This is a digital document from the collections of the Wyoming Water Resources Data System (WRDS) Library.
For additional information about this document and the document conversion process, please contact WRDS at [email protected] and include the phrase “Digital Documents” in your subject heading.
To view other documents please visit the WRDS Library online at: http://library.wrds.uwyo.edu
Mailing Address: Water Resources Data System University of Wyoming, Dept 3943 1000 E University Avenue Laramie, WY 82071
Physical Address: Wyoming Hall, Room 249 University of Wyoming Laramie, WY 82071
Phone: (307) 766-6651 Fax: (307) 766-3785
Funding for WRDS and the creation of this electronic document was provided by the Wyoming Water Development Commission (http://wwdc.state.wy.us)
JOHNSON-FERMELIA Co. Inc.
CONSULTING ENGINEERS, ARCHITECTS AND SURVEYORS
LEVEL I1 - FEASIBILITYSTUDY PHASE II REPORT
BITTW CREEK TRIBUTARY FLOOD STUDY ROCK SPRINGS, WYOMlNG
WYOMlNG WATER DEVELOPMENT COMMISSION
15 15 Ninth Street h Associatim With Rock Springs, Wyoming 82901 WESERN WATER CONSUUANE, INC. And Phone (307) 362-7519 WAfTS & ASSOCIATES, INc.
NOVEMBER 1991 1515 NINTH STREET ROCK SPRINGS, WYOMING 82901 PHONE (307) 362-7519
FAX NO. (307) 362-7569 JOHNSON-FERMELIA CO. INC. CONSULTING ENGINEERS, ARCHITECTS AND SURVEYORS October 31, 1991
Wyoming Water Development Commission 4th Floor West 122 West 25th Street Cheyenne, WY 82002
Attention: Mr. Patrick Erger Re: Phase I1 Report - Level I1 Feasibility Study Bitter Creek Tributary Flood Study Dear Mr. Erger:
We are transmitting herewith fifty (50) copies of the above Report and fifty (50) copies of an Executive Summary together with one unbound reproducible original of both the Report and the Executive Summary. We thank you for the assistance that you provided to us during the course of this work. Please call me if you have any questions. a-Sincerely, Wayne d Johnson, and LS Project Manager Enclosures - INDEX PAGE NO.
I INTRODUCTION 1 I1 SUMMARY 4
I11 FLOOD CONTROL FEATURES 6 IV HYDROLOGIC AND HYDRAULIC ANALYSIS V PERMITTING AND LAND ACQUISITION/EASEMENTS VI COSTS VII ECONOMIC ANALYSIS VIII FEDERAL INTEREST IX CONCLUSIONS AND RECOMMENDATIONS
APPENDIX
FIGURES
1. DAMAGE-FREQUENCY RELATIONSHIPS FOR REACH I OF KILLPECKER CREEK 2. DAMAGE-FREQUENCY RELATIONSHIPS FOR REACH I1 OF KILLPECKER CREEK 3. DAMAGE-FREQUENCY RELATIONSHIPS FOR KILLPECKER CREEK TABLES 1. RESULTS OF RESERVOIR ROUTINGS FOR VARIOUS FLOOD EVENTS 2. WHITE MOUNTAIN TRIBUTARIES PROPOSED STORM WATER DETENTION PONDS - HYDROLOGIC CRITERIA FOR DESIGN 3. WHITE MOUNTAIN TRIBUTARIES ELEVATIONS AND DIMENSIONS DEAD HORSE CANYON CREEK LAND ACQUISITION ALONG CREEK THROUGH TOWN DEAD HORSE CANYON CREEK DETENTION RESERVOIR NO. 2 CONSTRUCTION QUANTITIES AND COSTS DEAD HORSE CANYON CREEK DETENTION RESERVOIR NO. 3 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES DETENTION RESERVOIR NO. 4 CONFIGURATION #1 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES DETENTION RESERVOIR NO. 4 CONFIGURATION #2 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES DETENTION RESERVOIR NO. 4 CONFIGURATION #3 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #1 DETENTION RESERVOIR NO. 1 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE if1 DETENTION RESERVOIR NO. 2 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #1 DETENTION RESERVOIR NO. 3 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #1 DETENTION RESERVOIR NO. 4 CONFIGURATION #3 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #2 STORM WATER CHANNEL CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #2 DETENTION RESERVOIR NO. 2 CONSTRUCTION QUANTITIES AND COSTS WHITE MOUNTAIN TRIBUTARIES ALTERNATE #2 DETENTION RESERVOIR NO. 3 CONSTRUCTION QUANTITIES AND COSTS 41 WHITE MOUNTAIN TRIBUTARIES ALTERNATE #2 DETENTION RESERVOIR NO. 4 CONFIGURATION #3 CONSTRUCTION QUANTITIES AND COSTS 42 KILLPECKER CREEK LAND ACQUISITION ALONG CREEK THROUGH TOWN 44 KILLPECKER CREEK DETENTION POND AT SITE NO. 2 CONSTRUCTION QUANTITIES AND COSTS KILLPECKER CREEK DETENTION POND AT SITE NO, 3 CONSTRUCTION QUANTITIES AND COSTS 46 KILLPECKER CREEK REACH I CHANNEL IMPROVEMENTS FOR DETENTION SOLUTION CONSTRUCTION QUANTITIES AND COSTS 47 DAMAGE ESTIMATES FOR THE 1989 DEAD HORSE CANYON FLOOD VEHICLE COUNTS IN THE KILLPECKER CREEK FLOOD PLAIN 24. EXPECTED ANNUAL FLOOD DAMAGE (EAD) COMPUTATION FOR KILLPECKER CREEK WITHOUT PROJECT 67
25. EXPECTED ANNUAL FLOOD DAMAGE (EAD) COMPUTATION FOR KILLPECKER CREEK WITH PROJECT 67
PLATES
DRAWINGS BITTER CREEK TRIBUTARIES STUDY PgASE I1
INTRODUCTION
A. Project Study Area
The Study area includes the following tributaries to Bitter Creek and their watersheds in the Rock Springs Area:
1. White Mountain Tributaries 2. Dead Horse Canyon Creek 3. Killpecker Creek
These tributaries and watersheds are depicted on the accompanying Plate 4.
B. Level I1 - Feasibility Study - Phase 1A Report - Background
A Level I1 Feasibility Study and Phase 1A Report were completed in March 1989 that addressed flooding and possible solutions thereof on the Bitter Creek tributaries referenced above. That study and report were completed pursuant to a contract with the Wyoming Water Development Commission dated June 24, 1988. The work embraced in this Study was authorized by Amendments NO. Three and Four to that same contract. The work enumerated herein has been completed in association with Western Water Consultants, Inc. (WWC) and Watts & Associates, Inc. (WAI). WWC performed hydrologic and hydraulic analyses and WAI economic analyses.
The Phase 1A Report identified a number of possible alternatives for flood control on each of the subject tributaries and provided insight and information on the economic feasibility that ultimately led to the selection of the features described in this Report.
The flood control projects that are described and evaluated in this Phase I1 Report were selected from a number of different alternatives that were studied and presented in the Phase IA Report. The alternatives that are explained herein were selected on the basis of a feasibility analysis that was performed in the Phase IA Study.
The selected alternatives, or project features were toured by personnel from the City of Rock Springs, the City Flood Control Committee and representatives of the Wyoming Water Development Commission. The City recommended moving the detention reservoir near Smith's Food King farther away from Foothill Boulevard at the suggestion of a City Councilman. The site was relocated to a parcel of ground of lower value than property fronting on Foothill Boulevard to reduce project costs.
The group of people that toured the sites referenced above also toured the site of a detention reservoir that was proposed on Bitter Creek East of Rock Springs.
The Phase IA Report also addressed flood control solutions on Sweetwater Creek, another tributary to Bitter Creek. However, private interests completed various flood control work on that creek and it was subsequently deleted from this Phase I1 Study by the Wyoming Water Development Commission.
An additional phase of work (Phase Ib) was considered on the main channel of Bitter Creek; however it was deleted form JFCO. ' s Contract. The Phase Ib study was ultimately initiated by the U.S. Army Corps of Engineers, (C.0.E.). The Co0.E. undertook that portion of the study because Bitter Creek proper would qualify for Federal interest; or Federal funding for flood control works. The City of Rock Springs agreed with the C.0.E. pursuing the investigation on the main channel of Bitter Creek; however, the CoOoEo study was terminated when the construction of a large detention reservoir on Bitter Creek east of Rock Springs was found to be infeasible. Subsequently the City of Rock Springs engaged JFCO. to complete another report addressing flood control improvement on the main stem of Bitter Creek through Rock Springs.
That report, the Level 11, Bitter Creek Channel Improvement Study, was published in September, 1991. The information contained therein has bearing on the objectives and purposes of this Report. An evaluation of the overall flood problems in the Rock Springs area will require assessing the information in that report simultaneously with this Report.
C. Phase I1 Analysis
The work required and provided for in Amendments No. Three and Four to the Consultant's contract specified that the following work be completed for the selected alternat&es on Dead Horse Canyon Creek and the White Mountain Mall Tributaries: 1. Additional Mapping 2. Dambreak Analysis 3. Permitting and Land Acquisition/Easements 4. Economic Analysis
The Amendments specified that the following work be performed for Killpecker Creek: 1. Refinement of Hydrology and Flood Boundaries 2. Economic Analysis 3, Assessment of Federal Involvement
The improved mapping and dambreak analysis resulted in refinement of the design and costs of the flood works on Dead Horse Canyon Creek and White Mountain Tributaries. Also, a more detailed review of permitting easements and land acquisition was completed.
The refined hydrology on Killpecker Creek led to a better definition of flood boundaries and an improved economic analysis that not only defined the benefit cost ratio but allowed for a more accurate appraisal of the possibility of federal involvement in any possible solution to flooding on that tributary.
The following flood control features were evaluated on each of the subject tributaries: A. Dead Horse Canyon Creek 1. Detention Reservoir No. 2* 2, Detention Reservoir NO. 3*
B. White Mountain Tributaries Two Alternatives were examined on the White Mountain Tributaries, which are referred to as Alternates 1 and 2 below. Alternate 1 1. Detention Reservoir NO. I* 2, Detention Reservoir NO. 2* 3. Detention Reservoir NO. 3* 4, Detention Reservoir No, 4*
Alternate 2 1. Storm Water Channel 2, Detention Reservoir NO. 2* 3. Detention Reservoir NO. 3* 4. Detention Reservoir No. 4*
* As identified in Level I1 Feasibility Study - Phase 1A Report
Capital costs, flood control benefits, -and benefit-cost ratios for the project works considered on each of the tributaries are presented in the following Table:
TRIBUTARY PROJECT FLOOD CONTROL BENEFIT- CAPITAt COST BENEFIT COST RATIO 1. Dead Horse $744,637 $3,281,400* 4.41 Canyon Creek 2. White Mountain Mall Tributaries Alternate $1 $1,710,167 $20,900,000* 12.22 Alternate #2 $1,607,600 $20,900,000* 13.00 3. Killpecker $2,583,838** $459,750*** .18 Creek
* Using 3.7 percent discomt rate as required by State of Wyoming.
** From Phase 1A Report. *** Applying federal discount rate of 8.75 percent.
The benefit-cost ratios for Alternates #1 and #2 on the White Mountain Tributaries were not initially developed by using the upgraded mapping that was prepared for the dambreak analysis. Furthermore, the analysis of flood damages was completed using data that included the flooding event in 1989 and maps of a much smaller scale. However, there have been two flooding events in the summer of 1991 in that area. Therefore, additional hydrologic and hydraulic analysis was completed incorporating the use of better maps and information about the flood events of this summer to accurately redefine flood boundaries. These same maps and this information, together with additional data that was obtained from appropriate field reconnaissance and surveying, was used to recalculate flood control benefits and benefit-cost ratios for the White Mountain' Tributaries.
