This report discusses the potential effects of the proposed water storage dam on the flow regime. Construction and operation of the proposed Lee Water Storage Dam has the potential to have some adverse impacts on water quality. Further ecological work will be required to support a consent application to build the dam.
INTRODUCTION
FLOW RELATED ISSUES
Wairoa River at Irvine’s flow recorder site
The effects of the proposed strengthening scheme on the Wairoa River below the confluence with the Lee (and thus the Waimea River) are likely to be largely positive. This is especially the case as there will still be shallow, slow water habitat at the stream edges even at higher flows. Habitat modeling focused on this reach could shed light on this, but is arguably unnecessary given the short distance between the Lee Confluence and the section of.
There is a very small reduction in flow compared to the natural flow in the periods immediately after augmentation, especially longer episodes. These flow reductions are so small that they are unlikely to have any environmental impact. Overall, the proposed enhancement scheme is expected to have very little effect on the flow regime experienced in the Wairoa River at Irvine's and downstream areas, apart from increasing minimum flows (Table 1).
This is evident in the very small differences in median flow predicted during summer to early winter (January to March), when most of the flow increase is likely to occur, and during autumn to early winter (April to June) when refilling the storage reservoir is most likely to reduce downstream. Comparison of flow statistics under the natural flow regime and proposed flow augmentation plan for the Wairoa River at the Irvine flow recording site.
Lee River below the proposed dam site
- Rapid Flow fluctuations
Any negative effects of the enhancement scheme will also be more pronounced in the Lee River than further downstream. This would have removed the potential for these floods to scour and flush excessive periphyton growth as well as other detritus that may have accumulated in the downstream river during the preceding stable low flows. Excessive periphyton growth is associated with a reduction in the diversity of invertebrate communities, as well as the diversity of the periphyton community itself.
The timing of these low discharges (in autumn and winter) is likely to minimize any increase in water temperature due to reduced flow, and as a result periphyton growth is likely to be slower than when these periods of low discharge occur were to take place earlier in the year. The potential for periphyton proliferation in the Lee River is reduced by the relatively low natural concentration of dissolved nutrients (Hay & Young 2005b), meaning that periphyton growth in this river may be nutrient limited. The duration of flushing flow required to remove excess algal biomass may be relatively short, perhaps on the order of several hours.
Provision should be made in the design specifications for the dam for the release of such a large volume through the dam outlet, as these discharges would be necessary when the water storage behind the dam is below the spillway. This flow regime results from the need to balance changes in flow contribution from other tributaries to maintain minimum flow plus abstraction volume in the lower Wairoa and Waimea rivers.
Sediment regime
WATER QUALITY ISSUES
Current water quality
The only guidelines exceeded from the Lee at Meads Bridge SoE monitoring site were turbidity and water clarity, and these occurred only occasionally (i.e. on <8 % of sampling occasions). Tasman District Council's SoE monitoring also includes observations of periphyton using a visual assessment where the percentage cover of different algal types is weighted according to their pollution tolerance, and then combined to give an overall score for the site ranging between 1 and 10 (1 indicating a site with highly impaired water quality and a score of 10 indicating a healthy site with good water quality1) (Biggs & Kilroy 2000). The minimum score recorded from the Lee River at Meads Bridge site in 14 sampling occasions between April 2001 and November 2004 was 7.53.
The maximum value recorded during this period was 10 and the median was 9.80, indicating that the periphyton community in this reach is generally indicative of good water quality. Even during an extremely low discharge period in late summer 2001, when some other sites in the Waimea watershed recorded scores <5, the Lee at Meads Bridge site still scored relatively high (>9). This suggests that algal growth in the Lee River may be limited by low nutrient concentrations, rather than controlled by flow fluctuations.
Potential effects of the dam on downstream water quality
- Increased sediment load during construction
- Water temperature
- Deoxygenation of bottom water
- Release of nutrients
The worst-case scenario for the detention time in Lee Reservoir can be estimated as the time required to completely empty the dam (13 million m3) assuming a minimum discharge of 470 L/s, which would require approximately 46 weeks. The effects of increased sediment loading are likely to be relatively short-lived in the River Lee and will be largely confined to the construction phase, although this may last up to two years. Given the relatively frequent high flood flows generated in the catchment, deposited sediments must be flushed out of the system relatively quickly once construction disturbances are complete and the reservoir is filled.
