Construction Monitoring (Cont.)

highly project dependent. Fredette et al. (1990b) contains guidelines on

available equipment and techniques. Monitoring programs may only

consist of physical measurements that include bathymetry, cap thickness,

sediment physical properties (e.g., grain-size distribution and density),

wave and current conditions, etc. Depth sounders, side-scan sonar and

subbottom profilers, sediment sampling and coring devices, sediment

profiling cameras, and instruments for measuring engineering properties

of the sediment are required to make these physical measurements.

Navigation and positioning equipment are needed to accurately locate

sampling stations or survey tracks in the disposal-site area. The accuracy

requirements for monitoring are similar to those for placing the

contaminated material and cap. See the discussion on navigation and

positioning in Chapter 5.

Precision bathymetric surveys are perhaps the most critical monitoring

tool for capping projects. Such surveys allow determination of the

location, size, and thickness of the contaminated material mound or

deposit and cap. A series of surveys should be taken before placement of

contaminated material, immediately following (and perhaps during)

placement of the contaminated material, and immediately following

placement of the cap. The differences in bathymetry as measured by the

consecutive surveys yield the location and thickness of the deposits.

Because relatively small changes in mound elevation are of prime interest,

highly accurate bathymetric surveys are required. Lillycrop et al. (1991)

discuss interdependence of tidal elevations or bathymetry measurements

and equipment capabilities and their effect on measurements. Acoustic

instruments such as depth sounders (bottom elevations accurate to 0.6 ft

under favorable conditions), side-scan sonar (mapping of areal extent of

sediment and bedforms), and subbottom profilers (measures internal

mound and sea-floor structure) are used for these physical measurements.

Survey track spacing can be 50 to 200 ft depending on the areal coverage

of the mound.

The attainable accuracy of bathymetric surveys limits the area and

thickness of the deposit that can be detected. Limits of accuracy are

governed by a variety of factors, which include accuracy of positioning

systems, water depth, wave climate, etc. Engineer Manual (EM) 1110-2-

1003 contains detailed information on hydrographic survey equipment and

techniques and should be consulted in estimating the accuracy limitations

of surveys. Other monitoring tools such as side-scan sonar, settlement

plates, or SPCs must be employed to detect thinner deposits of

contaminated and capping material.

Most methods for monitoring ocean-bottom depths from the ocean

surface (air/water interface) are not accurate to within 20 cm. Waves

bobbing the ship on which measurement equipment is attached,

inaccuracy in local tidal elevation, and inaccuracy in latitude/longitude

location add to the natural error of the instruments in measuring the

bottom depth. In addition, the sediment/water interface is not clearly

defined. During relatively quiescent periods, during which most

measurements must be made, there is often a nephloid layer that blurs the

sediment water interface. This layer can be classified as bottom sediment

with a high water content or water with a high sediment content. This

103

Chapter 9 Monitoring Considerations for Capping