Designing forest stream crossings

Designing forest stream crossings
 November 2021   

Stream crossings are an important component of our forestry infrastructure. “We build about 1500 to 2000km of new road in our plantation forests each year, and we need about one to two stream crossings per kilometre. This adds up to about 3000 crossings per year that can range from multi-span engineered crossings across larger rivers, all the way through to the many single culvert installations,” notes Professor Rien Visser, who heads Forest Engineering at the School of Forestry, University of Canterbury. 

They not only need to be the right diameter (or height) to pass a flood flow, but it is also important to protect the natural ecosystem by ensuring fish passage. Two final year forest engineering students, Luke Wilson and Drew Wood, have worked on research projects to help the industry improve their design practices. 

While stream crossing sizing practices are set out in guides like the New Zealand Forest Road Engineering Manual (NZFOA 2020), it is not an easy process, with highly variable outcomes. You need to calculate the flood flow, and then match it to the stream crossing opening. It depends not only on the chosen approach, but also the person doing it, and this can lead to uncertainty. Most of the flood flow calculation factors are relatively easy to measure, such as catchment size and stream slope from maps, and rainfall data from the National Institute of Water and Atmospheric Research (NIWA) website. However, all of the culvert sizing equations require a ‘Catchment Factor’ that reflects the vegetation and soil type and hence the expected flood flow response from a larger rainfall event. This Catchment Factor will range between 0, where none of the rainfall will exit the catchment, and 1 where all of the rainfall will come out in the flood. 

This is where Luke Wilson has made a major contribution. He has developed a Geographic Information System (GIS) Tool that semi-automates the process of culvert design with minimal input from the designer. Developed using ArcGIS’s model builder, it uses a LiDAR- derived Digital Elevation Model (DEM), a Land Cover Database and a Fundamental Soil Layer for soil drainage information (both from Land Resource Information System). These GIS layers provide the slope, land cover and soil contributions so that a weighted runoff coefficient representing the entire catchment can then be calculated. In comparison, doing this manually, the designer would be expected to just estimate a single factor for the whole catchment.

The benefit of building this GIS Tool is that it also allows a sensitivity analysis to be undertaken. For example, we know that a forested catchment discharges less water during a rainstorm than one that is harvested, but what is the scale of this effect? In the figure (right) is a catchment of 36ha for which we want to design a culvert. When still fully forested, the Catchment Factor is 0.45 leading to a culvert size of 1.8m. For the same catchment when harvested, the Factor increases to 0.7, an expected increase in flood flow of 36% leading to...

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