Rangiriri Spillway and Lake Waikare Hydraulic Review · During normal conditions, lake levels are controlled by the Lake Waikare Gates. Normal lake levels vary between 5.4m and 5.6m

Prepared by: José Beyá For: Environment Waikato PO Box 4010 HAMILTON EAST 13 July 2005 Document #: 983654/v4

Environment Waikato Technical Report 2005/33

Rangiriri Spillway and Lake Waikare Hydraulic Review

Doc # 983654/v4 Page i

Peer reviewed by: Murray Mulholland Initials Date 17/11/05

Approved for release by: Ghassan Basheer Initials Date 17/11/05

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Table of Contents

1 Background 1

2 Methodology 2

3 Model description 3 3.1 Main Model (Mike 11 1D) 3 3.2 Rangiriri Ponding Zone Model (Mike flood 1D-2D) 4

4 Calibration 7 4.1 Data Availability 7 4.2 Rainfall Runoff model 8 4.3 Hydrographs Analysis 9 4.4 Hydraulic Model Simulations 11 4.5 Lake Mass Balance 12 4.6 Lake Waikare Gate 14 4.7 Comments 15

5 Modelling flood events 16 5.1 Design Storm Results 19 5.2 Above design Storm Results 19 5.3 Flooded Areas and the effects of the Rangiriri ponding zone 21

6 Earthworks 23

7 Conclusions 25

8 Recommendations 26

References 27

APPENDIX 28

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1 Background The Rangiriri spillway and Lake Waikare are part of the Lower Waikato Waipa Control Scheme (LWWCS). The Rangiriri spillway is approximately 1800m long and it is located on the right side of the Waikato River. It is designed to spill water from the Waikato River to Lake Waikare in large flood events. The Rangiriri spillway is expected to operate for flood events equal to or greater than the 50yr return period event. When the spillway operates its overflow crosses State Highway 1 and reaches a ponding zone created by the Highway and Railway embankments. From this ponding zone the water flows into the lake through the Te Onetea and Rangiriri streams railway bridges. For the design storm the Rangiriri spillway is expected to attenuate the Waikato River peak flow by 15%. Lake Waikare is used as a storage area when the Waikato River is at its peak. After the flood recedes the water is released to the Whangamarino wetlands through the Waikare Gate and Northern Outlet Canal located in the northern part of the lake. Additionally the lake has a spillway along its northern foreshore which is designed to act as a safety valve for storms greater than design. This is to prevent overtopping of the northern foreshore stopbank. The overflow from the northern spillway flows into the Swan Road drainage area and then the Whangamarino Wetland as it did before the construction of the scheme. Between the lake and the Whangamarino wetland there are two ponding areas defined by the Waerenga Rd and the Swan Rd stopbank. During normal conditions, lake levels are controlled by the Lake Waikare Gates. Normal lake levels vary between 5.4m and 5.6m and the surface area of the lake is 34.7Km2. During the July 1998 flood, the Rangiriri spillway operated for approximately 3 days and the Lake Waikare reached its highest level since the scheme was built. The objectives of this investigation are:

• To develop a reliable hydraulic (numerical) model of the Rangiriri Spillway and Lake Waikare.

• To calibrate the Rangiriri Spillway and Lake Waikare model. The model will be

developed in MIKE11 - DHI. The 1998 flood will be used as a calibration event.

• To assess the performance of the Rangiriri Spillway under design flood conditions against the original Scheme design objectives: namely attenuation of design peak flows in the Waikato River above the spillway of 1840 m3/s down to 1560 m3/s below the spillway.

• To assess lake levels for the design and above design storm events. This will be carried out using the calibrated model.

• To provide design options to ensure the lake will not overtop the northern

foreshore stopbank during an event greater than design. The calibration standard has been set to ensure a realistic prediction of peak flows and total volume through the spillway. Refer to Doc # 974698.

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2 Methodology The performance of the Rangiriri spillway and the Lake Waikare flood protection system was assessed using Mike 11 (1D) and Mike flood (1D-2D) hydraulic models. The Mike 11 model was calibrated for the 1998 flood event. During this event the Rangiriri spillway operated and the Lake Waikare reached its highest levels since the Lower Waikato Waipa Control Scheme was constructed. The calibrated model was used to estimate the effects of the 2% AEP, 1% AEP, Design and above design events. The hydrological inputs at the boundaries of the hydraulic model were as follows: 1) Waikato River at Huntly.

a) The 2% and 1% AEP event peak flows were estimated based on frequency analysis of the Waikato River at Ngaruawahia and Rangiriri. The hydrographs were scaled from the design hydrograph set out in the 1983 Scheme Review.

b) The hydrograph for the design event was the design hydrograph set out in the 1983 Scheme Review

c) The larger than design event with a peak flow of 2100 m3/s has an AEP of 0.05% (Average recurrence interval of 2000 years). Again the hydrograph was obtained by scaling the design hydrograph from the 1983 Scheme review.

2) Lake Waikare tributary inflows were modelled directly from rainfall using the NAM model in Mike11.

The downstream boundary for the hydraulic model was the Rangiriri Bridge. A q-h boundary condition was used in the model based on the rating curve for the Rangiriri recorder site. There was special concern for the ponding zone located between the Rangiriri spillway and the Railway embankment. A Mike flood model was set up to estimate the effects of this ponding zone on the performance of the system. The above range of events was used to provide different options for the location and size of the northern foreshore spillway. The downstream effects of these options were also assessed.

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3 Model description

3.1 Main model (Mike 11 1D) The model of the Rangiriri spillway and Lake Waikare has been developed in Mike 11, a 1-D hydraulic modelling package. The model includes the reach of the Waikato River between Huntly and Rangiriri. The Rangiriri spillway was modelled by a series of 5 broad crest weirs that connect the Waikato River to the Lake Waikare. The length and height of the weirs were selected from even and reasonably flat reaches of the spillway. See Figure 1. The upstream and downstream boundary conditions of the Waikato River are the Huntly hydrograph (inflow) and the Rangiriri Levels (rating curve or observed values) respectively. The lake Waikare is modelled as a 5 km length channel. The Lake “cross sections” were estimated in a way that matches the hypsographic curve (Depth, Volume) of the lake. Inflows, Direct Rainfall and Evaporation are set in the model as point sources connected to the middle point of the channel that simulates the lake. The Waikare gate is included in the model as a control structure and it is linked to the northern outlet canal. Levels of the Whangamarino River at Falls Rd are the downstream boundary of the northern outlet canal. The Northern Foreshore spillway is modelled as a single broad crest weir connected to a steep channel. The downstream boundary condition of this channel is a constant water level. Error! Reference source not found. shows the scheme of the model.

