This investigation analyses historical bed-level change at seven cross-sections along the lower Rakaia River between around SH1 Bridge and the coast using data from ground surveys undertaken in 1976 and 1988/89 and from airborne LiDAR flown in 2010. It is concluded that with a few exceptions, mean bed levels along this reach appear to have been stable - at least within the level of detection and allowing for natural variation in bed levels associated with normal braided riverbed dynamics. A slight degradation trend of a few mm/yr is consistent with a response to long term coastal retreat, at least for the river within about 18 km of the coast. It is likely that much of the 0.75 m of degradation apparent over the active braidplain of the South Branch 10 km upstream from the coast between 1988 and 2010 is an artefact of errors in re-locating the section line. The investigation included: 1) Securing the historical ECan survey records and entering them into a spreadsheet. 2) Extracting the 2010 bed-levels from the LiDAR along the ECan section lines 3) Post-processing the LiDAR topography to remove remnant bias from returns off riparian vegetation. 4) Using flow-gauging data to develop a relationship between wetted cross-section area and river flow rate for use in adjusting the LiDAR mean bed-levels to include wetted bed areas 5) Plotting the overlaid sections and calculating changes in mean bed-level reporting.

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.