IN THE MATTER of the Resource Management Act 1991

AND

IN THE MATTER of a Board of Inquiry appointed under s149J of the Resource Management Act 1991 to determine an application for resource consents sought by Watercare Services Limited for its River Take and Discharge Proposal.

STATEMENT OF EVIDENCE OF SELENE ALEXANDRA CONN ON BEHALF OF WATERCARE SERVICES LIMITED

Sedimentation

1. Introduction, Qualifications and Experience

1.1 My name is Selene Alexandra Conn. I am a Technical Director for fluvial geomorphology and an ecologist (specialising in riparian and wetland vegetation) at Tonkin and Taylor Limited, based in Tauranga. I have been employed by Tonkin and Taylor Limited for three years and seven months.

1.2 I hold the following qualifications relevant to this assessment:

(a) Bachelor of Science (Biology and Geography (2007), Bachelor of Science Honours (Geography (2008), Master of Science (Geography (2009) from the University of . (b) I have been a Certified Environmental Practitioner (#1219) since 2018, and received the Geomorphology Specialisation Accreditation (#001) in February 2021. I am currently the vice chair of the New Zealand Rivers Group, a technical interest group of Engineering New Zealand, and have been on the committee since 2017.

1.3 I have eleven years’ experience in the fields of ecology and fluvial geomorphology. Prior to my role at Tonkin and Taylor, I was a senior ecologist at Wildland Consultants based in Tauranga specialising in wetland and riparian vegetation surveys and stream habitat enhancement plans. Between 2012 and

2016, I was employed as an environmental scientist at Alluvium Consulting in Townsville, Australia specialising in riparian vegetation surveys, riparian planting plans for large scale waterway diversions and geomorphic assessments for a range of clients and project objectives. During this time, I also served on the Marine Conservation Board partnering with the Great Barrier Reef Marine Protection Authority providing technical advice and guidance on terrestrial sedimentation issues affecting the reef. Between 2010 and 2012 I was employed as an environmental scientist at Morphum environmental specialising in erosion assessments and management plans and stream enhancement plans, as well as a riparian technical co-ordinator at EnviroMatters in Auckland.

1.4 Examples of recent projects I have been involved in include:

(a) A geomorphic assessment of the Burdekin River, Queensland, Australia. (b) A geomorphic assessment of the Waiohine River for Greater Wellington Regional Council. I presented the findings at the Water NZ Conference in 2019. (c) A geomorphic assessment of the Manawatū and Ōroua Rivers for Horizons Regional Council. This work was presented at the New Zealand Freshwater Sciences Society, New Zealand Hydrological Society and New Zealand Rivers Group joint conference in 2020. (d) A geomorphic assessment of the streams in the Waikite Valley feeding into the Whirinaki Arm of Lake Ohakuri. (e) A high-level geomorphic assessment of the Mangaohoi and Mangapiko Streams for Waipa District Council.

1.5 This evidence is provided in support of Watercare’s application for all necessary consents to enable the taking of up to 150,000 cubic metres (net) of water per day (m3/day) from the for municipal water supply purposes (Project).

1.6 In terms of my involvement in this Project, I assisted in providing the specialist technical input into the possible effects of an increase in water take on sediment transport and bedform processes contained in Sections 4.4 Sediment and 5.2

Sediment Effects of the River Hydrology Assessment provided in support of Watercare’s updated application, lodged with the EPA on 11 December 2020.

2. Code of Conduct

2.1 My qualifications as an expert are set out above. I confirm that I have read the Expert Witness Code of Conduct set out in the Environment Court's Practice Note 2014. I have complied with the Code of Conduct in preparing this evidence. Except where I state that I am relying on the evidence of another person, this evidence is within my area of expertise. I have not omitted to consider material facts known to me that might alter or detract from the opinions expressed in this evidence.

3. Executive Summary

3.1 The sedimentation effects (transport and bedform processes) due to predicted changes in flow depth, and velocity in the river due to the proposal are in my opinion minimal.

