ERDC/CHL CETN-II-43
June 2000
Sediment Transport Analysis from
OBS/EMCM during Storms
by Joseph Z. Gailani and S. Jarrell Smith
PURPOSE: The Coastal Engineering Technical Note (CETN) herein provides information and
procedures for analyzing instantaneous measurements of currents and suspended sand to estimate
longshore sand transport during storms. The information and procedures described in this CETN
will assist others in developing similar and improved measurement techniques.
BACKGROUND: Longshore sand transport is a primary factor in many coastal engineering
studies and influences coastal erosion, impoundment at structures, channel infilling, and
morphological behavior of ebb shoals, spits, and capes. A significant portion of longshore
sediment transport occurs during coastal storms. To quantify and characterize longshore
transport induced by the combined influences of waves and currents, the Sediment Transport
Processes (STORM) Work Unit has measured currents and instantaneous concentration of
suspended sand across the surf zone during storms.
The STORM data referenced in this CETN were collected during an extratropical event at Duck,
NC in October 1997. Data were collected using the Sensor Insertion System (SIS), which is a
crane that moves along the pier at the U.S. Army Corps of Engineers Field Research Facility
(Miller 1999). This system provides the opportunity to place an instrument array at any cross-
shore location at approximately 20 m updrift of the pier pilings. Instrumentation on the SIS
included optical backscatter sensors (OBS) for measuring sand concentration, electromagnetic
current meters (EMCM) for measuring current velocity, a sonic altimeter for referencing
instrument location relative to the bottom, and a pressure sensor for measuring water depth.
Positioning of instruments, methods of data collection, and procedures for analysis of
measurements are critical for accurate estimates of longshore transport.
After data collection, concentration and velocity data are analyzed, the water column is
discretized into vertical bins with representative velocity and concentration values, and unit
transport is estimated. If sufficient cross-shore stations have been measured, total longshore
transport can be estimated from the cross-shore distribution of unit transport.
MEASUREMENTS: Velocity and concentration in the surf zone may change significantly both
spatially and temporally. To appropriately estimate longshore transport, the spatial and temporal
variability of these parameters must be considered.
Sand concentration and velocity variations occur on a relatively large scale in the cross-shore
direction. Longshore transport in the swash zone may vary significantly on the order of meters or
less, while in the surf zone significant variation may occur on the order of tens of meters or
more. Transport during storms may vary significantly between the regions of the nearshore:
swash zone, surf zone inshore of the trough, the trough, the bar, and offshore of the bar.
Transport within each of these regions must be accurately defined to estimate total longshore
1
Previous Page
