HECRAS Culverts Jon Fripp NDCSMC 2016 Module 7

HEC-RAS Culverts Jon Fripp NDCSMC 2016

Module 7: Modeling Culverts • Understand how culverts are modeled in HECRAS • Input culverts in a HEC-RAS river reach

What about culverts? • Hydraulically short pipe capable of transporting water underneath a roadway • Bureau of Public Roads (1950’s), nomographs and charts • Federal Highway Administration HEC-5 (1965), more charts • Federal Highway Administration HEC-10 (1972), more charts • Federal Highway Administration HEC-13 (1972), and more charts • Federal Highway Administration HDS-5 (1984), Hydraulic Design of Highway Culverts • HY-8 computer program for design

Typical Culvert Profile

Remember “inlet control” and “outlet control”? For a given flow profile, what physical features of the culvert dominate flow capacity? 1. Features of the culvert entrance? INLET CONTROL 2. Features at the downstream end? OUTLET CONTROL

Culvert Rating Curve This slide reinforces the fact that inlet controls at the lower flows and outlet control takes over for the higher flows, as a general rule. Note that higher tailwater not caused by flow through the culvert can alter this curve. Once overtopping of the road embankment begins, a large increase in flow can occur with a small increase in the headwater.

Culvert Hydraulics - Inlet Control • Occurs when flow capacity of the culvert entrance is less than the flow capacity of the culvert barrel. • Depends primarily on the geometry of the culvert entrance. Sharp entrance has less capacity. • Control section is just inside the entrance. • Flow passes through critical depth at this location or just downstream of this location. • Headwater is calculated assuming the entrance acts as a weir or as a sluice gate. • Usually occurs during lower flows.

Inlet control Flow is less than the barrel capacity because inlet features cause too much energy loss. Flow passes through critical depth near the inlet. (This is the control section. ) Hydraulic jump occurs in the barrel if downstream slope is subcritical

Culvert Hydraulics - Outlet Control a. Occurs when flow capacity is controlled by the tailwater conditions or the flow capacity of the culvert barrel. a. Entrance, exit, and friction losses are computed in determining controlling headwater elevation. a. Bernoulli equation is used to compute the change in energy through the culvert.

Outlet control Barrel capacity can be reached and flow limited only by downstream tailwater. Flow is generally subcritical and controlled by barrel friction unless. . . tailwater is low enough due to a downstream supercritical slope. Then flow passes through critical near the outlet (the control section).

How does HEC-RAS deal with Culverts? • Same as FHWA – computes headwater due to inlet control and outlet control and selects the higher value – Outlet control FHWA chart only consider case where culvert is flowing full. HECRAS allows culvert to operate partially full under outlet control.

How does HEC-RAS deal with Culverts? • • Weir flow and culvert flow as a combination Direct Step Method for culvert flow Standard Step Method for weir flow Same modeling approach as a bridge – once upstream EGL and WSE is calculated, computations proceed using standard step method – includes inlet control (FHWA) – Uses different equations for low and pressure flow

Culvert Types in HEC-RAS

Inlet and Outlet Control (details) 1. For a given flow rate, HEC-RAS calculates the required upstream energy head for the two cases, inlet or outlet control, assuming first one then another. 2. In general, whichever requires the higher head will be the controlling scenario. 3. The energy head under outlet control is a function of barrel characteristics (size, shape, length, roughness) as well as inlet geometry and tailwater conditions. 4. The energy head under inlet control is not a function of barrel characteristics, but entirely inlet geometry. 5. Hec. Ras calculates energy head assuming the inlet acts like a sluice gate or weir. 6. If the inlet control energy head is higher, Hec. Ras doublechecks to see if an hydraulic jump occurs in barrel. If so, orifice flow is assumed. This is an outlet control scenario.

Adding a Culvert(s) into the Model

Cross Section Locations The culvert must be bounded by two established cross sections (2 and 3) as well as sections that represent full channel flow (1 and 4).

Cross Section Locations (4 of them) Similar to bridge discussion on ineffective flow. The same number of cross sections are required, 1 through 4. 1 – flow fully expanded downstream of culvert 2 – Just downstream of culvert outlet 3 – Just upstream of culvert inlet 4 – flow full expanded upstream of culvert -similar to a bridge

Specify contraction & expansion coefficients. There is a possible range of values. Use larger values is a very abrupt transition section 4 (furthest u. s. ) Exp. 0. 5 ( to 0. 8) Contr. 0. 3 (to 0. 6) section 3 0. 5 ( to 0. 8) 0. 3 (to 0. 6) section 2 0. 5 (to 0. 8) 0. 3 (to 0. 6) section 1(furthest d. s. ) 0. 3 0. 1 ! s e g d i r b o t r a l i m i s

Input Requirements for Culverts • Roadway Deck Information • Culvert Structure Data

Adding the Culvert(s) • Establish the geometry • Select the Brdg/Culv button from the geometry data window:

Select “Options” from the Bridge Culvert Data editor then select “Add a Bridge and/or Culvert…” 21 HEC-RAS

. Adding the Culvert(s) • After you give the culvert a river station Hec. Ras shows you the two bounding sections • Next, select the Deck/Roadway button from the Bridge data window: You might want to add a description.