FLOOD CONTROL FEATURES
The following features and work were identified for each of the three tributaries involved in the Study. Each flood control facility was identified initially in the "Level I1 - Feasibilitv Studv - Phase 1A Re~ort"and examined in greater depth and more detail in this effort.
A. Dead Horse Canyon Creek 1. Detention Reservoir NO. 2. 2. Detention Reservoir NO. 3.
Both of these reservoirs would be incorporated into one project on Dead Horse Canyon Creek. The dual reservoir concept is necessary to achieve sufficient storage to reduce the peak flood flows downstream to non-damaging levels. The design and location of both reservoirs is shown on the accompanying drawings.
B. White Mountain Tributaries Alternate 1 1. Detention Reservoir NO. 1 2. Detention Reservoir NO. 2 3. Detention Reservoir NO. 3 4. Detention Reservoir NO. 4
All four reservoirs would be integrated into one project on the White Mountain Tributaries. The concept of constructing four reservoirs was necessary to develop sufficient total storage to reduce flood flows downstream to non-damaging levels. The design aspects and location of the four reservoirs are presented on the accompanying drawings.
Alternate 2 1. Channel Improvements 2. Detention Reservoir No. 2 3. Detention Reservoir No. 3 4. Detention Reservoir No. 4
Item 1 above, Channel Improvements, would be constructed in this Alternate instead of Detention Reservoir No. 1. The channel improvements were not presented in the Phase 1A Report. They were evaluated in this effort and are thought to be a suitable and practical approach to help solve the flood problems in the White Mountain area. These channel improvements, together with the three detention reservoirs, would be incorporated into one project on the White Mountain Tributaries under this Alternate. The design features of the channel improvements and reservoirs and their location are shown on the accompanying drawings.
C. Killpecker Creek 1. DamsiteNo. 2 2. DamsiteNo. 3 3. Channel Excavation
Both damsites (detention reservoirs) and the channel excavation are described in the Phase 1A Report. The detention reservoirs and channel excavation work would be included in one project on Killpecker Creek. The project would require two detention reservoirs in order to create sufficient storage to reduce peak flood flows downstream to non-damaging levels. The excavation of a reach of the stream channel is necessary to increase channel capacity to carry the non-damaging flow throughout the entire length of the Killpecker Creek channel in the developed area of Rock Springs. The design features and location of the detention reservoirs and the extent of the channel excavation are shown on Plates 1, 2, and 3, bound in the back of this Report, which are taken directly from the Phase 1A Report. IV HYDROLOGIC AND HYDRAULIC ANAIJYSTS
A. Introduction
A dambreak analysis was completed for the detention reservoirs on Dead Horse Canyon Creek and White Mountain Tributaries to assess the optimum design criteria and establish flood boundaries for design flows and dambreak or overtopping conditions. The 114, 112, 314 and Probable Maximum Floods were analyzed to assess the consequences of flooding conditions, spillway capacity, and property acquisition in concert with applicable standards as promulgated by the State Engineer's Office. These analyses established design concepts, property requirements and their associated costs that are presented later in this Report.
A somewhat different approach was taken on Killpecker Creek where a refinement in the hydrology was completed to better establish flood boundaries to determine flood control benefits and complete an accurate economic analysis to evaluate the possibility for federal involvement in a flood control project on Killpecker Creek. The economic analysis is presented later in this Report.
B. Dambreak Analysis and Spillway Sizing Methodology
This section presents the results of the dambreak analysis performed for proposed stormwater detention ponds located on Dead Horse Canyon Creek and in the White Mountain drainage basin. This analysis was undertaken to determine spillway sizes for the proposed reservoirs. These activities were coordinated with the Dam Safety Division of the Wyoming State Engineer's Office.
Dambreak analysis studies for the proposed detention
8 ponds involved development the 114, 112, 314, and full Probable Maximum Flood (PMF). All floods were developed using rainfall-runoff techniques. Following flood hydrograph synthesis, reservoir routing and dambreak modeling were performedto evaluate the downstream effects of an overtopping induced failure for the embankments. The resulting flood hydrographs were then routed downstream and an incremental damage analysis was performed to determine the required spillway capacity.
Flood Hydrograph Development
Unit Hvdroara~h All flood hydrographs were developed using unit hydrograph techniques. The unit hydrograph represents 1 inch of direct runoff from a rainfall of a defined unit duration. Unit hydrographs for the drainage basins were developed using the average Arizona-Colorado-Wyoming S-curve produced by the U.S. Army Corp of Engineers (C.O.E.) (1973 and 1984).
)Basin Parameters Unit hydrograph analysis requires the use of drainage basin parameters. All geomorphic parameters were obtained from USGS 7.5 minute quadrangle maps of the basin. Loss rate parameters were those used by the C.0.E. in their hydrologic analysis of these watersheds and are: Initial LOSS = 0.5 inches Constant Loss Rate = -25 inches per hour
Probable Maximum Preci~itation (PMP) The estimated time of concentration for the basins were less than one hour, so the one hour PMP event was modeled when developing the inflow floods. Sensitivity analysis examining the 1, 6 and 24 hour PMF events confirmed that the one hour PMP produced the highest peak discharges for the given drainage basins. The PMP values were determined from Hydrometeorological Report No. 49 (National Weather Service, 1984) . The values, adjusted for elevation and areal reduction factors, are presented below:
Basin 1 hour PMP Dead Horse Canyon Creek 6.7 inches White Mountain Tribs. 7.1 inches
These one hour precipitation amounts were then distributed usingthe United States Bureau of Reclamation One- Hour Thunderstorm, Zone I1 precipitation distribution (1977).
Probable Maximum Flood (PMF) Development of the PMF was accomplished using the c.0.E.'~ HEC-1 computer model. Input information included basin parameter data, unit hydrograph ordinants, loss rates, and precipitation amounts and distribution. The 114, 112, and 314 PMF values were derived as fractions of the full PMF.
Routing Studies
Reservoir Routinq Reservoir elevation-capacity-discharge data were developed from topographic maps of the proposed reservoir locations. These maps portrayed 2 foot contour intervals and were at a 1 inch to 100 foot scale. Outlet works discharges were limited to the non-damage discharges specified in the Phase Ia report. Reservoir routings were performed using the Modified Puls technique.
Channel Routinq The hydrographs created by HEC-1 were routed downstream using the Muskingum hydrologic routing technique. It is recognized that routing of a dambreak flood wave may be more accurately performed using unsteady flow equations. However, the solutions to these equations are typically very unstable, especially when there are dramatic changes in the channel cross-section characteristics such as encountered in the downstream reaches for the proposed dams in this study. The programs available for the unsteady flow solutions also tend to be much less flexible for defining cross-section locations and characteristics, thereby reducing their sensitivity to changes in channel shape and slope. Thus, use of hydrologic routing techniques was deemed reasonable for the dambreak routing requirements of this report.
Dambreak Analysis Analysis of an overtopping-induced breach of the proposed detention dams were performed to evaluate the risk of severe damage or loss of life in the downstream reaches. The dambreak hydrographs were developed using the c.0.E.'~ HEC-1 Flood Hydrograph Package.
Breach Parameters Selection of parameters for describing in advance the characteristics of an earthen dam breach is recognized as an imprecise art. This is primarily due to the many factors potentially influencing the speed and size of a dambreak. Therefore a dambreak modeling study requires selecting reasonable values from tables or nomographs prepared from the study of past failures. Such procedures are discussed by MacDonald and Langridge-Monopolis (1984).
Crest Elevation The dambreak analysis was performed on a five minute time step interval to increase breach hydrograph and routing accuracies. The model assumed an empty reservoir at the beginning of the precipitation event. This is a reasonable assumption as the proposed ponds are to have uncontrolled outlet works and will be empty except during flood events. The dam crest was set 5 feet higher than the elevation required to retain the 100-year flood with no spillway. This scenario was chosen because it represents the most conservative case. The absence of a spillway will result in less attenuation of the PMF event and as a result a larger flow will occur from PMF plus dambreak discharge.
Incremental Damaae Assessment Methodolow The purpose of the incremental damage assessment is to determine if the difference between the PMF flood event plus the associated dambreak (PMF+Dambreak) discharge is significantly larger than the naturally occurring PMF event alone, without the proposed dam. The incremental damage assessment was performed in accordance with tlDownstreamHazard Classification ~uidelines" (Bureau of Reclamation, 1988). This document presents figures relating the incremental differences in depth and velocities with Hazard Zone classifications such as curves for Houses built on foundation, Mobile Homes, Automobiles, Adults, and Children. For the purposes of this study it was assumed that the curves for Adults and Children would not apply. It is reasonable to assume people would seek shelter in their home or automobile during a rainstorm as severe as the PMP events. Incremental differences in flow depths and average channel velocities were applied to the remaining appropriate curves where the modeled PMF event resulted in an inundation of residential structures or roadways. Flood levels were computed using the HEC-2 computer model and mapping was accomplished using contour maps dated September 23, 1988.
Dambreak Analysis and Spillway Sising Results
Routed hydrograph peaks were quantified at several locations below the damsites. Attenuation of the floods is indicated by the reduction in peak flows in the downstream direction. In some cases, because of the small reservoir sizes, the initial incremental damage assessment was conducted for the 1/2 PMF event. If no incremental damages existed the 114 PMF event was then analyzed.
C. Dead Horse Canyon Creek
Updated topographic maps at a scale of 1" = 100 feet and a contour interval of 2 feet were completed and used in the hydrologic/hydraulic analysis for Dead Horse Canyon Creek.
An incremental dambreak and spillway sizing analysis was completed for the detention ponds on Dead Horse Canyon Creek. These analyses demonstrated that no incremental damages would result from the 112 and 114 Probable Maximum Floods (PMF); therefore, the dams were designed for the 500-year flood.
The non-damaging discharge was also calculated for the creek below the proposed detention ponds as part of the analysis.
The Level I1 - Phase 1A Report suggested a non-damage discharge of 1,100 cf s. This flow was governed by the channel capacity at the existing UPRR bridge. However, the non-damage flow was increased to 1,300 cfs in light of the UPRR committal to remove the bridge where flow was restricted to 1,100 cfs. Won-damaging channel capacities below the bridge are in excess of 1,400 cfs.
Available storage versus required fill of each of the two proposed damsites was estimated. Damsite #3 was found to be much more efficient and economical than Damsite #2 considering storage potential. This resulted in a reassessment of the allowable design discharge for the proposed detention ponds.
