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Appendix H: Hydraulic / Hydrologic Guidelines

Guidelines for Development Adjacent to State Highways

Highway Hydraulics Division
February, 1993
Revised – August, 1999
Revised – October, 2003

Table of Contents




This manual has been developed to outline the State Highway Administration’s policies, criteria and methodologies regarding hydrologic/hydraulic design of proposed developments adjacent to state highways.

Reference to this manual will provide applicants and engineers definitive guidelines for preparing thorough hydrologic/hydraulic design analyses in conformance with the requirements of the State Highway Administration’s Highway Drainage Manual. Close adherence to the provisions of this manual will reduce the review process time frame for hydrologic/hydraulics approval.


The general policy of the State Highway Administration (SHA) is that the SHA reserves the right to review any development adjacent to state highway facilities to ensure that: 1) the proposed development will not adversely affect the state facilities and 2) improvements within SHA R/W are in conformance with State Highway Administration design criteria, construction specifications and standard details. Adverse impacts may include flooding, erosion, structural damage and/or any safety hazard that may occur. The SHA Highway Hydraulics Division will review developmental plans and computations to confirm that:

  • All inlets, pipes, channels and other storm drain facilities constructed within SHA’ s right-of-way meet SHA criteria.
  • The hydraulic design of proposed drainage facilities, which connect directly to SHA drainage facilities either upstream or downstream, are adequate.
  • On - site drainage systems for proposed developments, which do not connect directly to SHA drainage, will not indirectly result in an adverse impact upon the SHA facilities.
  • Cross sectional elements of proposed improvements do not create safety or erosion hazards.
  • Grading within right-of-way does not cause increases in flow toward the highway.

The SHA’s hydrologic/hydraulic approval will not be issued until a complete set of final plans and computations as specified in the criteria are received and reviewed. These plans and computations must also be reviewed and approved by the local stormwater management-approving agency (town, city, county, etc.). Since most local agencies will not give their approval prior to Highway Hydraulic’s approval, a conditional approval can be given if the local SWM and E&S approvals are the only outstanding comments remaining. SHA is not an approving agency for stormwater management facilities. The applicant must also supply other appropriate approvals as requested; e.g., Non-Tidal Wetland and Waterway Permits, Tidal Wetlands License, Army Corps of Engineers Permit, etc. Most importantly, the SHA's review or lack of review does not release the applicant applicants consultant from liability.


The following material (if appropriate) shall be submitted for hydrologic/hydraulic review by the SHA:

  1. County approved stormwater management (SWM) plans and computations or documentation from the County regarding their waiver of SWM.
  2. Pre-development drainage area map for the entire watershed, clearly indicating the existing contours both on site and off site. (1”=200’ scale or smaller)
  3. Post-development drainage area map for the entire watershed, clearly indicating the proposed contours both on site and off site. (l”=200’ scale or smaller)
  4. A set of the latest approved post-development site plans, clearly indicating existing topographical features, proposed structure details, typical sections, pipe profiles and contour grading.
  5. Complete storm drainage plan and profiles of both the existing and proposed storm drainage systems.
  6. Complete hydraulic analyses for cross-culverts under the state highway for the design year storm associated with the functional classification of the highway, utilizing the Soil Conservation Service (SCS) Method.
  7. Complete inlet spacing investigation for proposed curb and gutter along the state highway for the 2-year storm utilizing the Rational Method.
  8. Complete storm sewer design computations for proposed closed storm drainage system for the 10-year storm utilizing the Rational Method.
  9. Complete hydraulic gradient computations for proposed and/or existing storm drainage systems for the 25-year storm, utilizing the Rational Method.
  10. Complete analyses, utilizing the SCS Method, for:
    1. Channels adjacent to the state highway for the 10-year storm.
    2. Inlet/ outlet channels to culverts for the functional storm.
  11. Complete hydrologic analysis/back-up data (eg.: tc, tc path, curve numbers, soil types, TR-20 Schematic Diagrams, land uses, etc.).

The following State Highway Administration standard forms shall be used for the design computations:

  • Cross-culvert Analysis
    HY-8 Computer Analysis
SHA 61.1 - 490
  • Inlet Spacing
SHA 61.1 - 491
  • Storm Sewer Design
SHA 61.1 - 492
  • Hydraulic Gradient for Storm Sewers
SHA 61.1 - 493
  • Peak Discharges using TR-55
    TR-20 Computer Analysis
SHA 61.1 – 494

Standard form SHA 61.1 - 494 shall also be used to show the derivation of the Runoff Curve Number (RCN) for the TR - 20 computer input forms. Supplementary computations sheets showing the derivation of “times of concentration” (tc) shall also be submitted. TR-55 worksheets may be used to show these computations.

