Science Centres: Freshwater

Pahurehure Inlet

Integrated modelling

Subdivision of Pahurehure inlet catchment area into sub-catchments

Detailed view of Mike 3Dfm hydrodynamic and sediment transport model grid and bathymetry for the Pahurehure Inlet and inner Manukau harbour.

Sediment core from the Pahurehure inlet.

C-CALM.

C-CALM model development and structure

Two identical treatment ponds have been built for this study and receive equal volumes of inflow from the same catchment area.

Technician Pete Pattinson comtemplates sediment rich sheet flow from the earthworks on 29 March 2007.

Ponds at full capacity and in danger of overflowing during the event.

Samples taken with automatic water samples at the outlets of the ponds

The raingarden resemples a landscaped bed of native plants.

The main inlet is enclosed with a bay ensuring all inflow flows through the v-notch weir.

The outlet is a standpipe raised above the ground surface.

The Auckland Regional Council (ARC) has contracted NIWA to model the accumulation of the contaminants sediment, copper and zinc in the South Eastern Manukau Harbour over the period 1950 to 2100.  This work is in collaboration with the Hamilton based Coastal Processes and Ecosystem Modelling groups.

The cornerstone of NIWAs approach is the use of the Urban Stormwater Contaminant (USC) model to synthesise outputs from a suite of other models and so project forward from past and current known conditions to predict future contaminant accumulation. These other models will provide input information on: the delivery of flows and sediment to the harbour (GLEAMS); estimated contaminant loads associated with different development and stormwater management scenarios (ARC Catchment Load Model spreadsheet, CLM); and the transport and deposition of sediment within the harbour study area (a suite of DHI models).  USC was used by NIWA for a similar project for the Upper Waitemata Harbour.

The primary focus of work during the 2006-07 financial year has been the definition of the model domains and the collection and processing of essential field data and other information for subsequent model development and implementation. The subdivision of the study area into harbour sub-estuaries and contributing sub-catchments has provided the framework for the implementation of the USC model. It has also defined the spatial distribution of outputs requried of the three contributing modelling programmes, since these outputs will feed directly into the USC model runs. Progress has been made in the definition and testing of the harbour hydrodynamic model (DHI model suite) and this is ready to run once all the necessary input data is available.

In parallel with the development and definition of the harbour models, a significant data collection programme has been undertaken. Sediment cores and surface sediment samples have been taken from locations within the harbour and its estuaries to provide data for the calibration and validation of the USC model. The surface samples have been analysed for metal concentrations and particle size distribution whilst additional analyses of the cores have been undertaken to determine sediment accumulation rates (from radioisotope analysis) and information on bioturbation and erosional processes (from X-ray imagery).

The various models also require time series input data in the form of rainfall and climate records. Data from rain gauge and climates sites in the catchment has been evaluated, selected and processed. The sediment load modelling will use rainfall records from Ardmore and a site in the Ngakaroa Stream catchment whilst the DHI models will use input climate data from Auckland Airport. This climate data has been processed into a series of discrete weather event types. Multiple runs of the DHI models will be undertaken to predict the pattern of sediment transport and deposition under each of these weather event types.

Work in 2007–8 will focus on model implementation, calibration and ,ultimately, predctions under a range of development and stormwater management scenarios.

Catchment Contaminant Annual Load Model
C-CALM

This project is under a sub-contract from Landcare Research(funded by the Foundation for Research and Technology). The main objective is to develop a GIS-based (Geographical Information System) spatial model of long-term stormwater generation and contaminant loads given various treatment options. The model will test the ability of stormwater quality treatment devices to improve the water quality of catchment discharge. The model has been dubbed C-CALM (Catchment Contaminant Annual Load Model). C-CALM will be developed for the Auckland area initially but will eventually be applicable nationwide.

The best management practices (BMPs, see the stormwater page) simulated in C-CALM are are commonly used in New Zealand to minimise the impacts of stormwater on receiving environments. Those to be included are detention devices (wet ponds and wetlands), raingardens, infiltration devices (e.g., swales, permeable paving) and media filters. The C-CALM will support planning, design, and assessment of the effectiveness of stormwater-related infrastructure and will also allow users to change landuse to simulate future city-scapes. It will be SIMPLE to use and have MINIMAL data requirements. C-CALM is not intended to be an alternative to established packages such as SWMM (US EPA) or MOUSE (DHI) which simulate both catchment hydrology and hydraulics including flow through the sewer and stormwater reticulated networks. These packages require a long set-up time, high resolution rainfall data, long run-times and a high degree of user expertise. Rather, C-CALM will give annual contaminant loads at the catchment scale.

C-CALM will be able to locate the relative sources and sinks of sediments and contaminants (at this stage zinc and copper). Annual yields for the catchment (determined using a similar method to the ARC Catchment Load Model, CLM, spreadsheet) will be modified using performance rules determined for a range of BMPs. Parameters for will include slope, landuse (e.g., presence of major roads, roofing materials, percentage permeable vs. impervious surfaces) and soil type. The GIS model will not explicitly simulate flows through the catchment.

