Tidal elevation is a major concern affecting different aspects of life in the Tidewater area. Students and faculty alike have experienced trouble navigating through Norfolk due to flooding of streets and walkways. The rise of the water level can be attributed to several causes including flooding due to storms, tidal waves, sea level rise due to icecap melting, and others. This paper will present and discuss the history behind leveling, current issues related to tidal elevation, and methods for tidal leveling.
Tide or tidal effect can be defined as the: ‘alternate rising and falling of the surface of the ocean and of water bodies (as gulfs and bays) connected with the ocean that occurs usually twice a day and is the result of differing gravitational forces exerted at different parts of the earth by another body (as the moon or sun)’ by Merriam-Webster. This effect is not new to civilization but has been an ongoing effect on the bodies of water since Earth was created. Tidal effects can occur ‘semidiurnal (semi-daily), mixed, and diurnal (daily)’ (NOAA, 2001). In the semidiurnal effect, the earth rotation with the moon lapse a 24.84 hour day. Throughout this period, the tide experiences two high and two low levels. For the mixed effect, it can be observed that the two high levels differ and the two low also differ in level height. Diurnal type corresponds to one high and one low water level per tidal day. Other tidal effects also exist due to orbital paths and universal motions 4.
Tidal effect is significant in cities with low elevations and that are located along the coast of seas, estuaries, rivers, and other bodies of water. Norfolk in particular is surrounded by a major estuary (Elizabeth River) and is in constant threat of sea level rise due to tidal effects combined with storms, precipitation, and hurricanes1. To mitigate damage, private property owners are encouraged to build property accounting for sea level rise while governments invest in strengthening infrastructure and implementing storm barriers. For any mitigation to take place, for any implementation to occur, research must be conducted, and data must be produced.
The National Oceanic and Atmospheric Administration (NOAA) is a branch of the United States Department of Commerce that conducts much of the oceanic research and data tracking along with the National Ocean Survey (NOS). Their work extend to many fields in the civil engineering discipline to including environmental, coastal, structural, and foundation. NOAA is a great source for locating benchmarks, datum, and even instructions and learning material for surveying 2.
Tidal elevations are measured throughout a period of time. Datums and benchmarks serve as a standard known elevation/location for other measurements to be done. Sea level is constantly changing, therefore some other measurements are obtained and calculated. These measurements enable us to make predictions on critical rise of water levels to mitigate grave damages 3. Measuring water levels greatly assists the engineer in designing solutions, mariners in their navigation, scientists in their studies of marine life and effects, and the public in assessing environmental hazards due to sea level rise. For this to be accomplished, measurements must be accurate, up to date, and uniform throughout all departments 4.
Tidal elevation are measured in the following stages: Highest Astronomical Tide (HAT), Mean Higher High Water (MHHW), Mean High Water (MHW), Diurnal Tide Level (DTL), Mean Tide Level (MTL), Mean Sea Level (MSL), Mean Low Water (MLW), Mean Lower Low Water (MLLW), and Lowest Astronomical Tide (LAT). These measurements are standard in the United States and are used to determine other values such as the Great Diurnal Range, Mean Range of Tide, Mean Diurnal High Water Inequality, and others alike. In turn, these values apply in monitoring the flux in tides, currents, territorial geography, oceanography, and water levels in general for marine projects and operations.
Image from the Coastal Engineering Technical Note. USACE.
Mean Seal Level, Mean Low Water, and Mean Lower Low Water are usually used as datums for measurements. These datums are linked to the National Geodetic Vertical Datum. The National Ocean Survey is tasked to update these information on a yearly basis. In turn, the data is used to make predictions on tidal rise and fall. Datums are calculated after measuring the rise and fall for a year, but the USACE allows data from three good months of collection. Furthermore, it is important to note that all datums reference back to NGVD 1929. This Datum is the initial reference point for all vertical control datums in the United States.
Datums and benchmarks are determined by methods common in the surveying field. The main method demonstrated by NOAA is the differential leveling, where rods are read to determine elevations. Benchmarks are determined, and other elevations are based off the benchmarks. It is notable that the National Water Level Program has standards published to conduct leveling on water. Another method for determining water levels is the use of tidal gauges. These can be either analog or digital, and can be connected to a tracking mechanism/system to collect continuous data as the water level fluctuates. Historically, surveyors had to utilize Analog Digital Recorders (ADR) with punched-paper-tape to compute their data. In this day and age, high accuracy computer systems are in place to work along digital imaging, satellite photography, and others to create databases. One in particular is the Next Generation Water Level Measurement System (MGWLMS) by NOS. This system uses automated functions in conjunction with platforms, terminals, satellites, processors, and sensors to create life-feed data collection 4.
Tidal elevation is constantly changing, and it is just another supplement to the sea level rise due to ice caps melting, storms, hurricanes, precipitation, and other natural and/or manmade influences. Sir Francis Bacon said ‘knowledge is power.’ Data is collected to better understand the changing atmosphere and prepare for an uncertain future. As engineers, we can benefit from more accurate data to create a better approach in solving the civil problems we face. Various methods are laid out, and information is available for the taking.