LGS Impoundment Inflow Flood Design Final · 3.0 Design Basis / Flood Control System 3 ... report serves as the inflow design flood control system initial plan for an existing CCR

Inflow Design Flood Control System Plan for Louisa

Generating Station CCR Impoundment

MidAmerican Energy Company

October 10, 2016

Inflow Design Flood Control System Plan for Louisa Generating Station CCR

Impoundment

Prepared for

MidAmerican Energy Company

Muscatine, Iowa

October 10, 2016

Prepared by

Burns & McDonnell Engineering Company, Inc. Kansas City, Missouri

COPYRIGHT © 2016 BURNS & McDONNELL ENGINEERING COMPANY, INC.

INDEX AND CERTIFICATION

MidAmerican Energy Company Inflow Design Flood Control System Plan for Louisa Generating Station CCR

Impoundment

Report Index Chapter Number Chapter Title

Number of Pages

1.0 Introduction 1 2.0 Existing Conditions 1 3.0 Design Basis / Flood Control System 3 4.0 Hydrologic and Hydraulic Capacity 7 5.0 Results 16.0 Periodic Assessment and Amendment 1 7.0 Record of Revisions and Updates 1 Appendix A Site Plan 1

Certification

I hereby certify, as a Professional Engineer in the State of Iowa, that the information in this document was assembled under my direct supervisory control. This report is not intended or represented to be suitable for reuse by the MidAmerican Energy Company or others without specific verification or adaptation by the Engineer.

Kira E. Wylam, P.E.

Date: 10/10/2016

Kira E. Wylam License Number 23129

My license renewal date is December 31, 2016

Pages or sheets covered by this seal: As noted above.

Inflow Design Flood LGS CCR Surface Impoundment Table of Contents

MidAmerican Energy Company TOC-1 Burns & McDonnell

TABLE OF CONTENTS

Page No.

1.0 INTRODUCTION ............................................................................................... 1-1

2.0 EXISTING CONDITIONS .................................................................................. 2-1

3.0 DESIGN BASIS / FLOOD CONTROL SYSTEM ............................................... 3-1 3.1 Hazard Potential Classification ............................................................................ 3-1 3.2 Inflow Design Flood System Criteria .................................................................. 3-1

3.2.1 Capacity Criteria ................................................................................... 3-1 3.2.2 Freeboard Criteria ................................................................................. 3-1

3.3 Flood Routing Design Criteria ............................................................................. 3-2 3.4 Model Scenarios................................................................................................... 3-2 3.5 Project Mapping ................................................................................................... 3-2

3.5.1 Mapping Sources .................................................................................. 3-2 3.5.2 Vertical Datum ...................................................................................... 3-3 3.5.3 Horizontal Coordinate System .............................................................. 3-3

4.0 HYDROLOGIC AND HYDRAULIC CAPACITY ................................................ 4-1 4.1 Calculation Approach .......................................................................................... 4-1 4.2 Hydrology ............................................................................................................ 4-1

4.2.1 Recurrence Interval and Rainfall Duration ........................................... 4-1 4.2.2 Rainfall Distribution and Depth ............................................................ 4-1 4.2.3 Watershed Delineation and Hydrologic Characteristics ....................... 4-3 4.2.4 Process Inflows ..................................................................................... 4-6 4.2.5 Hydraulic Analysis................................................................................ 4-6

5.0 RESULTS .......................................................................................................... 5-1 5.1 Assumptions ......................................................................................................... 5-1 5.2 Scenario 1 – Normal Operating Conditions ......................................................... 5-1 5.3 Scenario 2 – Federal Design Flood Event ............................................................ 5-1 5.4 Summary .............................................................................................................. 5-2

6.0 PERIODIC ASSESSMENT AND AMENDMENT ............................................... 6-1

7.0 RECORD OF REVISIONS AND UPDATES ...................................................... 7-2

8.0 REFERENCES .................................................................................................. 8-1

– SITE PLAN

Inflow Design Flood LGS CCR Surface Impoundment List of Abbreviations

MidAmerican Energy Company i Burns & McDonnell

LIST OF ABBREVIATIONS

Abbreviation Term/Phrase/Name

BMcD Burns & McDonnell Engineering Company, Inc.

