SH3 Awakino Tunnel Bypass. Assessment of Environmental Effects Stormwater

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1 SH3 Awakino Tunnel Bypass Assessment of Environmental Effects Stormwater

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3 i Contents Executive Summary... a 1 Introduction Description of Project, Key Features and Constraints Assessment and Design Criteria Study Input Data and Available Information Available Catchment and Drainage Data Design Rainfall Data and Climate Change Groundwater Levels Site Inspection Factors Influencing Stormwater Management Scale of Project in Proportion to Overall Awakino River Catchment Traffic Volume Steep Terrain Potential Stormwater-Related Effects Short-term Environmental Effects (during construction) Long-term Environmental Effects (during operation and routine maintenance) Proposed Mitigation Measures Increased Peak Flows Fish Passage Culverting and Modification of Watercourses Prevention of Erosion in Receiving Environments Mitigation of Contaminant Discharges Erosion and Sediment Control during Construction Assessment of Residual Effects Following Mitigation Conclusion References \\opus\s\proj\nz\23\ SH3 Awakino Tunnel Bypass\Home\4 - Consenting and Statutory Processes\AEE\Stormwater and ESC\Final reports\ AEE Report Stormwater

4 Assessment of Environmental Effects a Executive Summary The SH3 Awakino Tunnel Bypass Project is located 50km south of Te Kuiti, roughly half way between Hamilton and New Plymouth, in the Waikato Region. The project will reduce fatal and serious crashes and highway closures, as well as improve the performance of the network. Parts of the site are highly constrained by poor access, steep terrain and the narrow existing highway, as well as the meandering Awakino River. The proposed two lane realignment to the north of the existing single lane Awakino Tunnel will be about 2 km long. This report identifies the potential stormwater-related effects associated with the proposed realignment and the works proposed for their mitigation. Once consents have been issued and the designation confirmed the proposed stormwater works will be incorporated in the Specimen Design. The works will also be described in a set of Principal Requirements (PRs) that specify minimum design criteria for the eventual contractor. The PRs will become the basis of a design and construct contract where the successful contractor will transform and innovate the Specimen Design into their own final detailed design for construction. Thus it is important that the consents are sufficiently flexible to allow for variations of the specimen design through the process leading to the final design, while still ensuring agreed environmental outcomes will be achieved. The Project encompasses a pavement area of approximately 33,200m 2 (0.033km 2 ). It is predicted that the runoff from the new impervious surfaces will not create a perceptible increase in Awakino River flows or flooding as the Project area is so small in relation to the overall river catchment (225km 2 ). However, there are a number of other stormwater-related effects that need to be addressed. Without mitigation the proposed highway has the potential to cause both short-term and long-term adverse stormwater-related environmental effects. Short term effects occur during construction, and arise as a result of earth-working activities. Stormwater flowing over the site can mobilise sediment and carry it off-site into the Awakino River and surrounding streams and tributaries. To mitigate the effects that this sediment can cause, erosion and sediment control measures will be implemented in accordance with the Waikato Regional Council Technical Publication, Erosion and Sediment Control Guidelines for Soil Disturbing Activities (2009). Long term effects occur after construction has been completed, during the life of the highway. A Best Practicable Option (BPO) approach has been adopted to prevent, minimise or mitigate these effects, which may arise in the form of: Increased peak flows; Loss of fish passage; Habitat impact resulting from culverting and modification of water courses; Erosion of the receiving environments; Release of contaminants; Constructed swales, cut-off drains, stream enhancement and habitat creation, erosion control and appropriate fish passage will be used to mitigate these environmental effects. Treatment is proposed on the deviation sections, and on existing highway sections where it is readily achievable.

5 Assessment of Environmental Effects b Provided the proposed mitigation measures are implemented, the project will result in effects ranging from neutral to a net positive (this being due to the transfer of traffic from a currentlyuntreated highway to a mostly treated alignment).

6 Assessment of Environmental Effects 1 1 Introduction New Zealand Transport Agency (Transport Agency) is seeking to obtain resource consents from Waikato Regional Council (WRC) to enable the conveyance, diversion and treatment of stormwater for the State Highway 3 (SH3) Awakino Bypass Project (Project). The Transport Agency will also serve a Notice of Requirement (NOR) for a designation to cover the proposed realignment based on this AEE. Opus International Consultants (Opus) have been engaged to assess the potential effects of the proposed realignment, including impacts caused by modification of waterways, construction of new or modified culverts and discharging of treated or untreated stormwater from the designation area. This report covers stormwater drainage aspects of the Project including: Potential stormwater-related effects, both short and long-term; Proposed mitigation measures proposed to be incorporated; and An assessment of the residual effects. 2 Description of Project, Key Features and Constraints The project specific objectives for the SH3 Awakino Tunnel Bypass are to improve safety, resilience and network performance (freight throughput). The project is expected to reduce fatal and serious crashes and road closures, as well as improve the performance of the network. Parts of the site are highly constrained by poor access, steep terrain and the narrow existing highway, as well as the meandering Awakino River. The proposed two lane bypass to the north of the existing single lane Awakino Tunnel will be about 2km long. The project involves: approximately 2.3 km of new two lane road typically 10m wide (plus batter slopes), which bypasses the existing single lane Awakino Tunnel; approximately 675m of northbound passing lane and two truck pull off areas (one in each direction); two new bridges across the Awakino River, one 76m long and the other 63 m long; approximately 190,000 m3 of earthworks cut up to about 30 m high, covering about 400 m length of the new highway; approximately 600m length of embankment up to about 6m high, including a section of fill supported on timber piles due to underlying soft ground; approximately 600 m length of new retaining walls up to about 8 m high at various locations along the realigned highway; changes to existing farm entrances and access tracks, including provision of a new farm underpass; a rest area with a footpath to the tunnel and access to the river; and landscape treatment and ecological enhancement planting.

