Design Criteria & Standards

Hydraulic design considers environmental standards with respect to channel equilibrium and aquatic organism passage as well as hydraulic design criteria, as detailed in the VTrans Hydraulics Manual. Hydraulic and environmental standards are summarized as follows:


Headwater Depth

Headwater depth pertains to the depth of water immeadiately upstream of the inlet of a structure. Allowable headwater depth is compared to the depth of the structure opening (also called rise or clear height). The allowable headwater to depth ratios (HW/D) for the respective structure size at the design frequency, are tabulated in Hydraulics Manual Table 6-3, and are summarized as follows:

  • Culvert Rise ≤ 36 inches: HW/D ≤ 1.5
  • Culvert Rise > 36 inches: HW/D ≤ 1.2
  • Culvert Rise > 60 inches: HW/D ≤ 1.2, in addition HW/D ≤ 1.5 at the 1% AEP storm event

More Information: Hydraulics Manual, Section Allowable Headwater and Backwater


Figure 3: Culvert profile in high water. In this figure, headwater depth (HW) exceeds that of the waterway opening depth (D). The ratio of headwater depth to opening depth (HW/D) at the design frequency is one of the key factors in determining the hydraulic adequacy of the structure. This figure also displays the tailwater depth (TW), the depth of water immediately downstream of the structure outlet; and the structure invert, the bottom chord of a closed bottom culvert. When computing HW/D ratios for an embedded culvert, it is important that headwater and structure depth be measured from the streambed surface. This means that a culvert with a total rise of 6 feet, with the structure invert embedded 2 feet below the streambed surface (infilled with 2 vertical feet of E-stone), provides a 4 foot waterway opening depth (D). This opening depth, also termed clear height, is thus relative to the same datum as the headwater, providing a fair comparison for determining hydraulic adequacy. 


Figure 4: A large headwater to depth ratio indicates that the structure does not have sufficient hydraulic capacity. When a structure is hydraulically undersized, headwater depths will rise past the allowable threshold in the design event, putting the roadway at risk of washing out and perpetuating water surface elevations upstream. When water works its way through fill material coving a structure, it can carry sediment downstream, ultimately degrading the road and subbase material, posing a risk to infrastructure and the traveling public. 



Freeboard is the vertical distance between the water surface elevation (WSE) and the lowest point on the bottom of a bridge (low chord). To meet the VTrans hydraulic standard, there must be no less than 1 foot of freeboard at the design flow. This standard is applicable for all open bottom structures, regardless of structure type.

More Information: Hydraulics Manual, Section Freeboard


Figure 5: Warren Covered Bridge, constructed in 1880 (left) and the Waitsfield Covered Bridge (Great Eddy Bridge), constructed in 1833 (right). Freeboard is shown for the Warren bridge in a low flow condition. By contrast, the Waitsfield structure is experiencing flooding from the July 1st, 2017 event. Minimal freeboard is known to cause issues with debris jams. If not rectified, this can cause more debris to build up, ultimately reducing the capacity of the structure and creating a dam, raising floodwaters upstream. This structure was shut down for just one night while local responders worked to cut the tree in half. The bridge was declared safe for traffic the following morning. 



The vertical height from the streambed to the roadway may be a limiting factor, even when headwater to depth criteria is satisfied. Overtopping occurs when the headwater elevation exceeds the roadway elevation. Overtopping is not permissible at the design flow.

More Information: Hydraulics Manual, Section Hydraulic Adequacy


Figure 6: Water overtopping during flood flows is not permissible at the design flow but may occur at flows exceeding this threshold. VTrans calculates a check flow for each structure, generally the 1% AEP event (freeways use the 0.2% event as the check flow). Although the structure is not designed for this magnitude event, further mitigation can be taken to minimize impacts on the structure and surrounding landscape (eg. stone to dissipate energy and protect against erosion, installation of a relief structure to reduce headwater depths, etc.).


