Modelling from land to water

Author: Monica Miller Rodgers

Climate change is impacting bodies of fresh-water supplies, worldwide. The Colorado River, running through seven states in the U.S. to Mexico where it empties into the Sea of Cortez, is one such example.

Classified as the most endangered river in the U.S. in 2015, water flow is expected to reduce by 10 to 30 per cent by 2050. Warmer weather, less snowfall, and more than hundred dams built into the river’s path have placed significant stress on its ecosystem. This detrimentally impacts fish and wildlife habitat, along with the:

• $26 billion recreational economy,
• 40 million people who depend on the river for daily water use,
• 2 million hectares of farmland irrigation.

With so much riding on the health of Colorado River, the U.S. Bureau of Reclamation (BOR), tasked with overseeing water resource management and the delivery of water, needed to better understand the basin geomorphology.

“We have an ongoing effort to understand what is happening with the Colorado River,” said Carrie Scott, chief of BOR’s Yuma Area Office technical support office. “We model the river to understand sediment deposits throughout the shallow water, and in turn this helps us identify any water delivery issues such as degradation of banklines or ‘choke points’ in flow.”

As with any river’s nature, Colorado River’s meandering is to change its natural course. This poses a threat to nearby structures, such as roads, agricultural and residential communities. BOR strives to keep the river on its course through dredging and armouring bank lines.

“As the water masters of the River, we need to ensure continuous water delivery. We have an ongoing program to contract for a study to map the river by reach to understand how the riverbed is changing,” said Scott. “The data we receive helps us to better model the surface and sub-surface of the river. The better quality data we have, the better we’re able to model and understand the impact of various water flows and produce our engineering designs for projects along the river.”

Turning to Allied GIS, a mapping firm specialising in GIS, imagery and database applications, the firm was tasked to build a digital orthophotography and a topographical map that met the U.S. national mapping accuracy standard of 30 cm. Allied GIS then worked with the Bureau of Economic Geology in the Jackson School of Geosciences at The University of Texas at Austin (the Bureau) to acquire airborne bathymetric LiDAR data and another survey firm to provide topographic LiDAR and ground truthing support.

From bathymetry to topography

A long-time user of Leica Geosystems airborne sensors, researchers at the Bureau used their Leica Chiroptera airborne system to collect data of the lower Colorado River basin in California and Arizona.

Chiroptera has two independent LiDAR scanners:

1. A near-infrared (red-wavelength) for topographic LiDAR
2. A visible (green-wavelength) for bathymetric LiDAR

Chiroptera is designed to acquire data concurrently with both scanners, enabling surveys in complex landscapes with shallow water bodies. The river survey was conducted at an altitude of 400 m above the ground level, which is the optimum altitude for collecting bathymetric LiDAR data. The topographic scanner acquired data simultaneously with a repetition rate of 300 kHz to distinguish the water-surface, and to map the immediate shoreline.

“Chiroptera allowed us to capture bathymetric aspects of the river and features of the immediate terrain,” said Kutalmis Saylam, the lead research scientist for the Bureau. “We’ve been using our Chiroptera for more than four years now, and we can rely on this robust and intuitive system in the field. That’s very important for us as reliable sensors are a must for our work.”

Weather conditions in the survey location were mostly ideal, and a total of nine missions were completed. Flight plans were adjusted to maintain safe aircraft manoeuvring especially in the north end of the survey area with high mountain peaks. The entire survey of 133 flight lines covering an area of 265 km² was completed with 21 hours of airborne time.

Seeing clearly through

Part of the mission included collecting bathymetric data from an area of the deepest dam in the world. The Parker Dam, between the states of Arizona and California has a retaining wall height of 98 m, and discharges water with a varying rate, depending on the season. In coordination with the dam authority, the Bureau scheduled the airborne missions during lower discharge period (146 versus 252 m³/sec) for possible lower turbidity levels in the water-column. Because turbidity absorbs light beams and prevents them from penetrating into deeper water column, it is an essential part of quality assurance procedures for conducting a successful ALB survey. The average reading was 0.41 nephelometric turbidity unit (NTU) in the northern sections of the river, which indicated overall low levels of turbidity in the water-column. In the south, the river was shallower, running through a desert-like environment and losing much of its water to irrigation. These conditions contributed to higher turbidity levels of up to 6.25 NTU and created difficulties for LiDAR beams.

“The strong light beams of Chiroptera easily penetrated the clear waters of the northern sections of the river. We created a seamless water-bottom representation where depths exceeded 10 m,” said Saylam. “In the south, where turbidity levels were higher, we used available returns and interpolation methods to estimate and create a water-bottom representation.

Leica LiDAR Survey Suite LSS v2.3 software contains an updated turbid-water algorithm that filters the backscatter noise created by low or moderate levels of turbidity and selects the most distinctive peak as the water-bottom representation.

“This algorithm enabled the discovery of more returns from deeper waters of the river, and we measured an overall improvement of 81 cm compared to normal returns - a significant achievement in LiDAR bathymetry,” said Saylam.

Facilitating the study of the lower Colorado River basin geomorphology and further understanding of the effects of climate change, the Bureau was able to provide an accurate and detailed map.

“We were able to quantify the water-column depth, map the bottom of the river, the shoreline, and the overall topography of the river as requested by the U.S. Bureau of Reclamation” said Saylam. “This valuable knowledge enables us to undertake similar projects in the future, and the reliability of Chiroptera allows us to continue focusing on similar scientific studies.”

BOR will further use the digital orthophotography and the topographical map to match its in-house survey team’s measurements from boat and ground means. The LiDAR study conclusions will continue to help BOR to keep Colorado River flowing in the right direction and supporting those who depend on it.

“The data we received from the LiDAR was calibrated to our in-house team’s surveys,” said Scott. “This information is important to our ongoing operation to continue to improve our models so we can correctly identify what’s changing what could hap-pen and to develop our projects according to conditions.”