Case Studies
Bhutan: “Oversizing” Drainage Culverts for Asian Elephants Under the Southern Highway Corridor – Raidak-Lhamoizingkha Road
As one of five highway segments constructed as part of the new 183 km East-West National Highway, the 18 km Raidak- Lhamoizingkha Road is a two-lane highway that passes through Asian elephant habitat outside of designated protected areas along the India-Bhutan border. To avoid habitat degradation, fragmentation and barrier effects to the free movement of elephants, drainage culverts at three of four perennial stream crossings were “oversized” to increase the height and widths to better accommodate elephant passage. Prefabricated, corrugated metal plate arches were constructed to better accommodate passage of elephants as they use the stream corridors especially for seasonal movement.
The projects are the first elephant underpasses constructed in Bhutan and range in height from 5.6 to 7.6 meters. During two years of monitoring in 2015–2017, 70 groups of elephants encompassing 145 individuals were recorded with 76% passing through the structures (Chogyel et al., 2017). This monitoring has been expanded by ADB to include a wider range of underpass sizes; elephant use of structures with heights <5 meters have been documented.
(1-3) Varying underpasses in Bhutan that are monitored with cameras; (4) Elephant using underpass © Norris Dodd/Asian Development Bank
References:
Chogyel, K., Dodd, N. and Yangzom, K. (2017). ‘Wildlife use of highway underpasses in southern Bhutan’. 2017 Proceedings of the International Conference on Ecology and Transportation, Center for Transportation and the Environment. Raleigh, NC, USA: North Carolina State University.