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8.72

8.60

8.70

8.80

8.90

9.00

9.10

9.20

9.30

9.40

9.50

9.60

9.70

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Stopbank Chainage (m)

Red

uced

leve

l (m

)

Downstream Upstream

S1 S2 S3 S4 S5

Figure 1 Rangiriri Spillway long section levels, survey data 1998. Horizontal arrows indicate parts of the spillway that are modelled as weirs.

Figure 2 Main model scheme includes Waikato River, Rangiriri and Northern Foreshore spillways, Waikare Gate, Modelled Runoff and the lake Waikare.

3.2 Rangiriri ponding zone model (Mike flood 1D-2D) The main Mike11 model does not include the effect of the ponding zone located between the Rangiriri spillway and the railway embankment. See Error! Reference source not found.. The main model assumes that this ponding zone is not a significant restriction and therefore water from the Waikato River reaches the lake immediately after spilling over the Rangiriri spillway.

Lake Waikare

Inflows (Direct Rainfall, Matahuru stm, ungauged inflow)

Huntly Flow

Rangiriri Levels

Waikato River

Spillway

Northern Foreshore Spillway

Gate

Whangamarino Levels

Evaporation

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Railway Embankment

State Highway 1

Main waterways from ponding zone to Lake

Waikato River

Lake Waikare

Rangiriri Spillway

In order to verify the validity of this assumption the ponding zone was modelled in Mike Flood. This model couples Mike 11 (1D hydraulic model) and Mike 21 (2D hydraulic model). The ponding zone is represented in Mike21 using a 20m rectangular grid produced with LIDAR data. The boundaries of the ponding zone are “closed”. The flow from the spillway was obtained from the results of the general model. The spillway flow was simulated with three different “sources” located along the spillway in the 2D model. The Rangiriri and Te Onetea bridges were simulated in Mike 11 as 197m and 150m length branches. Each branch has one cross section at the railway bridge. The Railway embankment is 10m height and therefore the bridges inverts are not considered a restriction. The rest of the system, including the lake and the inflows were simulated in Mike11.

Figure 3 Rangiriri ponding zone. The blue area shows the extent of the ponding zone.

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Figure 4 Mike flood Rangiriri ponding zone model scheme

Lake

Northern Foreshores Spillway

Inflows

Outflows

Spillway flow

Rangiriri

Te Onetea

Ponding Zone (2D)

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4 Calibration The 1998 flood is used as the calibration event. During this period the Rangiriri spillway operated and the Lake Waikare reached its maximum level since 1958. The operation of the Rangiriri spillway started on 12/07/1998 at 12:00 and stopped on approximately the night of the 16/07/1998. This information was obtained from EW staff that experienced the flood. The lake Waikare Gate was opened on 20/07/1998. The simulation period is from the 01/07/1998 at 0:00 until the 31/08/1998 at 21:00.

4.1 Data availability The data required for the model is listed below:

• Flows and levels at “Waikato at Huntly”. • Levels at “Waikato at Rangiriri”. • Rainfall (Clarke’s Rd, Mangaukawa and Speedy’s) • Lake Levels • Peak levels of the Waikato River at the Spillway (pegged levels). • Waikare Gate opening • Whangamarino Levels

Figure 5, shows the location of the measuring sites.

Figure 5 Gauge, level and rainfall stations location

Figure 6 shows the availability of information for the calibration period. The Huntly level recorder did not operate during the peak flow because the recorder was literally “under water”. This gap of missing information lasts for approximately 4 days.

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Huntly level records were completed using the levels at Ngaruawahia. The ratio between these two stations was interpolated from the start of the missing period until the estimated peak and from the peak until the end of the missing period. The peak level at Huntly was estimated from the debris mark at Huntly. The level was 11.24m. The peak time was obtained from the Ngaruawahia gauge station assuming a time lag of 3 hrs between this station and Huntly. This assumption is based on the observation of the time series during previous floods.

1Jul6Jul

11Jul

16Jul

21Jul

26Jul

31Jul

5Aug10Aug

15Aug

20Aug

25Aug

30Aug

Lake Waikare Level (m)

Lake Waikare NorthernControl Gate (m)

Waikato at RangiririLevels (m)

Whangamarino RiverFalls Rd (m)

Waikato at Ngaruaw ahia(m3/s)

Waikato at Huntly Levels(m)

Matahuru (m3/s)

Clarke Rd Rainfall (mm)

Maungakaw a Rainfall(mm)

Figure 6 Data availability period 01/01/1998-31/08/1998.

4.2 Rainfall runoff model The lake inflows were simulated in Mike11-NAM. The hydrological model includes three catchments: Matahuru stream, ungauged catchment and Direct Rainfall. The parameters for the Matahuru catchment were calibrated using the period 01/07/1997-31/12/1998. These parameters are shown in Table 1. The ungauged catchment parameters were assumed to be equal to the Matahuru parameters except for the catchment area. The rainfall parameters were set in a way that represents the direct rainfall on the lake. The catchment area corresponds to the lake area. The stations used in the calibration of the Matahuru catchment are the Maungakawa and Clarke’s Rd with weighting factors of 0.7 and 0.3 respectively. The same is applied for the ungauged catchments.

Table 1 NAM calibrated parameters

Parameter Matahuru Ungauged Direct rainfall

Area (Km2) 106.53 70.85 34.7

Umax 2.6 2.6 0.1

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Lmax 200 200 0.1

CQOF 0.55 0.55 1

CKIF 3.30E+06 3.00E+05 1

CK1,2 20 20 0.1

TOF 0.55 0.55 0

TIF 0.55 0.55 0

TG 0.5 0.5 0

CKBF 5500 5500 1

U/Umax 1 1 0

L/Lmax 1 1 0

QOF 4 4 0

QIF 1 1 0

BF 0.5 0.5 0

The initial condition parameters were not obtained from the calibration. However they were set considering a saturated ground scenario. These parameters are displayed in the last 5 rows of Table 1.

