What is hydrology?

The branch of science concerned with the properties of the earth's water, especially its movement in relation to land.


What is hydraulics? 

The branch of science and technology concerned with the conveyance of liquids through pipes and channels, especially as a source of mechanical force or control.


How do hydraulics and hydrology impact transportation design?

The purpose of hydraulic engineering is to design a structure with the proper capacity to divert or remove water from the roadway and pass collected water under the roadway. The design of a hydraulic structure requires knowing how much water is associated with the design storm (hydrology) and calculating the velocity, depth, and type of flow (hydraulics) that must be accounted for in the design.


What services does the Hydraulics Unit provide?

The hydraulics unit provides engineering guidance for stream crossing structures within the State of Vermont. This is commonly achieved by means of a hydrologic and hydraulic (H&H) analysis. Precipitation records are used to develop statistical recurrence intervals for rainfall events. This information is then applied over the land surface, allowing engineers to determine the volume of water that would be expected during the particular rainfall event at a specific location. Once geometric features of the stream have been identified, this volume of water can be applied within the local system. With the hydrology completed, the engineer can then utilize design tools to recommend resilient infrastructure that is cost-effective, constructible based on site constraints, and provides the conditions necessary for stream equilibrium and organism passage.


How can I submit a hydraulics request?

Contact the VTrans district office for your community (http://vtrans.vermont.gov/operations/districts)


What is aquatic organism passage and how is this encompassed during the design process?

The interaction of our infrastructure system can adversely impact the local river ecosystem, restricting passage and providing barriers, limiting the movement of spawning fish and other aquatic organisms. Stream crossing structures within perennial streams (a stream that continuously conveys surface water) are extremely important as they serve to connect environmental transportation networks for fish and other aquatic organisms. Aquatic organism passage (AOP) is an essential feature in bridge design that serves to ensure the passage of these organisms within perennial streams.

Barriers to AOP are commonly related to velocity, depth and 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 and filled to the streambed level with E-stone (stone fill/gravel mix), graded to match the natural bed material, allowing flow to be kept above the surface, providing the conditions necessary for aquatic organism passage.


What is bankfull width and why is this an important design parameter for sizing structures?

Bankfull width (BFW) in its general form is the most active portion of the channel.  By designing structures to span BFW, we 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. Alternatively, a structure that spans larger than the BFW is more prone to fill in with streambed material, restricting the waterway area and hydraulic capacity of the structure. 


What is scour and how can it be prevented?

Scour is the most common cause of bridge failure (NCHRP). This phenomenon occurs when moving water erodes material from around a bridge pier or abutment, surrounding banks or streambed. Sediment supports pier and abutment foundations and is essential to the stability of bridges. There are a variety of scour countermeasures in existence, including: vegetation, in-stream flow control structures, geotextile filters, and the most common countermeasure, the use of stone fill placed along the piers, abutments and surrounding stream banks. These retrofits serve to dissipate energy and the erosive force from the flow of water.


How is the recommended low beam elevation of a bridge determined for state and town highway projects?

The low beam elevation of a bridge is one of the most important conclusions of a Hydraulic and Hydrologic study. Each roadway is given a functional classified based on ownership and traffic usage, which also corresponds to a design event that is based on the probability of annual occurrence. In general, state routes are designed based on rainfall that has a 4% chance of occurring each year, also called the 4% annual equivalence probability (AEP). State routes are designed for the 2% AEP, which has a 2% chance of occurring each year, a rainfall event that statistically occurs once within fifty years. Accompanying the hydrology, geometric features, such as stream slope and bankfull width (BFW) are determined by survey or field visits. New structures are then designed to accommodate the BFW of the stream. Using the volume of water given by the design storm and the necessary width of a replacement structure given by the BFW, engineers can then determine the elevation of the water at the inlet of the proposed structure. The low beam elevation is then determined by adding one foot to this depth. In this way, a new bridge on a state route is capable of passing a 2% AEP rainfall event with one foot of clearance between the depth of water and the low chord of the structure.