3.2 The reduction in median flow and mean annual flow are negligible and are unlikely to change sediment dynamics or effect bed form processes at intake site or in the downstream reaches.

3.3 Water level fluctuations could affect surface water gradient and/or shear stress resulting in localised changes in sediment transport around the intakes themselves, which is discussed in more detail in Mr Robert Keller’s evidence. The predicted volume of sediment removed through the intake could be up to an approximate maximum of 3,800 m3/year11 which is less than 4 % of the projected on-going annual extraction from Winstone Aggregates Smeeds Quarry across the river, and less than 2 % of annual authorised volume. Therefore, any sediment extracted due to the proposed water take will have a negligible effect on sediment dynamics in the lower Waikato River.

3.4 The lower Waikato River can be considered to be dynamic regarding mobile bedform processes and sediment, and underlying degradational trends. The intake screens are also located in a relatively dynamic area of the river planform. This all suggests that bed levels at the intake structure are going to be variable

(as shown in the data), due to any number of possible reasons. However, as per Mr Keller’s evidence, the chosen location of the intake screens is considered appropriate. Any changes in bed level observed through on-going monitoring may not necessarily be a result of effects from the proposed take and should be assessed against the catchment and reach-scale temporary, short, medium, and long-term geomorphic processes of the Lower Waikato.

4. Scope of Evidence

4.1 My evidence addresses the sedimentation effects of the proposed new water take from the Waikato River.

4.2 I have undertaken a desktop assessment of literature relating to sediment budgets, geomorphic processes and bedforms. To inform my assessments, I have relied on information contained in the reports set out in Appendix A.

4.3 My evidence is structured as follows:

(a) An overview of the sediment regime in the Lower Waikato River, looking at sediment budgets, and activities that are modifying the sediment regime. (b) A short description of the underlying trends of sediment transport and bed levels within the Lower Waikato River, identifying drivers of change, and effects on bed levels. (c) A description of the sediment type within the Lower Waikato River, and what this means for its transport behavior. (d) A brief description of sediment dynamics within the river, with regard to changes in flow. (e) A description of the bed levels at Watercare’s Waikato intake site, based on monitoring undertaken between 2003 and 2019, and a brief description of possible drivers/causes that may contribute to any bed level fluctuations that may be observed. (f) A summary of the potential effects associated with the Project.

4.4 I am aware of, and can confirm that I have read, Mr Keller’s evidence and believe that our evidence is in broad agreement in regards to wider changes in the Lower Waikato River.

5. Waikato River sediment regime

5.1 The Waikato River is a highly modified and regulated river with regard to sediment loads and sediment transport due to the presence of eight hydro- electricity dams between Lake Taupō at its source, and . Other human activities, such as sand extraction, have also modified the sediment regime.

5.2 The estimated total pre-dam suspended sediment load for the Waikato River is 526,000 t/year, with approximately 37 % (194,620 t/year) from the upper Waikato and (coincidentally) 37 % (194,620 t/year) from the Waipa River1 (Table 5.1).

1 Hicks M and Hill R (2010). Sediment Regime – sources, transport, and changes in the riverbed. In K J. Collier, D P Hamilton, W N B Vant, and C Howard-Williams (Eds.) The Waters of the Waikato: Ecology of New Zealand's longest river (pp. 71 to 92). Hamilton, New Zealand: Environment Waikato and the Centre for Biodiversity and Ecology Research (The University of Waikato).

Table 5.1: Averaged annual rates for the Waikato River bed material budget (1964-1998) for ‘natural’ pre-dam levels (taken from Hicks et al5) and for ‘actual’ post-dam levels (taken from Hicks and Hill1).

(a)

(b) ‘Actual’ Sediment budget tabulated (from Hicks and Hill1) Mid-Waikato (Karapiro to Ngaruawahia) Lower Waikato (Ngaruawahia to the coast) Upstream inputs (across the 0m3 Upstream inputs (across +141,000m3 upstream boundary at Lake the upstream boundary at Karapiro) Ngaruawahia) Tributary inputs +63,000m32 Tributary inputs +27,000m3 Yield from bed degradation +101,000m3 Yield from bed +417,000m3 degradation Extracted -23,000m3 Extracted -484,000m3 TOTAL reach bed load +141,000m3 TOTAL reach bed load +101,000m3

5.3 It is estimated that the eight hydro dams upstream of Lake Karapiro trap 100 % of the bed material load, and up to 30% of the suspended sediment load of the Waikato River1. For the purposes of this evidence, bed load is considered to be the main contributor to changes in bed form morphology.

