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Montgomery County GovernmentDrainageDesign CriteriaDepartment of TransportationNovember 2013 Rockville, MarylandRevised FinalJune 10, 2014
TABLE OF CONTENTS1INTRODUCTION . 11.1PURPOSE OF THE DRAINAGE DESIGN CRITERIA . 11.2LAWS ORDINANCES AND POLICIES . 11.3RIGHT-OF-W AY, EASEMENTS, OWNERSHIP, AND MAINTENANCE . 31.3.1 General Policy . 31.3.2 Storm Drain Easements . 51.3.3 Channels and Outlets . 61.3.4 Maintenance & Access . 61.3.5 Connection to existing Storm Drain Systems . 72SUBMISSION AND APPROVAL PROCEDURES . 82.1DRAINAGE STUDY . 82.1.1 General Information . 82.1.2 Drainage Area Map . 92.1.3 Hydrologic Calculations . 102.1.4 Hydraulic Calculations . 102.1.5 Miscellaneous Calculations . 112.2PLAN PERMIT SUBMITTALS . 112.2.1 Permit Submittal requirements . 112.2.2 EasementsStorm Drain Plans . 112.2.3 Storm Drain Profiles . 132.2.4 Plan Notes . 132.3DISCREPANCIES AND REVISIONS . 142.3.1 Plan Discrepancies . 142.3.2 Plan Revisions . 142.4UTILITIES . 142.4.1 General . 142.4.2 Utility Identification . 152.4.3 Utility Verification Plans & Coordination . 152.4.4 Clearance with Other Utilities . 153HYDROLOGIC ANALYSIS . 163.1GENERAL. 163.2TIME OF CONCENTRATION . 173.2.1 Rational Method . 173.2.2 NRCS (SCS) TR-55 . 18PEAK DISCHARGE COMPUTATIONS. 213.2.3 Rational Method . 213.2.4 NRCS Method . 263.3HYDROGRAPH METHODS . 313.3.1 TR-55 Method. 313.3.2 TR-20 Method. 313.4GIS HYDRO 2000 . 323.5CONCURRENT FLOOD ANALYSIS . 323.6BANKFULL DISCHARGE ESTIMATES . 334HYDRAULIC DESIGN. 344.1GENERAL. 344.1.1 On-site and Off-site Considerations . 344.2PAVEMENT DRAINAGE . 354.2.1 Street Capacity (Spread) . 35iRevised FinalJune 10, 2014
4.2.2 Inlet Spacing, Size and Capacity. 364.2.3 Grates . 394.2.4 Manholes, Inlets, and Field Connections . 404.3STORM DRAIN DESIGN . 414.3.1 Determination of Conveyance Size . 414.3.2 Hydraulic Gradient . 434.3.3 Pipe Design Requirements . 474.4BRIDGES AND CULVERTS . 494.4.1 General Considerations . 494.4.2 Classification and Review . 504.4.3 Design Considerations . 504.4.4 Site Inventory. 544.5ENTRANCES AND EXITS IN STORM DRAIN SYSTEMS . 554.5.1 Storm Drain System Entrances and Stabilization . 554.5.2 Outlet Structures. 564.5.3 Outlet Velocity . 574.5.4 Types of Inlet and Outlet Stabilization for All Structures . 574.6OPEN CHANNELS . 594.6.1 Design Requirements . 594.6.2 Design Channels . 594.6.3 Natural Channels . 604.6.4 Roadside Ditches . 614.7RIPRAP . 634.7.1 Riprap Outfall Protection . 634.7.2 Riprap Classification . 654.7.3 Riprap Bank Protection . 654.8RESIDENTIAL LOT DRAINAGE REQUIREMENTS. 664.8.1 Drainage Swales and Surface Drainage Easements . 665MISCELLANEOUS DRAINAGE . 675.1SUMP PUMP DISCHARGES . 675.1.1 Types of sump pump discharges allowed: . 675.1.2 Alternative Discharges of sump pumps . 675.2DRIVEWAY CULVERTS . 676COMPUTER SOFTWARE . 696.1FEDERAL AND STATE APPROVED SOFTWARE. 696.1.1 Hydrologic Software: . 696.1.2 Storm Drain Software: . 696.1.3 Bridge and Culvert Software: . 696.3THIRD PARTY SOFTWARE . 706.3.1 Hydrologic Software: . 696.3.2 Storm Drain Software: . 696.3.3 Bridge and Culvert Software: . 69iiRevised FinalJune 10, 2014
Table of TablesTable 1-1 - Minimum Right-of-Way . 5Table 2-1 – Pipe and Culvert Material Abbreviations . 13Table 3-1 - Context Sensitive Design Classification . 16Table 3-2 – Hydrologic Methods by Drainage Area. 17Table 3-3 – Small Drainage Area Time of Concentration . 17Table 3-4 – Common Overland Flow Roughness Coefficients (n) . 19Table 3-5 – Runoff Coefficients . 23Table 3-6 – Correction Factor . 25Table 3-7 – Intensity.-.Duration - Frequency (IDF) Data . 25Table 3-8 – NRCS Curve Numbers . 28Table 3-9 – Coefficients for 24-hour Rainfall. 29Table 3-10 – Adjustment Factor (Fp) for pond and Swamp Areas . 29Table 3-11 – Storm Duration (24-hr Rainfall) . 30Table 3-12 – Concurrent Flood Ratios . 32Table 4-1 – Site Considerations . 34Table 4-2 – MCDOT & MSHA Gutter Widths and Cross Slopes . 35Table 4-3 – MCDOT Standard Inlets . 37Table 4-4 – MCDOT & MSHA GRATES . 40Table 4-5 – Access Spacing . 40Table 4-6 – Elliptical and Circular Pipe . 49Table 4-7 – Culvert Service Life . 52Table 4-8 – Material Rating For Service Life. 52Table 4-9 – Ditch Lining & Velocity . 62Table 4-10 – Riprap Classification . 65Table of FiguresFigure 4-1 – Bicycle Safe Grate Options . 39Figure 4-2 - Hydraulic Grade Line . 43Figure 4-3 - Headloss through a Junction . 46Figure 4-4 - Head Loss Through a Field Connection . 47Figure 4-5 - Culvert Skew Length. 53Figure 4-6 - Culvert Skew Length. 53AppendicesAppendix A – County ChecklistsAppendix B – Design TablesAppendix C – RCP Loading and Bedding DetailsAppendix D – Design SpreadsheetsiiiRevised FinalJune 10, 2014
