37
Drainage Delineation on Vertical Photographs
Boundaries and size of drainage areas are easily determined by use
of aerial vertical photographs. On this photograph the dashed line
indicates the high ground demarcation between major drainageways
feeding into a single channel and crossing & proposed highway route
indicated by the solid line.
The boundaries of the drainage area are easily transferred to a
piece of clear plastic for measurement of the area by use of a pla-
nimeter or other convenient method. In addition, such runoff fac-
tors, as types of ground cover and soils may be interpreted and
steepness of slope measured. These data are used to determine the
size of structure opening which will provide efficient and adequate
passage of surface waters under the completed highway.
The physical features of the highway ascertained in detail from the photo-
graphs and maps and utilized in comparing each possible location on the sur-
veyed route include horizontal curvature (sharpness, and summation of the
angles through which the curves extend); directness, including total length;
steepness and lengths of gradients; summation of rise and fall; passing and
stopping sight distances; and for controlled access highways--character and
suitability of access by location and interval of spacing along highway, both
topographically and for service to traffic entering and leaving, from and to,
other roads; multiple lanes in opposite directions at variable levels, and
with variable medians for horizontal separation; and frontage roads. Physical
features of the topography which are avoided wherever possible are poor soils
foundations, such as swamps, plastic soils, slides, existing or likely slides,
and soils with water table near surface of ground. Other physical features
determined and considered are exposure to sunshine in the winter; elevation of
subgrade in relation to elevation of water table in adjacent ground, high
water in bordering rivers and large streams, esthetic appearance in urban and
rural areas, and avoidance of property severance.
