Estimating river cross-sections for hydrodynamic models from space-borne and airborne LIDAR altimetry

Hydrodynamic models are vital for water resource management and flood forecasting, but their application is limited by available data sources. In addition to observations of water surface elevation (WSE) and discharge, river channel and floodplain geometry must be estimated to calibrate, validate and operate hydrodynamic models.

While traditional terrain surveying is limited by physical access, political boundaries, safety, or cost, remotely sensed terrain data provides an alternative in data-scarce areas. Digital Elevation Models (DEMs) based on regional or global products have been used for river and floodplain geometry, but their accuracy is limited by low resolution and the inability to estimate geometry in the submerged section of the river channel. Airborne LIDAR missions, where available, provide high resolution point clouds of terrain and water surface elevation. In addition, novel satellite missions provide new opportunities for sensing hydraulic parameters remotely, when airborne LIDAR is not accessible. Estimating hydraulic parameters from these LIDAR datasets allows for the development of hydrodynamic models in flood-prone areas where it was previously not possible to reach sufficient accuracy for effective operation.

With the launch of ICESat-2, water surface slope (WSS) observations became available on a global scale. The LIDAR instrument on ICESat-2 records both terrain and water surface elevation, and the six LIDAR tracks provide 6 simultaneous measurements of WSE, allowing for a WSS estimate. The spatial resolution of just 0.7 m allows for cross-section delineation. But, both ICESat-2 and airborne LIDAR observations reflect strongly on water, hindering observations of submerged channel geometry.

We present a method of combining airborne or ICESat-2 LIDAR observations of the exposed cross-section and WSS with discharge to estimate the conveyance curve for the submerged part of the cross-section. The 1D de Saint-Venant equations are solved while assuming diffusive wave conditions, where acceleration terms are neglected. Under these conditions, water surface slope is equal to the friction slope. Manning’s equation can then be solved for conveyance in the submerged section using observed discharge and water surface slope. With an assumed shape and Manning’s resistance number, a full cross-section is delineated.

The method was initially developed for use with ICESat-2 altimetry measurements but has been extended to work with Airborne LIDAR point clouds when available. The method is shared in an open-source python package, containing functions for processing ICESat-2 or airborne LIDAR data, calculating WSS and producing cross-sections in a preferred data format, using a discharge input from observations or from a hydrological model, provided by the user. The package will allow users to estimate cross-sections in data-scarce areas, ready to be implemented in hydrodynamic models.