For example, water temperatures in Cobb Reservoir were consistently higher than temperatures recorded in the river upstream between December 1999 and July 2000 (Young et al. 2000). This depends on the ability to select the level in the water column in the impoundment from which the water released downstream originates. However, the critical periods when iron and manganese concentrations in bottom waters are high probably correspond to warm water temperatures in the upper layers of the reservoir.
Holding water in the reservoir can result in an increase in nutrient concentration in the water released from the reservoir (Young et al. 2004). For example, ammonium nitrogen concentrations were elevated in Cobb Reservoir during a period of low lake levels and may have been responsible for stimulating abundant growth of filamentous green algae downstream of the plant (Young 2001).
Potential water quality issues within the reservoir
- Fluctuating water levels behind the proposed dam
- Sediment
- Algal blooms
Removal of terrestrial vegetation and topsoil from the dam footprint prior to filling will help reduce the chances of deoxygenation within the reservoir. There may also be some ability to limit the amount of iron- and manganese-rich water released from the reservoir. However, Greig (1973) noted that some animals, especially caddisflies, did not occur in the littoral zone, except at the bottom of the drawdown zone where water cover was greater.
Fluctuating water levels in the proposed Lee River impoundment are likely to delay the establishment of macrophyte communities and their associated macroinvertebrate communities around the shallow margins, which are generally recognized as the most diverse and. The sediment load in the Wairoa River (downstream of the Lee River) is relatively low to moderate and the design of the dam has taken into account the amount of sediment that will be trapped within the reservoir (Tonkin & Taylor 2006). During investigations for an alternative potential dam site in the upper Wairoa, concerns were raised about the toxicity of metal-rich sediments derived from ultramafic rocks in the catchment (Tonkin & Taylor 2006).
However, there is no ultramafic material in the catchment area of the proposed Lee reservoir and therefore no adverse effects from toxic sediments are expected. The low nutrient concentrations of water in the Lee River would also indicate that algal blooms are unlikely to occur in the proposed reservoir.
FISH PASSAGE ISSUES
Existing fish community in the vicinity of the proposed scheme
Such blooms can result in unsightly accumulations of algal material and water quality problems, such as reduced water clarity and deoxygenation of bottom waters. One group of phytoplankton (called cyanobacteria) are of particular concern because they produce toxins that can cause illness, and sometimes death, in humans and animals that drink the water or ingest algal material. Algal blooms are largely controlled by nutrient concentrations and are most common in nutrient-rich lakes (Schallenberg 2004).
Changes to fish community post scheme commissioning
Fish passage mitigation options
Of the two options, a functioning pass is preferred as it is less expensive to maintain and would be permanently in place to provide access at any time when fish are attempting to migrate. However, traps have the advantage of being comfortable because people can see and count what is being transferred, regardless of the biological significance of the transfer. Now that koaro and eel have gained access above the dam, some attention needs to be paid to providing them with return access downstream.
Therefore, a downstream route that is free of obstacles such as turbines is unlikely to improve survival of koaro larvae and is therefore not necessary. Depending on how they exit the dam, they may suffer some damage, although in the absence of turbines or screens, this is likely to be less of a problem. There are likely to be few options for increasing the migration of larval koaro or adult eels other than releasing some flow from the reservoir during autumn freshets, when these fish will be most likely to seek entry downstream. .
With the intake tower set back from the dam wall, releasing water through the outlet instead of the intake can give fish, especially eels, a better chance of finding the exit. However, many natural fall freshets can be "trapped" in the reservoir as water levels recover after flow increases over the summer.
RECOMMENDATIONS FOR FURTHER WORK
Abundance and distribution of the benthic fauna of three upland reservoirs in the Kananskis River in Alberta. Siltation of stone surface periphyton in rivers by clay-sized particles from low concentrations in suspension. A study of the littoral fauna of two man-made lakes (Lake Aviemore and Lake Waitaki).
Analysis of instream habitat flows for the Waimea River and preliminary minimum flows for proposed dam sites in the upper Wairoa and Lee catchments. The effect of suspended coal particles on the life forms of the water moss Eurhynchium riparioides (Hedw.).
