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HomeMy WebLinkAboutPR 23728: AWC, MASTER DRAINAGE PLAN City 01 orl rlltur www.PortArthur" INTEROFFICE MEMORANDUM Date: April 26,2024 - To: The Honorable Mayor and City Council Through: Ron Burton; City Manager From: John Cannatella,PE,City Engineer RE: PR No. 23728:Arceneaux, Wilson& ole, LLC of Port Arthur,Texas: Master Drainage Plan—Acceptance of Master Drainage Plan. Project No.:DR1P05. Introduction: The intent of this Agenda Item is to authorize the City Manager to accept the Master Drainage Plan from Arceneaux, Wilson& Cole,LLC, (AWC) of Port Arthur,Texas. Background: The City of Port Arthur proposed the development of a citywide master drainage plan and policy guide that identified potential drainage concerns throughout the City. The City advertised for engineering firms to submit their qualifications regarding their ability to develop a citywide master drainage plan. Arceneaux, Wilson & Cole, LLC, (AWC) of Port Arthur, Texas, was selected to develop this plan. Pursuant to Resolution No. 21-345, the City of Port Arthur City Council approved entering into a contract with AWC for the not-to-exceed amount of$1,285,300.00 for a period of three hundred sixty- five (365)days. AWC has provided the final Master Drainage Plan which includes Drainage Reports, Drainage Maps, and Drainage Criteria for the purposes of stormwater management and the protection of life and property. Budget Impact: There is no budgetary impact associated with acceptance of the Master Drainage Plan. Recommendation: It is recommended that the City of Port Arthur City Council approve PR No.23728 with Arceneaux, Wilson & Cole,LLC, of Port Arthur,Texas,authorizing the City Manager to accept the Master Drainage Plan. "Remember,we are here to serve the Citizens of Port Arthur" 444 4th Street X Port Arthur,Texas 77641-1089 X 409.983.8182 X FAX 409.983.8294 5:\Engineering\PUBLIC WORKS\PW-DRAINAGE PROGRAM\MASTER DRAINAGE PLAN AWC\Agenda Memo AWC Accept Plan.dou PR No. 23728 4/26/2024 mje Page 1 of 3 RESOLUTION NO. A RESOLUTION AUTHORIZING THE CITY MANAGER TO ACCEPT THE CITYWIDE MASTER DRAINAGE PLAN FROM ARCENEAUX, WILSON & COLE, LLC, OF PORT ARTHUR, TEXAS. FUNDS ARE NOT IMPACTED WITH ACCEPTANCE. PROJECT NO. DR1POS. WHEREAS, the City of Port Arthur proposed the development of a citywide master drainage plan that identified potential drainage problems with the goal of developing a road map for future drainage-related activities and stormwater management; and, WHEREAS, Arceneaux, Wilson & Cole, LLC, of Port Arthur, Texas was selected by staff as the most qualified submitter to produce the master drainage plan and policy; and, WHEREAS, pursuant to Resolution No. 21-345, the City of Port Arthur City Council approved entering into a contract with Arceneaux, Wilson & Cole, LLC of Port Arthur, Texas, to develop a citywide Master Drainage Plan, for the not-to-exceed amount of $1,285,300.00; and, WHEREAS, Arceneaux, Wilson & Cole, LLC of Port Arthur, Texas has provided the final Master Drainage Plan submitted for the purposes of stormwater management and the protection of life and property; See Exhibit A; and, WHEREAS, approval of acceptance of the Master Drainage Plan submitted by Arceneaux, Wilson & Cole, LLC, of Port Arthur, Texas, is herein deemed an appropriate action; now, therefore, BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF PORT ARTHUR: THAT, the facts and opinions in the preamble are true and correct; and, THAT, the City Council hereby authorizes the City Manager to accept the Master Drainage Plan submitted by Arceneaux, Wilson & Cole, LLC, of Port Arthur, Texas; and, PR No.23728 4/26/24 mje Page 2 of 3 THAT, a copy of the caption of this Resolution No. be spread upon the Minutes of the City Council. READ, ADOPTED AND APPROVED this the day of , A.D. 2024 at a meeting of the City of Port Arthur, Texas by the following vote: Ayes: Mayor: Councilmembers: Noes: Thurman Bill Bartie Mayor ATTEST: Sherri Bellard City Secretary APPROVED AS TO FORM: APPROVED FOR ADMINISTRATION: Yti1,_. James M. Black Ron Burton, CPM Interim City Attorney City Manager PR No.23728 4/26/2024 mje Page 3 of 3 APPROVED AS FOR AVAILABILITY OF FUNDS: Lynda "Lyn" Boswell, M.A., ICMA-CM Director of Finance John C a Ila, PE City Engin er E4&;,& Clifton Williams, CPPB Purchasing Manager EXHIBIT A l xtftri > `�. O oC az \\\\\\\.11/1/:////_.,./' o o ,u 44 Z.? ..''' 7/' 1. � .oti• m r•s�,l l� i2.4a•z N.) o% %i••p m CO) ••y% II yO z • ,. � • �111XkW•``_ dui `V • .� y •� . . 3 . n •.,il%.� o a'"1 l\ iz•� A rl a .o� .fi %r•y N p .ii 10.. 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PURPOSE OF THE DRAINAGE CRITERIA MANUAL 5 1.2. DRAINAGE POLICIES 6 1.2.1. Zero Impact(No Adverse Impacts) 6 1.2.2. Level of Protection 6 1.2.3. Stormwater Detention 6 1.2.4. Floodplain Storage 7 1.2.5. Primary and Secondary Drainage Facilities 7 1.3. THE NATIONAL FLOOD INSURANCE PROGRAM 7 2. REVIEW AND APPROVAL OF DRAINAGE PLANS 9 2.1. ADVISORIES 9 2.1.1. Engineering Judgement 9 2.1.2. Deviations 9 2.1.3. Requirements of Other Jurisdictions 9 2.2. OVERVIEW OF THE APPROVAL PROCESS 9 2.3. TYPES OF SUBMITTALS 10 2.4. ACTIVITIES FOR WHICH SUBMITTALS TO THE CITY ARE REQUIRED 10 2.5. GENERAL REQUIREMENTS FOR VARIOUS SUBMITTALS 11 2.5.1. New Development 12 2.5.2. Hydrologic Studies 12 2.5.3. Hydraulic Studies of Primary Drainage Facilities 12 2.5.4. Hydraulic Studies of Secondary Drainage Facilities 13 2.5.5. Detention Studies 14 2.5.6. General Engineering Report Requirements 15 2.6. REVIEW&APPROVAL OF SUBMITTALS TO THE ENGINEERING DEPARTMENT 17 3. HYDROLOGIC AND HYDRAULIC CONCEPTS 19 3.1. DEFINITIONS OF BASIC TECHNICAL TERMS 19 3.2. BASIC HYDROLOGIC CONCEPTS 20 3.2.1. Manning's Equation 20 3.2.2. Conveyance 20 3.3. EFFECTS OF URBANIZATION 21 4. HYDROLOGY 23 4.1. DRAINAGE AREAS UP TO 200 ACRES 23 4.1.1. Introduction 23 4.1.2. The Rational Method 23 4.1.3. Establishing the Drainage Area 24 4.1.4. Determining Runoff Coefficients 24 4.1.5. Establishing the Time of Concentration 25 4.1.6. Computation of Rainfall Intensity 26 4.2. DRAINAGE AREAS GREATER THAN 200 ACRES 27 4.2.1. Watershed Boundaries 27 4.2.2. Rainfall Data 28 4.2.3. Infiltration Losses 28 4.2.4. Initial Abstraction 30 4.2.5. Percent Impervious Cover 31 4.2.6. Loss Rate Computations in HEC-HMS 32 4.2.7. Unit Hydrograph Methodology 33 4.2.8. Streamflow Routing 34 5. HYDRAULICS OF PRIMARY DRAINAGE FACILITIES 37 5.1. GENERAL DESIGN REQUIREMENTS FOR PRIMARY DRAINAGE FACILITIES 37 5.1.1. Design Storm Frequencies 37 5.1.2. Design Requirements for Earthen Channels 38 5.1.3. Minimum Design Requirements for Trapezoidal Concrete-Lined Channels 38 5.1.4. Design Requirements for Rectangular Concrete Low-Flow Sections 40 5.1.5. Design Requirements for Transitions, Bends,and Confluences 40 5.1.6. Design Requirements for Culverts 41 5.1.7. Structural Requirements for Culverts 42 5.1.8. Design Requirements for Bridges 42 5.1.9. Enclosed Drainage Systems 43 5.1.10. Maximum Flow Velocities 43 5.1.11. Maintenance 44 5.2. HYDRAULIC ANALYSIS OF PRIMARY DRAINAGE FACILITIES 44 5.2.1. Acceptable Open Channel Design Methodologies 44 5.2.2. Acceptable Bridge and Culvert Design Methodologies 45 5.2.3. Acceptable Enclosed Drainage System Design Methodologies 45 5.2.4. Flow Data 45 5.2.5. Boundary Conditions 45 5.2.6. Cross-Section Data 46 5.2.7. Manning's Roughness Coefficient 47 5.2.8. Bridge&Culvert Data 51 5.2.9. Floodway Analysis 54 6. HYDRAULICS OF SECONDARY DRAINAGE FACILITIES 57 6.1. GENERAL DESIGN REQUIREMENTS FOR STORM SEWERS 57 6.1.1. Design Storm Frequencies 57 6.1.2. General Design Requirements 58 6.1.3. Extreme Event Design 58 6.2. HYDRAULIC ANALYSIS OF STORM SEWERS 59 6.2.1. Acceptable Storm Sewer Design Methods 59 6.2.2. Peak Runoff Rates 59 6.2.3. Storm Sewer Slopes 59 6.2.4. Friction Losses 62 6.2.5. Minor Losses 62 6.2.6. Hydraulic Grade Line 63 6.3. DESIGN OF ROADSIDE DITCHES 64 6.3.1. Design Storm Frequencies 64 6.3.2. General Design Requirements for Roadside Ditches 65 6.3.3. Peak Runoff Rates 65 6.4. DESIGN OF OTHER SECONDARY DRAINAGE FACILITIES 66 6.4.1. Types of Facilities 66 6.4.2. General Design Requirements 66 6.4.3. Peak Flow Rates 66 6.5. LOT GRADING AND DRAINAGE 66 7. DETENTION ANALYSIS 71 7.1. GENERAL DESIGN REQUIREMENTS 71 7.1.1. Design Storm Frequencies 71 7.1.2. Detention Basin Location and Geometry 71 7.1.3. Maintenance Berms 72 7.1.4. Maintenance 73 7.1.5. Pumped Detention Facilities 73 7.1.6. Multi-Purpose Design 73 7.1.7. Extreme Event Overflow Structures 73 7.2. PEAK DISCHARGE RATES 73 7.2.1. Methodology 73 7.2.2. Allowable Peak Discharge Rates 74 7.2.3. Off-Site Flows 74 7.3. DETENTION ANALYSIS FOR DRAINAGE AREAS UP TO 200 ACRES 74 7.3.1. Detention Analysis for Drainage Areas Up to 200 Acres 74 7.3.2. Design of Outfall Structures for Drainage Areas up to 200 Acres 76 7.4. DETENTION ANALYSIS FOR DRAINAGE AREAS GREATER THAN 200 ACRES 78 7.4.1. Acceptable Detention Routing Software Programs 78 7.4.2. Inflow Hydrographs 78 7.4.3. Stage-Storage Relationship 78 7.4.4. Outfall Structure 78 7.4.5. Acceptable Results 79 7.5. DOWNSTREAM IMPACTS 79 8. EROSION AND SEDIMENT CONTROL 81 8.1. EFFECTS OF EROSION AND SEDIMENTATION 81 8.2. AREAS WITH HIGH EROSION POTENTIAL 81 8.3. SLOPE PROTECTION METHODS 81 8.3.1. Turf Establishment 82 8.3.2. Slope Paving 82 8.3.3. Riprap 83 8.3.4. Acceptable Velocities for Various Slope Treatments 83 8.4. REQUIREMENTS FOR CHANNEL BENDS AND CONFLUENCES 84 8.5. REQUIREMENTS FOR STORM SEWER OUTFALLS 85 8.6. CHANNEL BACKSLOPE DRAIN SYSTEMS 85 8.7. INTERCEPTOR STRUCTURES 85 8.8. STORMWATER POLLUTION PREVENTION PLANS 85 8.9. SPECIAL ENERGY DISSIPATION STRUCTURES 86 Chapter 1 Purpose and Policies 1. PURPOSE AND POLICIES 1.1. PURPOSE OF THE DRAINAGE CRITERIA MANUAL The primary purpose of this Drainage Criteria Manual is to establish standard principles and practices for the analysis, design, and construction of drainage systems within the jurisdiction of the City of Port Arthur (City). This Drainage Criteria Manual is intended to support the Master Drainage Plan and Drainage Regulations that the City has adopted. The manual is for users with knowledge and experience in applying standard engineering principles and practices of drainage design and management.The purpose of this Drainage Criteria Manual is to outline criteria and guidance to be used by developers, engineers, and land surveyors in the design of drainage measures to manage rainfall/runoff. Unless otherwise approved by the City's Director of Public Works,these criteria shall be used. Stormwater management is an essential component of community infrastructure and provides both increased convenience and protection of life and property. A properly designed system will detain and/or carry away runoff from more frequent rainfall events while allowing the movement of vehicles to homes and businesses. Such a system will also detain and drain stormwater from an infrequent"extreme rainfall event" so that habitable structures are not damaged,and major streets are passable to public safety vehicles. Providing the inhabitants within the City's jurisdiction with an effective stormwater management system that allows sustainable community growth is a continuing challenge. It involves setting minimum standards, planning for future detention basins and drainage channels, working with private development interests, coordinating with governmental agencies, and maintaining the existing system's efficiency. Recognizing that adopted policies and criteria should guide stormwater system development, the City launched a planning process aimed at setting consistent standards responsive to the needs of property developers and design engineers and compliant with federal and state regulations. This Drainage Criteria Manual applies to all areas within the City's jurisdiction and overrides all less stringent city or county codes of ordinance covered within this Manual. If the project falls within the jurisdiction of the Jefferson County Drainage District No. 7 (DD7), submittals related to the project must initially be submitted to DD7 in accordance with their drainage regulations (accessible at www.dd7.org) for approval. DD7 will determine approval whether the project follows its Drainage Criteria Manual. This manual is not intended to interfere with, abrogate, or annul any other ordinance, rule, regulation, statute, or other provision of law. Where any provision imposes restrictions different from those set by DD7 within the City's jurisdiction that is also within the jurisdiction of DD7, whichever provisions are more restrictive or impose higher standards shall control. 