Design and Permitting for Closure of a Fly Ash Reservoir High Hazard Dam Modification

 Geosyntec is assisting the client with closure design of a fly ash pond including lowering of a high hazard dam, dewatering, and management of total suspended solids during construction Geosyntec is assisting the client with closure design of a fly ash pond including lowering of a high hazard dam, dewatering, and management of total suspended solids during construction

Geosyntec's client, a power plant in Ohio, generates electricity from two, 1,300-MW, coal-fired units. In anticipation of upcoming federal coal combustion residuals (CCR) regulations, the client decided to initiate closure design for a 300-acre fly ash pond. The site also features a high hazard dam. Acid mine drainage (AMD) from around the reservoir reduces the pH of the water and is currently treated in the reservoir.

The client's objective for the project was to close the landfill in a manner consistent with anticipated CCR regulations in a cost-effective manner, while overcoming numerous technical challenges – including lowering the existing high hazard dam to the maximum extent practicable. Geosyntec designed and successfully permitted the adjacent residual waste landfill. Geosyntec has completed the permit modification design submittal as of December 2013.

Geosyntec’s Scope of Services

Geosyntec conducted a subsurface investigation to characterize the site and then designed a new cover, integrating data on the subsurface conditions and the results of stormwater modeling to mitigate downstream impacts. The new cover topography is designed to mimic the natural ground topography. The closure design also includes: characterization of AMD chemistry and flow rate; completion of the AMD collection system design for use during closure construction; sealing of all penetrations through the dam; and treatment system designs for post-closure conditions when the pond will no longer exist.

In order to lower the high hazard dam as the pond's water level is lowered and fly ash is covered with a final cover, Geosyntec utilized slope stability engineering analyses. Geosyntec completed a 70% design of the lowered dam and conducted a stability analysis and hydraulic modeling of the modified dam to design the spillway and assess potential impacts on downstream and upstream conditions. Final design is underway in 2014. Geosyntec has completed hydrologic and hydraulic modeling to estimate flow through the spillway at different stages of the closure and completed a permit-level design for automated total suspended solids monitoring and controls.

Geosyntec's scope of services also included a conceptual design that developed the regulatory strategies for the pond closure, dam lowering, AMD management, stormwater management, and National Pollutant Discharge Elimination System (NPDES) permitting. The project identified design criteria that were then documented in a conceptual design report that included all documents for the NPDES permit modification necessary to close the pond.

Notable Accomplishments

Geosyntec was able to address the significant regulatory issues and develop design criteria for a cost-effective closure in the conceptual design and completed the permit modification design for submittal to the two responsible regulatory agencies.

Project Overview

  • Location: Ohio, USA
  • Client: Confidential Electric Utility
  • Project Practice Areas: Water and Wastewater, Geotechnical
  • Type of Facility: Power Plant
  • Services Provided: Dam Slope Stability Analysis Ash Pond Closure Design, National Pollutant Discharge Elimination System Permit Modification
  • Type of Work: Coal Ask Pond Closure
  • Company: Geosyntec Consultants
  • Governing Regulation: Clean Water Act
Geosyntec Consultants
Geosyntec is a consulting and engineering firm that works with private and public sector clients to address new ventures and complex problems involving our environment, natural resources, and civil infrastructure.

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