0

5

10

15

20

25

30

35

01-07-98 04-07-98 07-07-98 10-07-98 13-07-98

Date

Flo

w (

m3/

s)

Observed

Simulated

Figure 7 Observed and simulated Matahuru Stream flows during the 1998 flood.

Simulated hydrograph is obtained with the calibrated parameters shown in Table 1.

4.3 Hydrographs analysis The Rangiriri Level recorder is located approximately 30m downstream of the northern end of the spillway. This recorder should provide a good indication of the period of spillway operation. In order to determine the period of time the spillway operated during the 1998 flood, levels for the Rangiriri recorder were compared against the minimum level of the Rangiriri spillway (8.72m), see Figure 1. According to this, the spillway would have operated for 6 days from the 12/07/1998 12:00 until the 18/07/1998 21:00. However, in reality, the spillway only operated from the 12/07/1998 12:00 until 16/07/1998 at night (verbal source). From Figure 9 it is possible to see the height of the debris

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accumulated along the crest of the spillway. This would have caused a decrease in the spillway capacity and a shortening in period of operation. After the 1998 flood the fence was moved to the foot of the spillway embankment.

18/07/1998 21:0012/07/1998 12:00

8

8.2

8.4

8.6

8.8

9

9.2

12Jul 13Jul 14Jul 15Jul 16Jul 17Jul 18Jul 19Jul 20Jul

Flo

w (

m3/

s)

Rangiriri Level Min Level Rangiriri Spillw ay

Figure 8 Levels at Rangiriri compared against the minimum level of the spillway during the 1998 flood.

Figure 9 Photo showing debris accumulated along the Rangiriri spillway after the 1998

flood. This is likely to have decreased the spillway capacity. This fence was shifted to the foot of the spillway embankment after the flood.

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4.4 Hydraulic model simulations The Rangiriri spillway calibration was carried out using a shortened Mike 11 model that only includes the Waikato River and the Rangiriri spillway. The 5 weirs are placed at the beginning of individual branches that connect the River with the Lake. These branches contain steep channels with the lake levels as the downstream boundary condition. This is to ensure that there are no back water effects that could affect the performance of the weirs.

Three different approaches have been used to estimate the flow over the spillway during the 1998 flood. The different approaches are listed as follows:

1. First run. The five weirs were set in the model in a way that the width-height relationship matched the survey data. Results from this simulation predict that the total volume of water that flowed over the spillway was 47 Millions of m3.

2. A mass balance was carried out for the Lake Waikare in Excel and checked with the Mike11 simulations. These calculations indicate that the amount of water from the spillway that effectively reached the lake was 7 Millions of m3.

3. The hydraulic model was run considering that the spillway had 30cm of debris blockage. This was included in the model modifying the width-height relationships of the weirs. The total spilled volume obtained in this simulation was 7.26 Millions of m3.

Initially when looking at the approaches in 1 and 2 above it was apparent that the mass balance for Lake Waikare was very poorly predicted by the model. A number of possible explanations were considered, including infiltration losses to the peat soils and intermediate ponding between the spillway and the railway embankment. The most credible explanation is considered to be the effect of debris blockage of the spillway crest. When this effect is allowed for in the model (approach 3) a good mass balance is obtained for the lake.

It is likely that part of the water spilled never reached the lake. In fact, the ponding zone between the State highway and the Railway embankment remained flooded for approximately 10 days after the spillway stopped operating as described by EW staff that experienced the flood. Evaporation and infiltration losses occurred in this ponding zone could have reduced the total volume of water.

0

50

100

150

200

250

12Jul 13Jul 14Jul 15Jul 16Jul 17Jul 18Jul 19Jul 20Jul

Date

Flo

w (

m3/

s)

No debris

Estimated from mass balance

300mm of Debris

Figure 10 Simulated Rangiriri Spillway Flow. (Red) Clean spillway (no debris). (Green) from lake mass balance (Excel calculations). (Blue) spillway blocked with 300mm of debris.

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Figure 11 shows a comparison between surveyed and simulated peak levels. The maximum difference between pegged and simulated levels is 2.5 cm.

8.6

8.8

9

9.2

9.4

9.6

82000 82200 82400 82600 82800 83000 83200 83400 83600 83800 84000

No Debris 300mm Debris Spillw ay levels Surveyed Peak Level

Figure 11: 1998 Flood levels along the Rangiriri spillway. (Blue) Simulated spillway with no debris. (Blue dashed) Simulation Spillway with 300mm of debris blockage. (Red) Surveyed levels (only the two extreme points generate this line).

4.5 Lake mass balance As discussed in the previous section the mass balance for Lake Waikare predicted by the hydraulic model was poor. It was only when allowance for debris blockage along the spillway crest was considered that a good mass balance was obtained. The final, the mass balance for Lake Waikare for the period 01/07/1998 to 31/08/1998 produced lake levels that agreed with the observed values. See Figure 12.

5

5.2

5.4

5.6

5.8

6

6.2

6.4

24-Jun 4-Jul 14-Jul 24-Jul 3-Aug 13-Aug 23-Aug 2-Sep 12-Sep

Lake

Lev

els

(m)

Observed Estimated

Figure 12 Comparison between observed and estimated lake levels. Estimated levels

were obtained from the mass balance equations.

The formulation behind the mass balance calculations is explained as follows.

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Mass balance formulation The mass balance is calculated through the following equation:

outIn QQt

V −=∂∂

Where Qin = Matahuru inflow + Direct Rainfall + Ungauged Inflows + Spillway Flow

Qout = Evaporation + Gate Outflow

Figure 13: Lake Waikare mass balance scheme

The lake levels are converted into lake volume through the hypsographic curve (Volume-height relationship). The ungauged inflows are estimated multiplying the Matahuru flow by 0.67. This 0.67 factor is obtained from the ratio between the ungauged catchment and Matahuru catchments areas. Direct rainfall is estimated multiplying the Clarke’s Rd rainfall and the lake area (34.7 Km2). The ground water flow is neglected. Due to the influence of the lake levels in the Matahuru stream level recorder, accurate flows cannot be estimated utilizing a standard discharge curve. The following equation was applied to correct the Matahuru flows.

