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Harding Lawson Associates

TASK ITEM 19 ASSESMENT OF HYDROLOGIC ANDHYDROGEOLOGICAL CONDITIONS OF THE

SUNRISE MOUNTAIN LANDFILL

i. x>-lff" ( 2027694

Engineering and Environmental Services

TASK ITEM 19 ASSESMENT OF HYDROLOGIC ANDHYDROGEOLOG1CAL CONDITIONS OF THE


2027694

Republic DUMPCOSunrise Mountain Landfill

TASK ITEM 19 ASSESSMENT OF HYDROLOGIC ANDHYDROGEOLOGICAL CONDITIONS OF THE


Prepared By

Harding Lawson Associates5145 South Arville St. Suite A

Las Vegas, Nevada, 89118

Prepared ForRepublic DUMPCO, Inc.770 East Sahara Avenue

Las Vegas, Nevada 89104

TASK ITEM 19 ASSESSMENT OF HYDROLOGIC AND HYDROGEOLOGICALCONDITIONS OF THE SUNRISE MOUNTAIN LANDFILL

1.0 INTRODUCTION

1.1 Objectives

The ultimate objective for this task is tocomplete and submit for approval to the EPAa full report on the hydrologic andhydrogeological conditions of the SunriseMountain Landfill (SML), certified,stamped, and signed by a qualified andappropriately licensed professional engineeror hydrogeologist which provides sufficientinformation to support the following:

Reassessment and selection of site-specificdesign criteria for storm water events usingall available meteorological data, includingcapacity to handle the intense, short durationstorms which occurred at the Landfill from1995 to the present.

Identification of Best Management Practices(BMPs) necessary for compliance with theIndustrial Storm Water General Permit asoutlined in Paragraph 22 of the Order.Develop and submit for approval to EPA aStorm Water Pollution Prevention Plan(SWPPP).

A technical evaluation and analysis of slopeerosion stability for materials used orproposed to be used as part of any BMP orin the final cover at the Landfill, and

Characterization of the potential for leachatesurface seep discharge.

1.2 Background

Republic DUMPCO requested HardingLawson Associates (HLA) assist in theacquisition of data and the development ofwritten reports and plans to respond to theUS Environmental Protection Agency (EPA)requirements as stated in Item 19 of theFindings of Violation and Order ForCompliance, Docket No. CWA-309-9-99-14.

Additional data acquisition will be necessaryin order to complete the Item 19 tasksrequested by the EPA. This section describesthe data available and data gaps togetherwith the work to be accomplished to reassessexisting data, develop new data, furthercharacterize hydrologic and hydrogeologicsite conditions, reassess of hydrologic designcriteria, provide technical evaluations ofslope erosion stability, and identify BestManagement Practices (BMPs) necessary tocomply with Industrial Storm Water GeneralPermit requirements. The following sectionsdescribe information available, neededinformation and data, and general approachto conducting investigations necessary tocomply with Item 19 of the Order.

1.3 History Regarding Storm WaterHydraulic Design Criteria (Item 19a)

As described in the 1994 closure documentsfor the SML, the hydrological design criteriaagreed upon between DUMPCo, ClarkCounty Public Works Department (CCPW),Clark County Health Department (CCHD),US Bureau of Land


Task Itom19 Assessment of Hydrologic and Hydrogeological Conditions

Management (BLM), and the NevadaDivision of Environmental Protection(NDEP), resulted in a storm drain systemdesign to provide sufficient hydrauliccapacity for on site drainage systems to passa 25-year frequency storm event withoutappreciable damage of overtopping. At thetime of closure it was agreed that the bypasschannel located along the eastern boundaryof the site, was to be designed with sufficientcapacity to pass a 100-year flood withoutfreeboard capacity.

The hydrologic and hydraulic design wasbased upon the criteria set forth by the ClarkCounty Regional Flood Control District'sHydraulic Criteria and Drainage DesignManual. Design reviews were performed andapproved by the CCPW, its independentdrainage design review consultant,CCHealth, BLM, and the NDEP. Closure ofthe SML was based upon approval of criteriaset forth in the closure plan. (See SMLClosure Plan, April 14, 1994).

Since the time of closure, two known stormwater runoff events, one in late May 1995and a second on September 11, 1998produced flow volumes which exceededdesign capacity of various components of theSML drainage control system. During the1995 storm water event, impact was inflictedto the on-site drainage system, while thebypass channel remained intact as designed.The 1998 flood event impacted both the on-site system and the bypass channel.

A review of the September 11, 1998 floodevent by the Clark County Regional FloodControl District (CCRFCD) and the NationalWeather Service (NWS) resulted in theconclusion that this event was of significantnature. HLA's "Evaluation of the SMLSurface Water Management Plan Followingthe September 11, 1998 Storm", concludedthat the event exceeded the theoretical 100-

year frequency flood event at the subject site.This data is sufficient to answer questionsregarding the size and nature of the largestrunoff event to impact the SML since 1995.

Based upon the hydraulic design criteriautilized in the 1994 closure design, there is a4 percent chance that the capacity of the on-site storm water runoff system will beexceeded in any one year. Over the last 5years two events have been estimated to haveexceeded the 25-year flood frequency runoffevent. The 1994 bypass channel design wasbased upon a once in 100-years frequencystorm water discharge event. There is a 1-percent chance that the capacity of thebypass channel storm water runoff systemwill be exceeded in any one year. During the100 years following closure it was expectedthat, on the average, one annual runoff eventwould exceed the capacity of the bypasschannel. Over the last 5 years one event, theSeptember 11, 1998 flood event has beenestimated to have exceeded the 100-yearflood frequency runoff event.

The 1994 storm water drainage design forthe SML was based upon the best availablehydrologic criteria established by theCCRFCD and NWS and as applied by stormwater runoff frequency guidance given inCFR 258.6, at that time. On the basis ofactual runoff events occurring since the timeof closure, EPA has requested that additionalinformation evaluation of drainage riskfactors affecting the SML be gathered andevaluated to establish design criteria for longterm drainage protection.

A number of informational reports and datasources are available to assist therespondents in establishing final designcriteria at the SML. Presently availableinformation includes SML Closure Designand Closure


Task Iteml9 Assessment of Hydrologic and Hydrogeological Conditions

Plan, dated April 14, 1994; Code of FederalRegulations (40CFR258.6); Nevada RevisedStatutes (NRS); CCRFCD HydrologicCriteria and Drainage Design Manual(CCRFCD Manual) (see CCRFCD ManualContents, attached as Appendix A), RainfallEvent Report, September 11, 1998,CCRFCD; NWS, October 21, 1998 letterreport; NOAA hydrologic regressionequations; recent hydrologic reassessment ofon-site discharge values, completed byHarding Lawson Associates (See AppendixB), and other information. Of thesedocuments, the CCRFCD Manual establishesprocedures for estimating runoff volumesspecific to the Clark County, Nevada region.

Currently, CCPW is completing topographicmapping of the SML and FrenchmanMountain area. CCPW is also in the processof reevaluating watershed hydrologyupstream of the SML. Both items ofinformation are necessary to assist inestablishing runoff volume criteria for SMLhydraulic capacity design.

Ultimately, the selected hydrologic designcriteria for use in evaluation of the SMLdrainage system and application of additionalstorm water BMPs will be based upon aselection of an acceptable level of risk ofstorm water system capacity being exceeded.For every statistical annual return frequencyof runoff volume there is a chance that theselected storm volume will be exceeded.Therefore, the final design criteria for stormwater runoff will necessarily be a risk-basedselection process.

1.4 History of Best ManagementPractices for Compliance withIndustrial Storm Water GeneralPermit (Item 19b)

The 1995 closure plan for the SMLestablished a design and construction

approach for installing BMPs necessary tohandle storm water generated on-site and inwatersheds above the SML, for purposes ofprotecting the downstream areas fromcontamination created by uncontrolled stormwater discharge from the subject site. Basedupon historical events and damage to thesubject site, the EPA has requested re-evaluation of the previously approved stormwater Best Management Practices (BMPs) inItem 22 of the Order as follows:

"Development and submit for approval toEPA a Storm Water Pollution PreventionPlan (SWPPP) including all elements offacility information, Best ManagementPractices (BMPs), and periodic evaluationsas specified in the Industrial Storm WaterGeneral permit, including andimplementation schedule for BMPs withestimated costs. Upon approval, implementthe SWPPP in accordance with the scheduleset forth therein.

Acceptable BMPs shall include, but not belimited to:

Relocation of the main drainage channelaround the Landfill on native material andreconstruction of the existing main drainagechannel bottom and side slopes to isolate anyunavoidable contact with refuse, using non-erodible material where bedrock is absent;

2. A new sediment/flow-equalizing basinwith easy clean-out access prior to enteringthe main water drainage channel;

3. Other new drainage structures as requiredto divert storm water runon around the

Landfill, as well as structures to drain stormwater runoff from the Landfill, with sedimentbasins accessible for clean-out havingproperly engineered capacities and


Task Item19 Assessment of Hydrologic and Hydrogeological Conditions

inlet/outlet structures,dissipater devices.

including energy

4. Minimum 2% slope on the Landfill top,

5. Maximum 3:1 slope on side slopescombined with erosion control blankets orother equally effective erosion controls,

6. Stabilization of haul roads and unstablecover areas using gravel or other equallyeffective stability controls,

7. Reduction of long runoff distances acrossthe Landfill too less than 600 feet byincreasing the number of dikes/swales,benches, and/or drainage conveyances asnecessary,

8. Prevention of sediment buildup in drainageconveyances by using non-destructive clean-out methods, increasing the size and/ornumber, and/or by increasing the gradient,and

9. A debris control plan, including quarterlyperimeter inspections within at least 500yards of the lease boundary, and continuingfurther downstream as required to removevisible pocketed accumulations in anycontinuous wash, with collection and properdisposal of any accumulated waste.

Develop and submit for approval to the EPAa new inspection and maintenance plan,including ongoing surveys of BMPs, ofsettlement during dry weather periods, and oferosion immediately after it rains. This planshall include provisions to document timelycorrective actions taken.

Establish a SWPPP maintenance,modification, inspection, corrective action,and annual inspection documentation and

record keeping system. The system is to beutilized to maintain conformance with theSWPPP and compliance certifications. "

As per the National Pollutant DischargeElimination System (NPDES) provisions ofthe Clean Water Act, the EPA requires, asoutlined in the Order that a SWPPPdeveloped for the SML. The content of theSWPPP will follow the guidelines of theNDEP General Discharge Permit for Stormwater Discharges Associated with IndustrialActivity.

In order to establish an adequate SWPPP forapplication at the SML, more hydrologic andsite information is needed.

First, as outlined in Section 1.3 above,current storm water runoff conditions,capacity criteria, and storm water flowcapacity criteria need to be reevaluated,design criteria recommended and agreedupon by responsible parties in order toadequately prepare storm water controldesigns of sufficient capacity for the subjectsite.

Second, current topographic mapping wascompleted prior to the 1994 construction atthe SML. Both construction activities andpossible landfill settlement have modified sitetopography. Therefore, adequate newtopographic mapping, currently beingcompleted by the Clark County PublicWorks Department, needs to be evaluated toassess present day, on-site conditions.

Third, a recommendation is requiredregarding the placement of upstream stormwater detention alternatives. Currently,CCPW, through an independent consultant,is conducting a reassessment of the northeastcanyon hydrology and developing hydraulicdesign criteria for installation of a stormwater detention basin upstream of the SML.



This decision will impact the size andcapacity of the bypass channel. This newdesign criteria then needs to be completedand reviewed and appropriately applied toadequately prepare storm water controldesigns for the bypass channels of the subjectsite. Acquisition of the new northeast canyonhydrologic data is expected to be completewithin approximately 60 days.

Fourth, based upon information which willbe gathered, storm water flow designsmeeting BMP needs, must be completedwhich will meet the CCRFCD criteria whichis hereby incorporated by reference. FinalBMPs selected for incorporation into aSWPPP, and for implementation on site, mayvary from those initially selected as interimcontrol measures, based upon the outcome ofanalysis of the above defined additionalrequired site data.

1.5 History of Slope Erosion Stability(Item 19c)

Several past storm water runoff eventsoccurring since 1995 have caused sheet andrill erosion at several locations in the uppererosion layer of the landfill slopes. Erosionstability at the SML should be based uponminimizing the potential for sheet orconcentrated overland flow to generatesurface or rill erosion. The ability for surfacewater runoff to generate erosion is basedupon topographic conditions of the SML,rates of runoff for selected hydrologic designcriteria, the nature of the cover or channellining materials used, and the present slopeof the landfill or channels which conveyrunoff. At the SML, the drainage designcriteria selected and the BMPs chosen forimplementation will also affect theparameters impacting slope stability.

