cover short grass prairie. - Used Cabba fine silt loam as soils input. - Chose one location draining most of Bluff B (most heavily altered of the mined bluffs) for ...
Improving Reclamation Design through Runoff and Erosion Modeling of High-Intensity Precipitation Events at the Riley Pass CERCLA Site, Custer National Forest, Harding County, South Dakota James A. (Andy) Efta- Hydrologist, USDA Forest Service- Custer National Forest Dan Seifert- Geologist/On Scene Coordinator, USDA Forest Service- Custer National Forest
Background: Lack of vegetation, fine soil textures, contiguous oversteepened slopes and a flashy runoff regime have combined to aggravate rill and gully erosion on hillslopes at Riley Pass. Below the mined bluffs, runoff is forming deep gullies, impacting water quality and transporting contaminated sediments onto adjacent private lands. Purpose: 1) Identify the magnitude of runoff events, especially events greater than 100 year recurrence interval, through analysis of existing records and further statistical analysis 2) Model the effects of runoff and erosion events using process-based modeling tools 3) Evaluate modeled effects against tolerable rates of surface erosion and mine waste transport
Methods: Runoff and erosion modeling: To evaluate runoff and erosion under existing landscape conditions, the Water Erosion Prediction Project (WEPP) was used (Flanagan and Nearing, 1995). WEPP is a process-based, distributed erosion prediction model. The GeoWEPP interface to WEPP (Renschler, 2003) provides the added utility of using GIS inputs to model hillslope and catchment runoff and erosion processes in three dimensions. - Used 1 meter LiDAR-derived DEM for elevation input - Newell, SD climate was modified using PRISM (Daly et al., 1994) for Riley Pass location and elevation. - Surface land management was modeled as 18% ground cover short grass prairie - Used Cabba fine silt loam as soils input - Chose one location draining most of Bluff B (most heavily altered of the mined bluffs) for initial modeling effort Single storm event analysis: 1) Plotted, calculated and/or interpolated partial duration precipitation values using Rainfall Atlas of the United States 2) Calculated 500- and 1000-yr recurrence interval events for multiple durations 3) Exported one example hillslope modeled in GeoWEPP to WEPP for Windows, modeled hillslope for multiple single storm extreme events
Results: Table 1. Peak runoff rate, detachment rate, flow shear stress, and max soil loss for a single hillslope under multiple single storm precipitation events.
Study Area: Riley Pass falls within the North Cave Hills in northwestern South Dakota approximately five miles west of Ludlow, SD. Almost all drainages within the North Cave Hills are ephemeral, only conveying water during flashy runoff events such as high intensity thunderstorms. Soils have been highly altered as a result of the mining activity, but adjacent soils are Cohagen and Cabba fine sandy loams (loamy, mixed, superactive, calcareous, frigid, shallow Typic Ustorthents).
References available upon request
Conclusions: ₋ Nearly 50% of all mapped acres are eroding at rates beyond tolerable soil loss (T) for Cabba/Cohagen soil series ₋ Flow shear stress from runoff was similar for extreme events across an order of magnitude ₋ Maximum modeled average annual sediment delivery for a single hillslope was 12.3 ton/ac/yr Future Work: ₋ Model single storm events of 6-, 12-, and 24-hr duration ₋ Model hillslopes and watersheds using rill/interrill erodibility and hydraulic conductivities from altered soils within mine perimeter ₋ Compare pre- and post- mining land surface and runoff/erosion model outputs