7 Assessment of Environmental Effects 2 3 Assessment and Design Criteria The Resource Management Act 1991 (RMA) provides for approaches to discharge consenting which are based upon means of mitigating effects that are demonstrably a Best Practicable Option (BPO) approach. This approach is commonly applied in the case of stormwater discharges. This assessment will seek to demonstrate that environmental effects from the Project will be prevented or minimised through the application of appropriate best practice stormwater management measures. The Transport Agency s Stormwater Treatment Standard for State Highway Infrastructure is considered to represent current best practice, as are the other documents referenced in Table 1. Criteria Design and/or Mitigation References Design Rainfall NIWA 2016, High Intensity Rainfall Design System (HIRDS), Version 3.0 Climate Change Projections for New Zealand (Ministry for the Environment, 2016) Waterway Crossings (except bridges) Water Quality (WQ) Stream Channel Erosion Control Peak Flow Control Sediment and Erosion Control during construction Transport Agency Bridge Manual, 3rd edition (Transport Agency, 2013) Transport Agency Stormwater Treatment Standard for State Highway Infrastructure, (NZTA, 2010). Transport Agency P46 Stormwater Specification (Transport Agency, 2016) Transport Agency Fish Passage Guidance for State Highways (Transport Agency, 2013). Best Practice Guideline for Waterway Crossings ((Environment Waikato, 2006/25R) NZWERF On-Site Stormwater Management Guideline (New Zealand Water Environment Research Foundation, October 2004) Transport Agency Stormwater Treatment Standard for State Highway Infrastructure, (Transport Agency, 2010). NZWERF On-Site Stormwater Management Guideline (New Zealand Water Environment Research Foundation, October 2004) Transport Agency P46 Stormwater Specification Transport Agency Stormwater Treatment Standard for State Highway Infrastructure, (Transport Agency, 2010). Transport Agency P46 Stormwater Specification Transport Agency Stormwater Treatment Standard for State Highway Infrastructure, (Transport Agency, 2010). Transport Agency P46 Stormwater Specification WRC Guidelines - Erosion & Sediment Control Guidelines for Soil Disturbing Activities, (WRC, 2009). Table 1 - Stormwater Design and/or Assessment Criteria In addition to the above, the following design standards have been adopted as a means of mitigating effects and providing a satisfactory level of service.

8 Assessment of Environmental Effects 3 Culverts and bridges to convey a 4% AEP design storm (including climate change) to pass without the structure sustaining damage (Transport Agency Bridge Manual, 2013). Culverts to convey the 10% AEP design storm event flow without surcharge of the pipe for the MPD scenario with climate change (Transport Agency P46 Stormwater Specification, 2016). Culverts under the highway to convey the 1% AEP flow (including climate change) with 500mm freeboard to the edge of the pavement (Transport Agency Bridge Manual, 2013). A minimum diameter of 375mm is required for pipes crossing a live traffic lane (Transport Agency P46 Stormwater Specification, 2016). The design shall allow for regional / catchment scale flood issues, consent requirements and not create unacceptable adverse effects on upstream and downstream properties outside the designation or land owned by Transport Agency for events up to the 1% AEP design flood (Transport Agency P46 Stormwater Specification, 2016). Stream diversions shall mimic the stream environment being replaced by replicating or improving channel substrate, gradient, sinuosity, bed form, morphology and habitat features. Grade control structures shall be utilised as deemed necessary (eg weirs, etc) where stable grades are not achievable. (Transport Agency P46 Stormwater Specification, 2016). For fish passage the invert level of culvert shall be below the natural bed level of the waterway to allow the culvert to be filled with streambed material which increases bed roughness and imitates the natural stream bed. (Fish Passage Guidance for State Highways, v1.0 Transport Agency, August 2013). If a waterway is reduced, the velocity along the banks at normal flow should be maintained at less than 0.3 m/s to allow for the passage of indigenous fish (Fish passage guidance for State Highways v1.0 NZ Transport Agency, August 2013). To mitigate scour at the interface between culvert and stormwater discharge to natural or man-made waterways, appropriate energy dissipation and erosion control measures shall be incorporated (Transport Agency P46 Stormwater Specification, 2016). Erosion and scour control measures shall be sized for at least the 20 year ARI storm event, However, if there is a risk that erosion or scour could cause a serious failure with environmental, social or economic consequences, or where access for future maintenance is difficult, a 100 year ARI design standard for erosion protection is to be adopted (Transport Agency P46 Stormwater Specification, 2016). Swales, open channels and overland flow paths shall have a 50 year design life with planned maintenance that provides the lowest whole of life cost (Transport Agency P46 Stormwater Specification, 2016). Swales to have minimum hydraulic residence time of nine minutes (Stormwater Treatment Standard for State Highway Infrastructure, 2010) Swales to have maximum side slope of 4H:1V (Stormwater Treatment Standard for State Highway Infrastructure, 2010). Maximum velocity of 0.8m/s through swales for water quality storm (Stormwater Treatment Standard for State Highway Infrastructure, 2010).

9 Assessment of Environmental Effects 4 Minimum swale length of 30m (Stormwater Treatment Standard for State Highway Infrastructure, 2010). Swale to have maximum bottom width of 2m (Stormwater Treatment Standard for State Highway Infrastructure, 2010). 4 Study Input Data and Available Information 4.1 Available Catchment and Drainage Data Scheme design alignment including standard cross-sections. RAMM Data indicating locations of existing highway culvert crossings. LINZ 20m contours for catchments extending beyond LIDAR data obtained. 4.2 Design Rainfall Data and Climate Change Rainfall data is required to calculate runoff volumes and peak flow rates for stormwater management, water quality management and culvert design. HIRDS rainfall data has been adapted to incorporate climate change to 2120 (adjusted for a predicted 2.45 degree Celsius increase in temperature by 2120). 4.3 Groundwater Levels Due to the combination of steep terrain and poorly-drained river flats, soakage has not been incorporated in the conceptual design. 4.4 Site Inspection Several site inspections have been undertaken to assess the current conditions. These visits along with photographs have assisted in the design of the stormwater system. 5 Factors Influencing Stormwater Management 5.1 Scale of Project in Proportion to Overall Awakino River Catchment The Awakino River upstream of the Project has an approximate catchment of 225km 2, whereas the Project has a zone of influence of approximately 2km x 0.25km = 0.5km 2. There is no other significant development in the catchment that might lead to cumulative effects. Accordingly, the highway s contribution (both existing and proposed) to river flows, flooding, and contaminant generation will be small. 5.2 Traffic Volume The highway has a traffic volume (AADT) of approximately 2,200 vehicles per day (Vpd) currently, projected to rise to less than 3,000 Vpd by There are no other high volume highways in the catchment.

