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# To John Howlett!! RMA2 reflection/absorption BC?

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John Howlett!!

I knew you are familiar with SMS very well through this forum,may i ask you some questions?

RMA2 was employed to simulate the tidal current in the estuary, in the downstream, water elevation BC was used,

but in the upstream nearby the estuary, no precise flowrate data. The reflection/Absorption BC was recommanded in the RMA2 mannual.

but how to specify the BRA card? mainly about the SSWSE ,TAREA and ACIX. thanks!!

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Are you able to extend the domain further upstream? If you are, I believe this would be preferable to using the reflection/absorption BC. The user manual also offers the warning, "This boundary condition has been found to work far better when applied to a one-dimensional element rather than to a two-dimensional element."

Although the reflection / absorption boundary condition does not require transient water surface elevation data (how the water surface elevation changes with time), it DOES require other information, such as the steady state water surface (SSWSE) and flowrate (AC1X) to be known / specified. As stated in the manual, SSWSE is the mean value about which the progressive or reflective wave crossing the boundary oscillates.

You said no precise flowrate data is available for the upstream boundary. In this case, I don't think you will be able to use the Reflective / Absorption boundary condition since flowrate information is required as part of the specification of a reflection / absorption boundary condition.

From page 145 of RMA2 user manual

VARIABLE, VALUE, DESCRIPTION

AC1X -, 0, + Average (net) river discharge ft3/sec or m3/sec, (Variable Qo in the equation below).

BETA 0 to 1 Reflection/Absorption coefficient used to calculate A2 in the rating curve equation below. BETA = 1 specifies total reflection at the boundary. BETA = 0 specifies total absorption at boundary.

SSWSE -, 0, + The steady state solution for the water surface elevation at the first node on the specified continuity line number. If SSWSE is less than or equal to zero, RMA2 will incorporate the steady state solution (Eo) before advancing to dynamic. Note: If Hotstarting a steady state run, use a value of –1. If Hotstarting a non-steady state (dynamic) run, you must supply SSWSE with at least 4 significant figures past the decimal.

THETA -, 0, + Direction of flow into the mesh in radians measured counterclockwise from the x axis. Note: The program adjusts the boundary directions to maintain flow parallel to the edges for nodes at the ends of the continuity line.

TAREA 0, + Surface area of tidal storage beyond the boundary. Used in defining standing wave flows. TAREA is the maximum tidal discharge divided by the maximum fall rate of water surface for a given time step. TAREA = Velocitymax * Width * Depth / Rmax Rmax = Δ WSELV / (ΔT * 3600)

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thanks for John Howlett's reply, Now I can get the water surface elevation data of the upstream, also a tidal current data near the upstream(but just 26 hours), Can I use the reflection/absorption BC in this situation? or maybe should I employ other modules in SMS? such as ADCIRC.

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I believe ADCIRC may be more stable when wetting and drying is occurring. However, if you want to use your hydrodynamic solution to run RMA4, you will need an RMA2 solution. It sounds like you have most of the data you would need to use the reflection / absorption BC - can you also get TAREA? You may also want to consider extending the mesh if possible so you don't need to use a reflection / absorption BC. It is more difficult to get a solution from models with wetting / drying - I also believe the reflection / absorption BC can be difficult, and it is hard to know what value to use for BETA.

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