  The Spallation Neutron Source (SNS) is a pulsed source of neutrons and, as a
byproduct of this operation, an intense source of neutrinos via stopped-pion
decay. The COHERENT collaboration uses this source to investigate coherent
elastic neutrino-nucleus scattering (CEvNS) with a suite of detectors. To
enable precise cross-section measurements, we must address an estimated 10\%
uncertainty in our flux calculation associated with the lack of data for
$\pi^\pm$ production from 1 GeV protons on an Hg target. We present here our
Geant4 simulation of neutrino production at the SNS and our plans to
experimentally normalize this flux with the development of a 670 kg D$_2$O
detector. Using the precise cross section calculations for neutrino
interactions on deuterium, we will dramatically reduce our flux uncertainty.
