New River Estuary - CLUES Estuary analysis
Prepared for Environment Southland
January 2017
Prepared by: David Plew
For any information regarding this report please contact: David Plew Scientist (Hydrodynamics) Hydrodynamics Group +64-3-343 7801 [email protected]
National Institute of Water & Atmospheric Research Ltd PO Box 8602 Riccarton Christchurch 8011
Phone +64 3 348 8987
NIWA CLIENT REPORT No: 2016004CH Report date: January 2017 NIWA Project: FWCE1603
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Contents
Executive summary ...... 5
1 Introduction ...... 6
2 Methodology ...... 7 2.1 CLUES estuary ...... 7 2.2 Tidal prism model ...... 7
3 Input data ...... 9 3.1 Estuary properties...... 9 3.2 Freshwater inputs ...... 10 3.3 Nutrient loads ...... 11 3.4 Ocean boundary conditions ...... 15
4 CLUES-Estuaries analysis ...... 17 4.1 Sensitivity to input parameters ...... 17 4.2 Summer conditions vs winter conditions ...... 23 4.3 Wastewater treatment plant ...... 24
5 Acknowledgements ...... 25
6 References ...... 26
Tables Table 3-1: Comparison of estuary volume and tidal prism from Coastal Explorer and from the bathymetry survey. 10 Table 3-2: Summary flow statistics for the gauged inflows into New River Estuary. 10 Table 3-3: Estimates of annual Total Nitrogen loads to the New River Estuary. 12 Table 3-4: Rate of increase in total nitrogen concentrations in five tributaries to the New River Estuary. 14 Table 3-5: Rate of increase in nitrate-nitrite nitrogen concentrations in five tributaries to the New River Estuary. 14 Table 3-6: Rate of increase in TP concentrations in five tributaries to the New River Estuary. 15 Table 4-1: Summer and winter median river flows and mean total nitrogen concentrations. 23
New River Estuary - CLUES Estuary analysis
Figures Figure 3-1: Time-series of estuary volumes from a 45 day Delft3D simulation of the New River Estuary . 9 Figure 3-2: Distribution of monthly median flows for the gauged inflows to the New River Estuary (refer to Table 3-2 for the periods over which the median flows were calculated). 11 Figure 3-3: Monthly mean concentrations of (top) Total Nitrogen and (bottom) Nitrate + Nitrite concentrations in 5 tributaries. 13 Figure 3-4: Annual distribution of monthly median total nitrogen concentrations from Oreti Beach. 15 Figure 3-5: Mean monthly nitrate concentrations from the CARS climatology extracted for the grid point closest to the New River Estuary. 16 Figure 4-1: Effect of return flow fraction on (top) dilution and salinity, and (bottom) predicted potential total nitrogen concentrations in the New River Estuary. 17 Figure 4-2: Map of the New River Estuary showing locations of the Invercargill City Council water quality monitoring sites. 18 Figure 4-3: Effect of tidal prism on (from top to bottom) dilution, salinity and potential total nitrogen concentration. 20 Figure 4-4: Effect of freshwater inflow (rivers) on dilution, salinity and potential TN concentration in the estuary. 21 Figure 4-5: Effect of freshwater inflow on potential TN concentrations in the estuary if the concentration of the river inflow is maintained constant. 22 Figure 4-6: Predicted potential total nitrogen concentrations in (top) summer and (below) winter. 24
New River Estuary - CLUES Estuary analysis
Executive summary Total nitrogen (TN) concentrations in the New River Estuary are predicted using the CLUES-Estuary approach. CLUES-Estuary is a GIS-based tool that predicts nutrient concentrations in an estuary using a combination of land-use models to predict nutrient loads, and analytical estuary models to calculate the dilution between sea-water and riverine water in the estuary. The model returns a single estimate of time and volume averaged potential total nitrogen (TN) concentration in the estuary for a given nutrient load, tidal prism and freshwater inflow. Potential TN concentrations represents the concentration that would occur in the absence of biological uptake and denitrification, and is an indication of the ecological pressure on the estuary.
The estuary nutrient concentrations predicted by the model are relatively insensitive to river discharges within typical flow ranges (total inflows up to 90 m3/s are considered). Estuary nutrient concentrations scale roughly linearly with catchment loads, with a minimum value determined by the ocean TN concentration and the loading from the Waste Water Treatment Plant. The size of the tidal prism (neap vs spring tide) also influences the predicted TN concentration. Predicted TN decreases as tidal prism increases, with greater dilution and flushing of the estuary at spring tide. The mean tidal prism is used for most of the analysis as this represents a ‘typical’ tidal range.