The majority of the existing data necessaryfor analysis of slope stability is outdated or isbeing updated. Therefore, new data must be

gathered pertaining to current topographicconditions, rates of runoff for selected stormwater hydrologic design criteria, the natureof the cover or channel lining materials to beused, the slope of the landfill or channelswhich convey runoff, and technical review ofrecommended Best Management Practices tobe applied at the SML must be evaluated andapplied to develop recommendations forcompleting drainage designs required toreduce landfill slope soil erosion at thesubject site.

Design methods presented in the CCRFCDManual, sheet flow erosion estimationtechniques utilized by the US NaturalResources Conservation Service (NRCS),and field analysis of erosion rill generationlengths experienced at the SML will be usedto determine necessary BMPs to controlslope erosion.

1.6 History of Characterization ofLeachate Surface Seep DischargePotential (Item 19d)

To date no information has been developedand little comprehensive data is available ofthe hydrogeologic condition of the SML. Inorder to understand the character andpotential for leachate to discharge to thesurface of the landfill, characterization of thehydrogeological condition of the SML site isnecessary. This data will be collectedfollowing the Work Plan submitted by SCSEngineers. Needed information to completethis effort is identified in RCRA-7003-09-99-005, Item IV.6.B.1.



2.0 APPROACH TO ADDITIONALSITE CHARACTERIZATION,AND AND HYDROLOGICAL /HYDRAULIC ANALYSIS OFTHE SML

2.1 Storm Water Discharge DesignData Gaps

To fill storm water discharge data gaps anddevelop storm water design the followingtask items:

Using new topographic mapping of thesubject site, development, review andsummarization of precipitation-depth-frequency statistical values for the 25-, 50-,100-, 200-year precipitation from theNortheast Canyon area will be conducted.Hydrologic information and detention basinrecommendations currently being gathered byCCPW must be reviewed for consistencywith present designs, or applicability andinclusion in new channel designs, if newdesigns appear necessary. Procedures anddata to be included in the study also includeinformation available in the CCRFCDManual, and the Federal EmergencyManagement Agency (FEMA) regionalregression equations. This information willbe summarized in tabular form in aTechnical Memorandum.

Runoff depths and volumes will bedetermined using the CCRFCD Manualprocedures and hydrologic estimationtechniques including US Army Corps ofEngineers Hydraulic Engineering Circular 1and its new predecessors, as well as othertechnically accepted procedures to determineflow capacity requirements for the variouson-site and bypass channels. Flow depthswill be determined for each precipitation-depth-frequency value determined as

impacted by flow from the Northeast Canyonarea.

Preliminary channel and storm waterconveyance alternatives and designs for eachrunoff depth will be determined usingprocedures established in the CCRFCDManual including the application of the USArmy Corps of Engineers HydraulicEngineering Circular 2 and its newpredecessor, HEC-RAS for determining flowdepths in the bypass channel. For each flowdepth evaluated, preliminary constructioncosts will be projected. A comparativesummary of precipitation-depth-frequencyversus runoff volume versus implementationcost will be prepared and included in atechnical memorandum.

For each storage and conveyance alternative,both protection and risk projected damagesover time will be developed for each runoff-depth-frequency value identified above.

A technical memorandum summarizing theprecipitation-depth-frequency values, runoffvolumes, detention, conveyance, andprotection costs, and potential risk of landfilldamage will be developed.

Recommendations will be made foracceptable levels of precipitation-depth-frequency versus risk criteria to be applied tothe SML storm water drainage systemdesign. A technical memorandum will bedeveloped summarizing the drainage criteriaselection process and recommendationsmade. This technical memorandum should bethe final work product satisfying EPA'srequest, under item 19a, for reassessmentand selection of site-specific design criteriafor storm water events using availablehydrological data including the capacity tohandle the intense, short duration stormswhich have occurred at the SML from 1995to present.



2.2 Best Management Practices forCompliance with Industrial StormWater General Permit Data Gaps

Task Items (based upon data gathered):

Identification and review of available BestManagement Practices (BMPs) for StormWater Management applicable to the SML.A listing of potential BMPs applicable to thesite will be derived from existing CCRFCDManuals and EPA Storm WaterManagement Handbooks.

A comparison of BMPs previously appliedas part of the 1994 closure plan for the SMLto historical function will be conducted. Thecomparison will propose BMP modificationsor installation of additional standard BMPs,which will improve storm water runoffcontrol necessary to meet the goals of aStorm Water Pollution Plan for the subjectSML. The comparative analysis will besummarized and recommendations definedfor BMP application at the subject site.

Preliminary construction costs will bedeveloped for each identified BMP.Approximate degree of benefit derived byimplementation of each BMP will bedetermined. A comparative summary ofBMP benefit versus implementation cost willbe developed. Where an identified BMPproves not to be applicable or beneficial tothe site a technical memorandum of theanalysis will be included with the benefitversus cost analysis.

Using new topographic mapping of the SMLcurrently being developed, location andextent of selected BMP implementation willbe shown on site maps for inclusion in theSWPPP. The BMP selection and summary ofbenefit versus cost with attached BMPapplication map should serve as the finalproduct for identification of BMPs necessary

for compliance with the Industrial StormWater General Permit at the SML.

2.3 Slope Erosion Stability Data Gaps

Task Items include:

Completion of this task is contingent on theavailability of new topographic mapping ofthe SML as currently being completed byCCPW, selection of hydrologic and hydrauliccriteria to be accomplished in item 2.1 above,and identification and selection of applicablestorm water control BMPs to be completedunder item 2.2 above. This first subtask willbe to gather this information and summarizethe data applicable to the Slope ErosionStability technical evaluation task. Reviewof site topography will be conducted toidentify potential locations where slopeerosion stability is to be evaluated. Atechnical memorandum describing applicableslopes and criteria for further evaluation willbe developed and submitted to EPA forapproval.

Additional site review of slope areas andmeasurement of erosion rill generationconditions will be conducted at slopelocations identified above. A technicalmemorandum report describing the findingsin the field will be developed and submittedto EPA.

Using analysis procedures recommended inthe CCRFCD Manual and the NRCS, sheetand rill erosion potential and estimatedannual volume of silt movement will bedetermined for present site conditions andsummarized in a technical memorandum tobe submitted to EPA for approval.

Sediment production estimates based uponimplementation of applicable BMPsnecessary for reducing erosion potential willbe determined. A technical memorandum will


Task Item 19 Assessment of Hydrologic and Hydrogeological Conditions

be prepared which compares before and afterBMP installation sediment production rateswill be developed. Recommendationsregarding refinement in slope erosion controlBMPs application and installation will bedeveloped and attached to the technicalmemorandum. The comparative informationand BMP installation recommendationsshould complete the technical evaluation andanalysis of slope erosion stability formaterials used or proposed to be used as partof any BMP or in the final cover at theLandfill.

2.4 Characterization of LeachateSurface Seep Discharge PotentialData Gaps

Task Items Include:

Characterization of the hydrogeologicalconditions of the SML site will be conductedby contractors following the work plandeveloped by SCS Engineers. The data willbe reviewed and evaluated utilizingapplicable information for characterization ofpotential leachate surface seep discharge. Atechnical memorandum summarizing the dataand describing how it applies to the potentialfor leachate surface seep discharge andinstallation of applicable BMPs necessary toeliminate the condition will be developed.Submission of this technical report to theEPA should satisfy conditions forcharacterization of the potential for leachatesurface seep discharge at the SML.

Appendix A

Clark County Regional Flood Control District

Hydrologic Criteria and Drainage Design Manual

Contents

HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL

GENERAL INDEX

GENERAL INDEX

AMENDMENTS AND REVISIONS

ACKNOWLEDGEMENTS

SECTION 100 - GENERAL PROVISIONS

SECTION 200 - DRAINAGE PLANNING AND SUBMITTAL

SECTION 300 - DRAINAGE POLICY

SECTION 400 - DRAINAGE LAW

SECTION 500 - RAINFALL

SECTION 600 - STORM RUNOFF

SECTION 700 - OPEN CHANNELS

SECTION 800 - STORM SEWER SYSTEMS

SECTION 900 - STREETS

SECTION 1000- CULVERTS AND BRIDGES

SECTION 1100 - ADDITIONAL HYDRAULIC STRUCTURES

SECTION 1200- DETENTION

SECTION 1300 - EROSION AND SEDIMENTATION

SECTION 1400 - DEVELOPMENT ON ALLUVIAL FANS

REFERENCES

STANDARD FORMS

10/90

CLARK COUNTY REGIONAL FLOOD CONTROL DISTRICTHYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL

SECTION 100GENERAL PROVISIONS

TABLE OF CONTENTS

Page

101 TITLE 101

102 ADOPTION AUTHORITY 101

103 JURISDICTION 101

104 PURPOSE 101

105 ENFORCEMENT RESPONSIBILITY 102

106 VARIANCE PROCEDURES 102

107 INTERPRETATION 102

108 REVIEW AND APPROVAL 103

109 IMPLEMENTATION 103109.1 - Development of the Manual 103109.2-Updates 103109.3-Adoption 104109.4 - Reconciliation of Pre- and Post-Manual Studies 104

110 ACRONYMS 105

111 GLOSSARY 106

10/90 GENERAL PROVISIONS 100


SECTION 200DRAINAGE PLANNING AND SUBMITTAL

TABLE OF CONTENTS

Page

201 SUBMITTAL AND REVIEW PROCESS 201

202 DRAINAGE STUDY INFORMATION FORM 202

203 CONCEPTUAL DRAINAGE STUDY 202203.1 - Letter Contents 203203.2-Drainage Plan 203

204 TECHNICAL DRAINAGE STUDY 204204.1 - Study Contents 204204.2 - Drainage Plan 208204.3 - Calculations Exemption 209

205 HYDROLOGIC/HYDRAULIC CALCULATIONS ADDENDUM 209

206 IMPROVEMENT PLANS 210

LIST OF TABLES

201 DRAINAGE STUDY SUBMITTAL REQUIREMENTS

10/90 DRAINAGE PLANNING AND SUBMITTAL 200

_j


SECTION 300DRAINAGE POLICY

TABLE OF CONTENTS

Page

301 INTRODUCTION 301

302 BASIC PRINCIPLES 302302.1 - Stormwater Drainage System 302302.2 - Multi-Purpose Resource 302302.3 - Water Rights 302302.4 - Jurisdictional Cooperation 302

303 REGIONAL AND LOCAL PLANNING 303303.1- Reasonable Use of Drainage 303

303.1.1- Increase in Rate of Flow 303303.1.2- Change in Manner of Flow 304303.1.3- Diversion of Drainage 304

303.2 - Regional Master Planning 304303.3 - Local Master Planning 305303.4 - Drainage Improvements 305303.5 - Drainage Planning Submittal and Review 306303.6 - Floodplain Management 307303.7 - Storm Runoff Detention 307303.8 - Storm Runoff Retention 308303.9 - Water Quality - 309303.10- Drainage Facilities Maintenance 309

304 TECHNICAL CRITERIA 310304.1 - Stormwater Management Technology 310304.2- Design Storm Events 310304.3- Storm Runoff Determination 311304.4 - Streets 311304.5 - Culverts and Bridges 314304.6 - Floodproofing 314304.7 - Alluvial Fans 315

3/23/90 - Final Draft 300


SECTION 400DRAINAGE LAW

TABLE OF CONTENTS


402 HISTORICAL EVOLUTION OF SURFACE WATER DRAINAGE LAW 401402.1- The Common Enemy Doctrine 402402.2- Civil Law Rule 402402.3 - Reasonable Use Rule 403

403 NEVADA DRAINAGE LAW 403403.1- Civil Law Rule 404403.2 - Reasonable Use Rule 405403.3 - Surface Waters - Private Development 408

403.3.1 - Negligence 408403.3.2- Breach of Express/Implied Warranty 409403.3.3 - Fraud/Misrepresentation 409403.3.4- Strict Liability 410403.3.5- Punitive Damages 410

403.4- Surface Waters - Governmental Entity Liability - 412403.4.1- Sovereign Immunity 412403.4.2- NRS 41.032-Discretionary Immunity 413403.4.3- NRS 41.033-Failure to Inspect 414403.4.4- Limitation of Tort Damage Awards 415403.4.5- Inverse Condemnation-Eminent Domain 415