( )0,57.5max

196.58502.257307.2

*

*2

*

−−=+⋅−⋅=

lakeMatahuru

Matahuru

LLL

LLQ

Where QMatahuru= Discharge of the Matahuru stream, L*=Corrected level LMatahuru= Matahuru Level Llake= Lake Waikare Level

Lake Waikare

Matahuru Stm

Ungauged Inflows

Evaporation Direct Rainfall

Gate Outflow

Groundwater flux

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The evaporation was estimated as:

000,700,3410)/3(

)17(25.365

2cos59.156.2)/(

3 ⋅⋅=

⎟⎠⎞

⎜⎝⎛ −⋅+=

−EdaymnEvaporatio

ddaymmEπ

Where d is the day of the year and 34.700.00 is the lake surface in m2. The gate outflow is estimated as:

aStagey

y

wW

WbC

WC

CC

ygdyWCQ

cc

c

cd

dgate

−=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

⋅=

⋅+=

⋅⋅⋅⋅=

1

1*

*

*

11*

,1min

1

2

Where: Stage = Lake Waikare water level recorder (m) y1= depth upstream the gate (m) a= invert level of gate structure (m)=3.934m Cc= contraction coefficient w= gate opening (m) b= dimensionless constant = 0.166 c= dimensionless constant = -0.478 Cd= discharge coefficient d= width of gate structure(m)=12.192m g = gravitational constant = 9.81 m2/s Qgate= discharge (m3/s) It is important to highlight that the gate opening data obtained from HYDRO (EW Hydrological Database) was incorrect. Correct values were found in a hand written log book.

Spillway flow estimation The contribution of the spillway to the mass balance was estimated from the first simulation in Mike11. The spillway flow from this simulation was adjusted by a 0.15 factor in order to match the lake levels as shown in Figure 12. Error! Reference source not found. (green line) shows the adjusted hydrograph compared with the simulation Hydrographs.

4.6 Lake Waikare gate The lake Waikare gate was implemented in the Mike11 model as a radial gate in the control structures section. The parameters that define the gate are: No of gates=1 Gate Width=12.192 Sill level=3.934

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Max speed=10-6 Tune factor=0.55 Height=3.962 Radius=6.096 Trunnion=4.929 Weir coeff=0 Weir exp=1 Tran Bottom=-0.1 Tran Depth=0.2 Initial Value (Not activated) Max Value (Not activated) Max speed was chosen to be 10-6 due to instabilities observed when the default value (0.001) was applied. Gate levels are the observed gate opening plus the sill level. It was found that the gate flow formulation does not allow having the gate open completely. This led to errors and a wrong estimation of the flow. In order to correct this, the maximum gate opening was set to a lower value above the maximum water level in the lake. DHI NZ support is aware of this problem and state they will correct it in the next release of the software. In the meantime they have provided an unofficial version that corrects this problem.

4.7 Comments The Rangiriri spillway-Lake Waikare model has been calibrated. However attenuation in the ponding zone, water losses and the decrease in spillway capacity caused by the accumulation of debris, needed to be included in order to represent adequately the lake levels. Adjustments to the gate openings, Matahuru flows and the simulated spillway flows were necessary to calibrate the mass balance. The Mike 11 model does not represent the Waikare gate flows correctly for high gate openings. It is necessary to consider this in future simulations. The simulations carried out for this report are not affected by this problem since the model has been corrected with an unofficial version. Finally it is recommended that the Matahuru gauge station be relocated further upstream. At the moment it is influenced by Lake Waikare levels.

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5 Modelling flood events Different flood events were run using the calibrated Mike 11 model. These include the 2% AEP, 1% AEP, design and the above design events The difference between the events is determined by the variation of the upstream boundary condition (hydrograph at Huntly) and the rainfall distribution (for the NAM model input). Additionally, the different events were modelled under the following considerations:

• The Lake Waikare gate is closed except for the “above design event”. • The Rangiriri spillway is simulated using actual levels and not design levels. It

assumes that no debris will affect the performance of the spillway. • The rainfall distribution was obtained by scaling the 72 hr design rainfall

temporal distribution, to the different AEP events. Total rainfall data for the 72hr duration storm was obtained from HIRDS v.2. See Figure 14.

• The upstream boundary for the Waikato River at Huntly was obtained by scaling

the design hydrograph to the different peak flows. Figure 15 shows the peak flows for the different events.

• The Waikato River downstream boundary condition is the rating curve at

Rangiriri obtained from HYDROL. See Figure 16. • The Lake inflows were simulated using the calibrated NAM model and the

rainfall distribution for the different events. • The modelling period is 15 days.

The Waikato river peak flows for the 1 and 2% AEP events were estimated from a frequency analysis carried out for Ngaruawahia and Rangiriri. This frequency analysis includes recent data and it takes into account the changes in the hydrology resulting from the construction of the Power generation dams in the upper Waikato. The boundary condition at Huntly is based on the design hydrograph at Taupiri obtained from the 1983 review scaled to reflect the different flood peak magnitudes.. The peak flow used for the “above design event” is the full capacity of the Waikato River upstream of the Rangiriri spillway to stopbank crest level. This peak flow is estimated at 2100m3/s and it has an annual exceedance probability of approximately 0.05%. The 1% AEP 72hrs duration rainfall was used for the design event. For the above design event, the 1%AEP 72hrs, rainfall was increased by 20%. This is to take into account future climate changes. Error! Not a valid bookmark self-reference. shows the total rainfall applied to the NAM model for the different events. These values are distributed in time using the design storm hyetograph shown in Figure 14.

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Table 3 shows a summary of results for the different flood events, including peak flows at Huntly, Rangiriri (downstream of the spillway) and flows over the Rangiriri spillway, and Lake Waikare and Rangiriri ponding levels.

Table 2: 72hr duration total rainfall adopted for different modelling scenarios.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 10 20 30 40 50 60 70 80 90

time (hrs)

Rai

nfal

l Int

ensi

ty (

mm

/h)

Figure 14: Rainfall distribution used for the different flood events. This distribution is the 3 days

rainfall event occurred in the 1907 year. This was obtained from the 1983 LWWCS review.