10 Assessment of Environmental Effects Steep Terrain The Project passes across river plains and through a narrow gorge where hillsides have slopes up to 66%. When considering opportunities for stormwater treatment, the competing need to minimise the Project footprint must also be considered. For example, at a slope of 66%, provision of space for a 6m wide swale at highway level could add many tens of metres to the extent of the cut batter. 6 Potential Stormwater-Related Effects Environmental effects that are associated with the realignment of SH3 can be divided into short term effects that occur during construction, and long term effects that occur during the operation and maintenance of the proposed new alignment. This report has identified the potential stormwater effects of the Project through: Our own assessment, local knowledge and professional judgement; Consultation with the design teams, Waitomo District Council, Waikato Regional Council and the Transport Agency; Consideration of site topography, hydrology, geology and existing stormwater infrastructure; Consideration of the Project stormwater drainage designs. 6.1 Short-term Environmental Effects (during construction) Short-term effects arise during construction as a result of construction activities, and initial trafficking of stabilised areas to facilitate construction staging. Stormwater flowing over the proposed works can mobilise sediment, and carry it off-site into the Awakino River or surrounding streams and tributaries. This sediment-laden runoff reduces water quality, and as it settles out can smother the base of the water body, suffocating aquatic life. There can also be short-term flooding or nuisance effects arising from planned or inadvertent stormwater diversions. Short-term effects may also arise through the opening of new pavement areas to traffic before the associated stormwater devices are commissioned. Since the highway and traffic patterns already exist, these short-term effects (e.g discharge of contaminants) are likely to be similar in scale and intensity to the pre-developed (i.e. current) effects. 6.2 Long-term Environmental Effects (during operation and routine maintenance) The creation of impervious surfaces causes stormwater to be discharged at a faster rate, and in greater volumes. This can cause flooding, and increase the rate of erosion within the receiving watercourses. The creation of new pavement can also result in traffic-generated contaminants such as heavy metals and hydrocarbons to be discharged into the Awakino River and surrounding stream and tributaries. The new alignment can also have long-term environmental effects due to the obstruction of overland flow paths, fish passage and streams.

11 Assessment of Environmental Effects 6 7 Proposed Mitigation Measures This section describes how the stormwater management concept has been designed to reduce, prevent or minimise the stormwater effects identified in Section 6. Section 8 then describes the residual effects (i.e those remaining after mitigation) to the extent that these are unable to be designed out. The stormwater mitigation measures will address: Increased peak flows; Provision of fish passage; Culverting and modification of water courses; Erosion of the receiving environments; Release of contaminants; Erosion and discharge of sediment during construction. The mitigation measures have been designed with reference to standards outlined in Section 3 of this report. 7.1 Increased Peak Flows The Project encompasses a pavement area of approximatley 33,200m 2 (0.033km 2 ), including the rest area. Increased runoff from the new impervious surfaces will not create a perceptible increase in Awakino River flows or flooding as the Project area is so small in relation to the overall river catchment (225km 2 ). However, potential effects on tributaries deserve greater scrutiny. Most tributaries have satisfactory conveyance capacity, with any increased flow being accommodated via a slight increase in depth within their existing cross-section. Flow velocities will be assessed by the contractor during design, and if they indicate that erosion is likely, additional rock armouring will be provided. 7.2 Fish Passage The proposed new alignment will have an impact on the existing environment by the obstruction of fish passage. Where existing culverts are being modified, fish passage will be considered if not already in place. New culverts should provide fish passage where there is a reasonable expectation (in the opinion of a qualified ecologist) that fish will be present. Significant conveyance measures are outlined in section 7.3 below. At this stage the proposed culvert at chainage 1080, the proposed culvert at chainage 1490 and removal of existing culvert at this location, and the proposed culvert under the footpath south of the alignment at chainage 720 have been identified as requiring suitable fish passage measures.

12 Assessment of Environmental Effects Culverting and Modification of Watercourses The proposed new alignment will have an impact on the existing environment by the modification of watercourses (e.g. culverting or stream relocation) resulting in loss of habitat and obstruction of fish passage. The above effects will be mitigated by: Culverts being designed to have sufficient capacity to ensure both the specified highway freeboard is met and any increase in headwater ponding on neighbouring properties is minimised during the 1% AEP design storm. New culverts will also be designed to have suitable capacity to cope with the 10% AEP design storm event with no surcharge at the inlet. Cut-off drains have been located at points above the highway where water from uphill catchments would otherwise run directly onto the highway or where the new alignment is blocking existing overland flow paths. These locations are exclusively ephemeral gullies, where fish passage and habitat considerations are less critical. Where relocation of an existing natural watercourse is considered unavoidable, the new watercourse will be constructed to mimic the stream environment being replaced by replicating or improving channel substrate, gradient, sinuosity, bed form, morphology and habitat features. The conveyance measures that are considered significant for this design are detailed below. See Appendix A for stormwater plans showing these measures. Stream diversion between chainage 580 and 660 At chainage 580 an existing watercourse that runs into the Awakino River will be blocked by the proposed realignment. This watercourse was probably man-made to improve pasture drainage and has relatively low ecological value. It is proposed that it is diverted to run parallel to the proposed realignment and enter the Awakino River just upstream of the proposed Bridge 2 at chainage 680. It is estimated that the stream diversion will mimic or improve on the current conditions. Culvert crossing on footpath south of alignment south of chainage 720 The proposed footpath crosses a stream before it enters the Awakino River. The culvert will be sized appropriately and suitable habitat creation and fish passage measures will be implemented at this location. Culvert crossing at chainage 910 The proposed realignment crosses a low point in pasture that drains into an existing tributary of the Awakino River at chainage 910. Therefore a culvert is proposed under the highway to convey water to the tributary on the south side of the highway. See Appendix B for calculations on culvert sizing. The 0.5ha drainage basin is ephemeral, with no defined channel at the beginning of the crossing point and accordingly no fish passage is proposed. Culvert crossing at chainage 1080