10/90 DRAINAGE LAW • 400


SECTION 500RAINFALL

TABLE OF CONTENTS

Page


502 RAINFALL DEPTH-DURATION-FREQUENCY RELATIONS 501502.1- Rainfall Depth-Duration-Frequency Maps 501502.2 - Refinement of Values Obtained from Rainfall Maps 502502.3 - Rainfall Depths for Durations from One to Six Hours 502502.4- Adjustments to NOAA Atlas 2 503

503 DEPTH-AREA REDUCTION FACTORS 503

504 DESIGN STORMS 504504.1 - General 504504.2 - Six-Hour Design Storm Distribution 504

505 TIME-INTENSITY-FREQUENCY CURVES FOR RATIONAL METHOD 505505.1 - General 505505.2 - Time-Intensity-Frequency Curves 505

506 RAINFALL DATA FOR McCARRAN AIRPORT RAINFALL AREA 505506.1- General 505506.2 - Rainfall Depth-Duration-Frequency 505506.3 - Time-Intensity-Frequency Data 505

507 EXAMPLE APPLICATIONS 505507.1 - Introduction 505507.2 - Example: Six-Hour Design Storm Distribution 506507.3- Example: Time-Intensity-Frequency Curves 508

10/90 RAINFALL 500


LIST OF TABLES

501 PRECIPITATION ADJUSTMENT RATIOS502 SIX HOUR DEPTH-AREA REDUCTION FACTORS503 SIX-HOUR STORM DISTRIBUTIONS504 FACTORS FOR DURATIONS OF LESS THAN ONE-HOUR505 DEPTH-DURATION-FREQUENCY VALUES FOR McCARRAN AIRPORT

RAINFALL AREA506 TIME-INTENSITY-FREQUENCY VALUES FOR McCARRAN AIRPORT

RAIN FALL ARE A507 TIME-INTENSITY-FREQUENCY VALUES FOR EXAMPLE IN SECTION 507.3

LIST OF FIGURES

501 RAINFALL DEPTH-DURATION-FREQUENCY 2-YEAR 6-HOUR502 RAINFALL DEPTH-DURATION-FREQUENCY 5-YEAR 6-HOUR503 RAINFALL DEPTH-DURATION-FREQUENCY 10-YEAR 6-HOUR504 RAINFALL DEPTH-DURATION-FREQUENCY 25-YEAR 6-HOUR505 RAINFALL DEPTH-DURATION-FREQUENCY 50-YEAR 6-HOUR506 RAINFALL DEPTH-DURATION-FREQUENCY 100-YEAR 6-HOUR507 RAINFALL DEPTH-DURATION-FREQUENCY 2-YEAR 24-HOUR508 RAINFALL DEPTH-DURATION-FREQUENCY 5-YEAR 24-HOUR509 RAINFALL DEPTH-DURATION-FREQUENCY 10-YEAR 24-HOUR510 RAINFALL DEPTH-DURATION-FREQUENCY 25-YEAR 24-HOUR511 RAINFALL DEPTH-DURATION-FREQUENCY 50-YEAR 24-HOUR512 RAINFALL DEPTH-DURATION-FREQUENCY 100-YEAR 24-HOUR513 McCARRAN AIRPORT RAINFALL AREA514 DEPTH-AREA REDUCTION CURVES515 SIX-HOUR DESIGN STORM DISTRIBUTIONS516 DEPTH-DURATION-FREQUENCY CURVES FOR McCARRAN AIRPORT

RAINFALL AREA517 TIME-INTENSITY-FREQUENCY CURVES FOR McCARRAN AIRPORT

RAINFALL AREA . vf "518 HYPOTHETICAL BASIN FOR NON-URBAN, LARGE BASIN EXAMPLES519 HYPOTHETICAL BASIN FOR URBAN, SMALL BASIN EXAMPLES _ , j520 PRECIPITATION DEPTH VERSUS RETURN PERIOD FOR EXAMPLE IN

SECTION 507.2521 TIME-INTENSITY-FREQUENCY CURVE FOR EXAMPLE IN SECTION 507.3

10/90 RAINFALL


Table of Contents - continued

608 CHANNEL ROUTING OF HYDROGRAPHS 619608.1- Muskingum Method 619608.2- Kinematic Wave Method 621

609 RESERVOIR ROUTING OF HYDROGRAPHS 621609.1- Modified Puls Method 622

610 STATISTICAL ANALYSIS 622

611 EXAMPLE APPLICATIONS 624611.1- Example: Time of Concentration (Urban) 624611.2- Example: Rational Formula Method 625611.3- Example: SCS Unit Hydrograph Method 626611.4- Example: SCS TR-55 Graphical Peak Flow Method 628611.5- Example: Kinematic Wave Method 631

LIST OF TABLES

601 RATIONAL FORMUAL METHOD RUNOFF COEFFICIENTSAND AVERAGE PERCENT IMPERVIOUS AREA

602 RUNOFF CURVE NUMBERS

603 TABULAR HYDROGRAPH METHOD UNIT DISCHARGES

604 LAG EQUATION ROUGHNESS FACTORS

LIST OF FIGURES

601 OVERLAND TIME OF FLOW

602 TRAVEL TIME VELOCITY

10/90 STORM RUNOFF 601


SECTION 600STORM RUNOFF

TABLE OF CONTENTSPage

601 INTRODUCTION 603601.1- Basin Characteristics 603

602 TIME OF CONCENTRATION 603602.1- Urbanized Basins 605

603 PRECIPITATION LOSSES 606603.1 - Introduction 606603.2 - SCS Curve Number Method 607

603.2.1- CN Determination 608

604 RATIONAL FORMULA METHOD 609604.1 - Methodology 609604.2 - Assumptions 609604.3- Limitations on Methodology 610604.4- Rainfall Intensity 610604.5- Runoff Coefficient 610604.6 - Application of the Rational Formula Method 61 1604.7- Major Storm Analysis 611

605 SCS TR-55 METHOD 612605.1 - Methodology 612

605.1.1- Graphical Peak Discharge Method 612605.1.2- Tabular Hydrograph Method 613

605.2- Limitations on Methodology 614605.3- Basin/Sub-Basin Sizing 614

606 SCS UNIT HYDROGRAPH METHOD 615606.1 - Methodology 615606.2- Assumptions 616606.3- Lag Time 616

606.3.1- Roughness Factor 617606.4- Unit Storm Duration 617606.5- Sub-Basin Sizing 617

607 KINEMTATIC WAVE METHOD 618607.1- Basic Concepts 618607.2- Solution Procedure 619

10/90 STORM RUNOFF 600


SECTION 700OPEN CHANNELS

TABLE OF CONTENTSPage

701 INTRODUCTION . 704

702 OPEN CHANNEL HYDRAULICS 704702.1 - Uniform Flow 705702.2- Uniform Critical Flow Analysis 705702.3- Gradually Varying Flow 707702.4- Rapidly Varying Flow 707702.5 - Transitions 708

702.5.1 - Introduction 708

703 MAXIMUM PERMISSIBLE VELOCITIES 708

704 DESIGN SECTIONS AND STANDARDS FOR NATURAL CHANNELS 709704.1 - Introduction 709

704.1.1 - Natural Unencroached Channels 710704.1.2 - Natural Encroached Channels 710704.1.3 - Bank Lined Channels 710704.1.4 - Partially Lined Channels 710

704.2 - Natural Channel Systems 711704.2.1 - Natural Channel Morphology and Response 712

704.2.1.1 - Slope 712704.2.1.2 - Degradation and Aggradation 713704.2.1.3 - General Scour 714704.2.1.4 - Local Scour 715704.2.1.5 - Total Scour 716

704.2.2 - Design Considerations 716704.2.2.1 - Stable Channel 716704.2.2.2 - Design Discharge 717704.2.2.3 - Sediment Supply (Upstream Reach) 717704.2.2.4 - Bed Form Roughness 718704.2.2.5 - Erodable Sediment Size 718

704.2.3 - Data Requirements 719704.2.4 - Design Procedure 720704.2.5 - Floodplain Management of Natural Channels 721

705 DESIGN SECTIONS AND STANDARDS FOR IMPROVED CHANNELS 722705.1 - Introduction 722705.2 - Permanent Unlined Channels 722705.3 - Non-reinforced Vegetation Lined Channels 723

705.3.1 - Design Parameters 723705.3.1.1 - Longitudinal Channel Slopes 723705.3.1.2 - Roughness Coefficient 724

10/90 OPEN CHANNELS 700



705.4

705.5705.6

705.7

_-,- -

705.8

705.3. 1.3- Cross Sections705.3.1.4 - Low Flow Channel or Underground

Low Flow Storm Drain705.3.1.5 - Bottom Width705.3.1.6 - Flow Depth705.3.1.7 - Side Slopes705.3.1.8 - Vegetation Lining705.3.1.9 - Establishing Vegetation

- Riprap Lined Channels705.4.1 - Types of Riprap

705.4.1.1 - Loose Riprap705.4.1.2 - Grouted Riprap

705.4.2 - Riprap Material705.4.3 - Bedding Requirements

705.4.3.1 - Granular Bedding705.4.3.2 - Filter Fabrics

705.4.4 - Channel Linings705.4.5 - Roughness Coefficients705.4.6 - Rock Sizing and Lining Dimensions705.4.7 - Toe Protection

705.4.8 - Channel Bend Protection705.4.9 - Transitions Protection705.4.10- Concrete Cutoff Walls

- Gabion Lined Channels- Soil-Cement Lined Channels

705.6.1 - Soii-Cement705.6.1.1 - Portland Cement705.6.1.2 - Aggregate705.6.1.3 - Proprotioning

705.6.2 - Placement- Concrete Lined Channels

705.7.1 - Design Parameters705.7.1.1 - Concrete Lining Section705.7.1.2- Concrete Joints705.7.1.3 - Concrete Finish705.7.1.4- Concrete Curing705.7.1.5 - Reinforcement Steel "'705.7.1.6 - Earthwork - - ^705.7.1.7 - Bedding ' -'• f -705.7.1.8 - Underdrain and Weepholes705.7.1.9 - Roughness Coefficients705.7.1.10- Low Flow Channel705.7.1.11- Concrete Cutoffs * "'J '?v *»"'"-

705.7.2 - Special Considerations for Super-Critical • ••'«--Flow " ' ->-v. ">>#»"' rv.i,- •

- Other Channel Linings " • "̂ °* <K<rr., ->i .,

Page

724

724724724724724725725725725726726727727728729729729730730730731731731732732732732733733733733734734734734735735735735736736

736737

ff~ 10/90 OPEN CHANNELS 701



Page

706 HYDRAULIC DESIGN STANDARDS FOR IMPROVED CHANNELS 738706.1 - Sub-Critical Flow Design Standards 738

706.1.1 - Transitions 738706.1.1.1 - Transition Energy Loss 738706.1.1.2 - Transition Length 739

706.1.2 - Bends 740706.1.3 - Freebroad 740

706.2 - Super-Critical Flow Design Standards 740706.2.1 - Super-Critical Transitions 740

706.2.1.1 - Contracting Transitions 741706.2.1.2 - Expanding Transitions 743

706.2.2 - Bends 743706.2.3 - Circular Transition Curves 743706.2.4 - Freeboard 744706.2.5 - Super-elevation 744706.2.6 - Slug Flow 744

707 CHANNEL APPURTANCES 745707.1 - Low Flow Channel or Storm Drain 745707.2- Maintenance Access Road 745707.3 - Safety Requirements 745707.4 - Outlet Protection 746

707.4.1 - Configuration of Protection 746707.4.2 - Rock Size 746707.4.3 - Extent of Protection 747707.4.4 - Multiple Conduits 748

708 EXAMPLE APPLICATIONS . 749708.1 - Equilibrium Slope Calculation 749708.2 - Super-Critical Contracting Transition 752

LIST OF TABLES

701 GEOMETRIC ELEMENTS OF CHANNEL SECTIONS

702 MAXIMUM PERMISSIBLE MEAN CHANNEL VELOCITIES

703 CONSTRAINTS FOR SEDIMENT TRANSPSORT EQUATION

704 CHECKLIST OF DATA NEEDS FOR NATURAL CHANNEL ANALYSIS

705 SEDIMENT SIZE DISTRIBUTION FOR EXAMPLE IN SECTION 708.1

10/90 OPEN CHANNELS . 702


SECTION 800STORM SEWER SYSTEMS

TABLE OF CONTENTS


802

803

804

805

DESIGN802.1 -802.2 -802.3 -802.4 -

802.5 -

PARAMETERSAllowable Storm Sewer CapacityAllowable Storm Sewer VelocityManning's Roughness CoefficientStorm Sewer Layout802.4.1 - Vertical Alignment