5 Chapter 1 Purpose and Policies 1.2. DRAINAGE POLICIES 1.2.1. Zero Impact(No Adverse Impacts) An impact is defined as a change in the response of a watershed to a storm event. The most common impacts are changes in the volume of runoff, changes in the rate of runoff, and changes in flooding depths. Impacts may be adverse or beneficial. Adverse impacts are those which increase the potential for flooding damages. Beneficial impacts, on the other hand, reduce the potential for flood damage. The term zero impact is normally defined as the absence of adverse impact.The City maintains a strict zero-impact policy in all watersheds located wholly or partially within the jurisdiction of the City. This means that neither increases in upstream flood levels nor in downstream flow rates are allowed in areas where there is the potential for flooding damages from rainfall events with a 4%annual chance (25-year)recurrence interval. Adverse impacts associated with new development must be identified and mitigated. Acceptable mitigation measures may include stormwater detention, the creation of new floodplain storage, channel improvements, and improvements to channel structures. A "zero impact" policy will be enforced by the City.No adverse impacts on downstream peak flow rates or upstream flood levels will be allowed because of new residential,commercial,or public or private developments,and no net loss in existing floodplain storage will be allowed with the City's jurisdiction. 1.2.2. Level of Protection The level of protection is generally regarded as the storm recurrence interval in which future primary drainage facilities, such as open channels, roadway culverts, and detention facilities, are designed to accommodate without significant flooding damages. For example, providing a 100- year level of protection would indicate that the future primary drainage facilities are designed to carry storm runoff from a 100-year storm event without significant flooding of homes and other buildings. For the analysis and design of future primary drainage facilities,the City would have to adopt a 100-year level of protection to be consistent with DD7. 1.2.3. Stormwater Detention Stormwater detention refers to the temporary storage of storm runoff in ponds or other storage facilities. The provision of this temporary storage allows storm runoff to be discharged to a receiving stream at a lower rate, thereby protecting downstream areas from increased flooding damages associated with increased flow rates and higher flood levels. The City recognizes the value of stormwater detention in reducing the potential for flood damages and allows the use of detention facilities in addition to adding conveyance capacity for mitigating impacts associated with new development and drainage improvements. 6 Chapter 1 Purpose and Policies 1.2.4. Floodplain Storage Floodplain storage is defined for the purposes of this manual as the space below the 100-year flood levels. This space is available for the temporary storage of flood waters during extreme storm events. Preservation of this space is extremely important because floodplain storage serves to reduce downstream peak flow rates. The City prohibits reductions in existing floodplain storage along all streams which pass through the jurisdiction of the City. 1.2.5. Primary and Secondary Drainage Facilities For the purposes of this manual, primary drainage facilities include open channels, bridges, culverts, and enclosed drainage systems (i.e., open channel that has been enclosed). Secondary drainage facilities include storm sewer systems, roadside ditches, and associated structures, and other facilities such as sheet flow swales, small culverts,and other structures which typically serve relatively small drainage areas, as well as lot grading and drainage requirements. In practice, the responsibility for the provision and maintenance of drainage facilities is uniquely divided between DD7, the City and private property owners and homeowner's associations, and similar entities in the following general manner: • DD7 is generally responsible for the major drainage arteries within its boundaries,all major tributaries and tributary drainage outfall ditches that lead to the bayous,creeks,gullies,and marsh, some of the ditches that convey stormwater away from subdivisions, and any regional, sub-regional, and/or other detention reservoirs constructed and accepted by the District, all within the boundaries of the District. • The City is generally responsible for underground storm sewers, open ditches and pipe- covered ditches that extend alongside City roads and streets, neighborhood stormwater collection ditches located within the City's jurisdiction, and any detention facilities constructed and/or accepted by the City. • Homeowners' association(s) and/or private property owner(s) are responsible for site grading, on-site drainage swales, and neighborhood detention facilities and ditches along private streets. 1.3. THE NATIONAL FLOOD INSURANCE PROGRAM The City is a participant in the National Flood Insurance Program (NFIP). This program provides federally subsidized flood insurance to those cities and counties which elect to participate. The program is administered by the Federal Emergency Management Agency (FEMA), which is headquartered in Washington, D.C. Flood insurance data for participating cities and counties is published by FEMA in two formats: bound flood insurance studies, which describe the results of flooding studies completed for significant streams, and Flood Insurance Rate Maps (FIRMs), which provide data on 100-year flood levels, illustrate the boundaries of the floodway, 100-year flood plain, and 500-year flood plain, and designate flood hazard zones for insurance purposes. 7 8 Chapter 2 Review and Approval of Drainage Plans 2. REVIEW AND APPROVAL OF DRAINAGE PLANS 2.1. ADVISORIES The City requires that the requisite engineering submittals be prepared and approved for all regulated developments which may affect the rate, direction, or volume of stormwater runoff or the depth and velocity of flow in primary drainage facilities and other infrastructure within the City's jurisdiction. 2.1.1. Engineering Judgement The design requirements, criteria, and schematics included in this manual establish uniform practices for the design of drainage associated with subdivisions and developments. However,the requirements of this manual neither replace the need for engineering judgment on behalf of designers nor does it preclude the use of methods not presented. Other accepted methods and procedures may be used with prior approval of the City's Director of Public Works. 2.1.2. Deviations Deviations from the City's drainage regulations and this manual, if known or anticipated, shall be identified and discussed at a pre-submission conference or by other means. Deviations are to be identified in the drainage report, and the technical justification for such deviations, including computations as appropriate, shall be provided. The acceptability of the deviations authorized by the City shall be determined by DD7 upon their review. 2.1.3. Requirements of Other Jurisdictions It is the responsibility of the Applicant/Owner to obtain all approvals required by the City, DD7, the other municipalities,or any other agency of the State of Texas or the United States of America. Evidence that such approvals have been applied for or obtained may be required by the City prior to the issuance of an Approval. 2.2. OVERVIEW OF THE APPROVAL PROCESS The following is an overview of the review and approval process. A pre-submission conference is required to be scheduled with the City, inclusive of available project information. The City acknowledges that drainage concerns,the adequacy of the existing drainage system and access for maintenance, and solutions to address inadequacies and flooding that may be exacerbated by new development vary from location to location. The purpose of the pre-submission conference is to improve understanding of the existing drainage system in the vicinity of and downstream of the proposed subdivision or development site and to discuss measures that are necessary and appropriate to address drainage and flooding. The City's engineering staff may provide information, data, and computer models from City engineering studies, and evidence of drainage 9 Chapter 2 Review and Approval of Drainage Plans and flooding concerns based on observations and data collected from past storm and flood events. DD7 reserves the right to require a pre-submission conference of its own. 2.3. TYPES OF SUBMITTALS The types of engineering submittals typically made in connection with new development or drainage studies include the following: • Engineering Reports: These documents, which may take the form of letter reports or more extensive and formal bound reports, normally describe the results of analyses of existing and/or proposed drainage conditions. Engineering reports may be submitted as a basis for a better understanding of existing conditions(i.e., a flood plain revision report),to support a request for approval of construction documents for a proposed facility(i.e., a preliminary engineering report for a roadway improvement project), or to serve as a plan for future conditions(i.e., a master drainage report for a given watershed). • Construction Documents: These include engineering drawings and specifications for a proposed facility or development which will affect stormwater drainage or flood control. • Permit Applications: These are applications for development permits, flood plain fill permits, pipeline or other crossing permits, and other permits required by the City. 2.4. ACTIVITIES FOR WHICH SUBMITTALS TO THE CITY ARE REQUIRED The City requires that engineering submittals be prepared for all activities which may affect the rate,direction or volume of stormwater runoff or