CCR Coal Combustion Residual

CFR Code of Federal Regulations

CFS Cubic Feet per Second

CY Cubic Yards

ELG Effluent Limitations Guidelines

EPA Environmental Protection Agency

GIS Geographical Information System

GPM Gallons per Minute

LGS Louisa Generating Station

MEC MidAmerican Energy Company

PMP Probable Maximum Precipitation

RCRA Resource Conservations and Recovery Act

U.S.C United States Code

Inflow Design Flood LGS CCR Surface Impoundment Introduction

MidAmerican Energy Company 1-1 Burns & McDonnell

1.0 INTRODUCTION

On April 17, 2015, the Environmental Protection Agency (EPA) issued the final version of the federal

Coal Combustion Residual Rule (CCR Rule) to regulate the disposal of coal combustion residual (CCR)

materials generated at coal-fired units. The rule is administered as part of the Resource Conservation and

Recovery Act [RCRA, 42 United States Code (U.S.C.) §6901 et seq.], using the Subtitle D approach.

The MidAmerican Energy Company (MEC) is subject to the CCR Rule and as such must meet the

hydrologic and hydraulic capacity requirements per 40 Code of Federal Regulations (CFR) §257.82. This

report serves as the inflow design flood control system initial plan for an existing CCR surface

impoundment, known as the CCR Impoundment or Bottom Ash Pond, at the Louisa Generating Station.

Per §257.82, the inflow design flood control system initial plan must contain documentation (including

supporting engineering calculations) that the inflow design flood control system has been designed and

constructed to:

Adequately manage flow into the CCR unit during and following the peak discharge of the

inflow design flood;

Adequately manage flow from the CCR unit to collect and control the peak discharge resulting

from the inflow design flood;

Handle discharge from the CCR surface impoundment in accordance with the surface water

requirements described in 40 CFR §257.3-3.

The seal on this report certifies that the initial inflow design flood control system plan provided herein

meets the requirements of 40 Code of Federal Regulations §257.82.

Inflow Design Flood LGS CCR Surface Impoundment Existing Conditions


2.0 EXISTING CONDITIONS

The Louisa Generating Station (LGS), owned by MidAmerican Energy Company (MEC) is located

approximately five miles south of Muscatine, Iowa. The CCR surface impoundment, herein referred to as

the Impoundment, is approximately 30 acres in size, and is composed of two areas: the main

Impoundment area and an overflow portion that is referred to as the “reclaim” area. To the east of the

Impoundment is the U.S. Army Corps of Engineers levee for the Mississippi River near river mile 447.5.

MEC property surrounds the perimeter of the Impoundment on the north, west, and south sides, as shown

on SK-001 in Appendix A.

Discharge from the Impoundment flows through an outfall control structure, which pumps water to

Outfall 002 on the Mississippi River. Plant flows that are directed to the Impoundment are the main plant

drains, ash recycle strainer, oil/water separator, economizer ash tank, pyrites holding tank, and bottom ash

hoppers. Flowrates from these processes were obtained from MEC. The Impoundment also receives

precipitation across its extent.

A normal pool elevation of 561.33 feet North American Vertical Datum of 1988 (NAVD 88), has been

assumed. This elevation was measured during survey activities on July 17, 2015, by HGM and

Associates. Per MEC, this elevation can be maintained using transfer pumps that are capable of pumping

from the reclaim area at a rate of 1,000 gallons per minute (GPM).

Based on available data from the Midwest Regional Climate Center, the area surrounding Muscatine,

Iowa, typically receives about 35.8 inches of precipitation annually. There were 17 and 47 inches of

evapotranspiration in 2014 and 2015 respectively, providing precipitation excess in 2014 and a deficit in

2015.

Inflow Design Flood LGS CCR Surface Impoundment Design Basis / Flood Control System


3.0 DESIGN BASIS / FLOOD CONTROL SYSTEM

3.1 Hazard Potential Classification

Per the CCR Rule compliance document titled, “Hazard Potential Classification Assessment for the

Louisa CCR Impoundment”, written in 2016, the Impoundment is classified as having a Low Hazard

Classification per §257.73(a)(2).

3.2 Inflow Design Flood System Criteria

3.2.1 Capacity Criteria

The CCR Rule requires that CCR surface impoundments must have adequate hydrologic and hydraulic

capacity to manage flows for the inflow design flood. Specifically, §257.82 (a) of the CCR regulations

states the following:

“The owner or operator of an existing or new CCR surface impoundment or any lateral expansion

of a CCR surface impoundment must design, construct, operate and maintain an inflow design

flood control system as specified in paragraphs (a)(1) and (2) of this section.

(1) The inflow design flood control system must adequately manage flow into the CCR

unit during and following the peak discharge of the inflow design flood.

(2) The inflow design flood control system must adequately manage flow from the CCR

unit to collect and control the peak discharge resulting from the inflow design flood.”