802.4.1.1 - Minimum and Maximum Cover802.4.1.2 - Manhole and Junction Spacing

802.4.2 - Horizontal Alignment802.4.3 - Utility Clearances

802.4.3.1 - Water Mains802.4.3.2 - Sewer Mains

Allowable Storm Inlet Types and Capacity Factors

CONSTRUCTION STANDARDS803.1 -

803.2 -803.3 -803.4 -

STORM804.1 -

804.2 -

STORM805.1 -805.2 -805.3 -

Pipe803.1.1 - Size803.1.2 - Material and Shape803.1.3 - Joint Sealants and GasketsManholesStorm Sewer InletsStorm Sewer Outlets

SEWER HYDRAULICSHydraulic Analysis804. 1.1 - Pressure Flow Analysis804.1.2 - Partial Full Flow AnalysisEnergy Loss Calculations804.2.1 - Pipe Friction Losses804.2.2 - Pipe Form Losses

804.2.2.1 - Expansion Losses804.2.2.2 - Contraction Losses804.2.2.3 - Bend Losses804.2.2.4 - Junction and Manhole Losses804.2.2.5 - Inlet Losses804.2.2.6 - Outlet Losses

INLET HYDRAULICSInlets on Continuous GradeInlets in a Sump ConditionInlet Spacing

803803803804804805805805805806806806807

807807807807808808808808

809809809810810810811811811812812813813

813814814815

10/90 STORM SEWER SYSTEMS 800


Table of Contents (continued)

806 STORM SEWER SYSTEM DESIGN 815806.1 - Initial Storm Sewer Sizing 815806.2 - Final Storm Sewer Sizing 816

807 EXAMPLE APPLICATIONS 816807.1 - Introduction 816807.2 - Example: Storm Sewer Hydraulic Analysis 817

LIST OF TABLES

801 STORM SEWER DESIGN AND ANALYSIS PARAMETERS

802 ALLOWABLE STORM INLET TYPES AND CAPACITY FACTORS

803 STORM SEWER ENERGY LOSS COEFFICIENTS

804 HYDRAULIC CALCULATIONS FOR EXAMPLE IN SECTION 807.2

LIST OF FIGURES

801 HYDRAULIC PROPERTIES OF CIRCULAR PIPE

802 HYDRAULIC PROPERTIES OF HORIZONTAL ELLIPTICAL PIPE

803 HYDRAULIC PROPERTIES OF ARCH PIPE

804 ALLOWABLE INLET CAPACITY - CONTINUOUS GRADE CONDITION -TYPE A INLET-Y<1 FOOT

805 ALLOWABLE INLET CAPACITY - CONTINUOUS GRADE CONDITION -TYPE A INLET - Y > 1 FOOT

806 ALLOWABLE INLET CAPACITY - CONTINUOUS GRADE CONDITION -TYPE B AND TYPE C INLETS

807 ALLOWABLE INLET CAPACITY - SUMP CONDITION - TYPE A ANDTYPE B INLETS

-* •" -

808 ALLOWABLE INLET CAPACITY - SUMP CONDITION - BEEHIVE DROPINLET ^ -

809 ENERGY LOSS COEFFICIENT IN STRAIGHT THROUGH MANHOLE

810 STORM SEWER PLAN FOR EXAMPLE IN SECTION 807.2

811 STORM SEWER PROFILE FOR EXAMPLE IN SECTION 807.2

10/90 STORM SEWER SYSTEMS 801


SECTION 900STREETS

TABLE OF CONTENTS

Page


902 FUNCTION OF STREETS IN THE DRAINAGE SYSTEM 901

903 DRAINAGE IMPACTS ON STREETS 901903.1- Sheet Flow 902903.2- Gutter Flow 902903.3 - Storm Duration 903903.4 - Temporary Ponding 903903.5 - Cross Flow 903

904 DRAINAGE IMPACT ON STREET MAINTENANCE 903904.1- Pavement Deterioration 904904.2 - Sedimentation and Debris 904

905 STREET CLASSIFICATION AND ALLOWABLE FLOW DEPTH 905

906 HYDRAULIC EVALUATION 905

907 EXAMPLE APPLICATION 906907.1 - Introduction 906907.2 - Example: Allowable Flow in 100 Foot ROW Street 906

LIST OF FIGURES

901 STREET CAPACITY CURVES - 48 FOOT ROW901 STREET CAPACITY CURVES - 51 FOOT ROW903 STREET CAPACITY CURVES - 60 FOOT ROW904 STREET CAPACITY CURVES - 80 FOOT ROW905 STREET CAPACITY CURVES -100 FOOT ROW906 STREET CAPACITY CURVES -100 FOOT ROW WITH MEDIAN


SECTION 1000CULVERTS AND BRIDGES

TABLE OF CONTENTS


1002 DESIGN STANDARDS FOR CULVERTS 1002

1002.1- Culvert Sizing Criteria 10021002.11- Design Frequency 10021002.1 2 - Allowable Cross Street Flow 10021002.13- Minimum Size 1003

1002.2- Construction Materials 10031002.3 - Velocity Limitations and Outlet Protection 10031002.4- Headwater Criteria 10041002.5- Alignment 10041002.6- Temporary Crossing 10041002.7- Multiple Barrel Culverts 1005

1003 CULVERT HYDRAULICS 1005

1003.1- Inlet Control Condition 10051003.2 - Outlet Control Condition 10061003.3- Hydraulic Data 10071003.4- Inlet and Outlet Configuration 10081003.5- Structural Design 1008

1004 DESIGN STANDARDS FOR BRIDGES 1008

1004.1- Bridge Sizing Criteria 10081004.2- Velocity Limitations 1008

1005 BRIDGE HYDRAULICS 1009

1005.1- Hydraulic Analysis 10091005.2- Inlet and Outlet Configuration 1009

1006 EXAMPLE APPLICATION 4009

1006.1- Example: Culvert Sizing 1009

10/90 CULVERTS AND BRIDGES 1000


SECTION 1100ADDITIONAL HYDRAULIC STRUCTURES

TABLE OF CONTENTS


1102 CHANNEL DROPS AND ENERGY DISSIPATION STRUCTURES 1102

1102.1 - Channel Drop Structures 11031102.1.1 - Sloping Riprap Drop Structures 1103

1102.1.1.1 - Criteria 11041102.1.2 - Vertical Riprap Drop Structures 1105

1102.1.2.1 - Criteria 11061102.1.3 - Gabion Drops 1108

1102.1.3.1 - Design Criteria 11081102.1.4 - Straight Drop Spillways 11101102.1.5 - Baffled Aprons (USBR Type IX) 1110

1102.2 - Energy Dissipation Structures 1111

1102.2.1 - Types of Energy Dissipation Structures 11111102.2.2 - Stilling Basins with Horizontal Sloping Aprons 11111102.2.3 - Short Stilling Basin (USBR Type III) 11121102.2.4 - Low Froude Number Basins (USBR Type IV) 11121102.2.5 - Impact Stilling Basin (USBR Type VI) 11131102.2.6 - Hydraulic Design 11131102.2.7 - Riprap Protection 11131102.2.8 - Design Flow Rates 11141102.2.9 - Trajectory Transition Section 1114

1102.3 - Example Applications 1115

1102.3.1 - Example: Sloping Riprap Drop Structure 11151102.3.2 - Example: Vertical Riprap Drop Structure , 11161102.3.3 - Example: Vertical Gabion Drop Structure " 11161102.3.4 - Example: Impact Stilling Basin 1118

10/90 ADDITIONAL HYDRAULIC STRUCTURES 1100


SECTION 1200DETENTION

TABLE OF CONTENTS

1201 INTRODUCTION 12021201.1 - Definition of Regional Facilities 12021201.2- Definition of Local Facilities 1202

1201.2.1 - Local Minor Facilities 12021201.2.2 - Local Major Facilities 1203

1202 DETENTION DESIGN GUIDELINES AND STANDARDS 12031202.1 - Regional Detention 12031202.2- Local Detention 1204

1202.2.1 - Local Minor Detention 12041202.2.2 - Local Major Detention 1205

1203 HYDROLOGIC DESIGN METHODS AND CRITERIA 12061203.1 - Inflow Hydrograph 1206

1203.1.1 - HEC-1 Method 12061203.1.2 - Rational Method 1206

1203.2- Detention Basin Design Outflow Limitations 12071203.2.1 - Regional Facilities 12071203.2.2 - Local Facilities 1207

1203.3 - Hydrologic Calculation Methods 12071203.3.1 - HEC-1 Method 12071203.3.2 - Rational Method 1208

1204 HYDRAULIC CALCULATIONS 12081204.1 - Low Flow Outlets 1209

1204.1.1 - Minimum Conduit Size 12091204.1.2 - Flow Calculations 1209

1204.2 - Spillways 12101204.2.1 - Sizing Requirements 12101204.2.2 - Flow Calculations . 1211

1205 DEBRIS AND SEDIMENTATION 12111205.1 - Trash Racks 12111205.2 - Sedimenation 1212

1206 DESIGN STANDARDS AND CONSIDERATIONS ' 12121206.1- Dam Safety 12121206.2 - Grading Requirements 12121206.3 - Depth Limits 12121206.4- Trickle Flow and Basin Dewatering 12121206.5- Embankment Protection 12121206.6- Maintenance Requirements 12131206.7 - Local Detention Basin Siting Guidelines 1213

10/90 DETENTION 1200



Page

1207 WATER QUALITY 12131207.1 - EPA Regulations 1213

1208 EXAMPLE APPLICATIONS 12141208.1 - Example: Detention Pond Outlet Sizing 12141208.2- Example: Rational Formula Detention Pond 1215

LIST OF TABLES

1201 INFLOW HYDROGRAPH FOR EXAMPLE IN SECTION 1208.1

1202 HEC-1 RUN FOR EXAMPLE IN SECTION 1208.2

LIST OF FIGURES

1201 BASIN GEOMETRY

1202 V-NOTCH WIER COEFFICIENTS

1203 OGEE-CRESTED WIER COEFFICIENTS

1204 TRASH RACK EXAMPLES

1205 HYDROGRAPH FOR EXAMPLE IN SECTION 1208.2

10/90 DETENTION 1201


SECTION 1300EROSION AND SEDIMENTATION

TABLE OF CONTENTS

Page

1301 DEBRIS CONTROL STRUCTURES AND BASINS 13011301.1 - Introduction 13011301.2- Debris Deflectors 13021301.3 - Debris Racks 13021301.4- Debris Risers 13021301.5 - Debris Cribs 13021301.6- Debris Dams and Basins 13031301.7- Sizing of Control Structures and Basins 13031301.8- Siting of Control Structures and Basins 1304

1302 CONTROL OF EROSION FROM CONSTRUCTION ACTIVITIES 13041302.1 - Introduction 13041302.2 - References 13051302.3 - Erosion, Sediment, and Debris Control Plans 13051302.4 - Performance Standards 1308

LIST OF TABLES

1301 DEBRIS STRUCTURE PERFORMANCE

LIST OF FIGURES

1301 TYPICAL DEBRIS DEFLECTOR

1302 TYPICAL DEBRIS RACKS

1303 TYPICAL DEBRIS RISER

1304 TYPICAL DEBRIS CRIB

10/90 EROSION AND SEDIMENTATION 1300


SECTION 1400DEVELOPMENT ON ALLUVIAL FANS

TABLE OF CONTENTS

Page


1402 ANALYSIS REQUIREMENTS 1403

1403 PENINSULA DEVELOPMENT 1404

1404 ADDITIONAL CONSIDERATIONS 1404

1405 ALLUVIAL FAN FLOOD PROTECTION MEASURES 14051405.1 - Whole Fan Protection 14051405.2- Subdivision or Localized Protection 14051405.3- Single Lot or Structure Protection 1406

1406 EXAMPLE APPLICATION 14061406.1 - Introduction 14061406.2- Example Development 14061406.3 - Example Analysis 1407

LIST OF TABLES

1401 EXAMPLE DEPTH AND VELOCITY ZONE BOUNDARY DETERMINATIONS

LIST OF FIGURES

1401 TYPICAL PENINSULA DEVELOPMENT ON AN ALLUVIAL FAN

1402 EFFECTS OF TYPICAL PENINSULA DEVELOPMENT

1403 EXAMPLE VIRGIN FAN WITH FLOOD HAZARD ZONES DEFINED

- 1404 EXAMPLE IMPACT OF DEVELOPMENT ON FLOOD HAZARD ZONES

1405 IMPACT OF DEVELOPMENT POSITIONING ON FLOOD HAZARD ZONES

e.10/90 DEVELOPMENT ON ALLUVIAL FANS 1400

Appendix B

Sunrise Mountain Landfill

On Site Drainage System

Hydraulic Calculations


April 7, 1999

42725

Mr. Alan GaddyEnvironmental Technologies of Nevada, Inc.770 East Sahara AvenueLas Vegas, Nevada 89104

SUMMARY OF HYDROLOGIC ANALYSISSUNRISE MOUNTAIN LANDFILL

Dear Mr. Gaddy:

Harding Lawson Associates (HLA) is pleased to provide this summary for the hydrologic analysis performed foronsite conditions at the Sunrise Mountain Landfill. The analysis reflects changes to the drainage pattern as aconsequence of the flood damages.

