Using topobathymetric Lidar for adaptive management

Using topobathymetric Lidar for adaptive management

Preservation of endangered species habitats along the Platte River

Topobathymetric Lidar technology has become an indispensable tool in environmental management, particularly for monitoring and preserving fragile ecosystems. In central Nebraska, along the Platte River, it plays a pivotal role in ensuring habitat suitability for endangered and threatened species such as whooping cranes, piping plovers and pallid sturgeon. This article delves into how the Platte River Recovery Implementation Program (PRRIP), in collaboration with NV5, is using state-of-the-art topobathymetric Lidar to monitor sediment dynamics, providing actionable insights for adaptive habitat management and conservation efforts.

Habitats in and along the Platte River in central Nebraska serve an important purpose in the life cycle of three endangered and threatened species – whooping cranes, piping plovers and pallid sturgeon. But changes in geomorphology, including erosion and deposition, which are eliminating shallow areas of water and creating deeper, fast-moving channels, are threatening the suitability of the river to support these species.

Since 2007, PRRIP – a collaboration between Nebraska, Colorado, Wyoming and the U.S. Department of the Interior – has focused on the efficient and effective management of water and riverine habitats for these species. To achieve its goals, PRRIP requires a detailed understanding of the area’s geomorphology. It needs to know how water flows impact the topography, bathymetry and riparian vegetation, as well as the changes resulting from both anthropogenic pressures and natural processes. Leveraging high-resolution topobathymetric Lidar data, PRRIP has gained valuable insights across the geography year-over-year, enabling adaptive management of sediment augmentation efforts to ensure the area remains suitable for the whooping crane and other species inhabiting the area.

Actionable intelligence from topobathymetric Lidar

To get an accurate look at sediment volume dynamics and to determine how habitat management work impacts water flows, PRRIP contracted NV5 to conduct multi-temporal acquisitions of topobathymetric Lidar. The survey posed several challenges, since it covered a 90-mile swath of the Platte River floodplain comprising a complex shallow river system with thousands of channels and low-gradient gravel bars. 

Beginning in 2016, NV5 conducted annual aerial surveys using cutting-edge technology that combined hydrographic and topographic Lidar sensors. This sensor package can deliver the high pulse rate and narrow beam divergence needed to accurately map the complex river morphology of braided channels and shallow land-water interfaces.

Figure 1: Endangered whooping crane with sandhill cranes in a Nebraska cornfield. (Image courtesy: Colleen Childers)

To provide PRRIP with the most accurate data, NV5 developed proprietary processes for water surface modelling and refraction correction. When this data was anchored to hundreds of field-surveyed control points distributed throughout the site, the spatial accuracy and thus the year-to-year comparability was improved. This level of accuracy and comparability is critical for PRRIP to obtain meaningful insights into precise sediment volume analyses of aggradation and degradation. While error rates of 15 centimetres are the accepted standard accuracy specification, this project pushed the limits of the technology to deliver vertical accuracies of around five centimetres.

Through this multi-year project, NV5 data has provided PRRIP with valuable insights into the Platte River floodplain, allowing it to optimally plan the use of land, water and fiscal resources to achieve the long-term goal of improving and maintaining target species habitats.

Figure 2: Relative elevation model derived from topobathymetric Lidar showing progression of incision and deposition downstream of augmentation. (Image courtesy: Platte River Recovery Implementation Program)

Sediment augmentation

A recent application of the data focused on a whooping crane habitat along the central Platte River. In this area, there is a well-known historical sediment deficit downstream from a hydroelectric plant. PRRIP was concerned that the river was deepening and narrowing because of the mining of sediment from the channel bed downstream by the hydropower return flows. It also worried that these changes would continue to migrate downstream and impact protected areas used by whooping cranes and other species.

Before the inception of PRRIP, there had been efforts to quantify the magnitude of the sediment deficit and develop plans to correct the issue. In 2017, PRRIP began a full-scale sediment augmentation experiment immediately downstream of the hydropower return. An average of 65,000 tons of sediment per year were augmented annually from 2017 to 2022 at a cost of as much as US$250,000 per year. During the same period, NV5 conducted regular aerial topobathymetric surveys to assess changes to the landscape. Using this data, PRRIP could analyse the effectiveness of sediment augmentation and determine whether its investments were helping to increase habitat suitability for whooping cranes, while not causing detrimental effects downstream in pallid sturgeon habitats.

Spatial comparisons were made using year-to-year bathymetry datasets along the targeted area. PRRIP discovered that, while erosion was still occurring, it had lessened as a result of its efforts. Upstream of the Overton Bridge, the channel bed had been degrading at a rate of 0.8 to 1.1 inches per year, averaged across the entire area. After augmentation, the rate of bed erosion upstream of the bridge decreased between 45% and 60% to 0.4 inches per year. Downstream of the bridge, the bed elevations remained stable.

Figure 3: Sand being augmented to a higher velocity pilot channel. (Image courtesy: Platte River Recovery Implementation Program)

Adaptive management

While the recovery programme has made commendable progress, the repeat data collected by NV5 supports adaptive management. After analysing the performance of sediment augmentation over five years, PRRIP managers can now refine the plan more effectively to meet future goals.

In addition to sediment augmentation, PRRIP uses topobathymetric Lidar and associated imagery from NV5 for various other purposes, including the annual quantification and monitoring of riparian vegetation type and structure. This vegetation mapping aids in monitoring endangered species habitats across the 13,000-acre protected area, enabling the evaluation of habitat needs and usage and the success of management efforts.

Geospatial data, including topobathymetry and aerial imaging, plays a crucial role in large-scale projects undertaken by PRRIP. Investing in the collection of highly accurate data enables programme managers to make informed decisions based on scientific knowledge and to adapt as they gain a better understanding of the evolving landscape.

Figure 4: Bulldozers pushing historic river terraces into incised river channel. (Image courtesy: Platte River Recovery Implementation Program)

Figure 5: Sediment augmentation under construction near Lexington, NE. (Image courtesy: Platte River Recovery Implementation Program)
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