Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 4 REFERENCES ON GEOMATICS AND LAND SURVEY SERVICES
4.16 LOCATING OF CROSS-SECTION PROFILES FOR HYDRAULIC ENGINEERING
(JADUAL 2001 ITEM 14.9 PART V)
4.16.1 Mixed Survey Methods
Obtaining cross-section profile of stream, adjoining bank and flood plain requires a combination of
survey methods. Hydrographic sounding surveys performed in the river must be combined with
conventional topographic, and or photogrammetric surveys in the adjacent over banks and flood
prone plain. Surveys of the flood plains are usually more efficiently conducted using air survey
(Digital photogrammetric) methods to create a Digital Elevation Model (DEM). Recently, airborne
LIDAR (Item 4.17) techniques have been developed to provide DEM of the flood plain. Conventional
topographic survey methods (leveling and digital/optical total station) will be required to fill in hidden
areas under cover of vegetation and to ascertain break lines in the final terrain models.
4.16.2 Guidance to Surveyors on Cross-Section Locations
Detailed guidance for determining the location and spacing of stream cross-sections is based on the
recommendations in the US Army Corps of Engineers, Engineers Manual “EM1110-2-1002” and
EM1110-2-1416”. Surveyors providing input for these studies should be aware of the hydraulic
considerations that dictate the intended placement and alignment of stream sections. Thus,
knowledge of the engineering rationale for locating cross-sections profiles is required by field
surveyors in order to make reasonable adjustments or recommend modification to the project
engineer to optimize the obtaining of basic field information on the river profile, the adjoining river
banks and the flood plain.
4.16.3 Guidelines on Locating Cross-Sections
Generally (not exhaustive) the locations of Cross-sections for hydraulic modeling should be
considered are:-
a. Points where roughness changes abruptly to provide channel roughness information
b. Closer together in stretches where water surface expands and in bends
c. Closer together in stretches where the flow of water changes greatly as a result of changes
in width, depth or roughness
d. Closer together at wide bends where the lateral distribution of water flow changes radically
with distance
e. Closer together in streams of very low gradient at lowlands which are significantly non
uniform, because the computations are very sensitive to the effects of local disturbances
and/or irregularities
f. At tributaries that contribute significantly to the main stem flow. Cross-sections should be
located immediately upstream and downstream from the confluence on the main river and
immediately upstream on the tributary
g. At regular intervals along waterway of uniform cross-section
h. Above, below, and within bridges at bridge sites including the soffit levels
i. On large rivers that have average slopes of 0.4 meter to 1.5 meter per kilometer or less,
cross-section within fairly uniform reaches may be taken at intervals of 1.5 km or more
j. More closely spaced cross-sections are usually needed to define energy losses in urban
areas, where steep slopes are encountered, and on relatively narrower streams. On small
streams with steep slopes it is desirable to take cross-sections at intervals of 500m or less.
4-8 March 2009