The design event is the 25-year frequency as established using Clark County Regional Flood Control District(CCRFCD) criteria. The watershed boundaries are shown on the enclosed site plan. Watershed areas are scaledapproximations. The time of concentration for most subbasins is assumed as 10 minutes. The losses are basedupon an SCS curve number of 87, as used in previous analyses. The results are provided in the attached HEC-1model output.

Ditch

Ditch 1Ditch 2Ditch 3Ditch 4Ditch 5Ditch 6Ditch 7Ditch 8Ditch 9Ditch 10Ditch 11Ditch 12Ditch 13

025

l lcfs32 cfs49 cfs18 cfs8 cfs

19 cfs23 cfs38 cfs22 cfs24 cfs59 cfs12 cfs79 cfs

Engineering andEnvironmental Services 5145South Arville Street, Suite A, Las Vegas, NV 891 18 702/251-5449 Fax: 701/251-3148

April 7, 1999Mr. Alan GaddyETNPage 2


HLA appreciates being of continued service to Environmental Technologies. If you have any questions, or needfurther information, please do not hesitate to call 702/251-5449.

Sincerely,

HARDING LAWSON ASSOCLYTES

Gerry A. Hester, P.E.Principal Engineer

Attachment

FLOOD HYDRCGRAPH ?AC<AGE (HEC-1) *

SEPTEMBER '?YJ *

VESSICM -.: *It

_ RLN DATE 01/07/199= TIME 10:55:59 •

U.S. ARMY CORPS OF ENGINEERS

HYDRCLCG1C ENGINEERING CENTER

iC9 SECOMD STREET

DAv!S, CALIFORNIA 95616

(916) 756-1104

X X XXXXXXX XXXXX

X X X X X

X X X X

XXXXXXX XXXX X

X X X X

X X X " X X

X X XXXXXXX XXXXX

XXXXX

X

XX

X

X

X

X

XXX

THIS PROGRAM REPLACES ALL PREVIOUS VERSIONS OF HEC-1 <NCWN AS HEC1 (JAN 73), HEC1GS, HEC103, AND HEC1KW.

THE DEFINITIONS OF VARIABLES -RTIMP- AND -RTIOR- HAVE CHANGED FROM THOSE USED WITH THE 1973-STYLE INPUT STRUCTURE.

THE DEFINITION OF -AMSKK- ON RM-CARO WAS CHANGED UITri REVISIONS DATED 28 SEP 81. THIS IS THE FORTRAM77 VERSION

NEW OPTIONS: OAHBREAK OUTFLOW SUBMERGENCE , SINGLE EVENT DAMAGE CALCULATION, DSS:WRITE STAGE FREQUENCY,

DSSrREAD TIME SERIES AT DESIRED CALCULATION INTERVAL LOSS RATE:GREEN AND AMPT INFILTRATION

KINEMATIC WAVE: NEW FINITE DIFFERENCE ALGORITHM

HEC-1 INPUT PAGE

LINE

1

23

456

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

ID

ID

ID

SUNRISE LAVDriLL

25 YEAR EVENT

CNSITE CONDITIONS

"DIAGRAM

IT

IO

?G

IN

?I

PI

PI

PI

PI

PI

PI

PI

KK

BA

PR

LS

UD

KK

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PR

LS

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KK

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PRLS

UD

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PR

LS

UD

KK

BA

PR

LS

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5 2334 1

1 2.53

5

.02 .037 .013 .017 .021

0 0 .003 .007 .002

.007 .002 .001 .001 .003

.005 .014 .008 .003 .002

.091 .12 .034 .037 .031

.008 .008 .012 ".022 .016

.003 .002 .002 .001 .003

.001 .001 : • '

AREA1

.0092

1

87

0.1

AREA2

.0321

87

0.2

ARE A3

.0491

87

0.2

AREA4

.01441

87.1

AREAS.0064

1

87

0.1

AREA7

.0141

87

0.15

.016 .006 0 0 0

.006 .01 .014 .009 .009

.01 .015 .012 .008 .002

.012 .027 .03 .057 .09

.007 .0:6 .016 .005 .004

.011 .013 .02 .006 .006

0 .001 .001 .003 0

HEC-1 UFUT PAGE 2

LINE !D 1 2 3 4 5 6 7 8 9 10

4647

48

49

50

51

52

5354

55

~ 56

57

— 585960

_ 61

62

63

64

65

— 6667

68

69

70

71

~~ 7273

— 7475

_- 76777879

~~ 80

81

— 828384

_ 8586

«

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0.1

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1

.1

891013

3

AREA11

.063

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.25

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87

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87

SCHEMATIC DIAGRAM CF STREAM vjjTU'O

NPUT

— • -IE

NO.

16

21

— 26

_ 31

36

41

46

43

53

57

~~ 62

— 64

_ 69

(V) ROUTING

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( - - - > ) DIVERSIC'i OR PUMP fLCW

( < - - - ) RETL'R'v :,- DIVERTED CR ? ?LCU

AREA2

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81

*) RUNOFF ALSO COMPUTED AT THIS LOCATION

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891013.

AREA11

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AREA 12

F.3CO HYDROGRAPH PAC<AGE (HEC-1)

SEPTEMBER 1990

VERSION 4 0

RL.N 3ATE 01/07/1999 TIME 10:55.59

U.S. ARMY CORPS OF ENGINEERS

HYOROLOG1C ENGINEERING CENTER

609 SECOND STREET

DAVIS, C A L I F O R N I A 953*6

(916) 756-1104

SUNRISE LANC- ..

25 YEAR EVENT

ONSITE CCSO'T CNS

5 10 OUTPUT CONTROL VARIABLES

IPRNT 4 =R\T CONTROL

[PLOT 1 ?LC- CONTROL

QSCAL o. HV;RCGRAPH PLOT SCALE

7 IN TIME DATA FOR INPUT T I M E SERIES

JXMIN 5 T".1-': 'NTERVAL IN MINUTES

JXDATE 1 0 S'iR~:vIG DATE

JXTIME 0 STARTING TIME

IT HYDROGRAPH TIME DATANMIN 5 MIMJTES IN COMPUTATION INTERVALIDATE 1 0 STARTING DATEITIHE 0000 STARTING TIME

NQ 288 NUMBER OF HYDROGRAPH ORDINATESNDDATE 1 0 E\DING DATENDTIME 2355 ENDING TIMEICENT 19 CENTURY MARK

COMPUTATION INTERVAL .08 HOURSTOTAL TIME BASE 23.92 HOURS

ENGLISH UNITS

DRAINAGE AREA SQUARE MILES

PRECIPITATION DEPTH INCHES

LENGTH, ELEVATION FEET

FLOW

STORAGE VOLUME

SURFACE AREA

TEMPERATURE

CUBIC FEET PER SECOND

ACRE-FEET

ACRES

DEGREES FAHRENHEIT

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

**************

* *16 KK * AREA1 *

**************

SUBBASIN RL.NOFF DATA

17 BA SUBBAS:v C H A R A C T E R I S T I C S

TAREA .01 SU33ASIN AREA

PRECIP ITATION D A T A

18 PR

0 FU

—19 LS

RECORDING STATIONS

HEIGHTS 1.00

SCS LOSS RATE

STRTL

C.R'ASR

RTIMP

.30 INITIAL ABSTRACTION

87.00 CURVE NUMBER

.00 PERCENT IMPERVIOUS A.REA

20 UC SCS DI^ENSIONLESS UNITGRAPH

TLAG .10 LAG

PRECIPITATION STATION DATA

STATION TOTAL

1 2.53

AVG. ANNUAL

.00

WEIGHT

1.CO

TEMPORAL DISTRIBUTIONS

STATION

.02

.00

.01

.00

.09

.01

.00

.00

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.00

.01

.03

.01

.00

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.01

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.00

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.02

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.02

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.01

.01

.06

.00

.01

.00

.00

.01

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.09

.00

.01

.00

20. 30. 13.

UNIT HYDROGRAPH

8 END-OF-PERIOD ORDINATES

5. 2. 1. 0.

* *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

21 KK

**************

* *

* AREA2 *

* *

**************

SUBBASIN RUNOFF DATA

BA SUBBASIN CHARACTERISTICS

TAREA .03 SUBBASIN AREA

23 PR

PRECIPITATION DATA

RECORDING STATIONS 1

WEIGHTS l . C O

SCS LOSS RATE

S'R'L

CRV.5R

.30 : S : * ; A L ABSTRACTION

87.00 C-R'.E NUMBER.00 ?=RCE\T [I'PERVICL'S AREA

25 SCS DIMENSIONLESS UNITCRAPi

TLAG .20 LA3


STATION TOTAL

1 2.53

VJG. ANNUAL

.00

WEIGHT

1.00

TEMPORAL -DISTRIBUTIONS

STATION

.02

.CO

.01

.CO

.09

.01

.00

.00

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.CO

.01

.03

.01

.CO

.02

.01

.00

.00

.04

.02

.00

.02

.00

.00

.00

.03

.02

.00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

UNIT HYDROGRAPH

16. 51.

1. 1.

64.

0.

51.

0.

14 END-OF-PESIOD

28. 16.

ORDINATES

9. 5. 3. 2.

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** **•* *** *** *** ***

26 KK

**************

* ** AREA3 *

* *

**************


27 BA SUBBASIN CHARACTERISTICS


PRECIPITATION DATA

28 PR0 PU


WEIGHTS 1.00

~9 LS SCS LOSS RATE

STRTL

CRVHBRRTIHP

.30 INITIAL ABSTRACTION87.00 CURVE NUMBER

.00 PERCENT IMPERVIOUS AREA

•>»>5-'̂ iJ J*J-VC ""

30 UD SCS DIME'iSiONLESS UNITGRAPH

TLAG .20 LAG

PRECIP ITATION STATION DATA

S T A T I O N TOTAL AVG. ANNUAL WEIGHT

1 2.53 .00 1.CO


STATION

.02

.CO

.01

.00

.09

.0*

.CO

-CO

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

' .04

.02

.00

.02

.00

.CO

.00

.03

.02

• .00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.Co

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

24.

1.

79.

1.

98.

1.

78.

0.

UNIT HYORCGRAPH

14 ENO-OF-PERICD ORDINATES

43. 25. 14.

31 KK

**************

* ** AREA4 *

**************


—32 BA SUBBASIN CHARACTERISTICS


PRECIPITATION DATA

33 PR

0 PW


WEIGHTS 1.00

34 LS SCS LOSS RATE

STRTL

CRVNBR

RTIMP


87.00 CURVE NUMBER


_35 UD SCS DJHENSIONLESS UNITGRAPH

TLAG .10 LAG


STATION

1

TOTAL

2.53

AVG ANNUAL

.00

WEICJ .T

1.CO

TEMPORAL D ISTRIBUTIONS

STATION

.C2

.CO

01

.CO

.09

.01

.00

.00

1, '-itIGH" = 1.0004

.00

.00

.01

.12

.01

.00

.00

.O1

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

.04

.02

.00

.32

.CO

.00

.00

.03

.02

.DO

.02

.01

.01

.01

.01

.01

.00

01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

32. 47. 21.

UNIT HYDROGRAPH


" 3. 1. 0.

