# Compute ordinariness of preserved strata¶

Several studies have discussed the “ordinariness” of stratigraphy  , which describes the observation that most fluvial outcrop exposures seem to record relatively normal river conditions, rather than extreme events.

This assertion of “ordinariness” has been examined in the context of vertical excursions of bed elevation (whereby more extreme excursions represent extreme events; ). We can go further than this with the pyDeltaRCM model outputs, which co-locate bed elevation change with velocity, in essence recording the actual magnitude of a flow event rather than bed-elevation change. With this paired information we can interrogate the velocity of deposits in the stratigraphic record.

```import numpy as np
import deltametrics as dm
import matplotlib.pyplot as plt

golfcube = dm.sample_data.golf()
golfcube.stratigraphy_from('eta')

def pick_velocities(sect):
"""Extract data.

Utility function to grab mean velocity of a section, returning the value
from the total section, and just the preserved section.

Note that both extractions are limited to where sediment has been
deposited/eroded in the model domain.
"""
_whr = [sect['eta'] != sect['eta'][0, :]]
_a = np.nanmean(sect['velocity'].data[tuple(_whr)])
_s = np.nanmean(sect['velocity'].strat.as_preserved().data[tuple(_whr)])
return _a, _s

# preallocate
_ys = np.arange(10, 90, step=2)  # sections to examine
_mall = np.full_like(_ys, np.nan, dtype=float)
_mstrat = np.full_like(_ys, np.nan, dtype=float)

# loop through all of the sections defined in _ys
for i, _y in enumerate(_ys):
_s = dm.section.StrikeSection(golfcube, distance_idx=_y)
_mall[i], _mstrat[i] = pick_velocities(_s)

# make the plot
fig, ax = plt.subplots()
ax.plot(_mall, 'b-', label='all values')
ax.plot(_mstrat, 'r-', label='preserved only')
ax.set_xlabel('distance from channel inlet')
ax.set_ylabel('mean velocity')
ax.legend()
plt.show()
```

This is not intented to be a formal scientific rebuttal to the references listed above! It’s just a simple demonstration of how to use DeltaMetrics to interrogate the stratigraphic record.

References