  We present a statistical study of chromospheric evaporation in solar flares
using simultaneous observations by the RHESSI X-ray telescope and the IRIS UV
spectrograph. The results are compared with radiation hydrodynamic flare models
from the F-CHROMA RADYN database. For each event, we study spatially-resolved
Doppler shifts of spectral lines formed in the transition region
(C\,II\,1334.5\,{\AA}) and hot coronal plasma (Fe\,XXI\,1354.1\,{\AA}) to
investigate the dynamics of the solar atmosphere during the flare impulsive
phase. We estimate the energy fluxes deposited by high-energy electrons using
X-ray imaging spectroscopy and assuming the standard thick-target model. Using
the RADYN flare models, the RH 1.5D radiative transfer code and the Chianti
atomic line database, we calculate C\,II and Fe\,XXI line profiles and compare
with the observations. While the RADYN models predict a correlation between the
Doppler shifts and deposited energy flux for both lines, this was only observed
in the C II data. Several quantitative discrepancies are found between the
observations and models: the Fe\,XXI Doppler shifts are substantially stronger
in the models than in the data; the C\,II mean blueshifts are absent in the
observations but predicted by the models. The transition energy between
`gentle' and `explosive' evaporation regimes estimated from the observations
($2-8\times{}10^{9}$~erg~cm$^{-2}$~s$^{-1}$) and derived from the models
($2.2-10.1\times{}10^{9}$~erg~cm$^{-2}$~s$^{-1}$) are comparable with each
other. The results illustrate relationships among the processes of
chromospheric evaporation, response of the colder layers, and the flare energy
flux deposited by high-energy electrons, although demonstrating discrepancy
between analyzed observations and RADYN models.