the depth and velocity of flow in primary drainage facilities and other infrastructure within the City's jurisdiction as applicable. Drainage criteria for secondary drainage facilities are shown in Section 6 of this manual but are subordinate to DD7's requirements. The City and DD7 will review the following three types of projects: 1. Construction of new projects, modification and/or improvement of existing facilities, or those which impact existing facilities which are maintained by the City or DD7, which include: a. Open channels b. Bridges, culverts, and other hydraulic structures associated with open channels c. Detention basins d. Pump stations 2. Construction of drainage facilities that are physically located in, on, over, under, or adjacent to a drainage facility maintained by the City or DD7: a. Land development projects 10 Chapter 2 Review and Approval of Drainage Plans b. Roads and highways c. Bridges and culverts d. Storm sewer outfall pipes e. Water and sanitary sewer lines f. Pipelines and public utilities g. Environmental features(tree plantings, landscaping, etc.) h. Recreation amenities(hike and bike trails, parks, etc.) i. Encroachments 3. Development or public projects that may affect the facilities maintained by the City or DD7, such as natural channels, lakes, drainage ways, etc., or future drainage facilities described in a drainage master plan: a. Proposed subdivision development b. Residential, commercial, and industrial site development c. Roads and highways The construction of projects that have been previously approved by the City, for which construction has been commenced within twelve (12) months after approval, is not subject to revised regulations that have been changed after the approval of such developments and shall remain subject to being reviewed by the City for compliance with regulations in effect at the time of approval or as required by said prior approval. The construction of residential homes within a subdivision that has been previously approved by the City, shall not be subject to revised regulations that have been changed after the approval of such developments. However, all construction shall remain subject to review, inspection, and enforcement of building restrictions, site grading, compliance with easements, and any other requirements that may be set forth in an approved Plat, Drainage Plan, homeowners' agreement or bylaws, or other document approved or filed related to the use or development of the property at issue. 2.5. GENERAL REQUIREMENTS FOR VARIOUS SUBMITTALS The various submittals presented to the City for review should be as complete and as well- documented as possible. Submittals to DD7 shall follow their drainage regulations which are accessible at www.dd7.org. The general requirements described in this section should be satisfied for all submittals and act as standards with priority over any other codes that are less stringent.The intent of these requirements is to ensure that the following aspects of the proposed activity are made clear to the reviewer: 11 Chapter 2 Review and Approval of Drainage Plans 2.5.1. New Development Submittals for all new development shall include the following-items: • a plat of the development illustrating property boundaries, individual lot boundaries. streets, drainage easements, etc.; • a hydrologic impact analysis that identifies the potential effects of the development on downstream peak flow rates; • if necessary, a hydraulic impact analysis which identifies the potential effects of the development on upstream flood levels; • a preliminary engineering report which presents the results of impact analyses, describes proposed mitigation measures, provides construction cost estimates, etc.; Preliminary construction plans for proposed streets, storm drainage facilities, utilities, and other features may be submitted along with the preliminary engineering report. Final construction plans should be prepared after the City and DD7 have completed their review of the report and issued written comments. 2.5.2. Hydrologic Studies Major watershed hydrologic studies will be summarized in a report which contains sufficient text, exhibits, and computer output to completely describe the methods, data, and assumptions used in the analysis, as well as the results obtained. Information provided in the report should include the following: • a description of the analysis and the results obtained; • tabulations of all hydrologic modeling parameters; • tabulations of all computed peak flow rates; • a watershed map that illustrates the borders of each sub-area included in watershed modeling; • a hydrologic parameter map that illustrates all watercourse lengths, drainage areas, and developed areas; • output from all hydrologic models used in the analysis; • USB storage media or similar electronic file storage containing input files for all hydrologic and hydraulic models. 2.5.3. Hydraulic Studies of Primary Drainage Facilities For hydraulic analyses and designs of primary drainage system components, an engineering report containing the following items should be submitted: 12 Chapter 2 Review and Approval of Drainage Plans • sufficient text to summarize the methods, data, and assumptions used in completing the analysis, as well as the results obtained; • calculations and other information supporting the flow rates used in the analysis; • tabulations of hydraulic modeling data and results; • vicinity and site maps which illustrate the location of the project area and the extent of the stream reach being analyzed; • a plotted stream profile(s); • plotted cross-sections of the stream with computed flood levels superimposed; • a copy of the effective Flood Insurance Rate Map (FIRM) for the project area and, as needed, a proposed conditions flood plain and floodway map which illustrates proposed changes in flood plain and floodway boundaries; • copies of all hydraulic calculations; • an analysis of the effects of proposed improvements on downstream peak flow rates and upstream flood levels; • recommendations for mitigating any adverse impacts associated with proposed improvements to channels or structures; • output from all hydraulic computer models used in the analysis; • USB storage media or similar electronic file storage containing input files for all hydraulic models. For studies involving improvements to open channels and hydraulic structures or designs of new open channels, a right-of-way (ROW) map should also be submitted. Preliminary construction plans may be submitted along with the engineering report.Final plans should be prepared after the City and DD7 have completed their review of the engineering report and issued comments. 2.5.4. Hydraulic Studies of Secondary Drainage Facilities For submittals involving the design of storm sewer systems, ditches, swales, and other secondary drainage facilities which will have a direct effect on any DD7 properties, rights-of-way, or facilities,the following items should be included: • a report which summarizes the methods, data, and assumptions used in completing the design analysis, as well as the results obtained; • vicinity and site maps which illustrate the location of the project area and the location and configuration of the proposed facilities; • a watershed map that illustrates the boundaries of all sub-areas included in the analysis of the proposed facilities; 13 Chapter 2 Review and Approval of Drainage Plans • calculations and other information supporting the flow rates used in the analysis; • hydraulic calculations used in designing the facilities and in assessing their hydraulic performance under design storm conditions; • an analysis of the effects of proposed improvements on downstream peak flow rates and upstream flood levels; • recommendations for mitigating any adverse impacts associated with proposed drainage improvements; • a plotted profile(s)of the storm sewer system, ditch, swale, etc.; • for ditches and swales, a typical cross-section(s); • output from computer programs used in the analysis; • USB storage media or similar electronic file storage containing input files for any computer programs used in the analysis. Preliminary construction plans may be submitted along with the engineering report. Final plans should be prepared after the City and DD7 have completed their review of the engineering report and issued comments. 2.5.5. Detention Studies The following information must be submitted in support of designs for detention facilities: • vicinity, site, and watershed maps which clearly illustrate the location of the facility, its physical extent and configuration, its drainage area, and the relationship of its drainage area to the overall boundaries of the major watershed in which it is located; • a ROW map that illustrates all existing and proposed ROWs in the immediate vicinity of the detention facility; • discharge calculations that identify peak flow rates for pre-development and post- development conditions for the design storm event; • hydraulic calculations on which the design of the detention discharge structure is based; • for facilities with a drainage area of fewer than 200 acres, calculations establishing the required detention storage volume; • for facilities having a drainage area of 200 acres or more,a detention flood routing analysis assesses the effectiveness of the detention basin in mitigating impacts on downstream peak flow rates; • calculations involving the required capacity of supplemental and emergency discharge structures; 14 Fr Chapter 2 Review and Approval of Drainage Plans • exhibits that illustrate the configuration of the detention facility, inflow structure, and discharge structure; • benchmark information; • a soil report which discusses the suitability of the soil for the construction of the proposed facilities. These items should be submitted in support of a written report which describes the proposed location and configuration of the detention facility,the methods used in the design of the facility, and the conclusions of the detention analysis regarding the effectiveness of the facility in mitigating increases in downstream peak flow rates. Preliminary construction plans may be submitted along with the engineering report. Final plans should be prepared after the City and DD7 have completed their review of the engineering report and issued comments. 