Stone Fill

E-stone, or environmental stone, is primarily used within perennial streams. E-stone has fine material that fill in voids, ensuring that flow is kept above the surface. VTrans stone fill has minimal fines, if used within the channel it has been known to cause subsurface flow which is not conducive to Aquatic Organism Passage (AOP).  VTrans stone fill is generally used to protect channel banks and/or abutments from erosion and scour. Stone fill should be larger than or equal to the size of unmoved natural stone. Current stone fill design practice emphasizes outlet velocity as a means of determining adequate stone fill sizing. The following resources are consulted when sizing stone fill based on velocity.

More Information: Hydraulics Manual

  • Section 5.3.2: Stream Channels
  • Section Outlet Protection
  • Section Embedded Materials


Figure 7: Stone fill applied to protect the roadway slopes and channel banks from erosion (left). Hydraulics Manual Table 5-1 provides guidance for sizing VTrans stone fill based on design flow velocity. This type of fill should be used to protect disturbed channel banks or roadway slopes at the structure’s inlet and outlet, up to a height of at least one-foot above the top of the opening. It should not constrict the channel or the structure opening. Type I stone is generally not used except for roadway ditches that require it. Guidance for roadway ditch design may be found in the Hydraulics Manual, Table 5-3.


Figure 8: Table for E-stone design size and the associated infill and structure embedment depths. E-stone includes fine material to fill voids between larger stone, preveting subsurface flow which would otherwise inhibit fish and other aquatic organism passage. E-stone is sized in accordance with the 2% AEP outlet velocity. Each stone size corresponds to a particular embedment depth. Closed-bottom structures on perennial streams should be embedded below the streambed at the corresponding depth, and filled level to the streambed with the specified E-stone.


Channel Equilibrium

State stream equilibrium standards require structures to span the natural channel bankfull width (BFW), which in its general form is the most active portion of the channel. Bankfull structures maintain natural channel velocities and increase the likelihood that the structure will pass natural debris. If a structure constricts the channel width, it is known to cause ponding at the inlet, increase stream velocity and scour at the outlet, and may also lead to erosion and failure of channel banks. By contrast, a structure that spans larger than BFW is more prone to fill in with streambed material (aggregation), restricting the waterway area and hydraulic capacity of the structure. For information on identifying bankfull width see Vermont Stream Geomorphic Assessment: Identification of Bankfull Stage.

More Information: Hydraulics Manual

  • Section Stream Cross Sections;
  • Section 5.2.4: VT Department of Environmental Conservation Guidelines


Figure 9: Identification of bankfull width. When sizing BFW, both upstream and downstream are surveyed to determine the natural channel width that should be maintained through a crossing structure. Historically, many structures in Vermont are undersized, which has led to issues with erosion and channel equilibrium. For more information on bankfull width and how this contributes to the five stages of channel evolution, see the Vermont Rivers and Roads Field Manual.


Aquatic Organism Passage

Stream crossing structures within perennial streams (a stream that continuously conveys surface water) are important transportation networks, connecting fish and other organisms to spawning habitat and allowing for genetic diversity. Aquatic organism passage (AOP) encompasses several features in bridge design that serve to ensure the passage of aquatic organisms within perennial streams. Barriers to AOP are commonly related to velocity, depth and/or turbulence. Maintaining geometric channel features through the structure is essential for providing the continuity and consistency needed for AOP. This is achieved by providing bankfull width structures, embedded at a designed depth (see Stone Fill) and filled to the streambed level with E-stone (stone fill/gravel mix), allowing flow to be kept above the surface, providing the conditions necessary for aquatic organism passage.

More Information: Hydraulics Manual

  • Section Aquatic Organism Passage
  • Section 5.2.4: VT Department of Environmental Conservation Guidelines


Figure 10: Aquatic organism passage is commonly associated with trout spawning habitat, but also encompasses connectivity for many other species, such as this Northern Two-Lined Salamander, pictured in the Mill River, at the outlet of Mount Holly Bridge 7. This species, along with the Northern Dusky and Spring Salamanders, are a few of the many commuters that regularly traverse perennial streams in Vermont. The Northern Two-lined and Spring Salamanders wander away from streams into adjacent woodlands to feed on rainy nights. To minimize the ecological impact, it is imperative that aquatic organism passage be facilitated for crossing structures within perennial streams.


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