Hydrology Overview of Lake Taupo and the Waikato River as it relates to the Waikato Hydro Scheme (WHS) (Ohakuri Site Visit)

Transcription

1 Hydrology Overview of Lake Taupo and the Waikato River as it relates to the Waikato Hydro Scheme (WHS) (Ohakuri Site Visit) Lake Taupo From 1905 to 1941 Lake Taupo was an unmanaged natural Lake. With the installation of the Taupo Gates, the Gates can now pass a greater flow for a given lake level than the natural outlet channel could. Lake Taupo received additional inflow via Tongariro Power Scheme water diversions from 1971 onwards. The Western Diversion was constructed in 1971 and the Eastern Diversion was constructed in The increased water inflow into the lake corresponds with an increased outflow from the lake to the Waikato River of approximately 29 m³/s, or 20% on average. Lake Taupo inflow is calculated via a water balance calculation. For the Waikato Hydro Scheme operating range in Lake Taupo of 1.4m and a lake area of 611 km 2 the usable volume for electricity generation purposes is Mm 3. This volume is used on average 5.8 times annually. All the water obtained as a result of rainfall is used for generation purposes. Mighty River Power has operated the Waikato Hydro Scheme since 1998 and during that time the annual inflow versus outflow of Lake Taupo has not varied by more than 5%. The aim is to have the lake around to masl at the start of the year for drought contingency purposes. If 7 cumecs was discharged via the gates constantly for one day the lake level would drop 1mm (ignoring inflows). The mean annual outflow from Taupo is cumecs (1980 to present). Under normal operation the outflow will range between 50 cumecs (the required minimum flow) and 319 cumecs which is the gate flow at masl (the Maximum Control Level). The natural outflow at masl is 188 cumecs The maximum daily lake level rise since 1980, marking the introduction of the Tongariro Power Scheme diversions, is 154mm for the 1998 flood event. Waikato Hydro System Mighty River Power completely funds or co-funds the operation of 11 water level, flow, rainfall recording stations throughout the Taupo and WHS dam catchments. The maximum outflow from Karapiro since it was constructed in 1947 is 662 cumecs (1996). For the 1998 flood Karapiro discharged 631 cumecs. MIGHTY RIVER POWER Page 1 of 9

2 The catchment area related to each WHS dam are: o Taupo 3429 Km 2 o Aratiatia 123 km 2 o Ohakuri 1481 km 2 o Atiamuri 308 km 2 o Whakamaru 576 km 2 o Maraetai 589 km 2 o Waipapa254 km 2 o Arapuni 249 km 2 o Karapiro 957 km 2. The mean annual station outflow from Karapiro (1980 to present) is 241 cumecs. The WHS does have the ability to store some flood flows which enables a lower peak discharge to be discharged but for a longer duration. Lower Waikato Catchment The minimum flow from Karapiro ensures that a higher flow than natural is maintained from approximately March onwards during drought events. During flood events flood management decision making is undertaken by the Waikato Regional Council and is supported by Mighty River Power to minimise flood effects overall and specifically to try and minimise the coincidence of flood peaks for the Waipa and Waikato Rivers. Management of flood peaks reduces water levels downstream of Ngaruawahia. Up to 150 cumecs of flood inflow can flow into the Waikato River (1998 flood event) between Karapiro and Hamilton city. MIGHTY RIVER POWER Page 2 of 9

3 Inflow (cumecs) B Supporting information for site visit overview 1 Inflows to Lake Taupo Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 1: mean monthly Lake Taupo mean inflow (1980 to present) Figure 1 illustrates how mean inflows vary throughout the year and indicates that relatively low inflows are received in the summer and autumn months and higher inflows during winter months. If we look at this against the level of Lake Taupo for a number of wet and dry years (Figure 2) we can get a sense of the variability in storage and how Mighty River Power must work with nature to balance its power generation against rainfall in the catchment. Water levels in Lake Taupo are determined by inflows including the additional 20% of inflows derived from the Tongariro Power Scheme. The amount of Inflow to Lake Taupo is the result of rainfall falling on the Lake Taupo and Tongariro catchments. The Tongariro Power Scheme does add an out of catchment inflow into Lake Taupo but the percentage of this additional foreign water inflow is also the result of rainfall falling on the Tongariro catchment. Therefore the amount of inflow is dependant purely on annual rainfall. As a consequence of the TPS inflows, the mean outflow from Karapiro Dam is 241 m 3 /s or cumecs (1980 to present). If the additional Tongariro Power Scheme inflow was not available the mean annual flow at Karapiro would be 211 cumecs (1980 to present). It is possible that resource consent requirements for the Tongariro Power Scheme could change and minimum or residual flows to diverted rivers could be increased thereby reducing inflows to Lake Taupo. Such inflows cannot be taken for granted and allocation regimes set for the Waikato River Catchment should account for this potential eventuality. Currently the additional inflows to Lake Taupo not only support power production but it also enables greater assimilative capacity for contaminants than may otherwise be possible MIGHTY RIVER POWER Page 3 of 9

4 Flow (cumecs) MW/h 2 Generation Generation from the Waikato Hydro Scheme follows the same pattern as inflows to Lake Taupo as a consequence of managing Lake Taupo levels within the bounds specified in resource consents. Figure 2 Mean monthly electricity generation 3 Relationship between Taupo Inflows, Generation and Karapiro Outflows Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months 0 Taupo Inflow Karapiro Outflow WHS generation Figure 3 Relationship between Taupo Inflows, Generation and Karapiro Outflows MIGHTY RIVER POWER Page 4 of 9