The concept behind C-CALM is that GIS can give spatial information about contaminant sources and sinks. C-CALM will allow uses to simulate the impacts of stormwater treatment trains on stormwater quality.

Three tiers of modelling are identified:

a. Development of continuous conceptual models of water flows and contaminant movement through a range of BMPs.

b. Development of performance rules by carrying out a series of “sensitivity analyses” using the BMP models run with long time series (e.g., 10 years artificial inflow record) to determine the effect of different stormwater control parameters (e.g., size, depth of treatment device) and catchment parameters (e.g. ratio of permeable to impervious surfaces) on removal efficiencies.

c. Incorporation of performance rules in the form of a look-up table or similar into the GIS contaminant model.

The user will be provided with a C-CALM toolbar in ARCMap GIS similar to that created for the CLUES model (Catchment Land Use for Environmental Sustainability) in a project led by NIWA. This interface will allow users to place a BMP with user specified dimensions at a specific location. The area contributing flow and contaminants to the BMP will be automatically determined from slope or theissen polygons(users will be able to redraw sub-catchment boundaries). Contaminant yields from the subcatchment will then be reduced according to the performance rules based on BMP type and size relative to the area of impervious surfaces of the selected sub-catchment.

C-CALM is not intended to be a full urban drainage model. It is meant to be simple to set up and used to give an indication of contaminant sources and sinks with landuse change and the long-term ability of BMPs to both prevent and mitigate contaminant transport.

Floc Ponds

The Auckland Regional Council (ARC) has contracted NIWA to evaluate the effect of adding a flocculant to sediment laden runoff from earthworks to aid particle settling in treatment ponds. The results of a previous study commissioned by the ARC (Technical Publication 227) indicate that a range of chemical treatments improved the efficiency of three sediment retention ponds. Flocculants added to the ponds promote the coagulation and settlement of fine sediments that would otherwise be discharged into the receiving environment. Based on sampling of inflows and outflows during storm events, ponds treated with Polyaluminium Chloride (PAC) were reported to have treatment efficiencies as high as 99% for removal of total suspended solids

Removing vegetation and topsoil during development leaves soils open to sheet erosion as runoff flows over the surface. Particles entrained in runoff pose a risk for both fresh water and marine receiving environments. There are two main problems caused by the deposition of sediments from earthworks: 1. if sediments are fine, the substrate can be changed from sandy to silty making it an unsuitable habitat for bed flora and fauna : 2. benthic communities can be smothered by sediments from a single event sedimentation if more than a few millimetres are deposited.

Sediment retention ponds are therefore employed as standard control measures on earthworks sites. Chemical treatments added to ponds aim to improve their treatment efficiency through the flocculation and settlement of fine particles. This project is an experimental study designed to determine: (a) the improvement of sediment retention achieved by a relatively common chemical treatment of a pond; and (b) the potential effects of the treatment on the receiving environment.

Two ponds, sharing a common catchment area and identical in their design, were constructed side-by-side at the ALPURT B2 motorway construction site near Orewa, north of Auckland for this project. One pond is dosed with PAC while the other receives no chemical treatment.

The inlets and outlets of the ponds are instrumented with weirs and water level recorders in order to monitor flows and to trigger automatic water samplers. Water samples are analysed for Total Suspended Solids (TSS) and particle size distribution. Since the dosing of ponds with PAC has the potential to result in the discharge of elevated quantities of aluminium, water samples taken from the pond outflows are also analysed for their aluminium concentration.

The ponds first filled during the storm event of 28-29 March 2007 (the same rainfall event that caused devastating floods in Northland), photos from this event are given below. Analysis of samples taken over the event show that the total sediment load discharged from the untreated pond is approximately four times that discharged from the treated pond.

Sampling during at least 5 further events, along with analysis of samples for particle size distribution and dissolved aluminium concentrations will provide for a more complete understanding of the effectiveness of PAC flocculent and its potential environmental effects.

Raingarden monitoring

The Auckland Regional Council (ARC) has contracted NIWA to evaluate the removal efficiency of a raingarden. Raingardens are a visually attractive subset of media filter that allow biological uptake of contaminants and evapotranspiration of stormwater. On the surface they look like vegetation beds and are often used as landscape features. However, below the surface, the filter media is able to trap particles and associated contaminants. There is also a possibility that dissolved contaminants can be removed as they bond with the media while the stormwater is retained.

Raingardens are often proceeded by a vegetated filter strip and/or a small pre-settling basin for temporary water storage. Unlike filters, they may not be lined allowing some interaction with local ground water. A perforated under-drain may be necessary to stop the raingarden becoming saturated, this is a perforated piping system within a gravel layer. Raingardens can be on or off-line and serve small drainage areas.