13 Assessment of Environmental Effects 8 The alignment crosses an existing stream at chainage The culvert will be sized to comply with Transport Agency s desired level of service and suitable habitat creation and fish passage measures will be implemented. Culvert crossing at chainage 1490 The alignment crosses an existing stream at chainage The culvert will be sized to comply with Transport Agency s desired level of service and suitable habitat creation and fish passage measures will be implemented. The stream will be diverted on the north side of the proposed new alignment and when construction of the new highway is complete the existing culvert on the south side of the proposed new alignment will be removed. The new stream channel will incorporate suitable bed form, meanders and habitat creation to mimic the original channel. Cut-off drains in cut slopes The alignment of the highway is in steep terrain in parts with cut slopes above the highway. Cut-off drains are proposed to collect water and convey it to the nearest discharge point, thus preventing the water from discharging onto to the highway with associated bank erosion and flooding issues. Cut-off channels should incorporate suitable armouring to prevent erosion and minimise future maintenance needs. None of these diversions are expected to warrant fish passage. See Appendix A for the proposed stormwater layout drawings. 7.4 Prevention of Erosion in Receiving Environments Another potential environmental effect is erosion of the receiving water bodies due to peak flows and runoff volumes. This is particularly applicable for outfalls from swales, cut-off drains or discharges from pipes. Also, there is potential for erosion at culvert inlets and outlets as well as on the side slopes of the swales. Erosion of tributaries is a greater risk than erosion at the main river as changes in highway runoff contribute a correspondingly larger proportion of total flow. Erosion effects in the main river are expected to be negligible. Potential erosion at outlet locations will be mitigated by the use of rock rip rap where required, together with appropriate energy dissipation measures. Side slopes of swales have been proposed to be stable in combination with the proposed planting measures. In summary, erosion mitigation measures should be limited to localised scour protection around outfall locations. There are several steep tributaries at either end of the project, but there is minimal increase in pavement width in these locations, so it is proposed that these locations are simply monitored during and post-construction, and that any erosion noted should be rectified. 7.5 Mitigation of Contaminant Discharges Highway runoff can transport a number of contaminants that will have a detrimental effect on the environment including sediment, heavy metals, hydrocarbons, and gross pollutants. Treatment swales to capture and treat highway runoff should be the principal form of treatment used for the new alignment. Given the rural environment it is believed this is the BPO for treatment. While

14 Assessment of Environmental Effects 9 it is technically feasible to provide comprehensive stormwater treatment for the entire new proposed layout we do not consider this is appropriate for the following reasons: Highway traffic volumes are modest (2,200 Vpd), and future traffic will be mostly freeflowing rather than stop-start, thus reducing contaminant generation. The highway comprises a small fraction of the overall Awakino River catchment, which is otherwise un-developed. Highway-generated pollutants will be highly diluted, and are therefore less likely to accumulate to levels that will impact on aquatic ecosystems. The Awakino River is a high-energy environment, which will aid contaminant transport and dispersal. Where swales are not provided for highway runoff, primary sediment interception will often be available in the sumps of catchpits. With the exception of sediment, contaminants in highway runoff are generally different to those in runoff from rural land (hydrocarbons, metals and some sediment vs organic pollutants and sediment). Therefore, the cumulative adverse effects of the two land-uses are likely to be minimal. Widening of the highway corridor for stormwater treatment at the base of steep slopes would require the cut batters to be considerably more extensive, introducing other undesirable effects that are considered to outweigh the benefits of treating these sections of road. For all these reasons, we do not consider that comprehensive stormwater treatment is justified in this location, and a more cost-effective treat-where-practicable approach has been provisionally adopted. This is in line with the RMA s BPO approach. Moreover, the present highway has no formal treatment, so any new treatment represents an improvement as compared to the existing situation. Swales will be designed in accordance with the relevant best-practice documents, in particular the Transport Agency s Stormwater Treatment Standard for State Highway Infrastructure (refer Section 3 for more details). Carriageway hydrology has been carried out using the Rational Method as this is deemed suitable for catchments under 50 hectares in area. Runoff coefficients of 0.3 for grassed areas and 0.95 for paved areas were adopted. Swale design is then carried out using the Manning s Formula for a trapezoidal swale to determine residence time and swale capacity. Wetland-planted swales are proposed to avoid the need for frequent mowing and the associated traffic control measures. The swales have been provisionally (and conservatively) sized to convey the 1% AEP design storm, with a 2m bottom width, 1 in 4 side batters and a maximum depth of swale of 100mm required. The geometry of the swales will be confirmed at the detailed design stage, however initial calculations indicate that the design is feasible and all the proposed swales meet the minimum residence time of 9 minutes and minimum length of 48m. Table 2 below indicates the length and slope of swale required per 100m of a two lane highway to obtain the minimum 9 minutes residence time for the water quality storm.

15 Assessment of Environmental Effects 10 Swale Slope Length of Swale Required for 9 minutes residence time 2% 48m 3% 54m 4% 62m 5% 70m Table 2 Length and slope of swale required for 9 minutes residence time per 100m of highway Where it is not feasible to have a treatment swale, kerb and channel will be used. In most cases the kerb and channel discharges to a swale will be treated before discharging to the Awakino River. Of the 2.3km of new highway, there is approximately 600m which is impractical to treat via swales due to limited highway width. As mentioned above this is a small portion of the overall Awakino River catchment and these areas are predominantly in locations of the existing two lane highway, meaning these areas aren t changing substantially. 7.6 Erosion and Sediment Control during Construction Short term effects that arise during construction as a result of earth-working activities will be mitigated by implementing erosion and sediment control practices that are consistent with the Erosion and Sediment Control Guidelines for Soil Distributing Activities (WRC 2009). Opus has prepared an indicative Erosion and Sediment Control Plan (ESCP) (see Appendix C). A final ESCP will be prepared by the appointed contractor as part of their Construction Management Plan. This plan will need to be certified by the WRC and will indicate the Contractor s specific construction methodology.