All computations are to be neatly prepared, well organized and appropriately labeled so they can be easily reviewed. The computations shall also include references to all design charts and publications used in the preparation of the computations.

The pre- and post-development drainage area information shall include:

  • An outline of the total drainage area to the point being analyzed and all sub-areas within the total watershed that are pertinent to the computations. The drainage area shall not be limited to the development site.
  • The soil types within the watershed.
  • The land uses within the watershed.
  • All tc flow paths investigated, showing the limits of overland flow, swale flow, ditch flow, pipe flow, stream flow, etc. The chosen path shall be clearly differentiated from any other paths investigated.
  • Existing and proposed storm drain and stormwater management systems (both on site and off site) serving the development and surrounding watershed.
  • Drainage area and cross section numbers that agree with the design computations and computer output data.


The following methods shall be used in performing hydrologic computations for development projects that are to be reviewed by SHA.

  1. The "United States Soil Conservation Service Hydrograph Method", utilizing either the TR-55 Tabular Hydrograph Method or the TR-20 computer program, shall be used for determining the discharges for design and analysis of cross-culverts, SWM facilities and open channels.
  2. The Graphical Peak Discharge Method may be used only when development falls in a homogeneous watershed (watershed subdivision not required) and reservoir routing is not required.
  3. The Rational Method shall be used for determining the discharges for design and analysis of closed drainage systems.
  4. The Rational Method shall be used for inlet spacing investigations.


The hydrologic computations shall be performed in conformance with the procedures included in the Highway Drainage Manual and as specified herein.

One or more of the following analyses may be required dependent upon the resulting adequacy of the drainage facilities:

  1. Post-development for the project site with full development in accordance with existing zoning for the off-site portion of the drainage area. Should this analysis demonstrate adequacy of the existing and/or proposed drainage facilities, no further analysis is required.
  2. Post-development for the project site with existing conditions for the off-site portion of the drainage area.
  3. Pre-development (existing).

The entire watershed to the point of investigation is to be included in the hydrologic computations.

The latest available version of TR-55 or TR-20 is to be used in determining runoff by the Soil Conservation Service Method. The standard SCS 24 hr. Type II rainfall distribution (Table #2) is to be used for the TR-20 program. A modified dimensionless hydrograph (SHA 61.1-403.99) is to be utilized on the Delmarva Peninsula. The selection of sub-areas and cross sections for the reach routings for development of composite hydrographs shall be suitably justified. The total length of overland flow should not exceed 100 feet and the time of overland flow shall be determined as prescribed by the latest version of TR-55 or TR-20.

The applicant shall determine a representative time of concentration based upon land use, slopes and soil groups. Several paths should be investigated in the process and the path representing the greatest contribution of runoff chosen (the most representative time is not necessarily longest time). All flow paths shall be indicated on the drainage area maps and supported by backup computations.

Special considerations in unique circumstances may require other additional methods of analysis. Contact with the Highway Hydraulics Division is recommended when the designer is considering special cases.


The applicant shall provide an analysis for all proposed cross-culverts under state highways and for all existing cross-culverts that may be affected as a result of the proposed development. This may include culverts that are located beyond the property boundaries. These analyses shall include a review of the stability/capacity of the downstream channel and design of outfall protection measures.

Definition of a cross-culvert - any culvert under a state highway with a headwall, end section or protective end whose primary function is to convey off site runoff through the highway right-of-way is considered to be a cross-culvert. This definition applies even if an extended downstream storm drainage system is connected to the culvert. In addition, an entrance culvert longitudinal to the state highway with a downstream storm drainage system connected to it may also be considered to be a cross-culvert. In these cases, a hydraulic gradient based upon the SCS methodology discharges will be required to determine the tailwater for the upstream culvert. The resultant headwater is then computed using the SHA standard form 61.1-490 or by the HY-8 computer analysis.

The flood frequency to be used for the design of new cross-culverts and analysis of existing culverts shall be as follows for the various functional classifications of highways (this storm shall be referred to as the “functional storm” in all subsequent discussion):

AASHTO Classification SHA Functional
Design Flood



Local Roads & Streets


Principal Arterial
Intermediate Arterials
Minor Arterial
Major Collectors
Minor Collectors
Local Streets


The functional classification of all State roadways can be located on the internet via the State Highway Location Reference located on the web at

SHA criteria states that the headwater pool for the design flood frequency of the highway classification not be higher than the edge of roadway shoulder based upon the proposed development and existing conditions for the off site portion of the watershed.

