Bosham Tide Data Sources & Methodology

Predicted High Water and Low Water times and heights for Bosham come from the UK Hydrographic Office via the Admiralty UK Tidal API. These are the same official tide-table predictions mariners use, not scraped from a third-party website.

Bosham is a secondary port referenced to Chichester Harbour (Entrance). Heights are metres above Chart Datum. We request about seven days of events so the outlook table and the next-tide panel can look ahead.

Station: Bosham (secondary port) · Reference: Chichester Harbour Entrance · Classification: authoritative prediction

Admiralty gives us the peaks and troughs. Between those points we need a continuous curve, for “how high is it now?” and for drawing when the road crosses the flood level.

Where available, we take the tidal curve shape from Chichester Harbour (Entrance), the reference port for Bosham, and apply it to Bosham's predicted High Water times and heights. That curve is modelled: the shape comes from the entrance, not a sensor on Shore Road, so harbour geometry can nudge the real water a little.

When a full curve isn't available, we fall back to a simple harmonic estimate of the cycle (see flood windows below).

Parking advice is not “how many minutes until high tide.” It asks two questions: will this High Water reach the Shore Road flood level, and, if so, when does the water climb above and fall back below that level?

The flood level we use is about 4.0m Chart Datum. That figure is a local working threshold for Shore Road flood risk, not a published UKHO constant, so treat it as guidance and verify against what you see on the ground.

Flood windows are resolved in this order:

1. Modelled curve crossings : for the current cycle, we interpolate the times the curve crosses 4.0m either side of High Water.
2. Harmonic estimate : for future tides without a detailed curve, we model the cycle as a cosine around High Water with a mean semi-diurnal period of 745 minutes, then solve for the threshold crossings. A marginal 4.1m tide floods for a short window; a 5.3m spring floods for hours.
3. Fixed ±2 hours : only if the tidal range is unknown or degenerate.

Status then follows the clock: outside the window (and outside the hour before flood start) reads safe; inside the window reads do not park. We also keep the status dangerous while the previous High Water's flood window is still draining, so a falling tide doesn't briefly look safe while the road is still under water.

When predicted High Water is within about 0.3m of the flood level, we show a weather-watch note: wind and low pressure can tip a borderline tide either way.

Spring and neap are properties of the Sun, Moon and Earth, not of a height number on the dashboard. Springs happen near new and full moon, when solar and lunar tidal bulges line up. Neaps happen near the quarter moons, when those bulges partly cancel.

We calculate that geometry ourselves (no separate moon API). Solar and lunar ecliptic longitudes follow Meeus, Astronomical Algorithms (2nd ed.), chapters 25 and 47. Against published new/full/quarter moon times for 2026, the calculation is accurate to roughly three minutes, far tighter than a day-level spring/neap label needs.

Peak tidal range lags the Moon by a couple of days, the age of the tide. The UK National Tidal and Sea Level Facility puts that lag at roughly 36–48 hours after new or full moon. We use 2.5 days, which also fits Admiralty predicted High Water heights for Bosham and independent spring/neap calendars (below).

The spring/neap label is evaluated at local midday, so every tide on the same day shares one label. Springs and neaps are a condition of the day, not of a single High Water.

Checked against:

• tides.today 2026 peak-range calendar : our peak days match within about half a day on average (exact for Aug 15 spring, Aug 22 neap, Aug 30 spring).
• Channel Swimming & Piloting Federation tide bands : every computed peak falls inside their corresponding spring or neap window for summer 2026.
• tides.today moon-phase listing : that page labels the new/full/quarter moon day itself as spring or neap. We deliberately label the later peak-range day instead, because that is when Shore Road actually sees the biggest (or smallest) tides.

The coefficient uses the conventional 20–120 scale: the tidal range as a percentage of the mean spring range for Chichester Harbour (MHWS − MLWS = 4.0m). Around 100 is an average spring tide; around 45 is an average neap.

Range comes from the predicted High Water height minus the nearest Low Water height (or an assumed Low Water of 1.4m if Low Water is missing from the feed).

Mean tidal levels for Chichester Harbour (Entrance), metres above Chart Datum, from the Admiralty chart 5605 pilotage table (Chichester to Ramsgate):

Those published means feed the coefficient and the Low Water fallback. They do not decide the spring/neap label; that comes from lunar geometry.

The Shore Road flood threshold (~4.0m Chart Datum) is a local working value for this site. It sits interestingly close to MHWN: on a true neap, the road is much less likely to flood.

When configured, we can also show a live observed water level from the Environment Agency Real-Time Flood Monitoring API. That reading is observational: what a nearby gauge is measuring now, not a prediction, and the nearest gauge may not sit on Shore Road.

Parking status is driven by Admiralty predictions and the flood window, not by the live gauge. The gauge is context only.

The live dashboard only has official Admiralty predictions for about the next seven days. The Bosham tide tables for 2026–2030 use a separate open-source timing model from drtonyr/tide, fitted specifically for Bosham. It is not Admiralty data.

The model starts from the roughly 12 hour 25 minute tidal cycle, then adds seven sinusoidal terms so the highs follow the real deviations from a simple tide clock. Its published typical timing error is around ±9 minutes (one standard deviation), an estimate, not a guarantee. Weather can move the actual tide further.

Those tables list high-tide times only. The model does not produce reliable heights, so we do not show metres above Chart Datum and we do not use these long-range times for Shore Road flood warnings.

Source: open-source Bosham configuration in drtonyr/tide · Classification: independent timing estimate · MIT licence

Predictions are guidance. Weather, atmospheric pressure, surge, wind set-up and drainage can move water above or below the forecast. Always judge conditions on arrival. Do not rely solely on this service for safety-critical decisions.

Times on the dashboard are shown in Europe/London (GMT/BST). Internal calculations use UTC.

Built by Harvey Woods. Tide predictions © UK Hydrographic Office / Admiralty. This site is independent and not affiliated with UKHO, Admiralty or Chichester Harbour Conservancy.

Local organisations, walking guides, photographers, tourism sites and press can use this description:

BoshamTide is a free local tide resource for Bosham, showing Admiralty-based tide predictions and estimated Shore Road flood risk.