16 Assessment of Environmental Effects 11 8 Assessment of Residual Effects Following Mitigation A noted in earlier sections, the Project has the potential to create adverse environmental effects. A suite of mitigation measures has been proposed to address these effects in whole or in part. This section assesses the likely residual effects, which are summarised in Table 3 below. Residual Effects Item Potential effect Mitigation through design Residual effect Increased peak flows Channel erosion Channel armouring as required. Less than minor Modification of channels Culverting or other modification of stream channels leading to loss of habitat Provision of fish passage and other habitat measures in new culverts and channels Minor Blockage of overland flow paths Damming or diversion of water where flow path is being blocked. Causes flooding and nuisance effects to other parties in extreme storm events. Diversion of streams and waterways to safely convey overland flow. Culverts crossing new highway designed to convey 1% AEP storm with 500mm freeboard below highway. Minor Runoff leading to erosion Rainwater runs off the new impervious surfaces at increased rates with potential to cause scour. Potential for scour to occur at outlets from pipes, culverts and swales. Localised scour protection where needed. Less than minor Discharge of contaminants Contaminants collect naturally on the highway surface and are washed into the stormwater system. These contaminants have potential to smother or poison aquatic life. Treatment swales where practicable. The proposed realigned highway is replacing an existing highway of similar length; thus there is little increase in pavement area, little change in traffic volume and therefore little change in Swales can adequately treat highway runoff to an acceptable standard, however they are incapable of removing 100% of contaminants. Swales have been used where possible but it is not practical or economically viable to treat the entire length of the proposed

17 Assessment of Environmental Effects 12 Sediment discharge during construction Uncontrolled discharge leading to stream discolouration and habitat loss Table 3 Assessment of Residual Effects contaminants being discharged. Best-practice ESC measures in accordance with WRC guidelines. realigned highway. Therefore, there will be a small residual effect, but the level of effect will be less than existing. Less than minor Provided the proposed mitigation measures are implemented the Project will have a net positive effect on contaminant levels entering the Awakino River (and surrounding streams and tributaries) compared to the existing situation, a neutral effect on flood levels and overland flow paths; and, a neutral effect on scour and erosion. 9 Conclusion The Project has potential to create adverse stormwater related environmental effects, both shortterm and long-term as explained in Section 3 above. This report is taking a BPO approach for potential stormwater related effects. Provided the proposed mitigation measures are implemented, the Project will have a neutral or net positive environmental effect.

18 Assessment of Environmental Effects References - Best Practice Guideline for Waterway Crossings, Environment Waikato, 2006/25R. - Best Practice Guidelines for Vegetation Management and In Stream Works, Environment Waikato Technical Report TR2007/41 - Bridge Manual, 3rd edition, New Zealand Transport Agency, Erosion & Sediment Control Guidelines for Soil Disturbing Activities, Waikato Regional Council, January Fish Passage Guidance for State Highways, New Zealand Transport Agency, High Intensity Rainfall Design System (HIRDS), Version 3.0, NIWA NZTA P46 Stormwater Specification, New Zealand Transport Agency, On-Site Stormwater Management Guideline, New Zealand Water Environment Research Foundation, October Stormwater Treatment Standard for State Highway Infrastructure, New Zealand Transport Agency, May 2010.

19 Assessment of Environmental Effects Appendix A Stormwater Plans

20 Assessment of Environmental Effects Appendix B Stormwater Calculations

21 Assessment of Environmental Effects SWALES Hydrology - Rational Formula (Flow arising from a typical 100m length of road, with passing lane) Q wq = CIA 2-year, 1-hour quality storm C (Road) = 0.95 C (Swale) = 0.3 Rainfall Intensity, I wq (mm/hr)= 27.2 HIRDS Climate Change Road Area, A R (ha) = 0.15 ha Swale Area, A S (ha) = 0.12 ha Q wq (m 3 /s) = m 3 /s 10 year design storm C (Road) = 0.95 C (Swale) = 0.3 Rainfall Intensity, I 10 (mm/hr)= 98.4 HIRDS Climate Change Road Area, A R (ha) = 0.15 ha Swale Area, A S (ha) = 0.12 ha Q 10 (m 3 /s) = m 3 /s 100 year design storm - 100m of Swale C (Road) = 0.95 C (Swale) = 0.3 Rainfall Intensity, I 100 (mm/hr)= HIRDS Climate Change Road Area, A R (ha) = 0.15 ha Swale Area, A S (ha) = 0.12 ha Q 100 (m 3 /s) = m 3 /s

22 Assessment of Environmental Effects Swale bed Slope Side Slopes Mannings n 2 year 1 hour quality storm - 100m of road Q required (m3/s) Base Width (m) Depth of swale required (m) Q capacity (m3/s) Velocity, V (m/s) Length of Swale Required (m) Swale bed Slope Side Slopes Mannings n 10 year storm m of road Q required (m3/s) Base Width (m) Depth of swale required (m) Q capacity (m3/s) Velocity, V (m/s) Swale bed Slope Side Slopes Mannings n 100 year storm - 100m of road Q required (m3/s) Base Width (m) Depth of swale required (m) Q capacity (m3/s) Velocity, V (m/s)

23 Assessment of Environmental Effects Awakino Tunnel Bypass Swale Capacity - ROW A Prep by: SM Date: Feb-17 Check by: JS Date: 20/02/2017 Swale Design - Peak Flow WQ = 6.3l/s Capacity Required = 81 l/s Flow: (max 2% AEP) 158l/s Flow (l/s) Q Solve Velocity (m/s) V 0.34 Grass Height 150 Bed Width 2.00 Side Slope 1: 4.00 Depth of Swale Required 0.10 Bed Gradient 1: Mod n Value = (Modified equation) Trapeziodal Section Dist (m) x y Height (m) Section Data Water Level Swale Length 190 m Retention Time 9 mins

