Waikato Stormwater Management Guideline Draft 28 February 2017

Waikato Stormwater Management Guideline “Draft” 28 February 2017

General comments • This is a draft guideline and subject to change based on internal council review and comments submitted during the public review process. • It is an evolutionary guideline that builds on previous guidelines done by the Auckland Regional Council, NZTA, HBRC, BOP and Tauranga City so there is some consistency between jurisdictions. • This workshop is not meant to be educational but rather informative on substantive changes that are being considered at this time.

Guideline Contents • • • • • Overview Discussion of the region Objectives Effects of land use on stormwater runoff Receiving environments Stormwater management concepts Choosing a stormwater management approach Hydrology and water quality Stormwater management practice design Bringing the elements together Industrial site management WRC administered drainage areas Rural residential development Innovative products Outlet design Contaminant load estimation Construction related issues Operation and maintenance Landscaping guidance Retrofitting considerations

Significant Areas of Difference from TP 10 • Low impact design is blended in throughout the guideline. • Receiving environments have defined stormwater issue prioritisation. • Regulatory information requirements spelled out. • A scoring matrix for stormwater management approach. • Retention of the first 5 mm of runoff to offset loss of predevelopment initial abstraction. • Revegetation as a stormwater design practice. • Soil conditioning to mitigate for potential compaction. • A section devoted specifically to industrial site management. • Criteria for WRC administered drainage areas. • Criteria for rural residential development. • Significant changes to innovative practices. All of these areas will be discussed in the following slides.

Best Practical Options (BPO) approach Using the BPO approach along with these stormwater management guidelines is possibly the clearest path to obtaining discharge consent, but these guidelines are not requirements but rather provide a recommended approach to obtaining a consent. An applicant can propose an alternative approach to site development and obtaining a discharge consent but the application shall demonstrate that a comparable outcome is obtained that limits adverse downstream impacts to the same extent. This includes quantity (including volume of runoff), quality and aquatic resource impacts.

Facts about the Waikato Region • The region supports over 35, 000 km of streams and rivers, • Before European settlement freshwater wetlands covered approximately 1, 100 km 2 or 5% of the region, • Today approximately 280 km 2 or 1% of the region is in wetland vegetation, • There approximately 103 natural ‘named’ lakes in the Waikato Region plus five hydro-power lakes , and • The region has abundant aquatic resources and those resources are vulnerable to degradation by our activities, both rural and urban.

Impacts associated with stormwater runoff Conversion of land to urban use results in the establishment of significant impervious surfaces. These surfaces prevent rainfall from soaking into the ground and cause impacts related to the increased stormwater runoff from those surfaces. Impervious surfaces also convey contaminants efficiently into drainage systems where they are transported to receiving systems. Additionally increased stormwater runoff occurring on a more frequent basis causes stream channel physical structure degradation.

Contaminants Parameter Suspended sediments Oxygen demanding substances Description Soil and organic particles entrained in stormwater flows Soil organic matter and plant detritus which reduce the oxygen content of water when they are consumed by bacteria Pathogens are disease-causing bacteria and viruses, usually derived from sanitary sewers. Faecal coliform and enterococci are often used as indicators of the presence of pathogens Metals Can be in particulate or soluble form. Most commonly measured metals of concern are zinc, lead, copper and chromium. Metals are persistent and do not decompose Hydrocarbons and oils Generally associated with vehicle or industrial use Toxic trace organics and Compounds found in New Zealand waters including organic pesticides polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides Nutrients Usually considered for nitrogen and phosphorus Litter Often referred to as gross pollutants. It has a high visual and amenity impact

Channel enlargement associated with impervious surfaces Where bankfull stream discharge in a rural catchment may occur once every 1. 5 - 2 years, urban streams can flow at full stage a number of times a year. Less rainfall generates more runoff, which increases the amount of work done on stream channel boundaries.