The revised discharges for the deign event (COEgs3-hr; 100-year event) are as follows: petention Pond Outlet Pi~eSize Design Discharae #2 72" CMP 540 cfs %3 36" CNP 125 cfs
Routing studies were performed that included the effects of runoff from areas below the proposed damsites for the design event (COEts 3-hr; 100-year event). The following results were obtained from these routing studies:
DISCHARGE - DESIGN EVENT (COE8s 3-hr: 100-year event)
Location Discharge
Above Pond #2 1,147 cis Below Pond #2 538 cfs Above Pond #3 627 cfs Below Pond #3 125 cis
@ I.P.5 (Confluence of Dead Horse Canyon Creek and Trib. #2) 1,270 cfs
@ 1/2 Way (Between I .P. 5 and I.P.11, approx. UPRR Bridge) 1,302 cfs 1.P.ll (Confluence of Dead Horse Canyon Creek and Bitter Creek) 1,394.cfs
A variety of flood events were routed through the proposed detention ponds using these revised outlet capacities. The results are presented in the following table: TABLE 1
RESULTS OF RESERVOIR ROUTING8 FOR VARIOUS FLOOD EVENTS Event Parameter I Pond #2 I Pond #3 3-hr 100-vr Event: (COE precip.l.89 in.) Flood Peak Inflow (cfs) Flood Volume (af) I 129 I 68 Reqtd Storage Volume (af) Reqld Water Surface Elev. (ft) Maximum Discharge (cfs) 538 125 Outlet Pipe Diameter (inch, CMP) I 72 1 36 1-hr 500-vr Event: (NOAA precip.l.42 in.) Flood Peak Inflow (cfs) * Flood Volume (af) 120 I 64 I I Reqtd Storage Volume (af) Reqld Water Surface Elev. (ft) Maximum Discharge (cfs) 549 124 Outlet Pipe Diameter (inch, CMP) I 72 1 124 3-hr 500-vr Event: (COE precip. 2.95 in.) Flood Peak Inflow (cfs) 2275 1244 Flood Volume (af) 277 14 6 Reqtd Storage Volume (af)" 143 110 Reqtd Water Surface Elev. (ft) 6452.87 6443.67 Maximum Discharge (cfs) 632 155 Outlet Pipe Diameter (inch, CMP) 72 36 1-hr 114 PMF Event: (HMR 49 PMP= 6.7 in.) Flood Peak Inflow (cfs) Flood Volume (af) Req'd Storage Volume (af) 119 90 Reqld Water Surface Elev. (ft) 6451.24 6441.92 Maximum Discharge (cfs) 616 148 Outlet Pipe Diameter (inch, CMP) 72 36 Recommended Desicm: Recommended Crest Elevation (ft) Elevation of Dam Base (ft) Maximum Dam Height (ft) An objective of the Study was to satisfy FEMA requirements that, together with a review of the above data, led to the conclusion that the ponds should be designed to contain the COEgs 3-hr; 500-year event. The volume of this event is larger than the volume of the 1-hr, 1/4 PMF, which is suggested by the State Engineer's Office, considering dam safety and incremental damages. The proposal was therefore put forth that runoff from the 500-year event be contained or stored by increasing the crest elevation of the proposed plan instead of designing the spillway to pass the same event. Discussions with Mr. Dave Benner and Mr. Russ Dahlgren of the Wyoming State Engineer's Office indicated that no freeboard would be required if flood protection was developed for 1/4 PMF. Setting the dam crest elevation at the COE's 3-hr; 500- year event high water line is therefore proposed. No separate spillway structures will be needed under this design basis. The cost of raising the dam crest is less than building a spillway to pass the 500-year event or constructing a roller- compacted concrete (RCC) dam to accommodate overtopping to pass the PMFo
Embankment Slope Compliance with the State Engineer's guidelines are proposed. These guidelines call for a 3:l slope on the upstream face and 2: 1 slope on the downstream face of the dam. Both faces should be vegetated to grass; however, some concern exists about erosion and the ability to establish vegetative covers on the 2: 1 downstream face of the embankment. This should be checked out with the Wyoming Transportation Department to see what problems they may have encountered on similar slopes and what solutions they implemented. Possible alternatives are:
o Make inspection and possible repair of eroded areas an annual maintenance item.
.II Use an erosion fabric or mat to help establish vegetation. - Increase the downstream face slope to 3:l. Ri~rap No riprap will be required on the dam itself. A small amount of erosion protection and possibly a plunge pool will be required below the outlet works.
Outlet Works The concern over plugging of the outlet works should be addressed. The sizes of the culverts typically required should eliminate the possibility of sediment plugging them up. The sizes of the culvert would also make manual cleaning possible if it is required. Regular inspection would be a prudent recommendation. Installation of a trash rack and emergency riser section on the outlet works is suggested. As far as the possibility of the outlet works becoming plugged by car bodies, etc. washed down during a flood event, these improvements, in combination with annual inspection, would prevent the outlet works from becoming plugged.
D. White Mountain Tributaries
Updated topographic maps were also completed for this tributary at a scale of 1' = 100 feet and a contour interval of 2 feet. These maps were then used in the hydrologic/ hydraulic evaluating process on the White Mountain Tributaries.
The incremental dambreak and spillway sizing effort indicates that incremental damages do exist for some of the proposed detention ponds. The possibility of replacing detention pond NO. 1 with a pipeline was also explored.
Pond design was considered to hold the COE's 3-hr; 500- year event to satisfy FEMA requirements, even though this event is larger than the 1/4 PMF event and is not required fromthe State Engineer's Office dam safety incremental damage point of view. Storage of the 500-year event by increasing the crest elevation of the proposed dams is recommended in some cases instead of developing spillway capacity to pass the same event. No freeboard is required if protection is provided for the 1/4 PMF as determined from conversations with Mr. Dave Benner and Russ Dahlgren of the Wyoming State Engineer's Office.
Setting the dam crest elevation sc that reservoir capacity is developed to contain the COE8s 3-hr; 500-year flood is therefore proposed. A separate spillway structure is not required by designing on this basis. The cost of constructing the dams higher in some instances should be more economical than building a spillway to pass the 500-year event or building a RCC dam to pass the PMF by overtopping.
Pond #l Redeeign/Replace with Pipeline
Analysis of floods expected to occur at the Pond #1 location show that the COE8s 3-hour, 100-year flood peak would be 400 cfs. This assumes that the proposed detention Ponds 13 and 14 which are upstream of Pond #1 are in operation. When the area between Pond #1 and Ponds #3 and #4 (essentially the area between Foothill Boulevard and Sweetwater Drive) is developed the peak discharge can be expected to rise to approximately 500 cfs. The ditch below Dewar Drive was estimated to have a capacity in excess of 600 cfs in the Phase IA report and after field measurement it was determined to have a bank-full capacity of approximately 900 cfs. This is more than sufficient to handle the 100-year flows into the ditch, including a flow of 500 cfs from the Pond fl area. The existing storm drain pipes from north of Foothill Boulevard to the outfall ditch below Dewar Drive have a capacity of about 100 cfs.
The proposed Pond #1 would have controlled the inflow to the storm sewer such that the 100 cfs would not have been exceeded. This existing storm water system consists of a 60- inch diameter pipeline from the ditch to the intersection of Foothill Boulevard and Dewar Drive at which point it divides into two 53" x 34" Elliptical Concrete Pipe (ECP) culverts, one extending up Dewar Drive and the other along Foothill Boulevard. The total capacity of the pipeline system is approximately 140 cfs. The configuration of the inlets restricts the capacity of this system to about 100 cfs. However the nuisance cost of a large detention pond amidst sucht a highly developed area, and the estimated land acquisition costs suggested that another solution might be more preferable. An alternate to the pond would be to enlarge the storm sewer pipelines. Analysis indicated that the 100-year flood event could be handled by constructing a new storm sewer from Foothill to the ditch below Dewar Drive. This pipe should have an open area of approximately 40 square feet to handle the existing 100-yr event or 55 square feet for the future/developed 100-yr event. The 53" x 34" ECP along Dewar Drive will also be necessary. The ECP along Foothill may be abandoned, if desired.
Pond #1 Spillway Si~ing/IncrementalDambreak Analysis
The original plan called for construction of the proposed detention Pond #I. The results of incremental damage analysis of the proposed detention pond showed that no incremental damages would exist below the pond for the Full and 112 PMF events. As a result of reservoir routing studies it was determined that the COE8s 3-hour, 500-year event produced a higher water level in the Pond #1 than the 1-hour, 114 PMF event. Therefore, the recommended design for Pond #1 calls for the crest elevation of the dam to be set to store the COE1s 500-year event. The results of the flood routing analysis are presented in Table 2. Recommended design criteria are presented in Table 3.
Pond #2 Spillway Siaing/Incremental Dambreak Analysis
Incremental damage analysis showed that no damages existed for this proposed detention pond for the Full and 112 PMF event. It was therefore decided to store or pass the COE'S 3-hour, 500-year event or the 1-hour, 114 PMF event, whichever proved to be more critical. Reservoir routing studies showed the COE1s 500-year event to be more critical when evaluating the option to provide protection by storage, and the 114 PMF to produce larger discharges for use in the design of a suitable spillway fer this pond. Analysis of the storage available at this damsite showed that it would be very costly and impractical to develop required storage volumes. A spillway is therefore recommended sized to safely pass the 1/4 PMF event. The results of the flood routing analysis are presented in Table 2. Recommended design criteria are presented in Table 3.
Pond #3 Spillway Sising/Incremental Dambreak Analysis
Incremental damage analysis showed that no damages existed for this proposed detention pond for the Full PMF event. Incremental damages did exist at the 314 and 112 PMF event levels. This is due to flooding which would occur at the County Library which is located just downstream of Pond #3, above Sweetwater Drive. Resulting Flood Levels are:
Water Surface Elev. EstimatedFloodDe~th PMF event plus Dambreak 6,304.6 1.6 ft. PMF alone 6,302.9 0.0 ft. 314 PMF plus Dambreak 6,304.0 1.0 ft. 112 PMF plus Dambreak 6,302.9 0.0 ft. Ground elevation @ Library 6,302.0 Estimated first floor Elevation 6,303.0
These flood depth are not life threatening but could cause damage to the contents of this facility, especially if a basement exists. However, as this is a public facility it is felt that the liability assumed by the City of Rock Springs does not warrant the expense of a dam capable of handling a PMF event. As a result of reservoir routing studies it was determined that the COE1s 3-hour, 500-year event produced higher water level in the Pond $3 than the l-hour, 114 PMF event. Therefore the recommended design for Pond #3 calls for the crest elevation of the dam to be set such that the reservoir would store the COE1s 500-year event. The results of the flood routing analysis are presented in Table 2. Recommended design configurations are presented in Table 3.
Pond #4 Bpillway Sising/Incremental Dambreak Analysis
Incremental damage analysis for the proposed detention Pond #4 were conducted. This pond did exhibit incremental damages at the full PMF level. For this event water surface elevations due to the PMF plus dambreak are about six inches deep through a housing development on the south side of the channel, above Sweetwater Drive. Flows for the PMF event alone are contained within the existing channel causing no damages.
Thus, while there would be incremental damages, they certainly are not life threatening and damages would be of limited economic concern primarily to basements.
For this proposed detention pond three options for design were developed. These include:
- Design the dam to store the 1/4 PMF with the recognition that there might be some damages to private property in the event of a PMF event. - Design the dam to retain the full PMF
- Design a dam capable of being overtopped which will have a spillway capacity to pass the PMF event.
The selection of the preferred option is dependant on the estimated construction costs and the concerns of the City of Rock Springs and possibly the local homeowners. Hydrologic Design Criteria
The following table presents the proposed hydrologic design criteria lor the White Mountain Tributaries detention ponds : TABLE 2 WHITE MOUNTAIN TRIBUTARIES PROPOSED STORM WATER DETENTION PONDS HYRROLOGIC CRITERIA FOR DESIGN
3-hr 100-vr Event: 1 Precipitation ( inches) 1.90 1.90 1.90 1.90 Flood Peak Inflow (cfs) 396 270 643 458 Flood Volume (af) 79 61 56 37 Req'd Storage Volume (af) 28 28 47 29 Req'd Water Surface Elev.(ft) 6249.81 6320.53 6333.99 6352.95 Maximum Discharge (cfs) 100 80 50 50 Outlet Pipe Dia. (inches CMP) 36 33 24 24
3-hr 500-vr Event:
Precipitation (inches) 2.97 2.97 2.97 2.97 I Flood Peak Inflow (cfs) 755 830 1235 870 Flood Volume (af) 122 130 70 80 Req'd Storage Volume (af) 57 36 105 69 Req'd Water Surface Elev.(ft) 6253.18 6322.63 6333.33 6357.36 Max. Spillway Discharge (cfs) N.A. 689 N.A. N.A.