The post-development headwater pool elevations shall be determined taking into consideration the following:

  • Storage at the inlet of the cross-culvert if appropriate.
  • Flow over the highway.
  • Overflow into or from an adjacent drainage basin.
  • Tailwater elevations from downstream drainage, headwater pools, floodplains and SWM facilities.
  • Extension of the culvert to accommodate highway widening.
  • Outflow from upstream SWM facilities.

Storm drain extensions that are proposed downstream of an existing cross-culvert under a state highway shall be adequately sized to handle the ultimate development of the watershed regardless of the capacity of the cross-culvert. The plans must include profiles of all proposed culvert extensions.

Any development adjacent to a state facility must ensure that an existing flooding problem on the state facility will not be worsened as a result of the proposed development. In the event of increased discharges, applicants may (subject to county and state approval) upgrade or supplement an existing culvert to reduce the WSE to pre-development levels or below. Adequacy of the outfall must be addressed should this option be pursued. Should this prove to be infeasible, a SWM facility may be required.

As a rule, headwalls shall be provided on pipes 36” in diameter or larger or if subject to baseflow. All cross culverts must have a minimum diameter of 18” for a length of < 60’ or 24” for a length of > 60’. End sections must be used at the ends of all other pipes. As with all drainage design, traffic safety is paramount in considering the location of culvert headwalls and end sections.

Please note that with this or any other special circumstance, coordination with the Highway Hydraulics Division is recommended.


Any proposed development adjacent to a state highway shall provide a storm drainage system along the said highway that will intercept existing and proposed gutter flow and outfall the system in accordance with SHA criteria. Waivers to this policy must be reviewed and approved by the Highway Hydraulics Division.

Complete inlet spacing design computations shall be submitted for all proposed curb and gutter along a state highway to determine the number of inlets or curb openings that are required. Bypass flow from upstream inlets shall be taken into consideration in performing the inlet design computations.

The inlet spacing for the inlets on grade shall be established to meet the maximum allowable gutter spread of 8 ft. or half of the travel lane, whichever is less, and percent interception of 85% or better for the 2-year storm, as specified in the Highway Drainage Manual. In addition, the maximum allowable flow across entrances is 1.0 cfs and onto downstream property owners (from the end of curb and gutter) is 0.5 cfs. For high speed roadways where inlets must be placed adjacent to the left lane, spread shall be confined to the shoulder. In sumps, flanker inlets should be provided.

The applicant shall also prepare a pre- and post-development analysis for existing inlets when runoff from the proposed development is directed toward the highway. Additional inlets will be required if flow from the development significantly increases spread along the curb and gutter or if interception by existing inlets is significantly decreased.

The applicant shall utilize one of the following options if SHA's review finds that an adequate number of inlets have not been specified:

  1. Provide additional inlet capacity.
  2. Revise the site plan to reduce runoff to the state highway. This may include re-grading for the proposed development and/or additional inlets on-site.

Inlet Selection

The type of inlet selected must be appropriate to the roadway. The following is a basic guideline to inlet selection:

First preference should be given to using COG/COS inlets. A local inlet depression of 1.5 in. (a=1.5) shall be used for COG inlets. When there is a utility conflict or other extenuating circumstances, grate inlets may be considered.

  1. Curb Opening Inlets
    • Preferred when heavy truck traffic is anticipated in a turning/parking lane
    • SHA capacity charts assume a one foot gutter pan with a 1.5 inch local depression
    • Standard COG for on grade. COS in Sump.
    • Where guide rail is present, only the 5 foot opening may be used
    • Shallow COG/COS versions are available (see details)
  2. Grate Inlets
    • Grate inlets within the roadway shall be safely traversable by bicycles
    • Does not apply to facilities where bicycles are prohibited (i.e. expressways)
    • Curved-Vane Grates ( LS and LE) fitting Type 'S' and Type 'E' inlets are preferred and approved (see details)
    • Reticular grates (WR, WRM, NR,NRM) are approved
    • Other grates may be approved by Chief, Highway Hydraulics Division (i.e. sinusoidal )
    • For combination inlets, the curb opening is a factor of safety against clogging
    • In a sump, reduce the perimeter length to 75% to compensate for clogging
  3. Curb Openings
    • New Shallow COG/COS (MD 374.68) will be used in lieu of Curb Openings.
    • New COG trough detail may also be used if there are still clearance problems with MD 374.68. (Standard pending)
For each project, the applicant should consider the following:
  • The contribution of runoff from the development.
  • What runoff is being introduced from off site?
  • What runoff is being directed toward the downstream property?
  • Compare pre-development and post-development conditions.
  • The spread and interception requirements and the flow limit across entrances.
  • Interception of flow on site.