The Regulatory Framework (Policy Setting) • • • The vision and strategy Waikato Regional Policy statement (WRPS) Relevant WRPS Policy Provisions Waikato Regional Plan Healthy Rivers Wai Ora: Plan for Change He Rautaki Whakapaipai All of these items are discussed in further detail in the guidelines

Technical Objectives The primary objectives therefore relate to the removal of contaminants from stormwater, reducing peak discharges, and reducing site run-off by volume control. However, prevention is better than cure. To fully meet stormwater objectives, stormwater management solutions will be recommended that are integrated with development where all opportunities are taken to prevent and minimise stormwater effects. This would necessitate the use of low impact design (LID) approaches in site design and in catchment master planning.

A key objective is to use a holistic approach to addressing stormwater issues and not rely on only mitigative approaches, which have limitations. These limitations include the following items. • Lack of site design flexibility, • Altered site hydrology, • Expense, • Loss of site area, • Potential increased impacts to site and catchment natural resources, • Configuration of development, • Connection of impervious areas, • Disregard of site resource conservation benefits, and • Maintenance obligations

Low Impact Design Advocacy of prevention in conjunction with mitigation Site design flexibility Limited site hydrologic change Reduced impact on catchment natural resources Incorporation of site natural features in stormwater management planning • Reduced maintenance obligations • • •

Throughout the guideline there will be emphasis on an approach to site development that incorporates: • Use of building materials that do not increase contaminant discharge downstream. • Source control via alternative approaches to site development that reduce the generation of stormwater runoff. • Use of natural drainage systems such as swales or filter strips to the degree that they can be incorporated. • Use of stormwater practices to provide an overlay to the first three items, as needed.

Receiving Environment Issue Prioritisation 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 Floodplains Peak flows need to be considered downstream of development Channel stability is Not an issue considered as an issue of concern Wetlands Ground High priority Not an issue depending on overflow High priority Not an issue High priority Karst areas Estuaries Harbours Open Coast Lakes High priority Not an issue Could be an issue if increased stormwater runoff increases lake water levels, even temporarily. Not an issue Tributary and outlet channel stability High priority Moderate priority High priority Geothermal areas Not an issue Protecting existing land cover in areas adjacent to geothermal areas

Stream hierarchy and regional percentages of Waikato streams Stream Order Length (m) 1 2 3 4 5 6 7 Total 26, 718, 000 7, 879, 637 4, 486, 129 2, 336, 487 1, 181, 286 522, 597 435, 126 43, 559, 262 Length in % of total length 62 18 10 5 3 1 1 100

Regulatory Information Requirements • General context information (surrounding land, catchment location, site size) • Ancillary benefits considered (crime prevention, energy efficiency, ecology, landscape) • Site natural features (wetlands, streams, floodplains, riparian, site vegetation , soils, groundwater, slope, cultural issues) • Receiving environment factors considered (coastal, sensitive areas, etc. ) • Hydrological factors (flow through site, erosion, runoff reduction, revegetation, connection of drainage system) • Stormwater issues related to the receiving environments (from Table) • Building programme (water, sewer, total number of units, lot density and flexibility) • Lot configuration consideration (lot size reduction, clustering, natural feature protection) • Impervious surface reduction considerations (road configuration, driveways, parking ratios and sizes, kerbing requirement) • Site disturbance minimisation (disturbed area reduction, natural or cultural features, site compatibility, revegetation potential) • Design calculations (CN reduction, volume reduction, time of concentration maintenance) • Mitigation alternatives (stormwater integration with site plan, prevention rather than mitigation, finally mitigation approach) • Long term operational considerations (responsibility, whole of life costs)