Summer and winter conditions were compared using river concentrations, river flows and ocean TN concentrations typical of December-February (summer) and June-August (winter). Predicted TN concentrations are higher in winter ~534 mg/m3 than in summer ~154 mg/m3. This is due to a combination of higher river TN concentrations and flows resulting in much higher nitrogen fluxes into the estuary, higher ocean TN concentrations in winter, and a small reduction in dilution (flushing) in winter due to the higher river inflows.
The Waste Water Treatment Plant contributes about 6.5% of the annual TN load to the estuary. Diverting the WWTP effluent would reduce the potential TN concentrations by about -18% in summer, and -4.6% in winter. This assumes that the effluent loading is constant over the year.
New River Estuary - CLUES Estuary analysis 5
1 Introduction This brief report illustrates the use of CLUES-Estuary for predicting potential nutrient concentrations (specifically total nitrogen) in the New River Estuary. CLUES-Estuary is a GIS-based tool that combines catchment models that predict mean annual loads of total nitrogen, total phosphorus, sediments and E. coli w with simplified hydrodynamic models that calculate dilution within estuaries.
In this study, the estuary component of the tool is run ‘off-line’ so that the sensitivity of the estuary nutrient concentrations to different inputs can be evaluated. Specifically, we investigate the sensitivity to river flow, size of tidal prism (spring vs neap), and to a calibration factor that accounts for incomplete spatial mixing and return flows back into the estuary on flood tides. Decoupling the estuary component from CLUES as done here enables us to test the sensitivity of and tune the estuary model.
CLUES estuary and the underlying analytical hydrodynamic model are described in Section 0, and an explanation of how various input conditions were obtained given in section 0. Results are presented in three parts: in 4.1 the response of the model to different input conditions is assessed; a comparison of summer vs winter conditions is made in section 4.2; and the influence of the wastewater treatment plant discharge on total nitrogen (TN), relative to the catchment loading, is given in section 4.3.
6 New River Estuary - CLUES Estuary analysis
2 Methodology
2.1 CLUES estuary CLUES-estuary is an add-on component to the CLUES (Catchment Land Use and Environmental Sustainability) GIS software (Elliott, Semadeni-Davies et al. 2016) that gives predictions of nutrient concentrations within estuaries. The estuary component consists of either of two flushing/mixing models: a two-layer box model for deep or strongly stratified estuaries, and a modified tidal prism model used for most tidally dominated estuaries, such as the New River Estuary. The flushing/mixing models provide a time-averaged, volume-averaged prediction of potential nutrient concentrations, specifically total nitrogen (TN) and total phosphorus (TP). Potential nutrient concentrations are the concentrations that would occur if there were no biological uptake or denitrification. Potential nutrient concentrations provide an indication of the loading applied to the estuary, and may be more useful than actual (measured) concentrations, which are influenced by denitrification and biological uptake. A high algae biomass, as expected in eutrophic conditions, may take up much of the available nutrients, resulting in low measured concentrations relative to the applied loading. Potential nutrient concentrations are expected to be higher than actual concentrations.
While the CLUES-estuary GIS tool could be used for this analysis, for the purpose of this report it was more convenient to apply the estuary model in the form of a spreadsheet to allow the inputs to be varied and different scenarios tested more rapidly.
2.2 Tidal prism model The New River Estuary is tidally dominated, so was modelled using a modified tidal prism model developed by Luketina (1998). This model treats the estuary as completely mixed, but includes a tuning factor which accounts for a portion of the tidal inflow consisting of water that had been discharged from the estuary on the previous outgoing tide. The model also accounts for the asymmetry between the duration of the ebb and flow tides that increases as the river discharge into the estuary increases relative to the tidal prism.
The model calculates a dilution factor D, where 1/D is the fraction of freshwater inside the estuary (averaged over both time and space). A value of D = 1 means the estuary is entirely freshwater, while a value of D = 100 means that the estuary is 1% freshwater, and 99% sea water.
The dilution factor is calculated as follows:
Q T P1 b F 1 b D 2 QFT
3 3 where P = tidal prism (m ), QF = freshwater inflow (m /s), T = tidal period (s), and b is the tuning factor or return flow fraction (the fraction of the incoming tidal flow that consists of water discharged from the estuary on the previous tide). The tidal period is taken as the M2 tide (12.42 hrs).
New River Estuary - CLUES Estuary analysis 7
The concentration of a substance in the estuary C is calculated as
C C D 1 C R O D where CR is the concentration in the freshwater inflow, and CO the concentration in the ocean. For estuaries with multiple freshwater inflows or point nutrient sources, the riverine concentration is calculated as the total nutrient load divided by the total inflow.
