PAST AND FUTURE SHAPING OF MOUNTAINTOP REMOVAL MINED LANDSCAPES BY GULLYING

Initiation and expansion of ephemeral channel (gully) networks follow human landscape disturbances and prevent sustainable land management. To mitigate gullying in disturbed landscapes, we need to understand where and how quickly gullies form. Here we quantify and subsequently forecast gullying after mountaintop removal (MTR) coal mining—one of Earth’s most significant human landscape disturbances—to assess where and when erosion occurs post-mining. MTR dramatically rearranges the hypsometry of drainage basins, producing novel patterns of topographic slope and flow accumulation—two key factors in gully formation. We first constrain the effective erodibility of post-MTR landscapes by using measured slopes, drainage areas, and incision depths of gullies mapped from lidar to calculate the efficiency of ephemeral channel erosion. We then use those erodibility estimates to drive landscape evolution models that predict the next 10 kyr of erosion and deposition in five mined watersheds. Calculated erosion rate constants from >100 gullies range from 10^-4 to 10^-2 yr^-1. We attribute this wide range to the variable vegetation and subsurface material property conditions that exist in post-mining landscapes. Model runs using the observed range of erosion rate constants predict consistent spatiotemporal patterns in ephemeral channel network development. Gullies initiate most efficiently on the margins of valley fills, waste rock structures that fill previous headwater stream valleys, due to their combination of steep slopes and high drainage areas. Gully erosion slows over time as erosive portions of the landscape return towards a pre-mining slope-area equilibrium. Analyzing the sensitivity of gully development to the extent of post-mining vegetation recovery reveals that while the locations of ephemeral channel growth are predictable by simple geomorphic analysis, rates and magnitudes of erosion are highly sensitive to post-disturbance landscape management and recovery. Our results will help inform improved reclamation and management of mined lands, which will be critical as surface mining expands its footprint during the ongoing renewable energy transition.