Scenario Lake Rainfall (mm)

Maungakawa Rainfall (mm)

Clarke's Rd Rainfall (mm)

2% AEP 174.7 196.5 185.9 1% AEP 205.8 231.7 219.0 Design Event 205.8 231.7 219.0

Above Design Event 247.0 278.0 262.8

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0

500

1000

1500

2000

2500

1 3 5 7 9 11 13 15days

Dis

char

ge (

m3/

s)Above Design 2100m3/s

Design 1840m3/s

1% AEP 1570m3/s

2% AEP 1445m3/s

Figure 15: Waikato River at Huntly upstream boundary condition hydrographs for different flood

events. Design hydrograph was obtained from the design hydrograph at Taupiri obtained from 1983 LWWCS review.

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12

Level(m)

Flo

w(m

3/s)

Figure 16: Rating curve at Rangiriri (Max level 10.01m). This curve is the downstream boundary

condition of the Waikato River in the model.

Table 3: Summary of modelling results for different flood events.

Scenario Peak flow at Huntly

(m3/s)

Peak Flow at

Rangiriri (m3/s)

Peak flow over the Rangiriri spillway (m3/s)

Maximum Lake

level (m)

Maximum level at

Rangiriri Ponding Zone (m)

2%AEP 1445 1400 42 6.39 7.00 1% AEP 1570 1450 118 6.70 7.53 Design 1840 1537 299 7.36 8.53 Above design 2100 1612 486 * 9.57

Note: * this level varies depending on northern spillway design see section 5.2.

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5.1 Design Storm Results The design storm simulations give similar results to the values obtained in the 1983 scheme review. Simulations show that the spillway causes 16% attenuation in peak flow for the design event. See Figure 17. The 1983 report mentions that the Rangiriri spillway is designed to attenuate the Waikato River peak flow by 15%. The model predicts a maximum lake level of 7.36m for the design storm. In the 1983 report the design level is 7.37m. For the design storm, lake levels are not expected to overtop the Northern foreshore spillway which has a minimum level of 7.45m.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Day

flow

(m

3/s)

U/S

D/S

1837 m3/s

1537 m3/s

Figure 17: Attenuation of the design hydrograph due to the Rangiriri spillway. Figure shows the modelled hydrographs upstream and downstream of the Rangiriri Spillway. Attenuation is 16% in peak flow.

5.2 Above Design Storm Results During an event greater than design, the Northern Foreshore spillway operates as a safety valve for Lake Waikare. The “above design event” was run for several options. These options include different gate operations, lowering and widening the existing Northern Foreshore spillway and the inclusion of a new spillway near the gate.

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The addition of a new spillway in the vicinity of the gate is technically possible. However it involves additional works in the areas downstream of the gate, and would require land purchase and permission to flood different areas. For these reasons this option has been discarded. Table 6 in the appendix shows modelling results for the different options: The options considered are as follows:

1. Keep the Northern Foreshore spillway at existing actual levels and operating with the gate closed*.

2. Keep the Northern Foreshore spillway at existing actual levels and opening the gate after the Lake level reaches Design Flood Level (R.L. 7.36m.)

3. Lowering the Northern foreshore spillway to Design Flood Level (R.L. 7.36m) and keeping it at its existing width of 73m. Operating with the gate closed*.

4. Lowering the Northern foreshore spillway to Design Flood Level (R.L. 7.36m) and keeping it at its existing width of 73m. Open the gate when the lake reaches Design Flood level (7.36m).

5. Lowering the Northern foreshore spillway to Design Flood Level (R.L. 7.36m) and extending the width to 100m. Operating with the gate closed*.

6. Lowering the Northern foreshore spillway to Design Flood Level (R.L. 7.36m) and extending the width to 100m. Open the gate when the lake reaches Design Flood level (7.36m).




10. Filling the Northern Foreshore spillway up to stopbank levels and opening the gate after lake level reaches Design Flood Level (R.L 7.36m).

In the previous list, the options that operate with the gate closed are listed with an *. This is to indicate that the gate was closed during the peak but opened after the Rangiriri hydrograph receded to 1150m3/s. This is recommended condition to open the Whangamarino and Lake Waikare gates after the peak flow has passed (See 1983 LWWCS Review). When the Northern foreshore spillway starts operating, the excess of water from the lake flows towards the Whangamarino wetland. Before the water reaches the wetland it fills two ponding zones. The ponding zone 1 has a storage volume of 0.944x106 m3 and it is delimited by the Northern foreshore stopbank and the Waerenga Rd. The ponding zone 2 has a storage volume of 12.72x x106 m3 and is delimited by Waerenga Rd and the Swan Rd stopbank. Figure 18 shows the location of these ponding areas.

Doc # 983654/v4 Page 21

Figure 18: Location of the Ponding Zones between Lake Waikare and Whangamarino Wetland.

The Whangamarino wetland and the ponding zones described above have not been included in the Mike 11 model. A mass balance was carried out in order to estimate the volumes and time of flooding for these ponding zones and the Whangamarino wetland for the 9 operation options. Results from the mass balance calculations are shown in Figure 21(1-9). These figures show volumes and time to flood for the different ponding zones. Time to flood is expressed in hours after the Northern foreshore spillway starts operating. The calculations were carried out considering that for an event of this magnitude the Whangamarino levels will be at design level of 5.85m. The available storage is the volume existing between the design flood level and the design crest level of the stopbanks which is 6.31m From the calculations it is possible to conclude that ponding zones 1 and 2 and the Whangamarino wetland are likely to overflow. The time of flooding for these ponding areas vary from 9 to 28 hours for ponding zone 1, from 38hrs to 143hrs for ponding zone 2 and 116hrs to 222hrs for the Whangamarino wetland.

5.3 Flooded Areas and the effects of the Rangiriri ponding zone Flooded areas in the lake vicinities were estimated for the 2% AEP, 1% AEP, design and above design events. The ponding zone between the Rangiriri spillway and the railway embankment was modelled in Mike Flood. The lake was modelled in Mike11. These results are shown in Figure 22 in the Appendix.