Implementation elements Typical components Source control maximised Water reuse LID Scoring Matrix Minimum total score is 15 Maximum Individual score Total score for each item 0 -3 depending on % of runoff capture Site disturbance reduced from a 0 -3 depending on % of conventional development runoff capture approach Impervious surfaces reduced 0 -3 depending on % of from a traditional approach runoff capture Use of building or site materials that do not contaminate 0 or 1 for residential 0 -3 for commercial or industrial 0 or 3 Existing streams and gullies located on site (including ephemeral) are protected and enhanced. The entire stream other than possible crossings shall be protected to qualify for points. Riparian corridors are protected, 0 -3 enhanced or created Protection and future preservation of existing native bush areas 0 -2 depending on percentage of site area LID stormwater practices used Swales and filter strips Tree pits Traditional mitigation Constructed wetlands Wet ponds Innovative devices Detention ponds (normally dry) 0 -6 depending on % of runoff capture 0 -3 depending on % of site covered 0 -6 depending on % of runoff capture 0 -3 depending on % of runoff capture 0 -6 depending on % of runoff capture 0 -4 depending on % of runoff capture 0 -1 depending on % of runoff capture 0 Infiltration practices to reduce runoff volume Revegetation of open space areas as bush Bioretention Total score 48 or 50 (depending on site use)

How the Table Works This table is the driver behind getting developments to consider and implement low impact design approaches on their sites. The draft guideline does have detailed scoring values for each of the parameters in the table. When the draft guideline is available to the public, the detailed guidance can be seen. The example of source control scoring is provided as an example of the approach. The purpose of todays workshop is to acquaint you with the principles behind the preferred approach to site development. Once the total score is calculated, the minimum score in terms of acceptability is 15. Scores lower than that will have to justify rejection for those items not incorporated. At least 6 points are required for source control and 6 points required for LID practices individually. Highway projects are different from normal development projects and the ability to do source control is limited. As a result highway projects must still consider LID and traditional mitigation practices and must achieve a total score of at least 8.

Source control scoring approach Water reuse • Flow detention only is 1 point. • Site use for garden, toilet and laundry is 2 points. • Site domestic use for all household needs is 3 points. Site disturbance reduced from a conventional development approach • 10 % reduction from a conventional development is 2 points. • 20% and greater reduction from conventional development is 3 points Impervious surfaces reduced from a traditional approach • 5% reduction is 2 points. • 10% reduction is 3 points. Use of building or site materials that do not contaminate • Residential roofs, gutters, down spouts made of non-contaminant leaching materials is 1 point. • Commercial roof, gutters, down spouts made of non-contaminant leaching materials is 3 points. Existing streams and gullies (including ephemeral streams) are protected and enhanced • Preservation and protection of natural streams and gullies is 3 points. Riparian corridors are protected, enhanced or created • Riparian corridor protection scores depend on the width of corridor provided. 10 metres on either side of the stream is 1 point, 20 metres is 2 points and greater than 20 metres is 3 points. Protection and future preservation of existing native bush areas • Protection, preservation and, if needed, enhancement of native bush areas that exceed 10% of the site is given 2 points.

Typical LID practices and approaches

Hydrologic design method Hydrologic analyses for all stormwater management purposes should be done according to the Waikato Guideline for Stormwater Runoff Modelling. The route through regulatory requirements can be more readily demonstrated when using the local guideline in the designated manner. However there is scope for a consultant or other entity to request use of an alternative method or computer model. The consultant must be able to demonstrate that it is robust and provides comparable outputs. The primary situation where alternative methods of design may be used, with council concurrence, is when catchment-wide analyses are done. Communication between the individual proposing an alternative method of design and WRC should be done prior to modelling being initiated to ensure there are no disagreements on the method of analysis.

Recommendations for peak flow control There are five requirements related to peak discharge control: • Where there are existing flooding problems the post-development peak discharge for the 100 -year storm for a new project be limited to 80% of the pre-development peak discharge. • In terms of intermediate storm control, the 2 - and 10 -year postdevelopment peak discharges shall not exceed the 2 - and 10 -year predevelopment peak discharges. • In addition, the rainfall data for the post-development 2 -, 10 - and 100 year storms should account for climate change. • These recommendations only apply to projects located in the top half of catchments avoiding concerns over coincidence of peaks aggravating downstream flooding concerns. • Rainfall events for the 2, 10 and 100 year storms shall use the 24 hour storm for calculations.

