i

Phase II Bond Release Application

N11 and J19 Coal Resource Areas, Kayenta Mine

TABLE OF CONTENTS

Section 4. Phase II Bond Release Supporting Information – Suspended Solids Outside

of the Permit Area

Introduction 4.1

EASI Model Development 4.1

EASI Model Sensitivity Analysis 4.2

J1/N6 EASI Sediment Yield Model 4.4

J19 EASI Sediment Yield Model 4.5

Total Suspended Solids 4.5

Suspended Solids Outside of the Permit Area 4.6

Alluvial Valley Floors 4.7

Surface and Subsurface Water Pollution 4.7

References Cited 4.7

List of Tables

 4.1 Total Ground Cover Values for Reclaimed Conditions Used in Previous

 EASI Sediment Models 4.4

List of Figures

4.1 Critical Velocity for Movement of Quartz Grains on a Plane Bed at

 a Water Depth of One Meter

4.2 Variation of Sediment Yield With Climate in the United States

List of Attachments

 4.1 Surface Water Modeling of the Reclaimed Parcels at Black Mesa Complex

 J1/N6 and N6 East Central Coal Resource Areas

4.2 Surface Water Modeling of Reclaimed Parcels at the J19 Coal Resource

 Area, Kayenta Complex
Section 4. Phase II Bond Release Supporting Information

Suspended Solids Outside of the Permit Area

Introduction

Beginning in the early 1980’s, Peabody Western Coal Company (PWCC) collected numerous 

measurements of suspended solids (Total Suspended Solids – TSS) in runoff events at sites 

established on the main washes and at small watersheds located on both reclaimed and un-mined 

areas within the leasehold. TSS values collected in runoff from runoff plots and small flumes 

contributed to the development of a surface water model (EASI) used to predict runoff and 

sediment loads from both un-mined and reclaimed mined lands at the Kayenta Mine. The following 

sections summarize the development of the EASI model and reference recent EASI modeling reports 

for reclaimed parcels adjacent to or within the Phase II parcels subject to this Phase II 

application (N-11 and J-19). Comparisons of measured and predicted sediment discharges and 

TSS concentrations collected at main channel monitoring sites, small un-mined watersheds, and 

in small, reclaimed parcels located within the Black Mesa and Kayenta Mines are also summarized. 

Based on the following discussions, PWCC is confident that runoff from these parcels will not 

contribute additional suspended solids to stream flow outside the permit area.

EASI Model Development

PWCC initiated a Small Watershed Study (SWS) monitoring program on Black Mesa in 1985, and continued 

monitoring through 1992. Details regarding study objectives and monitoring associated with the 

study are provided in Attachment 4 in Chapter 16, Hydrologic Monitoring Program in the AZ-0001F

Permit Application Package (PAP). Several small watersheds located within reclaimed and undisturbed 

areas were instrumented with supercritical flow flumes and continuous flow recorders for collecting 

runoff, sediment (TSS) and water quality data. Rainfall data were collected using Belfort automated 

tipping bucket rain gauges located at the centroids of each watershed and direct reading rain 

gauges set up at various locations within each watershed. Total overland runoff and sediment yield 

data for individual storm events were collected from hill slopes in each watershed using runoff 

plots. Small flumes were also installed downstream of the plots and were instrumented with 

continuous stage recorders and automated samplers to measure runoff rates and TSS concentrations 

during runoff events. In addition, runoff rates and TSS concentrations were collected at sites 

located in the main channels (e.g., Moenkopi Wash) over many years as part of historic monitoring 

commitments contained in the Hydrologic Monitoring Program during the 1980s into the mid-1990s. 

The data results were utilized to calibrate the physically based runoff and sediment yield model

4.1
named EASI (Erosion And Sediment Impacts - Zevenbergen et al., 1990; WET, 1990). EASI has been 

used to support Termination of Jurisdiction (TOJ) applications for mined areas reclaimed under the 

initial program rules (30 CFR Part 715) and bond release applications for mined areas reclaimed 

under the permanent program rules (30 CFR Part 816). The modeling results were used to support 

the first TOJ application submitted for the Kayenta Complex in March 1994 for the N-1/N-2 and J-27 

interim program reclaimed areas (PWCC, 1994). The 1994 TOJ application included the final report 

for the modeling project completed in August of 1993 (RCE, 1993).

The model was calibrated and verified using a two-step process and site-specific data collected as 

part of the Small Watershed Study. The EASI model was first calibrated and validated using total 

runoff volumes and sediment yields measured in the runoff plots along with rainfall data, followed 

by simulation of actual runoff hydrographs and corresponding sediment concentrations collected from 

the flumes considering measured storm durations and intensities. Soils and vegetative cover data 

measured in each plot and at select points in each watershed were also used in the model development 

process. Parameters that influence the model’s predictions of runoff and sediment were calculated 

from observed data or estimated through model testing. Other theoretical parameters such as 

rainfall interception storage and Manning’s “n” were estimated based on previous experience in the 

application of EASI at other surface mines in the Colorado Plateau region (WET, 1990). 

EASI Model Sensitivity Analysis. The 1993 report provides a discussion of the influence of several 

key input parameters on its ability to duplicate measured hillslope and channel responses. Runoff 

and sediment yields (TSS) predicted by EASI are controlled by the short-duration, high-intensity 

rainfall events common to the area. The model tends to underpredict runoff and sediment yield 

response for small rainfall events (< 0.1 inches), especially on hillslopes where antecedent 

moisture, looseness of surface soils, wind and temperature can vary appreciably. For larger events, 

the small watershed study runoff plot and flume data were in good agreement with EASI model 

predictions based on the calibration and validation process utilized for optimizing model inputs.

The sensitivity of the EASI model to several input parameters was performed after completing the 

calibration and validation work. The analysis evaluated calibrated values for soil hydraulic 

conductivity, total ground cover, and both overland flow (hillslope) and channel flow detachment 

coefficients (erosion) by varying the input parameter values by percentages. Model response to 

these variations was evaluated on a unit runoff (inches) and sediment yield (tons/acre) basis at 

both hillslope and watershed scales. The analysis indicates runoff is not appreciably affected by 

cover at either a hillslope or watershed scale. For larger events, rainfall intensities are far 

4.2
higher than infiltration rates.

However, sediment yield from pre-mining and reclaimed hillslopes is highly sensitive to total 

ground cover and less sensitive to infiltration (hydraulic conductivity) and erosion (detachment 

coefficients). On a watershed scale, the differences between pre-mine and reclaimed sediment yield 

are less pronounced because channel sediment transport processes dominate at the watershed outlet. 

Many of the required EASI model input parameters used for modeling runoff and sediment yield from 

watersheds at the Kayenta Mine were developed during the calibration and validation process because 

direct measurements were difficult to obtain and not readily available. However, percent ground 

cover values for modeling un-mined and reclaimed areas are based on field measurements of vegetative 

ground cover, litter and rock. These values are measured directly in the field and are required 

for demonstrating successful establishment of vegetation growth in the reclaimed parcels subject 

to this Phase II bond release application. Because predictions of sediment yields (including TSS 

concentrations) using EASI are sensitive to values of total ground cover, and are readily available, 

it follows that measurements of total ground cover in reclaimed areas may be used to indicate 

whether reclaimed areas are generating sediment yields, expressed as tons/acre on a unit basis or 

as individual TSS concentrations (mg/L), that may result in appreciable contributions of suspended 

solids to streamflow outside the permit area. 

Table 4.1 presents average total ground cover used in previous EASI models to predict sediment 

yields in numerous reclaimed areas throughout the leasehold and provides a general description of 

the reclaimed areas modeled, drainage area, and average total ground cover used for modeling 

purposes. The values range from 38.2 percent to 65.6 percent. Of note, the EASI models that were 

developed for all reclaimed areas listed predicted average annual sediment yields less than or 

equal to pre-mining conditions. Importantly, the processes that dominate the sediment yield 

predictions involve sediment transport in channels, not erosion from hill slopes. Measurements of 

total ground cover during 2020 and 2021 in the reclaimed parcels subject to this application 

averaged 55.2 percent in N-11 and 60.4 percent in J-19 (see Table 3.2 in Section 3.0). Accordingly, 

absent application of the EASI model to these parcels, the average total ground cover values 

indicate average annual sediment yields from these areas will be less than or equal to conditions 

that were present prior to mining these parcels.

4.3
Table 4.1. Total Ground Cover Values for Reclaimed Conditions used in Previous EASI Sediment 

Models

Reclaimed Area Modeled Model Date

(Month-Year)

Drainage Area (acres} Total Ground Cover1 

(percent}

N1/N2 Aug-93 2732.5 41.2

J27 Aug-93 178.9 43.9

N7/N8 Jul-01 946.0 53.9

N14 Jul-08 1580.6 46.5

J21-D/J21-E Aug-08 68.9 65.6

J16-E/J16-F Aug-08 148.5 61.0

N6-C/N6-D/N6-F Aug-08 280.9 38.2

J7-CD/J7-E/J7-F Aug-08 99.8 48.5

J21-A Apr-09 111.2 52.7

N6-G Apr-09 37.9 55.6

J7-K/J7-M Jun-09 37.3 55.2

N5-D/N5-E Aug-09 28.3 48.9

J1/N6 and N6 East 

Central Sep-09 1533.3 46.2

J21 Sep-10 2832.0 59.4

J7-A/J7-B1/J7-G/J7-H/ 

J7-I/J7-J/J7-R/J7-R1

Feb-11 440.0 55.2

J19 Sep-11 943.4 55.8

J3 Nov-12 95.5 39.9

J7 Nov-12 1194.7 48.7

Total Drainage Area Modeled = 13289.7

1 Total Ground Cover = Vegetation Ground Cover + Litter + Rock

Following the 1994 TOJ application submittal, sixteen additional EASI models were developed for 

reclaimed parcels located within the Kayenta and Black Mesa Mines, including reclaimed watersheds 

upstream of temporary sediment ponds that were permitted as outfalls in the Kayenta Complex NPDES 

Permit No. NN-0022179. As of 2016, a total of 13,289.7 acres of reclaimed areas had been modeled 

using EASI. The combined total of topsoiled and seeded areas at both mines at the end of 2016 was 

15,584 acres, of which approximately 85 percent were modeled using EASI. The following sections 

discuss EASI models that have been developed proximate to the N-11 and J-19 reclaimed parcels 

subject to this application.