Lake Waikare

Waerenga Rd R.L. 7.3m

Ponding Zone 1 Storage= 0.94 x 106 m3

Ponding Zone 2 Storage= 12.72 x 106 m3

Swan Rd Stopbank R.L. 6.31m Whangamarino

Wetland

Northern Foreshore spillway and stopbanks

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The Rangiriri ponding zone has a storage capacity of 5.0 x 106 m3. This volume was calculated from the LIDAR survey for levels below 8.72m, which is the actual crest level of the Rangiriri spillway. Figure 19 compares the main Mike11 and the Mike flood design event simulations. This comparison shows that the effect of the ponding zone between the Rangiriri spillway and the Railway embankment does not affect significantly the performance of the system for the design event. The water levels at the pond are lower than the spillway crest and the lake level does not differ in more than 0.01m for both modelling methods. However Table 3 shows that the level at the Rangiriri ponding zone for the above design event is higher than the level of the Rangiriri spillway. This indicates that the performance of the spillway could be affected by the ponding. This effect has not been considered in the main Mike 11 model.

5

5.5

6

6.5

7

7.5

8

8.5

9

3Jul 4Jul 5Jul 6Jul 7Jul 8Jul 9Jul 10Jul 11Jul 12Jul

Date

Wat

er L

evel

(m

)

Ponding Zone Level Mike Flood

Lake Level Mike Flood

Lake Levels Main Report

Northern Foreshore Spillw ay

Rangiriri Spillw ay

Figure 19: Design event. Lake levels comparison between results of the Rangiriri ponding zone model (Mike flood) and the Main Model. Figure shows that lake levels for both simulations are similar with a maximum variation of 0.01m. Levels at the ponding zone are below the Rangiriri spillway level. Therefore the performance of the spillway is not likely to be affected.

Doc # 983654/v4 Page 23

6 Earthworks The “above design event” has been modelled for a series of operation options. These options involve different gate operation procedures and dimensions of the Northern foreshore spillway. To achieve the proposed options, earthworks in the Northern spillway and stopbanks are required. The Northern foreshore stopbanks should be designed to contain the “above design event” without overtopping. The recommended design crest level for these stopbanks is the maximum lake level plus 300mm freeboard. However in practice this level cannot be higher than 8.31m, which is the level of the gate wing walls. Earthworks cut and filling volumes were calculated for the different options using the LIDAR survey and the stopbanks and spillway crest level survey. A bulking factor of 1.3 was used to calculate loose volumes. Results of these calculations are shown in Table 4 and Figure 20. For construction and maintenance purposes, the design of the spillway should consider the following aspects.

• Crest width between 10m and 30m. This is to minimize flat areas where in normal conditions the local runoff could pond and affect the grass on the spillway.

• Maximum flow velocity 2.5m/s, See Table 5. • Maintain the spillway well grassed to avoid possible erosion.

Stopbank filling volume (m3)

Spillway dimensions

Spillway Cut volume (m3)

Option Lake Level (m)

Design crest level (m)

Freeboard (m)

Compacted Loose Crest Level (m)

Width (m)

On site

Loose

1 8.19 8.31 0.12 368 478 actual actual 0 0

2 8.05 8.31 0.26 368 478 actual actual 0 0

3 8.14 8.31 0.17 368 478 7.36 73 2,907 3,779

4 7.99 8.29 0.3 321 417 7.36 73 2,907 3,779

5 8.1 8.31 0.21 368 478 7.36 100 11,390 14,807

6 7.96 8.26 0.3 258 335 7.36 100 11,390 14,807

7 8.1 8.31 0.21 368 478 7.36 150 23,638 30,729

8 7.99 8.29 0.3 321 417 7.36 200 35,778 46,511

9 7.95 8.25 0.3 239 311 7.36 250 58,262 75,741

Table 4: Cut and fill volumes to raise stopbanks up to design crest levels and widen spillway to values given in options 1 to 9.

Page 24 Doc #983654/v4

5

6

7

8

9

10

11

0 500 1000 1500 2000

Chainage (m)

Red

uced

Lev

el (

m)

SpillwayEastern Stopbank Western Stopbank

Stopbank Level

Ground Levels

8.31m

8.25m7.36m

Figure 20: Long section of Northern Foreshore Stopbank and Spillway. Range of stopbank

design crest level is shown.

Table 5: Northern foreshore Spillway flow velocities. Return slope velocities were calculated using the manning equation with a manning coefficient of 0.040.

Return slope velocities (m/s) Option

Critic Velocity

(m/s) 1% 3% 5% 10% 15%

1 2.1 1.7 2.4 2.8 3.4 3.9

2 1.4 1.1 1.5 1.7 2.1 2.4

3 2.1 1.7 2.4 2.7 3.4 3.8

4 1.9 1.5 2.1 2.4 3.0 3.4

5 2.1 1.6 2.3 2.7 3.3 3.7

6 1.8 1.4 2.0 2.3 2.9 3.3

7 1.8 1.4 1.9 2.3 2.8 3.2

8 1.9 1.5 2.1 2.4 3.0 3.4

9 1.8 1.4 2.0 2.3 2.9 3.3

Doc # 983654/v4 Page 25

7 Conclusions The calibration of the Rangiriri spillway and Lake Waikare model was achieved successfully. The spillway operation could not be calibrated due to the accumulation of debris on the spillway crest. When allowance for debris accumulation was made in the model satisfactory calibration of Lake Waikare levels was achieved. The results of the design storm event simulation match the figures of the 1983 report. This validates the calibration of the model. The existing system operates during a design event as proposed in the 1983 scheme review. Water from the Rangiriri spillway is contained in the Lake Waikare without overtopping the northern foreshore structures, (i.e. spillway, stopbanks and gate). The design event is estimated to have an annual exceedance probability of 0.2% event for the Waikato River. The possibility of including an additional spillway in the vicinity of the gate is technically possible. However this option was discarded for being uneconomic. There is a stopbank located in the northern foreshore of the lake for which actual levels are likely higher than design. This stopbank is not in the system and it should be added. Contours levels of this zone are shown in the appendix (unnamed stopbank). The opening of the gate after the lake level reaches the design level of 7.36m is an operation measure that is recommended. This provides extra capacity to evacuate water from the lake in an above design event and decreases the risk of overtopping the Northern Foreshore stopbank. In an above design event general the overflow from the lake via the northern foreshore spillway is likely to cause flooding of the Swan Road drainage area and overtopping of the Swan Road stopbanks. The modelling of the Rangiriri ponding zone showed that the Te Onetea and Rangiriri bridges are not likely to affect significantly the performance of the system for events equal or lesser than design. However for larger events, these bridges could represent a constriction in the flow from the Rangiriri spillway to the lake and affect the spillways performance.

