Mobile LiDAR and Flood Impact Studies

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On April 11th 12th 1979, widespread rainfall ranging from 8-10 inches fell in the headwater regions of the Pearl River in northern and central Mississippi–with heavier localized amounts in excess of 20 inches. The Jackson area, having already experienced heavy rainfall during the preceding weeks that saturated the soil and filled waterways, endured several inches of new rain, and witnessed a rapid rise of the Pearl River. The local rain fall, coupled with the additional necessary discharge to ensure the integrity of the rapidly filling Ross Barnett Reservoir, created a monumental flooding event in downtown Jackson and surrounding areas. The event, infamously known as the ’79 Easter Flood, resulted in substantial loss of property, in excess of $1.5 billion in present terms, and forced the evacuation of more than 15,000 residents from the surrounding region.

Over the past few decades, few flood control improvements were made to the levee system, outside of new construction in northeast Jackson and neighboring City of Pearl , which still allows regular flooding during moderate rain events on tributaries within Jackson, and continues to expose vulnerabilities to large areas should another exceptional event occur.

The One Lake project is a local initiative, spearheaded by a coalition of local leaders, Chambers of Commerce and private business leaders, to develop a secondary water body downstream from the Ross Barnett Reservoir along the Pearl River to protect residents, property, and the local economy against future flooding disasters. Supporting the required environmental and economic impact studies, detailed topographic survey information, including First Floor Elevations (FFE) of structures residing within the 500-year flood zone, were required for analysis. To service these requirements in an accurate, cost efficient, and timely manner, project stakeholders and partners employed a proven Mobile LiDAR solution to develop integral information for the impact studies.

Project Planning, Survey and Collection
The well-documented footprint of the ’79 Easter Flood closely resembled the limits of the current 500-year flood zone. Using a geographic buffer of these areas to ensure completeness of collection and processed information, four (4) distinct sub-regions were created using natural and man-made boundaries. The Pearl River divided the project into Eastern and Western halves, while Interstate 20 was utilized to create the North and South halves. The four (4) quadrants provided an efficient mechanism to schedule project activities and provide intermediate deliverables to other team members to support concurrent work efforts.

Several weeks prior to commencement of Mobile LiDAR collection activities, Baker’s local survey crews began setting and observing control targets. Where possible, crews used existing paint markings to observe redundant real-time kinematic (RTK) GPS observations. Points were observed and photographed, similar to documentation methodologies employed for photo-identifiable aerial targets. These were used for spatial constraint of the LiDAR data prior to supplemental post-processing. While surveying, the crews would check-in to other known control points as well as randomly observe multiple hard and soft target measurements for secondary quality control measures and reporting. Each day, observed control points were downloaded and plotted for review and validation against the master layout plan established during mission planning efforts. In all, more than 750 control points and 200 check points were observed to ensure accurate and defensible project results.

The size of the project and accelerated schedule required a detailed mission planning initiative, which included the development of custom map books, route planning, progress tracking routines, streamlined field notation processes, and automated progress updates to stakeholders. The Mobile LiDAR collection commenced in high density residential areas shortly after ground control had been established. Efficiency of LiDAR collections yielded completion of large areas on a daily basis, and produced an instant backlog for downstream processing by a team of trained technicians.

Comprehensive methodologies were commissioned to ensure all requirements were met, which included often trivialized procedures for the daily review of vehicle trajectories to verify completeness and communicate progress, as well as the creation of detailed maps denoting gated subdivisions, apartment complexes or other areas of restricted access (schools and large private properties) that would require additional public outreach. By leveraging the robust mission planning effort, the entire project area was collected over the course of fifteen (15) days, and promoted the capture of over 700 miles of LiDAR data. As an added value, additional areas adjacent to, but not within, the flood zone were captured during the field collection effort so that the information would be readily available if requested by the client for additional analysis. These included the newly constructed levees and the majority of downtown Jackson, including the Capital and other government complexes.

Mobile LiDAR Processing
Prior to this project, Baker had completed several similar FFE projects around the country. We understood the challenges that would be encountered during feature extraction routines to determine the actual threshold point representing the FFE. These challenges comprised both permanent and temporary obstructions such as vegetation, and parked vehicles, and other complexities attributed to distance from the structure and non-visible points of entry.