2 54,000m3 from the , and 9,000m3 from other tributary sources.

5.4 All bed load in the Waikato River downstream of Lake Karapiro comes from tributaries (the Waipa River being the largest contributor), and material eroded from the bed and banks of the main stem of the Waikato River (downstream of Lake Karapiro) and transported downstream (discussed further in Paragraph 5.11 and 5.12 below).

5.5 Historic sand extraction from the lower Waikato River has also altered the sediment regime further. Between 1953 and 2006 the volume of sand extracted was over three times the average rate of bed material entrapment in the hydro- lakes1, peaking in 1974 with over 1 million m3 extracted from the reaches between Hamilton and Puni (downstream of ).

5.6 Data from Waikato Regional Council3 suggests the rate of sand extraction has reduced between 1974 and 2010. R. J. Keller and Associates4 note that mean annual sand extraction is presently approximately 100,000 m3/year (Figure 5-1). Winstone Aggregates commenced sand extraction operations at Smeeds Quarry, Pukekawa, on the true left bank just downstream of the Watercare intake in 2013. Its consented extraction is 200,000 m3 of sand annually. However, the actual extraction is more variable, with between 70,000 m3/year to 140,000 m3/year initially, levelling out to an approximate 100,000 m3/year average, which is expected into the future4.

3 Waikato Regional Council (2012). Lower Waikato River main channel sand abstraction policy report. Prepared for River and Catchment Services Group. 4 R. J. Keller and Associates (2020). Waikato River Take Proposal, Lower Waikato Bathymetry Assessment – Changes Consequent to Development. For Watercare Services.

Figure 5-1: Lower Waikato River historical and forecast sand extraction volumes (WRC, 2012)

Underlying sediment trends in the Waikato River

5.7 Historic, and current sand extraction, as well as the hydro-power schemes have resulted in a deficit of bed load material downstream of Lake Karapiro compared to pre-dam levels.

5.8 Based on the bed material budget prepared by Hicks and Hill1 for the lower Waikato River (Table 5.1), sand extraction appears to have been the greatest contributor to historic bed degradation trends from Ngaruawahia to the coast, with averaged extraction rates between Ngaruawahia and equalling almost 83 % of the overall sediment inputs.

5.9 However, some evidence suggests that localised bed levels can recover quickly following the cessation of extraction activity1 5.

5.10 While sand extraction appears to be the primary driver of bed degradation in the lower Waikato River, there is also evidence that the Waikato River is attempting to ‘recover’ the deficit of its sediment load downstream of Lake Karapiro by eroding its own bed1 (bed degradation). This has created what has been

described as a wave of degradation moving downstream16. By 1980 the degradational wave was estimated to have reached as far as Ngaruawahia5. This ‘wave’ is likely preceded by a slight increase in bed levels, as sediment is mobilised from the degrading upstream reaches into those immediately downstream. But overall, it is estimated that some bed degradation in the lower reaches of the Waikato River may be due to (or likely to be in the future) from sediment starvation arising from the capture of bedload in the hydro-power schemes.

5.11 Hicks et al5 estimated that over the decade between 1990 and 2000, the river had ‘recovered’ (by eroding its bed) approximately 24 % of its bed material deficit from between Hamilton and Ngaruawahia and the remaining 76 % from between and Meremere, based on cross-section monitoring.

5.12 Hicks and Hill1 identified five main reaches in the Lower Waikato, and present data on the annual rates of change based on cross-section monitoring data ( 5.13 Table 5.2). While annual rates of change are not provided for the Mercer to Tuakau, and Tuakau to the coast reaches, these have both experienced substantial bed level fluctuations in the past (Figure 5-2) and are sensitive to changes in discharge and sediment load.

