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.
• The invert of a jack and bored pipes will be set to a minimum of 1 ft above the invert of the existing pipe to be lined.  If this is not feasible due to site constraints, please coordinate with the INDOT Hydraulic Engineering.
• All liner options should have the inverts of the pipes raised by the liner thickness.  Backwater should then be calculated by taking the existing culvert's backwater and subtract the difference of the headwaters.
• 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.
• Jack and bore pipes as part of a culvert liner project under the interstate are required to have a minimum inner diameter of 18 inches.
• For consistency of sump requirements and without more detailed information, if the soil map unit description contains the word sand, the required sump from IDM Fig 203-2E should be based off of the "Sump Required for Stream Bed of Sand" column.
• Reinforced concrete boxes and 3 sided structures under 18 foot span should incorporate corner haunches as produced by the manufacturer.  The spreadsheet provided below, named Specialty Structure Coordinate Generator under Other Links and Forms, includes the haunch data from the manufacturers.
• 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.
  • Semi-smooth options will no longer be required to be given as replacement options for small structures.
• All three-sided structures are no longer required to be sumped unless requested by the county surveyor for legal drains.
• 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.
• When designing cured in place pipe liners (CIPP), CIPP dimensions assumed in hydraulic memos are based on the dimension reductions listed in the IDM 203-2.02(13). The actual thickness of the CIPP liner should be designed by the road designer and the liner manufacturer to meet the structural requirements of the liner and meet INDOT specifications and any applicable special provisions for CIPP liners. If the liner decreases the size beyond that shown in IDM 203-2.02(13), hydraulic analysis needs to be reevaluated to make sure it still works hydraulically.
• Proposed liner sizes listed in hydraulic memos are based on field measurements of the existing structure(s) by INDOT Hydraulics or others prior to the creation of hydraulic memos.  Due to the potential for deterioration of existing structures after the measurements were taken, and the possibility of unmeasured internal deformations, it is recommended that the culvert size is verified, and it is confirmed that the proposed liners can be constructed properly.   If the assumptions made at the time of analysis are not representative of the conditions of the existing host pipe at the time the design plans are prepared, INDOT Hydraulics should be contacted.
• The inner dimensions for liners listed in hydraulic memos are the minimum required based on the hydraulic model.  The designer should verify the fit of the liner sizes based on the condition of the host pipe at the time design plans are prepared. If a smaller inner dimension is required for fitting, INDOT hydraulics should be contacted for a revised analysis.

• If a dual hydrologic soil group is within a watershed, then the soil should be considered undrained (D-soil) unless it can be shown that the water table is more than 24 inches below the ground surface. If desired, the groundwater depth can be found using the USDA Websoil survey.

When considering a smooth steel liner, the following guidelines should be followed:

1. Consider a steel liner when a CIPP is over 6 ft in span or if the district has specifically requested the option.
2. When modeling, the minimum gap between the inside of the liner and inside of the host pipe is 5 inches on top and 4 inches along each side. This is to account for the grout and wall thickness of the liner.
   • For circular steel liners, the inside diameter of the existing structure can be reduced by 8 inches to calculate the proposed inside diameter.
   • For corrugated metal pipe arches (CMPA), the Coordinate Generator for CMPA Liners is recommended to be used to obtain modeling coordinates of the proposed liner.
3. A thickness of 1” is assumed for the steel liner options.  The actual thickness should be designed by the road designer and the liner manufacturer to meet the structural requirements of the liner. The steel liner should also meet INDOT specifications for steel pipe and any applicable special provisions. If the liner design requires a thickness greater than 1” the road designer should contact INDOT hydraulics for a revised memo.
   • The liner is expected to lay directly on the host pipe resulting in an invert rise of 1 inch for modeling purposes.
4. A Manning’s n value of 0.012 should be used for analysis and design of the proposed steel liner.
5. The inlet condition should match the existing structure unless an improved inlet is being recommended.
6. For the Hydraulics Memo, please describe the assumed thickness of the liner and the minimum internal dimensions (I.D.)
   • Example: 1-inch thick Smooth Steel Liner, 143” (I.D.) Span x 94” (I.D.) Rise.

All median drainage should be submitted to INDOT Hydraulics for review. The IDM is limited in guidance for median drainage design and submittals. The following should be used in conjunction with the IDM to produce an optimal median drainage design for INDOT purposes.

