1) Design of the Rod SET (RSET)
2) Components of the Rod SET
3) Components of the deep benchmark
4) Components of the shallow benchmark

We have developed a type of SET called a Rod Surface Elevation Table (RSET). The RSET works on the same principle as the Surface Elevation Table (Cahoon et al. 2002b) but permits the determination of elevation change occurring over different portions of the sediment profile because it can be attached to bench marks that are driven to both deeper and shallower depths than the SET.

A detailed description of the rod SET, deep benchmark and the shallow benchmark can be found in:
Cahoon, D. R., J. C. Lynch, B. C. Perez, B. Segura, R. Holland, C. Stelly, G. Stephenson, and P. Hensel. 2002b. A device for high precision measurement of wetland sediment elevation: II. The rod surface elevation table. Journal of Sedimentary Research. Vol. 72, No. 5. pp. 734-739.

Cahoon and others have demonstrated that subsurface processes can exert significant influence over sediment elevation in many wetland systems (Cahoon et al. 1995; Cahoon et al. 1999, Cahoon et al. 2000). The question arises as to whether sediment elevation in a given wetland is controlled by a biological, hydrological, or geological process. One approach to determining which process is driving sediment elevation is to quantify elevation change over different portions of the sediment profile (e.g., the root zone versus the entire profile between the sediment surface and base of the bench mark). Unfortunately, the original SET design does not lend itself well to measuring elevation change over depths of the sediment profile typically less than 1 m or greater than 10 m. The 3" aluminum bench mark pipes to which the original SET is attached generally preclude going much deeper than 10 m, both because of surface resistance provided by the pipe and by the length of pipe which can be practically handled in the field. Use of thin-walled aluminum pipe for shallow bench marks is also precluded for two reasons. First, the weight of the original SET (about 15 LB, 6.5 kg) will cause a shallow bench mark pipe to sink in unconsolidated sediments. Secondly, the SET is not balanced because the center of gravity is located over the horizontal arm. Hence, the SET would cause a bench mark shallowly embedded in unconsolidated sediments to lean away from the vertical and become unstable.

Instrument: The RSET is smaller and lighter than the original SET (about 6.5 LB, 3.0 kg) and is designed to couple with a benchmark. The device is constructed out of aluminum and stainless steel and measures about 40" (1m) in length. The RSET couples with a Stainless Steel collar and provides up to 8 sampling positions. The instrument is only attached during sampling and then is removed.

NOTE: If you are using the newer design benchmarks, there will also be an 8 hole "Insert Collar" which stays in the case with the RSET (see below for details).

Pins - Like the original SET, the RSET uses 9 fiberglass pins (3/16" diameter) for measuring the distance to the marsh surface. Pins are typically 2.5 - 3.0 ft in length though other lengths can be used depending on the setup. Badge clips are used to hold the pins in position after they are placed.

Carrying Case - The SET is only used when taking readings so it's important to have a good carrying case to keep the instrument from getting damaged. Since the RSET is much smaller than the original SET, it can easily fit into any standard 4' gun case ranging in cost from $10-$75.

Stainless Steel Rods - The RSET is designed to attach to a benchmark driven into the marsh (or other substrate). We typically use 9/16" (15 mm) diameter stainless steel sectional rods to establish the deep benchmark.

Gateway NRA, Sandy Hook, NJ USA

Each rod is 4 ft (1.2 m) in length and threaded at both ends. A bench mark is established by driving a rod into the substrate and adding rod sections until it cannot be driven any deeper. In a given wetland, the rod benchmarks can typically be driven deeper than the SET pipe bench marks because the 9/16" (15 mm) diameter rods encounter less resistance.

Original Design Collar - A custom built collar is affixed to the bench mark rod by 4 set screws (2 each on opposite sides) and metal glue (Loctite 262 ®). The collar is made of stainless steel for strength and has a flange on the top with 8 evenly spaced holes 45 degrees apart. Top of Collar Bottom of Collar This is the point of attachment for the RSET. The RSET is slid onto the rod and lowered onto the collar where 2 diametrically opposed collar pins extending down from the base of the RSET are inserted into the holes in the collar. Thus, the RSET can be attached to the rod at up to 8 different fixed positions, assuring reoccupation of the same reference plane and point on the sediment surface during each sampling. The RSET is held firmly to the collar by 2 spring clamps ("Pony" clamp, Model 3201). NOTE - This is the mechanism used in the original design of the RSET.

Recent Modifications:
The original design of the above collar assembly for the Deep RSET have been modified to improve deployment in areas with:

1) high public access and/or high incidents of theft and vandalism.
2) Areas which experience ice and the affects of ice rafting.

We have had incidences where the top section of rod was unscrewed and removed, along with the collar and cement which anchored it in place. This is partly due to the visible nature of the collar and rod which can extend 1-3 feet above the marsh surface. The height and exposed nature of the benchmark and collar has also raised concerns in tidal areas where ice rafting in the winter could damage the benchmark or alter sediment deposition.

Changes were made to eliminate the possibility of unscrewing the sectional rods and to decrease the visibility and vertical height of the SET benchmark. The intent of these modifications was to improve the long-term stability and security of the benchmark that is established.

Rod SET benchmark (older design)

Modified Rod SET benchmark (new design)

These changes do not alter the operation of the SET and how the measurements are taken. The major changes are involved in the installation of the benchmark rods and the mechanics of how the SET attaches to this benchmark. This is the design we are currently using for all new installations. It is more expensive but leads to a much more secure sampling station.

Stainless Steel Rods - The newer setup uses the same 9/16" rods as described above. The rods are driven to depth as usual the only difference being that the rod is cut at or near the surface with bolt cutters when finished. The benchmark rod does not stick up above the surface.

Receiver - A custom built stainless steel receiver is attached below the sediment surface to the SS benchmark rod by 4 stainless steel bolts. Once attached to the SS rod, the entire rod and receiver are covered with cement inside of a 6" PVC pipe. Only the top of the receiver protrudes 3-6" above the cement. The receiver has a pvc cap which protects the threaded mechanism which couples with the "Insert Collar". The receiver has a notch which aligns with an "alignment" pin on the insert collar. This ensures the collar attaches in the same manner every time.

Conceptual diagram showing the features of the modified deep benchmark. Deep rod benchmark (white), receiver (yellow) and the insert collar (gray)
Receivers	Receiver attached to SS rod SS Rod in PVC pipe

Insert Collar - The insert collar is a piece of 9/16" SS rod which has an 8 hole collar welded to it. The SS rod has a threaded pvc cap and alignment pin which couples with the receiver/benchmark. The Insert Collar is not permanently attached to the receiver and stays in the case with the RSET when not in use.

Insert Collar Assembly	Coupling of Receiver and the Insert Collar
	Note Alignment Pin and retaining ring above.
RSET and Insert Collar on a deep benchmark	Threaded Cap couples both pieces together

Platform and Collar: The shallow platform is custom built out of aluminum. The newer design described below is built to accept the "insert collar", removing the need to attach a collar to the benchmark. The platform is approximately 18" x 18" in size weighs about 4.0 lbs. The RSET is attached to the shallow benchmark in the same way is is attached to the original deep benchmark.