Erosion control criteria 1. It is recommended that the difference between the pre- and post-development total volume is retained for smaller storms up to and including the 2 -year ARI event. There will be many situations where that volume cannot be retained on site due to slope or soil conditions. In those situations, a minimum retention of 5 mm of runoff from site impervious surfaces is required to offset the loss of the initial abstraction of 5 mm of rainfall that uncompacted pre-development pervious areas had. If soil conditioning is not provided for pervious areas that have been earthworked then 5 mm of runoff from the entire site shall be retained. 2. Check the 2 -year stream velocities against Table 8‑ 3 to ensure that velocities are non-erosive. If they are non-erosive in the post-development condition assuming ultimate development of the catchment then no extended detention is required. If stream velocities are predicted to be erosive then criteria are provided in item 3 below. 3. Implement extended detention or volume control according to the following: • If the stream is stable under the existing development condition, design detention or retention storage for a 24 -hour release of an equivalent volume to the water quality storm. • If the stream is not stable, multiply the water quality volume by 1. 2 to determine the 24 -hour extended detention volume.

Retention of the first 5 mm of runoff a minimum retention of 5 mm of runoff from site impervious surfaces is required on all new development sites to offset the loss of the initial abstraction of 5 mm of rainfall that uncompacted predevelopment pervious areas had. If soil conditioning is not provided for pervious areas that have been earthworked then 5 mm of runoff from the entire site shall be retained.

Stormwater runoff volume control A given volume of runoff might be specified for retention and that runoff must pass through the retention system and infiltrate in a given period of time, which would depend on the inter-event time period during that time of year when the average inter-event dry period is least. An example of this is that storms in the region during winter months occur approximately every 2 -3 days (2 days for Coromandel and Pukekohe, 3 days for the rest of the region).

Water quality control The following recommendations are made: • The water quality volume is the stormwater runoff volume determined by calculating the runoff volume from 1/3 of the 2 year 24 hour rainfall at a given location, • The water quality volume should be used for determining storage volumes and flow rates in sizing stormwater management practices, • In areas where the rainfall for the water quality event is greater than 30 mm, a rainfall depth of 30 mm can be used to determine the water quality volume for water quality treatment. This only applies to water quality. Extended detention will require design for the full-unadjusted amount. • Where lakes in the downstream catchment have existing nutrient issues, at least two practices should be used in conjunction with one another to improve removal of nitrogen from the stormwater discharge.

Source control Prior to any consideration of stormwater treatment, consideration should be given to source control and a series of questions answered. • Have building materials been used that minimise leaching of contaminants? • Has existing vegetation been preserved to the degree practicable or has vegetation been re-established upon project completion? • Are flow velocities and volumes increased downstream (energy dissipation)? • Has slope disturbance been minimised and have disturbed slopes been vegetated and slope lengths minimised through the use of cut-off drains? • Can concentrated flow areas be minimised? • Are any cross drains combined and considered for erosion protection? When these types of questions have been considered and addressed, the stormwater management practice selection process then moves on to flow and treatment control.

Benefits of practices in series While these practices provide individual benefits for removal of contaminants, their use in series can provide greater benefit than those used only individually. A simplified equation for the total removal of a given contaminant for two or more stormwater management practices in series is the following : R = A + B – [(A x B)/ 100] Where: R = total removal rate A = Removal rate of the first or upstream practice B = Removal rate of the second or downstream practice The use of this equation is easiest when considering removal percentages rather than using effluent limits as data on performance of practices for effluent limits can be highly variable.

