Shoreline Change at Mokau River Mouth

The sand spit at Mokau is experiencing significant long-term coastal erosion. Cameras were placed at Mokau Spit to monitor changes in the beach and river mouth/bar. A Master of Science study was initiated to utilise images gathered to isolate the natural processes controlling spit dynamics and the time scales over which they occur. Abstract from thesis: Sub-aerial video images from a video camera overlooking the seaward side of the spit were gathered by the National Institute of Water and Atmospheric Research (NIWA) 'Cam-Era' project. High and low tide shorelines were digitzed from these images using a technique described by Aaminkof et al. (2003). However, although some success with this technique occurred it was consistent due to either the rich titnomagnetite sand or the Mokau Rivers colour caused by high sediment load. Four Real Time Kinetic-Global Positioning System (RTK-GPS) surveys were undertaken on the 22nd December 2003, 22nd March 2004, 5th July 2004, and the 15th December 2004. Differences between digital elevation models created using these surveys show seaward hotspots of change occurring consistently at two locations over the timescale of the surveys, on either end of the spit. This suggests seaward and landward movement of the spit was in response to the interplay between river discharge and waves. Low tide images were used to classify the spit into three distinct geomorphic states. The presence of a seasonal 'Swash Bar' was the main distinction between the states with river flow rates being the main mechanism of change between them. High and low tide images were then used to assess variation of shorelines along transects located at the 'hot spots' found from the RTK-GPS surveys. Both high and low tide images showed seasonal variations between summer and winter shoreline locations. Low tide shorelines showed an overall erosion at the more northely transect of 0.064m/day and accretion at the more southerly transect of 0.028m/day during this study. Low tide shorelines followed normal beach behaviour accreting over summer and eroding over winter and it was found that the river and waves were the most controlling mechanisms for the shoreline variations. High tide shorelines showed the reverse in behaviour with accretion in winter and erosion in summer with the shoreline along the northern transect eroding 0.016m/day and the shoreline along the southern transect accreting 0.064m/day during the time of this study. There was a lag effect as shoreline responded to storm events some days or weeks later. This indicated that, in fact, the combination of successive storms and associated high wave energy did cause shoreline erosion at high tide, albeit somewhat delayed. Purpose: Properties at Mokau spit are vulnerable to coastal erosion. Shoreline change data has been gathered since 1999 at Mokau River mouth, using two cameras (part of the Cam-Era network) to investigate beach and bar dynamics. This thesis analyses camera data to investigate shoreline change at Mokau and interactions between river flow, bar dynamics and beach change.

Analysis of Cam-Era data from two cameras at Mokau River mouth.

Mokau Spit and River Mouth

RTK-GPS survey 22/12/03, 22/03/04, 5/07/04 and 15/12/04. Cam-era images from 1999 to 2004.

Images taken automatically every hour are then stored in a central computer at the National Institute of Water and Atmospheric Research (NIWA). Cameras at the remote sites are mounted on poles or on buildings overlooking the monitoring site. The cameras have rain hoods and lens-washers to improve image quality. Each site must have a power supply, a computer to control the system and a phone connection so that it can communicate with the base station in Hamilton. The Mokau site has the remote computer housed in the Mokau museum away from the camera, with a radio link connecting the two. Further details are given at www.niwa.co.nz/services/free/cam-era/tech. Frequency of collection: Cam-era images collected every hour (600 images taken over a ten minute period each hour).

Limitation are associated with the quality of the image. Gaps in collection: There are periods when the camera or images have been faulty. These gaps in data are detailed in the thesis or are available from NIWA. Data quality: Some images at Mokau are of poor quality due to light levels, sun angle and the quality of the radio link. Attribute accuracy: Accuracies are discussed in Needham (2005). Completeness: One Mokau camera (camera B) is still in place, but there are gaps in the dataset due to technical problems associated with camera housing damage and the radio link.

Images are held electronically by NIWA.

Understanding shoreline change at Mokau. MSc Thesis.

The images for each site are then archived (for subsequent research purposes) and placed on the web (www.niwa.co.nz/services/free/cam-era). Full archives of images are held by NIWA.

Data collection is ongoing at camera B.

Related information: Other camera sites nationally - see www.niwa.co.nz/services/free/cam-era.