1-hr 114 PMF Event:
PMP (inches) 7.10 7.10 7.10 7.10 Flood Peak Inflow (cfs) 937 1135 1509 1097 Flood Volume (af) 96 112 102 68 Req'd Storage Volume (af) 49 38 93 59 Req'd Water Surface Elev.(ft) 6252.21 6322.94 6337.68 6356.51 Max. Spillway Discharge (cfs) N.A. 792 N.A. N.A. Elevations and Dimensions
The following table presents the elevation, dam height, and outlet pipe diameter that are required to satisfy the design criteria given in the foregoing table:
TABLE 3 WHITE MOUNTAIN TRIBUTARIES $LEVATIONS AND DIMENSIONS
1 Alto 6256 6242 14 36" CMP Alternative dam alignment. 2 6324.25 6306 18.25 33" CMP Provide spillway for 114 PMF (Spillway Crest Elev. 6320.5) . 3 6340 6314 26 24" CMP Store COEQs 3-hr 500 yr event. 4 (Alt 1) 6366.4 6330 36.4 24" CMP Store full PMF. 4 (Alt 2) 6353 6330 23 24" CMP Store COEQs 3-hr 100-yr event only with RCC dam. 4 (Alt 3) 6358 6330 28 24" CMP Store COEQs 3-hr 500-yr event. +).
Killpecker Creek
Hydraulic modelling of Killpecker Creek was performed for the 100, 500, loo-, and 500-year events using the COE HEC-2 computer model. This modelling was done to refine backwater surface profiles for pre- and post-flood control project construction that is presented in the Level 11, Phase 1A Report. The project would include two detention ponds in the Killpecker Creek watershed and limited improvements to the creek channel. The channel improvements consist of excavating the channel to increase its capacity through a portion of length, as shown on Plate 2. The detention ponds are depicted on Plate.,1.
Backwater surface profiles were used to refine and accurately determine flood water depths at individual buildings along the creek that are subject to flooding. Building elevations were also surveyed. Water depths were then compared to building elevations to define and quantify flood damages under existing conditions.
The proposed flood control works would prevent flood damages that would result from the 100-year event and reduce flooding from a 500-year event. The anticipated flow depths at each building along the creek, with and without the proposed improvements in place, were used in the economic analysis section of this Report.
PERMITTING AND LAND ACOUISITION EASEMEN!CS
This section describes the necessary permits and land acquisition requirements for projects on Dead Horse Canyon Creek and White Mountain Tributaries. Permitting and land acquisition for the Killpecker Creek Project are not required to be addressed pursuant to the Contract between WWDC and JFCo.
Topographic mapping at a scale of 1 inch = 100 feet and contour intervals of two feet were developed for Dead Horse Canyon Creek and White Mountain Tributaries. This mapping was used to identify property requirements and land ownership.
A. Dead Horse Canyon Creek
1. ~ermittinq Permits to store water in the detention reservoirs will have to be filed with the Wyoming State Engineer's Office. Sweetwater County planning and Zoning Commission should also be provided with location maps and construction plans for these reservoirs. Detention Reservoir No. 2 is on BLM land.
2. Land Acquisition
Construction on the detention reservoirs will require the procurement of the following land areas:
Along Creek through Town...... 16 acres Detention Reservoir No. 2...... 25 acres Detention Reservoir No. 3...... 21 acres TOTAL...... 62 acres
Three landowners would be involved in the land acquisition associated with this project.
B. White Mountain Tributaries
1. Permittinq
Two alternatives have been identified in the project area . The first consists of the construction of four detention reservoirs; the second alternative consists of channel improvements and three detention reservoirs. Water right permits will have to be filed with the Wyoming State Engineer's Office for the four reservoirs if Alternate No. 1 is selected and for three reservoirs if Alternate No. 2 is selected. Location maps and construction plans would also be submitted to Sweetwater County and the City of Rock Springs.
2. Property Acauisition
The following land areas will have to be procured for a flood control project on White Mountain Tributaries: Alternate #1 Detention Reservoir No. 1...... 19 acres Detention Reservoir No. 2...... 14 acres Detention Reservoir No. 3...... 26 acres Detention Reservoir No. 4...... 30 acres TOTAL...... 89 acres
Alternate #2 Storm Water Channel...... Easement Detention Reservoir No. 2...... 14 acres Detention Reservoir No. 3...... 26 acres Detention Reservoir No. 4...... 30 acres TOTAL...... 70 acres
Fourteen landowners would be involved with property procurement for Alternate #1 and twelve for Alternate 82.
COSTS
A. Dead Horse Canyon Creek
The following tables present the estimated quantities, unit costs, and total costs for the Dead Horse Canyon flood control works.
TABLE 4 DEAD HORSE CANYON CREEK JdWD ACQUISITION ALONG CREEK THROUGH TOWN
DESCRIPTION QUANTITY COST/UNIT TOTALS b
LAND ACQUISITION 16 Acres 5,000.00 80,000.00 SUBTOTAL $80,000.00 I TABLE 5 DEAD HORSE CANYON CREEK
DETENTION RESERVOIR NO. 2 CONBTRUCTION OUANTITIES AND COSTS
50,500 C.Y.
ACCESSORIES 200 L.F. TABLE 6 DEAD HORSE CANYON CREEK DETENTION RESERVOIR NO. 3 CONSTRUCTION QUANTITIES AND COSTS
EARTHWORK 31,886.74 C.Y. 4.00 127,546.96 in Place PIPE 36" CMP 180 L.F. 80.00 14,400.00 ACCESSORIES 180 L.F. 28.00 5,040.00 (35%of Pipe Cost) STILLING BASIN Lump Sum 3,500.00 3,500.00 6" GRAVEL CAP 10,350 S.F. 0.34 3,519.00 ACCESS ROAD Lump Sum 5,000.00 5,000.00 SEEDING 15 Acres 1,500.00 22,500.00
SUBTOTAL 192,005.96
TOTAL PROJECT COST - DEAD HORSE CANYON CREEK
TOTAL CONSTRUCTION COST $ 588,64Sm96
PLUS 10%ENGINEERING
SUBTOTAL $ 647,510.56
PLUS 15%CONTINGENCY
TOTAL PROJECT CAPITAL COST $ 7441637m14 ------B. Whits Mountain Tributaries
The Hydrologic and Hydraulic Analysis section of this Report addresses three different concepts for Detention Reservoir No. 4. These are briefly described as follows:
Confiauration #1 A dam with sufficient capacity to contain the PMF.
Confiauration 52 A dam to also contain the 100-year flood.
Confiauration 13 A dam to contain the 3-hr, 500-year event with some resulting damage in the event of a PMF dambreak.
The cost of these three different configurations are presented in the following tables: TABLE 7 WHITE MOUNTAIN TRIBUTARIES DETENTION RESERVOIR NO. 4 -'CONFIGURATION #1 CONSTRUCTION OUANTITIES AND COSTS
EARTHWORK 80,000 C.Y. 4 .00 320,000.00 in Place PIPE 24" CMP 190 L.F. 50.00 9,500.00 ACCESSORIES 190 L.F. 17.50 3,325.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 3500.00 3,500.00 6" GRAVEL CAP 21,400 Sol?. 0.34 7,276.00 ACCESS ROAD Lump Sum 10,000.00 10,000.00 SEEDING 45 Acres 1,500.00 67,500.00
721,101.00 L TOTAL TABLE 8
WHITE MOUNTAIN TRIBUTARIES
DETENTION RESERVOIR NO. 4 - CONFIGURATION #2 CONSTRUCTION QUANTITIES AND COSTS
EARTHWORK 4,500 C.Y. 4.00 18,000.00 in Place PIPE 24'' CMP 50 L.F. 50.00 2,500.00 ACCESSORIES 50 L.F. 17.50 875.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 400.00 400.00 ROLLER COMPACTED 8,750 S.F. 50.00 437,500.00 CONCRETE ACCESS ROAD Lump Sum 16,000.00 16,000.00 SEEDING 5 Acres 1,500.00 7,500.00
TOTAL 622,775.00 r TABLE 9
WHITE MOUNTAIN TRIBUTARIES
DETENTION RESERVOIR NO. 4 - CONFIGURATION #3 CONSTRUCTIOW QUANTITIES AND COSTS
EARTHWORK 23,768 C.Y. 4.00 95,072.00 in Place PIPE 24" CMP 170 L.F. 50.00 8,500.00 ACCESSORIES 170 L.F. 17.50 2,975.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 400.00 400.00 6" GRAVEL CAP 12,240 S.F. 0.34 4,161.60 ACCESS ROAD Lump Sum 14,000.00 14,000.00 SEEDING 20 Acres 1,500.00 30,000.00
TOTAL 305,108.60 P Detention Reservoir No. 4, constructed in conformance with Alternate Configuration #3, is clearly the least axpensive and is deemed as the most feasible design to bet- included in the overall project on White Mountain Tributaries.