2.5.6. General Engineering Report Requirements Engineering reports shall be prepared in such a manner as to include all the necessary information without referencing previous submittals. Each report should utilize text, tables, and exhibits to thoroughly document the methods, data, and assumptions used in completing analyses of the proposed activity as well as the results obtained. Detailed computations and computer printouts should be attached to the report in the form of appendices. All reports should be bound to ensure that the report text, exhibits, and attachments stay together. All reports and accompanying materials should be submitted in a manageable format. Drawings and maps should be 24" x 36". All drawings, maps, and other exhibits must be legible, and information should be presented clearly and concisely. The following exhibits and calculations should be submitted with engineering reports as appropriate: • Vicinity Map: A map showing the project site with respect to recognizable landmarks in the vicinity. This could be a city map with the boundaries of a new development, or the limits of a channel improvement project indicated to mark the project location. • Site Map: This is a detailed map of the project site that illustrates the type and extent of activities proposed to be completed.For new developments,a plat with all proposed streets, lot boundaries, etc., may be used to satisfy this requirement. • Watershed or Drainage Map: A watershed or drainage map illustrates all drainage boundaries, flow directions, and computation points. • Discharge Calculations: Calculations specifying computed discharges at critical locations, with comparisons of existing and proposed discharges where appropriate. Drainage areas, runoff coefficients, rainfall depths and intensities, infiltration loss parameters, unit hydrograph parameters, and other applicable hydrologic data should be included and clearly documented. For computer applications, printouts should be attached. 15 • Hydraulic Calculations: Hydraulic calculations specifying the methods used in analyzing channels, storm sewers, and other hydraulic structures and providing a summary of the results obtained. Cross-section data, roughness coefficients, flow rates, and other data should be documented. For computer applications, printouts should be attached. • Benchmark Information: A description of the benchmark used to establish existing and proposed elevations in the project area, including the exact location, the elevation, and the source of the elevation. • Right-of-Way Map: A map that illustrates existing and proposed channel and utility ROWs and easements. Include both underground and overhead utilities and all drainage easements. Sufficient ROW must be set aside to allow for the construction of the most extensive permanent drainage facilities proposed to pass through the development in the future. These facilities may include open or enclosed channels, storm sewers, ditches, or swales. For channels, the width of the ROW must be adequate to provide for the channel itself, plus minimum maintenance berm widths. For enclosed systems considered to be secondary facilities in the City's jurisdiction, the minimum ROW width is equal to the widest dimension of the underground conduit plus two times the maximum depth from finished ground to the invert of the conduit, or 20 feet,whichever is greatest. For enclosed systems considered to be primary facilities,the minimum width is 30 feet. • Soils Report: A soil report prepared by a qualified geotechnical engineer, which identifies the existing soil types and assesses the suitability of the soil for the proposed activity. The soil report should address erosion and slope stability in areas subject to the action of storm runoff. • Plotted Conduit or Stream Profile:A profile of the subject conduit or stream which includes computed water surface profiles, existing and proposed flow-line profiles, the locations of existing and proposed bridges, culverts, and utility crossings, the locations of tributary confluences and major storm sewer outfalls in or near the project area,and the locations of hydraulic structures such as dams,weirs, and drop structures. • Plotted Cross-Sections: Typical cross-sections of the subject conduit or stream for both existing and proposed conditions. • Flood Plain Maps: A FIRM showing the boundaries of the existing 100-year flood plain and floodway in the project area and a separate map that illustrates proposed changes in the floodplain or floodway boundaries. • Facility Layout Map: Plan,elevation,and cross-section views of drainage facilities such as storm sewers, detention basins, roadway culverts, and bridges. • Erosion Control: All drainage facilities must be designed and maintained in a manner that minimizes the potential for damage due to erosion. Storm sewers entering ditches or detention basins shall follow acceptable standards of entry.No bare earthen slopes will be allowed.Various slope treatments,including turf establishment,concrete slope paving,and 16 riprap,are accepted. Flow velocities should be kept below permissible values for each type of slope treatment. Interceptor structures and backslope swale systems are required to prevent sheet flows from eroding the side slopes of open channels and detention facilities. 2.6. REVIEW & APPROVAL OF SUBMITTALS TO THE ENGINEERING DEPARTMENT Upon receiving an engineering submittal, representatives of the City and DD7 will check it for completeness and will request additional information as needed. Upon receiving all the information necessary to thoroughly evaluate the submittal,the City and DD7 will complete their review. Written comments will be forwarded to the submitter, who will make any corrections or adjustments to the analysis and re-submit a final package. Upon determining that all necessary corrections and adjustments have been made, the City and DD7 will prepare written acceptances of the submittal. 17 18 Chapter 3 Hydrologic and Hydraullic Concepts 3. HYDROLOGIC AND HYDRAULIC CONCEPTS 3.1. DEFINITIONS OF BASIC TECHNICAL TERMS • conveyance:the ability of a channel or conduit to carry water in the downstream direction. • cross-sectional area: the total area available to carry flow, measured at a vertical plane (cross-section), which cuts across a channel or conduit perpendicular to the direction of flow. • flood plain: an area inundated by flood waters during or after a storm event of a specific magnitude. • friction loss: a loss in energy associated with friction between flowing water and the sides of a channel or conduit • hydraulic radius: a parameter computed as the cross-sectional area divided by the wetted perimeter • hydrology:the study of the processes through which atmospheric moisture passes between the time that it falls to the surface of the earth as rainfall and the time that it returns to the atmosphere. • hydrograph: a graph that relates the rate of flow and time. • infiltration: the process by which rainfall soaks into the ground. • Manning's Equation: a mathematical formula that relates the velocity or rate of flow in a channel or conduit to the physical characteristics of the channel or conduit. • minor loss: a loss in energy associated with changes in flow direction or velocity. • probability: the chance, usually expressed in percent, that a storm event of a particular intensity and duration will occur in any given year;equal to the reciprocal of the recurrence interval. • rainfall intensity: the rate at which rainfall occurs, typically expressed in inches per hour. • Rational Method: a mathematical equation used to determine the volume of stormwater run-off; • recurrence interval: the average period of time which will elapse between storms of a particular intensity and duration (equal to the reciprocal of the probability). • roughness coefficient: a number that represents the relative resistance to flow in a channel or conduit. 19 Chapter 3 Hydrologic and Hydraulic Concepts • runoff: precipitation that does not infiltrate into the ground but instead makes its way to a stormwater drainage facility. • storm event: a single period of heavy rainfall,normally lasting from a few minutes to a few days. • time of concentration: the time required for water to travel from the most remote point in a watershed to the point at which a peak flow rate or runoff hydrograph is to be computed. • unit hydrograph: a runoff hydrograph that represents the response of a watershed to 1 inch of runoff. • watercourse:a path which water follows from the boundary of a watershed to the watershed outlet. • wetted perimeter: the total distance along a channel or conduit cross-section that is in contact with water that is flowing in the channel or conduit. 3.2. BASIC HYDROLOGIC CONCEPTS 3.2.1. Manning's Equation Manning's equation is a commonly used formula that relates the hydraulic capacity and the physical condition of an open channel,a storm sewer pipe,or a box culvert.The equation is written as follows: Q = (L49) 213112 n where: Q = the flow rate(cubic feet per second); a roughness coefficient related to the relative condition of the channel or n = structure; A = the cross-sectional area of flow(square feet); R = the hydraulic radius, which is computed as the flow area divided by the wetted perimeter(feet); S = the slope of the channel or structure; Equation 3-1 3.2.2. Conveyance 20 Chapter 3 Hydrologic and Hydraullic Concepts Conveyance is a measure of the capacity of a channel, flood plain, or hydraulic structure to carry stormwater. As indicated in Equation 3-2, conveyance increases with the cross-sectional area of flow,the depth of flow in the structure,and the smoothness of the surfaces with which water comes into contact. For example, enlarging a drainage channel will increase the conveyance and the rate of stormwater flow within the channel. Clearing away trees and brush from a channel will have the same effect. Replacing a corrugated metal pipe (CMP)with a reinforced concrete pipe (RCP) of the same diameter also results in an increased conveyance because of the smoother interior of the RCP. 1.49 K = AR213 n where: K = Conveyance (cubic feet per second) Equation 3-2 3.3. EFFECTS OF URBANIZATION Urbanization includes activities such as land clearing, new development, roadway construction, improvements to drainage systems, changes in natural land topography, placement of fill in flood plains,and construction of pavements and other impervious surfaces.These types of activities have significant effects on the response of a watershed to rainfall. 21 22 Chapter 4 Hydrology 4. HYDROLOGY The purpose of this chapter is to provide detailed information on the hydrologic analyses required by the City.This chapter is divided into two main sections. Section 4.1 describes requirements for the hydrologic analysis of drainage areas up to 200 acres,while Section 4.2 describes requirements for the hydrologic analysis of drainage areas greater than 200 acres. 4.1. DRAINAGE AREAS UP TO 200 ACRES 4.1.1. Introduction This section describes the methods to be used in hydrologic analyses of drainage areas up to 200 acres. These analyses may be completed using the Rational Method. However, a HEC-HMS hydrologic analysis can also be performed for drainage areas up to 200 acres using the methodology described in Section 4.2. 4.1.2. The Rational Method The Rational Method relates the runoff rate from a watershed to drainage area, land use, and rainfall intensity. The basic equation used in the Rational Method to compute the runoff rate is: Q = CXCaXIxA where: Q = the peak runoff rate (cubic feet per second); C = a runoff coefficient dependent on land use; a runoff coefficient adjustment factor dependent on the storm recurrence __ Ca interval; I = the rainfall intensity (inches per hour); A = the drainage area(acres). Equation 4-1 23 Chapter 4 Hydrology 4.1.3. Establishing the Drainage Area Drainage areas for Rational Method analyses should be established using topographic maps, storm sewer layouts, and other available information. At each computation point, the drainage area is defined as the total area contributing to runoff at that location. 