There are many aspects of trail design that combine to create a smooth-flowing, appealing, and sustainable trail, and where implemented successfully. This document summarises trail grade and alignment design principles for sustainable trails.
One of the most important design considerations for long-term sustainability is the alignment of a trail relative to the site topography and soils. Trails should be located across the side slope along the contour, and perpendicular to fall line (shortest path down a hill). High slope alignment of the trail will also improve the trail sustainability (Marion & Olive, 2006).
Trail grades are the slopes expressed as a percentage, and significantly influence the trails susceptibility to erosion and water diversion. Maximum sustainable grades have been developed to determine the steepest section of a trail that will be sustainable for local conditions, based on trail grade, soil type and multiple other factors.
In general, for most soil types, trail grade less than 10% is sustainable to minimise user-caused erosion whilst allowing for design flexibility. This is known as the 10% Average Rule.
A widely accepted design principle is ‘the half rule’ which ensures water is shed off the trail at an appropriate angle to prevent erosion and maintain trail flow. The half rule advises that the trail grade does not exceed half the grade of the side slope at the trail location, see image below.