8 New River Estuary - CLUES Estuary analysis
3 Input data
3.1 Estuary properties The estuary model requires the size of the tidal prism as an input. Estimates of the New River Estuary tidal prism were available from two sources: the Coastal Explorer database (Hume and Herdendorf 1988; Hume, Snelder et al. 2007), and from the DELFT3D model1 that used bathymetry data from a survey conducted by Environment Southland in 2015.
The Coastal Explorer estimates of tidal prism are calculated from the average of the surface areas at high and low tides multiplied by the tidal range. Volumes at low water are obtained by multiplying the estuary surface area at LWS by an estimate of mean depth. The depth estimates in Coastal Explorer are to the nearest metre (1 m for New River Estuary), which means that the calculated low water volumes are not likely to be very accurate. However, the tidal prism derived from Coastal Explorer should be sufficiently accurate for modelling the New River Estuary (Table 3-1).
Volumes and tidal prisms from the Delft3D model were obtained using outputs of estuary water volume from a 45 day simulation from 9 Aug 2015 to 23 Sep 2015 (Figure 3-1). Volume at low water spring was taken as the minimum low tide volume during this period, and at low water mean as the average volume at low tide from all low tides during the 45 day period. Similarly, high water spring volume was assumed to be the maximum volume at high tide during this period.
The tidal prism for each tide was calculated as the difference between water volume at high tide and the following low tide. Tidal prism spring tide was calculated as the largest tidal prism during this 45 day period, and tidal prism neap tide the smallest tidal prism. Tidal prism mean is the average tidal prism volume during the 45 day period.
Figure 3-1: Time-series of estuary volumes from a 45 day Delft3D simulation of the New River Estuary . The simulation period extents from 9 Aug 2015 to 23 Sep 2015. The blue line shows estuary volume output at 2 hr intervals, while orange and grey marks show high and low tide respectively.
The Coastal Explorer estimate of tidal prism at spring tide is within 5% of that derived from the Delft3D model (Table 3-1). The low water volume in Coastal Explorer is 21% lower, while the High
1 A 3D hydrodynamic model has been set up and calibrated for the New River Estuary. This model will be described in a separate report currently in preparation.
New River Estuary - CLUES Estuary analysis 9
Water volume is inconsistent, being less than the tidal prism. The high water volume was calculated by digitising chart NZ681 (the same calculation gives a low water volume of 5,442,669 m3, significantly lower than observed).
Table 3-1: Comparison of estuary volume and tidal prism from Coastal Explorer and from the bathymetry survey. Coastal explorer values were calculated from digitising bathymetry charts, and are the default values used by CLUES-estuary. The Delft3D model volumes are derived from the 2015 bathymetry data provided by Environment Southland.
Parameter Coastal Explorer Delft3D model Tidal Prism (spring tide) 69,607,863 m3 73,102,315 m3 Tidal Prism (neap tide) 35,745,854 m3 Tidal Prism (mean) 50,744,000 m3 Volume (high water spring) 60,268,977 m3 102,935,087 m3 Volume (low water spring) 23,129,736 m3 29,241,618 m3 Volume (low water mean) 32,576,978 m3
When running CLUES estuary, the estuary volume is used for calculating residence time, but does not influence the dilution or concentrations in the estuary, which are primarily determined by freshwater inflows, tidal prism and the return flow factor.
3.2 Freshwater inputs There are two main inflows to the New River Estuary
. Waihopai River
There are also ungauged inflows, such as Mokotua Stream and Waimatua Creek, which we assume are small compared to the above.
The Makarewa River joins the Oreti River upstream of the estuary. Both the Makarewa and Oreti rivers are gauged upstream of their confluence. There are (presumably smaller) ungauged inflows into both rivers downstream of these two gauging stations (e.g., Wainiwa, Otakau, Tomoporakau creeks, Waikiwi Stream). Flow data for the Waihopai River are taken from site 78504 Waihopai at Kennington, and from site 78601 Oreti at Wallacetown for the Oreti River. Summary flow statistics are reported in Table 3-1.
Table 3-2: Summary flow statistics for the gauged inflows into New River Estuary. MALF = mean annual low flow (average of the lowest daily flow from each year), MALF 7 days = mean of the annual low flow averaged over 7 days, MAM = mean annual maximum flow.
