从陆地到水的建模

Author:莫妮卡·米勒·罗杰斯（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 toreduce by 10 to 30 per centby 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,
• 4000万人依靠河流每天用水，
• 200万公顷的农田灌溉。

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,”Bor Yuma Area Office技术支持办公室的负责人Carrie Scott说。“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.”

与任何河流的性质一样，科罗拉多河的蜿蜒曲折是改变其自然路线。这对附近的建筑物构成了威胁，例如道路，农业和住宅社区。博尔（Bor）努力通过挖泥和武装银行线路将河水保持在路线上。

“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 accuracystandard 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 theirLeica Chiroptera空降系统收集加利福尼亚和亚利桑那州下科罗拉多河流域的数据。

Chiroptera有两个独立的激光扫描仪：

1. 地形激光雷达的近红外（红波长）
2. 测深痛的可见（绿波长）

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高于地面的400 m，这是最佳海拔用于收集测深痛数据。地形扫描仪以300 kHz的重复速率同时获得数据，以区分水面，并绘制直接的海岸线。

“ chiroptera允许我们捕捉河流的测深方面和近距离地形的特征”该局的主要研究科学家库塔米斯·塞拉姆（Kutalmis Saylam）说。“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 of133条飞行线covering an面积265公里²完成了21 hours of airborne time.

清楚地看到

任务的一部分包括收集来自世界上最深的水坝. 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）在水柱中可能较低的浊度水平。由于浊度吸收光束并防止它们渗透到更深的水柱中，因此它是进行成功的ALB调查的质量保证程序的重要组成部分。河流北部部分的平均读数为0.41肾小管浊度单元（NTU），这表明水柱中总体低水平的浊度。在南部，这条河很浅，穿过沙漠状的环境，将大部分水损失到灌溉中。这些条件导致高达6.25 NTU的浊度较高，并为激光束带来了困难。

“骨翅目的坚固光束很容易渗透到河北部的晴朗水域。我们创建了一个无缝的水底表示，深度超过10 m，”塞拉姆说。“在浊度较高的南部，我们使用了可用的回报和插值方法来估计和创建水底代表。

Leica LiDAR调查套件LSS v2.3软件包含一种更新的浑浊水算法，该算法过滤了低或中等水平的浊度产生的反向散射噪声，并选择最独特的峰作为水底表示。

“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,”塞拉姆说。

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”塞拉姆说。“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.

“我们从LIDAR收到的数据已校准了我们内部团队的调查，”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.”