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Louisiana Parish Reduces Flood Potential
By Kevin P. Corbley For more than a generation, residents of West Jefferson Parish, Louisiana, had taken for granted the series of canals and levees that are supposed to protect them from rising flood waters that follow hurricanes. But in 1998, a tropical storm striking land 300 miles away jarred people from their complacency.
Officials in the parish, many of whom had warned of potential disaster for years, turned to GIS technology to fully assess the flood danger. They then used this technology to create a graphical illustration for state and federal agencies, showing them what might happen if a storm were to hit the parish head-on.
The GIS models made such a persuasive case that the parish received both public and private support for building corrective measures to protect the West Bank of the Mississippi from tidal surges related to hurricanes. Protecting the Parish
West Jefferson Parish is situated southwest of downtown New Orleans, straddling the Mississippi River. Running north-and-south through the western edge of the parish lies the Harvey Canal, an 11-mile man-made channel that contains a portion of the Gulf Intercoastal Waterway.
Levees line both banks of the canal to protect homes and businesses from high water. The western side of the levee is a solid, continuous structure built with federal funds by the Army Corps of Engineers in 1991. Containment structures on the eastern side, however, are a hodgepodge of wooden bulkheads, cement sea walls, consolidated earthworks, and even a derelict barge.
Officially, maintaining these structures is the responsibility of the respective property owners. Most of these back-levee owners are industrial businesses involved in construction, fishing, and offshore petroleum operations.
"The eastern side is not really a levee system, and there's a difference in elevation between the two sides," said Giuseppe Miserendino, deputy director of the West Jefferson Levee District (WJLD). "Water would spill over the eastern side of the canal if there were a tidal surge."
Tidal surges pose a serious danger in areas of Louisiana where residential and commercial areas lie below sea level. In fact, the entire system of canals, levees, and river locks in the area have been constructed to protect New Orleans and its outlying suburbs from Mississippi River and Gulf of Mexico tidal surges that can follow a major storm.
"GIS modeling showed that West Jefferson Parish would fill with water like a bowl all the way [east] to the Mississippi if the Harvey levees were significantly breached," said Dan Heiken, general manager of Professional Engineering Consulting Corp. (PEC), the Baton Rouge firm that was contracted to build the GIS.
WJLD, which is responsible for maintaining more than 80 miles of levees on the Mississippi River's western bank, had tried to convince people of the flood threat ever since the early 1980s, but their arguments fell on deaf ears. Local government agencies had become more concerned with protecting the area against rain-induced flooding, not hurricane surges.
"There has been a whole generation that has grown up not knowing what it's like to go without running water or electricity after a storm," said Chip Cahill, WJLD president. "The last big storm to hit was Hurricane Betsy in 1965, and it came when Harvey Canal was at low water. There was no flooding, and people seemed to think we were immune."
Minds were changed when Tropical Storm Frances hit Houston in September 1998. Although the storm made landfall 300 miles away and was rated as less than a 25-year-level event, the surge forced water over and through the Harvey levees in half a dozen places. The damage was minimal, but the point was made-the levees on the eastern side had to be enhanced. Building a Levee System GIS
At the suggestion of the New Orleans District of Louisiana DOT, the levee district contracted the Biloxi, Miss., office of PEC to survey the eastern bank of the canal and develop a GIS for asset management and emergency response purposes. PEC utilized the GeoMedia GIS suite of products from Intergraph Corp. (Huntsville, Ala.) to complete nearly every phase of the GIS project, from GPS survey through its final presentation in front of local politicians.
"Although WJLD didn't have an existing GIS, they were savvy to what the technology could do for them," said Heiken. "One of their primary objectives was to have us create a thematic representation of elevation data to show where the levee was inadequate and where the water would go in the event of a 25-, 50- and 100-year storm."
PEC subcontracted the GPS survey to Automated Measuring Systems (AMS) of Monroe, La. Using Sokkia rover GPS units and a base station for differential post-processing, AMS collected 375 survey points with an accuracy of 3mm on the eastern side of the six-mile-long levee.
The eastern side levee is not continuous. Instead, it is divided into 137 pieces that cross 78 different parcels of private property. GPS points were collected at high and low points on each section. Special attention was made to survey levee defects, which were defined as pipes or gaps in the structure that could allow water to flow through or across to the other side.
"Defects usually occurred at property lines where two different types of levee structures did not join up properly," said Heiken. "We found 16 of these defects."
Accurate elevation values were extremely important during GPS collection. The Army Corps of Engineers had concluded that the levee needed to extend at least six feet above sea level to adequately protect the back levee area from a storm surge. The survey revealed that 106 of the 137 eastern side levee sections did not meet the Corps' requirement.
"During the survey, we gathered other information to populate the GIS," said Heiken. "Digital photos were taken of each section and of any defects."