4.1.4. Determining Runoff Coefficients Table 4-1 provides a summary of runoff coefficients for various land uses, overland slopes, and soil types. The appropriate runoff coefficient may be selected by establishing the land use and consulting this table. For example, an area developed as an apartment complex on land that slopes at less than one percent would have a runoff coefficient of 0.75. Land use data may be obtained from zoning maps, aerial photographs, and site visits. TABLE 4-1: RATIONAL METHOD COEFFICIENTS FOR 2-TO 10-YEAR STORMS Basin Slope Description of Area < 1% 1% -3.5% >3.5% Single-Family Residential Districts Lots greater than 1/2 acre 0.30 0.35 0.40 Lots 1/4 to 1/2 acre 0.40 0.45 0.50 Lots less than 1/4 acre 0.50 0.55 0.60 Multi-Family Residential Districts 0.60 0.65 0.70 Apartment Dwelling Areas 0.75 0.80 0.85 Business Districts Downtown 0.85 0.87 0.90 Neighborhood 0.75 0.80 0.85 Industrial Districts Light 0.50 0.65 0.80 Heavy 0.60 0.75 0.90 Railroad Yard Areas 0.20 0.30 0.40 Cemeteries 0.10 0.18 0.25 Playgrounds 0.20 0.28 0.35 Streets Asphalt 0.80 0.80 0.80 Concrete 0.85 0.85 0.85 Concrete Drives and Walks 0.85 0.85 0.85 Roofs 0.85 0.85 0.85 Lawn Areas Sandy Soil 0.05 0.08 0.12 Clay Soil 0.15 0.18 0.22 Woodlands Sandy Soil 0.15 0.18 0.25 Clay Soil 0.18 0.20 0.30 Pasture 24 Chapter 4 Hydrology Sandy Soil 0.25 0.35 0.40 Clay Soil 0.30 0.40 0.50 Cultivated Sandy Soil 0.30 0.55 0.70 Clay Soil 0.35 0.60 0.80 Table 4-1 For drainage areas with multiple land uses, runoff coefficients and drainage areas associated with each land use shall be determined. The composite runoff coefficient shall then be computed using Equation 4-2: V � (Ci xAt) Cu' = L AT where: Cyt, = weighted runoff coefficient; Ci = runoff coefficients for various land use; Ai = drainage areas corresponding to values of Ci(acres); AT = total drainage area(acres). Equation 4-2 As indicated previously, a runoff coefficient adjustment factor (Ca) shall be used to adjust peak runoff rates for various recurrence intervals. Table 4-2 lists the runoff coefficient adjustment factors for storm recurrence intervals ranging from two to 100 years. TABLE 4-2: RATIONAL METHOD RUNOFF COEFFICIENT ADJUSTMENT FACTORS Storm Recurrence Interval(years) Adjustment Factor(Ca) 2 - 10 1.00 25 1.10 50 1.20 100 1.25 Table 4-2 4.1.5. Establishing the Time of Concentration The time of concentration (Tc) is defined as the time (in minutes) required for all portions of the watershed to contribute runoff at the computation point. The Tc is normally calculated by identifying the longest flow path within the watershed and estimating the time required for runoff to travel the entire length of this path. Stormwater runoff may pass through a range of flow conditions as it moves along the longest flow path. Overland sheet flow is characterized by very 25 Chapter 4 Hydrology shallow depths of less than two inches.Within a short distance of about 100 to 300 feet,stormwater runoff begins to flow at greater depths to collect in streets, swales,and small ditches or gullies and is commonly known as concentrated overland flow. Finally,the=runoff collects in storm sewers, creeks, and drainage channels in which flow depths may reach several feet. To estimate Tc, the longest flow path is divided into reaches that represent the various types of flow conditions, and the flow velocity for each individual reach is estimated. For example, the longest flow path may include overland sheet flow,concentrated flow in a roadside ditch,and flow in a drainage channel. Flow velocities for overland sheet flow and some concentrated flow conditions may be estimated using the Uplands Method, which relates flow velocity to overland slope and land use. This method was developed by the U.S. Department of Agriculture Soil Conservation Service (SCS). Exhibit 4-1 provides a graphical representation of the Uplands Method. For storm sewers, creeks, and channels, flow velocities may be estimated using Manning's equation or a HEC-RAS model (see Chapter 5). The length of each individual reach is divided by the flow velocity to obtain the time of travel required for water to pass through the reach, and TC is calculated as the sum of the individual travel times. 4.1.6. Computation of Rainfall Intensity The rainfall intensity(I)for a particular frequency used in the Rational Method may be determined from Equation 4-3, which was developed by the Texas Department of Transportation (TxDOT) from the U.S. Weather Bureau publications Technical Paper No. 40 and Hydrometeorological Report No. 35. The values are currently being updated by TxDOT based on NOAA Atlas 14, volume 11. b I = (TT + d)e where: I = rainfall intensity (inches per hour); Tc = time of concentration (minutes); b, d, e = rainfall intensity parameters from Table 4-3. Equation 4-3 If the calculated Tc is less than 10 minutes, then a Tc of 10 minutes should be used in Equation 4-3. 26 Chapter 4 Hydrology TABLE 4-3: RAINFALL INTENSITY PARAMETERS FOR JEFFERSON COUNTY,TEXA CONSTANTS FOR JEFFERSON COUNTY Storm e b d Frequency 2-Year 0.8004 67.1216 13.2828 5-Year 0.7718 80.2163 13.4688 10-Year 0.7520 88.3236 13.4318 25-Year 0.7271 97.2164 13.3299 50-Year 0.7068 101.3770 13.0479 100-Year 0.6872 105.5701 12.9873 Table 4-3 Note:The rainfall data presented above is the latest available as of the date of the Drainage Criteria Manual issuance. The City and/or DD7 may adopt revised data not reflected in this table. It is the engineer's responsibility to ensure that current accepted rainfall intensity information is being utilized for the analysis. 4.2. DRAINAGE AREAS GREATER THAN 200 ACRES This section describes the methods to be used in hydrologic analyses of drainage areas greater than 200 acres. These analyses shall be completed using the HEC-HMS computer program developed at the Hydrologic Engineering Center of the U.S. Army Corps of Engineers (USACE). This software program can be downloaded from the USACE's website (http://www.hec.usace.army.mil/software/hec-hms/hechms-download.html) at no charge. The Hydrologic Modeling System HEC-HMS User's Manual,the Hydrologic Modeling System HEC- HMS Applications Guide, and the Hydrologic Modeling System HEC-HMS Technical Reference Manual developed by the USACE can be used for further reference. These manuals can also be downloaded from the USACE's website (http://www.hec.usace.army.mil/software/hec- hms/hechms-document.html)at no charge. The hydrologic parameters discussed in Sections 4.2.1 to 4.2.5 are the basis for developing HEC-HMS models. 4.2.1. Watershed Boundaries Topographic information, storm sewer layouts, and other available information shall be used to provide the level of detail necessary to delineate additional sub-area boundaries as needed. These boundaries may be delineated by hand or with HEC-GeoHMS,which is a Geographic Information Systems (GIS) program that works in conjunction with ArcView to compute hydrologic parameters. However, HEC-GeoHMS sub-area boundaries should be closely reviewed by an engineer familiar with the topography of the drainage area. The number of sub-areas required for the HEC-HMS analysis is a function of the number of computation points, which are typically established at confluences with tributaries, roadway crossings, or other points of interest (lakes and reservoirs,etc.).Normally,there is one sub-area above the first analysis point and one or more 27 Chapter 4 Hydrology between each pair of successive analysis points. In addition, there is at least one sub-area for each tributary. 4.2.2. Rainfall Data The rainfall depth-duration-frequency data of the City and DD7 listed in Table 4-4 was acquired from NOAA Atlas 14, volume 11, Version 2, Port Arthur SE TX, and shall be used in HEC-HMS hydrologic analyses. The curves are currently being updated based on the Atlas 14 data. The rainfall depth data and exceedance probability associated with the design storm event shall be entered in the meteorological model of HEC-HMS. A one-percent exceedance probability would be entered for a 100-year storm event, four percent would be entered for a 25-year event, and twenty percent would be entered for a 5-year event. A maximum intensity duration of five minutes shall be used for the analysis regardless of the design storm event. A value of 67 percent is used as the peak center, and the HEC-HMS program automatically distributes the rainfall over a 24-hour period in such a manner that the maximum rainfall intensity occurs at approximately two-thirds of the storm event. Rainfall leading up to and following the period of maximum intensity is distributed in a manner that produces a balanced rainfall distribution. Since the use of the total area option in HEC-HMS is problematic for many types of hydrologic analysis, point rainfall data will be used, and a total storm area of 0.01 square miles, or other approved area, is used to compute runoff hydrographs. A base flow of zero shall be used unless project-specific considerations warrant the use of this parameter. TABLE 4-4: RAINFALL DATA FOR CITY OF PORT ARTHUR NOAA Atlas 14,volume 11,Version 2, Port Arthur SE TX Recurrence Rainfall Depth (inches) for Given Duration Interval (years) 15 MIN 30 MIN 1 HR 2 HR 3 HR 6 HR 12 HR 24 HR 2 1.23 1.77 2.37 3.04 3.45 4.16 4.88 5.63 5 1.52 2.17 2.94 3.88 4.49 5.54 6.54 7.56 10 1.76 2.50 3.40 4.61 5.43 6.84 8.15 9.49 25 2.08 2.95 4.04 5.66 6.81 8.79 10.6 12.5 50 2.32 3.28 451 6.49 7.94 10.5 12.8 15.1 100 2.57 3.62 5.01 7.40 9.21 12.3 15.3 18.3 500 3.22 4.59 6.52 10.1 12.9 17.9 22.7 27.6 Table 4-4 4.2.3. Infiltration Losses Infiltration losses shall be accounted for using the Green & Ampt method, which is a conceptual representation of the infiltration process, was developed in 1911 by Green & Ampt. This method estimates infiltration losses based on a function of soil texture and the capacity of the given soil type to convey water. The advantage of this method is that the parameters can be estimated based 28 Chapter 4 Hydrology on soil type. The parameters should be applied on- a watershed-wide basis, similar to the _. exponential and initial/uniform loss methods from the original-Flood Hazard tudy. Table 4-5 shows the volume moisture deficit, wetting front suction, and hydraulic conductivity parameters for various soil textures and types. The hydrologic soil group to which a particular soil belongs may be determined by consulting the Soil Survey or the Soil Survey Geographic Database (SSURGO) for Jefferson County, Texas. TABLE 4-5: GREEN&AMPT LOSS PARAMETERS Volume Wetting Front Hydraulic Soil Classification Moisture Suction Conductivity Deficit (inches) (inches/hour) Soil Texture Sand 0.417 1.95 9.276 Loamy Sand 0.402 2.41 2.354 Sandy Loam 0.412 4.33 0.858 Loam 0.436 3.50 0.520 Silt Loan 0.486 6.57 0.268 Sandy Clay Loam 0.330 8.60 0.118 Clay Loam 0.389 8.22 0.079 Silty Clay Loam 0.431 10.75 0.079 Sandy Clay 0.321 9.41 0.047 Silty Clay 0.423 11.50 0.039 Clay 0.385 12.45 0.024 Soil Group A(freely draining) 0.417 1.95 9.276 B (intermediate) 0.436 3.50 0.520 C (intermediate) 0.389 8.22 0.079 D(poorly draining) 0.385 12.45 0.024 Table 4-5 29 11R'. Chapter 4 Hydrology 4.2.4. Initial Abstraction Initial abstraction losses shall be accounted for using the Soil Conservation Service (SCS) Curve _r Number