naughton north ash pond hazard potential … · i hereby certify that this initial hazard potential...

Status: Rev. 0 01 September, 2016 Document No: NP-TR-015 Naughton North Ash Pond Hazard Potential Classification Assessment

NAUGHTON NORTH ASH POND HAZARD POTENTIAL CLASSIFICATION ASSESSMENT

Prepared for: PacifiCorp, Naughton Power Plant Project Manager: Chad Tomlinson Date: 01 September 2016

Document No.: NP-TR-015

Quality Assurance Statement

Office Address MWH 2890 Cottonwood Parkway, Suite 300 Salt Lake City, UT 84121

Prepared by Jacob Trauscht Reviewed by Alex Edstrom Approved for Issue by Chad Tomlinson

Revision Schedule

Rev No. Date Description

Prepared

By

Reviewed By

Approved

By

0 09/01/2016 Issued for Use J. Trauscht A. Edstrom C. Tomlinson Comments:

This document has been prepared for the benefit of PacifiCorp in accordance with the generally accepted practices and standards in use at the time it was prepared. No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other person.

This disclaimer shall apply notwithstanding that this document may be made available to other persons for an application for permission or approval to fulfill a legal requirement.

CERTIFICATION

I hereby certify that this Initial Hazard Potential Classification Assessment for North Ash Pond located at PacifiCorp’s Naughton Power Plant has been prepared in accordance with §257.74(a)(2) – Periodic hazard potential classification assessments of the Coal Combustion Residuals (CCR) Rule - 40 CFR Parts 257 and 261, “Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals From Electric Utilities; Final Rule published in the Code of Federal Regulations (CFR) on April 17, 2015 with an effective of October 19, 2015.

By:

Date: 09/01/2016

Chad Tomlinson, P.E.

Wyoming P.E.# 15375

Status: Rev. 0 2 01 September 2016 Document No: NP-TR-015 Naughton North Ash Pond Hazard Potential Classification Assessment

REPORT OF §257.73 (A)(2) – SURFACE IMPOUNDMENT HAZARD POTENTIAL CLASSIFICATION ASSESSMENT

1. INTRODUCTION This document presents the initial hazard potential classification for PacifiCorp’s North Ash Pond (NAP) located at the Naughton Power Plant, approximately six miles southwest of Kemmerer, Wyoming. As stated in the Environmental Protection Agency Code of Federal Regulations (EPA CFR) §257.73(a)(2), this document does not involve the analysis of the characteristics and construction quality of the impoundment in question, but instead provides a recommendation for the appropriate hazard classification relative to the potential impacts resulting from a failure and the uncontrolled release of the NAP and/or NCWP contents. The impacts associated with the different hazard classifications are described by the Federal Emergency Management Agency (FEMA) Hazard Potential Classification Guidelines (FEMA, 2004) and are shown in Table 1.

Table 1 - Description of the FEMA Hazard Classifications

Hazard Classification Potential Loss of Human Life Economic, Environmental, and Lifeline Losses

Low None Expected Low and generally limited to the owner

Significant None Expected Yes

High Probable. One or more expected. Yes (but not necessary for this classification)

The Washington State Department of Ecology (WSDOE) provides guidelines for developing peak flood estimates and the development of hazard classifications of earthen dams (WSDOE, 2007). These guidelines were developed from the Bureau of Reclamation (USBR) guidelines (USBR, 1988). The WSDOE guidelines provide improved guidance for determining economic and environmental losses in the event of a dam breach. However, the FEMA classification is more rigorous in stipulating that the probable loss of 1 life receives a “High” hazard classification. The WSDOE stipulates a “High” hazard classification in the event of 7 to 30 probable lives lost in a dam breach.

MWH utilized the WSDOE approach in determining peak flows during the dam breach event, as well as in classifying economic and environmental losses. However, the FEMA classification system approach was used, where a “High” classification was reserved for probable loss of life of 1 or more person. Determination of “Significant” or “Low” hazard levels was accomplished using WSDOE guidelines which provide additional context to “Low” or “Significant” hazard levels.

2. SITE BACKGROUND The NAP consists of an ash settling pond and a clear water storage pond, separated by an intermediate dike. The surface area of the NAP and North Clear Water Pond (NCWP) at normal operating capacity


(maintaining five feet of freeboard) is 107 acres with a capacity of 2,181 acre-feet and 64 acres with a capacity of 1,360 acre-feet, respectively. Overtopping of the NAP is controlled through a drop decant structure and a weir discharge structure where clear water is discharged to the North Fork of Little Muddy Creek.

