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Rod SET
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
 
Dipper Harbour, NB Canada
Design of the Rod SET

We have developed an SET called the Rod Surface Elevation Table (RSET). The RSET works on the same principle as the original 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 benchmarks that are driven to both deeper and shallower depths. We have been using this design for over 10 years and it is what we recommend for colleagues looking to get an SET.

RSET Blackwater
RSET at Blackwater NWR, MD USA

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.

Components of the Rod SET

A) Instrument: The RSET is considerably 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 attaches to the benchmark (using an Insert Collar and Receiver) and provides up to 8 sampling positions (9 pins per position). The instrument is only attached during sampling and then is removed.

NOTE: A "receiver" and "insert collar" are now used with most RSET installations which differs from the older design indicated in the below diagram (see below text for details).

RSET diagram
(From Cahoon et al. 2002b)
RSET with clamps
Blackwater NWR, MD USA

B) Pins - Like the original SET, the RSET uses 9 fiberglass pins (Usually 1/4" or 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. NOTE: most new RSET instruments used 1/4" pins.

C) Insert Collar - The insert collar is a piece of 9/16" SS rod which has an 8 hole SS 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
Insert Collar Assembly

Insert Collar and Receiver
Coupling of Receiver and the Insert Collar

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

D) 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.

carrying case
RSET carrying case
Components of the Deep Benchmark

A) 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.

SS rods
Gateway NRA, Sandy Hook, NJ USA

Each rod is 4 ft (1.2 m) in length and threaded at both ends. A benchmark is established by driving a rod into the substrate and adding 4' rod sections until it cannot be driven any deeper. The rod is cut at or near the surface with an electric angle grinder (or bolt cutters) when finished. The benchmark rod does not stick up above the marsh surface.

B) Receiver:
Deep RSET installations use a "receiver" and "insert collar". Earlier designs of the RSET benchmark 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.

benchmark
Rod SET benchmark
(older design)

Modified benchmark
Rod SET benchmark using a "receiver"
(improved 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 used for all recent SET installations. It is more expensive but offers much better security and long-term viability of the benchmark.

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 and bolted-on 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.

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


Components of the Shallow Benchmark

A) Platform and Collar: The shallow platform is custom built out of aluminum and is designed to create a stable benchmark that is typically about 0.5 - 1.0 meter in depth. 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.

shallow benchmark
Blackwater NWR, MD USA
shallow benchmark image
Download above graphic - (PDF 43k)
B) Legs: The shallow RSET uses thin walled, 3" aluminum irrigation pipe for the legs. The length of these legs will depend upon how far you wish them to go into the substrate and how high you want the platform and collar above the surface.

Blackwater NWR, MD USA (NOTE - new design of platform)
Blackwater NWR, MD USA


OOB RSET
Old Oyster Bayou, LA USA (older design Shallow benchmark)
 
U.S. Department of the Interior, U.S. Geological Survey,
Patuxent Wildlife Research Center | URL: http://www.pwrc.usgs.gov/
Last Updated: 28 January 2010
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