indicated on the site maps for the purpose of estimating flood flows and distributions across the
river cross section.
8.1.4 Aerial and Other Photographs
It is highly desirable in preparing vicinity and site maps that aerial photographs be obtained.
Multi-image cameras use different ranges of the light spectrum to assist in identifying various
features such as sewer outfalls, groundwater inflows, types of vegetation, sizes and heights of
sandbars, river thalwegs, river controls and geologic formations, existing bank protection
works, old meander channels, and other features. Topographic information can also be
developed from aerial photographs for vicinity and site maps where such information is not
readily available. Historic aerial photographs are a valuable resource to assess river planform
stability.
Land photographs (as opposed to aerial photos) of existing structures near the crossing are
always helpful in documentation and evaluation of potential effects of highway construction.
Photographs of water intake works likely to be affected by the highway project should be
obtained, and specific data should be noted and briefly discussed. High water marks recorded
photographically along with dates of occurrence are useful. Photographs aid the designer, who
may not have the opportunity to visit the site, to visualize crossings and encroachments, and
they aid documentation.
Conditions of the river channel in the river reach of concern are easy to record
photographically, and such pictures can be very helpful in analysis of the river reach.
Vegetation on floodplains and seasonal variations of vegetation should be recorded
photographically. Notable geologic formations should be photographed as well and
supplemented with adequate notes. All photographs should be referenced on the site or on
vicinity maps.
8.1.5 Field Inspection
A field inspection of potential highway encroachment sites of rivers should be made prior to the
analysis. This has been implied in the foregoing paragraphs but is emphasized again because
of the underlying importance of making first hand appraisals of specific sites before
conclusions and recommendations are advanced for possible highway routes. Of course, they
are important in making detailed designs as well, but it is not always feasible to provide
opportunities for site inspection by the entire design staff. Forms and checklists for a detailed
geomorphic reconnaissance of the stream corridor are provided in HEC-20 (Lagasse et al.
2001).
8.1.6 Geologic Map
A geologic vicinity map, on which geophysical features are indicated, is a basic need. The rock
formations, outcroppings, and glacial and river deposits which form control points on rivers are
valuable in analysis of rivers. Soil type determines the size of sediment in transport, infiltration
rates, and groundwater flows. Channel geometry and roughness are important factors in river
mechanics.
8.2
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