For this analysis, the above criteria was interpreted to mean that the top of the Impoundment dike should

not be overtopped during the inflow design flood.

3.2.2 Freeboard Criteria

The CCR documentation further discusses that operating freeboard must be adequate to meet performance

standards, but a specific freeboard is not defined. As stated previously, the CCR criteria is interpreted to

mean that the top of the Impoundment dike should not be overtopped during the inflow design flood.

The State of Iowa regulation includes a freeboard requirement (IDNR, 2009). Iowa State regulations state

that for dams without an emergency spillway, the top of dam elevation shall be two feet higher than the

peak flood elevation expected to occur during passage of the freeboard design flood. The suggested

freeboard design flood is one-half of the probable maximum flood. The probable maximum flood is

derived from Equation 1, below.



Equation 1

Rainfall = P100 +0.12(PMP-P100).

where PMP is the probable maximum precipitation, and P100 is the 100-year event

A PMP storm duration and 100-year storm duration of six hours is recommended by the state criteria.

3.3 Flood Routing Design Criteria

To evaluate the criterion discussed above, the inflow design flood rainfall event was considered. Per

§257.82, the inflow design flood is based on the hazard potential classification of the Impoundment as

required by §257.73. The inflow design flood for this analysis was a 100-year flood event, due to the

Impoundment’s low hazard classification.

3.4 Model Scenarios

Three (3) modeling scenarios were completed as part of this evaluation. These scenarios were developed

to determine normal operating conditions and to evaluate the above outlined criteria.

Scenario 1 - Normal Operating Conditions. This scenario considered the Impoundment conditions

with no rainfall event occurring (refer to Section 5.2).

Scenario 2 – Inflow Design Flood. The Impoundment was analyzed under a 100-year, 24-hour

event to determine peak stage and freeboard (refer to Section 5.3).

Scenario 3 - Freeboard analysis, using IDNR state recommendations. A precipitation event,

described by Equation 1, was precipitated onto the watershed of the surface impoundment to

evaluate peak stage in comparison to top of dike (refer to Section 5.4).

3.5 Project Mapping

Project mapping for this analysis consisted of a comprehensive inventory of stormwater assets that

contribute to the Impoundment. This included stormwater structures, piping, culverts, and drainage

ditches. To develop the characterization of the existing stormwater system, two primary sources of

information were utilized: a survey and field investigation.

3.5.1 Mapping Sources

Survey data utilized for this analysis was obtained from a survey performed by HGM and Associates in

2015. Background data such as aerial images and information from the National Hydrography Dataset

was obtained from the Iowa Geographic Map Server (IDNR, 2016).



3.5.2 Vertical Datum

Mapping sources referenced were in the North American Vertical Datum of 1988 (NAVD 88).

3.5.3 Horizontal Coordinate System

North American Datum (NAD) 1983 State Plane Iowa South (US Feet) coordinate system was utilized as

the basis for mapping and modeling efforts.

Inflow Design Flood LGS CCR Surface Impoundment Hydrologic and Hydraulic Capacity


4.0 HYDROLOGIC AND HYDRAULIC CAPACITY

4.1 Calculation Approach

Based upon the simplicity of the water mass balance flows into and out of the Impoundment, the use of

sophisticated modeling software was not required. A spreadsheet analysis that includes all known process

inflows and outflows was used to create a quantitative estimate of the volume and stage of the

Impoundment during design flood conditions.

4.2 Hydrology

4.2.1 Recurrence Interval and Rainfall Duration

The inflow flood design event for this study, as dictated by the hazard potential classification, was a

100-year flood event. Since a storm duration was not specified under §257.82 or other pertinent inflow

flood design sections, a 24-hour storm duration was utilized. This is an industry standard duration and

produces a more conservative rainfall depth than shorter duration storm events. The calculated depth from

Equation 1 was used in the analyzing the State of Iowa requirements, separate from the CCR

requirements. The duration of the storm under the Iowa regulations is six hours.

4.2.2 Rainfall Distribution and Depth

The water mass balance based spreadsheet analysis used to calculate stage and storage within the

Impoundment during design flood conditions does not require the use of rainfall distribution. The

assumption is that the rainfall occurs at once, which is a more conservative approach than distributing the

rainfall over a longer period of time.

