**PILS: Low-Cost Water-Level Monitoring**

Samuel Russ, Bret Webb, Jon Holifield and Justin Walker *University of South Alabama United States of America* 

## **1. Introduction**

136 Environmental Monitoring

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The estuarine environment is important both to global ecology and to human economy. Estuaries are the place where freshwater meets saltwater, and so they typically contain a bounty of marine species, and are essential to the life cycle of many marine organisms. For similar reasons, they often contain sea ports and carry commerce of great value.

In order to study estuaries in more detail, we have developed two sets of low-cost sensors using off-the-shelf technology combined with innovative new low-cost circuits. The first, nicknamed "Jag Ski", is a highly mobile water craft for navigating estuarine and littoral areas and providing real-time data. The second, named "PILS", is a network of stationary sensors for making long-term water-level measurements. This paper describes the construction of both, along with actual measurements.

#### **2. Survey of literature**

Sensing the environment can be carried out through remote measurements (e.g. satellites (Villa & Gianietto, 2006)) and through in situ measurements (e.g. wireless sensor networks (O'Flyrm et al., 2007; Thosteson et al., 2009)). Both have been demonstrated successfully as means of measuring characteristics of water.

An example of one real-time water-sensor architecture is the Land/Ocean Biogeochemical Observatory (LOBO) system developed by Satlantic and the Monterrey Bay Aquarium Research Institute (MBARI) (Comeau et al., 2007; Jannasch et al., 2008) and has been installed in the field (Sanibel-Captiva Conservation Foundation, 2009). Others include the Ocean Observation Initiative (OOI) (Frolov et al., 2008; National Research Council, 2003; U.S. Commission on Ocean Policy, 2004), NOAA tide gauges for storm surge (Luther et al., 2007), and sonar-based water-level measurements (Silva et al., 2008). Specific to environmental monitoring in the coastal ocean, mobile field assets typically include profiling floats (Roemmich et al., 2004), autonomous underwater vehicles (AUVs) (Rudnick et al., 2004), and unmanned underwater vehicles (UUVs) (Freitag et al., 1998; Frye et al., 2001).

This work is in line with these earlier systems. We have adapted the mobile sensor platform to a highly maneuverable manned platform to navigate shallow-water areas proficiently. The sensor network is designed for relatively low cost and for unattended measurements. It also contains novel sensors for pressure and salinity.

PILS: Low-Cost Water-Level Monitoring 139

developed to meet broader goals and objectives in the area of coastal, water resources, and

The Jag Ski contains a SonTek/YSI RiverSurveyor M9 Acoustic Doppler Current Profiler (ADCP) with an integrated Real Time Kinematic Differential Global Positioning System (RTK DGPS) for georeferenced measurements (Fig. 2). The M9 ADCP has a profiling range of 6 cm to 40 m, and is capable of measuring velocity magnitudes up to 20 m/s. The resolution of the velocity measurements is as low as 0.001 m/s, and vertical bin sizes can be as small as 2 cm, or as large as 4 m. The horizontal resolution of the samples is a function of the reported sample rate (generally 1 Hz) and vessel speed (preferably equal to or less than the water velocity). A nominal speed of 1 – 2 m/s is maintained when using the M9 ADCP

on the Jag Ski, so a typical horizontal resolution is, accordingly, 1 – 2 m.

Fig. 2. SonTek/YSI RiverSurveyor M9 ADCP and RTK DGPS base station.

transitioning between the two extremes.

The M9 ADCP contains a dedicated 500 KHz vertical beam for depth measurements and bottom tracking, four slanted 1 MHz beams for sampling in deeper water, and four slanted 3 MHz beams for sampling in shallower waters (Fig. 3). This dual-frequency functionality is unique in the ADCP market, and along with its integrated GPS system for vessel-corrected measurements to account for the moving reference frame, makes it attractive for applications in Mobile Bay (Fig. 4). The bay is a broad, mostly shallow (< 4 m), drowned river mouth estuary that is incised by a navigation channel dredged to a maintenance depth of about 15 m. The depth of the channel in the main entrance to Mobile Bay can reach 20 m or more, and is flanked to the west by a broad, shallow area with depths less than 3 m. The dual frequency M9 ADCP performs well when

Aside from the technical capabilities of the RiverSurveyor M9 ADCP, the instrument comes with a well-developed, integrated software package for setup and data collection. The RiverSurveyor Live (RSL) software is loaded on the onboard PC, and is fully interactive using the touch screen LCD display. Some very helpful features of the software include dynamic icons that quickly report the status of various systems, like GPS and bottom

environmental engineering.

This work is motivated by the fact that computer models of estuaries need refinement. For example, there is disagreement whether wind forcing or river discharge dominates the dynamics of Mobile Bay (Schroeder & Wiseman, 1986; Kim et al., 2008). Data obtained using the sensors will be used to parameterize a linear approximation of a static momentum balance of the estuary (Van Dorn, 1953) to improve simulation and forecasting accuracy.