All proposed grading adjacent to a state highway must comply with the requirements contained in AASHTO’s most recent revision of “A Policy on Geometric Design of Highways and Streets”, and supplements thereto, or as directed by SHA.


The applicant shall provide an analysis for all existing and proposed storm drainage systems under or adjacent to a state highway which may be affected as a result of the proposed development. This frequently includes systems that are located beyond the property boundaries.

The 10-year post development storm, utilizing the Rational Method, shall be used for storm drainage pipe designs. Pipes should be initially sized to convey this discharge at or below full flow.

The submittal for review shall include the following:

  • Plans and profiles for the proposed system to meet design requirements for minimum cover, utility clearance, etc.
  • Utility test pit information for clearance whenever a storm drain crosses a utility line.
  • Completed Storm Sewer Design form (SHA, 61.1-492) for the 10-year storm frequency. Longitudinal pipes shall not be less than 15 in. in diameter between the first two structures and not less than 18 in. in diameter thereafter. Minimum size requirements for transverse pipes are the same as specified for cross-culverts.
  • Hydraulic gradient computations using form (SHA 61.1-493) should be completed for the 25-year storm.
  • Hydraulic gradient plotted on the pipe profiles should be below top of grate or manhole cover.

Channel conditions downstream from the storm drain outlet pipe and/or the Hydraulic Grade Line (HGL) of an existing downstream storm drain shall be carefully reviewed to determine the beginning elevation for the hydraulic gradient computations. The method for choosing a beginning elevation for various conditions is included in Part 1, Chapter 4, Section C of the Highway Drainage Manual. Additionally, when a proposed system is directed toward a stormwater management facility, the 25-year water surface elevation must be used as the controlling tailwater elevation. A complete discussion of assumptions and backup computations shall be provided.

The 25-year storm hydraulic gradient shall be developed for the proposed storm drainage systems for post-development conditions to determine if flooding of the highway occurs. An analysis of the existing system must be performed to determine if the flooding occur or worsen as a result of the proposed development. If so, appropriate design revisions must be made.

The surface overflow flood route shall be plotted for all projects for the pre- and post-development conditions to identify potential flooding hazards. This is of particular importance for sump inlets.

Some general design requirements for designing pipes within SHA right-of-way include:

  • Use Class IV or higher concrete (RCCP, HERCP) pipes.
  • Use 14 gauge or heavier for CMP’s.
  • Metal pipes larger than 84 in. shall be structural plate
  • Pipe cover shall be in accordance with the Highway Drainage Manual. For some projects, encasement of the pipe in concrete must be considered.
  • The 25-year storm hydraulic gradient must be below the top of manhole covers and inlet grates.
  • All structures within the state right-of-way must be state standard structures where possible.
  • Use of polyethylene pipe is limited to locations outside of the pavement section. Only double walled (Type ‘S’) pipe is allowed. Gasketed joints must be specified. Polyethylene end-sections are not allowed.
  • Closed storm drain systems shall be designed with a minimum number of necessary structures to minimize maintenance problems.
  • Storm drain trunk lines and cross pipes running underneath travel lanes must be kept to a minimum and used only when necessary to avoid design restraints (utilities, etc)
  • If the applicant’s storm drain extends from SHA R/W to the applicant’s, then a manhole structure must be placed just outside SHA R/W to set the maintenance limits.
  • Corrugated steel pipe shall be Type 2 aluminized as per M~274.
  • Metal pipe larger than 60 in diameter shall have 3"x1" corrugations.
  • All pipes shall have gasketed joints appropiate to the pipe material.


The applicant shall provide an analysis for all proposed channels adjacent to state highways and any existing channel that may be affected as a result of the proposed development. This may include channels that are located beyond the property boundaries. All proposed channels are to be designed such that no adverse impact to the state highway occurs.

Manning's equation shall be used for the hydraulic analysis. Reference is made to the "Highway Drainage Manual".

The water surface elevations for new open channels shall be a minimum of 9 in. below the edge of shoulder elevation for a 10-year frequency storm.

Inlet and outlet ditches shall be designed to accommodate the same storm frequency discharge as the culvert serviced.