Site Start date End date Mean Median MALF MALF 7 days MAM (m3/s) (m3/s) (m3/s) (m3/s) (m3/s) 78634 Makarewa at Counsell Rd 17/4/1981 1/1/2015 15.593 7.743 1.5658 1.7674 184.9 78601 Oreti at Wallacetown 7/10/1977 1/4/2015 39.775 27.578 6.4321 7.0956 495.8 78504 Waihopai at Kennington 11/11/1991 1/1/2015 2.631 1.227 0.2145 0.2343 38.82 Total 57.999 36.548 8.2124 9.0973 719.52
10 New River Estuary - CLUES Estuary analysis
The CLUES land-use model also gives a prediction of mean annual river flow into the estuary; this value is 67.236 m3/s. The CLUES prediction includes ungauged tributaries and runoff from the catchment below the three gauged sites listed above. Assuming that mean and median flows scale similarly, the median flow into the estuary is estimated by scaling the total median gauged flow by the ratio of the CLUES predicted mean flow to the gauged mean flow. The median freshwater discharge to the estuary is estimated as 36.548 x 67.236/57.999 = 42.37 m3/s (i.e., 16% higher than the gauged flows).
The other measured discharge into the estuary is from the Invercargill City Council Waste Water Treatment Plant (WWTP). The mean discharge over the period 1/7/2004 to 30/6/2015 was 23,163 m3/d (0.268 m3/s), which is negligible in comparison to the river discharges.
Median river flows are highest in June (totalling 64.03 m3/s across the three gauged sites) and lowest in February (totalling 17.46 m3/s). The distribution of monthly median river flows is shown in Figure 3-2.
Figure 3-2: Distribution of monthly median flows for the gauged inflows to the New River Estuary (refer to Table 3-2 for the periods over which the median flows were calculated).
3.3 Nutrient loads Annual nitrogen loadings to the New River Estuary are given below. Estimates of catchment loadings from CLUES and provided by Environment Southland (calculated by Aqualinc – although this may be an under-estimate at high flows, N. Ward pers. com.) show good agreement (~3% difference). The WWTP provides about 6.5% of the total nitrogen load.
New River Estuary - CLUES Estuary analysis 11
Table 3-3: Estimates of annual Total Nitrogen loads to the New River Estuary.
Source Annual TN load (t/yr) Catchment (Aqualinc estimate) 3736.0 Catchment (CLUES) 3617.5 Invercargill City Council Waste Water Treatment Plant (2010- 250.9 2014 average)
CLUES-estuary typically uses annual total nitrogen loads and mean flows, and provides an annual mean potential concentration for the estuary. However, it is also possible to calculate seasonal values (e.g., summer, winter) if seasonal values for nutrient loads and flows are available.
Environment Southland provided riverine nutrient data at the following sites on tributaries:
. Oreti River at Wallacetown
. Makarewa at Wallacetown
. Waikiwi Stream at North Road
. Waihopai River upstream of Queens Drive
. Otepuni Creek at Nith Street
All five sites show a seasonal trend with higher TN and nitrate + nitrite concentrations in winter (Figure 3-5).
12 New River Estuary - CLUES Estuary analysis
Figure 3-3: Monthly mean concentrations of (top) Total Nitrogen and (bottom) Nitrate + Nitrite concentrations in 5 tributaries.
A simple linear regression was used to determine trends in TN and NNN (nitrate + nitrite nitrogen) over time. All five sites show increases in TN over time. The increases are statistically significant (P<0.05) at three sites: Oreti (2.4% per year), Waikiwi (1.1% per year) and Waihopai (1.7% per year). NNN increased at all sites except Makarewa, which shows a small (-0.39%) decrease per year. However, this was not statistically significant (p=0.49). Statistically significant increases in NNN occurred at Oreti (2.8% per year) and Waihopai (1.3% per year).
Using a simple linear regression in this manner does not take into account any seasonality in changes in nutrient concentrations in a robust manner. For example, it does not indicate if the observed increasing trend is primarily driven by higher winter concentrations, or if instead concentration increases are occurring year-round. However, for the purpose of this report, this simple analysis is adequate to indicate that nutrient concentrations have been generally increasing, and that present- day estuary nutrient concentrations should be based on recent river concentrations (or at least weighted towards more recent values) rather than averaged over the entire record. There is considerable year-to-year variability in river nutrient concentrations, therefore, in this report, we use the linear trends to predict 2015 equivalent concentrations (Table 3-4 to Table 3-5), which are, in effect, averages weighted to ‘present day’ conditions.
