Automatic surface elevation table (auto SET)

An automatic surface elevation table (Auto SET) for measuring ground surface movements and land surface subsidence. The Auto SET includes a free-floating foot resting on a sediment surface. A benchmark rod passes through a center of the free-floating foot, and is driven into the sediment surface and extends above the sediment surface. A water level recorder system is attached to the benchmark rod. The water level recorder system includes a water level recorder attached to a top of the benchmark rod, a double block pulley system having an upper end connected to the water level recorder and a lower end connected to the free-floating foot, and a counterweight attached to the upper end of the double block pulley system. The water level recorder system measures movement of the free-floating foot to determine ground surface movements.

BACKGROUND

The present invention relates in general to measurement of ground surface movements and land surface subsidence.

River floodplains are some of the most dynamic surfaces of our landscape. In the normal course of a year, the sediment surface can be flooded by several meters of water for months and then crack when the water recedes and the sediment dries. Over time, deposited sediments decompose, dewater, and compact resulting in subsidence that is sometimes offset by subsequent deposition. In situations where deposition does not occur, net elevation loss results. Historically, shallow subsidence (<30 m) has been measured with Surface Elevation Tables (SETs)100(Cahoon et. al, 1999), such as the manual rod SET (rSET) illustrated inFIG. 1. The distance between a reader arm102and a land surface104is measured repeatedly over time and is compared with deposition in the area to determine subsidence rates and net elevation loss or gain.

The inventor has used rSETs in riparian environments and has observed large (greater than 10 mm) surface elevation variations during rSET measurement. The observed variations were assumed to have resulted from rainfall and changes in river stage, but periodic measurements from the rSETs were inadequate to fully understand what had occurred (Kroes and Hupp2010). As illustrated inFIG. 2, in most cases, polyvinyl chloride (PVC) sleeves202and marker pipes204associated with the SETs and other sediment monitoring locations showed dark, organic staining206of the upper sections and pure white lower sections208. In one observation, the distance from the sleeve202at ground level210to the top of the white portion208was about 100 mm. These marks were assumed by the inventor to be from the unconsolidated sediment's shrink and swell, that is, from sediment swelling during high water that covered the sleeve202and prevented organic rich water from coming into direct contact. Strict conformity of environmental conditions were observed in order to minimize these variations. Close attention was paid to river stage, rainfall, and the time of year. The SETs were read during low flow, no rain for a week, and just before the leaves fell in autumn, and still had unexplained inconsistencies in surface levels. The inventor has observed that traditional rSETs located in the Atchafalaya River Basin in Louisiana have measured rates of subsidence ranging from 2.3 mm/yr to 25 mm/yr and deposition ranging from 6 to 14 mm/yr during 2010-2012.

Measurement of shallow subsidence in a river floodplain has been problematic with relatively large measurement errors if a very specific set of conditions were not closely followed. Often years of data were unusable due to what appeared to be a change in the hydration state of the floodplain. The large range in water levels on a floodplain and the typical lack of usable well casings made dry land extensiometers useless. Thus, an improvement is needed in measuring shallow land surface subsidence.

SUMMARY

An automatic surface elevation table (Auto SET) is described herein that measures shallow land surface subsidence. It measures the movement of the ground surface around a benchmark rod automatically over time. The Auto SET records the movement of the land surface in a way that allows the user to compensate for and remove unwanted surface movements. In areas prone to changes in soil moisture, it measures the shrink and swell movement of the ground surface at time intervals in order to more accurately determine subsidence rates.

The Auto SET allows for precise measurements of ground surface movements in an automated fashion using a water level recorder system that is a modified float and counterweight shaft potentiometer. The water level recorder system is free-floating on the shrouded benchmark rod. Instead of using a float, however, one end of the water level recorder system is attached to an aluminum foot that is neutral in weight to the sediment density. Surface movements are mechanically advantaged at a ratio of 5:1 through a block and pulley system.