Open channels shall be checked for velocity, depth of flow and type of lining for the design storm at locations in the channel where:

  • other swales and ditches outlet into the channel
  • the typical section of the channel changes significantly (eg: the channel changes from a “vee” section to a trapezoidal section, the bottom width increases 2 feet or more, etc.)
  • the grade of the channel changes (either flattens or steepens)

Specific items, which the applicant shall take into consideration in the design and analysis of the channels, include:

  • Depth limitations as stated above.
  • All channels have linings, which are not erodible at design velocities.
  • Tributary channels shall be designed to intersect the roadway side ditch at an angle of between 30o and 60o.
  • Ditches are not to convey runoff down a slope in close proximity to headwalls or end sections.
  • Typical section is consistent with safe traffic conditions.


The applicant shall provide an analysis for all proposed pipes under entrances to state highways and for any existing entrance pipe that may be affected as a result of the proposed development. This may include existing pipes that are located beyond the property boundaries.

A 10 yr. design flood frequency shall be utilized for the design and analysis of entrance pipes. The headwater surface elevation shall be maintained 9” below edge of shoulder (10-year storm). When the entrance pipe is in the vicinity of a cross-culvert under a state highway, an analysis based upon the functional storm may be required.

The minimum diameter for entrance pipes within SHA Right-of- Way is 15 inches. Corrugated metal pipes shall be a minimum of 14 gauge under entrances when used within the SHA R/W. All concrete pipes shall be a minimum of Class IV strength when used within SHA R/W. Height of fill limits and cover shall be in accordance with Section I-3-C of the Highway Drainage Manual.

End treatment (end sections or endwalls) shall be provided to protect the ends of entrance pipes from crushing. End sections shall not be used on pipes 36” in diameter or larger or in cases where base flow exists. A riprap apron shall be provided at all pipe outlets.


SWM approval is the responsibility of the local approving agency. The SHA is not an approving agency for SWM facilities. SHA is solely concerned with potential impacts to state facilities due to inadequate SWM. SWM facilities created by private developers in SHA right-of-way that are designed to handle only runoff from SHA right-of-way will be reviewed and commented on by SHA reviewers. All other facilities (existing or proposed) in SHA right-of-way are required to be reviewed by MDE and subject to Joint Use Fees (see below).

The applicant shall provide complete design computations and construction plans for all proposed SWM facilities that are adjacent to a state highway. All SWM facilities are to be designed or analyzed with the SCS Hydrograph Method. No other methods will be allowed. The SCS "Short-cut Method" is not acceptable. The SWM computations shall address the appropriate pre- and post-development discharge rates. In addition, computations based on the functional storm may be required so that SHA can perform a complete evaluation of the development. All soils data for water quality management shall be included with the SWM computations.

All SWM facilities for private development should be located outside the SHA right-of-way. Special circumstances may allow surface storage within state right-of-way; however, stage/storage calculations shall not include storage volumes within SHA right-of-way or Joint Use fees may be required.

The following should be considered when designing SWM facilities:

  • Downstream ponds shall be adequately sized so that the hydraulic operation of upstream drainage systems for the state highways is not impacted.
  • Emergency spillways directing flow onto a state highway will not be allowed. The emergency spillway must discharge flow away from the state highway or into an adequate channel. Alternately, the principal spillway should be increased so that the functional classification design storm does not cause any discharge through the emergency spillway.
  • All pond designs must comply with the requirements contained in the Soil Conservation Service Publication MD-378.
  • Water quality inlets will not be allowed on SHA R/W.
  • Use of existing SHA SWM facilities by private applicants is generally not allowed. (See joint-use below)


To create a new SWM facility or utilize an existing one in SHA right-of-way is called joint-use. SHA is regularly approached by developer’s seeking the approval to use SHA right-of-way for their own stormwater management needs. To be able to do this, the developer must provide a substantial benefit to the SHA. This can be achieved in several ways, including providing a water quality credit for SHA, making improvements to the existing pond or paying a joint-use fee of $30,000 that will be used for perpetual maintenance of the facility by SHA. It is not considered joint-use if the SWM facility is being used for improvements to SHA right-of-way only.

A facility cannot be constructed in SHA R/W that will only benefit the developer. SHA must benefit in some manner whether it is quality for improvements to SHA property that the developer is providing or as a quality credit for existing pavement. MDE must review all designs and they must conform to the 2001 MDE Regulations.

Any existing facility that is utilized must be reconfigured and designed to comply with the 2001 MDE Regulations if it isn’t so already. Even if an existing facility is proven to be sized large enough to handle runoff from a development, it must be reviewed by MDE and still must follow all rules set by the Joint-Use policy.

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