1. The minimum pipe size is a 12 inch inner diameter.
2. Medians for added travel lanes should be designed so that no spread enters the edge of travel lane for a 100-year storm event.
3. Minimal Rational C value used for pavement shall be 0.90.
4. For unpaved medians, Type N inlets shall be used in the median unless they physically cannot geometrically fit for non-paved medians. In that case a Type P inlet will be allowed.
5. For paved medians, inlet spread calculations shall include 50% clogging factor for all inlets on grade. The clogging factor shall not be required for sag inlets with flanking inlets, for P-12 or N-12 inlets and for inlets with curb openings.
6. Flanking inlets are required at each sag inlet location. The flanking inlets are required on both sides of the sag inlet if flow is from both directions. If the flow is from one direction only, then one flanking inlet is required. If the required distance from the sag inlet to the flanking inlet is greater than the distance to the next adjacent upstream inlet, then dual inlets may be provided as an exception. The dual inlets should be connected by a 6’ section of pipe. The flanking inlets are not to be used in the spread calculations.
7. No detention is allowed within the median or under the roadway pavement.
8. No drainage from the outside ditch shall be brought into the median drainage system. Where drainage from the outside ditch or flow from outside of the right-of-way is to be conveyed to the opposite side of the road with a separate culvert that is not connected to the median drains.
9. Each median drain shall have a separate pipe draining to the outside. Flanking inlets will be permitted to be connected to the sag inlet. The inlet on the opposite side of the barrier wall at the same station will be allowed to be connected. Median drainage structures shall drain directly to outside ditches as soon as practical.
10. New median or storm sewer drain outlets shall be 12” above the ditch flowline elevation. A minimum of 6” may be allowed, but a hydraulic grade line (HGL) check shall be required with the appropriate side ditch water elevation and submitted to INDOT Hydraulics for review and approval. If these requirements cannot be met, coordinate with INDOT Hydraulics to obtain a design exception.
11. Median outlets shall be above the Q100 elevation of detention facilities and receiving waters. If this cannot be achieved, an HGL analysis of the median drain with the high tailwater at Q100 elevation shall be required and submitted to INDOT for review and approval.
12. Slotted drains may not be used in the drainage calculations. Trench drains may be used in the drainage calculations for spread, if the minimal grate width is 12”.
13. Proposed drainage area maps shall be provided. The proposed drainage area maps shall include proposed grading at 1 ft contour intervals, proposed median inlet structures and piping and flow direction arrows.
14. Existing drainage area maps shall be provided. The existing drainage area maps shall include existing grading at 1 ft contour intervals, existing median inlet structures and piping and flow direction arrows.
15. Spreadsheet submittals for median drains should include the following beyond the typical HGL calculations required for storm sewers in the IDM, this includes, hydraulic grade line, edge of travel lane elevation, structure rim elevation, structure invert elevation, tailwater elevation (define if different than pipe outlet depth), receiving ditch flowline elevation, etc. 
    • A sample table of these requirements along with a column on whether or not an HY8 model is required for each structure is included as an informed check in each analysis. The sample table can be found here.
16. HY-8 or programs and spreadsheets that use HEC-22 methodology are acceptable for calculating pipe HGL. If HY-8 is used, the software should be Version 7.2. Using only the Manning’s equation is not acceptable for analysis. The Division of Hydraulics has also provided spreadsheets to calculate the HGL of on grade and sag inlet grates. These spreadsheets can be found at the Division of Hydraulics website under “Related Links and Documents.”

An infiltration facility collects and retains runoff for the purposes of infiltrating into the soil to outlet into groundwater. Runoff is designed to be removed by means of infiltration through a permeable bottom. Infiltration is not preferred as a method of stormwater management and should only be considered at locations where site conditions limit other means of stormwater management and allow for infiltration. The Office of Hydraulics should be consulted to ensure that infiltration is the appropriate means of stormwater management for a select project area.

Soil characteristics are the primary concern in designing an infiltration pond. Infiltration facilities can be subject to failure due to plugging with fine material and debris running off of the site and require routine maintenance.

The criteria to use when considering using infiltration include the following:

1. Infiltration rate shall be determined by a qualified geotechnical company in the field.  Laboratory infiltration tests are not acceptable. The Percolation and Infiltration Testing Guidelines provided by INDOT Geotechnical Engineering shall be followed.
2. The native soil shall be sand or sandy loam;
3. The minimum tested infiltration rate shall be 2 inches per hour;
4. The annual high groundwater shall be a minimum of 2 feet below the bottom of the storage facility as determined from the NRCS and the soil borings; and
5. Only the bottom area for an infiltration basin shall be used to calculate the outflow rate;
6. Overflow routing shall be considered for infiltration design;
7. If an acceptable emergency overflow outlet is not available, the pond shall be sized for 1.5 times the total volume required, plus 1 ft of freeboard;
8. A maintenance plan shall be developed to include the necessary cleaning and debris removal to keep the facility functioning as designed;
9. The basin area needs to be protected from sediment and compaction during construction.

An infiltration pond will require additional right-of-way. It should be located outside the clear-zone for safety purposes.

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.

Inlet configurations for the pipe end sections: grated box types I and II, metal end sections, precast concrete, and safety metal culvert end sections for smooth pipes is recommended to have a the inlet configurations similar to that of a headwall.

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.

Scour protection for bridge design and rehab is based primarily on the following criteria (based on IDM Fig. 203-2J and Fig 203-3B):

• The abutments shall be protected with a minimum size rip rap to that of the average velocity of the bridge.
• The piers shall be protected with a minimum size rip rap to that of the max velocity of the bridge determined by velocity distribution.

The minimum pipe size for a paved invert rehabilitation is for the existing pipe to have a minimum span and rise of 4 ft.

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.

When using live bed scour for standard stream crossings, please use the minimum standard 0.01 mm for the d₅₀ soil size. If the structure is an overflow structure, then clear water scour should be used and HEC-18 updated in 2012 now states to use 0.2 mm as a minimal size for the d₅₀ soil size. If the structure is both a natural stream and an overflow structure for a larger stream, then both scour types should be checked under their respective flow rates and soil sizes and the larger scour result should be used.

Please send all median drain reports, plans and computations to the office of hydraulics for review.  Reference to the INDOT Hydraulics presentation on median drains is recommended.

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.

The elevation to use for the flowline in the hydraulics memo should follow the figures shown in the Assumed Flowline Elevation document. This document provides specific scenarios for when the invert of the pipe does not match the flowline of the channel.