Tidal Characteristic Numbers of Current

Graphical representation of characteristic numbers of current velocity (with German text only). This graphical representation is also available in Encapsulated PostScript format.

Graphical representation of characteristic numbers of volume fluxes (with German text only). This graphical representation is also available in Encapsulated PostScript format.

Graphical representation of eulerian residual current and residual path (with German text only). This graphical representation is also available in Encapsulated PostScript format.

Motivation

Computation as well as graphical display of several tidal characteristic numbers of current add aditional insight into tidally induced dynamic processes which are reflected in the current velocity and its related derived quantities. This type of tidal analysis should therefore go hand in hand with the calculation of tidal characteristic numbers of water level.

• The computation is carried out separately for the periods of flood and ebb current duration. Therefore it yields information about the asymmetry of the tide-induced currents. An asymmetry can result in different durations of ebb and flood currents on the one hand side and on the other hand side in different maximum flood and ebb current velocities. This imbalance might be a result of different speeds for the fall and rise of the water level which causes a distinguished acceleration of the flow. Different values of the maximum flood and ebb current velocities may be a main cause for residual transport (up- or down-estuary) of bedload material or suspended sediments.
• Unequal slack water times of flood or ebb current are mainly due to the varying significance of the various dynamic driving forces (e.g. batropic and baroclinic pressure gradient as well as bottom friction versus inertia forces). They mainly depend on water depth as well as on the precise location of the water volume in the water column.
• Different times of slack water dislocation are related to the character of the tidal wave. In a purely propagating deep water wave there exists a phase shift of approximately 90 ° between current and water level elevation. Whereas in shallow water, bottom friction and reflexion of the tidal wave causes a much smaller phase shift. Remember: for a purely standing wave the phase shift is zero.
• The periods of slack water are good indicators for the times of mild current climate. During these periods suspended sediments may settle to the bottom and become deposited. Unsymmetrical periods of slack water can result in net up- or down-estuary transport of suspended sediments.
• Tidal characteristic numbers of current which are representative for the whole tidal cycle may give some insight into the residual transport paths for the water masses travelling along an estuary. For example the Eulerian residual current is a first order approximation to the respective Lagrangian one. It should be therefore interpreted with some care.

Automatic analyses for all of the above mentioned tidal characteristic numbers of current are carried out for 2D- as well as for 3D-data by the program TDKVF.

Definition of Tidal Characteristic Numbers of Current

Flood Current

Maximum Flood Current Velocity

Maximum value of the flood current velocity during the flood current duration

Mean Flood Current Velocity

Mean value of the flood current velocity during the flood current duration

Flood Volume

Volume of water, which flows through a certain cross section during the flood current duration

Example: Flood Volume for the Inner Außenweser

Flood Current Duration

Timespan between slack water time of flood current and the next slack water time of ebb current

Slack Water Time of Flood Current

Official time of slack water occurrence at the end of a flood current duration

Slack Water Duration around Slack Water Time of Flood Current

Timespan around slack water time of flood current for which the flow velocity does not exceed a certain value

Slack Water Dislocation of Flood Current

Timespan between high water occurrence and slack water time of flood current

Differences of Time of Maximum Flood Current between Different Locations

Differences of time of maximum flood current occurrence between different locations within the area of data analysis

Example: Time of Maximum Flood Current for the Inner Außenweser

Flood Path

Vectorial summation of the path length for a certain location during the flood current duration

Ebb Current

Maximum Ebb Current Velocity

Maximum value of the ebb current velocity during the flood current duration

Mean Ebb Current Velocity

Mean value of the ebb current velocity during the flood current duration

Ebb Volume

Volume of water, which flows through a certain cross section during the ebb current duration

Example: Ebb Volume for the Inner Außenweser

Ebb Current Duration

Timespan between slack water time of ebb current and the next slack water time of flood current

Slack Water Time of Ebb Current

Official time of slack water occurrence at the end of a ebb current duration

Slack Water Duration around Slack Water Time of Ebb Current

Timespan around slack water time of ebb current for which the flow velocity does not exceed a certain value

Slack Water Dislocation of Ebb Current

Timespan between high water occurrence and slack water time of ebb current

Differences of Time of Maximum Ebb Current between Different Locations

Differences of time of maximum ebb current occurrence between different locations within the area of data analysis

Example: Time of Maximum Ebb Current for the Inner Außenweser

Ebb Path

Vectorial summation of the path length for a certain location during the ebb current duration

Tidal Cycle

Residual Current

Vectorial summation of the flow velocities for a certain location for a whole tidal poriod

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