The following tables present the total project cost for the White Mountain Tributaries:
TABLE 10 WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #1
DETENTION RESERVOIR NO. 1 CONSTRUCTION OUANTITIES AND COSTS
EARTHWORK 15,544 C.Y. 4.00 62,176.00 in Place PIPE 36n CMP 80 L.F. 80.00 6,400.00 ACCESSORIES 80 L.F. 28.00 2,240.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 800.00 800.00 6" GRAVEL CAP 11,350 S.F. 0.34 3,859.00 ACCESS ROAD Lump Sum 1,000.00 1,000.00 SEEDING 15 Acres 1,500.00 22,500.00
SUBTOTAL 478,975.00 TABLE 11
WHITE MOmJTAIN TRIBUTARIES - ALTERNATE #1
DETENTION RESERVOIR NO. 2
CONSTRUCTION QUANTITIES AND COSTS
EARTHWORK 14,000 C.Y. 4.00 56,000.00 in Place PIPE 33" CMP 115 L.F. 70.00 8,050.00 ACCESSORIES 115 L.F. 24.50 2,817.50 (35% of Pipe Cost) OVERFLOW CHANNEL Lump Sum 10,000.00 10,000.00 SPILLWAY STILLING BASIN Lump Sum 10,500.00 10,500.00 6" GRAVEL CAP - 9,000 S.F. 0.34 3,060.00 -TOP OF DIKE AND ACCESS ROAD ACCESS ROAD Lump Sum 4,000.00 4,000.00 SEEDING 10 Acres 1,500.00 15,000.00
SUBTOTAL 179,427.50 TABLE 12
WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #l
DETENTION RESERVOIR NO. 3 *
EARTHWORK 47,630 C.Y. 4.00 190,520.00 : in Place PIPE 33" CMP 180 L.F. 70.00 12,600.00 ACCESSORIES 180 L.F. 24.50 4,410.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 600.00 600.00 6" GRAVEL CAP - 15,500 S.F. 0.34 5,270.00 TOP OF DIKE AND ACCESS ROAD ACCESS ROAD Lump Sum 15,000.00 15,000.00 SEEDING 20Acres 1,500.00 30,000.00
SUBTOTAL 388,400.00 TABLE 13
WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #l
DETENTION RESERVOIR NO. 4 - CONFIGURATION #3 CONSTRUCTION OWANTITIES AND COSTS
9 EARTHWORK 23,768 C.Y. 4.00 95,072.00 in Place PIPE 36" CMP 170 L.F. 50.00 8,500.00 ACCESSORIES 170 L.F. 17.50 3,975.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 400.00 400.00 6" GRAVEL CAP 12,240 S.F. 0.34 4,161.00 ACCESS ROAD Lump Sum 14,000.00 14,000.00 SEEDING 20 Acres 1,500.00 30,000.00
SUBTOTAL 305,108.60 r TOTAL PROJECT COST
WHITE MOVNTAIN TRIBUTARIES - ALTERNATE #l
DETENTION RESERVOIR
DETENTIONRESERVOIRNO. 2 ...... 179,427.50
DETENTIONRESERVOIRNO. 3 ...... 388,400.00
DETENTION RESERVOIR NO. 4 ALTERNATE CONFIGURATION #3 ...... 305,108.60
TOTAL CONSTRUCTION COST oooooooooooooooo $1,351,911o10
PLUS 10%ENGINEERING ...... 135,191.11
SUBTOTAL
PLUS 15%CONTINGENCY ...... 223,065.33
TOTAL PROJECT CAPITAL COST 000000000000 $1,710,167.54 ALTERNATE #l ------TABLE 14
WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #2 STORM WATER CHANNEL (400 cfs) CONSTRUCTION QUANTITIES AND COSTS
Easements Lump Sum 2,000.00 2,000.00 Existing Asphalt 170 SOY. 3.70 629.00 Concrete Roadways 90 COY. 195.00 17,550.00 Curb and Gutter 40 L.F. 2.98 119.20 CONSTRUCTION Excavation 2,900 COY. 5,800.00 2.00 Haul 2,900 C.Y. 3.00 8,700.00 102" Concrete Pipe 1,100 L.F. 300.00 330,000.00 Arch Replace Concrete 270 SOY. 30.00 8,100.00 Pavement Replace Curbing 40 L.F. 10.90 436.00 Replace Asphalt 170 SOY. 8.00 1,360.00 - Ditch Improvements 5,050 C.Y. 4.00 20,200.00 Outlet Works Lump Sum 3,000.00 3,000.00
SUBTOTAL 397,894.20 WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #2
DETENTION RESERVOIR NO. 2
CONSTRUCTION QUANTITIES AND COSTS
EARTHWORK 14,000 C.Y. 4.00 56,000.00
PIPE 33" CMP 115 L.F. 70.00 8,050.00 ACCESSORIES 115 L.F. 24.50 2,817.50 (35% of Pipe Cost) OVERFLOW CHANNEL Lump Sum 10,000.00 10,000.00 SPILLWAY STILLING BASIN Lump Sum 10,500.00 10,500.00 6" GRAVEL CAP - 9,000 S.F. 0.34 3,060.00 TOP OF DIKE AND ACCESS ROAD ACCESS ROAD Lump Sum 4,000.00 4,000.00 SEEDING 10 Acres 1,500.00 15,000.00
SUBTOTAL 179,427.50 WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #2 DETENTXON RESERVOIR NO. 3 CONSTRUCTION OUANTITIEB AND COSTS
I EARTHWORK 47,630 C.Y. 4.00 190,520.00 in Place PIPE 33n CMP 180 L.F. 70.00 12,600.00 ACCESSORIES 180 L.F. 24-50 4,410.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 600.00 600.00 6" GRAVEL CAP - 15,500 S.F. 0.34 5,27Oo00 TOP OF DIKE AND ACCESS ROAD ACCESS ROAD Lump Sum 15,000.00 15,000.00 SEEDING 20 Acres 1,500.00 30,000.00
SUBTOTAL 388,400.00 TABLE 17
WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #2
DETENTION RESERVOIR NO. 4 - CONFIGURATION #3
CONSTRUCTION QUANTITIES AND COSTS
DESCRIPTION QUANTXTY COST/UNIT TOTALS
LAND ACQUISITION 30 Acres 5,000.00 150,000.00 EARTHWORK 23,768 C.Y. 4.00 95,072.00 in Place PIPE 36@@CMP 170 L.F. 50.00 8,500,OO ACCESSORIES 170 L.F. 17.50 2,975.00 (35% of Pipe Cost) STILLING BASIN Lump Sum 400.00 400.00 6" GRAVEL CAP 12,240 S.F. 0.34 4,161.00 ACCESS ROAD Lump Sum 14,000.00 14,000.00 SEEDING 20 Acres 1,500.00 30,000.00
SUBTOTAL 305,108-60 TOTAL PROJECT COST
WHITE MOUNTAIN TRIBUTARIES - ALTERNATE #2
STORM WATER CHANNEL ...... $ 397,894.20
DETENTION RESERVOIR NO. 2 ...... 179,427.50
DETENTION RESERVOIR NO. 3 ...... 388,400.00
DETENTION RESERVOIR NO. 4 ALTERNATE CONFIGURATION #3 ...... 305,108.60
TOTAL CONSTRUCTION COST .~.~~.~~~~~~~ooo$1,270,830.30
PLUS 10% ENGINEERING ...... 127,083.03
SUBTOTAL ...... $1,397,913.33
PLUS 15% CONTINGENCY ...... 209,686.99
TOTAL PROJECT CAPITAL COST ..omoommmmme $1,607,600.33 ALTERNATE #2 ------Killpecker Creek
The following tables present quantities, unit costs, construction costs, and capital costs for flood control improvements on Killpecker Creek and in the watershed. These cost figures are taken directly from the Level 11, Phase 1A Report :
TABLE 18
KILLPECKER CREEK
LAND ACQUISITION ALONG CREEK THROUGH TOWN
DESCRIPTION QUANTITY COST/UNIT TOTALS
LAND ACQUISITION 43 Acres 5,000.00 215,000.00 SUBTOTAL $215,000. 00 TABLE 19 KILLPECKER CREEK
DETENTION POND AT SITE NO. 2
CONSTRUCTION QUANTITIES AND COSTS
DESCRIPTION QUANTITY COST/UNIT TOTALS
LAND ACQUISITION 90 Acres 500.00 45,000.00 COMPACTED 17,747 C.Y. 4.00 70,988.00 EMBANKMENT 3 PIPES 84" 441 L.F. 250.00 110,250.00 DIAMETER 147' LONG BANDS AND 441 L.F. 87.50 38,587.50 ACCESSORIES (35% of Pipe Cost) COMPACTED GRAVEL 10,200 S.F. 0.34 3,468.00 CAP SEEDING 45 Acres 1,500.00 67,500.00 RIPRAP 4,479 SOY. 18.00 80,622.00 FABRIC FILTER 4,479 SOY. 1.00 4,479.00 OVERFLOW CHANNEL LUMP SUM 10,000.00 10,000.00
STILLING BASIN LUMP SUM 15,000.00 15,000.00 .
SUBTOTAL 445,894.50 TABLE 20
KILLPECKER CREEK
DETENTION POND AT SITE NO. 3
CONSTRUCTION OUANTITIES AND COSTS
DESCRIPTION QUANTITY COST/UNIT TOTALS
LAND ACQUISITION 300 Acres 500.00 150,000.00 COMPACTED 61,063 C.Y. 4.00 244,252.00 EMBANKMENT 3 PIPES 84" 411 L.F. 250.00 102,750.00 DIAMETER 147' LONG BANDS AND 411 L.F. 87.50 35,962.50 ACCESSORIES (35% of Pipe Cost) COMPACTED GRAVEL 34,200 S.F. 0.34 11,628.00 CAP SEEDING 150 Acres 1,500.00 225,000.00 RIPRAP 13,018 S.Y. 18.00 234,324.00 FABRIC FILTER 13,018 S.Y. 1.00 13,018.00 OVERFLOW CHANNEL LUMP SUM 10,000.00 10,000.00
STILLING BASIN LUMP SUM 15,000.00 15,000.00 .
SUBTOTAL 1,041,934.50 TABLE 21
KILLPECKER CREEK
REACH 1 CHANNEL IMPROVEMENTS FOR DETENTION SOLUTION
CONSTRUCTION OUANTITIES AND COSTS
P DESCRIPTION QUANTITY COST/UNIT TOTALS
EXCAVATION 21,512 C.Y. 2.75 59,158.00 RIPRAP 14,767 S.Y. 18.00 265,806.00 FABRIC FILTER 14,767 S.Y. 1.00 14,767.00
SUBTOTAL 339,731.00 -
TOTALCONSTRUCTIONCOSTooooooooooooooooooo $2,042,560000 PLUS 10% ENGINEERING ...... 204,256.00
SUBTOTAL ...... $2,246,816.00 PLUS 15% CONTINGENCY ...... 337,022.40
TOTALPROJECTCAPITALCOST O.OOO.OOOO.OOOOO $2,583,838.40 ------VII ECONOMIC ANALYSIS
A. Qead Horse Canyon Creek
1. Introduction
The purpose of the Phase I1 economic analysis of flood control measures for Dead Horse Canyon was twofold:
to review the Phase IA estimates of flood control benefits for Dead Horse Canyon in light of data concerning damages from the 1989 flood; and
to incorporate the results of Phase I1 cost estimates for the preferred alternative into the analysis.
Data concerning the 1989 Dead Horse Canyon flood were obtained during a series of personal interviews with Sweetwater County and Rock Springs officials duringthe summer of 1990. Revised cost estimates for the preferred alternative (retention ponds NO. 2 and No. 3) were derived during the course of the Phase I1 study.
2. Analvsis and Results
An analysis of damages attributable to the 1989 Dead Horse Canyon flood indicates that total damages were approximately $2.9 million. This estimate is based upon damage estimates prepared by the City of Rock Springs, the Sweetwater County Emergency Management Agency, the Red Cross, and other local groups. These damage estimates are presented in Table 22, below. TABLE 22
DAMAGE ESdIMATES FOR THE 1989 DEAD HORSE CANYON FLOOD
Damage Structural Contents Other Total Category Damage Damage ama age' Damage
Commercial -... $1,520, 6302 $ 25,070 $1,545,700 Residential $120,000 920,000 .- 1,040,000 Public 58,000 Facilities
Vehicles 150,000 ow -- 150,000 Totals $328,000 $2,440,630 $111,770 $2,880, 4003
Other damage includes emergency costs, lost income, and cleanup costs.
Conrnercial damages have not beerr separated into contents and structural, but presunably consist primarily of the former.
Total damages do not include damages to the UP rai 1 line, telephone facilities, and certain businesses for which information is unavai table.
The Phase IA economic analysis for Dead Horse Canyon estimated that flood damages would be approximately $3.0 million for a 60-year flood (Johnson-Fermelia, 1989). That estimate is consistent with the $2.9 million damage estimate given in Table 22 and a judgmental assessment that the 1989 flood was less than a 100-year flood but greater than a 50- year flood. It thus appears that the Phase IA damage estimates are at least in the ballpark with respect to the actual damages that occurred along Dead Horse Canyon in 1989. What is somewhat surprising, however, is that emergency and other indirect costs constituted only a small portion of actual damages for the Dead Horse Canyon flood. The Phase IA projections had assumed that a larger proportions-of damage would accrue from indirect and emergency costs than was the case for the Dead Horse Canyon flood. Instead, physical damages appear to have been higher than projected and indirect damages appear to have been lower.
One reason for this discrepancy may be the fact that the Phase IA physical damage projections were based upon aerial photographs, from which it is impossible to determine exact building elevations relative to the flood plain. As discussed elsewhere in this report, the Phase IA analysis for Killpecker Creek underestimated the elevation of buildings in the flood plain, thus overestimating the benefits of flood control improvements. With respect to Dead Horse Canyon, however, it would appear that the Phase IA analysis may have overestimated the elevation of buildings in the flood plain, thus underestimating the physical damages to property from a 50- to 100-year flood.
Without detailed engineering studies of building elevations along Dead Horse Canyon Creek, however, there is no objective way of revising the Phase IA structural damage estimates. Furthermore, since the Phase IA total damage estimates are roughly comparable to damages that actually occurred during the 1989 flood, they were retained for purposes of the Phase I1 analysis.