J1/N6 EASI Sediment Yield Model. Attachment 4.1 contains an EASI model report entitled “Surface 

Water Modeling of the Reclaimed Parcels at Black Mesa Complex J1/N6 and N6 East Central Coal 

Resource Areas” (Ayres, 2009) for reclaimed areas situated south of the N-11 reclaimed parcels. 

The results indicate average annual runoff (0.28 inches) generated from reclaimed hill slopes and 

low-order channels is less than pre-mining conditions (0.42 inches). 

4.4
The difference is attributed to the creation of several internal draining impoundments in the 

eastern portion of the N-6 post-mining landscape. Reclamation methods including BMPs (e.g., 

vegetative cover) utilized in the N-11 reclaimed parcels were like those evaluated in the J1/N6 

EASI model. In addition, physical properties of the reclaimed watersheds within the J1/N6 areas, 

including mean channel slope, drainage density and mean hillslope gradients were similar to premining conditions. 

J-19 EASI Sediment Yield Model. Attachment 4.2 contains the EASI model entitled “Surface Water 

Modeling of Reclaimed Parcels at the J19 Coal Resource Area, Kayenta Complex” (Ayres, 2011). The 

model results indicate post-mine (reclaimed parcels) average annual sediment yields are about 29 

percent less than pre-mine levels. Hill slope and sub-watershed erosion rates, which are 

significant for sustaining the postmining land use, are 9 percent higher for the reclaimed 

landscape, yet are comparable to pre-mine levels and are less than 1.0 ton/acre/year. The reduction 

of sediment yield is due to comparable hill slope erosion combined with channel erosion control 

measures for the post-mine landscape. 

Total Suspended Solids

Soils replaced within the N-11 and J-19 reclaimed parcels, naturally occurring soils in surrounding 

undisturbed areas within the leasehold overall and in the arid/semiarid Southwest typically lack 

cohesion. Un-mined stream channels within and adjacent to the Kayenta Mine and PWCC leasehold 

consist of steep sided, deeply incised arroyos with loosely consolidated channel banks and finegrained sand bed channels. Figure 4.1 from Blatt, Middleton, and Murray (1972) shows these types 

of soils (unconsolidated clays, silts and fine-grained sands) are easiest to keep in suspension. 

The gray band shown in Figure 4.1 represents the flow velocity ranges necessary to keep particle 

types and sizes in suspension. Above the gray band are the velocities necessary to erode or entrain 

soil particles, whereas velocities below the gray band would be insufficient to transport the 

particles and deposition would occur. The bandwidths for the clay and silt particle sizes are 

quite wide because considerably higher velocities are necessary to erode consolidated and cohesive 

clays and silts. For the unconsolidated non-cohesive silts, clays and fine-grained sands found on 

the leasehold, velocities of less than 2 feet/second will erode and keep the particles in suspension. 

Typical flow velocities measured historically in the stream channels on the leasehold including 

Dinnebito Wash (sites CG34 and SW34) and the main channels along Yucca Flat Wash, Coal Mine Wash, 

and Moenkopi Wash where monitoring sites SW155, SW25, and SW26 are located, respectively, range 

from 8 to 12 feet/second.

4.5
In the semiarid Southwest, much of the precipitation is effective in terms of producing runoff. 

Most of the rainfall occurs in short duration, very high intensity storms that rapidly overcome 

soil infiltration and generate larger amounts of runoff. Total annual rainfall on the PWCC

leasehold ranges from 6 to 12 inches. Figure 4.2, from Langbein and Schumm (1958), shows the 

relationship of annual sediment yield to effective annual precipitation and cover in the U.S. Note 

the highest annual sediment yields occur where there is a combination of approximately 12 inches 

of effective precipitation and desert/shrub type cover. Both factors are consistent for the 

leasehold and for the undisturbed areas adjacent to the N-11 and J-19 reclaimed parcels. Because 

of the soil and rainfall characteristics and the vegetative cover for this geomorphic region, 

stream flows on the leasehold more closely approximate debris flows than they do stream flows.

Suspended Solids Outside of the Permit Area

Section 2.0, Comparisons with Measured Sediment Transport in both EASI model reports provided in 

Attachments 4.1 (Ayres, 2009) and 4.2 (Ayres, 2011) contain a discussion of measured sediment 

discharge and total suspended sediment (TSS) concentrations along with EASI-model derived sediment 

discharge and TSS concentrations. Measured values were collected over many years at main channel 

stream monitoring sites and at Small Watershed Study (SWS) flumes. Each EASI model report compares 

predicted values for sediment discharge and TSS concentrations for reclaimed areas modeled with 

measured values based on data plots (see Figures 2.1 and 2.2 in each model report). Overlap of 

model predictions for both pre- and post-mine conditions with measured data strongly indicate EASI 

model predictions are representative and reasonable. In addition, the plots indicate sediment 

loads and concentrations are dependent on the channel sediment transport capacity for small unmined and reclaimed channels as well as larger channels draining larger basins. Channel sources 

of sediment in the semi-arid environment of the leasehold are virtually unlimited. Accordingly, 

channel transport capacity and channel-derived sediment limits and governs sediment discharge and 

TSS concentrations from the small tributaries and large sand-bed channels (e.g., Moenkopi Wash). 

Section 2.2 of each EASI model report (Attachments 4.1 and 4.2) also discusses statistical analysis 

of the sediment discharge and sediment concentration plots provided in Figures 2.1 and 2.2. The 

analysis involved applying non-parametric statistics to determine if channels in reclaimed areas 

have similar sediment transport characteristics as background (un-mined) channels. The analysis 

showed data collected at un-mined SWS flumes can be combined with the main channel monitoring site 

data, and that sediment is being conveyed at or near capacity. In addition, reclaimed channel 

sediment discharge and TSS concentrations show the same characteristics of the data collected at 

un-mined SWS flumes and main channel monitoring sites even though the flow ranges are lower. The 

4.6
data plots and statistical analysis indicate that channel flows within and adjacent to the leasehold 

achieve the sediment transport capacity of the channel regardless of whether they are located 

within reclaimed areas or in small and large basins that drain background watersheds not impacted 

by surface coal mining activities. Accordingly, runoff from any of the reclaimed parcels located 

within the N-11 and J-19 parcels subject to this Phase II bond release application are not 

contributing additional suspended solids (TSS) to streamflow outside the permit area.

Alluvial Valley Floors

Chapter 17, Protection of the Hydrologic Balance in the AZ-0001F PAP provides a summary of early 

investigations of the existence of alluvial valley floors (AVFs) within or adjacent to the 

leasehold. The findings clearly indicate there are no AVFs within or adjacent to the leasehold.

Surface and Subsurface Water Pollution

The regulations set forth under 30 CFR Parts 780 and 816 require operators to minimize impacts to 

the prevailing hydrologic balance. PWCC conducted mining and reclamation activities at the N-11 

and J-19 reclaimed parcels subject to this Phase II bond release application in accordance with 

plans and procedures approved by the Office of Surface Mining Reclamation and Enforcement (OSMRE)

as provided in the PAP for Surface Mining Permit AZ-0001F, many of which were developed in order 

to ensure impacts to the hydrologic balance in the vicinity were minimized. The changes to ground 

water (subsurface) are largely based on long term monitoring of ground water in monitoring wells 

completed in the Wepo Formation and adjacent alluvial deposits along Moenkopi Wash and Coal Mine 

Wash. Changes to surface water (surface) are based on long term monitoring of runoff at stream 

sites located on Moenkopi Wash and Coal Mine Wash. Changes in water chemistry discussed above 

cover decades of monitoring in many cases and are within magnitudes and ranges representative of 

naturally occurring or background values. In summary, no pollution of surface or subsurface sources 

of water has been found within or adjacent to the subject reclaimed N-11 and J-19 parcels shown on 

Map 1.1. 

References Cited

Ayres Associates, 2009. “Surface Water Modeling of the Reclaimed Parcels at Black Mesa Complex 

J1/N6 and N6 East Central Coal Resource Areas”, Prepared for Peabody Western Coal Company.

Ayres Associates, 2011. “Surface Water Modeling of Reclaimed Parcels at the J19 Coal Resource 

Area, Kayenta Complex”, Prepared for Peabody Western Coal Company.

4.7
Blatt, H., Middleton, G., and Murray, R., 1972. Origin of Sedimentary Rocks, Prentice-Hall, 

Englewood, New Jersey, 634 p.

Langbein, W.B. and Schumm, S.A., 1958. Yield of Sediment in Relation to Mean Annual Precipitation, 

American Geophysical Union, Trans., Volume 39, p. 1076-1084.

Peabody Western Coal Company (PWCC), 1994. “Application for Release of Reclamation Liability N1/N-2 and J-27 Interim Program Indian Lands”, submitted to the Office of Surface Mining 

Reclamation and Enforcement Albuquerque Field Office on March 31, 1994.

Resource Consultants & Engineers, Inc. (RCE), 1993. “Surface Water Modeling of Reclaimed Parcels 

at the Black Mesa Complex”, Prepared for Peabody Western Coal Company.

Water Engineering & Technology, Inc. (WET), 1990. “Determination of Background Sediment Yield and 

Development of a Methodology for Assessing Alternative Sediment Control Technology at 

Surface Mines in the Semiarid West,” Prepared for Office of Surface Mining and the National 

Coal Association, Fort Collins, CO.

Zevenbergen, L.W., Peterson, M.R., and Watson, C.C., 1990. “Computer Simulation of Watershed 

Runoff and Sedimentation Processes”, Proceedings of the Billings Symposium; Planning, 

Rehabilitation and Treatment of Disturbed Lands.

4.8

 

SURFACE WATER MODELING OF THE RECLAIMED PARCELS AT 

BLACK MESA COMPLEX J1/N6 AND N6 East Central COAL 

RESOURCE AREAS 

Prepared for 

Peabody Western Coal Co. 