24 Assessment of Environmental Effects CULVERTS Assumptions/Criteria: Time of concentration is equal to storm duration Kirpich formula used for time of concentration Culverts designed for 100 year storm with climate change to 2020 Checked for no surcharge above top of pipe for 10 year storm Rational Formula used to estimate Q: Q =.00278CIA C (road) = 0.95 C (green area) = 0.3 Freeboard required for 10 year storm = 0.5m HY-8 used for culvert sizing Inlet and outlet intervals for existing culverts have been estimated as there is no data available survey of existing culvert will need to be undertaken Time of concentration Kirpich Formula Catch ment Culvert Chainag e Location Road Area, AR (ha) Green Area, AS (ha) tc (mins) Rainfall Intensity, I100 (mm/hr) Rainfall Intensity, I10 (mm/hr) Q100 (m3/s) Q10 (m3/s) Culver t Height Culve rt Widt h OK OK OK OK OK OK Culvert Inlet Level Culvert top of pipe level Culvet Outlet level Culvet length Culvet Slope Road Level at Inlet Headw ater elevati on for 10 year storm Headwa ter elevatio n for 100 year storm 10 year stor m 100 Year Stor m

25 Assessment of Environmental Effects Appendix C Erosion and Sediment Control Plan

26 SH3 Awakino Tunnel Bypass Erosion and Sediment Control Plan

27 SH3 Awakino Tunnel Bypass Erosion and Sediment Control Plan Hamilton Environmental Office Opus House, Princes Street Private Bag 3057, Waikato Mail Centre, Hamilton 3240 Prepared by. New Zealand Stephanie Mackle Environmental Engineer Telephone: Facsimile: Reviewed by. Date: 18 June 2017 Warren Bird Reference: Principal Environmental Engineer Status: FINAL 2017

28 .. i Contents 1 Introduction and Project Description Principles to Minimises Sediment Discharge from the Site Potential Sediment Hot-Spots Design of Erosion and Sediment Control Devices Culvert Construction and Watercourse Diversion Maintenance, Monitoring and Reporting Procedures Heavy Rainfall Response and Contingency Measures Procedures for Review and/or amendment to the E&SCP Identification of Specific Site Responsibilities Construction Timetable... 7 \\opus\s\proj\nz\23\ SH3 Awakino Tunnel Bypass\Home\4 - Consenting and Statutory Processes\AEE\Stormwater and ESC\Final reports\ Erosion & Sediment Control Plan

29 Erosion and Sediment Control Plan 1 1 Introduction and Project Description An Erosion and Sediment Control Plan (ESCP) is required for works proposed within the SH3 Awakino Tunnel Bypass Project (Project), based on local soil erosion risk and potential discharge of sediment into the Awakino River and surrounding tributaries resulting from earthworks ground disturbance. The intension is for this ESCP to be considered a specimen design to demonstrate that sufficient means are available to mitigate the potential sediment contamination effects that could arise from construction. Once appointed, the construction contractor will adapt this ESCP to suit its own preferred works methodology, while delivering an environmental performance at least equivalent to that described herein. The construction contractor s specific ESCP will need to be submitted and approved by Waikato Regional Council (WRC) prior to construction commencing. 2 Principles to Minimise Sediment Discharge from the Site ESCP Design Standards Erosion and Sediment Control Guidelines for Soil Disturbing Activities (WRC Technical Report No. 2009/02) cited below as Reference A. The principle of this ESCP is to identify approaches that reduce the potential for erosion and sedimentation effects of the proposed construction works; i.e. proactive approaches to: Minimise Disturbance: Fit land development to land sensitivity. Some parts of a site should never be worked and others need very careful working. Watch out for and avoid areas that are wet (streams, wetlands, and springs), have steep or fragile soils or are conservation sites or features. Adopt a minimum earthworks strategy (low impact design) - ideally only clear areas required for structures or access. Site Specific Detail Disturbance of land will be minimised as much as possible. Existing tracks and roads will be utilised where possible. Staged Construction: Carrying out bulk earthworks over the whole site maximises the time and area of soil that is exposed and prone to erosion. Construction staging, where the site has earthworks undertaken in small units over time with progressive revegetation, limits erosion. Careful planning is needed. Temporary stockpiles, access and utility service installation all need to be planned. Construction staging differs from sequencing. Sequencing sets out the order of construction to contractors. Site Specific Detail Construction work will be carried out in stages to minimise the time that areas of the site are exposed and prone to erosion. An indicative and proposed outline of the staging is indicated below. August 2017

30 Erosion and Sediment Control Plan 2 1. Install temporary bridges, temporary roads and temporary staging. 2. Construct permanent bridges. 3. Construct watercourse diversions between chainages 590 to 670, 1430 to 1480 and 1580 to Fill chainage 480 to 660 with cut from chainages 1760 to Fill chainage 720 to 920 with cut from 920 to 1040 and 1800 to Fill chainage 1040 to 1720 with cut from chainage 1840 to Fill chainage 1720 to 1760 (temporary road) with cut from chainage 1940 to Fill chainage 1720 to 1780 with cut from chainage 1960 to Construct chainage 2020 to Construct farm tracks and footpaths using progressive stabilisation. Protect Steep Slopes: Existing steep slopes should be avoided. If clearing is absolutely necessary, runoff from above the site can be diverted away from the exposed slope to minimise erosion. If steep slopes are worked and need stabilisation, traditional vegetative covers like top soiling and seeding may not be enough - special protection is often needed. Site Specific Detail A large cut slope is required at Hammond s Corner and between chainage 930 to Cutoff drains will be installed at the top of the slopes to direct flow away from the work area. The cut slopes will be mulched and seeded as soon as possible to prevent erosion of the cut face. Protect Water bodies: Existing streams, watercourses, and proposed drainage patterns need to be mapped. Clearing may not be permitted adjacent to a watercourse unless the works have been approved. Where undertaken, works that cross or disturb the watercourse are also likely to require resource consents. Site Specific Detail It is proposed that: Silt fences be used around the perimeter of earthworks near watercourses to contain any potential discharge from the site (as indicated in the Erosion and Sediment Control drawings in Appendix B). Further information on silt fence construction and operation is provided in Reference A, Section 3.2 and in Appendix C of this report. Temporary stockpiles be located away from the river edge or any major surface water flow paths further that they be surrounded by a silt fence to capture silt from run-off. Further information on silt fences is provided in Appendix C of this report. Proposed watercourse diversions be constructed in dry conditions, and fully stabilised before any water is allowed to flow through them. Bypass pumping may be required to complete diversion in some cases. August 2017