5.14 Hughes6 identifies that the reduction in bed level throughout the mid to lower reaches of the Waikato River has resulted in instabilities (corresponding bed degradation) in adjoining tributaries, but that this does not seem to have (yet) resulted in increased bank erosion rates along the main stem of the Waikato.

5 Hicks, D. M., Webby, M. G., Duncan, M. J., and Harding, S. (2001): Waikato River sediment budget and processes. Report prepared by NIWA for Mighty River Power. 6 Hughes, A. (2015). Waikato River Suspended sediment – loads, sources and sinks. Information to inform economic modelling for the Healthy Rivers Wai Ora Project. Report prepared by NIWA for Healthy Rivers.

Table 5.2: Bed level change in response to a reduction of sediment supply and sand extraction between 1998 and 2007 (from Joynes 2008; in Hicks and Hill 2010).

Reach Rate of change Overall process (mm/year) Mid-Waikato (Karapiro to Hamilton) 17 Slow degradation Hamilton to Ngaruawahia 25 Degradation Ngaruawahia to Mercer 12 Slow degradation Mercer to Tuakau N/A Aggradation Tuakau to the coast N/A Degradation

Figure 5-2: Smoothed mean bed level of the lower Waikato River from Ngaruawahia to near Port Waikato, 1964-98. Source: Waikato Regional Council (formerly Environment Waikato).

Sediment type

5.15 The dominant sediment type of the Lower Waikato reach has previously been characterised by T+T7 as “relatively coarse, well graded sand”.

5.16 Hicks and Hill3 summarise research on particle size in the Waikato River, and suggest that this sand is between 1 mm to 0.7 mm at Mercer. This sand is

7 Tonkin + Taylor Limited (2008). Waikato Water Intake Sedimentation Assessment. Prepared for Watercare Services Limited. 20973.800.

suggested to have a high pumice content, and a greater mobility because of this8.

5.17 Hicks and Hill3 also suggest that this bed material typically forms mid-channel bars with large dunes (1 m in height with wavelengths several tens of meters), features which are common downstream from Ngaruawahia.

5.18 Dunes were also observed migrating downstream during repeat surveys between February and April in 2008 during average flow conditions7.

5.19 As the bed is mobile, scour holes commonly form in response to various channel features, including at the outside of meander bends, channel constrictions, or around objects such as woody debris or structures on the bed8.

5.20 There was no discussion of riverbed armouring (i.e. with coarser, or more cohesive material on the bed protecting the finer sediments below) in the reaches of the Lower Waikato downstream of Mercer, based on the literature reviewed as part of this evidence. However, it is possible that localised areas of armour may be present (and unknown) or may be revealed in the future, considering the following:

(a) Armouring may have been present prior to sand extraction activities, and sand extraction activities have removed it (and it’s yet to re- establish). (b) Lenses of armouring may have been/or may be present in buried alluvium below the currently mobile sand bed. (c) Dynamic armouring could be present, with local accumulations of coarser material deposited/accumulated during certain flow events, protecting the finer material beneath for a short time before they are themselves mobilized downstream in larger events. This phenomenon was discussed by Smart9 in relation to hydro-power flow ramping between Karapiro and Ngaruawahia. (d) Changes in catchment conditions (such as large-scale land use changes or landslips), sediment sources, and/or channel geometry

8 Wood AP 2006. Morphodynamic channel and stability of the Waikato River: Karapiro to Ngaruawahia reach. Unpubl. MSc thesis. The University of Waikato, Hamilton; in Hicks and Hill (2010). 9 Smart G 2005. Analysis of stage II degradation studies Waikato River, Karapiro to Ngaruawahia. Report prepared for Environment Waikato. National Institute of Water and Atmospheric Research Ltd, Christchurch.

may also initiate a change in sediment composition, thereby introducing coarser material8.

Sediment dynamics

5.21 As identified in paragraph 5.15 above, the dominant sediment at Mercer is reasonably mobile, and (in my opinion) it follows that the sediment would be similar at the intake site.