36 « AREAS




PRECIPITATION DATA

38 PR0 PU


WEIGHTS 1.00

39 LS SCS LOSS RATE

STRTL

CRVNBR

RTIHP


87.00 CURVE NUMBER


40 UD SCS DIMENSIONLESS UNITGRAPH

TLAG .10 LAG


STATION TOTAL AVG. ANNUAL WEIGHT

1 2.53 .00 1.00


STATION 1, WEIGHT = 1.00

.02 .04 .01 .02 .02 .02 .01 .00 .00 .00

.00 .00 .00 .01 .00 .01 .01 .01 .01 .01

30

00

01

12

01

00

.00

.0'

.03

.3'

.03

.00

.00

.04

.02

00

.CO

.00

.03

.02

.00

.01

.01

.01

.01

.00

.02

.03

.02

.01

.00

.01

.03

.02

.02

.00

.01

.06

.00

.01

.00

.00

.09

.00

.01

.CO

14. 21.

uNIT ^CRCGRAPH

8 END-OF--ERIOD OROINATES

1. 1. 0.

41 « AREA7


42 3A SUBBASIN' CHARACTERISTICS

TAREA .01 SU33A3IN AREA

PRECIPITATION DATA

5 PR

0 PW


WEIGHTS 1.00

44 LS SCS LOSS RATE

STRTL

CRVNBR

RTIMP


87.00 CURVE NUMBER



TLAG .15 LAG


STATION TOTAL

1 2.53

AVG. ANNUAL

.00

WEIGHT

1.00


STATION

.02

.00

.01

.00

.09

.01

.00

.00

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

.04

.02

.00

.02

.00

.00

.00

.03

.02

.00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

UNIT HYDROGRAPH

9^-*v^tt-Ti&5s-^a*sSi.

11 ENO-OF-PERICD ORDINATES

13. 35 30. 15. 8. 4. 2. 1. 0. 0.

0.

46 K< * 5AND7

47 HC HYDRCCRAPH COMBINATION-:C\1P 2 NLM3ER OF HYDROGRAPHS TO COMBINE

48 <K * AREA6 *

* *

**************


_49 BA SUBBASIN CHARACTERISTICS


PRECIPITATION DATA

50 PR RECORDING STATIONS 1

0 PW WEIGHTS 1.00

51 LS SCS LOSS RATE

STRTL .30 INITIAL ABSTRACTION

_ CRVNBR 87.00 CURVE NUMBER

RTIMP .00 PERCENT IMPERVIOUS AREA


~ TLAG .10 LAG



1 2.53 .00 1.00



.02 .04 .01 .02 .02 .02 .01 .00 .00 .00

j^^

CO

,01

CO

C9

01

CO

CO

.00

.00

.01

.12

.01

.CO

.00

.CO

.00

.01

.03

.01

.GO

.01

.00

.00

.04

.02

.00

.00

.CO

.00

.33

.32

.:o

.01

.01

.01

.01

.01

.00

.01

.02

.03

.02

.01

.00

.01

.01

.03

.02

.02

.00

.01

.01

.06

.00

.01

.00

.01

.00

.C9

.00

.01

.00

35. 52. 23.

UNIT -iYDROGRAPH

8 END-OF-=ERICD ORDINATES3. i. 1.

S3 <K AREAS

**************

SUBBASIN RLN'OFF DATA

54 SA SUBBASIN CHARACTERISTICS

TAREA .01 SU3SASIN AREA

PRECIPITATION DATA

0 PT

0 PW

TOTAL STORM STATIONS 1

WEIGHTS 1.00

50 PR

~ 0 PW


WEIGHTS 1.00

55 LS SCS LOSS RATE

STRTL

CRVNBR

RTIMP


87.00 CURVE NUMBER



TLAG .10 LAG


STATION TOTAL

1 2.53

AVG. ANNUAL

.00

WEIGHT

1.00


STATION

.02

.00

.01

.00

.09

.01

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.01

.00

.00

.01

.03

.01

.02

.01

.00

.00

.04

.02

.02

.00

.00

.00

.03

.02

.02

.01

.01

.01

.01

.01

.01

.01

.02

.03

.02

.01

.00

.01

.01

.03

.02

.02

.00

.01

.01

.06

.00

.01

.00

.01

.00

.09

.00

.01

^^^^^^^^

.CO

.CO.00

.00

.00 .00 .00 .00 .00 .00 .00

28. 19.

UNIT ifORCGRAPH

8 END-OF-'ERiOO ORDINATES

3. 1. 0.

57 « AREA9

**************

SUBBASIN RL10FF DATA

58 3A SUBBASIN CHARACTERISTICS

TAREA .02 SU53ASIN AREA

PRECIPITATION DATA

59 PR

0 PW


WEIGHTS l.CO

<>0 LS SCS LOSS RATE

STRTL

CRVNBR

RTIMP


87.00 CURVE NUMBER


—51 UD SCS DIMENSIONLESS UNITGRAPH

TLAG .10 LAG



1 2.53 .00 1.00


STATION

.02

.00

.01

.00

.09

.01

.00

.00

1, WEIGHT = 1.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

.04

.02

.00

.02

.00

.00

.00

.03

.02

.00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

39. 58. 26. 10.

UNIT HYDROGRAPH


4. 1. 1.

,2 <K a A NO;

o3 riC HY3RCGRA = -i COMBINATION

ICC.'-1? 2 NUMSER OF HYDRCGRAPKS TO COMBINE

KK AREA 10



TAREA .02 SUB3ASIN AREA

PRECIPITATION DATA

J6 PR

0 PW


WEIGHTS 1.00

67 LS SCS LOSS RATE

STRTL

CRVN8R

RTIMP


87.00 CURVE NUMBER



TLAG .10 LAG



1 2.53 .00 1.00


STATION

.02

.00

.01

.00

.09

.01

1, WEIGHT = 1.00.04

.00

.00

.01

.12

.01

.01

.00

.00

.01

.03

.01

.02

.01

.00

.00

.04

.02

.02

.00

.00

.00

.03

.02

.02

.01

.01

.01

.01

.01

.01

.01

.02

.03

.02

.01

.00

.01

.01

.03

.02

.02

.00

.01

.01

.06

.00

.01

.00

.01

.00

.09

.00

.01

l"£~"-'i'r''-i-'%^£: -a c. r •••"• •-

,00 .00 .00 .00 .00 .00 .00 .00

U1IT -ifOROGRAPH

3 END-CF-=ERICD ORDINATES

28. 11. 4. 2. 1.

STTCS

.01 SUS3ASIN AREA

)NS 1

ITS 1.00

INITIAL ABSTRACTION

37.TO CURVE NUMBER.00 PERCENT IMPERVIOUS AREA

JNI_RAPH.10 LAG

TOT-m: AVG. ANNUAL WEIGHT

2.53 -00 1.00

WE 1HT = 1.00

14

lO ~

10

11

i211

10

10 _

.01

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

.04

.02

.00

.02

.00

.00

.00

.03

.02

.00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

.00 .00

UNIT HYDROGRAPH .01 -01

8 END-OF-PERIOD ORDINATES .01 .00

207 8. 3. 1. 0. 0. .06 .09.00 .00

.01 .01

.00 .00 .00 .00 .00 .00 .00 .00 .00

.00

LNIT HfDRCGRAPH

17 END-O: ==R;CD OROINATES18. o3 101 '01. 78. 47. 29. 19. 12. 7.5. 3 2. 1. 1. 0. 0.

31 « * AREA12 ** *

~ **************


82 3A SUBBASIN CHARACTERISTICS

TAREA .01 SL33ASIN AREA

PRECIPITATION DATA

83 PR RECORDING STATIONS 1

~ 0 PW WEIGHTS 1.00

'» LS SCS LOSS RATE

STRTL .30 INITIAL ABSTRACTION

CRVN3R 87.00 CURVE NUMBER

RTIMP .00 PERCENT IMPERVIOUS AREA


TLAG .10 LAG



1 2.53 .00 1.00



.02

.00

.01

.00

.09

.01

.00

.00

.04

.00

.00

.01

.12

.01

.00

.00

.01

.00

.00

.01

.03

.01

.00

.02

.01

.00

.00

.04

.02

.00

.02

.00

.00

.00

.03

.02

.00

.02

.01

.01

.01

.01

.01

.00

.01

.01

.02

.03

.02

.01

.00

.00

.01

.01

.03

.02

.02

.00

.00

.01

.01

.06

.00

.01

.00

.00

.01

.00

.09

.00

.01

.00

UNIT HYDROGRAPH


22. 32. 14. 6. 2. 1. 0.

RbNC" SUMMARY

FLOW IN CUBIC =EET PER SECOND

TIME !,N HCL-RS, AREA IN SGUARE MILES

OPERATION

HYDRCG.RAPH *T

HYDROGRAPH AT

HYOROGRAPH AT

HYOROGRAPH AT

HYDRCGRAPH AT

HYDRCGRAPH AT

2 COMBINED AT

HYDROGRAPH AT

HYDROGRAPH AT

HYDROGRAPH AT

2 COMBINED AT

HYDROGRAPH AT

HYDROGRAPH AT

3 COMBINED AT

HYDROGRAPH AT

HYDROGRAPH AT

S" AT ISM

A.REAl

AREA2

AREA3

AREA4

AREAS

AREA7

5AND7

AREA6

AREAS

AREA9

8AND9

AREA 10

AREA13

891013

AREA11

AREA12

PEA<

FLCU

11.

32.

49.

18.

8.

16.

23.

19.

16.

22.

38.

24.

17.

79.

59.

12.

TIME 0?=EA'<

3.50

3.58

3.58

3.50

3.50

3.58

3.53

3.50

3.50

3.50

3.50

3.50

3.50

3.50

3.67

3.50

AVERAGE

i-̂ iC'.R

: _

5.

7.

2.

1.

2.

3.

2.

2.

3.

4.

3.

2.

9.

9.

1.

=LCW FCS MAXIMUM

24-HCUR

0.

1 .

2.

1.

0.

1.

1.

1.

0.

1.

1.

1.

1.

2.

2.

0.

PER [CD

72-HCUR

0.

1.

2.

1 .

0.

1.

1.

1.

0.

1.

1.

1.

1.

2.

'2.

0.

BASIN

AREA

.01

.03

.05

.01

.01

.01

.02

.02

.01

.02

.03

.02

.01

.06

.06

.01

MAXtMUM TIME 05

STAGE MAX STAGS

*** NORMAL END OF HEC-1 ***

PART 2

Appendix C

An Evaluation of the

Sunrise Mountain Landfill

Surface Water Management Plan

Following The September 11, 1998 Storm

AN EVALUATION OF THESUNRISE MOUNTAIN LANDFILLSURFACE WATER MANAGEMENT PLANFOLLOWING THE SEPTEMBER 11,1998 STORM

Prepared for

Republic Silver State Disposal

HLAJobNo 42725

Prepared by

Gerry A. Hester, P.Managing Principal

Jos$hH.MiIazzo,P.E/Senior Engineer

October 22,1998

Harding Lawson AssociatesInfrastructure Inc.

Engineering, Planning & Construction Services5145 South Arville Street, Suite ALas Vegas, Nevada 89118 — (702) 251-5449

-*•- r _ •< ** <t f

j^A^;,.J~''ys. -

' •' v -„•* - vX'-y^^t^M^. _2-̂ : ̂ -_ • _

Evaluation of the Sunrise Mountain LandfillSurface Water Management Plan

Following the September 11,1998 Storm

1.0 INTRODUCTION

The following report presents a preliminary evaluationof the design of the storm water drainage systemimplemented as part of the closure plan at the SunriseMountain Landfill (SML), located east of the LasVegas Valley, in Clark County, Nevada. Thisdrainage system received high flood flows during aheavy thunderstorm rainfall-runoff event occurring inClark County on September 11, 1998. Following theevent, the Nevada Division of EnvironmentalProtection (NDEP) and the Clark County HealthDistrict (CCHD) ordered the Clark County PublicWorks Department (CCPW) and Republic Silver StateDisposal (RSSD) to present an evaluation of thedrainage system design prior to reconstructing thesystem.

In response to the order, RSSD retained HardingLawson Associates (HLA) on October 12, 1998 toperform the subject evaluation. Specifically HLA'sresponsibilities are to evaluate:

• flood event occurring on September 11, 1998.• adequacy of design and performance of the major

bypass channel located along the easternboundary of the SML;

• performance and condition of landfill surfacerunoff collection and conveyance system;

• erosion on the face of the landfill;• proposed erosion protection on the north face of

the construction debris cell located on thenorthern boundary of the subject site.