31 Erosion and Sediment Control Plan 3 Stabilise Exposed Areas Rapidly: The ultimate objective is to fully stabilise disturbed soils with vegetation after each stage and at specific milestones within stages. Methods are site specific and can range from conventional sowing through to straw mulching. Mulching is the most effective instant protection. Site Specific Detail It is proposed that: Stabilisation of earthworks be completed as work progresses. Completed surfaces will have top soil re-spread and be seeded. Further information on seeding and straw mulch use is provided in Reference A, Sections and Smaller areas will be constructed by rapid stabilisation, which involves stabilising the relevant working area at the end of each working day. Refer to Erosion and Sediment Control drawings in Appendix B for locations. Install Perimeter Controls: Perimeter controls above the site keep clean runoff out of the worked area a critical factor for effective erosion control. Perimeter controls can also retain or direct sediment laden runoff within the site. Common perimeter controls are diversion drains, silt fences and earth bunds. Site Specific Detail It is proposed that: Silty water diversion bunds/channels be used both within and near the lower perimeter of the site to collect silty water and direct it to a sediment pond. Temporary silt fences be used to contain any potential sediment discharge from staged work sites mostly in discrete areas that are not readily amenable to other approaches. Silt fence construction and operation will be in accordance with Reference A, Section 3.2. Clean water diversion channels be used to keep clean runoff out of the working area. Stabilised construction entrances be constructed at each entrance where earth-moving plant enters or leaves the site. Where permanent culverts are proposed, it would be appropriate to install these culverts at the beginning of that stage of work to convey clean water and simplify sediment control measures required. Employ Detention Devices: Even with the best erosion and sediment practices, earthworks will discharge sediment-laden runoff during storms. Along with erosion control measures, sediment retention structures are needed to capture runoff so sediment generated can settle out. The presence of fine grained soils means sediment retention ponds are often not highly effective. Ensure the other control measures used are appropriate for the project and adequately protect the receiving environment. Site Specific Detail Sediment retention ponds have been proposed to treat sediment-laden run off and reduce the volume of sediment leaving the site. Silty water diversion bunds will be used to transport run off from the earthworks area to these ponds. Experience and Training: A trained and experienced contractor is an important element of an E&SCP. These people are responsible for installing and maintaining erosion and sediment control August 2017

32 Erosion and Sediment Control Plan 4 practices. Such staff can save project time and money by identifying threatened areas early on and putting into place correct practices. A construction contractor has not yet been appointed. However, it is recommended that a pre-construction meeting takes place with the contractor, once appointed, and a representative from Waikato Regional Council (WRC). This meeting is to ensure that the requirements of WRC in terms of the implementation of the ESCP are understood and met by the contractor. The construction specification shall seek to ensure that an appropriately trained and experienced ESC representative is appointed by the contractor, and has sufficient authority to ensure that ESC practices are implemented and maintained effectively. Assess and Adjust: An intense storm may leave erosion and sediment controls in need of repair, reinforcement or cleaning out. Assessment of controls and making repairs without delay reduces further soil loss and environmental damage, Assessment and adjustment is an important erosion and sediment control practice, make sure it features prominently in the E&SCP. Site Specific Detail It is proposed that: All ESC devices be regularly monitored to identify rips, tears and movement in fabrics and that sediment build-up is cleared and disposed of in an appropriate manner. Silt fences, bunds and channels be adjusted and relocated as work progresses to maintain maximum effectiveness. It is the responsibility of the contractor to ensure that all devices are maintained fully operational for the entire period that un-stabilised earthworks are present on site. Management of materials: The effective management of materials on site will have a substantial effect on the transportation of sediment from site. Site Specific Detail It is proposed that: Temporary soil and material stockpiles be located on vegetated areas at least 20m back from watercourses. Stockpiles will have a maximum height of 4m and have a silt fence or similar sediment containment barrier installed around the base of the stockpile. Further information on silt fence use is provide in Appendix C of this report. If temporary stockpiles are not scheduled for removal within 48 hours, they should be surface compacted (track-rolled) to reduce the risk of water or wind erosion. Stockpiles that are present for durations greater than 60 days shall be mulched and/or sown with grass. If dust arises from stockpiles, additional measures such as watering or covers will be applied. On-site maintenance and re-fuelling will be carried out clear of watercourses, and in a location where any accidental spillage can be remedied with minimum environmental effect. August 2017

33 Erosion and Sediment Control Plan 5 3 Potential Sediment Hot-Spots The Universal Soil Loss Equation (USLE) is frequently used to help identify hot-spots in an earthworks site where there is an elevated risk of soil loss. After using the USLE in this way it quickly becomes evident that the factor having the greatest influence on potential soil loss is the slope-length factor, S, which represents a combination of the steepness of the earth-worked slope and the distance between interception channels. Since the distance between interceptor channels is normally fairly easily adjusted, the slope becomes the greatest single determinant of potential erosion. On the Project the steepest slopes are those of Hammond s Corner, chainage 930 to 1040 and the banks of the Awakino River, while cut and fill batters can be as steep as 2.5H:1V. 1 These are the areas that should be considered potential erosion hot-spots, where particular care should be taken with erosion control and spacing of sediment bunds/channels. Based on the above rationale no USLE calculations have been undertaken. 4 Design of Erosion and Sediment Control Devices The works will involve excavation (320,000m 3 ), fill, retaining wall construction, drainage devices, an underpass and two new bridges. The total disturbed area is approximately 106,000m 2. Staging of the works will significantly limit potential sediment discharge, in addition to the ESC measures proposed in this report. It is proposed that all significant earthworks areas utilise silty water diversion bunds/channels to convey site water to sediment retention ponds for treatment before discharge from the site. Silt fences should be used to treat runoff from small discrete areas that are not amenable to treatment via ponds, and also provide a last line of defence at critical locations (e.g. stream channels). Where traffic has to be reduced to one lane in order for work to be carried out on the adjacent lane, a hotmix bund can be used to separate discharge from the construction site from that of the trafficked lane. It is further proposed that site entrances should be controlled with stabilised construction entrances. Where an existing culvert needs to be extended to enable temporary works to take place (shown in Erosion and Sediment Control plans in Appendix B) it should be extended by using a lie-flat flume. All ESC devices will be designed in accordance with the Erosion and Sediment Control Guidelines for Soil Disturbing Activities (WRC Technical Report No. 2009/02), and may use addition guideline information for selected ESC measures. 4.1 Culvert Construction and Watercourse Diversion The construction sequence for watercourse diversions is proposed as follows: Excavate the new channel to its required dimensions leaving a plug of earth at either end so that excavation takes place in dry conditions; 1 Where steeper cut batters occur, they are in more-competent and therefore less-erodible rock. August 2017