Practice Swales Filter Strips Sand Filters Bioretention practices (normal) Bioretention practices (w/anaerobic zone) Infiltration Practices Dry Ponds (no extended detention) Dry Ponds (with extended detention) Wet Ponds Wetlands Green Roofs Water Tanks Oil Water Separators TSS 75 70 80 80 80 Removal rates (%) Nitrogen 20 20 35 40 50 Phosphorus 30 20 45 60 80 80 40 30 10 60 20 30 75 80 Volume reduction only 15 25 40 Volume reduction only 0 40 50 Volume reduction only 5

Practice Swales Filter Strips Sand Filters (with organic matter) Bioretention practices (normal) Bioretention practices (w/anaerobic zone) Infiltration Practices Dry Ponds (no extended detention) Dry Ponds (with extended detention) Wet Ponds Wetlands Green Roofs Water Tanks Oil Water Separators Removal rates (%) Zinc Copper 50 60 90 90 70 75 75 80 70 10 20 20 30 30 60 Volume reduction only 5 40 70 Volume reduction only 5

Flow and treatment control Specific design guidance is provided in this Section for the following practices: • • • Swales Filter strips Sand filters Bioretention Infiltration Wet ponds Wetlands Green roofs Revegetation Water tanks Conditioning of surface soils Oil and water separators

Revegetation as a stormwater management practice Bush revegetation reduces site runoff by providing leaf canopy interception, evapotranspiration and soakage into the organic ground cover: • Evapotranspiration • Soakage • Flow retardance

Soil Conditioning 1. Conditioning of surface soils is a three-step process: Conditioning of the parent material in both cut and fill areas. • Disaggregate and aerate the surface 150 mm of the soil and add gypsum. • This can be achieved by scarifying (break up the surface of) and, if necessary, subsoiling at a spacing of approximately 1. 5 m. 2. Placement of a 200 mm uncompacted layer of subsoil over the conditioned parent material. 3. Placement of a 100 mm uncompacted layer of topsoil over the ‘subsoil’. Excavation and regrading at individual lot level must also apply these three conditioning steps to rehabilitate any reworked areas outside the building and paving footprint.

Flow diagram of design progressive steps

Industrial Site Management The following items are discussed with detailed information provided: • Source control and site housekeeping, • Industries, contaminants of concern and appropriate treatment practices, and • Stormwater treatment for contaminant reduction and peak flow control. Each of these items is a significant element in developing an effective site management plan and the elements need to be done in conjunction with one another to minimise adverse impacts to receiving systems.

The importance of site plans A site plan has to be available that shows the following: • Buildings, • All outdoor areas, • Site boundaries and adjacent land use, and • Stormwater and wastewater systems.

Examples of Industries, Contaminants of Concern and Treatment Processes The guidelines have a complete list of various industries, contaminants of concern, likelihood of release and appropriate treatment processes.

Stormwater quantity and quality control Stormwater from new industrial sites must be considered from a peak discharge, stream erosion and water quality perspective the same way that all other potential discharges must comply with the criteria outlined for other types of development. The use of infiltration practices as a water quality treatment practice is discouraged on industrial sites. The ground is itself a receiving system and contaminants may migrate to groundwater and be discharged into another receiving system (stream, estuary, harbour, open coast, lake). Where infiltration of stormwater runoff is anticipated, treatment should be provided prior to the infiltration practice to prevent migration of contaminants to groundwater.

Stormwater quantity and quality control (cont. ) While source control should always be provided on industrial sites, treatment is also necessary as not all contaminants can generally be eliminated through source control. Treatment practices should be carefully selected to ensure that contaminants of concern are targeted for removal by a practice (or practices) whose functioning facilitates their removal. It must also be recognised that water quantity must also be addressed on new sites or where significant site modification is intended.

WRC Administered drainage areas

WRC Administered Drainage Areas Ideally these areas would remain rural; however land use intensification is occurring in the vicinity of some of these drainage areas, particularly around the fringes of Hamilton. The urban growth areas need outfalls and drainage area networks are being looked at to provide this. The drainage networks are not designed to take urban flows and the expansion of urban areas into these drainage systems causes complications. Conventional stormwater management approaches are generally used by consultants but that approach doesn’t work well in these conditions, as attenuation leads to extended duration of peak flow, which can exacerbate erosion and scour issues in low capacity drains, and can also exacerbate ponding duration on adjacent land.