3. HAZARD ASSESSMENT BACKGROUND Initial hazard classifications for ponds at the Naughton Power Plant were evaluated by Wyoming’s State Engineer Office (SEO) utilizing the FEMA hazard classification system for dams (URS, 2015). The State Engineer produced a “Low” hazard classification level for the NAP and NCWP.

Clough, Harbour, and Associates (CHA), on behalf of EPA, prepared an independent classification of the site using the EPA’s hazard classifications. The result of CHA’s analysis was a “Significant Hazard Potential” impoundment listing for the NAP and NWCP (2009). As specified in the CHA report (2009), the “Low” hazard classification assigned by the SEO did not “address potential environmental damage or economic losses from damages to adjoining lands from a release caused as a result of dam/dike failure.”

The “Significant” designation applied by CHA appears to be in line with similar hazard classifications adopted by the Tennessee Valley Authority (TVA) for evaluating CCP facilities as shown in Table 2 (TVA, 2009). Additionally, the TVA guidelines appear consistent with the WSDOE guidelines for economic and environmental loss.

Table 2 – Guidelines for TVA Evaluating FEMA Criteria

Hazard Classification Potential Loss of Human Life

Potential Environmental Impact

Potential Economic and Infrastructure Lifeline Losses

Low 0 Contained on [TVA] property or minimal off-property impact

No expected damages to public roads, powerlines, etc.

Significant 0 Off [TVA] property, may enter waters of the U.S.

Expected damages to public roads, powerlines, etc.

High 1 or more Any impact Any impact

4. CURRENT HAZARD CLASSIFICATION Given the fact that the NAP is located immediately upstream of the NCWP, it was assumed for the any failure of the NAP would result in a cascading failure of the NCWP. Therefore, this current hazard classification evaluated the potential inundation area produced by the simultaneous failure of the NAP and NCWP. The flood inundation area was used to evaluate potential Populations at Risk (PAR). Environmental and economic losses were also considered for the flood effected area. The flood inundation area produced by the dam breach estimated in this analysis is given in Figure 1.

There are no habitations within the estimated inundation zone and therefore no PAR. Potential environmental impacts include degradation of the local water quality, erosional damage to downstream


banks and ecosystems, and plant and wildlife exposure to coal combustion residuals (CCRs). The potential economic impacts include damage to one (1) uninhabited structures, commercial buildings, agricultural lands and crops, livestock, roads, highways, and utilities.

From these results, MWH believes a “Significant” hazard potential rating is appropriate due to the lack of PAR and the anticipated off-site environmental and economic impacts associated with the breach of the NAP. Flood wave analysis, locations of concern, economic losses, and environmental hazards are discussed below.

Figure 1 - Extent of Inundation along the NAP Drainage


4.1. Dam Breach Evaluations The following dam breach scenarios were evaluated:

• Scenario 1 – Failure of the NAP with normal water levels, 5.0 ft below the maximum crest elevation (6,898.5 ft above mean sea level (amsl));

• Scenario 2 – Failure of the NAP with water at the maximum crest elevation (6,903.5 ft amsl);

• Scenario 3 – Failure of the NCWP with normal water levels, 5.0 ft below the maximum crest elevation (6,885.5 ft amsl);

• Scenario 4 – Failure of the NAP with water at the maximum crest elevation (6,890.5 ft amsl);

• Scenario 5 – Cascading Failure of the NCWP into the NAP (NAP + NCWP) with both ponds at normal operating levels (6,898.5 and 6,885.5 ft amsl, respectively);

• Scenario 6 – Cascading Failure of the NCWP into the NAP (NAP + NCWP) with both ponds storing water at the maximum crest elevation (6,903.5 and 6,890.5 ft amsl, respectively).

The discharge flows resulting from each scenario are presented in Table 3.

Table 3 - Calculated Breach Peak Flows

Scenario

Normal Operating Condition Failure Maximum Water Surface Elevation Failure Peak Breach Flow

(ft3/s) Time to Peak Flow

(hours) Peak Breach Flow

(ft3/sec) Time to Peak Flow

(hours)

North Ash Pond 33,850 1.2 44,845 1.3

North Ash Clearwater Pond 23,017 1.1 29,340 1.2

NAP + NCWP 49,840 1.2 56,663 1.3

4.2. Breach Flow Calculations The Fread method was used for calculating the peak flows from an overtopping dam breach. The method is outlined by the WSDOE (2007) and was developed for small embankment dams. The calculations utilize a breach formation factor to calculate the volume of eroded material, breach geometry, and time for breach development. This method is suitable for dams with a breach formation factor (BFF) of 100 acre-ft2 or greater, making it suitable for this analysis given the NAP and NCWP BFF are 109,047 acre-ft2 and 62,528 acre-ft2 respectively (where the BFF is the product of the storage volume and height of breach).


