Open-File Report 2006–1311
U.S. GEOLOGICAL SURVEY
Open-File Report 2006–1311
The USGS, in cooperation with SNWA and Nevada Division of Water Resources (NDWR), operates and maintains a surface-water network of 6 continuous-record stream-gaging stations and 11 partial-record stations in the Warm Springs area near Moapa, Nev. (table 1). To determine the elevation of the water surface at each monitoring site, BMs, RMs, and RPs were established at strategic locations within the Warm Springs area. For this report, a BM is defined as a permanent marker that was installed in the ground or on a structure and that has an established elevation based on the North American Vertical Datum of 1988 (NAVD 88). The elevations of these markers were derived from survey-grade differential global positioning system (GPS) instrumentation. A RM is defined as a permanent marker installed in the ground or on a structure in the vicinity of a gaging station. The elevations of the RMs were determined by optical theodolite surveys from a nearby bench mark. A RP is a permanent marker installed on a structure at a gaging station that allows direct measurement of water-surface elevation using a graduated measuring tape or ruler. The elevation of the RP also is determined by optical theodolite survey from a nearby bench mark. The elevations of the RMs and RPs in this report also were based on the NAVD 88.
Water-surface elevations at each of the monitoring stations were determined by reading the water level (stage) at the staff plate installed at each site. The elevation of each staff plate was determined from optical theodolite survey using nearby reference marks as control. Because staff plates have their own scale, the elevation of a water surface is obtained by converting stage value to differential GPS elevation.
Permanent land-surface BMs used for this study were established by the LVVWD, Stantec (on contract with SNWA), and the USGS. BMs consist of either brass plates, steel bolts, or rebar set in concrete structures or survey-grade earth anchors. Photographs and descriptions of selected BMs used for this study are included in Appendix B and table 2, respectively. The locations of these BMs were selected on the basis of the long-term stability of the area or of the structure a BM was installed on and the proximity of each BM to each of the monitoring sites. The location of each BM is shown in figure 3.
Elevations and latitudes and longitudes for all the BMs in the study area were determined by applying DGPS surveying methods. The surveys consisted of setting up survey-grade DGPS equipment over each BM and processing the data through a National Geodetic Survey (NGS) web site. Equipment used by the LVVWD and Stantec and by the USGS for these surveys were the Trimble 5700™ and Ashtech ZExtreme™, respectively. According to manufacturer specifications, the relative accuracies of these units are 1 cm in the horizontal and 2 cm in the vertical.
The DGPS surveys of the study area ranged from just over 2 hours to almost 24 hours (table 2). When completed, the survey data were sent to the NGS Online Positioning User Service (OPUS) web site at http://www.ngs.noaa.gov/OPUS/. The GPS data files were then processed by OPUS using NGS computers and software to determine a BM positioning. The returned product, called the NGS OPUS Solution Report, lists the vertical datum, horizontal coordinates, and other related information. The accuracy of the OPUS corrected data is included with each individual sheet (Dave Doyle, National Geodetic Survey, oral commun., 2004). The vertical datum and horizontal coordinates for each BM are summarized in table 2. Copies of the OPUS Solution Reports are included in Appendix C.
BMs at Jones Spring Box (WSBM-1), Baldwin Spring (WSBM-3), and Warm Springs Road near Iverson flume (WSBM-4) are included in table 2 and figure 3 but were not used to determine RM elevations at USGS monitoring sites for this study. They are included in this report to document their location in the event other BMs are destroyed.
Following the installation and survey of the land-surface BMs in the Warm Springs area, permanent RMs or RPs were established at each of the current USGS monitoring sites. Existing or new RMs established at continuous-record stream gages typically included brass monuments or anchor bolts embedded in concrete-filled areas or on other stable structures adjacent to the site.
For the partial-record stations, staff plates were installed in the channel reaches where discharge measurements are periodically made. If there was no existing RM near the periodic site, fence posts were hammered into both banks of the channel. Where staff plates could not be installed, such as at site M-10, a RP with a 3/8-in. bolt was set into the vertical headwall of the culvert. The elevation of the water surface was then determined by measuring from the RP to the water surface using a graduated tape or ruler. Elevations and descriptions of the RMs and RPs for each site are given in table 3. Photographs of selected RMs and RPs are included in Appendix B for each site.
In addition to the RMs and RPs, optical theodolite surveys were done at each staff plate and at the top corners of each flume or weir. These additional points were surveyed to document the current conditions of existing structures and to help track changes that may occur in the future. This information also is included in table 3.
Optical theodolite surveys were used to transfer BM datum (NAVD 88) to the permanent RMs and RPs established at all the current monitoring sites. A detailed description of the concepts and procedures of optical theodolite surveying is given by Kennedy (1988). The identification number of the BM used for each survey is shown in table 3. The approximate locations of the BM relative to the USGS monitoring sites are shown in Appendix B.
The equipment used to complete the optical theodolite surveys included an engineer’s automatic, or self-leveling, instrument and a “Frisco-style” aluminum rod. The precision and accuracy for each survey were in accordance with procedures described by Kennedy (1988).
To ensure the accuracy of BM datums and coordinates, OPUS Solutions for each DGPS survey were run using the NGS GEOID configuration (1999 and 2003) for computing orthometric heights. In addition, three of the BMs—Jones Spring Box, Baldwin Spring, and L.D.S. gage were resurveyed by one of the other agencies. OPUS solution reports for these three sites are included in Appendix C.
LVVWD and Stantec surveyed Jones Spring Box at Apcar on February 6, 2004, and Baldwin springs near the pump house on May 5, 2004. Differences between the Jones Spring Box surveys were 2 cm in the vertical (orthometric height) and 3 mm in the north horizontal (UTM coordinate). Differences between the Baldwin Spring surveys were 1.4 cm in the vertical, 5 mm in the north horizontal, and 4 mm in the east horizontal (see OPUS reports, Appendix C).
Muddy Spring gage at L.D.S. Farm was surveyed by LVVWD and USGS on May 14, 2002, and June 2, 2004, respectively. Differences between the two surveys were 5 mm in the vertical, 2.1 cm in the north horizontal, and 5 mm in the east horizontal.
To ensure the accuracy of each optical theodolite survey, a series of operational checks were made on the instruments and rods. Visual inspections of equipment were made daily when in use, and peg tests were done to determine if instruments were in proper adjustment. A two-peg test was made before and after each series of levels. A complete description of the two-peg test procedure is given in a report by Kennedy (1988). At the same time the surveying instrument was peg-tested, the rod was checked by comparing it against an engineer’s ruler.
On August 17, 2004, water levels were obtained from the staff plates at all the USGS monitoring sites. The only exception was the water level at an unnamed spring-fed pool near well L.D.S Well East (fig. 2). The water-surface elevation of the pool which is located about 100 ft east of the well was measured using the optical theodolite survey for the BM at the well. In early July 2004, flow was observed to be discharging from several pools in the area and flowing into the Muddy River. By August 17, 2004, most of the pools had completely dried up and water no longer discharged from the area. On October 1, 2004, water was observed to be emanating from the surveyed well pond and other nearby pools and discharging to the Muddy River.
The elevation of each water surface was adjusted to NAVD 88 using the results of the differential stadia surveys. The location and water-level elevations of the readings made on August 17, 2004, are shown on figure 2 and included in table 4.
U.S.
Department of the Interior | U.S. Geological
Survey
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