To minimize variables, we had performed numerous tests to determine optimal driving speed, sensor orientation, and other collection parameters to foster full saturation to the fronts of structures with information dense enough to accurately identify the threshold. Knowledge gained from previous projects was also employed to develop detailed metrics to quantify the confidence that the identified FFE was consistently extracted at the desired location. This knowledge also promoted the establishment of documentation for our processing team to include a hierarchy of alternative locations to extract FFE values when the primary target was not identifiable. These procedures were heavily vetted and approved by the client. In all, these types of impact studies commonly strive to establish accurate FFE’s on 40% of the structures–our proven methodologies and techniques yielded 92% on a single pass.

During scope development and meetings with other project partners, we were able to identify additional attributes that could be assigned by our staff during the FFE extraction that would involve little effort, and would be required by the economists to calculate various impacts and flooding scenarios. While some of these attributes could be obtained from tax rolls, we observed that there were inconsistences in currency, completeness and correctness of the values between the three (3) counties (the project resides in Hinds, Madison, and Rankin Counties) due to significant new construction and frequency of update.

To combat these inconsistencies, we developed a custom toolbar within MicroStation to perform several tasks. The primary Add FFE tool provided a streamlined interface to facilitate the user to place a point representing the FFE; then present a user-friendly graphical user interface (GUI) to enable rapid population of type of structure (single family, multi-family, trailer/ mobile home, church, commercial, etc.), number of floors, type of commercial structure (eating & drinking establishment, professional services, grocery, public facilities, etc.), FFE quality rating, link to terrestrial photograph, and general comments. These tools greatly improved our efficiency in extracting FFE locations and standardized attribute population for the 12,000+ structures within the project limits.

Equally significant as the property damages incurred in the ’79 Easter Flood, many evacuation corridors were cut off by the rising flood waters. Interstate 55, the primary north/south corridor lying between downtown Jackson and the Pearl River, was cut off on the north side of the city, hampering evacuation and recovery efforts. In an effort to identify low-lying roads within the project boundary, the comprehensive LiDAR data promoted the effortless creation of approximately 325 miles of road profiles for each of the routes. This enabled the identification of areas that would be cut off during future flood events so that affected residents could be alerted about modified evacuation plans.

GIS Development and Analysis
Baker’s final deliverables included a comprehensive Esri Personal Geodatabase of information compiled and created from the scope of services. GIS technicians compiled tax assessment and parcel details for each of the properties within the project area. The attribute tables and field names were standardized among the three (3) counties to create a seamless feature class. FFE points were used to create a many-to-one relationship with the parcels to account for multiple structures on a singular tax parcel–i.e. medical complex, apartments, schools, commercial shopping districts–while digital photographs captured and archived for each structure were hyperlinked to each structure for easy review by stakeholders. Lastly, the roadway profiles were converted to three dimensional polyline features for map creation and analysis.

LiDAR Assessment and Accuracy
Baker’s scope stated the vertical RMSe as measured between the control points and the Mobile LiDAR point cloud needed to be less than one-tenth of a foot (<0.10′). Upon detailed analysis, the largest observed vertical error was 0.07′, and the observed RMS was approximately 0.03′. To ensure consistency of spatial positioning we also verified our data against the approximately 200 check points observed throughout the project area; for which an RMS of approximately 0.05′ was achieved. Lastly, an additional check was performed by evaluating the Mobile LiDAR derived FFE values against several FEMA Elevation Certificates for employees that lived within the study area (well outside the scope of work, but mere curiosity piqued our interest). Suffice it to say, Mobile LiDAR proved to be an exceptional tool.

While the Mobile LiDAR data was used to gather information on FFE’s and roadway profiles, our partners were using readily available aerial LiDAR for regional analysis, wetland delineation, and other applications. As was the case with the parcel data, the aerial LiDAR data came from different counties, each with different specifications and captured with different requirements.

In order to defend conclusions drawn from the data, Baker performed detailed review and assessment of the information using our custom LiDAR evaluation toolsets. The analysis included vertical accuracy assessments using our survey control points, point density calculations, data void review and summary of the LiDAR characteristics including classification, coordinate systems and metadata. These assessments were documented for each of the counties and reported to our client and partners.

Stephen J. Clancy is an Assistant Vice President with Michael Baker International and serves as program manager for Mobile LiDAR and Static Scanning. He maintains active survey licenses in Alabama, Florida, Louisiana and Mississippi, where he resides.

A 2.008Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE