High-Tech Flood Control
June 1, 2010
Wichita, Kan., and surrounding Sedgewick County are located at the junction of the Arkansas and Little Arkansas River Valleys. The area is flat and is interwoven with canals that move water, both during base flows and during periods of flooding. Supporting these canals are 97 miles of levees that cut across the city from north to south. These levees, built in the 1950s as part of the Wichita Valley Flood Control Project and a companion initiative, West Branch Chisholm Creek Local Flood Protection Project, provide millions of dollars of flood protection for the region’s nearly half a million residents from the excess water flows of the Arkansas River, the Little Arkansas River, area creeks and the 18-mile-long, manmade channel, known locally as the “Big Ditch.”
The area consistently receives heavy rainfall, with most of that rainfall occurring in the spring or fall. Storms have historically had a significant flood impact on the region, even with the complex levee system. On Oct. 31, 1998, a flood of Cowskin Creek in western Wichita due to extensive rainfall caused millions of dollars in damages. According to the USGS Water-Resources Investigations Report (03-4074), emergency management personnel and flood mitigation teams had difficulty efficiently identifying areas affected by the flooding, and no warning was given to residents because flood-inundation information was not available. Following that flood, the USGS, in cooperation with the City of Wichita and with assistance from the National Weather Service, developed a publicly accessible Web site that allows residents and business owners along Cowskin Creek to see in advance the expected extent of flooding. But the city’s leaders continuously sought additional ways to protect residents. When the Federal Emergency Management Agency (FEMA) mandated in 2005 that all U.S. levees had to be recertified to ensure the structural integrity of the levees and the safety of the surrounding communities, Wichita was one of the first cities to comply.
In reviewing the options for collecting the topographic data required for the levee assessments, Wichita, along with local and federal partners, decided that the use of LiDAR technology was the best way to gather high-fidelity elevation data. The data would more-accurately define the watershed boundaries as well as address stormwater issues and continued urban growth. The city awarded the contract to Merrick & Company, a multidisciplinary professional services firm headquartered in Aurora, Colo., with extensive experience in LiDAR data collection and processing.
Extraordinary Amounts of Data
The project had to be turned around quickly. The team would begin collecting data in March 2008, and the initial orthoimagery and LiDAR bare earth contours had to be delivered by Aug. 1, 2008, to meet FEMA’s deadlines. The timeline seemed feasible at the outset. However, everyone underestimated the level of work involved in processing the extraordinary amounts of data that would be collected for the project.
Merrick’s airplane was equipped to collect simultaneous LiDAR data and color digital aerial imagery through the use of co-mounted LiDAR and aerial photography sensors. The team collected more than 1,000 square miles of LiDAR and digital aerial imagery but focused specifically on a 33-mile stretch of the Arkansas River that consisted of levees, floodways, improved channels and control structures. The LiDAR data were collected at a 2-foot ground sample distance (GSD) in a 390-square-mile subset for the larger Wichita area and at a 5-foot GSD for the rural area. The remainder of Sedgewick County was flown at a 5-foot GSD.
Due to the numerous canals and levees in the region, a large number of hydro-enforced breaklines had to be collected. To accurately define the geometry of structures and hydrographic features that influence the flow of water, single breaklines were collected for streams, ditches and canals in which the bottom of the channel was less than 2 feet wide. For streams, ditches and canals with channel bottoms greater than 2 feet, dual breaklines were collected at the water/land interface. Breaklines were also collected for all dam outlets and culverts as well as around all water bodies greater then one-quarter acre in size. To increase accuracy, the z-value was taken from the bare-earth LiDAR data along the edge of the water, and the water body was flattened.
Collecting the data--980 gigabytes in all--proved to be the easy part. Since the LiDAR GSD was at a 2-foot distance for the urban areas, the project team had to process approximately 12 million elevation points for every square mile flown. The urban area covered 492 square miles, resulting in some 5.9 billion LiDAR points for the team to process for the Wichita area alone. This figure didn’t even include the remainder of the county area. The high-density LiDAR point cloud was calibrated to meet a 3-inch vertical RMSE.
Following data acquisition, all buildings, trees and electric lines had to be filtered out in order to provide accurate representation of the topography and drainage patterns. Merrick used its custom LiDAR processing software, Merrick Advanced Remote Sensing (MARS), to complete the ridged calibration, filtering and breakline steps.
The initial orthoimagery and LiDAR bare earth contours for the Wichita area were delivered by September 2008, a month behind the original due date. However, no one was willing to cut corners for the sake of speed; accuracy was paramount. To ensure that all of the stakeholders knew where the project stood at any given time, Merrick’s project manager provided written progress reports for each deliverable throughout the project. This level of communication helped ensure a successful outcome, despite the extra time required for data processing. “The City of Wichita was very satisfied with Merrick’s ability to educate city staff on the wide range of digital products that were delivered with the LiDAR,” says Scott Lindebak, PE, the city’s stormwater engineer.
A Common GIS Framework
To create a common GIS framework for storing stormwater data, the team developed an ESRI ArcHydro geodatabase model. ArcHydro accounts for stream networks and their associated topology and connectivity along with the drainage, channel and hydrography data. This sophisticated model was used to determine the flow of water and the drainage area over the project area. The database also provides a very accurate water resource geospatial layer for the city, including more-specific and accurate data on the quantity of water flowing through channels, the origins of the water, and the velocity of the water based on the slope data obtained during the LiDAR data collection.
The ArcHydro geodatabase specifications called for the development of catchments with an average size of five acres as well as the primary drainage line and outlet points for the catchments. The project also included the editing of the watershed boundary dataset (WBD) polygons and attribution using the catchments as the input layer to increase the accuracy and currency of the WBD. The project team developed specific parameters to create accurate flow lines and catchments that accurately portrayed how water flows across the landscape during storm events by integrating an existing GPS stormwater database that included catch basins, manholes and culvert points. The drainage lines provided an accurate stream network that is within feet of the actual location of the stream channel, or in wider stream channels, the centerline of the stream flow.
Developing the geodatabase required the creation of a 60-step process that was custom designed by the Merrick team along with the City of Wichita and AMEC, the firm retained by Wichita for the levee recertification engineering. Each step had to be tested with a variety of flow modeling parameters, blended with other steps, then retested and recombined as the project team determined the optimum flow of tasks for processing the data to develop the best information for the client. The final process involved a considerable amount of trial and error since this approach had not been used previously to create such a detailed catchment dataset.
The 60-step process was found to be invaluable in processing the considerable amount of data. The Merrick team plans on using it for future projects where it will be used for other clients and refined as needed, project by project. The database is also expected to be highly useful to the client. With the accuracy and detail of data, it can be used for identifying areas likely to flood and/or retain run-off; designing culverts and bridges; identifying paths of flood flows; assisting in new development site plans; and supporting overall stormwater mitigation.
Improved Planning Tools
The use of LiDAR technology and the 60-step ArcHydro geodatabase custom processing approach decreased the amount of time required to collect data while providing substantially more data at a lower cost (by about 50 percent) compared to ground surveying or traditional photogrammetry methods. Costs are reduced because of fewer staff involved in data collection, and the data are collected faster. The LiDAR data provides an accurate elevation database of information that can be applied to multiple needs and applications, thus saving governments, and the public, money. The model was created to be a dynamic representation of the stormwater system and, as such, can be updated with current conditions as development or other changes occur that influence the flow of water.
Through the use of scientific, unbiased data, Wichita’s levee recertification provides the public increased safety, protection of property and lives, and preservation of the social fabric of the community. Geospatial technologies played a crucial role in accomplishing this goal. As a result of this project, the levees can be more accurately recertified, future flooding potentials can be identified, and structures can be designed to mitigate future flooding. The city subsequently awarded Merrick an additional contract for the development of data that will aid watershed planning.
Importantly, as one of the first levee recertification projects in the United States, the Wichita project set a standard for accurately and scientifically substantiating engineering decisions for other FEMA levee recertifications. Through LiDAR and imagery collection, high-precision and technically accurate data can be easily and scientifically collected. Layers of geospatial data can be provided that more accurately define an area’s topography, slopes, drainage flow, and flooding issues that provides cost and time efficiencies on data input for engineering hydrologic and hydraulic modeling. With these scientific, nonbiased data, government agencies can provide greater substantiation for the decisions they are making that impact lives, communities and economies.
Sidebar: Managing a Multi-Agency TeamMerrick & Company was the prime consultant for this project. Savoy, a Wichita, Kan.-based surveying firm, provided ground survey and local support to the team. AMEC was retained by Wichita for the levee recertification engineering and was an active member of the team so that the final deliverable data could efficiently be used in the stormwater analysis process. Multiple client agencies were involved in the project, including the Wichita Stormwater Department, Sedgwick County, the National Geospatial Intelligence Agency (NGA), the USGS, and the cities of Maize, Park City, and Valley Center. Each of the clients also had multiple individuals involved in the review and approval process.
Combined with the rapid pace that was required to address FEMA’s deadline request, successful management of the process was crucial. A master schedule was developed at the beginning of the project that included key milestones for distributing data and reports to client contacts at specific times, with required review response deadline dates. Simultaneous distribution of data and reports to all client team members ensured a consistent and continuous method for collecting input and reviewing all client comments in light of each other when they were returned.