WRC Administered Drainage Areas There are two major issues in establishing site specific criteria: volume of runoff and the timing of the runoff. Criteria for urban development in WRC administered drainage areas is the following: • Total volume of runoff from the post-development 10 -year rainfall must not exceed the pre-development runoff volume. • The runoff depth that is released for the 10 -year storm shall have an extended detention time of 72 hours so as not to overload the receiving drainage channel. • Other criteria related to water quality treatment shall still be required. The requirement for the runoff volume to not exceed the pre-development runoff volume for the 10 -year rainfall event is stringent. As a result, meeting this requirement negates the need to provide peak flow or erosion control as normally required in Section 8. 4. Having the runoff volume not exceeding the predevelopment runoff volume in conjunction with the 72 hour detention will meet those objectives.

WRC Administered Drainage Areas (continued) The first two criteria are very stringent and may, on a case by case basis be an undue hardship when proposed land use of a given density is being promoted through a district plan. In those situations, only when WRC is satisfied that all available on-site options to retain runoff have been considered and rejected, there is latitude for a fee-in-lieu or impact fee to be applied, which will allow WRC to modify channel cross-sectional area in the drainage system so that existing lands will still meet the drainage criteria.

Rural Residential Development The Regional Plan provides for permitted activities subject to the following conditions: a) There shall be no adverse effect on water quality of the receiving water body. b) Any adverse erosion effects occurring as a result of the discharge to be remedied as soon as practicable. c) There shall be no adverse effects from increased water levels downstream of the discharge point. d) The Waikato Regional Council shall be notified in writing of the discharge, its volume, contaminant concentrations and the water quality of the receiving water body 10 working days prior to the discharge commencing.

Rural Residential Development (cont. ) The goals of the guidance are: • To minimise changes to the hydrological regime in order to protect the physical structure of streams and also to reduce potential downstream flooding; and • To reduce sediment discharges resulting from increased stream channel erosion and small scale rural development.

Rural Residential Development (cont. ) Recognising that stormwater issues may not be well understood by an individual rural residential property owner or developer, it is important to understand the process of runoff movement through a property to a receiving environment. This process depends on three levels of consideration: • Source, • Pathway, and • Receiving environment

Rural Residential Development (cont. ) Hydrologic analysis is broken into two parts: • An individual residence constructed in a rural area, or • A rural residential subdivision. If an individual residence is constructed in a rural zone, stormwater management requirements can be met through a series of practices that eliminate the need for a more detailed analysis. Those practices include: • Capture of roof runoff in a water tank that is used for domestic water supply. The overflow from the tank shall be discharged into an infiltration soakage pit, a bioretention practice or through a flow disperser. • Runoff from driveways, access roads shall be accounted for either through soakage, bioretention or bush revegetation. • If there are significant cuts and fills to facilitate construction, those disturbed cuts or fills shall be rehabilitated.

Rural Residential subdivisions Hydrologic analysis shall be done according to the Waikato Guideline for Stormwater Runoff Modelling. The same requirements as detailed in Section 8. 4 shall be designed for on these developments. This includes retention of the first 5 mm of runoff. The stormwater management practices have design criteria specified in the guidelines. Of particular applicability on rural residential sites the following are emphasised: • Swales, • Filter strips, • Bioretention, • Infiltration trenches and soakage pits, • Wetland swales, • Water tanks, • Bush revegetation, and • Green roofs.

Innovative Practices There are two potential avenues that an innovative product can take to determine the extent of its use for stormwater quantity or quality function. • Submission of an approval that has been given through another certification process. • Submission of information as detailed in the rest of this section. • The preferred option is submission of an approval that has been given through another certification process. There are several processes listed and particularly the processes in Switzerland two processes discussed from the United States are the most desirable and evidence of approval through those processes will form the basis of acceptability in the Waikato Region.

Innovative Practices WRC does maintain the right to not accept a specific certification if the source is not generally recognised by the engineering community as being objective or comprehensive.

