Tidal Harmonics Boundary

A tidal boundary is a head-time boundary which can be used as an alternative to a Head Time Boundary unit, by using numerical methods based upon the Admiralty Tide Tables and the Constituent Harmonics of the tide for a given location and date.

Data

Theory and Guidance

The Tidal Harmonics Boundary calculates the tide levels at a specified time and location from a set of up to 28 tidal constituents derived from the Admiralty Tide Tables (time zone referenced by Greenwich Mean Time (GMT)). The calculations are based on the Admiralty method of Tidal Prediction which uses the harmonic constants and table of tide angles and factors documented in the Admiralty Tide Tables.

In many estuaries, tidal properties can be an important factor to both the hydrodynamics and the water quality. A representation of the tides is therefore required if an understanding of the system is to be gained. A tidal wave can be represented mathematically by a combination of sinusoidal curves of varying phase and amplitude, namely the tidal constituents.

A tidal boundary is a head-time boundary which can either be generated by analyzing field data and entering the data as a Head Time Boundary, or by using numerical methods based upon the Admiralty Tide Tables and the Constituent Harmonics of the tide for a given location and date.

From version 4.0 onwards, it is possible to specify a varying mean sea level rate rise, to enable long term simulations to take into account sea level rise scenarios.

The time at which the tidal boundary is effective from may be specified in the Tidal Boundary (TIDBDY) unit itself, although it is preferable to set this in the event file by using real ("Absolute") times in the run forms interface. An option is available in the TIDBDY form ("Override date with event time") to toggle this option. If checked, the date/time is obtained from the event file. The recommendation to use event times is made so that the TIDBDY unit will not need changing if the simulation date/time changes.

When loading a data file into the Flood Modeller interface which contains a Tidal Boundary with the date/time set in the TIDBDY form, a warning dialogue box appears suggesting that the user uses an event date/time instead. There is also an option on this dialogue box to switch this warning off. The first such TIDBDY unit in the data file is highlighted, although it is then left to the user to change the field if desired.

Note: If "Event Times" are chosen, but no Absolute times are set in the event file, the times used will revert to those specified in the TIDBDY unit by default.

The Tidal Boundary unit is also compatible with the steady-state runs, although will use a single value, corresponding to the tide level at the real time equivalent to the time of simulation.

Equations

The total tide-raising force may be expressed as the sum of a number of cosine curves of different amplitude and frequency. The amplitude and phase of the contributory factors to tidal motion are known as tidal constituents, the principal of which are called M2 and S2 , relating to synodic tides of the Moon and Sun respectively; the subscript 2 indicates that they are both semi-diurnal constituents.

The contribution of every tidal constituent may be expressed as:

The quantities A and g form the tidal constants for a particular location. The tidal level at any particular time and location is given by the sum of the above expression derived for each tidal constituent.

Table of Major Harmonic Constituents

Semi-diurnal Constituents

Diurnal Constituents

Quarter-diurnal Constituents

Other Constituents

The storm surge component, S, is given by:

From version 4.0 onwards, it is possible to model more than one surge, with differing parameters, during a simulation.
Thus the water level at the Tidal Harmonics Boundary is given by:

(3) where: H = Water level (mAD [ft, US]) h = tidal component (m [ft,US]) S = surge component (m [ft, US]) X0 = Initial Mean Sea Level for the location (m AD [ft,US]) y  is  the current simulation time (as year, including factional part, e.g. 1992.5 denotes 0000hrs, 2nd July 1992),  yo,i is the year from which rate rise i applies,  ri is the MSL rate rise i (m/yr [ft/yr, US]), and the sum is performed over all i such that y >  yo,i

Datafile Format

LINE 1 - Keywords 'TIDBDY'

LINE 2 - Label

LINE 3 – z, nsurges, nrises

LINE 4.1 to LINE 4.nsurges [1] - sstart(i), sdur(i), samp(i), spower(i)

LINE 5 - x0

LINE 6 - hour, idat, iy, tidbdy_dates

LINE 7 - nh

LINE 8.1 to LINE 8.nh - ifharm, amp, phase

LINE 9.1 to LINE 9.nrises [2] – yrise(i), rrise(i)

Note: line 8 has a format width of 4 characters for ifharm and 10 characters for amp and phase (ie a4,2f10)

where

Label = Node label at boundary

z = Sea Bed Elevation (mAD) (not used in calculations)

nsurges – number of surges to be modelled during the simulation

nrises – number of different rate of sea level rises during the simulation

sstart(i) = Surge start time of i^th surge (hours from start of run)

sdur(i) = i^th Surge duration (hours)

samp(i) = i^th Surge amplitude (m)

spower(i) = i^th Surge cosine power (unitless)

x₀ = Mean Sea Level for the location, including the Seasonal Correction taken from Admiralty Tide Tables, vol II. (mAD)

hour = start hour of the tidal predictions in decimal on the 24 hour clock

idat = start date of the tidal predictions as day and month (eg for 21 July enter 2107)

iy = start year of the tidal predictions (eg 1995)

tidbdy_dates = 'EVENT' if times are to be taken from the ief file, if present; anything else, e.g. blank, if times are to be taken from the TIDBDY unit

nh = number of harmonics to be included

ifharm = Constituent name (left justified within a field width of 4 characters)

amp = Amplitude of Constituent (m)

phase = Phase lag constant (°)

yrise(i) – year from which the isea level rate rise occurs

rrise(i) - isea level rate rise (mm/yr)

[1] This (repeatable) line must occur at least once
[2] This (repeatable) line is absent if nrises=0