The precipitation depth used for the inflow design flood event is 7.41 inches, as required per §257.82 and

the assumed 24-hour duration. This precipitation data was acquired from the National Weather Service

(NOAA, 2016). The point precipitation location is shown in Figure 4-1. The table of rainfall depths for

various frequencies and durations is presented in Figure 4-2. The precipitation depth used for the State of

Iowa freeboard requirement was calculated using Equation 1, where the 100-year, 6-hour event is

5.74 inches and the 6-hour PMP is 26.25 inches, which resulted in a depth of 8.2 inches. This

precipitation data was acquired from the Iowa Department of Agriculture and Land Stewardship, as

specifically mandated in the State of Iowa design criteria for Iowa dams (IDALS, 1988).



Figure 4-1. Point Precipitation Location

Figure 4-2. NOAA Point Precipitation Frequency Estimates



4.2.3 Watershed Delineation and Hydrologic Characteristics

The watershed of the Impoundment was delineated using the mapping sources as discussed in Section

3.5.1. The watershed of the Impoundment is shown in Figure 4-3, below. Using Geographical Information

System (GIS) tools, the watershed was calculated to be 99.5 acres.

The simple water mass balance approach proposed in this document uses conservative assumptions, with

respect to timing. The NRCS TR-55 methodology of rainfall loss was used to estimate the total rainfall

depth after losses.

The site soils primarily belong to hydrologic soil group (HSG) A, characterized by having low run-off

potential and a high infiltration rate, even when fully wetted, as shown in Figure 4-4. The hydrologic soil

classification was obtained from the Iowa Geological and Water Survey. A curve number of 70 was used

to determine rainfall losses due to infiltration and was obtained from Table 3 in Section 2C-5 of the Iowa

Stormwater Manual (IDNR, 2009).



Figure 4-3. Watershed of Louisa CCR Impoundment



Figure 4-4. Hydrologic Soil Groups



4.2.4 Process Inflows

To accurately evaluate the inflow design flood control system, both stormwater runoff flows and process

flows were considered. Estimated inflows contributing to the Impoundment are summarized in Table 4-1,

below.

Table 4-1. Surface Impoundment Inflows

Source Flow (gpm) Flow (cfs)

Main Plant Drains 137 0.31

Ash Recycle Strainer ‐2036 ‐4.54

Oil/Water Separator 582 1.30

Economizer Ash Tank 134 0.30

Pyrites Holding Tank 409 0.91

Bottom Ash Hoppers 911 2.03

Pumps to River ‐1000 ‐2.23

Sum 137 0.31

Cubic Feet in 24 hours = ‐166,127

Acre‐Feet in 24 hours = ‐3.81

4.2.5 Hydraulic Analysis

The hydraulic component of the hydraulic analysis consisted of those elements necessary to account for

all inflows to the Impoundment. These elements, including watershed size, rainfall depth, rainfall loss,

process inflows, and pump outs, are described in previous sections.

4.2.5.1 Stage / Surface Area Information

Stage and surface area information for the Impoundment was developed from the survey data discussed in

Section 3.5. A plot of the stage versus surface area relationship is shown in Figure 4-5, below.



Figure 4-5. Stage and Surface Area Relationship for Impoundment

4.2.5.2 Storage

The water level in the Impoundment is typically maintained at an elevation of 561.33 feet. Therefore, at

stages of 561.33 feet and below, the Impoundment is considered to have no available storage for

stormwater runoff control. A plot of the stage versus the Impoundment storage relationship is shown in

Figure 4-6, below.

Figure 4-6. Stage and Storage Relationship for Impoundment

0

5

10

15

20

25

560 561 562 563 564 565 566 567

Surface Area (acres)

Stage (feet)

Louisa CCR Impoundment Surface Area

0

10

20

30

40

50

60

70

80

90

560 561 562 563 564 565 566 567

Volume Available (acres‐ft)

Stage (feet)

Louisa CCR Impoundment Available Volume

Inflow Design Flood LGS CCR Surface Impoundment Results


5.0 RESULTS

5.1 Assumptions

Analysis results described herein were based on the following additional assumptions:

The Impoundment is continually being filled with CCR and sedimentation; therefore, the

Impoundment storage is in constant flux. This analysis was based on the conditions of the

Impoundment at the time of the survey conducted by HGM and Associates in 2015.

Contributing process flows to the Impoundment were daily averages over a year. For this

analysis, the inflows were assumed to be constant.

The starting water surface elevation for the Impoundment was set to the normal operating

elevation.

This analysis was based on the assumption that the Impoundment water transfer pumps have

power and are operational.

5.2 Scenario 1 – Normal Operating Conditions

During normal operating conditions, based on a simple water mass balance analysis, the Impoundment

will remain at its normal operating level of 561.0 feet.

