Construction of an Impact Plate Geophone to Record Sediment Movement in New Zealand Gravelbed Rivers
Andrew J. Neverman1, Ian C. Fuller1, Jon Procter1, Ranvir Singh1 1Innovative
River Solutions, Institute of Agriculture & Environment, Massey University, Palmerston North, New Zealand
Background • Bedload transport drives erosion and deposition which creates a hazard for infrastructure and controls habitat and aquatic community composition. Understanding bedload transport should therefore be a high priority for effective management of gravel-bed rivers; both for maintaining ecological health and reducing impact on infrastructure. The accurate predication and quantification of bedload transport still eludes scientists, largely due to the inability to record bedload transport in natural channels at suitable spatio-temporal resolutions. Historical studies have relied on direct physical measurement techniques which often produce poor results. Surrogate bedload transport techniques offer significant advantages, but sensor arrays can be costly and require expertise to operate. Geophone-based impact plates are seeing increasing use in bedload transport studies as they offer a cheap solution and are relatively easy to use. Many of the impact sensors in the literature either require permanent support structures for installation or are installed in paving slabs in relatively lowenergy systems. Dataloggers are often stored in the impact plate itself resulting in lost data if the impact plate is washed away. These installation methods limit the application of impact plate geophones in dynamic gravel-bed rivers, such as those found in New Zealand. This paper demonstrates the development of a novel installation method for impact plate geophones in New Zealand’s relatively high-energy, dynamic gravel-bed rivers, which anchors the impact plate without large support structures. It also allows for easy adjustments to be made to adjust the sensor following bed level changes. Data is logged on the river bank, allowing data recovery even if the sensor is washed away.
Field Site
Method
Figure 1. Field site at Mais Reach on the Pohangina River, located 30 km from Palmerston North in the Manawatu Region, New Zealand.
• A 750 mm x 500 mm impact plate was constructed from 4 mm mild steel plate • Three securing rods were constructed from API .75 x .219 xS160 x 6 seamless line pipe. • Threaded rod is screwed in to the securing rods to bolt the impact plate down, or to add an extra extensions of pipe to allow rod lengths to be varied depending on local bed conditions (i.e. depth of penetrable gravel). • The securing rods were driven 1.5 – 2 m in to the bed. • A multi-core power and data cable is run from the impact plate along steel chain to a datalogger on the bank. A cable with a thick sheath is used to help protect the cable from abrasion damage. • A geophone is mounted underneath the steel plate. The geophone is housed in a steel pipe welded to the base of the impact plate. • An ultra-sonic velocity sensor is mounted inside a housing on the end of the impact plate. • The impact plate records a variety of metrics from the geophone signal at 1 Hz.
Figure 3. Installing the impact plate in the field during a maintenance inspection, Note the securing rods protruding from the bed.
Field Testing • • • • • •
Figure 2. CAD drawing of the impact plate.
An impact plate was installed in the Pohangina River, Manawatu, New Zealand for 1 year. The sensor was installed at a Horizons Regional Council gauging site known as Mais Reach, which is known for it’s mobile bed. The securing rods were driven between 1.5 - 2 m into the bed, and the impact plate bolted on top. Over a 1 year period of testing the impact plate remained in place, despite a 300 mm drop in bed level. The installation withstood a peak flow of 307 m3s-1, 18 times the median flow. During this time the sensor has also recorded several bedload events (figure 4). Field testing showed this novel approach to impact plate installation is viable for gravel-bed rivers with a relatively coarse mobile bed. Pros and cons are listed in table 1.
Table 1. Pros and cons for the installation method described in this paper. Pros Cons relatively cheap Sensor may be buried Support rods can be easily adjusted to suit Potential for power/data cables to be the site and bed level changes damaged Data logged on the bank for easy retrieval Requires maintenance for accurate operation Plate size easily adjustable to change spatial coverage On-board logging of velocity is a significant advantage over other sensors Relatively little expertise required for operation
This research is supported by:
ESL
Figure 4. Bedload impacts (blue) and stage (red) recorded using the impact plate at Mais Reach, Pohangina River. Impact values are given as a digital value of the impact signal magnitude (working name for units is Nevermans).
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