method, which is an empirical method developed by the U.S. Department of Agriculture. Equation 4-4 can be used to compute the initial abstraction for specific soil types. la = 0.2S where: Ia = the initial abstraction depth (inches); loon S = initial retention= — 10 CN CN = SCS Curve Number, from Table 4-6. Equation 4-4 The Curve Number is a function of soil structure, antecedent watershed moisture, and land use. Soil structure is defined by assigning individual soils to one of four hydrologic soil groups (A through D)that represent a wide range of soil porosities. Soils belonging to hydrologic soil group A are the most porous, while soils in group D are the least porous. The hydrologic soil group may be determined from the Soil Survey for Jefferson County, Texas, or the Soil Survey Geographic Database(SSURGO)for Jefferson County.Table 4-6 provides a summary of SCS Curve Numbers for various land uses,taken from the SCS National Engineering Handbook, Section 4. TABLE 4-6: SCS CURVE NUMBERS Hydrologic Soil Group Land Use Description A B C D Cultivated Land Without Conservation Treatment 72 81 88 91 With Conservation Treatment 62 71 78 81 Pasture or Range Land Poor Condition 68 79 86 89 Good Condition 39 61 74 80 Meadow: Good Condition 30 58 71 78 Wood or Forest Land Thin Stand, Poor Cover, No Mulch 45 66 77 83 Good Cover 25 55 70 77 Open Spaces,Lawns, Parks,Cemeteries Good Condition, 75%Grass Cover 39 61 74 80 Poor Condition, 50-75%Grass Cover 49 69 79 84 Commercial and Business Areas(85% Impervious) 89 92 94 95 30 Chapter 4 Hydrology Industrial Districts(72%Impervious) 81 88 91 _ 93 Residential Average Lot Size Average %Impervious 1/8 acre or less 65 77 85 90 92 1/4 acre 38 61 75 83 87 1/3 acre 30 57 72 81 86 1/2 acre 25 54 70 80 85 1 acre 20 51 68 79 84 Paved Parking Lots, Roofs, Driveways, Etc. 98 98 98 98 Streets and Roads Paved with Curbs and Storm Sewers 98 98 98 98 Gravel 76 85 89 91 Dirt 72 82 87 89 Table 4-6 For watersheds with varying land uses and soil types,composite Curve Numbers may be computed by determining the Curve Number and drainage area associated with each land use and/or soil category. The composite Curve Number may then be computed using the following equation: (CN, x Ai) CNW = AT where: CNw = weighted Curve Number; CN, = Curve Number for various land uses and soil types; Ai = drainage areas corresponding to values of CNi (acres); AT = total drainage area(acres). Equation 4-5 In HEC-HMS applications, cumulative totals for rainfall and infiltration are maintained. The total runoff is re-computed for every time step. 4.2.5. Percent Impervious Cover Percent impervious cover is a function of land urbanization and can be estimated from field observations, aerial photographs, and other supporting information on the drainage area. Table 4- 7 provides a summary of percent impervious cover values for different land use categories: 31 Chapter 4 Hydrology TABLE 4-7: PERCENT IMPERVIOUS COVER VALUES FOR JEFFERSON COUNTY,TEXAS Land Use Categories Land Use Descriptions % Impervious Undeveloped Unimproved,natural,or agricultural 0 Residential—Rural Lot >_ 5-acre ranch or farm 5 Residential Average Lot Size 1 acre 20 '/2 acre 25 '/3 acre 30 'A acre 38 '/8 acre or less 65 Developed Green Areas Parks or golf courses 15 Light Office parks,nurseries,airports,warehouses, 60 Industrial/ or manufacturing with non-paved areas Commercial High Density Commercial,business,industrial, 85 or apartments Transportation Highway or major thoroughfare corridors 90 Water Detention basins, lakes, channels, roadside 100 ditches Table 4-7 4.2.6. Loss Rate Computations in HEC-HMS The Green&Ampt loss rate parameters and percent impervious cover values discussed in Sections 4.2.3 to Section 4.2.5 are entered into the loss rate option of the HEC-HMS Subbasin Model Editor window. 32 Chapter 4 Hydrology 4.2.7. Unit Hydrograph Methodology Unit hydrographs shall be computed based on the Clark Urrit-Hydrograph-method,which is-one of the unit hydrograph methods available in HEC-HMS. The Clark Unit Hydrograph method uses three parameters to define a unit hydrograph for a watershed: the Tc, a storage coefficient, and a time-area curve. The Tc is defined as the time required for all portions of the watershed to contribute runoff at the computation point. Refer to Section 4.1.5 for more information on estimation of Tc. The storage coefficient (R) is an indicator of the available stormwater storage volume within a watershed within depressions, ponds, channels, and flood plains. The value of R varies directly with the relative amount of storage volume within a watershed (i.e., the greater the storage volume,the higher the storage coefficient).For DD7,R should be estimated from Equation 4-6 or any other methodology approved by the City and/or DD7. R = 3xTc Equation 4-6 The time-area curve relates the percentage of the watershed contributing runoff at the analysis point to the fraction of the Tc, which has elapsed since the beginning of runoff. The entire watershed is considered to be contributing runoff at the outlet when the elapsed time is equal to or greater than the Tc.This standard curve is applicable as long as extremes in watershed shapes(i.e., very large or very small ratios of watershed length to width) are avoided. Calculation of the time- area curve is handled internally by HEC-HMS with a standard time-area curve based on assumed watershed shape. Runoff hydrographs can be computed in HEC-HMS by selecting the Clark Unit Hydrograph method from the Transform option of HEC-HMS Subbasin Editor Window. In addition, the Tc and R parameters should be entered for each subbasin. The meteorological model data works in conjunction with the subbasin editor data to calculate a hydrograph for each subbasin. 33 Chapter 4 Hydrology 4.2.8. Streamfiow Routing Streamflow routing is the process by which the lagging and-attenuating-effects of travel time storage on runoff hydrographs are considered as flood flows move from-one analysis point-to- another. Although the HEC-HMS program offers several streamflow routing methods, the City and DD7 require the use of the Modified Puls method, where channel cross-sections and a HEC- RAS hydraulic model of the channel are available. For streamflow routing along channels without a HEC-RAS model,the Muskingum-Cunge Standard, Muskingum-Cunge 8-Point,or Muskingum methods should be used depending on which method is best suited to the specific application. However, if backwater conditions and/or overland flooding are anticipated, it is recommended that a HEC-RAS model of the channel be developed and the Modified Puls method be used. The Modified Puls Method explicitly accounts for the effects of storage volume within the flood plain and is based on a simple continuity equation: OS = 1 - 0 where: AS = change in storage volume within the routing reach; 1 = inflow to the routing reach; 0 = outflow from the routing reach. Equation 4-7 For the Modified Puls method, input to the HEC-HMS program consists of a set of flow rates and corresponding storage volumes, which are input in the basin model routing reach window. Additionally, the number of sub-reaches and initial flow conditions are selected in the same window. The Muskingum method approximates the continuity equation (Equation 4-7), where storage is modeled as the sum of prism and wedge storage. Required input parameters for this method include the Muskingum K,Muskingum X(ranges from 0.0 to 0.5),and the number of sub- reaches. Refer to the HEC-HMS documentation mentioned in Section 4.2 for additional information on these routing methods. HEC-HMS modeling input for the Muskingum-Cunge Standard method consists of the following physical parameters:the length and slope of the routing reach, Manning's roughness coefficient(n value),the shape of the channel (circular or prismatic),the bottom width or diameter, and the side slope ratio. This mathematical routing method provides an implicit accounting of storage within the channel.However, storage within the flood plain outside the defined channel is not considered. Although the same equations and solution techniques are used for the Muskingum-Cunge 8-Point method, the channel is described with eight station-elevation coordinates instead of a standard cross-section shape. Other required input items for this method are the reach length, energy slope, and"n"values for the channel and overbank. For additional information on these routing methods, refer to the HEC-HMS documentation listed in Section 4.2. 34 Chapter 4 Hydrology 1.00 0.90 0.80 0.70 0.60 0.50 ... 0.40 ...." 0.30 ..- ,...-.• ,,,,i.,. 0.20 , ...i.'"' -- -3,- _-- ..,..- .,.., z..-' .-.. - . :-. 4,t 0.08 0.07 .-..-` -:- . .. ,...: 7 ,..z..- .:- z, ,:". -:- , *-- - .7. .-, ....- -,...-----. _÷....,.: .' .4.-z, -...-- — '....=-. 0.04 — -7 =-- '` -..... $ ..:1- .i.-- _... 0.03 , -;••• zi ..;•.• F '77 .... 0.02 -..." 2..... ..5 :.'-` ..:: •-•:- ., , s.... ..... ...-.- ,.... ..-.,- --- •-...:-- ...- „.--- :„.- ....,- -... :NI 6 6—V: ..1.: 1*-: I. 1-X:-.77,,= 71 7: "7 ..r.:. .,.0 N. L:4, R. -6, 6 6 © 6. = 6: — Velocity((t/s) DRAINAGE CRITERIA MANUAL EXHIBIT 4-1 p •,,,prr SCS UPLAND METHOD 111111,' 411)))1.1 SCALE:NTS DATE NOVEMBER 2022 DSON BY:1 M W DRWN BY.N M F. SHEET NO 1 OF 1 35 36 Chapter 5 Hydraulics of Primary Drainage Facilities 5. HYDRAULICS OF PRIMARY DRAINAGE FACILITIES The purpose of this chapter is to provide detailed information on the hydraulic analysis and design of primary drainage facilities within the City's jurisdictions.As indicated in Chapter 1,the primary drainage facilities include open channels, bridges, culverts, and enclosed drainage systems (i.e., open channels that have been enclosed). 5.1. GENERAL DESIGN REQUIREMENTS FOR PRIMARY DRAINAGE FACILITIES The following design requirements are discussed in this section: • design storm frequencies; • design requirements for earthen channels; • design requirements for concrete-lined channels; • design requirements for rectangular concrete low-flow sections; • transitions, bends, and confluences; • design requirements for culverts; • structural requirements for culverts; • design requirements for bridges; • design requirements for enclosed systems; • Maximum allowable velocities. 5.1.1. Design Storm Frequencies The following design storm frequencies shall be used for the analysis and design of open channels, bridges, culverts, and enclosed systems: • Channels draining up to 100 acres shall be designed to convey 25-year peak discharges with a minimum freeboard of one foot. • Channels draining between 100 acres and 200 acres shall be designed to convey 50-year peak flow rates with a minimum freeboard of one foot. • Channels draining greater than 200 acres shall be designed to convey 100-year peak flow rates with a minimum freeboard of one foot. These channels shall also be analyzed using a 10-year design storm event. 37 Chapter 5 Hydraulics of Primary Drainage Facilities • For open channel studies involving the Federal Emergency Management Agency(FEMA) submittals, the 10-year, 50-year, 100-year, and 500-year storm frequencies_must_be analyzed. 