Quantcast Site Surveys

 
  
 
Monitoring and Maintenance of Stabilization Works
Interviews with local persons to obtain additional information, and encouraging
reliable local observers to report significant observations to those responsible for monitoring,
is often worthwhile.
11.2.3.2 Site Surveys
Surveys are particularly important where the water level does not permit visual
observation of the toe and lower bank slope, or where a primary concern is scour during high
flows. Cross-sections of the bank, extending well past the toe, are the most reliable and
useful type of survey, since toe deepening and undermining are the major factors in many
failures. Handling of large amounts of data can be expedited by the use of computers.
Thalweg profiles require less effort than bank cross-sections, but are less reliable as
to repeatability and accurate documentation of significant changes, therefore should be
supplemented by cross-sections at representative locations.
Surveys taken during low flow periods may not adequately document conditions,
particularly at the toe of the bank in bends, where scour and deposition may occur during high
and low flows, respectively. Surveying during high flow periods is more difficult and
expensive than during low flows, but may be advisable where adequacy of protection against
toe scour is questionable.
The level of survey detail required for routine monitoring will usually be less than that
which will be required if the need for repair work is detected. Monitoring surveys must be
comparable to previous surveys, but surveys for design of repairs should be tailored to the
specific one-time need. Routine monitoring surveys will, however, provide a basis for
defining the additional level of detail required for surveys to be used in designing repairs.
11.2.3.3 Geomorphic Observations
Changes in overall planform or local channel alignment can threaten the integrity of
the work. Application of the principles discussed in 6.1 will determine when such threats
exist. Also, the work itself may affect downstream conditions. The length of stream requiring
geomorphic observation will depend upon the overall stream stability and the type and extent
of protection work.
Aerial inspection is often a cost-effective technique for monitoring geomorphic
conditions. Oblique and/or vertical aerial photography is particularly useful in documenting
progressive changes. Vertical photography should provide sufficient overlap of adjacent
photographs to allow at least a qualitative stereoscopic analysis of changes in bar heights and
bank slope steepness. If detailed mapping is justified, vertical aerial photography can be
geodetically controlled to permit mapping of the channel topography above water.
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