New River Estuary - CLUES Estuary analysis 13
Table 3-4: Rate of increase in total nitrogen concentrations in five tributaries to the New River Estuary. A linear regression was fitted to the entire data series. The % increase is calculated relative to the mean concentration over the time period. The 2015 equivalent TN is forecasted from the trend analysis. Values in bold font indicate a statistically significant trend (P<0.05).
Site Time span Increase per Increase per year P value Mean TN 2015 year (%) (mg/m3) equivalent (mg/m3) TN (mg/m3) Oreti River at 26/1/1989-4/11/2014 27.8 2.4 3.39e-8 1170 1530 Wallacetown Makarewa River at 23/11/1999-4/11/2014 12.8 0.71 0.448 1800 1900 Wallacetown Waikiwi Stream at 10/12/1998-4/11/2014 36.9 1.1 0.011 3280 3580 North Road Waihopai River u/s 10/12/1998-4/11/2014 48.4 1.7 0.00970 2790 3180 Queens Drive Otepuni Creek at Nith 10/12/1998-4/11/2014 5.12 0.22 0.717 2290 2330 Street
Table 3-5: Rate of increase in nitrate-nitrite nitrogen concentrations in five tributaries to the New River Estuary. A linear regression was fit to the entire data series. The % increase is calculated relative to the mean concentration over the time period. The 2015 equivalent NNN is forecast from the trend analysis.
Site Time span Increase per Increase per year P value Mean NNN 2015 year (%) (mg/m3) equivalent (mg/m3) NNN (mg/m3) Oreti River at 4/4/1977-4/11/2014 25.7 2.8 1.34e-12 920 1330 Wallacetown Makarewa River at 4/4/1977-4/11/2014 -4.6 -0.39 0.491 1180 1130 Wallacetown Waikiwi Stream at 15/11/1995-4/11/2014 17.6 0.67 0.086 2620 2830 North Road Waihopai River u/s 18/6/1995-4/11/2014 27.8 1.30 0.020 2130 2400 Queens Drive Otepuni Creek at Nith 15/11/1995-4/11/2014 2.3 0.16 0.789 1480 1500 Street
14 New River Estuary - CLUES Estuary analysis
Table 3-6: Rate of increase in TP concentrations in five tributaries to the New River Estuary. A linear regression was fit to the entire data series. The % increase is calculated relative to the mean concentration over the time period. The 2015 equivalent NNN is forecasted from the trend analysis. Values in bold indicate a statistically significant trend.
Site Time span Increase per Increase per year P value Mean TP 2015 year (%) (mg/m3) equivalent (mg/m3) TP (mg/m3) Oreti River at 17/2/1997-4/11/2014 -0.57 -1.87 0.0619 30 22 Wallacetown Makarewa River at 16/2/1977-4/11/2014 -1.88 -2.24 0.0014 80 64 Wallacetown Waikiwi Stream at 10/12/1998-4/11/2014 0.06 0.12 0.912 50 52 North Road Waihopai River u/s 10/12/1998-4/11/2014 0.24 0.47 0.714 50 52 Queens Drive Otepuni Creek at Nith 9/11/1979-4/11/2014 -0.31 -0.47 0.760 70 63 Street
3.4 Ocean boundary conditions Concentrations of each nutrient in the ocean are required as a boundary condition for CLUES- estuary. These values set a minimum concentration for each in the estuary (assuming that the nutrient concentrations of the river inflows are higher than the ocean). The oceanic concentration should be obtained from a point sufficiently far from the estuary that the concentrations are not measurably affected by the estuary. Invercargill City Council (ICC) nutrient samples from Oreti Beach from 10 Jan 2013 to 04 Jun 2015 (57 samples) provide an indication of oceanic nitrogen concentrations. Median values of total dissolved nitrogen (TDN) vary between a low of 30 mg/m3 in Feb-Mar to a high of 710 mg/m3 in Jul. The annual mean concentration is 237 mg/m3 (S.D. 329) with median value of 110 mg/m3. Note that the ICC data report nitrate and ammonium concentrations, which have been added to obtain the total dissolved nitrogen concentration. This differs from Total Nitrogen (TN), which also includes particulate (not dissolved) forms of nitrogen. We make the assumption that the majority of oceanic nitrogen is in dissolved forms.
Figure 3-4: Annual distribution of monthly median total nitrogen concentrations from Oreti Beach. Calculated from 57 samples taken over the period 10 Jan 2013 to 4 Jun 2015.
New River Estuary - CLUES Estuary analysis 15
Some of the nitrogen concentrations appear very high for a coastal site, particularly in the winter months. It is likely that some riverine derived nutrients are detected at Oreti Beach due to proximity to the estuary, particularly during high flow events. As a consequence the Oreti Beach site might not provide a reliable indication of the true oceanic concentration.