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8 Recommendations

• The model indicates that the Rangiriri spillway performs as designed. No works are recommended for this spillway.

• In order to ensure that the Rangiriri and Te Onetea bridges will not restrict

significantly the flow from the Rangiriri spillway into the Lake Waikare, it is recommended to maintain the waterways underneath and in the vicinity of the bridges. Part of this maintenance includes vegetation clearance.

• Recommended works for the Northern foreshore spillway are: lowering the

existing 73m width spillway to R.L. 7.36m and raising the Northern Foreshore stopbanks up to R.L. 8.31m. The recommended return slope for the spillway is 3%.

• The northern foreshore spillway needs to be maintained. This involves grassing,

fencing, clearance of weeds and regular checks to ensure the levels are at design standard.

• When Lake Waikare levels exceed the Design Flood Level of 7.36 m (larger

than design event) opening the Lake Waikare gate is an operational measure that will reduce the risk of overtopping the northern foreshore stopbank. It is recommended that this be adopted as a flood management option for events larger than design.

• The Rangiriri Lake Waikare Mike 11 model developed in this study should be

included into the LWWCS model.

• The “unnamed stopbank” should be included in the system. Currently this stopbank is not in the Conquest database. However it protects a considerable portion of farmland and isolates the Lake Waikare from the Whangamarino system.

• The Matahuru stream gauge station is currently affected by lake levels. It is

recommended the location of this gauge station be shifted further upstream, where the effects of the lake levels will not affect the flow measurements.

Doc # 983654/v4 Page 27

References

• Mullholand M. 2005, Doc# 97350 Flood frequency analysis for Ngaruawahia and Rangiriri.

• Mullholand M, 1983, Lower Waikato Waipa Flood control scheme (LWWCS)

review Part A, Hydraulic evaluation.

• Mike 11 and Mike Flood User and Reference manuals.

• Joynes S. 2005, Rangiriri spillway and Northern Shoreline stopbank hydraulic appraisal.

• Lewis B. 2002, Farm Dams planning construction and maintenance. Land links

editorial.

• Chow V. 1988, Applied Hydrology. McGraw hills international editions. Civil engineering series.

Page 28 Doc #983654/v4

APPENDIX

Figure 21-1

Figure 21-2

Figure 21-3

Figure 21-4

Figure 21-5

Figure 21-6

Figure 21-7

Figure 21-8

Figure 21-9

Table 6: Gate and Northern Spillway flows and volumes for different options and operation scenarios. The closed* indicates that the gate was kept closed until the flow at Rangiriri was below than 1150m3/s during the recession curve of the hydrograph. Then it was fully opened and closed again when the lake level receded to 7.36m. On this the total volume is constant for both closed* and fully open gate operation scenarios.

Figure 22: Flooding areas for 1and 2%AEP, design and above design events.

Figure 23: Contour levels gate and surroundings

Figure 24: Contours of the Northern Foreshore spillway area.

Figure 25: Contour levels unnamed Northern foreshore stopbank

Figure 26: Contour levels Lake Northern Foreshore

Figure 27 Contour levels Ponding zone between Rangiriri spillway and Railway embankment Spillway

Spillway

Doc # 983654/v4 Page 1

APPENDIX

Figure 28Figure 29Figure 30Figure 31Figure 32Figure 33Figure 34Figure 35Figure 36Figure 37Figure 38Figure 39Figure 40Figure 41Figure 42Figure 43Figure 44Figure 45Figure 46Figure 47Table 7Table 8Table 9Table 10Table 11

Page 2 Doc #983654/v4

Figure 21: Effects downstream the Northern Foreshore spillway caused by the different operation options. Figures 1 to 9. Time of flooding and water volumes are indicated in rectangles. Arrows indicate the direction of the flow.

Figure 48-1

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

8.19m7.3m

6.31 m

5.85m

15.6x10^6 m30hrs

14.66x10^6 m328hrs

9.44x10^6m3222hrs

Gate

Northern Outlet

82hrs

40.7x10^6m3

Option 1: Northern Foreshore Spillway at current level, Gate closed until Rangiriri recedes to 1150m3/s

1.94x10^6 m3114hrs

Figure 21-2

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

8.05m7.3m

6.31 m

5.85m

5.5x10^6 m30hrs

4.6x10^6 m333hrs

9.44x10^6m3143hrs

Gate

Northern Outlet

-25hrs

50.8x10^6m3

Option 2: Northern Foreshore Spillway at current level, Gate opened after Lake reaches design level 7.36m.

8.2x10^6 m3143hrs

Doc # 983654/v4 Page 3

Figure 21-3

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

8.14m7.3m

6.31 m

5.85m

26.4x10^6 m30hrs

25.5x10^6 m317hrs

9.44x10^6m3182hrs

Gate

Northern Outlet

85hrs

29.9x10^6m3

Option 3: Northern Foreshore Spillway Crest level at 7.36m and 73m Width, Gate closed until Rangiriri recedes to 1150m3/s.

12.7x10^6 m375hrs

Figure 21-4

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

7.99m7.3m

6.31 m

5.85m

15.6x10^6 m30hrs

14.7x10^6 m318hrs

9.44x10^6m3144hrs

Gate

Northern Outlet

0hrs

40.7x10^6m3

Option 4: Northern Foreshore Spillway Crest level at 7.36m and 73m Width, Gate opened after Lake reaches design level 7.36m.