Highway 160, Navajo Route 41 

Kayenta, Arizona 86033 
 

SURFACE WATER MODELING OF THE RECLAIMED PARCELS AT 

BLACK MESA COMPLEX J1/N6 AND N6 EAST CENTRAL COAL 

RESOURCE AREAS 

Prepared for 

Peabody Western Coal Co. 

Highway 160, Navajo Route 41 

Kayenta, Arizona 86033 

P.O. Box 270460 

Fort Collins, Colorado 80527 

 (970) 223-5556, FAX (970) 223-5578 

Ayres Project No. 32-1304.01 

PEA-N6EC.DOC 

September 2009 
i Ayres Associates 

TABLE OF CONTENTS

1. Reclaimed Parcel Modeling........................................................................................1.1

1.1 Introduction .........................................................................................................1.1

1.2 Background.........................................................................................................1.1

1.3 Data ....................................................................................................................1.2

1.3.1 Soils .............................................................................................................1.2

1.3.2 Vegetation....................................................................................................1.2

1.3.3 Topography..................................................................................................1.2

1.4 Methodology........................................................................................................1.2

1.4.1 Synthetic Rainfall .........................................................................................1.8

1.4.2 Computation of Average Runoff and Sediment Yield ...................................1.8

1.5 Results................................................................................................................1.8

1.6 Discussion...........................................................................................................1.8

2. Comparisons With Measured Sediment Transport.....................................................2.1

3. References.................................................................................................................3.1

Exhibit 1 – Postmine Topography........................................................................................ -- 

Exhibit 2 – Premine Topography ......................................................................................... -- 
ii Ayres Associates 

LIST OF FIGURES

Figure 1.1. Reclaimed area soils trilinear graph.................................................................................................1.3 

Figure 1.2. Vegetative cover for CRA J1/N6 and N6 East Central premine condition .......................................1.4 

Figure 1.3. J1/N6 and N6 East Central postmine basins. ..................................................................................1.6 

Figure 1.4. J1/N6 and N6 East Central premine basins.....................................................................................1.7 

Figure 2.1. Observed and modeled sediment discharge and water discharge..................................................2.2 

Figure 2.2. Observed versus modeled sediment concentration and discharge. ................................................2.3 

Figure 2.3. Background measured sediment and water discharge....................................................................2.5 

Figure 2.4. Reclaimed measured sediment and water discharge. .....................................................................2.6 

Figure 2.5. Modeled premine sediment and water discharge for J1/N6 and N6 East Central ...........................2.7 

Figure 2.6. Modeled postmine sediment and water discharge for J1/N6 and N6 East Central..........................2.8 

LIST OF TABLES 

Table 1.1. Soils Data..........................................................................................................................................1.3

Table 1.2. Cover Sampling Data. .......................................................................................................................1.5

Table 1.3. Cover Data for J1/N6 and N6 East Central Watersheds...................................................................1.5

Table 1.4. Average Runoff and Sediment Yield Results. ...................................................................................1.9

Table 1.5. Average Physical Properties of the J1/N6 and N6 East Central CRA...............................................1.9
 Ayres Associates 1.1

1. RECLAIMED PARCEL MODELING 

1.1 Introduction 

The purpose of this project is to use a previously calibrated and validated runoff and erosion 

model EASI - Erosion And Sediment Impacts (Zevenbergen et al. 1990; WET 1990) for the 

Black Mesa and Kayenta Mines (combined as Black Mesa Complex in December 2008) to 

predict mean annual runoff and sediment yields from the reclaimed parcel J1/N6 and N6 

East Central. Since the model for the J1/N6 Coal Resource Area (CRA) was completed in 

2001, the objectives of this project are to review the completed J1/N6 model, develop a 

model for the neighboring N6 East Central CRA, and incorporate the newly developed N6 

East Central model into the existing J1/N6 model. The response of the reclaimed parcels 

was evaluated relative to undisturbed (premine) conditions in the corresponding undisturbed 

watersheds. All soils and rainfall input to the model are to be taken from models calibrated 

in the previous study (RCE 1993). The input variables that were calibrated to the mine areas 

and used in this study include soil infiltration parameters, erodibility parameters, and the 

grain size distribution. Parameters that are specific to this study are vegetative canopy and 

ground cover percentages from data collected on site. The model serves a tool for 

assessing the success of reclamation efforts to protect hydrologic balance (30 CFR 715.17 

and 30 CFR 816.41). 

The model calibration was conducted in a previous study (RCE 1993) using data obtained 

from instrumented watersheds and small hillslope plots collected under natural rainfall 

conditions. For a detailed discussion of data collection and model calibration, please refer to 

the previous study (RCE 1993). 

1.2 Background 

The J1/N6 and N6 East Central CRA that is the focus of this project was reclaimed between 

1981 and 2007. This reclaimed area is now eligible for termination of jurisdiction from the 

Office of Surface Mining Regulation and Enforcement (OSMRE). The fundamental purpose 

of this study was to quantify the expected behavior and hydrologic response of the current 

conditions of reclaimed areas relative to the conditions that existed prior to the occurrence of 

mining activities. 

Runoff and sediment yield response from the reclaimed lands should be managed by 

implementing Best Management Practices (BMP’s) in conjunction with an OSM approved 

sediment control plan in order to not adversely impact the prevailing hydrologic balance and 

to limit additional contributions of suspended sediment to streamflow or runoff outside the 

mine permit areas. BMP’s include regrading, replacing salvaged topsoil, revegetation, and 

other controls such as riprapped channel bottoms, check dams, and where practicable, 

contour terraces. The natural watersheds on the mesa contribute significant quantities of 

sediment to the channel system. It is expected that the postmine condition will also produce 

comparable amounts of sediment without adversely impacting the hydrologic balance. 

This section describes the data and procedures used to evaluate the CRA J1/N6 and N6 

East Central. This area was modeled to determine the average annual hydrologic response 

following the completion of reclamation activities and maturation of the reclaimed area 

vegetation taking into account BMP’s implemented as part of the reclamation process. 

Infiltration, runoff, and erosion processes from both hillslopes and channels within the CRA 

were modeled using EASI. Results were determined for concentration points at the outlets 

of the reclaimed watersheds. The locations of these points are shown in Exhibit 1. 

Modeling was also conducted to determine hydrologic response under premine conditions 

based on the topography, soils, cover, and other conditions that typified the undisturbed 

watersheds draining to each concentration point. Exhibit 2 shows the modeling endpoints 

for the J1/N6 and N6 East Central premining watersheds. 
 Ayres Associates 1.2

1.3 Data 

1.3.1 Soils 

Soils data used for the current study (CRA J1/N6 and N6 East Central) were based on data 

developed from the calibration of models used in the previous study for Coal Resource 

Areas (CRAs) N1/N2 and J27 (RCE 1993). The composition of postmine soil in the current 

study is depicted along with the composition of postmine soils from the previous study in 

Figure 1.1. This figure shows that the soil composition of CRA J1/N6 and N6 East Central is 

very similar to soils evaluated during model calibration. Therefore, the soil properties 

developed in the previous study are valid for this modeling project. These properties include 

calibrated parameters, such as infiltration and erodibility coefficients, and measured soil size 

distributions. Table 1.1 lists the premine and postmine soils data used during EASI 

modeling of CRA J1/N6 and N6 East Central. 

1.3.2 Vegetation 

Vegetative cover data representative of both pre- and postmine conditions in CRA J1/N6 and 

N6 East Central were supplied by PWCC. For the premine condition, land was 

characterized as being covered by sagebrush or pinon juniper. The spatial distribution of 

vegetative cover for the J1/N6 and N6 East Central CRA premine condition appears in 

Figure 1.2. Average cover properties for CRAs N1/N2 and J27 of the previous study and 

CRA J1/N6 and N6 East Central of the current study appear in Table 1.2. For the postmine 

condition, the reclaimed area was assigned the postmine cover type and the unmined area 

was assigned the same cover type as the premine condition. Table 1.3 lists the pre- and 

postmine vegetative cover data used in the EASI model runs generated for the J1/N6 and 

N6 East Central CRA. Note that if a unit contained significant portions of both sagebrush 

and pinon juniper cover types, it was classified as half pinon juniper and half sagebrush. 

1.3.3 Topography 

Pre- and postmine topography was supplied by PWCC in the form of ArcGIS geodatabase. 

Basin delineations, hillslope delineations, subwatershed delineations, as well as areas, 

slopes, and lengths of all units of the study area were defined and calculated using ArcGIS 

software. Figures 1.3 and 1.4 show the watershed delineation and numbers assigned to the 

basins used in the EASI model for the post- and premine conditions, respectively. Channel 

dimensions input to EASI were based on the topography supplied and limited field 

observations. 

1.4 Methodology 

Runoff and sediment yield in the semiarid western United States is largely governed by the 

occurrence of high-intensity, short-duration rainstorms of limited areal extent (Renard and 

Simaton 1975). Research has indicated that relatively few events may produce the greatest 

erosion (e.g., Hjelmfelt et al. 1986 reported that only 3 to 4% of rainfall events accounted for 

50% of long-term sediment yields). Although there is perhaps a relatively limited physical 

basis for definition of an "average annual" runoff or sediment yield in a semiarid 

environment due to the extreme variability in response and importance of single infrequent 

events, such a term does provide a useful basis for long-term comparison between 

reclaimed and undisturbed conditions. 
 Ayres Associates 1.3

Figure 1.1. Reclaimed area soils trilinear graph. 

Table 1.1. Soils Data. 

