  Land topography can affect air radiation dose rates by locating radiation
sources closer to, or further from, detector locations when compared to
perfectly flat terrain. Hills and slopes can also shield against the
propagation of gamma rays. To understand the possible magnitude of topographic
effects on air dose rates, this study presents calculations for ambient dose
equivalent rates at a range of heights above the ground for varying land
topographies. The geometries considered were angled ground at the intersection
of two planar surfaces, which is a model for slopes neighboring flat land, and
a simple conical geometry, representing settings from hilltops to valley
bottoms. In each case the radiation source was radioactive cesium fallout, and
the slope angle was varied systematically to determine the effect of topography
on the air dose rate. Under the assumption of homogeneous fallout across the
land surface, and for these geometries and detector locations, the dose rates
at high altitudes are more strongly affected by the underlying land topography
than those close to ground level. At a height of 300m, uneven topographies can
lead to a 50% change in air dose rates compared to if the ground were uniformly
flat. However, in practice the effect will more often than not be smaller than
this, and heterogeneity in the source distribution is likely to be a more
significant factor in determining local air dose rates.