5.1.2. Design Requirements for Earthen Channels The following minimum requirements shall be incorporated into the designs of earthen channels: • Channel side slopes shall be no steeper than 3 horizontals to 1 vertical (3:1). Flatter slopes may be required when soil conditions are conducive to slope instability. • The minimum channel bottom width shall be six feet. • A maintenance berm is required on both sides of the channel. For channels with top widths of 30 feet or less,the minimum maintenance berm width is 15 feet.For top widths between 30 feet and 60 feet, 20-foot maintenance berms are required. For channels with top widths greater than 60 feet, the minimum maintenance berm width is 30 feet. Additional width shall be provided to accommodate backslope swales if necessary. • Backslope drain swales and interceptor structures are required to prevent flow down the ditch side slopes. The maximum spacing for interceptor structures is 600 feet. • Channels, channel rights-of-way (ROW), and side slopes must be vegetated immediately after construction to minimize erosion in accordance with the erosion control requirements discussed in Chapter 8. • Flow from roadside ditches must be conveyed into open channels through standard roadside ditch interceptor structures as described in Chapter 8. • A geotechnical investigation and report on local soil conditions is required for all channel construction and improvement projects. Exhibit 5-1 illustrates a typical design cross-section for a trapezoidal earthen channel. 5.1.3. Minimum Design Requirements for Trapezoidal Concrete-Lined Channels Concrete-lined channels shall be designed to meet the following minimum requirements: • All concrete slope paving shall consist of Class A(3000 psi) concrete. • The minimum bottom width shall be eight feet. • The side slopes of the channel shall be no steeper than 1.5 horizontal to 1 vertical (1.5:1). • A maintenance berm is required on both sides of the channel. The berm width shall be at least 20 feet on one side of the channel and at least 15 feet on the other side. • All slope paving shall include a toe wall at the top and sides with a minimum depth of 18 inches. Toe walls shall also be included along the bottom of the channel, with a minimum depth of 24 inches for clay soils and 36 inches for sandy soils. 38 Chapter 5 Hydraulics of Primary Drainage Facilities • Backslope drain swales and interceptor structures are required in the channel maintenance berm to prevent overland flow down the bank of partially lined channels.These items shall be designed in accordance with the minimum requirements specified in Chapter 8. However, backslope drain swales and interceptor structures are not required on fully lined channels. • Channel maintenance berms must be vegetated immediately after construction in accordance with erosion control requirements discussed in Chapter 8. • Weep holes shall be used to relieve hydrostatic pressure behind lined channel sections.The specific type, size,and placement of the weep holes shall be based on the recommendations of the engineers. • Where construction is to take place under muddy conditions or where standing water is present, a seal slab of Class C (1500 psi) concrete shall be placed in the channel bottom prior to placement of the concrete slope paving. • Control joints shall be provided at a maximum spacing of approximately 25 feet.A sealing agent shall be utilized to prevent moisture infiltration at control joints. • Concrete slope protection shall have the minimum thickness and reinforcement indicated in Table 5-1. • A geotechnical investigation and report on local soil conditions is required for all channel construction and improvement projects. • Exhibit 5-2 illustrates a typical design cross-section for a trapezoidal concrete-lined channel. TABLE 5-1: MINIMUM THICKNESS AND REINFORCEMENT FOR CONCRETE SLOPE PAVING Channel Side Slope Minimum Thickness Minimum Reinforcement (H:V) (inches) Material Dimensions 3:1 4 inches welded wire 6 x 6 x W2.9 x W2.9 fabric* 2:1 5 inches welded wire 6 x 6 x W4.0 x W4.0 fabric* 1.5:1 6 inches reinforcement 4 x 4 x W4.0 x W4.0 *Rolled wire fabric not allowed Table 5-1 39 Chapter 5 Hydraulics of Primary Drainage Facilities 5.1.4. Design Requirements for Rectangular.Concrete Low-Flow ec ions. - --- --- - --- Rectangular concrete low-flow sections can be incorporated into designs for earthen and concrete- lined channels to provide additional capacity or depth in areas where channel ROW is limited. The following criteria shall be used for concrete low-flow sections: • All concrete slope paving shall consist of Class A(3000 psi)concrete. • The structural steel design should be based on the use of ASTM A-615, Grade 60 steel. • The minimum bottom width of the low-flow section shall be eight feet. • For bottom widths of 12 feet or more, the channel bottom shall be graded toward the centerline at a slope of 1/2 inch per foot(4.15 percent). • The maximum height of vertical concrete walls shall be three(3) feet. • Escape stairways shall be located at the upstream side of all roadway crossings.Additional escape stairways shall be located along the channel to keep the maximum distance between stairways below 1,400 feet between stairways. • For channels with concrete low-flow sections,the top of the vertical concrete wall shall be constructed in such a way as to provide for the possible future placement of concrete slope paving. • Weep holes shall be used to relieve hydrostatic pressure behind lined channel sections.The specific type, size,and placement of the weep holes shall be based on the recommendations of the geotechnical report. • Where construction is to take place under muddy conditions or where standing water is present, a seal slab of Class C (1500 psi) concrete shall be placed in the channel bottom prior to placement of the concrete slope paving. • Control joints shall be provided at a maximum spacing of approximately 25 feet.A sealing agent shall be utilized to prevent moisture infiltration at control joints. • Concrete low-flow channels may be used in combination with a maintenance shelf on one or both sides of the channel. The minimum width of the shelf shall be fourteen 14 feet. Pavement used on the shelf shall be capable of supporting maintenance equipment having a concentrated wheel load of at least 1,350 pounds. • All designs of concrete low-flow sections shall be supported by structural computations. • A geotechnical investigation and report on local soil conditions is required for all channel construction and improvement projects. 5.1.5. Design Requirements for Transitions,Bends,and Confluences Transitions, bends, and confluences shall be designed to meet the following minimum requirements: 40 Chapter 5 Hydraulics of Primary Drainage Facilities • Transitions in channel bottom widths or side slopes shall.be_designed,to create minimal_ flow disturbance and thus, minimal energy loss. Transition angles should be less than 12 degrees. • A warped or wedge-type transition is recommended for connecting rectangular and - trapezoidal channel sections. • Channel bends shall be made as gradually as possible.The minimum bend radius along the centerline of the channel is three times the top width of the channel at the maximum design water surface elevation(WSEL). Where smaller radii are required, erosion protection(i.e., concrete slope paving, riprap, interlocking blocks, etc.) is required as specified in Chapter 8. In no case shall the bend radius be less than 100 feet. • The maximum allowable deflection angle for any bend in an improved channel is 90 degrees. • Erosion protection shall be provided at channel confluences in accordance with the erosion protection requirements described in Chapter 8. 5.1.6. Design Requirements for Culverts • Culverts shall be designed to convey the fully developed peak discharge rates associated with the design storm frequency requirements provided in Section 5.1.1 while maintaining a minimum freeboard of one foot in the channel upstream of the culvert. In addition, the maximum allowable velocities discussed in Section 5.1.10 must not be exceeded. • Culverts shall be aligned parallel to the longitudinal axis of the channel to maximize hydraulic efficiency and minimize turbulence and erosion.At locations where a difference between the alignment of the channel and the culvert is necessary,the change in alignment shall be accomplished upstream of the culvert so that the culvert is aligned with the downstream channel. • The minimum allowable diameter for circular culverts is 18 inches for City projects. • The minimum allowable size of box culverts is 2 feet x 2 feet. • Concrete slope paving or riprap shall be used upstream and downstream of the culvert to protect earthen channels from erosion. • Culverts shall extend completely across road and railroad ROW at crossing locations. • Where hydraulic jumps are anticipated around culverts, the channel geometry shall be modified to force the hydraulic jump to occur in a portion of the channel protected with concrete slope paving. Hydraulic jumps are characterized by a rapid change in the depth of flow from a low stage to a high stage, which results in an abrupt rise in the WSEL. 41 Chapter 5 Hydraulics of Primary Drainage Facilities 5.1.7. Structural Requirements for Culverts Unless otherwise approved, all pipe and box culverts shall satisfy the following minimum structural design requirements: • All pre-cast reinforced concrete pipes shall be ASTM C-76. • All high-density polyethylene(HDPE)pipe culverts shall conform to the AASHTO M294 specifications.Bedding for HDPE culverts shall be designed and constructed in accordance with the manufacturer's recommendations. • All pre-cast reinforced concrete box culverts with more than two feet of earthen cover shall be ASTM C789-79. All pre-cast reinforced box culverts with less than two feet of earthen cover shall be ASTM 850-79. • All corrugated steel pipes shall be aluminized in accordance with AASHTO M-36. • AASHTO HS20-44 loading shall be used for all culverts. • Joint sealing materials for pre-cast concrete culverts shall comply with the "AASHTO Designation M-198 74 I, Type B, Flexible Plastic Gasket(Bitumen)" specification. • Two-sack-per-ton cement-stabilized sand shall be used for backfilling around culverts. • A six-inch bedding of two-sack-per-ton cement-stabilized sand is required for all pre-cast concrete box culverts. 5.1.8. Design Requirements for Bridges Bridges shall be designed to meet the following requirements: • Bridges shall be designed to convey the fully developed peak discharge rates associated with the design storm frequency requirements provided in Section 5.1.1 while maintaining a minimum freeboard of one foot in the channel upstream of the bridge. In addition, the maximum allowable velocities discussed in Section 5.1.10 must not be exceeded. • New bridges shall be designed to completely span the existing or proposed channel so that the channel will pass under the bridge without significant contractions or changes in the channel shape. Bridges constructed on existing or interim channels shall be designed to accommodate the ultimate channel section with minimum structural modifications. • Bridges shall be designed to intersect the channel at an angle of 90 degrees, if possible. • Pier bents and abutments shall be aligned parallel to the direction of flow in the channel. Pier bents shall be placed as far from the center of the channel as possible and wherever possible shall be placed within the channel side slopes instead of the channel bottom. • Concrete slope paving or riprap shall be used to protect earthen channels from erosion underneath, upstream, and downstream of bridges. 