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Unstable water surface elevation

Mon, 02/03/2003 - 11:05pm — george
"Unstable water surface elevation on day 1.500 , negative surface layer thickness using minimum timestep at iteration 0 Segment, Water surface elevation, m Bottom elevation, m Elevation difference, m 24 269.000 267.210 1.790 25 269.000 267.210 1.790 26 269.000 267.210 1.790 27 269.000 266.910 2.090 28 268.946 266.910 2.036 29 268.866 266.910 1.956 30 268.970 266.910 2.060 31 268.960 266.910 2.050 32 269.470 267.810 1.660 33 269.425 267.210 2.215 34 268.186 266.610 1.576 35 268.822 266.910 1.912 36 268.280 266.910 1.370 37 269.397 267.210 2.187 38 268.922 267.210 1.712 39 268.866 267.210 1.656 40 268.900 267.210 1.690 41 268.586 266.910 1.676 42 269.000 267.510 1.490" "Computational warning at Julian day = 1.500 timestep = 0.000 sec Computational warning at Julian day = 1.500 at segment 34 timestep = 1.000 water surface deviation [Z] = 0.524 m layer thickness = -0.224 m" Hi all, from the above error, can anyone tell me what really went wrong? thks Your sincerely, george [addsig]
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Hydrodynamics
Sat, 02/15/2003 - 12:40am — Scott

George Since this instability occurred at iteration 0, your initial water surface elevations must be out-of-whack. Were there any warnings in the pre.wrn file? Scott[addsig]
Mon, 02/17/2003 - 7:24pm — george

hi scott, "No structures defined [NSTR=0] for downstream flow boundary in branch 1" this is the warning in the pre.wrn.npt. can i ask what do the model mean by structure in this case? and what may be the possible error that i encountered below? one more thing to ask, i assumed that the inflow and outflow discharge of a waterbody should be balanced, is that correct? one problem that i faced is that the water elevation keep increasing throughout the simulation, does this indicated that there is error in the outflow discharge? i am currently working on a waterbody where the inflow is through the 2 tributaries and not from the branch, where the branch inflow is equal to zero, with this i set the total inflow equal to the outflow at the end of the branch, is this workable? for doing estuary, is it neccessary to input the external downstream head elevation file? with regards, george[addsig]
Thu, 02/27/2003 - 1:00am — Scott

George For 'No structures defined' - this just means that you do not have an outlet structure, such as turbine flow through a dam. You can ignore this warning. If the water surface kept increasing, you must have flow going in but less or none leaving your domain. You can set inflows=outflows - that can work - but if this is an estuary, you generally would have a flow and then a downstream head boundary condition - so yes you would need an external head dynamic water level (and temp and water quality) file. The user manual shows how to set these files up. Scott[addsig]