For our purposes, we will discuss tides as they affect navigation planning with a brief review of terminology.
Details of Tidal Theory is more in-depth than we need to explore here.
Terms and definitions important to understand regarding tides
These terms can sound complicated, but fear not; they are only here to provide a basic understanding of where they come from, but not necessary to memorize them. We will highlight the things to remember.
Truthfully, there is very little need to know all the datum measurements and the datum points. The key is understanding what you see on a chart as a depth and what it means as you navigate the area.
With that said, here are some of the terms you may come across:
The Mean is the average tide. It is a word used frequently in navigation.
Tides are measured from the surface of the water and the "tidal datum." All tides measured by the National Ocean Service (NOS) and the United States' National Oceanic and Atmospheric Administration (NOAA) use the mean lower low water (MLLW) as the Tidal Datum. We will explain in more detail what this means below.
It is important to make the direct distinction between "height of tide" and "depth of water." as these are not the same. Height of tide is the distance from mean lower low water (MLLW) and the water surface. Depth of water is the distance between the bottom of the surface.
Charted water depth is the distance between the bottom and the MLLW point.
Charted bridge clearance distances are measured from the Mean High Water (MHW) level and the bottom of the structure.
NOAA defines the Mean Lower Low Water (MLLW) as the average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch (The NOS and NOAA define the National Tidal Datum Epoch as a period of 19 years). For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.
This image provides a visual reference to tidal measurements used by NOS and NOAA.
So what does all this mean to us? Not much, but what we need to know is how deep is the water under us and how much clearance to we have above us.
The chart depths we see printed (also called soundings, because depths were actually measured at some point) are the depths of the water we can expect when the tide level is zero at that time and place. If the chart shows 15 ft, and the tide is currently running at 5 ft, then we should expect the water to be 20 ft deep at that time.
So, with the exception of when there is a negative tide, we can always expect the water to be deeper to some degree than what is listed on the chart. You can have confidence that all nautical charts you read work this same way.
From the NOAA Tidal Predictions Page you can also access station data pages and much more detailed information allowing you to gather tidal information for planning purposes.
The movement of tides causes sea level rises (also known as floods) and sea level falling (also known as ebbs) and produces tidal currents, one of the different types of currents we discuss in the next sections.