Roadway Width U. S. Bridge/Culvert Face to U. S. Bounding XS Roadway Data Roadway Embankment Side Slopes (Visual Only) Deck/Roadway Editor Weir Coefficient

24 HEC-RAS January 2009

Adding the Culvert(s) To enter the culvert data, first select the Culvert button:

Add in all your culvert info. Enter the data How many? This distance is from the u. s. end of the culvert to the next u. s. x-section. These stations relate to the cross-sections and tell RAS how to show the culverts in the plot

Enter the data Table 6. 3 or 6. 4, HRM, HDS 5 (for outlet control or click and select OK. . .

Adding Another Culvert Can add multiple culverts at a location or multiple barrels of the same culvert Up to 10 different culvert groups allowed per crossing. Any difference in shape, invert elevations, slope, roughness coefficient, or chart or scale number requires a new group. Up to 25 identical culverts may be in each group per crossing. Note: each individual culvert must have a constant • slope • cross section • material • type • length • discharge

Ineffective Flow Areas Again - like a bridge

…. concerning ineffective flow key question: how close to the culvert is the flow ineffective?

Where is the flow effective? Here? Don’t assume it’s always the same. Think about the site configuration. Set left and right station from culvert edge = distance of bounding cross section from culvert face.

Where is the flow effective? At what level of roadway overtopping does the whole section become effective?

concerning roadway overtopping if all of sec 3 becomes effective because of overtopping. . . then all of sec 2 should also become effective!

for the u. s. side of the culvert for the d. s. side set the elevation lower, by an amount similar to the head loss through the culvert

Cross Section Location Ineffective Flow Areas Cross-Section 3 Cross-Section 2 Ineffective flow limits are typically placed at crosssections 2 and 3, to the approximate width of the culvert plus the amount of contraction or expansion between the culvert and the adjacent cross section (2 or 3). Expansion and contraction ratios can be used to estimate these widths. Elevations associated with the ineffective flow areas are typically set at the top of road elevation on the upstream side (cross section 3), and a nominal amount below the top of road on the downstream side (cross section 2). A rule of thumb for this nominal amount could be half the HEC-RAS distance from the top of road to the crown of the culvert. January 2009

Once data is entered, check out the schematic of the culvert group Visually Check • Number • Shape • Size • Ineffective area …

Do not let the plot fool you If the invert elevation is below ground, a portion of the culvert area will appear to be ineffective on the plot. However, HEC-RAS uses the area specified regardless of horizontal or vertical position.

Culvert Output Several Methods of Viewing Output for Culverts: • Pre-defined Tables • Plots Note the detailed output table where you can view: • Control (outlet or inlet) • Q per barrel • Velocity • Depth • etc.

Adding Debris at Culvert(s) Partially Buried Culverts Multiple ‘n’ Values in Culvert

Debris at Culverts Debris is shown graphically.

…and concerning debris… key question: what will the culvert look like during your project flood? What about n-value? 1. debris plugs culvert 2. flood washes out embankment Do you need a higher entrance loss coefficient?

Is this a debris issue or a wildlife issue?

Examples for low-flow methods • For culverts with changes in slope, shape, etc. , the energy-based standard-step method is the most suitable approach. (Use cross sections with a lid option for multiple sections. ) Several sections can be taken through the culvert to model changes in grade or shape or to model a very long culvert. • For culverts modeled as cross sections, the inlet control solution can be checked by hand calculations.

Special Culverts • Culverts with changing cross-section or with changes in slope can be modeled by using cross-sections with lids. Ground geometry describes bottom half of culvert. Uses energy equation. Allows user to see profile through the culvert.

Adding a lid to a cross section Q: Why add a lid? A: Broken back (special) culverts. § Basically a form of ineffective flow area § Can be used to model open channel flow through a pipe (note: only open channel equations are used) § Do not use for modeling typical culverts § Once water surface comes in contact with the lid, subtracts out the area of the lid and adds wetted perimeter. The space between the high and low lid is removed from the conveyance computations. § HEC-RAS fits a straight line between adjacent lid points to define the lid Note: only open channel equations are used

Special Culverts • Under “Cross Section Data”, click on “Options” • Then “Add a Lid” • Enter the coordinates of the lid at that section • Look at the plotted section. • Does it make sense?

Culvert Warning! • HEC-RAS computes the upstream EGL that will pass the water through the culvert. • The road material may function as a dam. • Storage upstream of culvert may not be considered if it is modeled as steady flow. • Inflow = outflow? • Reservoir simulation?

How would you model this culvert group?

Multiple Culverts • How should this crossing be modeled?

How would you model this culvert?

How would you model this culvert group? What additional information would you want about this culvert set?

Is this a concern? Would this affect the hydraulics? How would you model it? How would you predict it?

Culvert Issues? This m ! a n d o y ti r s d e a u f q o k Tric tream s n w o is d Would steady flow be appropriate? What would an unsteady model tell us? W e Re ll – m no em t r res be ea erv r t lly oir hat a tr ro HE ick ut CR qu ing A est S pr is og no ion ram t a

How would you model this culvert?

The End Questions?