5.22 Smart9 suggested that the maximum moveable particle size in the Hamilton Reach was between 8.9 mm and 14.5 mm for flows between 250 and 300 m3/s at four sites assessed during his assessment of the effects of flow ramping on bed degradation. While these assessments were site specific, and undertaken in the Hamilton Reach, they provide an indication of the potential for particle movement near the intake.

5.23 Paragraph 5.15 above suggests the sand particle size at Mercer is between 1 mm and 0.7 mm, suggesting the bed would most likely be mobile during most flows at the intake site.

5.24 Smart’s9 research, and Hick and Hills1 subsequent interpretation suggest that rapidly changing flows may affect sediment transport, with smaller more turbulent flows able to transport more sediment. This has been attributed to the threshold-of-motion of sediment, changes in water surface gradient, and changes in bed shear stress (associated with water surface gradient and turbulence). While the findings were in relation to daily flow fluctuations associated with hydro-power, there could be similar localised changes associated with the increased water take.

5.25 The presence of dune and bar bed forms also suggests that average flows are most likely higher than the critical threshold for motion for the existing sediment type (e.g. the sediments are frequently mobile during low flows) due to the following:

(a) Dune and bar features are described as features that form in ‘lower’ flow regime condition, meaning they typically form in finer grained

particles during lower velocity flows10, as observed near the intake site by T+T7 over the low flow summer months. (b) Dune features are commonly washed out at high velocities (described by Brierley and Fryirs10 as occurring during intense bed load transport), which was presented as occurring downstream of Huntly in Hicks and Hill1.

5.26 Dune bedforms are also depth dependent, with Brierley and Fryirs10 stating that dune height is up to ⅓ of flow depth, and wavelength is 4 to 8 times flow depth.

5.27 As described in paragraph 5.19 scour holes are likely to form on the outside of meander bends and around obstructions on the bed. Conversely, immediately downstream of channel constrictions (e.g. where the channel narrows and then widens again), sediment is likely to be deposited.

5.28 The intakes are located on a tight, bedrock-controlled meander, with channel constrictions on both the upstream and downstream ends of the reach. This would suggest that the intakes may sit within an area prone to scour, especially during high flows. However, scouring will improve the long term resilience and sustainability of the intake screens against silting (refer to Section 5 of Mr Keller’s evidence for a more detailed discussion around the suitability of the chosen intake site).

5.29 Conversely, sediment deposited downstream of the upstream channel constriction (i.e. upstream of the intakes) are likely to form into dune bedforms, which will migrate downstream during low flow conditions.

Waikato intake bed levels

5.30 Survey data from 2007 showed that approximately 4.0 m of sediment had accumulated at the intake since 2004 and that the bed level was 0.5 m to 1.0 m below the underside of the intake screens.

10 Brierley, G. J. and Fryirs, K. A. (2005) Geomorphology and River Management: Applications of the River Styles Framework. Blackwell Publishing.

5.31 Regular surveys since 2008 and until 2019 indicate varying bed levels, with the clearance between the underside of the screens and the riverbed ranging between 0.2 m and 3.8 m.

5.32 Given what has been discussed in 5.28 and 5.29 above, it is entirely likely that bed levels at the intake will vary from time to time due to the following:

(a) Progression of dunes downstream during low-flow periods. (b) Scour on the outside of the bend near the intakes during high flow events. (c) Slow lowering of the bed levels due to long term degradational trends in the river (associated with sand extraction and hydro-schemes).

5.33 The suitability of the chosen proposed intake location is discussed in more detail in Section 3 of Mr Keller’s evidence.

Figure 5.3: Distance between Waikato intake Screen and river bed recorded by diver observations since 2003 until 2019.

6. Effects of the proposed take on sediment

6.1 In the wider river context, catchment and reach-scale sediment supply interruptions (hydro-lakes and sand extraction) are having the greatest effect on sediment transport, bed levels, and bedform processes. In comparison, the proposed new take is likely to have a negligible effect.

