Peak Flow calculator math errors/warnings

[gassc]

Somewhat sporadically, seeing warnings (not exceptions) popping up with the Peak Flow calculator

drainit\calculators\runoff.py:166: RuntimeWarning: divide by zero encountered in log10
  qu = 10 ** (CONST_0 + CONST_1 * numpy.log10(tc_hr) + CONST_2 *  (numpy.log10(tc_hr))**2 - 2.366)
drainit\calculators\runoff.py:166: RuntimeWarning: invalid value encountered in double_scalars
  qu = 10 ** (CONST_0 + CONST_1 * numpy.log10(tc_hr) + CONST_2 *  (numpy.log10(tc_hr))**2 - 2.366)