Condition Premine Postmine Rock Chutes 

Rainfall detachment 0.005 0.005 0

Overland flow detachment 0.44 0.44 0

Channel flow detachment 0.5 0.5 0

Initial soil moisture, % 70 70 70

Final soil moisture, % 90 90 90

Soil porosity, % 45 45 46

Temperature, *F 70 70 70

Hydraulic conductivity, in/hr 0.23 0.29 0.3

Capillary suction, in 3.7 2.6 2.6

 Particle Size Distribution 

(all conditions) 

 Size, mm % Finer 

0.001 0

0.004 18.0

0.016 27.4

0.062 36.6

0.125 56.2

0.250 64.3

0.500 72.4

1.000 80.5

2.000 88.6

4.000 92.4

16.000 100
 Ayres Associates 1.4

Figure 1.2. Vegetative cover for CRA J1/N6 and N6 East Central premine condition. 
 Ayres Associates 1.5

Table 1.2. Cover Sampling Data. 

Area Condition Cover 

Type 

Nonstratified 

Vegetation 

Cover (%) 

Vegetation 

Canopy 

Cover (%) 

Vegetation 

Ground 

Cover (%) 

Litter* 

(%) 

Rock 

(%) 

Total 

Ground 

Cover 

(%) 

N1/N2 Postmine Postmine 25.6 1.4 24.2 13.6 4.2 41.9

J1/N6 Postmine Postmine 20.6 0.3 20.4 21.6 4.2 46.2

N1/N2/J27 Premine Pinon Juniper 32.7 31.1 3.0 44.0 19.7 66.7 

J1/N6 Premine Pinon Juniper 16.9 14.6 2.7 18.8 17.3 38.8 

N1/N2 Premine Sagebrush 25.1 16.0 10.3 25.3 18.1 53.7 

J27 Premine Sagebrush 30.6 9.7 22.0 24.0 1.6 47.6 

J1/N6 Premine Sagebrush 12.4 1.3 11.2 24.7 2.5 38.3

*Including standing dead litter 

Table 1.3. Cover Data for J1/N6 and N6 East Central Watersheds. 

Condition Pinon Juniper Sagebrush 

Half Pinon JuniperHalf Sagebrush Postmine 

Canopy cover, % 14.6 1.3 8.0 0.3 

Ground cover, % 38.8 38.3 38.5 46.2 

Canopy storage, in 0.05 0.05 0.05 0.05 

Ground storage, in 0.05 0.05 0.05 0.05 

Depression storage, in 0.03 0.03 0.03 0.03 

Impervious area, % 0 0 0 0 

Manning n 0.07 0.07 0.07 0.05 

To make comparisons between reclaimed lands and associated undisturbed lands at the 

Black Mesa Mining Complex on the basis of average annual sediment yield, a procedure 

was used that considers the importance of infrequent storm events in defining sediment yield 

in the semiarid west. First, however, the site-specific rainfall data available for the Black 

Mesa Mining Complex were used to evaluate the frequency and magnitude of the measured 

events relative to existing predictions for rainfall depth-duration (Miller et al. 1973). The 

analysis of the rainfall data was performed as part of a previous study of the N1/N2 and J27 

CRAs (Resource Consultants and Engineers 1993). 

Comparisons between runoff and sediment yield from undisturbed and reclaimed areas in 

CRA J1/N6 and N6 East Central were developed for specific modeling endpoints shown in 

Exhibits 1 and 2. Mining and reclamation activities did not exactly replicate the topography, 

drainage network, or drainage areas that existed prior to mining. Consequently, direct 

comparisons of total runoff and sediment yield cannot be made between undisturbed and 

reclaimed response at a given point in a watershed. Comparisons were made on the basis 

of unit rates of runoff (inches) and sediment yield (tons/acre) at the various modeling 

computation endpoints. Although the same disturbance boundary was used to define the 

extent of both pre- and postmine conditions, the topographic differences that resulted after 

mining and reclamation occurred in the J1/N6 and N6 East Central CRA dictated that some 

areas would be included or excluded from the modeling. The total area modeled for premine 

conditions is 1499.7 acres (Exhibit 2) and for postmine conditions is 1533.3 acres (Exhibit 1). 
 Ayres Associates 1.6

Figure 1.3. J1/N6 and N6 East Central postmine basins. 
 Ayres Associates 1.7

Figure 1.4. J1/N6 and N6 East Central premine basins. 
 Ayres Associates 1.8

1.4.1 Synthetic Rainfall 

 

Synthetic storms of 2-, 5-, 10-, 25-, 50-, and 100-year return periods were used as input to 

the EASI model. Actual hyetographs were taken from the previous study (RCE 1993) and 

are based on both local data collection and the NOAA Atlas (Miller et al. 1973). 

1.4.2 Computation of Average Runoff and Sediment Yield 

The EASI model was used to evaluate runoff and sediment yield from a series of storm 

events having recurrence intervals of 2-, 5-, 10-, 25-, 50-, and 100 years. To define average 

annual conditions, the average annual runoff and sediment yield generated from storm 

events were computed using the commonly used equation of Lagasse et al. (1985). 

1.5 Results 

Figures 1.3 and 1.4 show the post- and premine basin delineations. Since the individual 

subareas differ in number, acreage and outlet locations, a direct comparison is not possible 

on a subarea basis. Therefore, the best way to compare the results is on an average basis 

for the CRA. Table 1.4 shows pre- and postmine drainage area, runoff, and sediment yield 

for the J1/N6 and N6 East Central CRA. Runoff is defined as the total volume of water 

leaving the CRA on an average annual basis and, therefore, does not include water stored in 

depression areas and ponds. For the premine condition, this is equal to the amount of water 

that drains off the hillslopes and subwatersheds because there are no ponds or significant 

depressions. For the postmine condition, this is equal to the amount of hillslope runoff less 

the amount stored in ponds. Similarly, the sediment yield is the amount of eroded material 

that leaves the CRA on an average annual basis computed using the equation of Lagasse et 

al. (1985). The sediment yield is the production from the hillslope areas and erosion from 

the channels. The amount of erosion is the sediment yield from the hillslopes and 

subwatersheds only and does not include channel erosion, channel deposition or sediment 

trapped in ponds. Sediment yield can be greater or less than erosion, depending on the 

amount of channel erosion and the capacity of the channel network to convey sediment off 

the leasehold. 

For the postmine condition, sediment yield is substantially less than the premine condition. 

Sediment yield is approximately one-third of the premine amount. Runoff is the same as the 

premine amount for the N6 East Central CRA, while runoff for postmine is much smaller than 

the premine amount for the J1/N6 CRA. The amount of hillslope runoff is virtually the same 

between pre- and postmine conditions and the difference between the runoff leaving the 

CRA is due to ponds and depressions storing water in the postmine condition. Hillslope and 

subwatershed erosion rates are lower for reclaimed (postmine) conditions due to more 

effective hydrologic cover and channel erosion control measures. 

1.6 Discussion 

Table 1.5 gives an overview of the geometric properties of the pre- and postmine 

topographies for the J1/N6 and N6 East Central CRA. The geometric properties for the 

postmine condition are similar to the premine condition. 
 Ayres Associates 1.9

Table 1.4. Average Runoff and Sediment Yield Results. 

Area Condition Drainage Area 

(ac) 

Runoff 

(in) 

Sediment Yield 

(t/ac/yr) 

Erosion 

(t/ac/yr) 

J1/N6 Premine 1024.8 0.42 3.79 1.74 

J1/N6 Postmine 1039.7 0.22 1.32 1.22 

 

N6 East 

Central 

Premine 474.9 0.42 3.68 0.80 

N6 East 

Central 

Postmine 493.6 0.42 1.61 0.65 

 

Combined Premine 1499.7 0.42 3.76 1.44 

Combined Postmine 1533.3 0.28 1.41 1.03 

Table 1.5. Average Physical Properties of the J1/N6 and N6 

East Central CRA. 

 Premine Postmine 

Total Area (ac) 1499.7 1533.3

Total Channel Length (ft) 112,844 116,293

Mean Channel Slope 0.0563 0.0576

Drainage Density (mi/mi2) 9.1 9.2

Mean Hillslope Length (ft) 269 320

Mean Hillslope Gradient 0.1171 0.1149

 
 Ayres Associates 2.1

2. COMPARISONS WITH MEASURED SEDIMENT TRANSPORT 

As discussed in Section 1, PWCC has monitored flow and sediment on the main channels, 

principal tributaries and small watersheds within the leasehold. These data, along with the 

runoff plots, were used to calibrate the EASI model soil erodibility and infiltration input 

variables. Figures 2.1 and 2.2 show sediment transport and sediment concentration versus 

discharge for measured unmined (background), measured reclaimed, J1/N6 and N6 East 

Central’s modeled unmined (premine) and modeled reclaimed (postmine) data. Although 

there is significant scatter shown in the data (as is expected with any sediment transport 

conditions), there are several conclusions that can be drawn from this data. 

The open symbols in both figures depict measured data and whether the data were collected 

from reclaimed areas (the small watershed study) or from unmined or background surface 

water monitoring stations. The range of flows is generally greater for the background data 

but there is significant overlap between the two data sets between 0.1 cfs and 100 cfs. This 

is because the reclaimed data are from small watersheds and the unmined data are from 

channels draining larger basins. These data show the same trend for sediment transport 

and sediment concentration over the entire range of flows and very close agreement in the 

area of discharge overlap. This, in itself, is strong evidence that (1) the sediment yields are 

channel transport capacity limited, (2) overlap of model predictions for both pre- and 

postmine conditions with measured data strongly indicate that EASI model predictions are 

representative and reasonable, and (3) sediment yields from reclaimed areas will not be 

additive to yields on the receiving streams. 

The closed symbols depict data from J1/N6 and N6 East Central’s pre- and postmine EASI 

model runs. They represent data generated by EASI for both subwatersheds and channels 

for peak discharges resulting from 2-, 5-, 10-, 25-, 50- and 100-year storms. Using the peak 

flows from extreme events results in discharges that generally exceed 10 cfs. The trend of 

the model-derived data is similar and the ranges of concentration and sediment transport are 

similar to the measured data and between pre- and postmine conditions. 

