Motueka WWTP RC Part-4 Appendix D

Appendix D

Motueka WWTP Upgrading: Concept Design Report for Proposed Marine Outfall (Ocel, December 2014)

TASMAN DISTRICT COUNCIL

MOTUEKA WWTP UPGRADING

CONCEPT DESIGN FOR PROPOSED MARINE OUTFALL

CONSULTANTS NZ LIMITED

49 Crownhill Street PO Box 151 New Plymouth 4340 Telephone 06 7512310 Fax 06 7512310 Email: [email protected]

OCEL House 276 Antigua Street

PO Box 877 Christchurch 8140

Telephone 03 3790444 Fax 03 3790333 Email: [email protected]

mailto:[email protected]

mailto:[email protected]

ISSUE PAGE

Draft for Review 12 Nov 2014 I W Goss

Final 1 December 2014 I W Goss

Final R1 10 December 2014 I W Goss

140906 Concept Design Final R1.docx 1 OCEL Consultants NZ Limited

INTRODUCTION This study was undertaken in response to a brief prepared by Beca to develop a concept design to discharge treated effluent from the Motueka Wastewater Treatment Plant (MWWTP) to the coastal marine area of south branch of the Motueka River at a site close to the existing disposal wetland area which presently discharges by seepage and overflow. The preferred discharge position was established as a result of mixing and dilution studies carried out by DHI, with a discharge coordinate proposed in their report. Improvements to the WWTP are being designed by Beca, to include the land pipeline to the commencement of the outfall. The objective of this component of the project is to provide a conceptual design solution to enabling mid channel discharge whilst providing appropriate anchoring and durability of the components to resist reasonably foreseeable conditions. BACKGROUND INFORMATION Information provided included a Beca topographical survey of the ponds and adjacent areas as far as the mean high water mark level at the south branch of the river. A profile of the south branch bed cross section has been derived from this survey and observations of tide level and appropriate depth across the channel at the nominated location. In addition two reports were provided, the MWH report Motueka Wastewater Treatment Plant Receiving Environments June 2012, and the DHI Report Motueka River WWTP River Mixing Study April 2014. These reports present the basis for selecting the nominated discharge location. AERIAL PHOTOGRAPHY AND HISTORICAL CHANNEL POSITIONS A range of historical aerial photographs of the site has been provided by Tasman District Council (TDC). These cover the initial aerial runs in 1948 to present and show the gradual northerly migration of the main Motueka River channel from the present position of the south branch. This information provides useful background on the recent history of channel positions and potential changes. In addition, TDC have an accumulation of historical river bed cross sections that, while not exactly applying to the proposed discharge position, provide an indication of low bed levels in relation to establishing appropriate pipeline levels to maintain burial. The appended aerial photographs, which are marked up with cadastral road boundaries and the proposed discharge location show the gradual migration of the main river channel from hard against the edge of the present wetlands in 1948, to split channels in 1978, one still against the wetland, and one well to the north. By 1989, when the main ponds were completed, the northern channel had become predominant but with a significant overflow channel remaining to the south. The latest photographs from 2012 and 2013 show the northern channel as dominant. While the southern channel remains as an overflow pathway and a tidal arm of Tasman Bay, it is, under normal river flow conditions, a predominantly tidal secondary channel. In terms of considering outfall design, this information shows that the dominant river channel has migrated to the north over the last 65 years and left the selected discharge location a relatively stable backwater. The driver for this northern excursion of the main channel is not obvious, and it is assumed that it could potentially return to the south. CHANNEL BED MATERIALS While no specific river bed geotechnical investigations have been made for this concept design, discussions were held with a representative of Concrete and Metals Limited which runs the gravel pit just upstream of the wetlands site. They have dug exploratory holes in the vicinity of the proposed outfall pipeline which suggest that the material is sandy gravel to depth, sandier than at their present site. On this basis, it has been assumed that the river bed can be satisfactorily excavated to install a buried outfall pipeline, and that support piles to effectively secure the pipeline can be installed.


Clearly a bed of this material type is susceptible to scour under river flood conditions. Figure No 1 shows a series of surveyed river cross sections in the general vicinity but upstream of the outfall position between 1968 and 2001 and illustrates the channel migration.

