Modeling the impacts of erosion and sediment control practices for roadway construction sites on water quality in Oklahoma

Abstract

Construction typically requires mass clearing and grading, causing project site areas to be unstable. As a result, they lack ground cover to protect against rainfall and runoff, which results in soil degradation and erosion. Erosion introduces nonpoint source (NPS) suspended solids pollution into water bodies, which diminishes water quality and reduces the lifetime of water resources. In the past few years, there has been a significant improvement in water quality using erosion and sedimentation best management practices (BMPs). Despite improvements in NPS management, many challenges remain due to the complexity of BMP implementation. Desirable environmental protection and appropriate drainage and erosion control are only achieved when drainage, erosion and sediment control (ESC) work together.

The Oklahoma Department of Transportation (ODOT) is responsible for implementing approaches to reduce environmental impacts of construction on thousands of bridges and culverts across the state. The goals of this project were to summarize existing BMPs for ESC and to estimate the impacts of BMP implementation on suspended solids loadings in Oklahoma.

The annual soil erosion rate for select Oklahoma counties was determined using ArcGIS Pro to run the Revised Universal Soil Loss Equation, Version 2 (RUSLE_2) based on soil, land use, elevation, and climatic data. All factors used in RUSLE_2 were calculated with local data from the United States Department of Agriculture, Natural Resources Conversation Service (USDA_NRCS). Moreover, the RUSLE_2 Graphical User Interface (GUI) was used with a database of ODOT projects to estimate sediment loadings from the roadway construction sites into Oklahoma waterways. Furthermore, the efficiency of BMPs for multiple combinations of different land surface conditions (soil erodibility and slope) were evaluated. The estimates of sediment yield from the ODOT construction sites were then compared with background sediment production in each county to estimate the impacts of construction and BMPs on water quality in Oklahoma.

The mean annual sediment yield for select Oklahoma counties from 2010-2017 was 0.05 and ranged from 0.002 to 0.3728 tons per hectare per year (t/ha/yr). The average annual erosion prediction for this study is similar to previous Oklahoma estimates of 0.027 to 0.0465 tons/acre/year for good to excellent land conditions. The results indicate that, in some cases, construction sites can increase the annual erosion rates up to 40 times the natural erosion rate. In some cases, by using ESC BMPs, sediment yields can be decreased up to 90 percent relative to unprotected construction site surfaces. The model results show a high correlation between slope steepness and the efficiency of erosion blanket BMPs. Temporary seeding with mulching BMPs acts better in the area with higher average rainfall. Finally, sediment production controls, such as silt fences and fiber logs, exhibited better efficiency in low to medium slope areas.