The sediment discharge plot (Figure 2.1) shows a stronger trend because it is plotting 

discharge (sediment) against discharge (flow). This is expected because the sediment 

discharge does depend on flow discharge. The concentration plot (Figure 2.2) shows the two 

separate variables and, therefore, a less significant trend. PWCC believes that data 

measurement may have some influence on the scatter (outliers were removed), but the 

process variability is probably the major influence. The majority of the data, however, fall in 

a group centered on 100 cfs and 100,000 mg/l, both in the observed data and in the model 

results. These plots support the use of the EASI model, the results of the modeling, the 

conclusion that sediment yields from reclaimed areas are not additive to receiving stream 

sediment loads, and that sediment impacts to the prevailing hydrologic balance have been 

minimized. 

From Figures 2.1 and 2.2 it is apparent that sediment loads and concentrations are 

dependent on the channel sediment transport capacity for both pre- and postmine 

conditions. Channel sources of sediment in this arid environment are virtually unlimited. 

Therefore, channel transport capacity and channel derived sediment limits and governs 

sediment yields from the small tributaries, large channels and the CRA as a whole. The 

similarity of sediment discharge (or concentration) between pre- and postmine conditions 

appears to be inconsistent with the lower rates of sediment yield shown in Table 1.4. 
 Ayres Associates 2.2

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Discharge (tons/day)

Background Measured

Reclaimed Measured

Model Premine

Model Postmine

Figure 2.1. Observed and modeled sediment discharge and water discharge. 
 Ayres Associates 2.3

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Concentration (mg/l)

Background Measured

Reclaimed Measured

Model Premine

Model Postmine

Figure 2.2. Observed versus modeled sediment concentration and discharge. 
 Ayres Associates 2.4

However, the sediment yield shown in Table 1.4 is the amount of sediment leaving the CRA 

whereas the sediment discharge shown in Figure 2.1 is the peak rate of sediment in 

transport occurring in any channel on the CRA, whether the channel is located upstream or 

downstream of a pond. Therefore, with or without the ponds trapping sediment or storing 

water, the mine reclamation is not contributing additional sediment to the receiving streams 

and sediment impacts to the prevailing hydrologic balance have been minimized. 

Smith and Best (2000) analyzed the measured data (background and reclaimed) shown in 

Figure 2.1 to develop an approach that can be used to determine if channels in reclaimed 

areas have similar sediment transport characteristics as background channels. The method 

that they used was to develop Sen lines (Sen 1968) and confidence intervals around the 

data. The slope of the Sen line is a non-parametric statistic computed as the median slope of 

all possible slopes determined from pairing all the data points. The Sen line is drawn through 

the median coordinate of the data. Smith and Best first showed that the large channel flume 

data (background) and the small watershed background data could be combined. They 

concluded that since the data from one data set fall within the Sen line bounds of the other 

data set then the two data sets are merely extensions of each other and could be combined. 

Also, because the main channel and background small watershed site data could be 

combined, it indicated there is an unlimited supply of sediment and the channels are 

conveying sediment at (or near) capacity. The Sen line and bounds are shown with the 

background measured data in Figure 2.3. 

They then plotted the reclaimed measured data (Figure 2.4) with the Sen line and bounds 

from the background data to show that the reclaimed data have the same characteristics 

even though the flow range of the measurements is lower. The data indicate that channel 

flows in this environment achieve the sediment transport capacity of the channel, whether in 

reclaimed or background conditions. 

Using the same approach with the modeled data generated for the CRA, Figures 2.5 and 

2.6 show the pre- and postmine computed sediment transport rates with the Sen lines and 

bounds. One difference between the plots is that the measured data occur throughout the 

flow hydrograph whereas the modeled data are tabulated at the peak of the simulation flow 

hydrograph. The premine data plot (Figure 2.5) shows the data grouped densely around the 

Sen line and well within the bounds. The postmine data (Figure 2.6) also plot closely around 

the Sen line and well within bounds. On these graphs data plotting below the Sen line 

indicate that there is less sediment in transport for a given discharge. 

Several conclusions can be drawn from these data plots: (1) EASI model well replicates 

erosion and sediment transport processes at the mine site for background and reclaimed 

conditions, (2) all data show similar trends and are within the same bounds, (3) data trends 

indicate that channels are transporting sediment at or near capacity, and (4) amounts of 

sediment leaving the CRA for postmine conditions are similar to premine conditions and 

within the range expected for the background conditions. Therefore, the overall conclusion is 

that the postmine reclaimed condition in the J1/N6 and N6 East Central CRA is not 

contributing additional suspended solids to receiving streams, and related impacts to the 

hydrologic balance have been minimized. 
 Ayres Associates 2.5

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Discharge (tons/day)

Background Measured

Sen Line

Upper Line

Lower Line

Figure 2.3. Background measured sediment and water discharge. 
 Ayres Associates 2.6

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Discharge (tons/day)

Reclaimed Measured

Sen Line

Upper Line

Lower Line

Figure 2.4. Reclaimed measured sediment and water discharge. 
 Ayres Associates 2.7

0

0

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Discharge (tons/day)

Model Premine

Sen Line

Upper Line

Lower Line

Figure 2.5. Modeled premine sediment and water discharge for J1/N6 and N6 East Central.
 Ayres Associates 2.8

0

0

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Discharge (cfs)

Sediment Discharge (tons/day)

Model Postmine

Sen Line

Upper Line

Lower Line

Figure 2.6. Modeled postmine sediment and water discharge for J1/N6 and N6 East Central. 
 Ayres Associates 3.1

3. REFERENCES 

Hjelmfelt, A.T., Kramer, L.A., and Spomer, R.G., 1986. Role of large events in average soil 

loss in Proceedings of the Fourth Federal Interagency Sedimentation Conference, March 24-

27, 1986, Las Vegas, Nevada, p. 3-1 to 3-9. 

Lagasse, P.F., Schall, J.D., and Peterson, M.R., 1985. Erosion Risk Analysis for a 

Southwestern Arroyo, Journal of Urban Planning and Management, American Society of Civil 

Engineers, v. III, no. I, November, 1985, Paper No. 20165. 

Miller, J.F., Frederick, R.H., and Tracey, R.J., 1973. "NOAA Atlas 2, Precipitation-Frequency 

Atlas of the Western United States," Volume VIII - Arizona, National Oceanic and Atmospheric 

Administration. 

Renard, K.G. and Simaton, J.R., 1975. "Thunderstorm Precipitation Effects on the RainfallErosion Index of the Universal Soil Loss Equation" in Volume 5 of Hydrology and Water 

Resources in Arizona and the Southwest, American Water Resources Assn., Arizona Section 

Arizona Academy of Science, Hydrology Section, Proceedings of April 11-12 meeting, Tempe 

Arizona, v. 47-55. 

Resource Consultants & Engineers, Inc., 1993. "Surface Water Modeling of Reclaimed 

Parcels at the Black Mesa Complex," prepared for Peabody Western Coal Co. 

Sen, P.K., 1968. "Estimates of Regression Coefficient Based on Kendall's tau: Journal of 

American Statistical Association, v. 63, p. 1379-1389. 

Smith, T. and Best, A., 2000. "Assessing Sedimentation and Protection of the Hydrologic 

Balance in Disturbed and Reclaimed Areas at the Black Mesa and Kayenta Mines, Arizona, 

Using Baseline Sediment Data Collected in Receiving Streams," presentation given at the 

Billings Mined Land Reclamation Symposium in 2001, Billings, MT. 

Water Engineering & Technology, Inc. (WET), 1990. "Determination of Background Sediment 

Yield and Development of a Methodology for Assessing Alternative Sediment Control 

Technology at Surface Mines in the Semiarid West," prepared for Office of Surface Mining and 

the National Coal Association, Fort Collins, CO. 

Zevenbergen, L.W., Peterson, M.R., and Watson, C.C., 1990. "Computer simulation of 

watershed runoff and sedimentation processes," Proceedings of the Billings Symposium; 

