Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 NIWA Client Report: HAM2011-015 February 2011 NIWA Project: BOP11203 Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 Max Gibbs Rod Budd Cliff Hart Scott Stephens Aslan Wright-Stow Scott Edhouse NIWA contact/Corresponding author Max Gibbs Prepared for Bay of Plenty Regional Council NIWA Client Report: HAM2011-015 February 2011 NIWA Project: BOP11203 National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone +64-7-856 7026, Fax +64-7-856 0151 www.niwa.co.nz All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Contents Executive Summary iv 1. Introduction 1 2. Methods 2 2.1 Current meters 2 2.2 Lake Rotorua 3 2.3 Lake Rotoehu 4 3. Results and discussion 6 3.1 Lake Rotorua July/August 2010 6 3.1.1 Current velocity 6 3.1.2 Current direction 7 3.1.3 Wind velocity and direction 7 3.1.4 Wind-water coupling 8 3.2 Lake Rotorua November/December 2010 10 3.2.1 Current velocity 12 3.2.2 Current direction 13 3.2.3 Wind velocity and direction 13 3.2.4 Wind-water coupling 15 3.2.5 Water column decoupling 16 3.3 Lake Rotoehu January/February 2011 16 3.3.1 Wind velocity and direction 17 3.3.2 Mid-lake site current velocity and direction 19 3.3.3 Southern lake site current velocity and direction 22 3.3.4 Lake Rotoehu current synthesis 25 4. Summary 27 4.1 Lake Rotorua fully mixed 27 4.2 Lake Rotorua stratified 27 4.3 Lake Rotoehu 28 4.4 Bottom line 28 5. Acknowledgement 29 Reviewed by: Approved for release by: Dr Chris Hickey Dr Ngaire Phillips Formatting checked Executive Summary The need to be able to apply sediment capping agents to defined target areas on the lake bed from the surface of the lake, necessitates determining the strength and characteristics of lake currents that will affect their settling. Bay of Plenty Regional Council asked NIWA to measure the lake currents in Lake Rotorua when it was fully mixed and when it was thermally stratified, and to measure the lake currents in Lake Rotoehu in summer when the lake was stratified. This report presents a summary of the current meter data together with the associated wind data to aid interpretation for the three deployment, Lake Rotorua mixed (July/August 2010), Lake Rotorua stratified (November/December 2010) and Lake Rotoehu stratified (January/February 2011). During fully mixed periods in winter, the whole water column of Lake Rotorua moves in the same direction at the same velocity. Mean water velocity was around 3 cm/s reaching a maximum of 20 cm/s under strong winds. The direction of flow at the current measurement sites was in the opposite direction to the wind direction measured at Rotorua Airport, implying that the water in the lake flows around Mokoia Island as a central axle in the lake. It took about 2 days to establish a current flow from a calm lake, and about 24 hours for the lake currents to reverse when the wind direction reversed. the residual lake currents dissipated after about 3 days when the wind stopped. During thermally stratified periods, the water column of Lake Rotorua became decoupled at the thermocline with water in the upper layers moving in the opposite direction to the water in the lower layers. In general, water velocities were lower than in winter with mean velocities of around 2 to 3 cm/s reaching a maximum of around 15 cm/s. At low wind velocity, water in the lake appeared to move in slowly rotating gyres. Threshold conditions required to establish the decoupled circulation pattern were a wind velocity of >5 m/s sustained for at least 24 hours. In Lake Rotoehu, the surface waters flowed with the wind. When the lake was mixed the whole water column moved in the same direction but the current velocity decreased towards the lake bed. When the lake was stratified, the water column became decoupled at the thermocline with water in the upper layers moving with the wind while water in the lower layers moved in the opposite direction. Mean water velocities were 1 to 2 cm/s reaching a maximum of around 18 cm/s during strong winds. Under low wind conditions there was a tendency for the water in the lower layers to move towards the south. Threshold conditions required to establish the decoupled circulation pattern were a wind velocity of >4 m/s sustained for at least 3 hours. Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 iv 1. Introduction To manage phosphorus releases from the sediments of Lake Rotorua and improve the water quality of the lake, one strategy is to apply an active barrier to the sediment surface i.e., a sediment cap. Typically sediment capping agents are applied at the lake surface and settle to the lake bed forming the active barrier against phosphorus release. However, preliminary data from earlier hydrology studies on Lake Rotorua indicated that the water column is not stationary and even small lake currents have the potential to move the capping material considerable distances from the point of application as it settles. To assess the magnitude of this issue, NIWA was asked by Bay of Plenty Regional Council to measure the currents in Lake Rotorua during periods when the lake was mixed and again when the lake was thermally stratified. Current measurements were also required in Lake Rotoehu to assess their likely effect on the use of aeration to manage internal P loads from sediment release. This report presents a summary of the three current measurement periods with graphical interpretation to provide visual representations of how water moves in these lakes and the role of wind forcing. Data presented in this report is available on CD- ROM with electronic files of the full data set from each period and the associated wind record from Rotorua Airport. Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 1 2. Methods 2.1 Current meters Current measurements were made using bottom mounted Acoustic Doppler Current Profilers (ADCP) which record the water velocity and direction at several depths from about 1.5 m above the sediments to about 1 m below the lake surface. The current is measured by bouncing sound off particles in the water and calculating the velocity from the Doppler shift in the return echo of each “ping”. Because the velocity of sound in water is essentially constant, the return echo can be timed to give separate data for selected distances (depths) from the instrument. However, because of the variable distances of particles from the acoustic head, the data is recorded for a depth layer or “bin” of finite thickness – usually 1 or 2 m. Also because it requires a finite time for the ping to stop ringing in the acoustic head, there is a dead zone, close to the acoustic head, in which no data is collected. The data close to the water surface may also be corrupted by wave action. The ADCP has three acoustic heads which allows the calculation of flow direction as well as velocity. The software in the instrument can be set to calculate the velocity and direction at different depths in the water column. In these deployments each current meter was set to record a burst of data (500 pings each of 0.5 second duration) and produces an average value for these data for each depth bin from the lake bed to the surface – excluding the dead zones. These measurements were repeated at 15 minute intervals for the period of each deployment. Note: By convention, water flow directions are given as “ going towards ” whereas wind flow directions a given as “ coming from ”. Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 2 2.2 Lake Rotorua Trial plot Buoy Airport ADCP positions Airport wind station Area deeper than 15 m Cores Bathymetry of Lake Rotorua (Adapted from Irwin, 1969). Figure 1: Site map of Lake Rotorua showing the locations of the ADCP current meters beside the monitoring buoy [Buoy] and the proposed sediment capping trial plot [Trial plot] relative to the meteorological station at Rotorua Airport. Currents in Lake Rotorua were measured for four weeks when the lake was fully mixed in winter [27/07/2010 to 25/08/2010] and when the lake was thermally stratified in spring [5/11/2010 to 8/12/2010]. Two study sites were used in Lake Rotorua in July/August 2010. The first was beside the remote monitoring buoy, in a depth of 21 m, and the second was at the location of a proposed sediment capping trial plot site in a depth of 18 m (Fig. 1). The ADCP site by the buoy was intended to allow direct linkage between water currents and wind forcing using the met station on the monitoring buoy. Unfortunately, the meteorological station on the buoy did not record during the August 2010 period of monitoring. Consequently, all current data Current measurements in Lakes Rotorua and Rotoehu 2010 and 2011 3 interpretation has used the wind data record from Rotorua Airport [meteorological station Agent 1770].