Following submittal and approval of the evaluationreport HLA is to develop final designrecommendations for reconstruction of the drainagesystem at the SML.

2.0 DOCUMENT REVIEW

Documents reviewed by HLA during the course of thedesign review included:s

• SML closure design, and SML Closure Plan datedApril 14,1994;

• Code of Federal Regulations, Part 258.6;• • Nevada Revised Statutes Section 444;• "Rainfall Event Report, September 11, 1998"

Clark County Regional Flood Control District(RFCD);

• "Hydrologic Criteria and Drainage DesignManual, 1990, Clark County Regional FloodControl District;

• "Finding of Violation and Order", October 5,1998, Nevada Division of EnvironmentalProtection, issued to Clark County Public WorksDepartment and Republic Silver State Disposal;

• "Corrective Action Order", October 8, 1998,Clark County Health District, issued to ClarkCounty Public Works Department, RepublicSilver State Disposal and U. S. Bureau of LandManagement.

3.0 SITE REVIEW AND FIELD SURVEY

HLA personnel initially toured the SML site,following the September llth flood event, onSeptember 16, 1998. Present at that tour were Mr.Gerry Hester. P.E., HLA and Mr. Alan Gaddy ofRSSD, Environmental Technologies Division. At thattime RSSD personnel and equipment operations wereunderway to repair flood damages within the bypasschannel, on the landfill face, and at a number of localdrainage system locations. High water marks in thebypass channel and at the entrance to the channel wereobserved at that time and noted for future reference.

HLA personnel, Mr. Gerry Hester, P.E. and Mr. ScottSmith, P.E., Ph.D., toured the site on September 30,1998 to observe ongoing RSSD construction activity,photograph various runoff damage and high watermarks, and discuss potential corrective actions.Following the on-site visit these two HLA personneltoured the Las Vegas Wash at Pabco Road and at theSouthern Nevada Water Authority (SNWA) pipelinecrossing to observe and evaluate flood and scourdamage to the Las Vegas Wash downstream of thesouthwest Frenchman Mountain drainages.

HLA's onsite observations and verbal discussionswith RSSD staff indicated the following damagesoccurred due to flooding at the SML:

• Bypass Channel: Severe riprap, liner, channelbedding displacement and refuse scour,throughout the length of the channel; rock gabiondrop structure scour and headwall movement;large diameter riprap displacement and landfillslope erosion along the westerly bank of thechannel downstream of the rock cut; downstreamscour and deposition of channel bed materialsbelow the channel outfall.

Harding Lawson Associates Pagel



• Onsite Drainage System: Culvert entrancestructure and adjacent levee erosion and scour,various locations; rip-rap channel downdrainscour and liner displacement, various locations;lateral erosion outside of half round culvertchannel sections, various locations; and sedimentdeposition in half-round culverts at points ofchannel slope change or in reaches of relativelyflat slope, various locations.

• Landfill Slopes: Rill erosion western facingslopes, various locations; shallow sheet and rillerosion, eastern facing slopes adjacent to bypasschannel, slope toe erosion at the constructiondebris cell (described to HLA by RSSD), andsevere bank erosion,

• Landfill Cap: Minor sheet erosion or runoffponding observed in areas of designed landfill capplacement; landfill surface was observed to havesettled in various locations over the last severalyears. Where past wind and water sheet erosionhas taken place, undisturbed surface areas appearto have become armored with remaining graveland rock materials.

On October 15, 1998, HLA conducted a field surveyof watermarks located in the bypass channel and in theupstream watershed. Although RSSD had been in theprocess of making repairs to the channel, channelcross section and channel geometry surveys wereaccomplished at three separate locations within andupstream of the bypass channel using the high watermarks as points of reference. Cross sections weresurveyed approximately 1/4 mile upstream of thechannel entrance, at the gabion drop structure, (Station24+47), and in the bedrock channel section (Station47+85). Stationing is based on the channel plan andprofile for the project (Figure 1, Site Map).

Additionally, high-water marks and other flow volumeindicators were observed in the upper watershed,above the bypass channel. Manning's 'n' values wereestimated in the field to range from 0.034 to 0.04 atthe surveyed cross sections. Although remedialconstruction activity following the subject storm hadobliterated the original design, channel slopesappeared to approximate the design slopes.

4.0 DRAINAGE SYSTEM DESIGN ANDDISCHARGE ANALYSIS

A review of the design documents and constructionplans indicate that on-site drainage improvementswere designed to convey a 25-year frequency runoffevent in accordance with the Code of FederalRegulations (CFR), Part 258.6 and Nevada RevisedStatutes (MRS), Section 444. Both federal and statecriteria for solid waste landfills specify design of floodcontrol facilities to convey the 25-year frequencydesign storm event. The duration selected for thisdesign storm event was the 6-hour duration inaccordance with the RFCD "Hydrologic Criteria andDrainage Design Manual", 1990. The 100-yearfrequency, 6-hour duration design storm was also usedto size the bypass channel to convey this eventwithout freeboard.

The hydrologic analyses for the SML bypass channelwas performed utilizing the above criteria and usingthe US Army Corps of Engineers (USCOE), HEC-1model. Soil Conservation Service (SCS) unithydrograph and soil-cover complex methods wereincorporated into the hydrologic analysis. Inaccordance with the NWS and the RFCD criteria 2.53and 3.15 inches of rainfall depths were used toapproximate the 25-and 100-year frequency, 6-hourduration storms, respectively. Accordingly, thehydrologic analysis for the SML bypass channelestimated the 25-year and 100-year frequency, 6-hourduration design peak discharge values to be 945 and1,310 cubic feet per second (cfs), respectively.

Hydraulic capacity analysis for the bypass channelwas performed for both the 25-and 100-year estimatedrunoff peak discharges using USCOE HEC-2hydraulic computer model. Results of the hydraulicanalysis indicated that the channel be designed as arip-rap lined trapezoidal section having 3:1 sideslopes, 10 foot wide bottom, and design depths of 4.5to 6.25 feet, depending upon channel slope. Channelriprap was designed in accordance with RFCDprocedures, with a d50 size of 12 inches to sustain the25-year discharge velocities.

The SML drainage design, supporting documents, andclosure plan were reviewed by the Clark CountyPublic Works Department, Clark County HealthDistrict, U.S. Bureau of Land Management, and theNevada Division of Environmental Protection, in1994, receiving approval on April 14th of that year.Following construction, on-site reviews by HLA and

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others indicated that the SML closure design had beeninstalled in substantial conformance with the plans.

Separate calculations, performed as a part of thisdrainage system review, using the U.S. GeologicalSurvey (1994) regression equations produced similarresults to the initial design study (attached).Hydrologic analyses of the on-site landfill watershedsalso appear to have produced reasonable results duringinitial design using standard engineering practices.

5.0 SEPTEMBER 11,1998 FLOODREVIEW

5.1 Precipitation

Severe weather moved through the Las Vegas Valleyand northeast Clark County beginning late in themorning of September 11, 1998, causing wide-spreaddrainage problems and other damages. A "RainfallEvent Report, September 11, 1998'', prepared by theClark County Regional Flood Control District(RFCD), describing the storm indicates that rainfallexceeding 1 inch depths with hail fell over 40% of theLas Vegas Valley and reports of tornadoes in theHenderson area occurred during the event. The RFCDestimates the largest depths of rainfall occurred in theGreen Valley area of Henderson and the east part ofthe Las Vegas Valley extending north to Nellis AirForce Base. On the east side of the Valley, SloanChannel, Flamingo Wash, and Las Vegas Wash allexperienced significant flows.

Four precipitation gauges operated by the RFCD inthe California and Muddy River drainages in theGlendale-Moapa area recorded in excess of 2 inchesof rainfall during the event. One of those gauges,located on State Route 169, near Valley of Fire StatePark recorded 3.19 inches of rainfall during the 12-hour recording period of the gauge

National Weather Service (NWS) precipitation radarcoverage for September llth indicates the band oftotal rainfall, in excess of 3 inches, extended northeastfrom the Green Valley-Henderson to the Glendale-Moapa areas of Clark County.

Regarding rainfall in the Frenchman Mountain Area,Kim Runk, Science & Operations Officer, NWS, inLas Vegas, indicated in a letter report of October 21,1998 that:

"Periods of heavy rain occurred from two successthunderstorms between 12:20-1:40 p.m., with

additional Light-to-moderate rain until about 2:10 p.m.During the heaviest rainfall, radar estimated rain ratesof 2.5-3.0 inches per hour. Rainfall totals for the entirestorm were maximized in the Sunrise Mountain area at4.4" total accumulation. While this may be slightlyoverestimated due to hail contamination, a storm totalof 3-4" is reasonable, most of which fell in a timeperiod of less than an hour and a half."

While the RFCD does not operate precipitation gaugesin the Sunrise Mountain area, three NWS type gaugesoperated by the RSSD recorded rainfall depths of 2.2,2.25, and 2.5 inches of rainfall during the event. Thegreatest measured depth of precipitation at 2.5 inchesoccurred at an RSSD gauge located in the northeastcorner of the SML site, near the mouth of the bypasschannel entrance.

While the RSSD gauges do not record rainfallintensities, RSSD personnel indicated to the RFCDthat the rainfall occurred in less than a 1-hour period(RFCD Rainfall Event Report). RSSD personnelconfirmed this data in verbal discussions with HLAstaff. A review of the NWS precipitation radarcoverage indicates that a precipitation cell generatingin excess of 3 inches of total rainfall was centeredover Townships 20 and 21 South, Ranges 62 and 63East, and Township 20 South, Range 64 East, directlyover the Sunrise Mountain area Additional radarinformation has been requested from Mr. K. Runk,Science Officer at the NWS, but has not beenprovided as of the time of this report submittal.

Following procedures presented in Section 500 of theRFCD Manual, and applying appropriate depth-areareduction factors for the watershed above the bypasschannel 100-year, 45-minute and 1-hour durationdesign precipitation depths were estimated at 2.19 and2.42 inches, respectively. Therefore actual rainfallmeasurements of between 2.5 and 3.0 inches in anhour or less during the first of two rainfall eventsindicate that the storm event of September 11, 1998was most probably much greater than a 100-yearfrequency event

5.2 Runoff

The RFCD Rainfall Event Report for September 11,1998 indicates that significant flows were recorded inmost of the major drainage channels on the east sideof the Las Vegas Valley and in the California Washand Muddy River drainages in the Glendale-Moapaareas. The RFCD indicates that flow in the Las VegasWash at Vegas Valley Drive, 2 miles west of the




SML, was within 1 foot of the maximum channelcapacity, and had a peak discharge of between 5000and 8000 cubic feet per second (cfs). The RFCDestimates that, based upon the adopted 100-year peakflow value of 7100 cfs adopted for the FloodInsurance Study for the Las Vegas Wash at VegasValley Drive, flood flow approximated the 100-yearfrequency flood. At the Lake Las Vegas the flow rateof the Las Vegas Wash exceeded capacity of the twobypass pipelines and a large volume of the flood flowdischarged directly into the lake.

Relatively high flood flow discharges were alsorecorded by the RFCD and the U.S. GeologicalSurvey in Duck Creek, Pittman Wash, FlamingoWash, Sloan Channel, as well as the Las Vegas Wash.Estimated peak discharge rates are depicted in theRFCD's Rainfall Event Report. On Duck Creek, highflows in the vicinity of the Stadium Mobile HomePark caused the collapse of approximately 100 feet ofblock wall, ruptured an 18-inch water line and a sewerline. Approximately 300 lineal feet of a 28-inchdiameter natural gas line was exposed at this locationas well.

To the northeast of the Las Vegas Valley, the RFCDestimates runoff may have been up to 10,000 cfs inCalifornia Wash at the Hidden Valley Road. TheRFCD indicates that above Glendale, Muddy Riverflood flows overtopped SR 168, and washed outGubler Avenue at the Muddy River crossing hiLogandale. According to the damage assessmentreport prepared by the American Red Cross 13 homesin the Overton area suffered major damages and 2mobile homes in the Glendale/Moapa area weredestroyed. The CCPW has estimated that over$400,000 of roadway damages resulted from theflooding in the Moapa-Glendale-Overton area alone.

On September 16, 1998 HLA staff toured the SMLsite and the length of Hollywood Boulevard(Telephone Line Road), running parallel to andeasterly of the Las Vegas Wash. HLA's observationsresulted in a conclusion that all of the local andregional watersheds of the southwest area of theFrenchman Mountain region discharged large volumesof runoff, depositing massive volumes of sedimentand debris on Hollywood Boulevard and in the LasVegas Wash.