Figure No 1 Indicative cross sections looking downstream showing channel invert levels

This information gives an indication of lower bed levels that have been observed in the lower reaches of the river over time. The sections are plotted looking downstream, and over the record period lowest bed levels have been recorded in the main (northern) channel to about 2.5 m below Mean Sea Level (MSL). This level has been selected as the maximum outfall pipeline level to provide a degree of protection, in case the main channel reverts to the southern position it evidently occupied in 1948. Installation of the pipeline section below this level for security requires a riser to allow discharge at existing bed level, and in turn requires piled support to hold the pipeline in position under flood flow conditions. DISCHARGE FLOWS AND OUTFALL CONFIGURATION The range of design flows advised by Beca for outfall discharge is maximum of 60 l/s, with lower flows to be implemented in stages at 40 and 20 l/s. The short outfall component, comprising a 23.5 m feeder pipeline below bed level and a riser to a discharge point just above the existing bed level, has been sized at 300 mm nominal bore to comfortably accommodate this flow range. The potential for the bed level to change through river flooding or channel migration makes a single riser discharge more practical than a horizontal diffuser which would suffer loss of support should the bed lower, or potential blockage should it rise. The proposed discharge configuration to reduce the effect of these potential issues is to use a tee on top of the riser to discharge horizontally through two ports perpendicular to the channel flow. The ports will be fitted with proprietary rubber duckbill check valves which close to prevent saltwater ingress to the outfall system and open gradually with effluent pressure to provide relatively high jet discharge velocities into the receiving waters over the discharge range, and thus optimise initial dilutions. The limited number of jet discharges resulting will also maintain scour holes around the riser should bed levels rise (within reason). The concept is shown on the attached drawing DR-140906-001.


Bed level change also requires that the outfall pipeline and riser are supported in case of bed level changes, and this has been provided by steel piles driven into the lower bed. Consideration of exposure and loadings applied by river flows in flood conditions (see Figure No 2) has resulted in the recommendation for the proposed outfall section to comprise a steel pipeline to provide a robust solution to resist such loads. Zinc metal spray would provide cathodic corrosion protection under salt water exposure as will be exposure in the estuary environment, and an appropriate effluent resistant lining will be required. HYDRAULIC PERFORMANCE Assessment of the hydraulic performance of the outfall components as outlined, and the 300 m 300 NB land line indicated that the expected head loss in discharging 60 l/s via the outfall pipeline with two 150 mm duckbill valves is 1.0 m. To ensure that the design flow is provided for long term, this head provided at the treatment plant must be in addition to allowance for spring tide (or Highest Astronomical Tide - HAT), storm surge, sea level rise, and river flood effects. CONSTRUCTION REQUIREMENTS Construction of the short outfall pipeline will require excavation of the sand and gravel bed to allow the installation of the preprepared pipeline to the designated level. The approach to achieving this installation should be left up to the contractor, but the large tidal range and relatively short low tide periods suggest that temporary support access for construction and excavation plant will be required. Access could take the form of a steel trestle, or a temporary bund made from concrete blocks or gravel fill. Whether the level of the accessway needs to be above high tide, or could be established at say MSL to provide an adequate window over each tide depends on the contractor’s approach and assessment of task times. The risk of river flood should also be considered, with the opportunity to consider periods of the year which show a lower probability of flooding.

Figure No 2 Aerial view of flood discharge – 2 yr return period


The installation of the outfall, including the piling which is expected to be installed using a vibro hammer, will not be a process of long duration once suitable access and construction procedure is established, and the components fabricated and on site. It is estimated that installation would easily be completed with 1 month on site, which would overlap with other works such as the land line section of the outfall pipeline. To ensure that adequate construction area is available to accommodate trenching and temporary access, it is suggested that a 40 x 40 m area covering the proposed outfall pipeline alignment is provided to the contractor. Access to this site, which would be predominantly below MHWS, could be provided through the treatment plant site, or via the Concrete and Metals gravel plant access road and adjacent pasture land. A similar area away from the river should be provided for storage and equipment. Note that as illustrated by Figure No 2, any nearby shore based construction area has the potential to be exposed to flooding. CONCLUSIONS The design concept presented provides a practical solution to allow treated effluent to discharge to the coastal marine area adjacent to the Motueka WWTP. The parameters considered in developing this configuration include: • Nominated discharge position • Nominated design discharge flows • Present and historical channel positions • Present and historical channel invert levels • Discharge position relative to present and potential bed levels • Hydraulic performance in relation to pipeline head losses


DRAWINGS – DR-140906-001 Rev 2


ELEVATION OF OUTFALLPRELIMINARY

SITE PLAN

W

S

N

E

Scale 1:5000

Scale 1:100

APPENDIX A – HISTORICAL AERIAL PHOTOGRAPHS


A 1 :4 ,000

MOTUEKA RIVER DELTA - AERIAL PHOTO 1989 --=::::::::m-c::::~m 0 40 80 120 160

- comprising existing wetlands/infiltration basins

(1989)

Appendix E

Motueka WWTP: Baseline Study and Water Quality Assessment for a Proposed Marine Outfall (Cawthron, December 2014)

Documents

Motueka WWTP RC Part-4 Appendix D