CAD/GIS tools help community improve flood preparation
Kevin P. Corbley
Principal, Corbley Communications Inc.
A generation of residents in West Jefferson Parish, Louisiana, grew up taking for granted the canals and levees built to protect them from floodwaters following hurricanes. But in 1998, Tropical Storm Frances struck land 300 miles away and doused their false sense of security.
Although it was rated only as a 25-year event, Frances hit Houston, Texas, with enough force to send a storm surge across the Gulf of Mexico and into the Mississippi watershed. Rising waters spilled over the levees and into the streets on both sides of the Harvey Canal, which splits the parish.
Damage was minimal, but the point was madeWest Jefferson Parish was not safe from flooding. And worse yet, parish officials asked themselves what would happen if a storm hit head-on.
The last big storm to hit was Hurricane Betsy in 1965, and it came when the canal was at low water, said Chip Cahill, President of the West Jefferson Levee District (WJLD). There was no flooding, and people seemed to think we were immune.
WJLD is responsible for maintaining 80 miles of levees on the Mississippi West Bank and had tried to convince residents since 1980 that the Harvey levees were not sufficient protection. No one listeneduntil Frances drove home WJLDs point.
With the support of the Army Corps of Engineers and a suddenly interested public, the levee district hired Professional Engineering Consultants (PEC) of Biloxi, Mississippi, to survey the canal and build a GIS. This system would have two purposes: first, to determine how the levee structures could be improved, and second, to manage the canal under both normal and emergency situations.
In developing the system, PEC utilized GPS field survey and mapping techniques to gather raw data that were used to create storm surge models in CAD-based civil engineering software. These results were then imported to the GIS for display and integration with other data layers necessary for remediation and management.
West Jefferson Parish is located southwest of New Orleans and straddles the Mississippi. The Harvey Canal, a man-made channel containing a section of the Gulf Intercoastal Waterway, borders the parish for 11 miles. Levees line both sides of the canal.
Even before mapping with GPS, the problem with the canal was obvious. The south half of the west side levee was a continuous structure built to Army Corps specifications in 1991. But the remainder of the levees was a hodgepodge of wooden bulkheads, cement sea walls, consolidated earthworks and even a derelict barge. Responsibility for maintaining the structure lies with the property owner, and many had not kept the containments up to specifications.
Our CAD models ultimately showed the parish would fill like a bowl if the Harvey levees were significantly breached, said Dan Heiken, PEC President.
Equipped with Sokkia GPS receivers, the PEC crew surveyed X and Y coordinates for the centerline of each levee section as well as Z values for high and low point elevations. These GPS units allowed the surveyors to enter codes that differentiated sets of points defining each specific structure. For example, one group of points might belong to a cement bulkhead while the next set identifies a section of sea wall.
After the field work was completed, the GPS points were uploaded into FieldWorks, a mapping software package developed by Intergraph Inc. (Huntsville, Alabama) to run on top of CAD systems. FieldWorks takes the GPS points and essentially connects the dots to build a CAD line map of the surveyed area.
The survey revealed that there are 137 separate sections comprising the east side levee. As expected, many gapsor low pointswere found in the structure. Otherwise, levee height ranged from three to six feet above the water level.
Once the line map of the levee system was built, PEC technicians transferred the file to InRoads, another CAD-based Intergraph product developed for highway design, but used for a variety of civil engineering modeling functions. InRoads enabled the technicians to display and analyze the levee structures in a three-dimensional CAD view.
The Louisiana Department of Transportation supplied us with InRoads files of the road networks behind the levees, and we got some digital USGS quad maps showing the layout of the parish, said Heiken. So, we had a pretty good representation of the whole area in three dimensions.
Still working in InRoads, the technicians built vertical alignments, which essentially were digital elevation models of the levee structures. Next, PEC obtained storm surge height information from the Army Corps of Engineers. These estimated the heights above sea level water in the Harvey Canal would reach under 25-, 50- and 100-year flood conditions.
We modeled the impacts of each flood event with InRoads by simply creating a profile for each storm surge and combining it with our levee alignments, said Heiken. It showed the exact spots on the levee that would be breached in different flood scenarios.
A stunning 106 out of 137 sections did not meet Corps height requirements. Not only would the levees allow water through, the model revealed there was little in the way of raised roads or other topography in the back-levee area to stop the water from sweeping all the way across West Jefferson Parish, home to 200,000 people.
CAD served as the link between the civil engineering modeling software and the GIS, said Heiken. The next step was to integrate the CAD and GIS so we could examine the modeling results within the context of other information.
The technicians imported the CAD files into Intergraphs GeoMedia GIS development environment. The levee maps, flood models, and road files became the spatial data elements in the GIS. With assistance from WJLD, PEC gathered other important non-spatial information, such as parcel layers, ownership records, ground photographs, and notes on levee building materials.
The primary strength of the GeoMedia software is that it allows all data sets, whether spatial or not, to remain in their native formats without requiring conversion or migration to a single database. This was especially important in this case because parcel data was in ArcView shape files, an older levee survey was in an NAD 27 map projection, and air photos were in GeoTiFF files. Tabular data was kept in numerous different database files.
Developing the GIS involved linking the spatial data to the other information so they could be analyzed together. The first function of the new GIS was to identify which properties and ownership records coincided with each insufficient levee sections.
WJLD sent letters along with GIS-generated diagrams to each owner explaining the dangers and the remediation alternatives. District officials imported GIS modeling displays directly into Microsoft PowerPoint to give presentations at public gatherings.
We met with the owners to present the findings and discuss the options, said WJLDs Cahill. The GIS was very convincing; most property owners rolled up their sleeves and got to work on repairs.
In the event of an emergency, such as a levee breach, the GIS will be accessed by WJLD staffers to find the problem point on the map and determine what danger it presents to life or property. By clicking on the levee section to access database information, they can learn what materials comprise that section, and they can even access InRoads calculations of how much material would be required to fill the breach.
As a result of the CAD modeling and GIS displays, state and federal officials have pledged more than $40 million to make additional improvements to the Harvey Canal levee structure.
For more information, contact Kevin Corbley at (303) 979-3232 or firstname.lastname@example.org.