Planning, Rehabilitation and Treatment of Disturbed Lands. 
EXHIBIT 1 

Postmine Topography 
6675

6670

6665

6660

6650

6655

6680

6685

6690

6695

6700

6705

6710

6715

6720

6725

6740

6745

6750

6755

6760

6765

6780

6785

6645

6640

6790

6800

6805

68106815

6820

6825

6830

6835

6795

6840

6845

6850

6855

6860

6865

6870

6730

6735

6635

6875

6880

6885

6630

6890

6625

6895

6900

6905

6910

6915

6920

6925

6620

6930

6775

6615

6610

6605

6770

6600

6595

6935

6940

6945

6950

6590

6585

6575

6570

6955

6565

6580

6560 6555 6550 6545

6960

6540

6535

6965

6970

6530

6975

6525

6980

6520

6985

6515

6785

6650

6760

6840

6670

6815

6605

6690

6720

6580

6620

6600

6630

6665

6625

6715

6650

6600

6845

6595

6560

6895

6685

6595

6690

6610

6825

6680

6870

6750

6640

6640

6640

6695

6715

6590

6620

6645

6665

6920

6655

6875

6605

6740

6690

6780

6600

6675

6585

6630

6920

6705

6670

6860

6540

6760

6640

6900

6775

6590

6945

6595

6675

6680

6600

6560

6610

6760

6660

6745

6600

6570

6565

6750

6700

6695

6835

6605

6875

6600

6680

6610

6900

6650

6750

6915

6560

6530

6645

6640

6770

6800

6890

6620

6795

6635

6965

6650

6600

6865

6640

6795

6600

6620

6895

6635

6675

6940

6575

6940

6805

6575

6785

6565

6725

6770

6665

6650

6600

6690

6770

6650

6630

6660

6650

6650

6740

6605

6580

6760

6680

6655

6705

6765

6615

6775

6880

6630

6610

6945

6660

6810

6650

6870

6810

6615

6630

6710

6580

6695

6615

6685

6600

6900

6655

6615

6630

6645

6580

6770

6955

6925

6770

6595

6770

6775

6590

6635

6955

6630

6585

6580

6660

6585

6705

6900

6715

6805

6590

6595

6695

6795

6640

6575

6955

6625

6640

6625

6575

6835

6780

6735

6590

6855

6710

6735

6615

6600

6620

6575

6640

6670

6590

6615

6610

6610

6805

6670

6725

6945

6635

6760

6825

6820

6590

6680

6775

6600

6845

6700

6655

6885

6815

6850

6775

6695

6750

6795

6880

6575

6620

6635

6625

6595

6950

6865

6915

6795

6605

6675

6970

6615

6650

6735

6740

6840

6570

6585

6810

6850

6655

6960

6595

6580

6610

6605

6950

6560

6605

6705

6750

6635

6600

6605

6765

6625

6740

6625

6930

6850

6745

6715

6880

6680

6830

6700

691

6

0

680

6620

6855

6630

6675

6800

6700

6570

6590

6655

6625

6765

6685

6755

6825

6665

6660

6610

6585

6775

6795

6585

6870

6640

6890

6690

6570

6575

6865

6675

6755

6635

6580

6890

6670

6730

6955

6800

6795

6730

6595

6680

6645

6735

6680

6960

6755

6845

6655

6960

6600

6675

6760

6775

6585

6610

6720

6890

6840

6695

6745

6730

6625

6600

6830

6845

6790

6730

6795

6835

6955

6780

6650

6715

6630

6795

6670

6950

6675

6590

6565

6875

6685

6690

6670

6645

6540

6940

6605

6615

6730

6700

6780

6695

6685

6745

6750

6610

6630

6810

6590

6705

6685

6635

6880

6780

6960

6555

6570

6635

6645

6930

6665

6755

6625

6775

6725

6840

6730

6670

6690

6730

6735

6600

6680

6930

6610

6855

6935

6585

6935

6745

6785

6740

6740

6785

6845

6690

6820

6655

6580

6895

6800

6940

6700

6680

6935

6690

6650

6770

6670

6785

6860

6720

6775

6680

J1/N6

N6 East Central

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

78

67

69

68

61

60

59

58

57

56

55

54

53

83

82

81

80

79

7677

75

74

73

72

71

70

0 1,500 3,000 4,500 750 3

Feet

Legend

End Points

Modeling Area Post

Exhibit 1. Postmine Topography

(5 foot contour)
EXHIBIT 2 

Premine Topography 
6770

6800

6720

6640

6740

6750

6840

6760

6680

6600

6890

6730

6710

6920

6700

6690

6670

6780

6790

6660

663

6

0

620

6810

6830

6610

6820

6650

6590

6850

6860

6870

6880

6560

6580

6900

6570 6910

6540

6550

6600

6870

6880

6660

6710

6920

6810

6630

6650

6680

6920

6720

6740

6760

6560

6720

6710

6610

6690

6600

6770

6670

6660

6610

6860

6730

6620

6580

661

6

0

600

6640

6700

6790

6870

6630

6800

6820

6740

6820

6800

6590

6690

6720

6640

6650

6780

6790

6730

6680

6840

6820

6680

6750

6670

6690

6740

6600

6740

6810

6910

6580

6800

6560

6760

6560

6810

6920

6650

6720

6630

6820

6650

6710

6720

6680

6680

6770

6720

6790

6650

6730

6820

6820

6650

6790

6660

6730

6830

6650

6750

6620

6770

6640

6810

6630

6750

6660

6670

6850

6690

6920

6670

6700

6830

6690

6720

6640

6690

6780

6810

6600

6660

6770

6670

6570

6670

6590

6730

6620

6620

6650

6730

6640

6860

6760

6680

6710

6810

6740

6640

6920

6830

6710

6700

6650

6720

6720

6

6800

730

6670

6690

6820

6840

6680

6660

6730

6700

6640

6610

6850

J1/N6

N6 East Central

65

66

67

68

69

70

71

72

73

74

75

76 51

52

53

54

55

56

57

58

59

60

61

62

63

64

15

16

14

13

12

11

0 1,500 3,000 4,500 750 3

Feet

Legend

End Points

Modeling Area Pre

Exhibit 2. Premine Topography

(10 & 40 foot contour)
SURFACE WATER MODELING OF RECLAIMED PARCELS AT THE 

J19 COAL RESOURCE AREA, KAYENTA COMPLEX

Peabody Western Coal Company

Highway 160, Navajo Route 41

Kayenta, Arizona 86033
SURFACE WATER MODELING OF RECLAIMED PARCELS AT THE 

J19 COAL RESOURCE AREA, KAYENTA COMPLEX

Peabody Western Coal Co.

Highway 160, Navajo Route 41

Kayenta, Arizona 86033

September 2011
TABLE OF CONTENTS

1. Reclaimed parcel modeling .........................................................................................1.1

1.1 Introduction ..........................................................................................................1.1

1.2 Background..........................................................................................................1.1

1.3 Data .....................................................................................................................1.2

1.3.1 Soils ..............................................................................................................1.2

1.3.2 Vegetation.....................................................................................................1.2

1.3.3 Topography...................................................................................................1.2

1.4 Methodology.........................................................................................................1.2

1.4.1 Synthetic Rainfall ..........................................................................................1.8

1.4.2 Computation of Average Runoff and Sediment Yield.....................................1.8

1.5 Results.................................................................................................................1.8

1.6 Discussion............................................................................................................1.8

2. COMPARISONS WITH MEASURED SEDIMENT TRANSPORT ................................2.1

2.1 Sediment Discharge and Concentration ...............................................................2.1

2.2 Statistical Analysis................................................................................................2.4

2.3 Conclusions..........................................................................................................2.4

3. References ..................................................................................................................3.1

Exhibit 1 – J-19 Post-Mine Topography..............................................................................3.2

Exhibit 2 – J-19 Pre-Mine Topography ...............................................................................3.3
LIST OF FIGURES

Figure 1.1. Reclaimed Area Soils Trilinear Graph..............................................................1.3

Figure 1.2. Spatial Distribution of Vegetative Cover Types for J19 Pre-Mine Condition. .....1.4

Figure 1.3. J19 Post-Mine Basins.......................................................................................1.6

Figure 1.4. J19 Pre-Mine Basins. .......................................................................................1.7

Figure 2.1. Observed and Modeled Sediment Discharge and Water Discharge.................2.2

Figure 2.2. Observed versus Modeled Sediment Concentration and Discharge. ...............2.3

Figure 2.3. Background Measured Sediment and Water Discharge...................................2.5

Figure 2.4. Reclaimed Measured Sediment and Water Discharge.....................................2.6

Figure 2.5. Modeled Pre-Mine Sediment and Water Discharge for J19. ............................2.7

Figure 2.6. Modeled Post-Mine Sediment and Water Discharge for J19............................2.8

LIST OF TABLES

Table 1.1. Soils Data. ........................................................................................................1.3

Table 1.2. Cover Sampling Data........................................................................................1.5

Table 1.3. Cover Data for J19............................................................................................1.5

Table 1.4. Average Runoff and Sediment Yield Results. ...................................................1.9

Table 1.5. Average Physical Properties of the J19 CRA....................................................1.9
1.1

1. RECLAIMED PARCEL MODELING

1.1 Introduction

The purpose of this project is to use a previously calibrated and validated runoff and erosion 

model EASI - Erosion And Sediment Impacts (Zevenbergen et al. 1990; WET 1990) for the 

Kayenta Complex (previously identified as the Black Mesa and Kayenta Mines) to predict 

mean annual runoff and sediment yields from the reclaimed parcel J19. The objective of this 

project included computation of runoff and sediment yields under premine conditions for the 

same area. The response of the reclaimed parcels was evaluated relative to undisturbed 

(premine) conditions in the corresponding undisturbed watersheds. All soils and rainfall input 

to the model were taken from models calibrated in the previous study (RCE 1993). The input 

variables that were calibrated to the mine areas and used in this study include soil infiltration 

parameters, erodibility parameters, and the grain size distribution. Parameters that are 

specific to this study are vegetative canopy and ground cover percentages from data 

collected on site. The model serves as a tool for assessing the success of reclamation 

efforts to protect the hydrologic balance (30 CFR 715.17 and 30 CFR 816.41).

The model calibration was conducted in a previous study (RCE 1993) using data obtained 

from instrumented watersheds and small hillslope plots collected under natural rainfall 

conditions. For a detailed discussion of data collection and model calibration, please refer to 

the previous study (RCE 1993).

1.2 Background 

The J19 Coal Resource Area (CRA) at the Kayenta Complex that is the focus of this project 

was reclaimed between 1989 and 2010. This reclaimed area is now eligible for Phase II 

Bond Release from the Office of Surface Mining Reclamation and Enforcement (OSM). The 

fundamental purpose of this study was to quantify the expected behavior and hydrologic 

response of the current conditions of reclaimed areas relative to the conditions that existed 

prior to the occurrence of mining activities.

Runoff and sediment yield response from the reclaimed lands should be managed by 

implementing Best Management Practices (BMP's) in conjunction with an OSM approved 

reclamation plan in order to not adversely impact the prevailing hydrologic balance and to 

limit additional contributions of suspended sediment to streamflow or runoff outside the mine 

permit areas. BMP's include regrading, replacing salvaged topsoil, revegetation, and other 

controls such as riprapped channel bottoms, check dams, rock down drains, and where 

practicable, contour terraces. The natural watersheds on Black Mesa contribute significant 

quantities of sediment to the channel system. It is expected that the postmine condition will 

also produce comparable amounts of sediment without adversely impacting the hydrologic 

balance.

The next sections describe the data and procedures used to evaluate the J19 CRA. This 

area was modeled to determine the average annual hydrologic response following the 

completion of reclamation activities and maturation of the reclaimed area vegetation taking 

into account BMP's implemented as part of the reclamation process. Infiltration, runoff, and 

erosion processes from both hillslopes and channels within the CRA were modeled using 

EASI. Results were determined for concentration points at the outlets of the reclaimed 

watersheds. The locations of these points are shown in Exhibit 1. Modeling was also 

conducted to determine hydrologic response under premine conditions based on the 

topography, soils, cover, and other conditions that typified the undisturbed watersheds 

draining to each concentration point. Exhibit 2 shows the modeling endpoints for the J19

premining watersheds.
1.2

1.3 Data

1.3.1 Soils

Soils data used for the current study (CRA J19) were based on data developed from the 

calibration of models used in the previous study for Coal Resource Areas (CRAs) N1/N2 and 

J27 (RCE 1993). The composition of postmine soil in the current study is depicted along with 

the composition of postmine soils from the previous study in Figure 1.1. This figure shows 

that the soil composition of CRA J19 is very similar to soils evaluated during model 

calibration. Therefore, the soil properties developed in the previous study are valid for this 

modeling project. These properties include calibrated parameters, such as infiltration and 

erodibility coefficients, and measured soil size distributions. Table 1.1 lists the premine and 

postmine soils data used during EASI modeling of CRA J19.