Montgomery CountyDepartment of TransportationDrainageDesign CriteriaPrefaceIn 1968 House Bill (H.B.) No. 629 was enacted by the General Assembly of Maryland, transferring thejurisdiction over storm drainage systems within Montgomery County from the Washington SuburbanSanitary Commission (WSSC) to Montgomery County. The portion of the City of Takoma Park, lyingwithin Montgomery County after June 30, 1968 was exempted from transference of the storm drainsystem. H. B. No. 629 went into effect on July 1, 1968. Pursuant to the legislation, Montgomery County'sDepartment of Public Works, which is now the Department of Transportation (MCDOT), approved its firstStorm Drain Design Criteria on July 1, 1968. Subsequently, some minor changes were incorporated intothe Storm Drain Design Criteria to reflect technological advances. Since the inception of the storm draindesign criteria in 1968, the criteria has been updated or revised in 1975 and 1988 and has now been ineffect for over thirty five years.Consequently, in an effort to keep up with the state of the art in storm drain design and to provide morecompatibility with the criteria of other local, State and Federal agencies, MCDOT has developed thisdocument to replace the 1988 criteria. Review of this set of criteria has been in conjunction with theMontgomery County Road Code Committee (ROCOCO). ROCOCO represents local engineers, landsurveyors, public utilities, developers and other government agencies.MCDOT and Montgomery County Department of Permitting Services (MCDPS) have the authority toimplement the Montgomery County Government Drainage Criteria as outlined in this manual. MCDOThandles the review of storm drain capacity & impact analyses at the preliminary plan stage to determine ifany improvements are needed by an applicant to the downstream County storm drain system; handlesthe design and construction of County storm drain systems through Capital Improvements Programprojects; and maintains the County storm drain systems within County rights-of-way and easements.MCDPS handles the review and approval of construction drawings for County storm drain systems (andthe connections of private storm drain systems with County storm drain systems), issues the necessarypermits, inspects the permitted construction, and then accepts the completed system for Countymaintenance. Drainage design approval is not the same as storm water management approval.Stormwater review shall be coordinated through the MCDPS Water Resources Section.Criteria in this document apply only to storm drain systems maintained by MCDOT; Maryland StateHighway (MDSHA) and municipalities within Montgomery County may require different design criteria.The drainage criteria in this manual are to be used in conjunction with the Montgomery CountyDepartment of Transportation's Design Standards and construction policies. This manual provides theguidelines for designing typical drainage systems and structures. The criteria does not address allunusual situations that require unique solutions by the Design Engineer and the need to allow users someflexibility with respect to its application is apparent. At their discretion, MCDOT or MCDPS will approvespecial designs in extenuating circumstances with adequate documentation presented by the DesignEngineer.Table A contains the municipalities within Montgomery County that are considered Special TaxingDistricts. (Tax District symbols are in parentheses). These municipalities have their own planningauthority which maintains their own roads and storm drainage systems. For those Special TaxingDistricts in which the storm drain system is maintained by Montgomery County (and the special TaxingDistrict pays a storm drain tax to Montgomery County), the MCDOT review will be limited to the stormdrain capacity and post-development impact analysis (as part of Executive Regulation No. 28-06AM).ivRevised FinalJune 10, 2014
Montgomery CountyDepartment of TransportationTable A – Montgomery County Municipalities Supported by Storm Drain TaxMAINTAINED BY COUNTYNON-MAINTAINED BY COUNTYBattery Park(M11)Barnesville(M14)Chevy Chase Section #3(M0
Jun 10, 2014 · Montgomery County Drainage Department of Transportation Design Criteria iv Revised Final June 10, 2014 Preface In 1968 House Bill (H.B.) No. 629 was enac