5.3 Scenario 2 – Federal Design Flood Event

As stated in Section 0, the depth of 100-year, 24-hour storm is 7.41 inches. With losses derived from the

curve number, the total runoff depth is 3.96 inches. Given the measured watershed of 99.5 acres, this will

result in a total of 32.8 acre-feet of runoff. If the process flows and pumping to the river were to continue

during this 24-hour event, a net of -3.81 acre-feet of flow would be pumped out of the Impoundment, for

a combined volume of 29.0 acre-feet. Based on the stage/storage relationship shown in Figure 4-6, this

volume could increase the stage of the Impoundment from a level of 561.33 feet to 563.21 feet. This stage

increase allows for over 2.5 feet of freeboard to the top of the dike surrounding the Impoundment.

5.4 Scenario 3 – State of Iowa Freeboard Event

As stated in Section 0, the depth of the rainfall derived from Equation 1 is 8.2 inches. With losses derived

from the curve number, the total runoff depth is 4.64 inches. Given the measured watershed of 99.5 acres,

this will result in a total of 38.4 acre-feet of runoff. If the process flows and pumping to the river were to

continue during this 6-hour event, a net of -0.95 acre-feet of flow would be pumped out of the

Inflow Design Flood LGS CCR Surface Impoundment Results


Impoundment, for a combined volume of 37.5 acre-feet. Based on the stage/storage relationship shown in

Figure 4-6, this volume could increase the stage of the Impoundment from a level of 561.33 feet to

563.96 feet. This stage increase allows for 2.04 feet of freeboard to the top of the dike surrounding the

Impoundment.

5.5 Summary

Results for all scenarios indicated that the Impoundment was not overtopped. Based on these results, CCR

regulations were considered to be met for the LGS Surface Impoundment.

Inflow Design Flood LGS CCR Surface Impoundment Periodic Assessment and Amendment


6.0 PERIODIC ASSESSMENT AND AMENDMENT

MidAmerican Energy must place this initial inflow design flood plan in the CCR Operating Record by

October 17, 2016. MEC may amend the plan at any time, and is required to do so whenever there is a

change in conditions which would substantially affect the written plan in effect. MEC must prepare

periodic inflow design flood control system plans every five years. Each periodic plan or amendment to

the written plan shall be certified by a qualified professional engineer in the State of Iowa. All

amendments and revisions must be placed on the CCR public website. A record of revisions made to this

document is included in Section 7.0.

Inflow Design Flood LGS CCR Surface Impoundment Record of Revisions and Updates


7.0 RECORD OF REVISIONS AND UPDATES

Revision Number Date Revisions Made By Whom

0 10/10/2016 Initial Issue Burns & McDonnell

Inflow Design Flood LGS CCR Surface Impoundment References


8.0 REFERENCES

Geographic Information Systems (GIS Section), Iowa Geological and Water Survey, Iowa Department of

Natural Resources. https://programs.iowadnr.gov/nrgislibx/. Accessed 3/7/2016.

Iowa Department of Natural Resources, 1990. Technical Bulletin No. 16, Design Criteria and Guidelines

for Iowa Dams. Des Moines, Iowa

Iowa Department of Agriculture and Land Stewardship, State Climatology Office. Climatology of Iowa

Series No. 2, Revised. Iowa Precipitation Frequencies. Paul Waite, 1988.

Iowa Department of Natural Resources. 2009. Iowa Stormwater Management Manual, Version 3.

http://www.iowadnr.gov/Environmental-Protection/Water-Quality/NPDES-Storm-Water/Storm-Water-

Manual

Iowa State University http://mesonet.agron.iastate.edu/, Muscatine (Muscatine County, FRUI4)

National Weather Service. Precipitation Frequency Data Server (PFDS). NOAA's National Weather

Service, Hydrometeorological Design Studies Center. [Online] [Cited: March 15, 2016.]

http://hdsc.nws.noaa.gov/hdsc/pfds/.

United States Department of Agriculture, Natural Resources Conservation Service. Urban Hydrology for

Small Watersheds, Technical Release 55. June 1986. (210-VI-TR-55, Second Ed., June 1986)

United States Environmental Protection Agency (EPA) Federal Register, 2015. Vol. 80. No. 74. April 17,

2015. 40 CFR Parts 257 and 261. Page 21480.

– SITE PLAN

Burns & McDonnell World Headquarters9400 Ward Parkway

Kansas City, MO 64114 O 816-333-9400 F 816-333-3690

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Documents

LGS Impoundment Inflow Flood Design Final · 3.0 Design Basis / Flood Control System 3 ... report serves as the inflow design flood control system initial plan for an existing CCR