At the SML, flood marks were observed at the top ofthe bypass channel indicating that the channel hadconveyed flood flows matching or exceeding physicalchannel capacity, without freeboard. Flood lines

further indicated that water and debris levels upstreamof the bypass channel were above local channel anddepression area storage capacities. At severallocations, within the upstream watershed, floodelevation marks indicated that debris dams might havebriefly formed in the various drainages and at the areaof depression storage, above the bypass channel.

Verbal conversations with RSSD personnel indicatethat peak flow rates and resulting damage in thebypass channel occurred early in the runoff event withhigh intensity rainfall continuing to occur on site afterpeak channel flow had occurred. These observationssupport NWS data which point to two separate rainfallevents with the first event being the most severe.There are indications that debris blockages in theupstream watershed, above the SML site, may havetemporarily blocked runoff flow during the firstrainfall event and that upon release, sent higher peakflood volumes instantaneously down stream early inthe runoff event, exacerbating flow rates and impactsto the bypass channel. HLA personnel furtherobserved that flood marks, channel scour andsediment deposition in the bypass channel watershedwere consistent with similar observations made inother off-site watersheds within the southwestFrenchman Mountain area.

The data collected in the field indicates that the depthof flow in the channel was approximately 6 feetChannel scour, upstream, through the channel anddownstream of the site suggest that flow velocitieswere greater than 10 feet per second.

Hydraulic analysis of the surveyed channel geometrywas performed by HLA to determine the approximatepeak flow rates (see attached). The analysis indicatesthat the peak discharge was approximately 1190 cfs1/4 mile upstream of the channel inlet, 1999 cfs at thelocation of the gabion drop structure at Station 24+47,and 1873 cfs at Station 47+85, hi the rock cutAveraging the channel flow rates thus indicates thatthe bypass channel carried approximately 1936 cfsduring the September 11, 1998 flood event.

6.0 CONCLUSIONS

Based upon historical accounts of the September 11,1998 flood event, visual review of the SML followingthe subject flood, appropriate design documents anddesign data, field surveys of the subject site, andengineering analysis conducted to date by HLA, thefollowing conclusions regarding the operation of theSML drainage system are reached:




• The precipitation-flood event occurring onSeptember 11, 1998 was a significant event,producing rainfall depths, runoff volumes, andflood impacts in eastern Clark County consistentwith flood conditions exceeding die theoretical100-year flood event in a number of watersheds.

• At the SML the September 11, 1998 flood eventcaused extensive impacts to the bypass channel,the on-site drainage system, and steeper slopes ofthe landfill.

• At the SML recorded rainfall events of 2.2, 2.25,and 2.5 inches, observed to have accumulated in atime frame of 45 minutes to 1 hour. The NationalWeather Service radar coverage of the subjectprecipitation event indicates that over 4.4 inchesof rainfall fell in less than 1 hour, 20 minutes,over the Frenchman Mountain Area during thesubject storm event Theoretical 100-year designprecipitation depths, calculated by following theprocedures outlined by the RFCD, for thewatershed area lying above the entrance to thebypass channel, are 2.19 and 2.42 inches for the45-minute and the 1-hour duration rainfall events,respectively. Therefore, on the basis of bothrecorded rainfall depths at the subject site andprecipitation data supplied by the NationalWeather Service, when compared to theoreticaldesign values, the September 11, 1998precipitation event produced actual rainfall thatapproached exceeded the 100-year designprecipitation depths.

• The bypass channel at the SML was designed tocarry the 25-year frequency, 6-hour runoff event,at 945 cfs, with freeboard, and the 100-year, 6-hour runoff event calculated to produce 1310 cfs,without freeboard. This was an acceptable designmeeting state and federal criteria

• The observed high water marks in the bypasschannel suggest that approximately 1190 cfsentered the channel and that the channel wassubjected to peak flows of between 1873 and1999 cfs throughout its entire length. Based upondesign capacities, the channel carried an averageof 148% and 205% of its respective 100-and 25-year design discharge values, respectively, basedupon the design estimation procedures presentedby the RFCD.

• On the basis of comparison of theoretical designvalues for both precipitation and discharge, to

precipitation depths and runoff volumes actuallyobserved in the bypass channel and on theremaining site, it can be concluded that the runoffevent of September 11, 1998 approached orexceeded design capacity of the SML drainagesystem.

On the conclusion that precipitation and runoffoccurring on September 11, 1998 approached orexceeded the design capacity of the SMLdrainage system, it is reasonable to expect thatdamage to the system would occur.

• Based upon observations of drainage systemoperation and damage experienced it may bepossible to improve system operation duringhigher runoff-discharge events than mandated,without major expenditure of capitalimprovement funds.

7.0 RECOMMENDATIONS

The following recommendations are based upon areview of observed runoff events, design plans, andsystem operation occurring at the SML site since thetime of closure. The following recommendations areintended to improve system performance and reducethe risk of catastrophic failure of onsite drainagesystems during discharge by events exceeding designcapacity.

• A floodwater detention and debris basin should beconstructed offsite, upstream of the entrance tothe bypass channel to ensure that the channel willbe subjected to predictable released flows whichwill remain within the design capacity of thebypass channel system.

• If an upstream detention basin is planned fordevelopment, the channel should be designed toconvey a minimum 25-year storm or larger tomatch hydraulic outfall characteristics of theupstream structure. If the upstream detentionbasin is not installed, the bypass channel systemshould be reconstructed to withstand the effects ofa 100-year or higher frequency storm, withoutmajor failure.

• To reduce channel lining unraveling during amajor runoff event, cutoffs should be designedinto the channel at regular intervals to combatlong reaches of channel liner failure. Channelriprap should be sized to withstand scour by a

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Following the September 11, 1998 Storm

100-year or larger flow event. It is recommendedthat channel redesign continue to offer a designsection which is flexible to withstand the longterm effects of underlying refuse settlement andchannel slope change.

• Channel lining materials should be buried at thelongitudinal outside edge of the channel with itscomposite riprap and lining section buried deepenough below the adjacent ground surface toallow lateral runoff to enter the channel over thetop of the riprap rather than be intercepted by thechannel section.

• Based upon observed operation during past floodflow events, the entrance to the bypass channelshould be enlarged to ensure that floodwater anddebris more efficiently enter the channel.

• The steep channel outlet downstream of the rockexcavated section should be reinforced on thewesterly side with concrete or deeply set largediameter rock rip-rap to better protect the adjacentlandfill from side channel erosion and scour. Thelower outlet of the channel should be excavatedinto a flat bottom and slightly realigned to lessenimpact to the westerly bank.

• Entrances to on-site channels should be designedas open channel systems rather than closedculvert systems and be reinforced with cutoffsystems and channel lining materials to protectsurrounding dike systems against erosion scour.

• In areas where rill erosion has been evidentduring the recent and previous flood events,intermediate interception drainage channelsshould be installed to intercept runoff and toshorten rill generation lengths.

• On-site drainage structures should be furtherevaluated as a part of final design for repair orrelocation on the basis of past performance duringrunoff events.

• On-site drainage system evaluation shouldincorporate consideration of means to convey,capture and store accumulated sediment.

• At locations where landfill slopes are subjected topotential erosion from off-site drainageconveyance, the toe of the slope should beprotected by keyed concrete grout or grouted

riprap slope protection barriers to reduce erosionscour.

• Where wind and water scour have left a protectivegravel surface layer over the areas of cappedlandfill, vehicle travel should be confined todesignated travel corridors in order to protect thecompetency of the protective cover.

• As with all landfill drainage systems,maintenance will be necessary in the future andmaintenance activities should occur on a regularlyscheduled basis.

• As with all landfill drainage systems, changes ingrade due to subsurface settlement and thedischarge of runoff events exceeding designcapacity will occur. Therefore drainage systemrepair will continue to be necessary from time totime as runoff events dictate.

8.0 CLOSURE

The above preliminary analysis and the conclusionsand recommendations contained herein representHLA's initial response to RSSD's request issued onOctober 12, 1998. Due to the relatively short timeavailable to develop this report, detailed engineeringevaluations necessary to determine appropriate finaldesign options were not undertaken. Further analysisof hydraulic and hydrologic characteristics of thestorm drain system should be completed as a part offinal design recommendations. Upon review of thisreport by RSSD, the Clark County Public WorksDepartment, the Clark County Health District, andothers HLA stands ready complete final design plansfor repair of the SML drainage system.

Should the RSSD wish to discuss HLA's conclusionsand recommendations further, contact Gerry Hester,P.E. or Joe Milazzo, P.E. at (702) 251-5449.

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U.S. DEPARTMENT OF COMMERCENational Oceanic and Atmospheric AdministrationNATIONAL WEATHER SERVICE7851 Industrial RoadLas Vegas, Nevada 89128

October 21,1998

Harding Lawson Associates5145 South Arville, Suite ALas Vegas, NV 89118

TO: Joe RumannSUBJECT: Radar Images and Event Summary for Heavy Rainfall at Sunrise Mountain on Septmeber 11,1998

Dear Joe,

Please find enclosed a series of radar images for the September 11,1998 rain event as per your request. Explanation follows.

Summary: Based on analysis of KESX WSR-88 Doppler Radar data, rain began in the vicinity of Sunrise Mountain between 11:45a.m. and noon, but was very light until about 12:20 p.m. PDT. Periods of heavy rain occurred from wo successive thunderstormsbetween 12:20-1:40 p.m., with additional light-to-moderate rain until about 2:15 p.m.

During the heaviest rainfall, radar estimated rain rates of 2.5-3.0 inches per hour. Rainfall totals for the entire storm were maximizedin the Sunrise Mountain area at 4.4" total accumulation. While this may be slightly overestimated due to hail contamination, a stormtotal of 3-4" is reasonable, most of which fell in a time period of less than an hour and a half.

Imagery: The radar images I've provided are of two types: reflectivity and rainfall estimates. The first type is a measure of reflectedradar energy back to the antenna in decibels of reflectivity (dBZ). Echoes shaded in reds and purples represent the strongest returns(>50 dBZ) and typically depict moderate to heavy rainfall. The second image type is rainfall estimate, an algorithm based onconverting reflectivity to hourly rain rate and storm total precipitation. The equation used during this event was Z=300RW, whereZ is reflectivity in dBZ, and R is rain rate.

The date/time group for each image is listed in the upper right, four rows down from the top. The date/time stamp in the top rowis the date which the image was printed, so ignore that. The second line is labeled "CMP REF", which means "CompositeReflectivity". This simply means that each pixel represents the highest three-dimensional reflectivity for that location at that time.The third line down is labeled "124 NM" and ".54 NM RES", defining the range and individual pixel resolution for that scan. Thefourth line, "09/11/98 19:21" represents the date and time in UTC. During the summer, our local time is UTC - 7 hours, so a timestamp of 19:21 represents 12:21 PDT.

The five reflectivity images indicate the time, duration, and intensity of rainfall over the Sunrise Mountain area on 11 SEP 98. The19:21 (12:21 PDT) image depicts an area of heavy rain approaching the western slopes of Sunrise Mountain from the direction ofBoulder Highway. At 12:34, heavy rain was clearly overspreading the mountain. The heavy rain associated with the firstthunderstorm crested the mountain and appeared to be moving downwind at 12:57 p.m. This is not to suggest rain had stopped atthis time, but had briefly diminished in intensity. By 1:15 p.m., rainfall intensified again with the onset of the next thunderstorm.This persisted for at least 15-20 minutes, then began to taper offinto light to moderate showers for another half-hour or so. The lastreflectivity image indicates the trailing edge of showers clearing the area of interest at 2:13 p.m.

The image labeled "1-HR PRECD?" depicts the radar-estimated hourly precipitation rate for the volume scan ending at 20:03 UTC(13:03 PDT). It shows a large area exceeding two inches/hour extedning from the McCollough Range across a narrow swath of theeastern side of the valley to the Sunrise/Frenchman Mountain region. Maximum rainfall was estimated on the western slopes ofSunrise/Frenchman peaks at 2.6 inches/hour.

The final image, labeled "STM PRECIP" represents an accumulated precipitation estimate for the entire storm event. Once again,the maximum (4.4 inches) occurred near Sunrise Mountain.

If you have any further questions, feel free to call or e-mail me.

Regards,

Kim Runk, Science & Operations Officer Phone: 263-9744 ext. 224National Weather Service - Las Vegas E-mail: [email protected]

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