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减少10％至30％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.

“作为河流大师，我们需要确保连续的水输送。我们有一项持续的计划，可以通过伸直人的研究签约，以了解河床的变化，以了解河流。”斯科特说。“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.”

转向专门从事GIS，图像和数据库应用程序的映射公司Allied GIS，该公司的任务是构建数字矫形器摄影和一张符合美国国家映射准确性的地形图标准30厘米. 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

Leica Geosystems空降传感器的长期使用者，该局的研究人员使用了他们的Leica Chiropteraairborne 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 of400 m above the ground level, which is the optimum altitudefor 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.“我们已经使用了我们的Chiroptera已有四年多了，我们可以依靠该领域的这种强大而直观的系统。这对我们来说非常重要，因为可靠的传感器是我们的工作。”

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 of133 flight linescovering anarea of 265 km²was completed with21 hours of airborne time.

Seeing clearly through

Part of the mission included collecting bathymetric data from an area of thedeepest 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.

“这种算法使河流深水的收益获得了更多的回报，与正常收益相比，我们的总体改善为81 cm，这是激光雷达测深的显着成就，”said Saylam.

该局促进了科罗拉多河盆地盆地盆地盆地的地貌学的研究，并进一步了解气候变化的影响，该局能够提供准确详细的地图。

“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将进一步使用数字矫形器和地形地图，以匹配其内部调查团队的测量，从船和地面手段进行测量。激光研究的结论将继续帮助BOR使科罗拉多河朝着正确的方向流动，并支持那些依赖它的人。

“The data we received from the LiDAR was calibrated to our in-house team’s surveys,”斯科特说。“此信息对我们正在进行的操作继续改善模型很重要，因此我们可以正确地确定正在改变什么可能是可以改变的，并根据条件开发我们的项目。”