GeoFabrics supports the inclusion of river bathymetry data as part of the DEM generation process. The river bathymetry data must be paired bathymetry point data and a polygon defining the extents of the river. These are specified by the river_bathymetry and river_polygons instruction file options as described in the Instruction file contents.

River bathymetry data can either come from measured observations, or be estimated using the RiverBathymetryGenerator processor module class. This page focuses on the estimation of river bathymetry data.

Estimating River Bathymetry

The river bathymetry of a given channel is estimated in several stages:

1. The slope and width of the channel are estimated from LiDAR derived DEMs.
2. The flow and friction of the channel are derived from River Environment Classification (REC) at the reach level.
3. The depth of the channel is calculated from the river geometry and flow characteristics
4. The bed elevation of the channel is calculated from the depth and water elevation.

A basic validation of the approach has been preformed as described at Bathymetry estimate validation

The pipeline including incorporation into a DEM

There is also an alternative pipeline were Open Street Maps is used to provide an aligned river centreline. This removes the need for the alignment step. The full pipeline of estimating river bathymetry data through including it in a DEM is shown below.

1. River width and slope estimation

The river width and slope (as well as water surface elevation) are estimated along a channel as detailed in the River Geometry Characterisation page.

2. Flow and friction estimation

Flow and friction at each river environment classification (REC) version 1 reach are estimated as described below. The values at each reach are then mapped from REC version 1 to REC version 2.

• Flow is estimated from the statistically based flood statistics web-map, which is based on the REC1 version 1 network. A 1 in 2.33 year flood return is used as the flow above which channel forming flow is caused. This is estimated from the provided flood returns using a Gumbel GFD.

• Friction is estimated for use in the uniform flow theory approach using USGS tables that map river type to n. The REC version 1 bed sediment classification is used as a proxy for reach type. It is a weighted average of proportional cover of bed sediment using categories: 1–mud; 2–sand; 3–fine gravel; 4–coarse gravel; 5–cobble; 6–boulder; 7–bedrock, predicted from a boosted regression tree model.

3. Depth estimation equations

Two equations based on different hydrologic assumptions as used to calculate depth given river width, slope, flow and in one case bed friction. Currently a minimum slope of 0.01% is assumed. Any slopes less than this are replaced by this slope.

Uniform flow theory

This is based on the published approach Estimating River Channel Bathymetry in Large Scale Flood Inundation Models by Neal et al (or on sharepoint for those with access).

Hydraulic geometry

This is based on the published critique Comment on ‘‘Flow resistance equations without explicit estimation of the resistance coefficient for coarse-grained rivers’’ by Rau ́lLo ́pez, JavierBarraga ́n, and M. A ́ngels Colomer by Rupp and Smart (or on sharepoint for those with access).

4. Bed elevation estimation

The depth equations give estimated river depths along a channel. This can be translated to a river bed elevation if you know the water surface elevation, which is estimated as part of the River Geometry Characterisation process. The river bed elevation along the river channel is simply:

river bed elevation = water surface elevation - river depth