The magnitude of the return period of the 1998 flood peak at the Waipoua River Mikimiki hydrological recording station is a critical factor in the assessment of potential flood plain inundation in Masterton. Using rating and slope-area information available at the Mikimiki site, and assuming subcritical flow and occurrence of bed scour, the peak discharge of the 1998 flood is calculated to be 400 ±40 m3/s. From historical flood information and rainfall intensity data, the 1998 flood is deemed to be the second largest peak since 1897 which implies that it has a return period of about 75 years. This estimate combined with flood frequency analyses of site annual maxima yields a return period of 70 ±7 years for the 1998 flood peak, a figure consistent with the results of regional flood frequency analysis. The largest flood since 1897 occurred in 1947 and has a return period of about 200 years. Both the 1947and the 1998 floods were safely passed by the existing Masterton Flood Protection Scheme. The following analytical output resulted from the data: (1) Plot of apparent bed level for flow of 400 L/s at Waipoua River at Mikimiki. (2) Frequency of annual maxima (1979-1982, 1996-2014) fitted by a Gumbel distribution (1998 flood peak = 356 m3/s). (3) Stage versus velocity relationship (1996‑2014) for Waipoua river at Mikimiki. (4) Frequency of annual maxima (1979-1982, 1996-2014) fitted by a Gumbel distribution (1998 flood peak = 400 m3/s). (5) Comparison of daily rainfalls for 1947 and 1988 storms.

Tidal currents were measured by a month-long deployment of an ADCP seaward of the existing farm in Te Pangu over a neap-spring cycle. Results showed that while water flow was greater close to the mouth of Te Pangu, recirculation and eddies generated by adjacent headlands suggested there would be no significant advantages in moving net pens within the near vicinity once drag from the net pens was considered.

The relevance of flow regimes for both in-stream and out-of-stream values means that national representations of flow regimes may be used in national environmental reporting of New Zealand’s Freshwater state. This report describes: (1) Observed river flow time-series and river width used in this report. (2)Indices that may be of use in national environmental reporting to represent various parts of the flow regime: (a) the 1 in 5 year low flow (b) the 7-day mean annual low flow (c) the mean flow (d) the proportion of flow in February (e) the frequency of events exceeding three times the median flow (f) wetted river width at the 7-day mean annual low flow (3) Methods for calculation of these indices from flow time-series. (4) Regression models used to relate each index to known catchment characteristics. (5) Testing and validation of predicted values for each index. (6) National maps for each index when predicted onto New Zealand’s river network. (7) The format of data files containing the predicted and observed data (8) For wetted width, model performance was quantified by comparing predicted values with an independent data set. Model performance for ungauged sites was assessed by comparing observed values with out-of-bag predictions for the remaining hydrological indices. All models showed a good ability to discriminate between-site patterns.

The Waipori Power Scheme uses the outflow from Lake Mahinerangi, passing this flow through four dams and power stations in the upper Waipori gorge. The data and associated report provide flow information in the lower Waipori Gorge below the last (No. 4) Power Station, and below the last Weir (No. 4). The flow recording site (74395) was located three kilometres below the No. 4 Power Station at grid reference H44:802714. The site was shifted upstream on 15 August 2003 to a location 200m downstream of the power station (H44:789715) to enable TrustPower to monitor the flow information by linking the station Program Logic Controller to the site. The flow recording station at H44:777721, site (74398) below the No. 4 Weir was installed on 19 November 2003 for monitoring of minimum flows and ramping rates downstream of the Weir. The following outputs are available from the quarterly data Oct to Dec 2014. Tabulation of daily mean flows at site 74395 for the Waipori at Below No. 4 Power Station from 1 January to 31 December 2014. Full range plot of flows at site 74395 for the Waipori at Below No. 4 Power Station for the reporting period from 1 October to 31 December 2014. A partial range plot (<1000 l/s) to better show the detail of the low flows at site 74395 for the Waipori at Below No. 4 Power Station from 1 October to 31 December 2014. A tabulation of daily mean flows at site 74398 for the Waipori at Downstream No. 4 Weir from 1 January to 31 December 2014. A full range plot of flows at site 74398 for the Waipori at Downstream No. 4 Weir from 1 October to 31 December 2014. A partial range plot (<500 l/s) to better show the detail of the low flows at site 74398, for the Waipori at Downstream No.4 Weir from 1 October to 31 December 2014.