6.2 Sedimentation effects (transport and bedform processes) and changes in flow and velocity in the river due to the proposal are considered minimal. This is due to the following:

(a) The incremental change in velocity associated with the increased take is very small, i.e. 0.003 m/s, and cumulatively 0.006 m/s (between 0.6 % and 1.1 % change in mean velocity at the intake). (b) The change in water depth is also minimal, with a slight cumulative

water level reduction of up to 82 mm (1.2 % of the q5 springtide water depth) upstream of the intake (and expected to be less at the wider channel sections downstream). (c) As threshold for motion is already frequently exceeded during low-flow conditions, the estimated changes in both water depth and velocity are unlikely to have an impact on sediment transport or bedform processes.

(d) As the dune bedforms are already less than ⅓ of q5 springtide water depth upstream of the intake, the minimal reduction in water depth is unlikely to have an observable effect on bedform morphology.

6.3 The reductions in flow during median and mean annual flow conditions are considered negligible and are unlikely to change sediment dynamics or effect bedform processes at the intake site or in the downstream reaches.

(a) Changes between q5 discharge and proposed discharge (with a cumulative take of 300 MLD) are approximately 1.7% or 5.65 m3/s for both a spring high and low tide. (b) As for the reasons discussed in 6.1 (c) and (d), these changes in discharge are unlikely to result in any observable changes to sediment transport or bedforms.

6.4 Water level fluctuations could affect surface water gradient and/or shear stress resulting in localised changes in sediment transport around the intakes themselves. As discussed in Paragraph 5.23, the effects of flow ramping on sediment transport presented by Smart9 and subsequently interpreted by Hicks and Hill1 may be applicable in regards to the processes at the intake. However, this phenomenon is unlikely to result in any discernible effect on the wider river processes, based on the findings of R. J. Keller and Associates11 which estimates that the annual volume of ‘sediment’ removed from the water column could be up to an approximate maximum of 3,800 m3/year, noted to be less than 4 % of the projected on-going annual extraction from Winstone Aggregates at Smeeds Quarry across the river, and less than 2 % of the annual authorised volume. Regardless of whether the sediment being entrained by the intake is suspended or bed load, at the rates predicted by R. J. Keller and Associates, sediment entrained into the intake structure due to the proposed additional water take will have a negligible effect on sediment dynamics in the lower Waikato River.

Selene Alexandra Conn 21 May 2021

11 R. J. Keller and Associates (2020b) Waikato River Take – Lower Waikato River – Changes Consequent to Development. Report prepared for Watercare Services (unpublished).

Appendix A: References

(a) Hicks, D. M., Webby, M. G., Duncan, M. J., and Harding, S. (2001): Waikato River sediment budget and processes. Report prepared by NIWA for Mighty River Power.

(b) Hicks M and Hill R (2010). Sediment Regime – sources, transport, and changes in the riverbed. In K J. Collier, D P Hamilton, W N B Vant, and C Howard-Williams (Eds.) The Waters of the Waikato: Ecology of New Zealand's longest river (pp. 71 to 92). Hamilton, New Zealand: Environment Waikato and the Centre for Biodiversity and Ecology Research (The University of Waikato).

(c) Hughes, A. (2015). Waikato River Suspended sediment – loads, sources and sinks. Information to inform economic modelling for the Healthy Rivers Wai Ora Project. Report prepared by NIWA for Healthy Rivers.

(d) R. J. Keller and Associates (2020a). Waikato River Take Proposal, Lower Waikato Bathymetry Assessment – Changes Consequent to Development. For Watercare Services.

(e) R. J. Keller and Associates (2020b). Waikato River Take Lower Waikato River – Changes Consequent to Development. For Watercare Services.

(f) Smart G (2005). Analysis of stage II degradation studies Waikato River, Karapiro to Ngaruawahia. Report prepared for Environment Waikato. National Institute of Water and Atmospheric Research Ltd, Christchurch.

(g) Waikato Regional Council (2012). Lower Waikato River main channel sand abstraction policy report. Prepared for River and Catchment Services Group.