42 Chapter 5 Hydraulicss-cf Primary Drainage Facilities • Where hydraulic jumps are anticipated around bridges, the.channel geometry shalt be__- modified to force the hydraulic jump to occur in a portion of the channel protected with concrete slope paving. 5.1.9. Enclosed Drainage Systems Enclosed drainage systems include pipe and box culverts used to replace segments of open channel longer than the typical width of a road or railroad ROW. • Enclosed drainage systems shall be designed to accommodate fully developed design peak runoff rates discussed in Section 5.1.1 while maintaining the hydraulic grade line elevations below adjacent natural ground elevations or street gutter elevations, whichever are lower for fully developed watershed conditions. • The minimum inside pipe dimension shall be 24 inches. • The minimum and maximum allowable velocities for design peak runoff rates shall be two feet per second and eight feet per second, respectively, assuming full pipe flow. • Structural requirements for enclosed systems are identical to those specified for pipe and box culverts in Section 5.1.7. • Manholes or junction boxes shall be located no more than 600 feet apart along the entire length of the system, and at all locations where changes in culvert size and shape occur. • Outfall structures shall conform to the requirements set forth for storm sewer outfalls in Chapter 7 of this manual. • The ROW width required for enclosed systems considered to be primary facilities will be set equal to the maximum pipe or box width plus two times the depth to the culvert invert or 30 feet, whichever is greater. 5.1.10. Maximum Flow Velocities The maximum allowable velocity in open channels and at bridges or in culverts shall be analyzed for the design storm event. As shown in Table 5-2, the maximum allowable velocity is related to the type of channel,the slope treatment,and the soil structure throughout the open channel section. If the maximum velocities listed in this table are exceeded during the design storm event,then the channel design shall be modified until acceptable velocities are attained. Alternatively, erosion protection(i.e.,riprap,concrete slope paving,or interlocking blocks)could be provided to increase the maximum allowable velocity in that portion of the channel (see Chapter 8). However, the erosion protection must extend upstream and downstream a sufficient distance to a location where the design storm velocity in the channel is below the maximum allowable levels for earthen channels without slope protection. 43 Chapter 5 Hydraulics of Primary Drainage Facilities TABLE 5-2: MAXIMUM ALLOWABLE VELOCITIES-IN OPEN CHANNELS Soil Description Slope Treatment Maximum Velocity (feet per second) Fine Sand None 1.50 Sandy Loam None 1.75 Silt Loam None 2.00 Clay Loam None 2.50 Stiff Clay None 3.75 Sandy Soils(Easily Eroded) Grass 4.00 Clay Soils (Erosion-Resistant) Grass 5.00 Sandy Soils(Easily Eroded) Riprap 6.00 Clay Soils (Erosion Resistant) Riprap 8.00 Sandy Soils(Easily Eroded) Concrete 8.00 Clay Soils (Erosion Resistant) Concrete 10.00 Bridges& Culverts 8.00 Table 5-2 5.1.11. Maintenance Provisions for adequate maintenance must be made in the design of all drainage facilities. Sufficient ROW must be set aside, slopes must be kept at or below maximum values, and slope treatments must be properly completed. Access to drainage facilities must not be impeded. 5.2. HYDRAULIC ANALYSIS OF PRIMARY DRAINAGE FACILITIES This section describes the methods to be used in the hydraulic analysis of open channels as well as associated bridge and culvert structures. 5.2.1. Acceptable Open Channel Design Methodologies The final open channel dimensions shall be determined by using the HEC-RAS computer program developed at the Hydrologic Engineering Center of the U.S. Army Corps of Engineers (USACE). The latest version of this software program can be downloaded from the USACE's website (http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html)at no charge. The HEC- RAS program has the capability to analyze unsteady flow conditions, transitions from subcritical to supercritical flow, and other complex hydraulic conditions. The hydraulic data discussed in Sections 5.2.1 to 5.2.8 should be compiled to facilitate the development of HEC-RAS models. Additional information on HEC-RAS can be obtained from the HEC-RAS River Analysis System User's Manual, the HEC-RAS River Analysis System Application's Guide and the HEC-RAS River Analysis System Hydraulic Reference Manual developed by the USACE. All these manuals can be downloaded free of cost from the USACE's website at http://www.hec.usace.army.mil/software/hec-ras/hecras-document.html. 44 Chapter 5 Hydraulics of Primary Drainage Facilities 5.2.2. Acceptable Bridge and Culvert Design1Viethodoingies--- -.-. Hydraulic analysis of bridges and culverts may be-performed -using-the HEC-RAS computer program. However, the nomographs developed by the Federal Highway Administration (FHWA) published in Hydraulic Design of Highway Culverts may be used for initial estimates of culvert size or to verify that the results obtained from HEC-RAS are reasonable. These nomographs can also be used to size culverts associated with roadside ditches. In addition, any other software programs which meet industry standards can be used, if prior approval is obtained from the City and/or DD7. 5.2.3. Acceptable Enclosed Drainage System Design Methodologies Hydraulic analysis of enclosed drainage systems that are part of an open channel system may be analyzed using HEC-RAS. In addition, any other software programs which meet industry standards can be used, if prior approval is obtained from the City and/or DD7. For stand-alone enclosed systems, the methodology described in Chapter 6 for storm sewer systems shall be used. For these calculations, full pipe flow may be assumed. Both friction losses and minor losses (i.e., losses due to transitions, bends,junctions, manholes, etc.) should be accounted for. 5.2.4. Flow Data The Rational Method may be used to compute the peak flow rates for drainage areas up to 200 acres. However, a HEC-HMS hydrologic analysis can also be performed for drainage areas up to 200 acres. For drainage areas greater than 200 acres, the HEC-HMS methodology discussed in Chapter 4 shall be used to compute peak discharge rates for the design storm frequencies specified in Section 5.1.1. These peak flow rates shall be used to develop the flow data in HEC-RAS. For unsteady flow analysis, the inflow hydrographs computed in HEC-HMS are used instead of peak discharge rates. These hydrographs should be entered into the Unsteady Flow Data editor of HEC-RAS for unsteady flow detention analyses(see Chapter 7)or the applicable portion of other approved software programs. 5.2.5. Boundary Conditions For HEC-RAS to perform computations, boundary conditions or starting WSELs must be defined. Boundary conditions are required at the downstream and upstream ends of the river system for subcritical and supercritical flow regimes, respectively. For mixed flow regimes, boundary conditions are required at both the upstream and downstream ends of the system. Subcritical flow typically occurs in undeveloped areas. This flow regime has a low velocity and appears tranquil, whereas the supercritical flow regime is characterized by shooting and rapid flows. For unsteady flow detention analyses (see Chapter 7), a variety of boundary conditions are available within the Unsteady Flow Data editor. Refer to the HEC-RAS River Analysis System User's Manual for additional information on the available unsteady flow boundary conditions.For open channel analyses, the downstream boundary conditions should be entered into the Steady Flow Data editor. If a HEC-RAS model of the receiving channel is not available, then normal depth should be used as the downstream boundary condition and the energy slope should be 45 Chapter 5 Hydraulics of Primary Drainage Facilities entered.The energy slope can be approximated as the slope of the bottom of the channel.If a HEC- RAS model of the receiving channel is available and the tailwater in this- channel can be determined, then the known WSEL downstream boundary condition should be selected. In order to determine the tailwater elevation in the receiving channel, the Frequencies of Coincidental Occurrence methodology described in the Texas Department of Transportation's (TxDOT's)Hydraulic Design Manual shall be used.This methodology is based on the assumption that the rainfall events within the drainage system being analyzed, and the receiving channel are neither completely dependent nor completely independent. As shown in Table 5-3, this method provides a basis for selecting an appropriate frequency for the tailwater elevation of the receiving channel versus the frequency for the tributary channel, storm sewer system,or detention basin.For example, a 100-year analysis of a tributary channel with a drainage area of 200 acres that discharges to an open channel with an associated drainage area of 2,000 acres would have a ratio of receiving channel to the tributary of 10:1. Therefore, the required tailwater elevation for the 100-year analysis of the tributary channel would be the 50-year WSEL in the receiving channel. Table 5-3: Design Storm Event for the Hydraulic Analysis of Tributary Area Ratio Channels, Storm Sewer Systems,or Detention Basins 2-year 5-year 10-year 25-year 50-year 100-year 10,000:1 2 2 2 2 2 2 1,000:1 2 2 2 5 5 10 100:1 2 2 5 10 10 25 10:1 2 5 10 10 25 50 1:1 2 5 10 25 50 100 Table 5-3 5.2.6. Cross-Section Data The cross-section data required by HEC-RAS includes elevation-station data; Manning's roughness coefficients (n values), which are described in Section 5.2.7; channel and overbank reach lengths; top of bank (TOB) locations; and expansion and contraction coefficients. If ineffective flow areas (IFAs), levees, or blocked obstructions exist, then geometric information regarding these items would also be entered in the Cross-Section Data Editor. The elevation-station data shall be obtained from recent field survey data and the cross-sections shall be extended far enough into the left and right overbanks so that all of the flow is contained within the defined cross-section, if possible. Although LIDAR data may be used to supplement field survey data and define the overbank areas of the cross-sections, field survey data is required to accurately define the channel because LIDAR does not penetrate water. Cross-sections shall be taken approximately every 500 feet along the channel unless project-specific considerations warrant otherwise. Near bridges and culverts, cross-section spacing shall adhere to recommendations in the HEC-RAS program documentation referenced in Section 5.2.1. If necessary, additional cross-sections can be interpolated by HEC-RAS, or field-surveyed cross- sections can be copied to achieve the required cross-section spacing around bridges and culverts. 46