In accordance with an embodiment of the invention, there is provided an automatic surface elevation table (Auto SET) for measuring ground surface movements. The Auto SET includes a free-floating foot resting on a sediment surface. A benchmark rod passes through a center of the free-floating foot, and is driven into the sediment surface and extends above the sediment surface. A water level recorder system is attached to the benchmark rod. The water level recorder system includes a water level recorder attached to a top of the benchmark rod, a double block pulley system having an upper end connected to the water level recorder and a lower end connected to the free-floating foot, and a counterweight attached to the upper end of the double block pulley system. The water level recorder system measures movement of the free-floating foot to determine ground surface movements.

In accordance with another embodiment of the invention, there is provided a method for measuring ground surface movements using an automatic surface elevation table. The method includes placing a free-floating foot on a sediment surface and passing a benchmark rod through a center of the free-floating foot. The benchmark rod is driven into the sediment surface, and an upper part of the benchmark rod extends above the sediment surface. A water level recorder system is attached to a top of the benchmark rod and to the free-floating foot. Movement of the free-floating foot is measured using the water level recorder system to determine ground surface movements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An automatic surface elevation table (Auto SET)300shown inFIG. 3provides needed improvement in measuring shallow land surface subsidence. The Auto Set300has a benchmark rod302made of stainless steel that is driven to refusal, for example, 13 m below a sediment surface304, and extends above the sediment surface304to above a normal flood stage. The benchmark rod302has a typical length of 16 m, which may vary depending upon the particular area being studied.

The benchmark rod302passes through the center of a free-floating foot306. In one embodiment of the invention, the foot306is constructed from 1 m by 2.5 cm aluminum angle bar308of 3 mm thickness forming an X-shape with a 20 cm square aluminum center310. The surface area of the foot306is 0.06 m2. The foot306rests on the sediment surface304and exerts an effective downward pressure of about 1 g/cm2on the sediment surface304, and it is buried by 1 cm of onsite sediment to prevent lifting by water or animals. The foot306may have other sizes and shapes (e.g., solid square, triangular, and circular).

A top of the benchmark rod302has a platform312on which is fitted a water level recorder system314. The water level recorder system314is a “geared-up” or modified float and counterweight-type shaft potentiometer water level recorder system. Rather than using a float, however, the foot306is used instead. The water level recorder system314has a water level recorder316that is attached to a double block pulley system318by a stainless steel beaded cable320. A lower end of the double block pulley system318is attached to the foot306using another stainless steel beaded cable322. The cable322is attached near the center of the square aluminum plate310. The double block pulley system318uses a line324that is 1.8 mm dyneema braid, 386 kg tensile strength, 0.2% stretch. A 230 g counterweight326is attached to the double block pulley system318using stainless steel beaded cable328. The counterweight326exerts 1.15 kg of lift on the foot306. The free-floating water level recorder system314is attached only to the benchmark rod302and measures the movement of the foot306, whose only contact points are the sediment surface304and the cable322.

A typical float and counterweight shaft potentiometer water level recorder is sensitive to movement of about 0.2 mm. However, the double block pulley system318operates as a motion multiplier (5×) to increase the sensitivity of movement of the Auto SET300to about 0.04 mm. Through the double block pulley system318, surface movements are mechanically advantaged at a ratio of 5:1. Other pulley systems may be used to provide desired gear ratios.

All parts of the Auto SET300involved with measurement are shielded from direct sunlight and vented in order to minimize heat expansion issues. The benchmark rod302is covered by a vented lower sleeve or shroud330, which is enclosed within a vented upper sleeve or shroud332. Referring toFIG. 4, the water level recorder system314may be enclosed within a housing structure402with a door404for easy access. Also, the Auto SET300may be framed within a supporting and protective structure406that supports the water level recorder system314and its housing402and stabilizes the benchmark rod302.

In another embodiment of the invention, a direct cable rather than the motion multiplier (i.e., the 5× double block pulley system318) is used because of the large range in surface level. In many situations, the individuals investigating subsidence are not interested in the small movements during daily cycles, but are interested in the yearly gross movement of the land surface. In those circumstances, the motion multiplier318would provide unnecessary resolution of ground surface movement, and the 0.2 mm resolution of movement by the typical shaft potentiometer water level recorder would be sufficient.

An example will now be described in detail below that serves to illustrate the preparation and testing of an illustrative embodiment. However, it will be understood that the present invention is in no way limited to the example set forth below.