1.3.2 Vegetation 

Vegetative cover data representative of both pre- and postmine conditions in CRA J19 were 

supplied by PWCC. For the premine condition, land was characterized as being covered by 

sagebrush or pinon juniper. The spatial distribution of vegetative cover for the J19 CRA 

premine condition appears in Figure 1.2. Average cover properties for CRAs N1/N2 and J27 

of the previous study and CRA J19 of the current study appear in Table 1.2. For the 

postmine condition, the reclaimed area was assigned the postmine cover type and any 

unmined area was assigned the same cover type as the premine condition. Table 1.3 lists 

the pre- and postmine vegetative cover data used in the EASI model runs generated for the 

J19 CRA. Note that if a unit contained significant portions of both sagebrush and pinon 

juniper cover types, it was classified as half pinon juniper and half sagebrush.

1.3.3 Topography

Pre- and postmine topography was supplied by PWCC in the form of ArcGIS geodatabase. 

Basin delineations, hillslope delineations, subwatershed delineations, as well as areas, 

slopes, and lengths of all units of the study area were defined and calculated using ArcGIS 

software. Figures 1.3 and 1.4 show the watershed delineation and numbers assigned to the 

basins used in the EASI model for the post- and premine conditions, respectively. Channel 

dimensions input to EASI were based on the topography supplied and limited field 

observations.

1.4 Methodology

Runoff and sediment yield in the semiarid western United States is largely governed by the 

occurrence of high-intensity, short-duration rainstorms of limited areal extent (Renard and 

Simanton 1975). Research has indicated that relatively few events may produce the greatest 

erosion (e.g., Hjelmfelt et al. 1986 reported that only 3 to 4% of rainfall events accounted for 

50% of long-term sediment yields). Although there is a relatively limited physical basis for 

definition of an "average annual" runoff or sediment yield in a semiarid environment due to 

the extreme variability in response and importance of single infrequent events, such a term 

does provide a useful basis for long-term comparison between reclaimed and undisturbed 

conditions. 
1.3

Figure 1.1. Reclaimed Area Soils Trilinear Graph.

Table 1.1. Soils Data.

Condition Premine Postmine Rock Chutes

Rainfall detachment 0.005 0.005 0

Overland flow detachment 0.44 0.44 0

Channel flow detachment 0.5 0.5 0

Initial soil moisture, % 70 70 70

Final soil moisture, % 90 90 90

Soil porosity, % 45 45 46

Temperature, *F 70 70 70

Hydraulic conductivity, in/hr 0.23 0.29 0.3

Capillary suction, in 3.7 2.6 2.6

Particle Size Distribution

(all conditions)

Size, mm % Finer

0.001 0

0.004 18.0

0.016 27.4

0.062 36.6

0.125 56.2

0.250 64.3

0.500 72.4

1.000 80.5

2.000 88.6

4.000 92.4

16.000 100

J19 AVG.
1.4

Figure 1.2. Spatial Distribution of Vegetative Cover Types for J19 Pre-Mine Condition.
1.5

Table 1.2. Cover Sampling Data.

Area Condition Cover 

Type

Nonstratified

Vegetation 

Cover (%)

Vegetation 

Canopy 

Cover (%)

Vegetation 

Ground 

Cover (%)

Litter* 

(%)

Rock 

(%)

Total 

Ground 

Cover 

(%)

N1/N2 Postmine Postmine 25.6 1.4 24.2 13.6 4.2 42.0

J19 Postmine Postmine 28.8 0.2 31.3 17.9 6.6 55.8

N1/N2/J27 Premine Pinon Juniper 32.7 31.1 3.0 44.0 19.7 66.7

J19 Premine Pinon Juniper 19.1 17.0 2.5 28.8 16.7 48.0

N1/N2 Premine Sagebrush 25.1 16.0 10.3 25.3 18.1 53.7

J27 Premine Sagebrush 30.6 9.7 22.0 24.0 1.6 47.6

J19 Premine Sagebrush 16.7 3.8 13.4 30.6 1.7 45.7

*Including standing dead litter

Table 1.3. Cover Data for J19. 

Condition Pinon Juniper Sagebrush

Half Pinon JuniperHalf Sagebrush Postmine

Canopy cover, % 17.0 3.8 10.4 0.2

Ground cover, % 48.0 45.7 46.9 55.8

Canopy storage, in 0.05 0.05 0.05 0.05

Ground storage, in 0.05 0.05 0.05 0.05

Depression storage, in 0.03 0.03 0.03 0.03

Impervious area, % 0 0 0 0

Manning n 0.07 0.07 0.07 0.05

To make comparisons between reclaimed lands and associated undisturbed lands at the 

Kayenta Complex on the basis of average annual sediment yield, a procedure was used that 

considers the importance of infrequent storm events in defining sediment yield in the 

semiarid west. First, however, the site-specific rainfall data available for the Kayenta 

Complex were used to evaluate the frequency and magnitude of the measured events 

relative to existing predictions for rainfall depth-duration (Miller et al. 1973). The analysis of 

the rainfall data was performed as part of a previous study of the N1/N2 and J27 CRAs (RCE 

1993).

Comparisons between runoff and sediment yield from undisturbed and reclaimed areas in 

CRA J19 were developed for specific modeling endpoints shown in Exhibits 1 and 2. Mining 

and reclamation activities did not exactly replicate the topography, drainage network, or 

drainage areas that existed prior to mining. Consequently, direct comparisons of total runoff 

and sediment yield cannot be made between undisturbed and reclaimed response at a given 

point in a watershed. Comparisons were made on the basis of unit rates of runoff (inches) 

and sediment yield (tons/acre) at the various modeling computation endpoints. Although the 

same disturbance boundary was used to define the extent of both pre- and postmine 

conditions, the topographic differences that resulted after mining and reclamation occurred in 

the J19 CRA dictated that some areas would be included or excluded from the modeling. 

The total area modeled for premine conditions is 943.4 acres (Exhibit 2) and for postmine 

conditions is 943.4 acres (Exhibit 1). 
1.6

Figure 1.3. J19 Post-Mine Basins.
1.7

Figure 1.4. J19 Pre-Mine Basins.
1.8

1.4.1 Synthetic Rainfall

Synthetic storms of 2-, 5-, 10-, 25-, 50-, and 100-year return periods were used as input to 

the EASI model. Actual hyetographs were taken from the previous study (RCE 1993) and 

are based on both local data collection and the NOAA Atlas (Miller et al. 1973). 

1.4.2 Computation of Average Runoff and Sediment Yield

The EASI model was used to evaluate runoff and sediment yield from a series of storm 

events having recurrence intervals of 2-, 5-, 10-, 25-, 50-, and 100 years. To define average 

annual conditions, the average annual runoff and sediment yield generated from storm 

events were computed using the commonly used equation of Lagasse et al. (1985). 

1.5 Results

Figures 1.3 and 1.4 show the post- and premine basin delineations. Since the individual 

subareas differ in number, acreage and outlet locations, a direct comparison is not possible 

on a subarea basis. Therefore, the best way to compare the results is on an average annual 

basis for the CRA. Table 1.4 shows pre- and postmine drainage area, runoff, sediment yield, 

and erosion for the J19 CRA. Runoff is defined as the total volume of water leaving the CRA 

on an average annual basis and, therefore, does not include water stored in depression 

areas and ponds. For the premine condition, this is equal to the amount of water that drains 

off the hillslopes and subwatersheds because there were no ponds or significant 

depressions. For the postmine condition, this is equal to the amount of hillslope runoff less 

the amount stored in ponds. Similarly, the sediment yield is the amount of eroded material 

that leaves the CRA on an average annual basis computed using the equation of Lagasse et 

al. (1985). The sediment yield is the production from the hillslope and subwatershed areas 

and erosion from the channels. The amount of erosion is the sediment yield from the 

hillslopes and subwatersheds only and does not include channel erosion, channel deposition 

or sediment trapped in ponds. Sediment yield can be greater or less than erosion, 

depending on the amount of channel erosion and the capacity of the channel network to 

convey sediment off the leasehold.

For the postmine condition, sediment yield is substantially less than the premine condition. 

Postmining sediment yield is approximately 71% of the premine amount. Runoff is the same 

as the premine amount for the J19 CRA. Hillslope and subwatershed erosion rates are 

about 9% higher for the reclaimed (postmine) condition than the premine condition. However, 

the erosion rates are comparable between both conditions, and remain below 1.0 

tons/acre/year. The reduction of sediment yield is due to effective hydrologic cover 

combined with effective channel erosion control measures in the postmining landscape.

1.6 Discussion

Table 1.5 gives an overview of the geometric properties of the pre- and postmine 

topographies for the J19 CRA. Postmine hillslopes are generally about 15% shorter and 7% 

steeper than premine hillslopes, postmine channels are slightly less steep than premine 

channels, and the drainage density of the postmine condition is about 13% greater than that 

of the premine condition. These properties agree with the postmine versus premine 

topography: the premine topography is fully dissected.
1.9

Table 1.4. Average Runoff and Sediment Yield Results.

Area Condition Drainage Area

(ac)

Runoff

(in)

Sediment Yield 

(t/ac/yr)

Erosion 

(t/ac/yr)

J19 Premine 943.4 0.42 3.13 0.75

J19 Postmine 943.4 0.42 2.22 0.82

Table 1.5. Average Physical Properties of the J19 CRA.