The ARC has guidelines for raingarden construction, however a recent ARC study of approximately 30 raingardens in Auckland suggested that their construction is poor and hence the performance of raingardens reported in the literature overestimates their treatment ability (Timperley & Reed, 2007; unpublished data). One of the raingardens thought to be operating correctly, that is, built to design criteria was chosen for further assessment. This device is located in Waitakere City at a vehicle testing station off Lincoln Road and treats runoff from the testing station carpark and roof.

Flow monitoring and sampling required the building a a flow divert channel to a single inflow equipped with a v-notch weir as inflow normally comes from three inlets. The outflow from the garden passes through a shallow manhole, and a 90 degree weir was installed in the manhole. Stilling wells were installed at both weirs with float and counterweight encoders logged stage every 1 minute to an accuracy of ±1 mm. Stage checks were performed periodically to ensure the logger datums remained constant. A stainless steel benchmark was installed in the manhole at the lowest point, and all the loggers were adjusted to the same datum. One other stilling well and encoder was installed in the garden itself to log water levels above the garden surface. A tipping bucket raingauge with a 0.2mm resolution was also installed, attached to a 2 m fence which runs adjacent to the garden’s north side by the inlet.

Six events have been monitored for flow and water quality since November 2006. Water quality has been analysed for particulate and dissolved copper and zinc and Total Suspended Solids (TSS). The raingarden has up to 79% reduction of TSS at the outlet and was also able to reduce metal concentrations.

New Innovations - Capability Funds

Breaking the urban contaminant transport chain

Urban stormwater carries elevated loads of total suspended solids (TSS, i.e., sediments) and contaminants such as metals (mainly zinc and copper) and hydrocarbons. A substantial part of stormwater is conveyed via roadside gutters and catchpits (i.e., drain inlets) to the reticulated pipe network and on to streams, estuaries and harbours. Roadside gutters and catchpits therefore represent an obvious point at which to intercept and remove contaminants. There are products on the market which recognise this and are designed for installation in existing catchpits. Ingal’s EnviroPod, for instance, is a filter bag which can be readily inserted and later removed for cleaning. Trials suggest they can trap up to 80% of TSS (Diffuse Sources, 2001), although they are designed neither to trap fine particles (<100 microns) nor treat dissolved metals.

However, studies internationally and in New Zealand have shown that a substantial proportion of metals in stormwater are attached to finer particulate matter or are in dissolved form.  There is therefore an opportunity to develop a treatment device which can improve on the design of existing catchpit filters. This could be a stand alone device or something which complements existing products. It is likely to involve the use of aggregate matrices to reduce flow velocities and promote the settlement of fine particulates and a chemically treated filter to remove dissolved metals. The development of such a device needs to consider:

• Effectiveness compared to existing options (cost / benefit)
• Re-use potential
• Ease of installation
• Ease of maintenance & waste disposal
• Simplicity / applicability
• Ease of customisation
• Compatibility with the hydraulic functioning of the stormwater system

OBJECTIVE:

To develop a roadside stormwater treatment device that can be retrofitted into existing stormwater networks and which results in an improved reduction of contaminant loads compared to systems currently available in New Zealand.

Socio-Economic Scenarios for Climate Change Impact Assessments Pilot Study:  Urban Drainage in the Auckland Region

There are four steps when assessing climate change impacts on society: 1) projection of future climate; 2) transformation of projected climate into bio-physical impacts; 3) transformation of bio-physical impacts into societal impacts; 4) identification of possible societal responses to these impacts.  Superimposed on these steps is the fact that society changes independently of climate as technologies develop.  Thus, along with climate change scenarios there is also a need for scenarios of change in urban areas for impact assessment.

A problem when creating scenarios for urban change is that the socio-economic future can not be projected using the same kind of physically-based or conceptual models used to assess changes to the natural environment.  Instead, one must draw on the tacit knowledge of a range of stakeholders, each with their own set of experiences and agendas.  This pilot study would concentrate on the impacts of urbanisation and climate change on urban drainage for the Auckland Region.

The way urban water is managed can have an impact on urban receiving waters at least as great as climate change.  Urban change can both exacerbate and mitigate the potential impacts of climate change.  The following are a few of the possible impacts of both urbanisation and increased winter storminess (projected for Auckland in NIWA climate change scenarios) and therefore stormwater reaching streams and receiving environments:

• increased stream bank erosion and sediment transport
• increased deposition of sediments in estuaries and harbours
• increased local flood risk for urban flood plains
• increased overflow frequency and volumes
• failure of stormwater devices, e.g. flushing of settled sediments from ponds

Taken together, climate change can exacerbate the impacts of urbanisation and vice versa.  However the trend towards low impact urban design and development could limit impacts of both independently and together.

OBJECTIVE:

1. To disseminate knowledge about the links between water management and climate change impacts to researchers in New Zealand.
2. To create urban-drainage scenarios for the Auckland region.
3. To show the feasibility of a wider programme to develop New Zealand wide regional landuse scenarios that can be used in tandem with climate change scenarios.