CLUES-estuary uses annual mean nitrate concentrations derived from the CSIRO Atlas of Regional Seas (CARS). The CARS 2009 climatology contains nutrient (nitrate, phosphate and silicate), temperature and salinity data interpreted on to a ½ degree grid (CSIRO 2011). Nitrate concentrations for the grid point nearest the New River Estuary are plotted in Figure 3-5. Nitrate concentrations from CARS range between 47 mg/m3 in April to 80 mg/m3 in July with an annual mean value of 64 mg/m3.
CARS only contains estimates for nitrate-N, and does not provide estimates of other forms of nitrogen, such as ammonium or particulates. Nitrogen in coastal waters is mostly in nitrate form (the ICC data shows on average 89% of the dissolved nitrogen from the Oreti Beach site is in nitrate form). This suggests that an appropriate annual mean TN concentration for the ocean boundary is likely to be around 70 mg/m3.
Figure 3-5: Mean monthly nitrate concentrations from the CARS climatology extracted for the grid point closest to the New River Estuary.
16 New River Estuary - CLUES Estuary analysis
4 CLUES-Estuaries analysis
4.1 Sensitivity to input parameters
4.1.1 Return flow fraction The value of the return flow fraction influences the predicted potential nitrogen concentrations. As the return flow fraction increases, the potential nitrogen concentration in the estuary also increases because riverine nitrogen concentrations are higher than oceanic concentrations, and increasing the return flow fraction decreases the dilution by seawater in the estuary. This is demonstrated in Figure 4-1, which shows dilution and salinity both decreasing as the return flow fraction is reduced, while predicted TN increases.
Figure 4-1: Effect of return flow fraction on (top) dilution and salinity, and (bottom) predicted potential total nitrogen concentrations in the New River Estuary. Calculations use surveyed mean tidal prism (50,744,000 m3), CLUES-derived mean annual flow (67.263 m3/s), oceanic TN of 70 mg/m3, and combined (catchment + WWTP) nitrogen load of 3868.4 t/yr TN.
New River Estuary - CLUES Estuary analysis 17
Calibration of the return flow factor is done in two ways: firstly using field data, and secondly using the DELFT3D model of the New River Estuary (R. Measures, in prep). The return flow factor can be obtained using the ratio of estuarine salinity to ocean salinity to calculate the dilution:
푆푂 1 퐷 = = 푆푂 − 푆 1 − 푆⁄푆푂 Then the return flow fraction is determined from
1 Q T D P F 2 b Q T F P 2
While there are salinity data from six sites within the estuary (Figure 4-2), these sites are located around the perimeter of the estuary. Some are in or near the inflowing river channel, while others are at the estuary mouth. All are surface samples, which will be lower salinity than a depth-averaged value. A simple average of these data may not give an accurate estuary volume average. Instead, the mean estuary salinity is calculated by estimating the proportion of the estuary represented by each sampling site. A weighted mean salinity of S/S0 = 0.67 is obtained giving b = 0.87. The Luketina model was developed for high tide conditions, and the weighted mean salinity at high tide is S/S0 = 0.77, giving b = 0.80.
Figure 4-2: Map of the New River Estuary showing locations of the Invercargill City Council water quality monitoring sites.
The DELFT3D model for mean flow conditions gives volume-average salinities ranging between S/S0 = 0.58 at low tide and 0.80 at high tide. Using the high tide value gives b = 0.76. The likely range for b of 0.76-0.80 is high compared to the default value of b = 0.5 recommended by Luketina (1998).
18 New River Estuary - CLUES Estuary analysis
Furthermore, the DELFT3D model shows that 29% of the incoming tidal prism consists of water that had been in the estuary at the previous high tide, indicating the return flow fraction is ~ 0.29, significantly lower than that calculated from salinities. The discrepancy is that the so called “return flow factor” is in fact a tuning factor that accounts for more than just a return of water to the estuary. By setting its value using salinities also accounts for the incomplete mixing in the estuary. There are large spatial variations in salinity, with lower salinities near the rivers, and higher near the mouth. Much of the volume leaving the estuary on the outgoing tide is the higher salinity water. Thus, to obtain an accurate volume averaged salinity requires setting the return flow fraction higher than the actual fraction returned to the estuary.
From here-on, we will use a range of return flow fractions to indicate sensitivity to this parameter, but make most comparison between different flow/nutrient conditions using b = 0.76.