Additional attributes collected in the survey included the type and condition of each levee section, as well as the location and size of defects. WJLD supplied parcel data containing the name, address, and contact details for property owners along the canal's eastern side. All of these attributes were compiled into the GIS. Assessing the Levees with GIS
For the data collection and modeling portions of the GIS development, PEC used Intergraph's FieldWorks and InRoads software packages. Survey points were loaded into FieldWorks, which processed the x, y and z coordinates and drew lines between related points to generate a plan-view vector map of the levee system.
PEC obtained a 1985 LaDOT survey of the area, using that data for comparison with the GPS elevation points. They also acquired aerial photos from Jefferson Parish, which were then compared to the GPS plan map to ensure that levee boundaries had been precisely mapped. In a few cases, the photos revealed survey errors near a towering bridge, which likely had interfered with precise GPS collection. These errors were corrected with traditional survey techniques.
Once all the GPS points had been processed and corrected, they were imported into InRoads, a civil engineering package typically used for elevation modeling and cut-and-fill calculations for the construction of highways, canals, and landfills. InRoads generated a DTM showing a vertical elevation profile of the eastern side levee system.
"Then we made DTMs of the flood water heights estimated by the Corps for 25-, 50- and 100-year storm surges and compared them alongside the levee DTM," said Heiken. "The results were pretty bad."
"Pretty bad" in this case meant the comparison of water levels with the levee profile showed that numerous sections of the structure could be breached by high water, even during a 25-year storm. Many more would be breached if a 50- or 100-year hurricane hit. Compiling Data in the GIS
As the DTMs were being generated, PEC concurrently built the GIS with the GeoMedia Professional development package. This Windows-compatible software was ideal for the Harvey Canal application because it was designed to integrate, process and display multiple types of geographic data in varied formats.
PEC had several different data layers to assemble in the GIS: aerial photos in GeoTiFF, parcel data in ArcView shape files, a 1985 levee survey in a NAD 27 projection, and the new GPS survey in NAD 83. The older survey was especially important to the project because it contained coordinate points for ground elevations, roads and building footprints in the back levee area, which were not surveyed in the 1999 project.
"We wanted to overlay the new and old surveys in the GIS to include buildings and terrain features in our flood assessment," said Heiken. "In another GIS, this would require rubber-sheeting to rectify the two projections, but GeoMedia aligns files on the fly and displays them in any coordinate system and projection."
The aerial photo serves as the base map for the GIS, and ground survey points can be overlaid for visual reference. The GIS enabled WJLD users to view where each breach point would be located at various surge levels. In addition, PEC hyperlinked the ground photos to the GIS so that WJLD could view ground level conditions by clicking on any section of the GIS levee map.
InRoads was also used to compute the volume of material required to fill each gap in the levee. The engineers converted these volumetric calculations into numbers of sandbags, and this information was stored as an attribute.
"If we get a call of a levee breach, we can punch a street address into the GIS and instantly see what the area looks like and find out how many sand bags are needed to plug it," said Miserendino. "More likely though, we would fill those defects in advance as the storm approached." Making a Case with GIS
With the breach points located on the digital map, PEC used InRoads hydraulic analysis tools to model the flow of water over and through the structures. The system showed which roads and buildings in the back levee would be inundated as flood waters rose in various parts of the parish. The model revealed that virtually any serious water breach could flood the entire bowl-shaped topographic depression between the Harvey Canal and the Mississippi River.
Because GeoMedia GIS is Windows compatible, the flood models and DTM profiles were exported directly into Microsoft PowerPoint for use in public presentations. WJLD's Cahill and PEC's Heiken made presentations before citizen groups, state legislators and Corps engineers to demonstrate the potential disasters that awaited the parish under certain high-water conditions.
WJLD also used the GIS to cross-reference levee defect and height inadequacy locations with parcel ownership records. This was crucial information because private property owners are responsible for levee maintenance in the parish. Sixty of the 78 eastern side owners were found to have insufficient levees.
The district sent hardcopy GIS maps and a cover letter to each owner, outlining the problem and suggesting how each particular levee section could be repaired to bring it up to the Corps' standards. WJLD also offered some building materials at little or no charge.
"We met with the owners to present the findings and discuss options," said Cahill. "The GIS was very convincing; most property owners rolled up their sleeves and got to work on repairs."
More importantly, state legislators pledged funding to build a flood control gate at the canal's north end, and a series of cement-capped walls in front of the levees. With matching funds from the federal level, WJLD received more than $40 million to make the improvements it had sought for the last 15 years. An estimated 250,000 people will be protected in the Harvey back-levee area once this work is completed.
"The GeoMedia GIS helped us get the funding, but its use doesn't end there," said Cahill. "We will continue to update it for use in everyday operations and emergency situations."
WJLD has contracted PEC to perform another GPS survey on the western side of the canal for inclusion in the GIS. Plans are also underway to link real-time tidal surge sensors in the canal to the GIS, publishing this information on the Web so that WJLD and Corps technicians can monitor rapidly changing storm situations from remote locations.
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