The breach flows calculated using the Washington State Dam Safety Guidelines are based on the following equation:

𝑄𝑄𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 3.1 𝑊𝑊 𝐻𝐻1.5 �𝐴𝐴

𝐴𝐴 + 𝜏𝜏√𝐻𝐻 �3 𝑓𝑓𝑓𝑓3

𝑠𝑠𝑠𝑠𝑠𝑠

Where:

Qpeak = Dam breach peak discharge (cfs)

W = Average breach width (ft)

H = Initial Height of Water (ft) over the base elevation of breach

τ = elapsed time for breach development (min)

A = 23.4Sa/W (acres), where Sa is the surface area of the NAP or NCWP

For cascading failures, the WSDOE provides the following modification for the surface area term:

𝑆𝑆𝑝𝑝ʹ = 𝑆𝑆𝑆𝑆 �𝑉𝑉𝑤𝑤 + ∑𝑉𝑉𝑤𝑤𝑤𝑤𝑤𝑤

𝑉𝑉𝑤𝑤 �

Where:

𝑆𝑆𝑝𝑝ʹ = The adjusted surface area of the downstream reservoir used for calculating “A” above (acres)

𝑆𝑆𝑝𝑝 = The surface area of the downstream reservoir (acres), [in this case the NAP]

𝑉𝑉𝑤𝑤 = The volume of water in the downstream reservoir, excluding the contribution from upstream dam failures (acre-ft), in this case the volume of water in the NAP

∑𝑉𝑉𝑤𝑤𝑤𝑤𝑤𝑤 = The sum of all upstream volumes of water released from the failure of all upstream dams (acre-ft), in this case the volume of water released from the NCWP

The breach formation time is calculated as follows:

𝜏𝜏 = 0.036 𝑉𝑉𝑚𝑚0.36

Where:

𝜏𝜏 = The breach formation time (hours)

𝑉𝑉𝑚𝑚 = The volume of material eroded during the breach (yds3)


The peak flow was assumed to coincide with the breach formation time. The results of this analysis were used as input to the inundation modeling.

4.3. Inundation Modeling The peak flow developed from the previous analysis was modeled as a 2-D, unsteady flow using the U.S. Army Corp of Engineers River Analysis System (HEC-RAS). Model parameters and boundary conditions are described below.

Topographic Data:

Site topography data for the areas near the ponds,

Stage-Area-Storage data as developed by WET (2015), and

1/3 arc-second resolution raster files obtained from the National Elevation Dataset developed by the United States Geological Survey to investigate downstream impacts. The root mean square error of the 1/3 arc-second map is 1.55 m (5.09 ft) overall and 2.22 m (7.28 ft) for shrub/scrub land use types, which dominate the area (Gesch et al., 2014).

Grid Spacing:

The two dimensional model was evaluated using 50’ x 50’ grid spacing. Finer grid spacing was evaluated but had no discernable changes to the resulting inundations area.

Land Use / Land Cover:

Land use designation was evaluated using the National Land Cover Database (NLCD) produced by the Multi-Resolution Land Characteristics Consortium (Homer et al., 2011). The land use raster map was assigned Manning’s roughness values using the trimmed mean classifications developed by Bent et al. (2013) as shown below in Table 4. The region is predominately classified as shrub/scrub.

Table 4 – Assigned Manning’s Roughness per NLCD Assigned Land Cover Class

NLCD Land Cover Class NLCD Index Number Assigned Manning’s n Barren Land Rock/Sand/Clay 31 0.033 Cultivated Crops 82 0.037 Deciduous Forest 41 0.12 Developed, High Intensity 24 0.06 Developed, Low Intensity 22 0.046 Developed, Medium Intensity 23 0.053 Developed, Open Space 21 0.027 Emergent Herbaceous Wetlands 95 0.089 Evergreen Forest 42 0.116 Grassland/Herbaceous 71 0.037 Mixed Forest 43 0.123


NLCD Land Cover Class NLCD Index Number Assigned Manning’s n Open Water 11 0.027 Pasture/Hay 81 0.039 Shrub/Scrub 52 0.054 Woody Wetlands 90 0.089

Boundary Conditions:

1. The upstream boundary condition is the assumed breach flow hydrograph. The hydrograph shape is not determined in Washington State guidelines; therefore, MWH assumed a triangular hydrograph shape as shown below (Figure 2). The area below the hydrograph equates is equal to the total volume of water above the dam breach and the time to peak and peak flow are maintained calculated using the WSDOE methods described previously.