During June 2012, three Auto SETs were installed in the Atchafalaya River Basin in Louisiana (Bayou Darby, Bayou Pigeon, and Buffalo Cove) to collect hourly surface elevations. They are co-located with previously installed traditional, manual rSETs and temperature, barometric pressure, and groundwater/surface water sensors in order to better understand the causes of variability in surface elevation. The auto SETs were deployed in an isolated cypress backswamp, a willow flat, and a cypress-tupelo swamp.

Groundwater monitoring wells were dug at each location to a depth of approximately −1 m NAVD 88 (North American Vertical Datum of 1988). Wells were cased with 5 cm diameter slotted PVC and screened with a fine plastic fabric. Water level and water temperature were monitored with an unvented pressure transducer that was lowered to the bottom of the well. Barometric pressure corrections and air temperature were recorded with another unvented pressure transducer that was located within the equipment box at the top of the structure. During the low water common during the summer, surface water levels were monitored by the closest U.S. Geological Survey/U.S. Army Corps of Engineers gage station (Ser. No. 07/381,567 (Buffalo Cove at Round Island near Charenton, La.), 073815963 (Murphy Lake near Bayou Sorrel, La.), 300507091355600 (Bayou Darby near Lake Fausse Point Cut), USACE 49630 (Bayou Sorrel Lock)). Rainfall events were derived from RADAR archives (Wunderground 2012, http://www.wunderground.com/wundermap).

The Auto SETs were in operation until late October 2012. During this time, water levels in the Atchafalaya River were at decadal low levels. Despite the small range in surface water and groundwater levels, the sediment surface moved average range of 13 mm downward and upward. Surface levels declined as groundwater levels declined and increased as groundwater levels increased at these lower river stages, as shown inFIGS. 5A, 5B, and 5C.FIGS. 5A to 5Cillustrate sediment surface levels (recorded by the Auto SET300) and water levels (recorded by the gage stations) for Bayou Pigeon, Buffalo Cove, and Bayou Darby, respectively. In a time frame of one week, the inventor observed daily cycling of surface levels. Surface levels decreased as groundwater levels decreased, as shown by the chart of surface and groundwater levels for Bayou Darby inFIG. 5D. The amount of groundwater rebound did not rebound an equivalent percentage as surface level over this short time frame. In the short time frame, the sediment surface cycles inversely with air temperature, and rainfall interrupts the cycle momentarily, as shown by the chart of surface level and daily temperatures for Bayou Darby inFIG. 5E. The three sites showed some differences in range of movement and rebound; however, the overall trends of motion were very similar.

The surface level movements recorded by the Auto SET300(seeFIGS. 5A to 5E) indicate that in order for surface movement to be understood and properly measured, high accuracy measurements need to be made at short time intervals. In the embodiment shown inFIG. 3with the 5:1 motion multiplier318, the Auto SET300is able to measure changes in surface level within a single day in response to changes in the environment (seeFIGS. 5D and 5E).

The apparatus described herein provides several advantages including, but not limited to, the following: (a) the Auto SET is self-compensating for temperature variations due to the similar thermal expansion rate of the stainless steel benchmark rod and stainless steel cable, (b) the Auto SET may be built to the specific range of site water levels present on most floodplains, (c) the Auto SET is sensitive to 0.04 mm of surface movement and its measurement rate allows detrending of data to find the true shallow subsidence rate, and (d) the accuracy of the Auto SET allows for the measurement of soil shrinkage due to daily evapotranspiration cycles.

It will be appreciated by those skilled in the art that modifications and variations of the present invention are possible without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

General Bibliography on the Subject

The following bibliography provides citations to the references cited in the above text. The references are provided merely to clarify the description of the present invention and citation of a reference either in the bibliography below or in the specification above is not an admission that any such reference is “prior art” to the invention described herein.Cahoon, D. R., J. W. Day, Jr., and D. J. Reed. 1999. The influence of surface and shallow subsurface soil processes on wetland elevation: a synthesis. Current Topics in Wetland Biogeochemistry 3:72-88.Kroes, D. E. and Hupp, C. R. 2010. The effect of channelization on floodplain sediment deposition and subsidence along the Pocomoke River, Md.: Journal of the American Water Resources Association 46:686-699.