Premine Postmine

Total Area (ac) 943.4 943.4

Total Channel Length (ft) 70,077 79,298

Mean Channel Slope 0.0662 0.0653

Drainage Density (mi/mi2) 9.0 10.2

Mean Hillslope Length (ft) 243 206

Mean Hillslope Gradient 0.1115 0.1197
2.1

2. COMPARISONS WITH MEASURED SEDIMENT TRANSPORT

2.1 Sediment Discharge and Concentration 

As discussed in Section 1, PWCC has monitored flow and sediment on the main channels, 

principal tributaries and small watersheds within the leasehold. These data, along with the 

runoff plots, were used to calibrate the EASI model soil erodibility and infiltration input 

variables. Figures 2.1 and 2.2 show sediment transport and sediment concentration versus 

discharge for measured unmined (background), measured reclaimed, modeled unmined (J19

premine) and modeled reclaimed (J19 postmine) data. Although there is significant scatter 

shown in the data (as is expected with sediment transport), there are several conclusions 

that can be drawn from this data. 

The open symbols in both figures depict measured data and whether the data were collected 

from reclaimed areas (the small watershed study) or from unmined or background surface 

water monitoring stations. The range of flows is generally greater for the background data 

but there is significant overlap between the two data sets between 0.1 cfs and 100 cfs. This 

is because the reclaimed data are from small watersheds and the unmined data are from 

channels draining larger basins. These data show the same trend for sediment transport and 

sediment concentration over the entire range of flows and very close agreement in the area 

of discharge overlap. This, in itself, is strong evidence that (1) the sediment yields are 

channel transport capacity limited, (2) overlap of model predictions for both pre- and 

postmine conditions with measured data strongly indicate that EASI model predictions are 

representative and reasonable, and (3) sediment yields from reclaimed areas will not be 

additive to yields on the receiving streams.

The closed symbols depict data from the J19 CRA pre- and postmine EASI model runs. 

They represent data generated by EASI for both subwatersheds and channels for peak 

discharges resulting from 2-, 5-, 10-, 25-, 50- and 100-year storms. Using the peak flows 

from extreme events results in discharges that generally exceed 10 cfs. The trend of the 

model-derived data is similar and the ranges of concentration and sediment transport are 

similar to the measured data and between pre- and postmine conditions.

The sediment discharge plot (Figure 2.1) shows a stronger trend because it is plotting 

discharge (sediment) against discharge (flow). This is expected because the sediment 

discharge does depend on flow discharge. The concentration plot (Figure 2.2) shows the two 

separate variables and, therefore, a less significant trend. PWCC believes that data 

measurement may have some influence on the scatter (outliers were removed), but the 

process variability is probably the major influence. The majority of the data, however, fall in a 

group centered between 10 and 100 cfs and between 10,000 and 100,000 mg/l, both in the 

observed data and in the model results. These plots support the use of the EASI model, the 

results of the modeling, the conclusion that sediment yields from reclaimed areas are not 

additive to receiving stream sediment loads, and that sediment impacts to the prevailing 

hydrologic balance have been minimized.

From Figures 2.1 and 2.2 it is apparent that sediment loads and concentrations are 

dependent on the channel sediment transport capacity for both pre- and postmine conditions. 

Channel sources of sediment in this arid environment are virtually unlimited. Therefore, 

channel transport capacity and channel derived sediment limits and governs sediment yields 

from the small tributaries, large channels and the CRA as a whole. The similarity of 

sediment discharge (or concentration) between pre- and postmine conditions appears to be 

inconsistent with the lower rates of sediment yield shown in Table 1.4. 
2.2

Figure 2.1. Observed and Modeled Sediment Discharge and Water Discharge.

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Discharge (tons/day)

Discharge (cfs)

Background Measured

Reclaimed Measured

Model Premine

Model Postmine

Sen Line

Upper Line

Lower Line
2.3

Figure 2.2. Observed versus Modeled Sediment Concentration and Discharge.

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Concentration (mg/l)

Discharge (cfs)

Background Measured

Reclaimed Measured

Model Premine - J19

Model Postmine - J19
2.4

However, the sediment yield shown in Table 1.4 is the amount of sediment leaving the CRA 

whereas the sediment discharge shown in Figure 2.1 is the peak rate of sediment in 

transport occurring in any channel on the CRA, whether the channel is located upstream or 

downstream of a pond. Therefore, with or without the ponds trapping sediment or storing 

water, the mine reclamation is not contributing additional sediment to the receiving streams, 

and sediment impacts to the prevailing hydrologic balance have been minimized.

2.2 Statistical Analysis

Smith and Best (2000) analyzed the measured data (background and reclaimed) shown in 

Figure 2.1 to develop an approach that can be used to determine if channels in reclaimed 

areas have similar sediment transport characteristics as background channels. The method 

that they used was to develop Sen lines (Sen 1968) and confidence intervals around the 

data. The slope of the Sen line is a non-parametric statistic computed as the median slope of 

all possible slopes determined from pairing all the data points. The Sen line is drawn through 

the median coordinate of the data. Smith and Best first showed that the large channel flume 

data (background) and the small watershed background data could be combined. They 

concluded that since the data from one data set fall within the Sen line bounds of the other 

data set then the two data sets are merely extensions of each other and could be combined. 

Also, because the main channel and background small watershed site data could be 

combined, it indicated there is an unlimited supply of sediment and the channels are 

conveying sediment at (or near) capacity. The Sen line and bounds are shown with the 

background measured data in Figure 2.3.

Smith and Best then plotted the reclaimed measured data (Figure 2.4) with the Sen line and 

bounds from the background data to show that the reclaimed data have the same 

characteristics even though the flow range of the measurements is lower. The data indicate 

that channel flows in this environment achieve the sediment transport capacity of the 

channel, whether in reclaimed or background conditions.

Using the same approach with the modeled data generated for the J19 CRA, Figures 2.5 

and 2.6 show the pre- and postmine computed sediment transport rates with the Sen lines 

and bounds, respectively. One difference between the plots is that the measured data occur 

throughout the flow hydrograph whereas the modeled data are tabulated at the peak of the 

simulation flow hydrograph. The premine data plot (Figure 2.5) shows the data grouped 

densely around the Sen line and well within the bounds. The postmine data (Figure 2.6) also 

plot closely around the Sen line and well within bounds.

2.3 Conclusions

Several conclusions can be drawn from these data plots: (1) the EASI model well replicates 

erosion and sediment transport processes at the mine site for background and reclaimed 

conditions, (2) all data show similar trends and are within the same bounds, (3) data trends 

indicate that channels are transporting sediment at or near capacity, and (4) amounts of 

sediment leaving the CRA for postmine conditions are similar to premine conditions and 

within the range expected for the background conditions. Therefore, the overall conclusion is 

that the postmine reclaimed condition in the J19 CRA is not contributing additional 

suspended solids to receiving streams, and related impacts to the hydrologic balance have 

been minimized.
2.5

Figure 2.3. Background Measured Sediment and Water Discharge.

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Discharge (tons/day)

Discharge (cfs)

Background Measured

Sen Line

Upper Line

Lower Line
2.6

Figure 2.4. Reclaimed Measured Sediment and Water Discharge.

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Discharge (tons/day)

Discharge (cfs)

Reclaimed Measured

Sen Line

Upper Line

Lower Line
2.7

Figure 2.5. Modeled Pre-Mine Sediment and Water Discharge for J19.

0

0

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Discharge (tons/day)

Discharge (cfs)

Model Premine

Sen Line

Upper Line

Lower Line
2.8

Figure 2.6. Modeled Post-Mine Sediment and Water Discharge for J19.

0

0

1

10

100

1000

10000

100000

1000000

10000000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment Discharge (tons/day)

Discharge (cfs)

Model Postmine

Sen Line

Upper Line

Lower Line
3.1

3. REFERENCES 

Hjelmfelt, A.T., Kramer, L.A., and Spomer, R.G., 1986. "Role of large events in average soil 

loss," in Proceedings of the Fourth Federal Interagency Sedimentation Conference, March 

24-27, 1986, Las Vegas, Nevada, p. 3-1 to 3-9.

Lagasse, P.F., Schall, J.D., and Peterson, M.R., 1985. "Erosion Risk Analysis for a 

Southwestern Arroyo," Journal of Urban Planning and Management, American Society of 

Civil Engineers, v. III, no. I, November, 1985, Paper No. 20165.

Miller, J.F., Frederick, R.H., and Tracey, R.J., 1973. "NOAA Atlas 2, Precipitation-Frequency 

Atlas of the Western United States," Volume VIII - Arizona, National Oceanic and 

Atmospheric Administration.

Renard, K.G. and Simanton, J.R., 1975. "Thunderstorm Precipitation Effects on the RainfallErosion Index of the Universal Soil Loss Equation," in Volume 5 of Hydrology and Water 

Resources in Arizona and the Southwest, American Water Resources Assn., Arizona Section 

Arizona Academy of Science, Hydrology Section, Proceedings of April 11-12 meeting, 

Tempe Arizona, v. 47-55.

Resource Consultants & Engineers, Inc. (RCE), 1993. "Surface Water Modeling of 

Reclaimed Parcels at the Black Mesa Complex," prepared for Peabody Western Coal Co.

Sen, P.K., 1968. "Estimates of Regression Coefficient Based on Kendall's Tau," Journal of 

American Statistical Association, v. 63, p. 1379-1389.

Smith, T. and Best, A., 2000. "Assessing Sedimentation and Protection of the Hydrologic

Balance in Disturbed and Reclaimed Areas at the Black Mesa and Kayenta Mines, Arizona,

Using Baseline Sediment Data Collected in Receiving Streams," presentation given at the

Billings Mined Land Reclamation Symposium in 2001, Billings, MT.

Water Engineering & Technology, Inc. (WET), 1990. "Determination of Background 

Sediment Yield and Development of a Methodology for Assessing Alternative Sediment 

Control Technology at Surface Mines in the Semiarid West," prepared for Office of Surface 

Mining and the National Coal Association, Fort Collins, CO.

Zevenbergen, L.W., Peterson, M.R., and Watson, C.C., 1990. "Computer simulation of 

watershed runoff and sedimentation processes," Proceedings of the Billings Symposium; 

Planning, Rehabilitation and Treatment of Disturbed Lands.
3.2
3.3