Outlet design Outlet protection for culverts, stormwater outfalls or ditches is essential to prevent erosion from damaging downstream channels and receiving environments. Outlet protection can be a channel lining, structure or flow barrier designed to lower excessive flow velocities from pipes and culverts, prevent scour, and dissipate energy. Good outlet protection will significantly reduce erosion and sedimentation by reducing flow velocities.

Outlet design Key design elements include: • Pipe grade • Outlet velocity • Riprap aprons • Engineered energy dissipaters • Flow alignment and outfall setback in freshwater receiving environments • Erosion control in coastal environments

Calculation of contaminant loads The purpose of this section is to provide a means to calculate water quality loadings related to land use and how implementation of stormwater management strategies and practices can reduce contaminant loadings to receiving systems. The calculations and contaminant unit loads are based on annual loads rather than event mean concentrations. It is primarily an educational tool so developers and consultants can become aware of contaminant loads from various land use activities and the ability of stormwater management approaches and treatments to reduce, but generally not eliminate contaminant discharge.

Calculation of contaminant loads There a variety of approaches that have been developed in New Zealand internationally. Several of the more appropriate approaches are discussed here and include the following methods. • Auckland Council’s Contaminant Load Model, • Modelling Nutrient Loads in Tweed Catchment, New South Wales, and • Nutrient Export Coefficient Model used in California, United States of America. A further section provides unit loadings for nutrients based on Mf. E guidance (Elliott, Sorrell, 2002) that is New Zealand based. Based on the review of appropriate methods, an approach is recommended to qualitatively predict water quality loads to receiving systems.

Calculation of contaminant loads The annual loadings for urban land uses for the Auckland Region are considered as good loading estimates but their assumption in developing the loadings was a uniform impervious surface runoff of 1, 000 mm (based on 1, 200 mm of rainfall). That assumption was considered reasonable for the purposes of their analysis. Rainfall in the Waikato Region has greater variability and needs to be accounted for in estimating annual loads. As a result the contaminant yields provided in Table 10 of the Auckland Contaminant Load Model should be increased by the ratio of local rainfall (mm)/1, 200 (mm). As an example, a project in Hamilton having 1, 300 mm of rainfall would use a ratio of 1. 1 times the contaminant yields provided in the Auckland Council’s Contaminant Load Model to more accurately reflect the difference in local area runoff.

Calculation of contaminant loads Land use Specific elements Roofs Galvanised steel Zinc/aluminium coated Concrete 1 k-5 k vpd 5 k-20 k vpd 20 k-50 k vpd 50 k-100 k vpd Residential Industrial commercial Urban grassland trees < 50 slope Roads Paved Pervious Contaminant yield (kg/ha/year) Total copper These values need to account for rainfall variation 50 22. 4 0. 003 TSS Total zinc Total nitrogen Total phosphorus 15 1. 1 50 50 2. 0 0. 2 0. 009 0. 016 15 15 1. 1 160 280 530 960 1580 320 220 320 450 0. 26 1. 1 2. 57 4. 7 1. 95 5. 9 0. 016 0. 033 0. 089 0. 37 0. 86 1. 57 0. 36 1. 07 0. 03 0. 003 15 14 20 26 30 14 91 1. 1 5. 5 4. 7 4. 0 3. 2 5. 5 21 Urban grassland trees Slope 5 -100 920 0. 032 0. 006 Urban grassland trees Slope >100 1850 0. 065 13

Calculation of contaminant loads Rural areas unit loadings (kg/ha/year mean values) Dairy Hill TN 25. 0 TP TSS Native Exotic 9. 0 Lowintensity pasture 5. 2 3. 0 2. 8 1. 0 1. 98 0. 46 0. 39 0. 37 26 0. 46 0. 15 0. 4 0. 01

Calculation of contaminant loads To use the tables and determine contaminant loading, do the following steps. 1. Determine the appropriate land use for a given site and take the values for the contaminants given in Table 16‑ 5 and Table 16‑ 6 to use for subsequent calculations. 2. Find the site location in the region and determine the ratio of the Waikato region rainfall/1, 200 mm. 3. Multiply the ratio value and the unit loadings for TSS, Zn and Copper to get the local unit loadings for the site. 4. Multiply the unit loadings calculated in item 3 by the area of the land use in hectares to get the site loadings. 5. To get the loadings that are exported from the site, the loadings must account for the stormwater management practices that are used to treat the site runoff. Table 9‑ 1 and Table 9‑ 2 are then used to determine the percentage reduction of the contaminants of concern. The percentages given in the tables are multiplied by the site loadings to obtain site contaminant export.

