Modeling Strategies

DAMBRK is a very complex program, and therefore getting the results you desire will require some troubleshooting. This guide was put together to give you some basic ideas of how to handle your DAMBRK problems, as well as take you through some of the most common problems.

Modeling Strategies

1. What is your time step?

o If it is too large, you will miss the peak value. For example, let’s say you have a cross-section every 1/4 mile, but your time-step is 1 hour.

If you "sample" every hour, then it is very easy to miss the peak water surface elevation values.

2. What is the minimum base flow value for your inflow hydrograph?

o DAMBRK is a "wet model," meaning it must have some base flow in it. It cannot start up dry (i.e., no flow in channel).

o Note that the default error tolerances are:

Q = 100 cfs.
WSEL = 0.01 ft

Hence, if your base flow discharge is less than this, your model can easily blow up due to lack of flow in the model.

First, ask yourself, what is the widest channel bottom invert in the entire model?

If it is very wide, then you may have to artificially insert a V notch bottom and/or increase the minimum base flow specified for the upstream inflow hydrograph.

o For example, when trying to get a model up an running, experiment with increasing the base flow to 1,000 cfs.

o Does the DAMBRK model now "spin up?"

o If you increase the base flow that much, how much does this really skew (or affect) your model results?

o Try to determine this effect by increasing the base flow another 1000 cfs. How much did your peak water surface elevation change? Hydrograph attenuation change?

o If there was little to no change, run with it.

o Be careful changing the error tolerance (in the Boundary Conditions screen).

o For example:

WSEL = 0.1 ft
Q = 1000 cfs

3. What is the slope of the inflow hydrograph?

o Abrupt changes in inflow hydrograph can cause the model to experience a numerical shock, causing convergence problems.

o May require that you perform some smoothing of the inflow hydrograph.

4. Are there abrupt changes in channel top-width?

o Is it reasonable to assume the entire top-width as available for flow? Or would it be wiser to count some of this increased top width as storage?

o "Rule of Thumb" 50% decrease, 100% increase in cross-section to cross-section. Allow cross-section top-width to increase by 100% (i.e., 50 ft to 100 ft) or decrease by 50% (i.e., 100 ft to 50 ft) from section to section. Remainder is accounted by defining as storage.

Table 1: Using the "Rule of Thumb"

Prior
XS1 ---------------- XS2
Elev Width Storage - Elev Width Storage
100 0 0 - 100 0 0
110 10 0 - 110 10 0
120 20 0 - 120 70 0
130 30 0 - 130 300 0
140 40 0 - 140 500 0

After
XS1 ----------------- XS2
Elev Width Storage - Elev Width Storage
100 0 0 - 100 0 0
110 10 0 - 110 10 0
120 20 0 - 120 40 30
130 30 0 - 130 60 240
140 40 0 - 140 80 420

5. Are there additional (internal) flow structures in your model (i.e., additional dams and bridges)?

o If so, first try removing all structures (simply undefine them in the cross section description screen). You will need to define the DAMBRK model with Problem Specification 7.

o If model continues to fail, problem lies further in the model. Need to look at some of the other items discussed here to get that portion of the model running.

o Then insert the structures, one at a time--starting at the upstream end of the model, run the model and make certain results seem consistent. Troubleshoot each structure in turn.

6. Do you have bridges in your model?

o Try treating the bridge as a dam with a spillway rating curve designed to mimick flow through the bridge opening.

o Conservative approach--assume the bridge gets washed out totally, hence completely removed from the analysis.

7. Simplify, Simplify, Simplify

o Try:

o No hydrograph, simply a constant Q
o 1 Structure
o Shorter time step
o Less cross sections
o Undefine your structure failure
For example, WSEL at time of failure = 20,000 ft

8. Develop a procedure (or system) for troubleshooting your model.

o Get a notebook and document what you have tried.

o Save your files and note what you have done to change it from the last run.

o Saving stages of your development efforts allows you to immediately return to the prior stage that was working when you hit a dead end.

9. Try different problem options.

1 Structure: Problem Specification Option 1, 11 and 13

2 Structures: Problem Specification Option 12 and 14)

o Option 1

o This will not work when there is a concern about tailwater submergence, since DAMBRK computes outflow from the structure first, without any tailwater effect and then routes it (2 step process).

o Note that option 1 is very similar to option 13, simply respecify problem option in Project Description input screen

o Options 11 & 13, and 12 & 14

o Fully dynamic (does dam failure analysis simultaneously with routing), and hence can account for tailwater submergence affects on dam failure.

o Option 11 assumes a storage/elevation relationship to define reservoir volume.

o Option 13 requires additional upstream cross-sections to define reservoirs, due to a negative wave traveling upstream.