The preferred flood control alternative for Dead Horse Canyon from the Phase IA study is No. 6, which consists of Detention Reservoirs Nos. 2 and 3. These detention reservoirs would attenuate a 3-hour, 500-year flooding event. The Phase IA cost estimate for this alternative was $644,000, and the ratio of benefits to costs was 3.07. One change was made in the benefit-cost calculations for the Phase I1 analysis. In the Phase IA analysis, it was assumed that Detention Reservoirs NOS. 2 and 3 would not attenuate flooding events larger than the 100-year flood. The Phase I1 design for these reservoirs, however, will attenuate up to a 500-year flood event. Thus, estimated flood control benefits increase from $1,979,300 (from Level 11, Phase IA Report) to $3,281,400. The Phase I1 cost estimate for the detention reservoirs is $744,637, and the resulting benefit-cost ratio is 4.41. It should be noted that this benef it-cost ratio was derived using a 3.7 percent discount rate, which is appropriate for evaluating projects funded in Wyoming. The benefit-cost would be somewhat lower if the federally mandated discount rate were used. This consideration is mooted, however, because Dead Horse Canyon Creek does not meet criteria for federal interest based upon the magnitude of its ten-year flooding event.
B. White Mountain Tributaries
1. Introduction
A re-evaluation of flood control benefits and costs per the White Mountain Tributaries was undertaken during the Phase I1 study. The re-evaluation was prompted by two flooding events that occurred in the area during the summer of 1991, and the fact that greatly improved mapping of the flood plain was developed for purposes of the Phase I1 dam break analysis. The re-evaluation is described below.
2. Analysis and Results
Costs were developed for two preferred alternatives for flood control in the White Mountain area during the Phase I1 study. They are: Alternate NO. 1
- Detention Reservoir No. 1, attenuating a 3-hour, 500-year event;
Detention Reservoir No. 2, attenuating a 3-hour, 500-year event;
- Detention Reservoir No. 3, attenuating a 3-hour, 500-year event;
- Detention Reservoir No . 4 (alternative configuration No. 3) attenuating a 3-hour, 500-year event.
Alternate No. 2
- channel improvements to attenuate a 3-hour, 100- year event;
- Detention Reservoir No. 2, attenuating a 3-hour, 500-year event;
- Detention Reservoir No. 3, attenuating a 3-hour, 500-year event;
Detention Reservoir No. 4 (alternative configuration No. 3) attenuating a 3-hour, 500-year event.
The Phase I1 cost estimates for these alternates are $1,710,167 for Alternate No. 1, and $1,607,600 for Alternate No. 2.
Benefit estimates for these alternatives were developed using improved flood plain mapping and HEC 2 model runs for the lo-, 500, loo-, and 500-year floods. To make the benefit estimates as precise as possible, surveys were performed to determine the exact elevation of all commercial property in the flood plain. The estimated value of commercial structures and their contents was based upon actual survey data for similar businesses in Killpecker Creek flood plain (see Section V1.C). The resulting combined value of $124 per square foot was used for purposes of estimating flood damage reduction benefits.
The benefit analysis was also expanded to include residential areas that were identified as being in the flood plain through improved mapping. Approximately 203 single family dwellings were identified as potentially being at risk. Because it was impossible to determine precise flood routings through the affected residential areas, it was assumed that the following percentages of dwellings would be affected at various flood frequencies:
w 10-year flood - 10 percent affected 50-year flood - 40 percent affected w 100-year flood - 60 percent affected m 500-year flood - 90 percent affected
Area wide sheet-flooding depths were then applied to affected structures to estimate damage reduction benefits.
The revised estimate of flood control benefits for the White Mountain Tributaries is $20.9 million, higher than that for any other tributary. The resulting benef it-cost ratios are 12.22 (Alternative No. 1) , and 13.00 (Alternative No. 2). These benefit-cost ratios include estimates of both direct (structure and contents) and indirect (emergency cost) savings that would accrue from flood control measures. As discussed in Section VIoA, however, an analysis of 1989 Dead Horse Canyon flood indicates-that the indirect benefit estimates may be too high based upon actual experience. Based upon direct benefits alone, however, benefit-cost ratios for khe White Mountain Tributaries are 7.25 (Alternative NO. 1), and 7.76 (Alternative NO. 2). These ratios are higher than those for any other tributary, making flood control improvements for White Mountain a high priority.
It should be noted that these benefit-cost ratios were derived using 3.7 percent discount rate, which is appropriate for evaluating projects funded in Wyoming. The benef it-cost would be somewhat lower if the federally mandated discount rate were used. This consideration is mooted, however, because Dead Horse Canyon Creek does not meet criteria for federal interest based upon the magnitude of its ten-year flooding event.
C. Rillpecker Creek
1. Introduction
The purpose of this Phase I1 economic analysis of flood control measures for Killpecker Creek is to determine whether there may be any federal interest in funding flood control improvements in that drainage. According to U.S. Amy Corps of Engineers (COE) criteria and regulations, several conditions must be met before federal funding can be made available for such purposes.
First, regulations preclude COE participation in portions of streams where the lo-year flood event is less than 800 cfs (Tucker, 1979). In addition, the proposed flood control improvements must benefit more than one landowner, and the COE generally does not get involved unless construction costs exceed $50,000 (Dixon, 1988). Finally, the proposed project must have a benefit-cost ratio greater than 1.0 using federal procedures for estimating benefits and costs. These procedures are described in a COE manual entitled National Economic Develo~mentProcedures Manual - Urban Flood Damaae (COE, 1988).
A preliminary appraisal of federal interest for each of the Bitter Creek tributaries is described in the Phase IA report for this project (Johnson-Fermelia, 1989). That appraisal indicates that only Killpecker Creek meets the COE regulation requiring a 10-year flood event in excess of 800 cfs. The appraisal also indicates that Killpecker Creek flood control measures might meet other criteria for federal interest. Specifically, flood control solutions would cost in excess of $50,000, they would benefit more than one landowner, and a preliminary benefit-cost assessment produced benefit- cost ratios in the range of 0.67 to 1.24.
Because of time and budget limitations, however, the Phase IA economic analysis for Killpecker Creek did not strictly follow federal procedures for estimating flood damage reduction benefits. For example, estimates of physical flooding damages were based upon aerial photography instead of field work. Federal procedures require extensive field survey work for estimating such damages. Furthermore, the Phase IA estimates of nonphysical (indirect) damages were based upon estimated percentages of physical (direct) damages. Federal procedures specifically exclude that approach and require estimates to be based upon actual nonphysical costs incurred in similar flooding events.
This Phase I1 economic analysis for Killpecker Creek was thus designed to closely follow procedures promulgated by the COE for estimating flood damage reduction benefits. The discussion that follows describes the methods that were employed and the results that were obtained in this analysis. 2. physical Flood Damaae Reduction Benefits
Valuing Structures in the Flood Plain
The Killpecker Creek flood plain contains three types of buildings: single family residences, mobile homes, and commercial buildings. Different federal rules apply to the evaluation of each building type.
According to the COE (1988), the appropriate measure for single family residences is de~reciated re~lacement value, which can be approximated by market values. To determine these values, the COE suggests that buildings be separated into groups according to the number of stories and whether they have basements. From each of these groups, a random sample of houses can be examined to establish an average value for all houses in that group. These market values can be established with the help of data from any of the following sources :
real estate assessment data used for local real estate taxes which must be separated by land and improvements;
recent sales prices recorded by the recorder of deeds and the assessor's office;
appraisals made with the help of Marshal Valuation Service documents.
Because there are only eight sinale family residences in the 500-year flood plain along Killpecker Creek, it was unnecessary to group them and draw a sample for the purpose of establishing values. Instead, the value of each structure was determined from local real estate assessment data and verified by on-site inspections. The value of contents for each residence was established through personal interviews with owners and tenants. The eight residences had an average structural value of $35,300 and an average contents value of $24,200, in 1990 dollars.
The 115 mobile homes in the flood plain were regarded as a group of single family residences, as suggested by COE procedures. A random sample of 14 mobile homes was drawn from this group for the purpose of valuation. Based upon real estate assessment data and on-site inspections, the average market value of mobile homes in the flood plain was determined to be approximately $6,800. This figure reflects the fact that the majority of mobile homes in the flood plain are relatively old and not in the best condition. Personal interviews were used to establish contents values for the mobile homes in the sample. The resulting estimate was $7,055 per unit. According to COE procedures, however, estimates of contents value for mobile homes cannot exceed the value of the structure. Thus, the estimate of contents value was reduced to $6,800 for purposes of the analysis.
Commercial and industrial buildincrs usually require detailed inspection for structural as well as contents value (COE, 1988) . Each property should be inspected, preferably by interviewing the manager, to obtain the following data:
structural value;
contents value; and
any damages that may have occurred during previous floods .
During the Phase I1 economic analysis for Killpecker Creek, 73 businesses and two vacant business locations were
Unless otherwise noted, all cost and benefit figures in this section of the report are in 1990 dollars. identified in the 500-year flood plain. These businesses occupied a total of 115 buildings. Personal interviews were conducted with the owners and/or managers of 60 of the 73 businesses to obtain data concerning structural and contents value and other data relevant to estimating physical flood damages. A copy of the interview questionnaire is given in the Appendix to this report.
In cases where reliable data concerning structural values could not be obtained through personal interviews, estimates were based upon 1989 real estate assessment data for that location and/or a $22 per square foot value derived from interviews with local real estate agents. The resulting average value for commercial structures in the flood plain was $243,500 per business. Contents value averaged $617,000 per business.
Vehicles are another type of property that can be physically damaged by floods. According to the COE, there are two acceptable ways to estimate potential damage to vehicles. The first involves determining the number of cars per thousand population from the U.S. Bureau of Census1 "Statistical Abstract of the United States." Since the flood plain along Killpecker Creek does not represent a typical ratio between residential and commercial uses, however, this method was considered inappropriate. The other method involves repeated counting of vehicles parked in the flood plain to determine the average number of vehicles present. For purposes of this analysis, vehicle counts were conducted on four different days during the fall of 1990. The results are depicted in Table 23. The counts revealed that an average of about 580 vehicles are parked in the flood plain at any one time. TABLE 23 VEHICLE COUNTS IN THE KILLPECKER CREEK FLOOD PLAIN
Tne of Vehicle pate/Time Cars and Vans Pick UP Trucks Trucks Totals
Thursday 182 September 13, 1990 9:00 p.m. Friday September 14, 1990 9:00 a.m. Sunday September 30, 1990 4:30 p.m. Tuesday October 2, 1990 2:00 porn.
Average 193 232 162 587 To account for the possibility that vehicle owners might have enough warning time to remove their vehicles from the flood plain, only one-third of the vehicles in the flood plain were assumed to be damaged in the event of a flood. The 1989 flood in the Dead Horse Canyon drainage occurred so suddenly that virtually no vehicles could be evacuated. Since the Killpecker Creek drainage is substantially larger than the Dead Horse Canyon drainage, however, it was assumed that some evacuation of vehicles would be possible. With this assumption, the following numbers of cars would get flooded at various flood stages:
500-year flood, without project: 193 cars flooded 500-year flood, with project: 58 cars flooded 100-year flood, without project: 39 cars flooded 50-year flood, without project: 19 cars flooded lo-year flood, without project: 4 cars flooded
For the 1989 flood in Dead Horse Canyon, the Sweetwater County Emergency Management Agency used $1,000 as an estimate of damages to each car that was flooded. That same value was used for purposes of this study.
The last category of possible physical flood damages involves public utilities. Interviews with the managers of all public utilities with facilities located in the Killpecker Creek flood plain revealed that no flood damage to any of these facilities is likely in the event of a 500-year flood.
Depth-Damage Relationships
Depth-damage relationships are used to estimate damages to structures and contents of various types of property at various flooding depths. According to the COE, there are two acceptable ways of estimating such relationships: extensive interviews about damages from previous floods; or
a computer oriented analysis.