4.1.2 Tidal prism Dilution increases linearly with tidal prism. Salinity also increases, while potential total nitrogen concentrations decrease. Figure 4-3 shows dilution, salinity and potential total nitrogen as a function of tidal prisms spanning between neap and spring tide for a range of return flow factors between 0.5 and 0.8. Mean annual flows and TN loadings from CLUES have been used, and the ocean TN concentration was set to 70 mg/m3. For the reference return flow factor of 0.76, the potential TN concentration varies between 331 mg/m3 at spring tide to 539 mg/m3 at neap tide. At mean tide, the predicted potential concentration is 425 mg/m3.
New River Estuary - CLUES Estuary analysis 19
Figure 4-3: Effect of tidal prism on (from top to bottom) dilution, salinity and potential total nitrogen concentration. The range of tidal prisms shown extends from neap to spring tide. The dashed vertical line shows the mean tidal prism. The values plotted are calculated for return flow factors of 0.6, 0.7, 0.76 and 0.8 (see legend). The graphs show the calculated values for a fixed total inflow of 67.263 m3/s, total annual nitrogen load of 3868.4 t/yr, ocean salinity of 34.4 ppt, and oceanic TN concentration of 70 mg/m3.
4.1.3 Freshwater inflow Dilution, salinity and potential TN concentration all decrease as river flow increases, assuming here that the total annual nitrogen load is held constant, such that the riverine concentrations increase or decrease as flow is decreased or increased (Figure 4-4). Under this assumption, the estuary concentration is relatively insensitive to river flow, suggesting that errors in determining flow inputs are less important that in determining total loads. For example, for a return flow factor of 0.76, halving the inflow from 67.263 m3/s to 33.63 m3/s increased the potential TN concentration from 426 mg/m3 to 468 mg/m3. If the concentration in the river inflows is held constant (such that the loading t/yr increases linearly with flow) then the concentrations in the estuary increase almost linearly with
20 New River Estuary - CLUES Estuary analysis
flow, as demonstrated in Figure 4-5. Halving the flow (and consequently the catchment TN load) by 50% reduces TN concentrations from 426 mg/m3 to 278 mg/m3.
Figure 4-4: Effect of freshwater inflow (rivers) on dilution, salinity and potential TN concentration in the estuary. The calculation of TN assumes the total annual load remains constant such that river concentrations decrease with flow. Ocean TN concentrations of 70 mg/m3 have been assumed.
New River Estuary - CLUES Estuary analysis 21
Figure 4-5: Effect of freshwater inflow on potential TN concentrations in the estuary if the concentration of the river inflow is maintained constant. The reference concentration for the river inflows is 1705 mg/m3.
4.1.4 Summary The analysis shows a high sensitivity to the return flow fraction. The data available suggest that the return flow faction may be relatively high (b ~ 0.76-0.80). Using annual mean flows, catchment loads and ocean boundary conditions, the predicted potential TN concentration increases from 261 mg/m3 to 480 mg/m3 as the return flow fraction is increased from b = 0.5 to 0.8. Both of these values are ecologically high. Based on 3D hydrodynamic modelling, we suggest using a value of b = 0.76. This is also similar to estimates obtained from field data.
Tidal prism also has an influence on the predicted estuary concentrations. The agreement between these two estimates of tidal prism (Coastal Explorer and the DELFT3D model based on the estuary survey) is quite good at within 5%. The difference in predicted TN concentration is small, 342 mg/m3 vs 331 mg/m3 for the Coastal Explorer and Surveyed spring tide prism, respectively. However, the mean tidal prism is much smaller than that at spring tide (mean tidal prism is 69% of that at spring tide), which results in a higher concentration of 425 mg/m3 (28% higher). The mean tidal prism is more representative of typical conditions in the estuary and should be used in preference. Provided the value of the tidal prism used is consistent, comparisons between different scenarios should still be useful. Using the larger (spring tide) tidal prism will result in higher dilution and a greater proportion of seawater in the estuary. This will result in smaller absolute changes in concentration for an increase in catchment load, but relative changes (ratios of concentrations from scenarios) will be slightly higher.
If the catchment load is known with sufficient accuracy, then the model is not highly sensitive to river flow (on a time-averaged basis). This means that the river inflows need not be calculated to a high degree of accuracy to provide a useful estimate of estuary concentration. However, it is clear that the catchment loading needs to be calculated accurately – loading received by the estuary being a function of flow and concentration in the river.
