# Estimate a flooding probability¶

In this example, we estimate the probability that the output of a function exceeds a given threshold with the FORM method. We consider the flooding model.

## Define the model¶

```from openturns.usecases import flood_model
import openturns as ot
import openturns.viewer as viewer
from matplotlib import pylab as plt

ot.Log.Show(ot.Log.NONE)
```

We load the flooding model from the usecases module :

```fm = flood_model.FloodModel()
```

We load the joint probability distribution of the input parameters.

```distribution = fm.distribution
```

We create the model.

```model = fm.model
```

## Define the event¶

Then we create the event whose probability we want to estimate.

```vect = ot.RandomVector(distribution)
G = ot.CompositeRandomVector(model, vect)
event = ot.ThresholdEvent(G, ot.Greater(), 0.0)
event.setName("overflow")
```

## Estimate the probability with FORM¶

Define a solver.

```optimAlgo = ot.Cobyla()
optimAlgo.setMaximumEvaluationNumber(1000)
optimAlgo.setMaximumAbsoluteError(1.0e-10)
optimAlgo.setMaximumRelativeError(1.0e-10)
optimAlgo.setMaximumResidualError(1.0e-10)
optimAlgo.setMaximumConstraintError(1.0e-10)
```

Run FORM.

```startingPoint = distribution.getMean()
algo = ot.FORM(optimAlgo, event, startingPoint)
algo.run()
result = algo.getResult()
standardSpaceDesignPoint = result.getStandardSpaceDesignPoint()
```

Retrieve results.

```result = algo.getResult()
probability = result.getEventProbability()
print("Pf=", probability)
```
```Pf= 1.1322019024606214e-06
```

Importance factors.

```graph = result.drawImportanceFactors()
view = viewer.View(graph)
plt.show()
```