34 Erosion and Sediment Control Plan 6 Stabilise the new channel and install any required bed form and riparian planting; Remove the downstream plug and temporarily stabilise, followed by the upstream plug, and effect the diversion by damming the existing channel with sand bags or similar; Create a permanent dam behind the sand bags and install any required bend armouring. Culvert construction will follow a generally similar sequence: Construct the new culvert alongside the existing watercourse keeping an earth plug at either end to create dry conditions. (Alternatively, stop flow in the watercourse by temporary damming or bypass pumping, so that the pipe can be constructed within the existing watercourse); Remove the earth plugs or sand-bag dams and introduce flow once the pipe is installed; Stabilise transitions and install any required rip rap armouring. For all diversions and culverting the contractor must make provision to stop work and stabilise the site sufficiently to safely convey any storm flow that arises. Suitable methods might include pinned geotextile, rock placement, or using the installed pipes as a flood conduit. 5 Maintenance, Monitoring and Reporting Procedures All ESC measures should be inspected daily for any obvious damage. A general inspection will be undertaken weekly on all sediment control measures to ensure they are effective, and after any significant rainfall event. All ESC measures should be inspected prior to any forecast significant rainfall event, and any sediment accumulated behind the silt fences will be removed. All-weather access will be maintained to the control devices and the fill sites. The performance of ESC measures should be reported monthly to the client as part of normal contract reporting requirements. 6 Heavy Rainfall Response and Contingency Measures The contractor shall register with NZ MetService for severe weather warnings and it is recommended that immediately upon receipt of any heavy rainfall warning the ESC measures be inspected and repaired/cleaned. Exposed surfaces shall be prepared by removing loose material and compacting the surface where practicable; and applying temporary surface stabilisation such as mulch, fabric covers or a suitable soil binder product in accordance with the manufacturer s instructions. The following contingency measures are proposed: August 2017

35 Erosion and Sediment Control Plan 7 Scheduled construction sequencing can be varied depending on likelihood of rainfall. Machinery and any loose materials should be removed from the construction area. Diversion channels should be installed to prevent stormwater surface flow discharge into a construction works area. In the event that excavations require dewatering, any sediment-laden water can be pumped through a filter bag or similar device, discharging to stable, vegetated terrain at least 20 m back from the river edge. Contingency ESC measures should be recorded on a Sediment Control Maintenance Sheet. 7 Procedures for Review and/or amendment to the E&SCP It is proposed that any minor changes will be discussed with the WRC representative during routine inspections, implemented by agreement, and recorded in the site inspection notes. Any major changes should be documented and an amended ESCP will be submitted to WRC for approval. 8 Identification of Specific Site Responsibilities The Contractor should be required to nominate a suitably trained and experienced Environmental Manager, who will be responsible for the implementation and maintenance of the ESC measures, and updating the ESCP as required during the works. The Site Supervisor or Foreman is likely to be responsible for the day-to-day maintenance of the sediment control measures under the oversight of the Environmental Manager. The implementation and performance of the ESC measures should be monitored by WRC or their appointed representative. 9 Construction Timetable Earthworks are likely to occupy two summer periods, beginning in January Construction of structures and other generally clean activities only proceed during the intervening winter. August 2017

36 Erosion and Sediment Control Plan Appendix A Sediment Retention Pond Sizing February 2017

37 Erosion and Sediment Control Plan Design Size of Pond (WRC Earthworks Series Erosion and Sediment Control Factsheet Sediment Retention Pond) - On earthwork sites with slopes less than 10 per cent and less than 200m in length, construct a sediment retention pond with a minimum volume of 2 per cent of the contributing catchment (200m 3 for each ha of contributing catchment). - On sites with slopes greater than 10 per cent and/or more than 200m in length, construct sediment retention ponds with a minimum volume of 3 per cent of the contributing catchment (300 m 3 capacity for each ha of contributing catchment). Therefore equation: SRP volume = 0.02 x Contributing Catchment (for sites with sloped less than 10%) SRP volume = 0.03 x Contributing Catchment (for sites with slopes greater than 10%) Sediment Retention Pond SRP1 SRP2 SRP3 SPR4 SPR5 SPR6 % of Slopes Contributing Catchment (m 2 ) Sediment Retention Pond Volume (m 3 ) February 2017

38 Erosion and Sediment Control Plan Appendix B Erosion and Sediment Control Drawings February 2017

39 Erosion and Sediment Control Plan Appendix C WRC Earthworks Series - Sediment Retention Pond (SRP), Silt Sock/Filter Log, Silt Fence, Catch Pit Protection, Stabilised Construction Entrance, Decanting Earth Bund February 2017

40 Opus House, Princes Street Private Bag 3057, Waikato Mail Centre, Hamilton 3240 New Zealand t: f: w:

41 Appendix D Site Visit Photographs

42 Figure 1 Culvert inlet at chainage 1,510 Figure 2 Culvert outlet at chainage 1,510, showing how culvert has been previously extended

43 Figure 3 Previously-diverted stream at outlet of culvert between chainages 1,460 and 1,510