5 From the above figure it can be seen that generation from the Waikato Hydro Scheme follows the same pattern as inflows to Lake Taupo and outflows from Karapiro. There is little discretionary water as is evident from the narrow storage range in Lake Taupo and the operational bounds specified in the resource consents for the Waikato Hydro System within which the company must operate. Karapiro outflows reflect the additional flows from tributaries downstream of the Taupo Gates over which there is no management control and reflect that once water leaves the Taupo Gates the Waikato Hydro Scheme is largely a run of river system. 4 Taupo Lake Levels Figure 4: Lake level seasonal variance graph Lake Taupo storage is limited to a range of 1.4m. As storage capacity is limited and annual inflows (rainfall and snowmelt) exceed this storage capacity water is cycled through the lake 5-6 times a year. This largely dictates how much water is released through the Taupo Gates and determines flows down the river. Downstream of the outflow from Lake Taupo the Waikato Hydro Scheme is a run of river system and together with flows from tributaries determines the catchment hydrology. Power production reflects the climatic variability and hydrology of the catchment each year. On average 4000 GWh of electricity is produced each year (July 1992 to June 2009) which is reflective of the mean flow at Karapiro Dam of 241 cumecs. Taking account of the dry and wet year variation production can range from 3058 GWh and 5373 GWh since FY1981 to FY2012. MIGHTY RIVER POWER Page 5 of 9

6 Looking at the mean lake level and comparing this with the annual average power production we find that the level of the lake will be at or about the same level at the start of the calendar year and the end. The level of Lake Taupo must be managed within the minimum and maximum operating levels shown on Figure 4. If the 2010 drought had continued for another 10 days the lake level would have dropped below the minimum operating level. If the lake level drops below the minimum operating level then the flow below Karapiro would likely be between 80 to 110 cumecs which would constrain many users ability to abstract water for industrial and public purposes given their current infrastructure. Only during flood events is the maximum control level exceeded. However, the active storage volume (namely that between the maximum and minimum lake levels) is equal to about 9 weeks of storage at the mean inflow rate of m 3 /s (based on Taupo inflow data for , including TPS diversions), assuming no further inflow. By contrast, the active storage in Lake Pukaki is approximately 23 weeks of inflows to the Pukaki scheme. 1 Flow through Taupo Gates will range from 50 cumecs ( the required minimum flow) and 319 cumecs during high flows with a mean annual outflow of 156 cumecs (1980 to present). Mighty River Power does manage the rate of flow from the gates to coincide with electricity demand. Flow is highly variable throughout each day, month and seasonally but as explained previously approximately the same amount of water is retained annually in Lake Taupo. There is limited storage in each reservoir as is shown in the following figures 5 and 6 comparing storage in Lake Taupo with the hydro reservoirs. Figure 5: Lake Taupo, WHS and individual dam storages. 1 Evidence Ross Woods to Environment Court re Variation 6 to Waikato Regional Plan MIGHTY RIVER POWER Page 6 of 9

7 Volume (Mm 3 ) Waikato Hydro System (WHS) 0.7 Aratiatia Ohakuri Atiamuri Whakamaru Maraetai Waipapa Araipuni Karapiro Whs and single dam storgaes Figure 6: WHS and individual dam storages. 5 The Middle Waikato Catchment The middle reaches of the Waikato River between the Taupo Control Gates and Karapiro Dam drain a total area of approximately 4563 km 2 and display unusual hydrological properties compared with catchments in other parts of the country. A key reason being that groundwater baseflows are relatively high (over 77% of the flow in the tributaries) due to the high percolation rates in the porous soils of the catchment. Both tributaries and the main stem are replenished from groundwater reserves which has a moderating effect on flows. As there is significant retention in groundwater systems during periods of high rainfall the flood magnitudes that they produce are extremely low considering the areas of catchment and conversely the low flows yielded during dry periods are remarkably well sustained compared with catchments of comparable size in New Zealand. For example, the natural catchment area of the river at Karapiro Dam is approximately 7,852 km 2, but the design flood outflow for the dam is only 850 m 3 /s. This contrasts with the Clyde Dam in central Otago, also fed by lakes with a total surface area of 637 km 2, which is very similar to the surface area of Lake Taupo. The area of the Clutha River catchment at Clyde is 53% larger, but the design flood for the Clyde spillway is nearly four times larger. These comparatively low design floods for the Waikato are attributable in part to the general lithology of porous volcanic ash which overlies much of the catchment area upstream of Karapiro. In contrast, most of the flood flows in the river below the Waipa confluence at Ngaruawahia issue from the Waipa catchment which has a different lithology that produces larger flood flows and smaller low flows for a given size of catchment area. 2 2 Evidence Ross Woods to Environment Court re Variation 6 to Waikato Regional Plan 2 MIGHTY RIVER POWER Page 7 of 9

8 In Figure 7 mean flows for the river are shown at each of the dams in the Waikato Hydro Scheme cascade. Inflows to the river are approximately split two thirds from Lake Taupo and one third from the tributaries in the middle catchment as measured at Karapiro Dam. Figure 7: Water balance Taupo to Ngaruawahia (1908 to 2014). MIGHTY RIVER POWER Page 8 of 9

9 6 Lower Waikato Catchment The minimum flow from Karapiro ensures that a higher flow than natural is maintained below Karapiro from approximately March onwards during drought events. During flood events flood management decision making is undertaken by the Waikato Regional Council and is supported by Mighty River Power to minimise flood effects overall and specifically to try and minimise the coincidence of flood peaks for the Waipa and Waikato Rivers. Management of flood peaks reduces water levels downstream of Ngaruawahia. 7 Mighty River Power Monitoring Sites Figure 8: WHS hydrological monitoring stations location map MIGHTY RIVER POWER Page 9 of 9