Flooding Data

What is Mean Lower Low Water?

Mean Lower Low Water (MLLW) as defined by NOAA as the average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch.  All data was collected from station 8575512 Annapolis, MD.  This benchmark is located within the U.S. Naval Academy’s grounds. The gauge and operators are located on Santee Pier in the Oceanography Laboratory Building.  All data used for this webpage is sourced from NOAA’s Tides & Currents.

According to NOAA a minor flood stage classification in Annapolis occurs when the MLLW level is ≥ 2.6 ft.  When the MLLW level is measured to be 3.3 ft. or more Annapolis is considered to be in a moderate flood stage.  Independent of NOAA, the Annapolis Office of Emergency Management categorizes any MLLW level equal to or greater than 3.0 ft. as significant.  

Read more about water levels, historic crests and flood impacts in Annapolis.

Annapolis MLLW Flood Data for 2020

Minor HoursSignificant HoursModerate HoursTotal Hours
Winter Hours126422
Spring Hours33131662
Summer Hours428050
Fall Hours496560
Annual Hours1363325194

*Analysis and Figure by: MIDN 2/C Julianne Barkholz, USN, OEM Intern 2021

Figure 1. Provides individual and cumulative flood hours per classification and season. In 2020, downtown Annapolis experienced a total of 194 hours of flooding.

Bar Chart for Flood Hours

Figure 2. Depicts minor (≥ 2.6ft), moderate (≥ 3.3 ft) and total flood hours for the past five years.  An increasing trend is visible from 2016-2019.

Pie Chart for Flood Hours


Figure 3. Shows that more than half of the Significant flooding (a MLLW level ≥ 3.0 ft.) hours in the past 5 years occurred during the Fall season.  Spring to fall, March to November, accounts for 87% of all Significant flood hours.

Winter includes December, January and February and Spring is March, April and May.  Summer is considered June-August and Fall is September-November. 


Significant Flooding in Annapolis


For more details and information on flooding in Annapolis from 2016-2020 please download Significant Flooding Events in Annapolis 2016-2020 and Significant Flooding Days in Annapolis 2016-2020 

The Significant Flooding Events chart states the event start and end date, duration and maximum MLLW level. A significant flood event can be described as consecutive hours in which the MLLW levels are consistently ≥ 3.0 ft.  A flood event may occur multiple times in a single day or even last multiple days. 

Whereas any day that significant flooding occurred is classified as a significant flood day.  The Significant Flooding Days chart displays data highlighting the specific day in which significant flooding occurred, the duration and maximum MLLW level. 

For example, on September 25th, 2018 Annapolis was significantly flooded from 3:00am-7:00am and again from 3:00pm-7:00pm.  As a significant flood event, technically two events occurred.  The first event was in the morning 5 lasting hours with a maximum MLLW level of 3.29ft.  The second event occurred in the evening for 5 hours with a maximum MLLW level of 3.28ft.  Whereas a significant flood day, September 25th, 2018 was flooded for 10 total hours reaching a maximum MLLW level of 3.29 ft.  

If you are interested in a specific event or date follow the Step-by-Step to Access and Utilize NOAA Tides & Currents (PDF) to plot verified water levels. You may find NOAA's Tides & Currents website here.