1.9x10^6 m3118hrs

Page 4 Doc #983654/v4

Figure 21-5

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

8.10m7.3m

6.31 m

5.85m

30.7x10^6 m30hrs

29.8x10^6 m314hrs

9.44x10^6m3164hrs

Gate

Northern Outlet

85hrs

25.6x10^6m3


17.0x10^6 m361hrs

Figure 21-6

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

7.96m7.3m

6.31 m

5.85m

18.7x10^6 m30hrs

17.8x10^6 m316hrs

9.44x10^6m3132hrs

Gate

Northern Outlet

0hrs

37.6x10^6m3

Option 6: Northern Foreshore Spillway Crest level at 7.36m and 100m Width, Gate opened after Lake reaches design level 7.36m.

5.0x10^6 m382hrs

Doc # 983654/v4 Page 5

Figure 21-7

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

8.10m7.3m

6.31 m

5.85m

36.2x10^6 m30hrs

35.3x10^6 m312hrs

9.44x10^6m3187hrs

Gate

Northern Outlet

85hrs

20.1x10^6m3


22.6x10^6 m349hrs

Figure 21-8

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

7.99m7.3m

6.31 m

5.85m

39.9x10^6 m30hrs

39.0x10^6 m310hrs

9.44x10^6m3127hrs

Gate

Northern Outlet

85hrs

16.4x10^6m3


26.2x10^6 m342hrs

Page 6 Doc #983654/v4

Figure 21-9

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

7.95m7.3m

6.31 m

5.85m

42.5x10^6 m30hrs

41.6x10^6 m39hrs

9.44x10^6m3116hrs

Gate

Northern Outlet

85hrs

13.8x10^6m3


28.8x10^6 m338hrs

Figure 21-10

Northern FS SB

Waeranga Rd

Ward SB

0.944 MMm3

12.72 MMm3 33.2 MMm3

7.95m7.3m

6.31 m

5.85m

42.5x10^6 m30hrs

41.6x10^6 m39hrs

9.44x10^6m3116hrs

Gate

Northern Outlet

85hrs

13.8x10^6m3

Option 10: No Northern Foreshore spillway Gate opened after Lake reaches design level 7.36m.

28.8x10^6 m338hrs

Doc # 983654/v4 Page 1

Table 12: Gate and Northern Spillway flows and volumes for different options and operation scenarios. Closed* indicates that the gate was kept closed until the flow at Rangiriri receded to 1150m3/s and then fully opened until the lake level receded to 7.36m. The total volume of water for all the options is constant.

Northern F/S spillway Gate

closed Gate Open Operation scenario Crest level

(m) Width (m)

Peak flow (m3/s)

Duration (hrs)

Volume (106m3)

Duration (hrs)

Peak flow (m3/s)

Duration (hrs)

Volume (106m3)

Lake Levels (m)

1 actual actual 72 249 15.6 189 72 194 40.7 8.19 2 actual actual 22 207 5.54 104 69 232 50.8 8.05 3 7.36 73 69 231 26.4 189 69 146 29.9 8.14 4 7.36 73 50 192 15.6 104 68 192 40.7 7.99 5 7.36 100 88 212 30.7 189 68 127 25.6 8.10 6 7.36 100 64 178 18.7 104 67 178 37.6 7.96 7 7.36 150 88 187 36.2 189 63 102 20.1 8.04 8 7.36 200 140 169 39.9 104 61 84 16.4 7.99 9 7.36 250 159 156 42.5 189 60 71 13.8 7.95 10 No spillway 0 0 0 104 70 263 56.3 8.08

Doc # 983654/v4 Page 1

Figure 49: Flooded areas for 1and 2% AEP, design and above design events.

Above Design

Design

1% AEP

2% AEP

Page 2 Doc #983654/v4

Figure 50: Contours of the Northern Foreshore Spillway.

Spillway

6 6.5 8 6

6.5

9.5 8

7.5 6

6.5 7.5

6.5 6

6.5 10 6.5 6 6.5 6

6.5 7 8.5 9.5 6.5 6 7

7.5 7 6.5

76.5

3300 3400 3500 3600 3700 3800 3900 4000 4101300

1350

1400

1450

1500

1550

1600

1650

1700

1750

1800

1850

1900

1950

000

Doc # 983654/v4 Page 1

Figure 51: Contour levels gate and surroundings

6 7 8.5 10

6 7 8.5 11

6 7 8 11

14

6 7 7.5 10 9 13

6

6.5 7

8.5 9

8.5 12

6.5

6

6.5

8.5 8 9 9.5 7.5

7

7.5 6

7.5 7

6.5 7 7.5

8 6.5 6 6.5 7 8

6.5 7

8.5

6.5 7 8

7 6

6.5 7

8

4500 4550 4600 4650 4700 4750 4800 4850 4900 4950 500(U it i t )

1500

1520

1540

1560

1580

1600

1620

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1940

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(Uni

ts in

met

er)

y y

Page 2 Doc #983654/v4

Figure 52 Contour levels unnamed stopbank at northern foreshore

6

7.5 11 14

17 20

6 6.5 7.5 12

17 186

6.5 711

14 17 16

6

6.5 8 10 14 13

7.5 7 6.5 7

8.5 13 11

8.5 11 10 7 7.5 11

12 14 13 96 8.5

10

14 15 13 9.5

7.5

8.5

9.5 9

12

15

13 10 8.5 6 7 8

15 14

11 9.5

7

6.5 7

2300 2350 2400 2450 2500 2550 2600 2650 27 1100

1120

1140

1160

1180

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1220

1240

1260

1280

1300

1320

1340

1360

1380

1400

1420

1440

1460

1480

1500

Doc # 983654/v4 Page 3

Figure 53: Contour levels Lake Northern Foreshore

Spillway

6

6.5 20

15

7.5 13

9.5 6

9 8.5 9 6 6.5 6

6.515 12 7

8 6.5 7 8

15 7.5 7 7.5

6.5 7

7.5

20 13 20 7.5

6.5 7 12 14

4.5 4 15 8.5 7

6.5

7.5 8 20

4 9.5 9 7.5 6.5 7 20

6.5 20

5000 5500 6000 6500 7000 7500 8000

10300

10400

10500

10600

10700

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Page 4 Doc #983654/v4

Figure 28: Contour levels Rangiriri Ponding Zone

Documents

Rangiriri Spillway and Lake Waikare Hydraulic Review · During normal conditions, lake levels are controlled by the Lake Waikare Gates. Normal lake levels vary between 5.4m and 5.6m