2. Outflow Hydrograph – set as channel flowing at normal depth with 0.1% slope as recommended by HEC-RAS.

Figure 2: Breach Hydrograph for Cascading Failure of the NAP and NCWP at Maximum Water Surface Elevation for both Ponds

The results of the HEC-RAS model were overlain onto an aerial image, showing the extent of the inundation (Figure 1), notably extending to the structures located 12 miles downstream of the NAP.

0100002000030000400005000060000

0 0.5 1 1.5

Dis

char

ge, m

3/s

Time, hours

Dam breach hydrograph


4.4. Failure Impact Assessment

4.4.1. Introduction

The failure impact assessment was used to assist in the assessment of PARs from a dam failure scenario. The results of which were used for determining the consequence categories of the NAP.

4.4.2. Locations of Concern

Nearby structures (locations of concern), which could represent potential PARs, were initially identified by using aerial images of the site (Figure 3) and by reviewing the information available through the Lincoln County Assessor (Account #R0000077).

Figure 3 – Houses and structures North of State Road 412, located approximately 12.5 miles to Southeast of the Naughton North Ash Pond (Source: Google Earth September 2014)

The location of concern is identified as 80 acres of rural land owned by Sims Sheep Company LLC. The 80 acres are divided into 79 acres of agricultural use and 1 acre of residential use. The structures identified by the assessor are shown in Table 5. None of the structures are listed as permanent or temporary inhabitance.


Table 5 – Structures Identified by the Lincoln County Surveyor in the Location of Concern

Building Sq. Feet Built Rooms/Bedrooms/Bath

Salvaged Farm Utility Building 640 1900 0/0/0

Farm Utility, Loafing Shed #478 1,500 1995 0/0/0

Farm Utility, Loafing Shed #478 864 1990 0/0/0

Barn #397 1,200 1980 0/0/0

Light Commercial Utility #1471 1,728 2005 0/0/0

Farm Utility, Loafing Shed 680 2010 0/0/0

Salvaged Masonry Stone Structure 360 1900 50/0/0

4.4.3. Inundation Assessment

The main focus of the inundation assessment was to determine whether and to what extent the uninhabited structures identified in Figure 3 would be inundated by a breach. Inundation modeling was performed using HEC-RAS to approximate the depth of flow at the locations of concern as described in Section 4.3. Inundation maps were generated showing the location of concern, off-site topography, extent of flooding, and possible flow directions for breach scenarios are presented as Figure 1 and Figure 4. The extent of inundation was modeled under the highest expected breach flows, showing the flows would inundate the structures by up to 20 feet of water near the drainage and 10 feet of water in the adjacent floodplain area (Figure 4).


Figure 4 - Extent of Inundation along NAP Drainage at the Location of Concern

4.4.4. Economic Loss Assessment The classification of economic losses is based on damages to inhabited dwellings, commercial and production buildings, agricultural lands and crops, livestock, roads, highways and utilities. The classification levels are (WSDOE, 2007):

• Low – Minimal economic loss. No inhabited structures, limited agriculture development.

• Significant – Appreciable economic loss. One (1) or two (2) inhabited structures, notableagriculture or work sites and/or secondary highway and/or rail lines.

• High – Major to extreme economic loss: Three (3) to more than one hundred (100) inhabitedstructures, low to high density suburban or urban areas with associated industry, property andtransportation features (i.e. primary highways and rail lines).

Roadways that would be affected by a dam breach include: US Highway 189, County Road 304, Wyoming State Road 412 (Carter Cutoff Road) and several dirt frontage roads. Based on the potential damage to one (1) uninhabited structures, and secondary highways and rail lines, the breach of the NAP dam results in “Significant” economic losses.

10’ below water surface

20’ below water surface


4.4.5. Environmental Hazard Assessment Environmental hazard classification of the NAP is determined by the harm to humans, aquatic life, or habitat resulting from the release of stored NAP materials in any phase (solid, liquid, or vapor). Only the environmental impacts associated with the dam break are considered here. Other potential environmental impacts, such as groundwater impacts, were not considered in this classification. The criteria for the three levels of environmental hazard classification are as follows (WSDOE, 2007):

• Low – No toxic or harmful materials in water

• Significant – Limited water quality degradation from reservoir contents

• High – Severe water quality degradation potential from reservoir contents and long-term effectson life

Water quality data from the NAP indicates that inundation of water from a dam breach would result in limited water quality degradation in downstream locations as a result of releases of CCRs. The expected environmental impacts are consistent with a “Significant” environmental hazard classification. A “Significant” classification is also consistent with the TVA guidelines (2009) as environmental impacts would not be contained on-site.

