Hydraulics

• CLARIFICATION OF IDM 203-2.02(02) 2.a.1.: The replacement structure does not have to have the same span as the existing structure when the existing structure is going from pipe to a proposed box type structure.  However, the cross-sectional structure area cannot be reduced.
  • Example:  An existing twin 8 foot diameter pipes may be replaced with a proposed 15x7 box structure, even though the twin pipes have a total span of 16 feet while the box has 15 feet, as long as the proposed total area is equal to or greater than the existing.  In this case, the existing area is about 101 square feet, while the proposed box (with haunches) has an area of 103 square feet.
  • Example 2:  An existing 6 foot diameter pipe cannot be replaced with a 5x5 box because the existing pipe has an area of 28 square feet, while the proposed box (with haunches) has an area of 23 square feet.
• HY-8 v.7.2 is currently the only version permitted for use by the INDOT Hydraulic Engineering.  Please click the following link to download the software. (Active Design Memoranda16-34).
  • Due to Window's updates, HY-8 v.7.2 will not install without first installing the  application that can be downloaded by clicking here. After this application is installed, HY-8 v.7.2 may be installed.
• A flow chart has been developed to help designers select which structure and liner options should be analyzed.  Modeling guidance has also been included.  This chart can also be found as Proposed Structures Flow Chart under Other Links and Forms below.
• When analyzing an existing structure that is showing overtopping, the designer should NOT artificially raise the road to analyze headwater elevation as it may exaggerate the backwater total, but rather make certain that the discharge through the liner/new pipe at overtopping is greater than the existing.

Slab-top structures are concrete slabs installed on two vertical abutments with the top of the slab normally being the driving surface.  Therefore, no pavement would typically be installed on top of it.  Comparatively, a 3-sided structure is precast structure with the walls and top seamlessly constructed together and a driving surface installed over the top of the structure.

Please note that the structural depths described below are only estimates and feasible for hydraulic design only.  To determine the actual structural depth, please contact the engineer of record.

• Thickness Estimation:
  • The structure depths described below are for the crown of a road and should be used to determine the overflow elevation. The shoulder will be based on the minimum slab thickness and used for serviceability determination of the roadway.
  • For spans up to and including 15’, that slab-top structure should have a minimum slab thickness of 12”.  When computing structure depth, it would be 12” + 2% * (1/2 width of the roadway + shoulder).
    • As an example, a 15’ span structure with a normal crown on a road with two 12’ lanes and 4’ shoulders, the minimum structure depth would be : 12” + 0.02*(12’+4’)*(12”/Ft) = 15.84” in thickness.
  • For spans over 15’ and up to and including 20', that slab-top structure should have a minimum slab thickness of 15”.  When computing structure depth, it would be 15” + 2% * (1/2 width of the roadway + shoulder).
  • Please contact INDOT bridge design to estimate the slab thickness for structure over 20'.
  • Wider roads (>32’ including lanes and shoulders) and longer spans (>15’) would likely need to have a larger structure depth.
• All spans have a minimum desirable structural free board of 1 ft.
• Slab tops should not be designed to be under pressure flow.

• When initially lining a pipe with CIPP, the inlet configuration should match whatever is in the field, unless the existing pipe is projecting. In which case, the liner should have an inlet configuration of mitered to conform to slope. 
• When initially lining a pipe with HDPE liner, the inlet configuration should have an inlet configuration of mitered to conform to slope even if the existing pipe has a headwall. If a headwall or bevel edge is needed for the HDPE liner to perform correctly, it will need to be constructed with the project.
• If you are putting in a 5" paved invert, the inlet configuration should simply match the existing configuration.  If you need to improve the inlet configuration, then adding a headwall or a headwall with bevel edge is acceptable with the proposal.

If the existing pipe has a reverse slope, we recommend bringing the inlet invert to the elevation of the outlet creating a flat pipe.

The 50% Huff Distribution Curves have been set in four locations within the state of Indiana.  For consistency purposes, this map - Huff Distribution Regions - shows which county belongs to which region.

According to the LTAP Stormwater Drainage Manual, the first quartile is set for any storm duration less than or equal to 6 hours.  The second quartile is set for any storm duration greater than 6 hours and up to and including a 12-hour storm.  The third quartile is for storms greater than 12 hours and up to and including a 24-hour storm.  The fourth quartile is for any storm greater than 24 hours.

A flow chart has been created to help determine if a CIF permit is required for a project.  CIF Permit Criteria Flow Chart

A 3 sided structure, whether it is a flat top, arch top, or slab top, should have its low structure elevation, flow line elevation, and sump labeled in the hydraulics memo with their respective line in the table.  The span should be labeled within the type of structure line.  To avoid confusion, the rise of the structure should not be stated as the flow line elevation, sump depth, and low structure elevation will be the necessary information for the structural and environmental engineer to use in the designs.

The proper way to submit projects with multiple types is to separate the types by category in a limited fashion. For example culverts will be their own submittal as well as detention in its own and storm sewers in its own.  Culvert submittals should be limited to ten structures.

The zip files can be opened using a free software called 7-zip and can be downloaded via the website https://www.7-zip.org/download.html.  It is recommended to use the most up to date version.

Three-sided structures, spill through structures, storm drain systems, and structures with inlet depressions should not be sumped. For other culverts, sumping is required unless the Waters Report determined the location not to be a Waters of the US nor Waters of the State.

Only CIPP liners should be used to line pipes that are already the minimum allowable size (before lining).  For example, if the existing is a 24” diameter cross pipe under I-465, then this pipe should not be lined because the existing is already less than allowable. However, a 36” cross pipe or a 12” median drain is allowed to have a CIPP liner.

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