CETN IV-15
Rev. September 1999
may not be directly related to the measurement process. Significant contributors to true
uncertainty enter through natural variability and unknowns in the measurement process. These
include temporal variability (daily, seasonal, and annual beach change); spatial variability
(longshore and across shore); definitions (e.g., shoreline orientation, direction of random seas);
and inability to quantify a process, such as the volume of material pumped to a beach or the
sediment pathways at an inlet. Other unknowns can enter, such as grain size and porosity of the
sediment (especially true in placement of dredged material). For further discussion and example
uncertainty calculations, the reader is directed to CETN-IV-16 (Kraus and Rosati 1998a), Kraus
and Rosati (1999), and Mann (1999).
Step 7: Formulate a Macrobudget. A macrobudget is a quantitative balance of sediment
inflows, outflows, volume changes, and engineering activities within the regional conceptual
budget. Essentially, the macrobudget solves the budget with one large cell (perhaps by
temporarily combining many interior cells) that encompasses the entire longshore and cross-
shore extents of interest. Balancing the macrobudget reduces the possibility of inadvertently
including potential inconsistencies in a detailed or full budget (Kraus and Rosati 1999).
Step 8: Refine Estimated Values and Uncertainties. Once the macrobudget has been
balanced, detailed analysis for all inflows, outflows, volume changes, and engineering activities
pertaining to each individual cell may commence. Values entered for the macrobudget can serve
as first estimates for the detailed budget, providing at least the expected order of magnitude for
final values. The types of data sets that are available for refining sediment budget quantities are
discussed below.
Aerial Photography. Interpretation of aerial photographs offers the best means of obtaining
broad qualitative understanding of the site. As examples, photographs of sites with relatively
clear water can identify the planform shape of the flood-tidal shoal to estimate its volume if
more quantitative data are unavailable. The pattern of wave breaking over the ebb-tidal shoal
indicates the planform shape of this feature, which might lend qualitative understanding of its
interaction with adjacent beaches and of sediment pathways. Overwash fans on adjacent
barrier islands indicate pathways for loss of sediment to the coastal littoral system, from
which quantification of volumes might proceed. Shoals adjacent to jetties might indicate
sediment transport over and through the structure as a potential sediment-transport pathway.
In a more quantitative analysis, controlled and rectified aerial photographs are commonly
interpreted to identify the berm or high-water line (HWL) shoreline position (see CETN-II-
39 "Interpretation of Shoreline-Position Data for Coastal Engineering Analysis," Kraus and
Rosati 1998b).
Beach-Profile Surveys. Volume change V in the beach can be obtained accurately through
repetitive surveys of the beach profile. The volume change for a given profile is typically
assumed to represent the region of beach of length x b e t ween adjacent profile lines. Both
the elevations B of the berm and of the profile closure depth DC can be estimated from beach-
profile surveys if the profile data are sufficiently accurate and well controlled. The active
berm crest is a discernible morphologic feature on the profile representing the upward limit
reached by the water under normal tide and water-level conditions. The profile may have
two berm crests if the beach has recently accreted, and the elevation of the seawardmost
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