The last major flood in the Killpecker Creek flood plain occurred prior to most development in that area. Therefore, historic data could not be used directly. Furthermore, the development of a computer model to simulate damages to structures along Killpecker Creek was beyond the scope of this study. Instead, a set of depth-damage relationships for various types of structures was obtained from the COE's Omaha regional office. These relationships are based upon extensive survey work associated with previous floods, and thus constitute an acceptable method of estimating such relationships.
Damage-Frequency Relationships and Expected AMual Damages
Combiningthe inventory of structural and contents values with the depth-damage relationships results in damage- frequency relationships. These relationships were developed separately for Reach I, the flood plain of Killpecker Creek between its confluence with Bitter Creek and the Stagecoach Blvd. bridge; Reach 11, the area between the Stagecoach Blvd. bridge and the Yellowstone Road bridge; and the entire flood plain along Killpecker Creek.
Damage-frequency relationships were also developed for with and without project scenarios for each of the above areas. For without project scenarios, damages were calculated for four flood stages: the 5000, loo-, 500, and lo-year floods. For other flood frequencies, points along the damage- frequency relationships were obtained by interpolation. For with project scenarios, however, damages were calculated for the 500-year flood only, because floods of lesser magnitude than the 100-year flood would be fully attenuated. For flood frequencies between the 100- and 500-year floods, damage- frequency relationships were obtained by interpolation. Figures 1 through 3 show the damage-frequency relationships derived by this method.
The areas under the curves in Figures 1 through 3 represent annual expected damages both with and without flood control improvements. The expected annual benefits of improvements are thus represented by the differences in areas under the two sets of curves. According to the COE, there are two acceptable methods of estimating these areas:
fitting equations to the data and using integral calculus to estimate benefits; and
using various methods of approximating the results of such integration Figure 1 Damage-Frequency Relationships for Reach I of Killpecker Creek
Damages (Millions)
$8-0 1
p.WJWWS
1 with project
0 100 200 300 400 500 600 Years Figure 2 Damage-Frequency Relationships for Reach II of Killpecker Creek
Damages (Millions) $8.0 1
without
.-.-.. I / ' oroiect
0 100 200 300 400 500 600 Years Figure 3 Damage-Frequency Relationships for Killpecker Creek
without Damages (Millions) ?- -r
0 100 200 300 400 500 600 Years For purposes of this Phase I1 analysis, a method approximating the results of integration was used to estimate expected annual flood control benefits from reducing physical damages. The results of these:calculations are presented in Tables 24 and 25. The results in Table 24 show that without flood control improvements, expected annual damages (EAD) are estimated to be approximatley $46,400. With flood control improvements, expected annual damages would be reduced to approximately $5,500. The magnitude of expected annual benefits is the difference between those two figures is approximately $40,850.
These annual benefit estimates are significantly lower than the benefit estimates derived during the Phase IA study (Johnson-Fermelia, 1989). In fact, the 1989 study estimated annual flood damage reduction benefits along Killpecker Creek to be approximately $230,000, compared to only approximately $41,000 for the current study.
There are two primary reasons for this difference. First, as stated previously, the 1989 damage estimates were based upon aerial photographs with flood plain boundaries superimposed. With this method it is impossible to determine the precise elevation of buildings relative to the surrounding flood plain. Instead, it was assumed that, depending upon the type of building, first floor elevations were in the range of 6" to 24" above the flood plain floor.
For purposes of this Phase I1 analysis, however, precise elevation estimates were developed for each building in the flood plain. These estimates indicate that, with few exceptions, significant land filling occurred along the flood plain prior to the construction of commercial buildings. TABLE 24 EXPECTED ANNUAL FLOOD DAMAGE (EAD) COMPUTATION FOR KILLPECKER CREEK WITHOUT PROJECT
Damage EAD Recurrence Frequency Frequency Damage Average for (In Years) (In %) Interval (In $) for Interval Interval
500 0 l 2 00008 8f086'462 4,267,310 34,138 100 100 448,158 .01 335,562 3,336 50 2.0 218,966 111,483 .08 8,919 10 10.0 4,000 0 EAD W/O Project $4g;555
TABLE 25 ECTED ANNUAL FLOOD DAMAGE (EAD) COMPUTATION FOR KILLPECKER CREEK WITH PROJECT
Damage EAD Recurrence Frequency Frequency Damage Average for (In Years) (In 8) Interval (In $) for Interval Interval
500 0.2 1,387,338 ,008 693,669 5,549 100 100 0 EAD with Project $ 5;5Z5 Total EAD $40,844 Thus, building elevations are, in reality, significantly higher than was assumed in the previous study, thus greatly reducing potential damages from floods. As a result, the benefit& from flood control improvements would be less than previously estimated.
A second difference in the benefit estimates is that the 1989 study assumed nonphysical (indirect) flood damages could be approximated as a percentage of physical damages. As a result, indirect benefits from flood prevention are incorporated in the 1989 benefit estimate of $230,000 annually. The COE specifically rejects this approach to indirect benefit estimation, however, and insists that such benefit estimates be determined by reference to specific flooding events. As a result, we attempted to estimate indirect flood control benefits for Killpecker Creek based upon data and insights developed from the 1989 Dead Horse Canyon flood. A discussion of that analysis is presented in the following section.
3. ~ssessmentof Non~hvsicalFlood Damacre Reduction
The COE manual states that: I1nonphysical flood damage reductions costs should be estimated from specific independent data, and not computed as a percentage of physical damagew (COE, 1988, p. VII-1) . In general, "the level of detail spent on estimating these costs should depend on whether there is good reason to believe these costs are significant and whether there is potential of significantly reducing these costs8@(p. VII-2). Nonphysical costs include potential income losses, emergency costs, floodproofing costs, the administrative costs of flood insurance, temporary location costs, and restrictions of use for flood-prone property. To estimate the magnitude of these costs for Killpecker Creek, we relied upon COE procedures and data and insights gathered from the 1989 Dead Horse Canyon flood. Jncome losses can occur if there is a halt in production or delivery of goods and services and postponing or transferring these activities is not possible. These losses can occur during flood warning,* inundation, and cleanup and restoration. The key to interpreting this definition from the COE's federal perspective is the phrase "and postponing or transferring these activities is not possible. Although a number of Rock Springs businesses were shut down for varying periods of time after the 1989 Dead Horse Canyon flood, it is doubtful that any of them were supplying goods or services that could not be provided elsewhere in the U. S. economy. Thus, although individual business owners may have suffered income losses, from the federal perspective these losses were probably offset by increased income elsewhere. For example, a contractor planning to build a home in Rock Springs would probably not cancel his plans if one of the local lumber yards were flooded and forced to close. Instead, he would probably buy his lumber elsewhere.
According to the COE procedures, it would thus be difficult to claim income losses due to flooding along Killpecker Creek unless it could be shown that one or more businesses there provide a unique good or service that is not available elsewhere. Based upon the survey of businesses in the flood plain conducted for this study, that does not appear to be the case. Thus, it is not appropriate to consider reductions in income losses as a project benefit if COE procedures are followed.
Emeraencv costs are the incremental costs of flooding due to:
a monitoring and forecasting floods;
actions taken by police, fire, and National Guard with respect to a flood;
a flood fighting efforts; costs incurred by emergency aid for flood victims;
evacuation costs;
administrative costs for delivering emergency services; and
costs of traffic rerouting.
The key to interpreting this definition from the federal perspective is the word llincremental.N For example, the costs of actions taken by the police and fire department in response to a flood would be considered emeraencv costs only if they involved calling in off-duty employees who would be paid an incremental amount over and above their normal salary. Such overtime payments would qualify as emergency costs under COE procedures.
An analysis of the 1989 Dead Horse Canyon flood indicates that the magnitude of most emergency costs, using the COE definition, was quite low. Actions taken by emergency agencies involved relatively little overtime payroll, and relatively few flood victims required emergency aid. According to the Rock Springs representative of the American Red Cross, only two single people needed to be relocated to motels, although an undetermined number of flood victims took refuge with family and friends in the local area. As a result, it would be difficult to justify any significant savings in emergency costs as a result of flood control efforts along Killpecker Creek.
One exception in this respect is the cost associated with traffic rerouting. A 500-year flood without flood control improvements would flood Elk Street south of the 1-80 intersection. This street is one of the main arteries leading from 1-80 and from northern outskirts into the downtown area. Additional operating cost and travel time cost would be associated with road inundation in this area. The magnitude of these costs was estimated by following a step-by-step procedure in the COE manual (COE, 1988).
The amount of time that Elk Street would be impassable was estimated to be three hours. Long and persisting floods are not likely to occur in small drainages such as Killpecker Creek. The 1987 Dead Horse Canyon flood lasted only about one hour. The Dead Horse Canyon drainage, however, is smaller than Killpecker Creek's, and the 1986 flood was significantly smaller than a 500-year event. Three hours of inundation for Elk Street, therefore, seems to be a conservative estimate.
To estimate the magnitude of increased travel costs, average traffic counts were obtained from the Wyoming Highway Department. During an average three hours, Elk Street would be used by 1,346 automobiles and 52 trucks. The length of Elk Street that would be made impassable by a 500-year flood is approximately 1.25 miles. The shortest alternative route is 4.75 miles. Thus, the additional mileage per vehicle is 3.50 miles.
Multiplying the additional mileage by the traffic counts yields total additional auto mileage of 4,711 and total additional truck mileage of 182. Multiplying these additional miles by operating costs of 30 cents for automobiles and 50 cents for trucks yields total additional operating costs of $1,504.30.
To estimate the additional travel time associated with traffic rerouting, it was necessary to make assumptions about traffic speed. Because part of the detour follows the interstate highway, an average speed of 40 mph was assumed for the detour. An average speed of 30 mph was assumed for Elk Street in the absence of flooding. Subtracting original travel time from detour travel time yields additional travel time of 104 hours for automobiles and four hours for trucks. Traffic observations led to the conclusion that one person per car is an appropriate estimate, so that the above numbers represent total additional travel time. The COE manual recommends using one-third of the average local wage rate as time costs for automobile driver and the local trucker wage rate as time costs for truck driver. Both of these rates were determined with the help of the "State and County Summary of Covered Employment and Total Payroll by Indu~try,~~ published by the Employment Security Commission of Wyoming. Rates were determined to be $4.00 for automobile driver and $9.80 for a truck driver. The resulting total delay costs were $416.00 for automobiles and $39.20 for trucks. Total delay costs were thus $455.20. Adding these to the additional operating costs results in cost of an inundated road of $1,959.50. These costs were added to the damages attributed to a 500-year without project flood in Table 24.
Flood~roofina costs were not estimated because the surveyed businesses in the flood plain revealed that hardly any buildings have structural features whose sole purpose is the prevention of flood damage. The only floodproofing features mentioned by the managers interviewed were drains in the floor of shops and storage rooms and doors on both sides of the building to allow flood water to run through the building instead of against it. Where building floors were elevated, this was done primarily for purposes other than floodproofing, e.g., to create ramps for truck loading.
Administrative costs of flood insurance were not estimated because only very few of the companies in the flood plain had flood insurance, and those that did usually had combined insurance for floods and other hazards, such as fire and storm.
Benefits from modified use of flood-prone pro~ertvcan be considered if a change in the actual use of a portion of a structure would make the property more valuable and the change would occur under with project conditions. The COE manual makes no suggestion how to quantify these benefits.
The survey of businesses in the flood plain did reveal some evidence that land uses might change on some properties when restrictions imposed on development within flood plains could be lifted. The nature and extent of these changes are difficult to predict, however, and as a result, the effects of such changes in land use were analyzed in the content of a sensitivity analysis as described in Section 5, below.
