Experimental Galveston Bay/Houston Ship Channel Nowcasting/Forecasting System

INTRODUCTION:

To further support NOS nowcast/forecast efforts in Galveston Bay, the NOAA Sea Grant Office funded a joint NOS-Texas A & M University ADCP/CTD survey of the HSC. The survey focused on the occupation of 5 transects at 1 nautical mile spatial increments along the HSC above Redfish Bar. Each transect was surveyed at 1-2 hour intervals over a complete tidal cycle. The location above Redfish Bar was selected in consultation with Mr. Dalton Krueger of the US Army Corps of Engineers (USACE), Galveston District, such that no interference with their on-going widening and deepening project would be encountered. The objective of the survey was to measure the current structure over the vertical across the entire width of several HSC cross sections. The HSC is dredged over a width of 400 ft to a project depth of 42 ft below USACE Project datum, which corresponds to Mean Low Tide (MLT). Channel side slopes are steep order 1:2.5 to 1:5 with several buried pipelines.

OBJECTIVES:

The magnitude of across channel velocity variation was to be evaluated over channel depth. In addition, the vertical salinity and temperature structures were to be assessed at mid-channel locations. Of interest, was to observe the magnitude and spatial variation of the density stratification along a section of the HSC. A secondary focus was to attempt to measure the vertical velocity structure to assess the validity of the hydrostatic assumption used in the present modeling approach.

SURVEY:

The survey was performed on 8-9 September 1999. These dates were selected to coincide with peak monthly predicted astronomical tidal currents in the vicinity of Redfish Bar. The five transects were occupied over a complete tidal cycle. At each transect an ADCP pass was made from east to west followed by a CTD pass from west to east. Along the CTD pass a single mid-channel profile was obtained. Dr. Matthew Howard, TAMU, served as Principal Scientist and was assisted by Dr. Stephen DiMarco, TAMU. Mr. Edward Webb, TAMU, performed the CTD measurements with the assistance of Mr. Philip Richardson, NOS and Ms. Karen Earwaker, NOS. Mr. Paul Devine, RD Instruments, performed the towed ADCP measurements using the 1200 KHZ Broadband set for 13 one meter bins and 1 s ensembles. A high pecision GPS was used in conjunction with the bottom tracking feature of the ADCP system. A SeaBird SBE 19 Seacat Profiler (V2.1E, SN 371) was used to perform the salinity/conductivity/temperature measurements at each transect.

DATA PROCESSING:

CTD datasets were post-calibrated and corrected by Mr. Edward Webb, TAMU. It was necessary to edit some of the bottom points in the files to remove instabilities in density. Edited files were also provided to Mr. Douglas Webb, Webb Research Inc., in support of a Small Business Independent Research Project addressing the measurement of average harbor water density. Program TRANSECT (Version 4.04l) developed by RD Instruments was used to playback the raw ADCP data (RFILES) obtained from TAMU in ASCII out mode to place navigation positions on the associated output files. These files were then scanned and edited such that at the times near each CTD cast a separate ADCP datafile was created. Within this ADCP datafile, all points are near the navigation channel markers and within +/- 6 minutes of the CTD cast time. In addition, certain bearing limits were imposed to insure a reasonable representation of the transect between the appropriate channel markers. Program ADCP_SCAN written by Dr. Richard Schmalz, NOS/CSDL was used to create these ADCP files; one for each CTD cast. The CTD passes were selected by NOS to reduce ship wake interferences. Note order 5 ADCP datafiles and associated CTD casts were available at the two end transects, with approximately 10 ADCP datafiles and CTD casts available for the three interior transects.

TRANSPORTS:

These data were used to compute the transports normal to the navigation channel markers (plus coinciding with the flood direction) using Program ADCP_TRANSPORT written by Mr. Philip Richardson and Dr. Richard Schmalz, NOS. At Transect 2, maximum flood and ebb transports were 690 m3/s and 458 m3/s, respectively. These transports represented peak flood and ebb conditions and are further investigated by use of an IDL contour plot program developed by Mr. Philip Richardson, NOS.

Additional Program and Processing Details.

To request survey data, please send your inquiries to
Mr. Philip H. Richardson
NOAA/NOS/Coast Survey Development Laboratory
Phone: (301) 713-2809 ext. 115
Fax: (301) 713-4501

Last updated on February 2, 2001