Construction related issues Construction inspection can be broken down into several components (Watershed Management Institute, 1997) including: • A clearly defined responsibility for proper construction, • Pre-construction activities, • Practice construction for each type of practice, and • Final acceptance responsibility These items are discussed individually to facilitate successful construction of stormwater management practices. Any inspection forms that are completed should remain with the project files in the event that future maintenance issues may need to refer back to them.

Operation and maintenance Stormwater management practices are expected to perform their water quality and quantity control functions as long as the land use they serve exists. There a number of reasons why this continued function is important. • Maintenance is necessary to ensure outcomes that the practices were constructed to achieve are, in fact, achieved, • Public safety may be compromised if maintenance is not done, and • The stormwater management practices may be required by an RMA consent and the eventual property owner will have a legal obligation to ensure continued practice function.

Operation and maintenance The following items are covered in this section: • When does a practice need to be inspected, • When does maintenance need to be done, • The importance of documenting and tracking inspection and maintenance, • Prioritising maintenance tasks, • Disposal of removed contaminants, • Safety issues, and • Location of inspection reports and stormwater management operational notes.

Landscaping is critical to improving both the function and appearance of stormwater management practices. It has aesthetic, ecological and economic value that is often not recognised during site design and construction. In almost all cases, compliance with regulatory requirements is the key driver and the issue of how a stormwater practice fits into the local landscape can often be overlooked. The objectives of landscaping stormwater management practices are to: • Improve their aesthetics, • Improve their water quality and ecological function, and • Increase the economic value of the site. Guidance is provided for all of the stormwater management practices discussed in the guideline.

Retrofitting problem areas When considering retrofitting, there a number of items that need to be considered: • Downstream flooding or stream channel erosion being adversely impacted by existing land use, • Receiving system impacts by upstream generated contaminants, • Appropriateness of a given stormwater management practice, • Practice size needed to provide a substantial benefit, • Land availability for the stormwater practice to be constructed, • Maintenance access, • Ability to get the drainage through the practice, and • Cost (design, construction and operation).

Overall process for retrofit selection

Feasibility studies Once project prioritisation has been done feasibility studies would need to be done to determine whether a project is a realistic possibility. Feasibility studies include the following items: • Magnitude of impact, • Appropriate practice availability, • Space availability, Retrofit of a dry pond with a small weir to • Ability to get runoff to the practice, create a wetland • Magnitude of benefit • Cost, and • Maintenance access

Taking advantage of opportunities Retrofitting can be done through an almost limitless number of ways. These include the following: • Retrofitting existing stormwater quantity control structures, • Using existing road crossings to impound stormwater, • Demonstration projects, • Use of new consenting to exceed individual project benefits, and • Retrofitting through education.

Demonstration projects

Auckland Botanic Gardens

General Comments The Guideline is at a semi-final stage of development and two public workshops (with this being the first) will be held prior to them being finalised. It is strongly felt that there has to be a shift away from the conventional/historic approach to site development if aquatic resources are to be protected from urban land uses. There is biological data that indicates that conventional approaches to stormwater control are not providing resource protection so a more active approach to runoff management is required. Thus the evolutional changes proposed in the draft guideline.

Where to from here • Both guidelines are to be peer reviewed externally. • Once peer reviewed, both guidelines will be made available to external stakeholders for information/comment. • Council and Earl will consider comments. • A second external stakeholders workshop to be held mid-2017 to release the final guidelines. • Once released, both guidelines are to be used instead of TP 10 and TP 108.

Questions?