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4.2 Summer conditions vs winter conditions Concentrations and flows are lower over summer months when algal growth will be highest because of increased light and temperature. The median flows (all years) and mean concentrations (2010- 2014) for December - February are used to represent summer conditions. Nutrient concentrations are averaged over the 2010-2014 period due to the generally increasing trend in river nutrient concentrations. Gauged river inflows accumulate to 23.05 m3/s, which we increased by 16% to 26.74 m3/s to account for ungauged inflows. The net inflow concentration of ~1100 mg/m3 is calculated by accumulating the mass flux from each river and dividing by the total of the river inflows. Coastal nutrient concentrations tend to be low in summer too. The median measured NO3+NH4 concentration at Oreti Beach in February is 30 mg/m3. The discharge from the WWTP is included assuming annual mean flows and concentrations.
For winter conditions, median flows (all years) and mean concentrations (2010-2014 only) from Jun- Aug are used. Gauged river inflows accumulate to 54.866 m3/s which we increase by 16% to 63.64 m3/s to account for ungauged inflow. The net inflow concentration is ~2500 mg/m3. The median 3 measured NO3+NH4 concentration at Oreti Beach is 470 mg/m , which is abnormally high for coastal 3 3 - waters. Instead, we use a value of 86 mg/m TN based on a Jun-Aug mean value of 76.7 mg/m NO3 from CARS, with the 12% increase allowing for NH4 based on the ICC data.
Table 4-1: Summer and winter median river flows and mean total nitrogen concentrations. Summer flows are calculated as the median of all data between December and February, while winter is from June to August. For nutrients, mean concentrations from Dec to Feb (summer) and June to August (winter) are calculated from 2010-2014. Note that the total flows in this table are used here for the purpose of calculating the volume- averaged inflow concentration. The total inflow is increased by 16% when calculating the inflows to the estuary to allow for ungauged inflows.
River Summer flow (m3/s) Summer TN (mg/m3) Winter flow (m3/s) Winter TN (mg/m3) Makarewa 4.109 1334 14.427 2806 Oreti 18.425 1020 37.848 2244 Waihopai 0.515 2626 2.591 4053 Total 23.049 54.866 Volume averaged2 1112 2477
Figure 4-6 shows predicted potential nitrogen concentrations for summer and winter for a range of riverine nitrogen concentrations. This suggest summer potential TN concentrations of 154 mg/m3, and winter concentrations of 534 mg/m3. Both values are enough to saturate Ulva growth. In other words, the growth rates of Ulva are unlikely to be limited by nitrogen.
2 The volume-averaged concentration is calculated as ∑ 푄푖퐶푖⁄∑ 푄푖 where Qi and Ci are the discharges and total nitrogen concentration from each inflow.
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Figure 4-6: Predicted potential total nitrogen concentrations in (top) summer and (below) winter. The dashed vertical line shows the estimated present day volume-averaged river inflow concentrations. The predictions include the flow and nitrogen load from the waste water treatment plant.
4.3 Wastewater treatment plant The wastewater treatment plant is responsible for ~6.5% of the current annual TN loading. Should the WWTP effluent be diverted from the estuary, potential TN concentrations in the estuary would decrease from 154 to 127 mg/m3 (-18%) in summer, and from 534 to 510 mg/m3 (-4.6%) in winter. Note, this assumes that the TN loading from the WWTP is relatively constant over the year.
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5 Acknowledgements Thanks to Environment Southland for providing river flow and nutrient data, and Wriggle LTD and Invercargill City Council for a summary of estuary nutrient data.
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6 References CSIRO (2011) CSIRO Atlas of Regional Seas (CARS). www.cmar.csiro.au/cars
Elliott, A.H., Semadeni-Davies, A.F., Shankar, U., Zeldis, J.R., Wheeler, D.M., Plew, D.R., Rys, G.J., Harris, S.R. (2016) A national-scale GIS-based system for modelling impacts of land use on water quality. Environmental Modelling & Software, 86: 131-144. http://dx.doi.org/10.1016/j.envsoft.2016.09.011
Hume, T.M., Herdendorf, C.E. (1988) A geomorphic classification of estuaries and its application to coastal resource management—A New Zealand example. Ocean and Shoreline Management, 11(3): 249-274. http://dx.doi.org/10.1016/0951- 8312(88)90022-7
Hume, T.M., Snelder, T., Weatherhead, M., Liefting, R. (2007) A controlling factor approach to estuary classification. Ocean & Coastal Management, 50(11–12): 905-929. http://dx.doi.org/10.1016/j.ocecoaman.2007.05.009
Luketina, D. (1998) Simple Tidal Prism Models Revisited. Estuarine, Coastal and Shelf Science, 46(1): 77-84. http://dx.doi.org/10.1006/ecss.1997.0235
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