What annual flood-day totals leave out

Open full-size comparison chart

A day contributes one to the annual total whether the gauge exceeds its threshold briefly or at many hourly sample times. In this year, Annapolis had 53% as many threshold-exceedance days as Sewells Point but 93% as many above-threshold hourly readings. Annapolis and Ocean City Inlet had similar day counts, but Annapolis had more than twice as many above-threshold hourly readings.

The original contribution is this matched-year, three-station comparison, its run-length analysis and its concentration checks. The broader idea that frequency and duration are different is established: Annapolis already publishes flood frequency and duration information using a different local threshold. This analysis is not a discovery that NOAA day counts are wrong. [4]

Annapolis had a maximum run of 21 consecutive above-threshold hourly readings, from 03:00 to 23:00 UTC on October 30, 2025. Those sample times span 20 elapsed hours, not 21 proven continuous flood hours. The corresponding maxima were 9 consecutive readings at Sewells Point and 6 at Ocean City Inlet.

Open full-size episode timeline

The annual contrast is concentrated, rather than evenly spread through the year. October 30–31 accounts for 27 of Annapolis’s 69 above-threshold hourly readings, or 39%. Adding October 2 brings the total to 42, or 61%. These are calendar windows, not independently classified storms. The analysis does not attribute the readings to a particular weather system or estimate a trend from one year.

The useful question is what a community needs to know beyond an annual day total: whether high-water readings occur in brief clusters or persist across many sample times. Local records can then establish what actually happened on roads and waterfronts. They cannot be inferred from the gauge alone.

For example, Ocean City’s October 14, 2025 storm recap reported road closures south of Route 50, but no significant structural damage or injuries. That is evidence of disruption during that event, not a duration estimate for those closures. Annapolis’s August 2026 City Dock update distinguishes tidal backflow from rainfall flooding and describes ongoing resilience work. Neither source establishes that a particular gauge reading caused an impact, nor that this analysis measures project effectiveness. [5, 6]

Moving each NOS threshold down or up by one centimeter leaves Annapolis with more above-threshold readings than Ocean City Inlet and a longer maximum run than either comparator. Annapolis ranges from 60 to 78 readings and a maximum run of 20 to 28; Sewells Point from 71 to 83 and a maximum of 9; Ocean City Inlet from 28 to 37 and a maximum of 6. Exact run lengths are threshold-sensitive. The near-equality between Annapolis and Sewells Point is a descriptive observation, not a formal equivalence test or risk ranking.

1. [1] NOAA CO OPS Data API

   hourly_height for stations 8575512, 8638610 and 8570283; water_level for the Annapolis six-minute check. Parameters and exact URLs are in sources.json.

2. [2] NOAA CO OPS Metadata API

   Station floodlevels.json, units=metric. Threshold snapshot, not a historical threshold series.

3. [3] NOAA Derived Product API

   htf_daily with explicit metric/STND parameters, plus the 2025 meteorological-year annual product as reference.

4. [4] City of Annapolis Flooding Data

   Existing frequency and duration treatment uses a local 3.0 ft MLLW threshold. Do not merge those totals with this NOS-threshold comparison.

5. [5] Town of Ocean City storm recap

   October 14, 2025. Corroborates disruption during the event, not closure duration.

6. [6] City of Annapolis City Dock update

   August 19, 2026. Context on tidal backflow, rainfall and construction; not evidence of project effectiveness.

7. [7] NOAA Coastal Inundation Dashboard

   Explains differing NOS and NWS flood thresholds.