” concepts.

Minimising impervious areas; clustering; minimizing

site disturbance; building layout; water reuse

Step 1: Determine the nature of the development and assess it against the relevant District Plan and the Regional Plan.

Meet with the relevant Council Officers at this stage to discuss your

development if you need assistance.

Step 2: Determine the technical/ design stormwater objectives and assess the discharge point/ receiving environment.

Flood Protection

Channel Protection

Volume Reduction

WQ Treatment

Step 7: Design your device. Also, assess and describe the future ownership and maintenance obligations (who will maintain the device, how, where, when, etc) and the cost of construction and maintenance.

Note: The design and practice selection may need to be reviewed at this stage if the cost is too high or the maintenance and ownership obligations cannot be resolved.

Part B

Part B

Part C

Part A

Part A

Step 6: Develop an erosion and sediment control plan for the site.

Step 8: Finalise site design and practice selection. Apply to Council for Consent.

Figure B11: Site Design and Stormwater Management Practice Selection Process


5 REFERNCES Auckland Regional Council. Undated(a). Riparian Facts: Streamside Planting Guide. (fact sheet) Auckland Regional Council. Undated(b). Erosion and Sediment Control on Construction Sites: Site Management for Permitted Activities. (fact sheet) Auckland Regional Council. 2000. Technical Publication 124: Low Impact Design Manual for the Auckland Region Auckland Regional Council. 2001. Riparian Zone Management: Strategy for the Auckland Region. Technical Publication 148. Auckland Regional Council. 2003. ARC Technical Publication 10: Stormwater Treatment Devices: Design Guideline Manual. Auckland Regional Council. 2006. Breathing Spaces – Creating memorable places with living infrastructure. http://www.arc.govt.nz. Accessed on 4 August 2008. Auckland Regional Council. 2007. Technical Publication 90: Erosion and Sediment Control – Guidelines for Land Disturbing Activities in the Auckland Region. Brisbane City. Undated. Erosion and Sediment Control Management Plans: Best Practice Guidelines for the Control of Stormwater Pollution from Building Sites. Fact Sheet 9. Gardiner, L., R., Armstrong, W. 2007. Identifying Sensitive Receiving Environments at Risk from Road Runoff. Land Transport NZ Research Report No. 315 McCuen, R., H. 1989. Hydrologic Analysis and Design. 2nd Edition, Prentice-Hall, Inc. The Stormwater Manager’s Resource Centre. 2008. Small Site Erosion and Sediment Control Guidance. http://www.stormwatercenter.net/Model%20Ordinances/esc_small_site_ordinance.htm. Accessed on 18 July 2008. URS Ltd. 2005. Countryside Living Methods: Version 3.0. Prepared for WCC and RDC.

Site Design and Prevention of Stormwater Effects

APPENDIX A

RIPARIAN PLANTING GUIDE

Site Design and Prevention of Stormwater Effects

APPENDIX B

INFORMATION CHECKLISTS

Checklist 1: Pre-Development Site Assessment (amended from URS, 2005)

Pre-Development Site Assessment

Please enter information or tick if the required information is attached

1. Size of site ha

2. Topography and site contours

3. Extent of existing vegetation cover and land uses m2

4. Existing drainage patterns, streams, wetlands and springs

5. Receiving environment description

6. Relationship (location) of site to the entire catchment area

7. Flood plain maps (if available)

8. Water supply source

9. Wastewater disposal (on-site or connected to system)

Checklist 2: Development Proposal Assessment (amended from URS, 2005)

Development Proposal Assessment

Please enter information or tick if the required information is attached

1. Number and size of proposed lots

2. Lot layout and roading layout

3. Landuse type (i.e. commercial, rural residential, etc)

4. Total impervious area ha

5. Breakdown of impervious area by type:

- roof area m2

- driveway areas m2

- parking areas m2

- other paved areas m2

6. Discharge location/ receiving environment

7. Required design objectives:

- flood protection

- channel protection

- volume reduction

- water quality treatment

8. Site analysis inventory (identifying natural areas)

9. Site constraints: catchment area

10. Site constraints: slope

11. Site constraints: soil type

Checklist 3: Site Design Assessment

Site Design Assessment Please tick if able to comply & information is attached

1. Site disturbance minimised

2. Impervious areas minimised:

- roads

- kerbing

- parking and streetscape

- clustering of lots/ alternative lot configuration

3. Natural areas/ bush retained

4. Riparian margins protected and planting proposed

5. Use of bush replanting to offset impervious area increases

6. Use of biofiltration practices (swales; filter strips; etc)

7. Source Control:

- control of building materials

- use of water tanks for reuse

8. Erosion and Sediment Control Plan