4. Benefit-Cost Ratios
The federal discount rate in effect at the time of this study was 8.75 percent. Project life was assumed to be 50 years. Based upon these parameters, the discuounted value of project benefits for the Killpecker Creek tributaries is approximately $459,750. Compared to project costs of $2,583,838, the resulting benef it-cost ratio is 0.18, which is too low to support federal interest flood control improvements. As mentioned in Section 2, this low benefit- cost ratio is primarily attributable to the fact that most commercial buildings in the flood plain have foundations that are raised above the flood plain floor, and the fact that preventing most of the indirect costs associated with flooding would not qualify as a benefit under COE evaluation procedures.
According to COE procedures, it is important to determine if benefit estimates are reasonable and what difference there might be in estimates of net benefits or the benefit-cost ratio if any of the major assumptions in the analysis are changed. The manual recommends three procedures for conducting such analyses: a sensitivity analysis;
a confidence limits for critical variables; or
• procedures to check average structure and content values.
Sensitivity analysis was chosen for this study because it can pinpoint situations where the outcome of the study is particularly sensitive to a change in the values of certain variables. To conduct a sensitive analysis, an interval is placed around the value of a variable, and project feasibility is recalculated with values from both ends of the interval. The results are then compared with the original estimate.
In this study, sensitivity analyses were performed with respect to the following:
• a change in expected annual flood damages; and
a change in the market value of land due to changed land uses.
Estimated annual flood damages have three components: the damage to buildings and their contents; damage to vehicles; and traffic delay costs due to the inundation of Elk Street. The latter two of these parts are of negligible magnitude compared with structural and content damage. Therefore, only building and content damage was examined with respect to its influence on the final benefit-cost ratio.
The magnitude of structural and contents damage is determined by three factors: the value of the building and its contents, the depth of inundation, and the depth-damage relationship. To achieve a benefit-cost ratio of greater than one for Killpecker Creek, it would be necessary to assume that actual building and contents values were over five times higher than those used in this analysis. Alternatively, a benefit-cost ratio of unity would require the assumption that average flood depths within buildings along the, flood plain were three feet higher than indicated by the HEC 2 model. Finally, a benefit-cost ratio of unity could be achieved by assuming the depth-damage relationships used in the analysis are 5.2 times too conservative. These possibilities all seem unlikely given that:
building and contents values were carefully estimated from personal interviews and assessment data;
w flooding depths were based upon survey elevations and computerized flood routing studies; and
the depth-damage relationships were developed by the COE.
Another factor that might change the outcome of the study is that the market value of structures and land along the flood plain might increase in the presence of flood control measures. To the extent that such values would increase (relative to other property values outside the flood plain), it would be evidence that flood control measures had made the property in the flood plain more desirable and/or productive. Increases in land values can be used as a proxy for this increase in productivity and counted as a project benefit.
In Section 2.0 we noted that the 75 businesses in the flood plain had an average structural value of $243,400, for a total structural value of approximately $18.3 million for all businesses combined. To achieve a benefit-cost ratio of unity, we would have to assume that the value of commercial structures in the flood plain would increase by at least 10 percent relative to other commercial property in Rock Springs if flood control measures were implemented. To achieve a benefit-cost ratio of 1.5, a 16 percent increase in property value $would be required.
It makes cornman sense to assume that property values along the flood plain may increase somewhat in the presence of flood control measures. Since such increases cannot be predicted in advance, however, it may be difficult to convince the COE to become involved in flood control measures that are justified primarily on the basis of property value increases. VIII FEDERA& INTEREST
The foregoing hydrologic and economic analysis examined the possibility of Federal interest in flood control projects on the three tributaries to Bitter Creek that were studied in this Phase 11 effort. A project that satisfies applicable COE criteria would be qualified to receive funding for construction from the United States Government. None of the projects examined herein qualify for Federal interest under the referenced COE criteria,
The following summary presents the reasons for which the projects do not meet the necessary criteria:
Proiect Comments
Dead Horse Does not meet criteria for Federal interest CanyonCreek considering the magnitude of the lo-year flooding event.
White Mountain Tributaries:
Alternate #1 Does not meet criteria for Federal interest based upon magnitude of lo-year flooding event,
Alternate #2 Does not meet criteria for Federal interest based upon magnitude of lo-year flooding event.
Kill~ecker Benefit-cost ratio, as compiled applying COE Creek criteria of 0.18 is inadequate to warrant Federal interest in flood control improvements. I CONCLUSIONS AND RECOMMENDATIONS Ao Conclusions
1. Dead Horse Canyon Creek
The hydrologic and hydraulic modelling, project design, costs, economic analysis, and benefit-cost ratio of 4.41 indicate that a flood control project on Dead Horse Canyon Creek is economically feasible.
2. White Mountain Tributaries
Modelling, project design, costs, revised economic analysis as presented herein indicate that a flood control project under Alternate #1 or #2 is economically feasible on the White Mountain Tributaries. The benefit cost ratios of 12.22 and 13.00 for Alternatives #l and #2 respectively indicate that a flood control project on this tributary should be considered as a high priority.
3. Kill~eckerCreek
Economic analysis of flood control costs and flood control benefits as analyzed under COE criteria do no warrant further consideration of a flood control project on this tributary.
Bo Recommendations
1. Dead Horse Canyon Creek
The benefit cost ratio of 4.41 for the flood control project described in this Report demonstrates that a project on this tributary is economically feasible; however, prioritizing such a project should consider the comparable benefit-cost ratio for a project on the White Mountain Tributary as presented below. The information and data presented in the "bevel I1 Bitter Creek Channel Im~rovement Studv ReDoxztW should also be incorporated into the deliberations concerning prioritization of a project on this tributary.
2. White Mountain Tributaries
The benef it-cost ratios presented in this Report for both Alternate $1 and #2 (12.22 and 13.00 respectively) indicate that a flood control project is justified on this tributary and the project should be classified as having a very high priority.
3. Killnecker Creek
No further consideration of a flood control project on Killpecker Creek should be pursued.
Confidential Confidential
KILLPECKER CREEK FLOOD CON!CROL SURVEY FOR BUSINESS
A. Backaround Data
1. Business Name:
2. Business Address:
3. Mailing Address: - - - - - (if different)
4. Business Owner:
5. Building Owner: (and address if different)
6. Person Interviewed:
7. Interview Date: Be Business Data
8. Type of Business:
9. Operating Hours: Monday-Friday: Saturday:
Sunday :
10. Number of Employees:
Full-time Salaried: Part-time Salaried:
11. Value of Inventory: Date/.Source Jtem Description Value of Estimate 12. Value of Equipment, Furniture, and Fixtures:
Item ~escription Value of Estimate
C. Buildins characteristics
13. Structural Characteristics (Brick, Wood, Metal, Etc. ) : 14. Interior Characteristics: Elevation Relative Square Percentage of to Ground Floor Description Footaae Inventorv/Fixtures Level Basement Ground Floor Second Floor Third Floor Fourth Floor
15. For the first floor (and for split levels below and above the first floor, if appropriate), please estimate the percentage of inventory and fixtures at each of the following levels: Elevation Relative Percentage of to Floor Inventorv/Fixtures
Below 2 Feet
From 2 to 4 Feet
From 4 to 6 Feet
Above 6 Feet
16. Value: Date/Source Item Description Value of Estimate Building Land D. Precautions
17. Have you undertaken any floodproofing or other precautions designed to avoid or reduce flood damages to your property? Yes No If yes:
(a) Please describe.briefly those precautions and the date they were implemented:
(b) What were the material costs for these measures?
(c) How many man-hours were necessary to implement these measures?
18. Are there any floodproofing measures in the building that were installed by others? If so, please describe briefly: 19. Do you have flood insurance? Yes If yes, what are your annual payments?
$ per year
20. Has your business ever been affected by a flood at this locat ion? Yes No If yes:
(a) What was the structural damage?
(b) What was the contents damage?
(c) How long were normal business operations interrupted due to the flood(s)?
21. Would a flood control project on Killpecker Creek affect in any way your future plans for your business? Yes No If yes, in what way? PLATES KIWECKER CREEK
DRAWINGS BITTER CREEK TRIBUTARY FLOOD STUDY PHASE I ROCK SPRINGS, WYOMING
SHUT NO.
0 NORTH
.JWNsON FERMEUA CO. M. ARCHCTECTS AND LAND SURVEYORS 1515 tWT'H STFWET ROCKSPMNGS,~82901 (307) 362-7519 6
LEGEND
@ %XTWW€R CITY
ROCK SPRING
W+D€X MAP INDEX MAP
BITTERFLOOD CREEK STUDY TWUTARY PHASE#
a.IWC. CO)JaUIIWCPlOlWEERS MlCHTECrS8SUMrWLS IM5 NINTH STREET ROCKsmNGs.WY62901
JOB No. 2340-0368 CbJoL133)(8 eeco-OHZ -ON BOP LEGEND
GRAVEL ROAD
TRAIL
LUI VERl
DRAINAGf L INF
luff
INDEX CONTOUR
INlfRMEDIATf CONIOUR
ROCK OUlCROP
SPOT LLfV*I IOh
HORIZUN~A, :Ih'QL
tkW1 ICAL :.UhTl+C,
DEAD HORSE CANYON DETENTION RESERVOIR #3
SEcrXm I, TI% A105W SWEETWATER CO, WYO. BITTER CEEK TRIBUTARY FLOOO STUDY MI
-UA -UA CO, INC. cmsuuwGEmmERS AmirrECTS 8 SURYEm IS15 NINTH STREET ROa< SPRWGS. WY 82901 LEGEND
PAVE0 ROAD - ---- GRAVEL ROAD PAVED PARKING
GRAVEL P4RU ING
PAVED OR1 VEWAY
-.- FENCE - - GATE
.REE
'REE COVEF
ATHLETIC IiEL?
VAL L
SWIMMING POOL
BUILDING
UNIOENTIFIEO STRUCTURE
CONCRETE SL AB
S I GN
MANHOLE
~ORIZONTAL CONTROL
rOPZ.\YERT. CONTROL
INDEX CONTOUR
INTERPIEOIATE CONTOUR
OEPRESSION CONTOUR
OBSCURED CONTOUR
SPOT ELEVAl'ON
ELEC~RICALPOLE w\LIGHI
f.EclRiC1~ POLE
STREET ,:Lm'
DOLL
- WHITE MOUNTAIN DETENTION RESERVOIR #1 ME QUARTER. SECTION 33. TI%. R05W , SWEETWATER CO, WYO. BITTER CREEK TMUTARY FUXX) STUDY PHASE #
JOWWN-FERMEtlA CO, NC. CnNsuUlNG~Rs ARCHITECTS a SURVEYORS lSlS NINTH STET ROCK SPRINGS. WY e2301
0.1. or Phet~(r.ph,' 9-23-68 / 2' \'7 / \ / & \ 2' /' 4 4 \ 4/ 3 '7 I \ "L.0 O"b& II JOB NO. 2340-0388 -3-13 LEGEND
PAVED ROAC
GQAVEL QJAC
OAVEG P4Rh I NG
~RAVELPARKING
------. 'RAIL
"4rEC DR iEu43
.- cENCE - - iA1E
BRIDGE
DRAINAGE :INE
.REE
'REE COVER
AlHlEll~6 IELL
14, 1
SilMNlNG P30~
3UiLOlYu
JN~DEN~:f IF@ SIRIJCIURL
-3NCRE 'E Si A8
5 I GN
IANHOLL
*ORIZONlA: CONTROL
MORZ ~VEt". LONIR[!l
:NOEN CON'CJUR
INIERMEDIAIE LONTOUR
DEPRESSION CONlOUR
OBSCURE0 CONTOUR
SPOT ELEVATION
tLECTRICAL POLE Y\L lG*'
i,ECTRl;A~ POLL
SlREII .:be'
OOL E