Waikato Stormwater Management Guideline “Draft” 28 February 2017

1. Waikato Stormwater Management Guideline“Draft”28 February 2017

2. 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.

3. 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

4. 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 pre-development 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.

5. 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.

6. 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 km2 or 5% of the region, • Today approximately 280 km2 or 1% of the region is in wetland vegetation, • There are 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.

7. 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.

8. Contaminants

9. 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.

10. 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 RautakiWhakapaipai • All of these items are discussed in further detail in the guidelines

11. 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.

12. 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

13. 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

14. 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.

15. Receiving Environment Issue Prioritisation

16. Stream hierarchy and regional percentages of Waikato streams

18. 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)

19. LID Scoring Matrix Minimum total score is 15

20. 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.

21. 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.

22. Typical LID practices and approaches

23. 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.

24. 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 post-development peak discharges shall not exceed the 2- and 10-year pre-development 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.

25. Erosion control criteria • 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. • 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. • 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.

26. 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 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.

27. 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).

28. 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.

29. 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.

30. 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.

33. 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

34. 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

35. 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.

36. Flow diagram of design progressive steps

37. 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.

38. 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.

39. 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.

40. 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.

41. 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.

42. WRC Administered drainage areas

43. 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. 

44. 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 pre-development runoff volume in conjunction with the 72 hour detention will meet those objectives.

45. 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.

46. Rural Residential Development • The Regional Plan provides for permitted activities subject to the following conditions: • There shall be no adverse effect on water quality of the receiving water body. • Any adverse erosion effects occurring as a result of the discharge to be remedied as soon as practicable. • There shall be no adverse effects from increased water levels downstream of the discharge point. • 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.

47. 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.

48. 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

50. 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.