4.4.6. Hazard Category Assessment Due to significant environmental and economic hazards, but no probable loss of life, the current hazard classification for the NAP is Significant (Table 6).

Table 6: Summary of Individual and overall Hazard Classification for the NAP

Hazard Category Current Hazard Classification Loss Of Life No probable loss of life

Economic Significant Economic Loss

Environmental Significant Environmental Hazard

Overall Hazard Classification Significant


5. PERIODIC ASSESSMENT AND AMENDMENT PacifiCorp must place the initial hazard potential classification assessment in the CCR Operating Record no later than October 17, 2016. PacifiCorp may amend the document as it sees fit, however, all amendments must be tracked for custodial purposes. Given that the NAP has been determined to be a significant hazard dam, a written Emergency Action Plant (EAP) will be prepared and placed in the Operating Record.

Additional Hazard Potential Classification Assessments will be completed every five years. This will be achieved by reviewing the current Assessment and amending the document if required. As with all documents required by the CCR rule, the deadline for submission of a new assessment is one year after the initial assessment has been completed. All subsequent assessments must be certified by a qualified professional engineer.


REFERENCES Bent, R.W., Brelsford, C.M., Tasseff, B.A., and D.B. Visarraga. 2013. Hydropolis Dam Failure: Impact

Assessment. 12th International Benchmark Workshop on Numerical Analysis of Dams. August 23, 2013.

Black and Veatch, 1993, PacifiCorp Construction Permit Application: Combustion Waste Disposal Expansion, Naughton Power Plant (Vol. 1)



CHA, Assessment of Dam Safety, Coal Combustion Surface Impoundments (Task 3), Final Report, 2009.

Department of Environment and Heritage Protection. 2013 “Manual for Assessing Consequence Categories and Hydraulic Performance of Structures”. EM635 Version 3.1. Queensland Government. 2013.

Department of Energy and Water Supply (DEWS), 2012. Guidelines for Failure Impact Assessments of Water Dams. 2012.

Froehlich. D. C. 1996a. Peak Outflow from Breached Embankment Dam. Journal of Water Resources Planning and Management. July/August 1996.

Gesch, D.B., Oimoen, M.J., and Evans, G.A., 2014, Accuracy assessment of the U.S. Geological Survey National Elevation Dataset, and comparison with other large-area elevation datasets—SRTM and ASTER: U.S. Geological Survey Open-File Report 2014–1008, 10 p., http://dx.doi.org/10.3133/ofr20141008.

Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, P., Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., and Megown, K., 2015, Completion of the 2011 National Land Cover Database for the conterminous United States-Representing a decade of land cover change information. Photogrammetric Engineering and Remote Sensing, v. 81, no. 5, p. 345-354

PacifiCorp Energy Memo to Environmental Protection Agency. PacifiCorp Comments Regarding the Assessment of Dam Safety Coal Combustion Surface Impoundments Final Report at the PacifiCorp Naughton Power Station. January 14, 2010.

Tennessee Valley Authority. 2009. Evaluation of Fossil Coal Combustion Products (CCP) Facilities for Dam Safety Hazard Classification. Accessed online at: https://archive.epa.gov/epawaste/nonhaz/industrial/special/fossil/web/pdf/tva-haz-pot.pdf

Washington State Department of Ecology (WSDOE). 2007. Dam Safety Guidelines – Technical Note 1 Dam Break Inundation and Downstream Hazard Classification. Water Resources Program. Dam Safety Office. October 2007.


Water and Environmental Technologies (WET). 2015. Naughton Power Plant CCR Pond Bathymetric Surveys. Part of the Naughton Power Plant IDF Control Plan. File No. MC-HH01-PERCM61.DWG. 11/25/15.

Webby. V. K. 1996b. Peak Flow from Dam Breach Embankment Dam. Journal of Water Resources Planning and Management. July/August 1996.

United States Bureau of Reclamation. 1988. Downstream Hazard Classification Guidelines. ACER Technical Memorandum No. 11. December 1988. Assistant Commissioner – Engineering and Research. Denver, Colorado.

URS Corporation, 2015 Coal Combustions Residuals Annual Inspection, Naughton Power Plant, North Ash Pond. December 2015.

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