Question

Provide the Introduction for the minor Project 1-Culvert Design. It should include at least the following information:

INTRODUCTION

A.General facts on culverts

B.Overview of Culverts

1.Shapes

1. Materials

2. Inlet types and selection

3. Considerations regarding Velocity at the culvert Outlet

4. Information regarding construction and culvert maintenance.

5. Culvert Hydraulic Programs.

Sketches and images to elucidate the information are also required

Answer 1

GENERAL :-

Culvert is a tunnel carrying a stream under a road or railway. A culvert may act as a bridge for traffic to pass on it. They are typically found in a natural flow of water and serves the purpose of a bridge or a current flow controller.

A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction from one side to the other. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. In the United Kingdom, the word can also be used for a longer artificially buried watercourse

Culverts are commonly used both as cross-drains to relieve drainage of ditches at the roadside, and to pass water under a road at natural drainage and stream crossings. A culvert may be a bridge-like structure designed to allow vehicle or pedestrian traffic to cross over the waterway while allowing adequate passage for the water.

INTRODUCTION CULVERT :-

Culverts are structures used to convey runoff from one side of the road to another and are usually covered with embankment and composed of structural material around the entire perimeter, although some are supported on spread footings with the streambed serving as the bottom of the culvert. For economy and hydraulic efficiency, culvert should be designed to operate with the inlet submerged during the flood flows , if conditions permit. Cross-drain are those culverts and pipes that are used to convey runoff from one side of highway to another

ESTIMATION: For all engineering works it is required to know beforehand the probable cost of construction know as the estimated cost. If the estimated cost is grater then the money available, then attempts are made to reduced the cost by reducing the work or changing the specification. From this the importance of estimate for engineers may be understood. In preparing in estimate, the quantities of different items of work are calculated by simple mensuration method and from these quantities the cost is calculated. The subject of estimating is simple, nothing much to understand, but knowledge of drawing is essential. One who understand and can read drawing may find out the dimension- length, breath, & height etc. In preparing an estimate one has to go in to details of each items, big or small, nothing can be left or missed. Accuracy in estimate is very important, if estimate is exceeded is becomes a very difficult problem for engineers to explain to account for and arrange for the additional money. Inaccuracy in preparing estimate, omission of items, change in designs improper rates etc.

Culverts are available in many and shape like round, elliptical, flat-bottomed, pear-shaped, and box-like constructions. Culverts are by their load and water flow capacities, lifespan and installation of bedding and backfill. The type is based on a number of factors including hydraulic, upstream elevation, and roadway height and other conditions.

Following are the different types of Culvert:

• Pipe culvert (single or multiple)
• Pipe-Arch culvert (single or multiple)
• Box culvert (single or multiple)
• Arch culvert
• Bridge culvert
• Metal box culvert

Pipe Culvert

Pipe culverts are the most common types of culverts due to competitive price and easy installation. They are found in different shapes such as circular, elliptical and pipe arch. Generally, their shapes depend on site conditions and constraints. Pipe culverts on a small scale represent normal pipes like concrete pipes.

Pipe culvert. Source: metal-culvert.com

Advantages of Pipe Culvert

The main features of pipe culverts are:

• It can be constructed of any desired strength by proper mix design, thickness, and reinforcement.
• They are economical.
• These pipes can withhold any tensile stresses and compressive stresses.
• The crossing of water is under the structure.

Disadvantages of Pipe Culvert

The main disadvantage of pipe culvert is that it can be easily corroded at the crown because of bacteria‘s organic matter and release of harmful gas, which is known as Crown corrosion.

Pipe-Arch Culvert (Single or Multiple)

Arch culverts are suitable for large waterway opening where fishes can be provided with a greater hydraulic advantage. Moreover, they provide low clearance and are definitely, much artistic. Pipe arches are particularly useful for sites where headroom is limited and also have a hydraulic advantage at low flows.

Pipe arch culvert. Source: http://www.theconstructor.org

Advantages of Pipe-Arch Culvert

The features of pipe arch culverts are:

• Limited headroom condition
• Improved hydraulic capacity at a low flow
• Aesthetic shape and appearance
• Lightweight
• Easy to install

Box Culvert

Box culverts are made up of concrete and especially, RCC (Reinforced Concrete). The most challenging part in constructing a box culvert is that dry surface is needed for installing it. However, due to the strength of the concrete floor, water direction can be changed when a large amount of water is expected. This feature makes box culverts, one of the most commonly found types of the culvert.

Box Culvert. Source: miller-miller-inc.com

Advantages of Box Culvert

Box Culverts are economical for the reasons mentioned below:

• The box culvert is a rigid frame structure and very simple in construction
• It is Suitable for non-perennial streams where scrub depth is not significant but the soil is weak.
• The bottom slab of the box culvert reduces pressure on the soil.
• Box culverts are economical due to their rigidity and monolithic action and separate foundations are not required.
• It is used in special cases, weak foundation.

Arch Culvert

An arch culvert is made up of metal, stone masonry, concrete, RCC etc. Construction does not take a lot of time and unlike box culvert, water diversion is not necessary, as it can be installed without disturbing the water current. Thus, it can be termed as a Low Profile Culvert. This type of culvert maintains the natural integrity of the wash bed.

Arch culvert. Source: http://www.visitgrey.ca

Advantages of Arch Culvert

The advantages of using arch culverts over traditional box culverts and pipe culverts are as follows:

• Cost savings
• Accelerated construction schedule
• Greater hydraulic efficiency
• Pleasing aesthetics
• Design-build advantage

Bridge Culvert

Bridge culverts serve a dual purpose. It acts both as a bridge and a culvert. Generally, rectangular in shape, bridge culverts are constructed on rivers and canals. A foundation is laid under the ground level and pavement surface is laid on top of the series of culverts. Generally, we can term it as a Multi-Purpose culvert.

Bridge culvert. Source: binghamtonprecast.com

Advantages of Bridge Culvert

Following are the main features of bridge culvert:

• Extension of the network by acting as a repeater
• Very strong
• Allows traffic to pass on it
• Highly strong foundation
• Most expensive river crossings

Metal Box Culvert

The metal box culvert is the economic alternative of the bridge. These bridges are manufactured from a standard structural plate or deep-corrugated structural plate. They are the perfect bridge replacement maintaining the same road grade level.

Metal Box Culvert. Source: conteches.com

Advantages of Metal Box Culvert

The advantages are as follows:

• Durability
• Shorter construction period and easy installation
• Deformation ability
• Long service life

Culvert Design Items

The following should be considered for all culvert designs where applicable:

1. Engineering aspects

a. flood frequency

b. velocity limitations

c. buoyancy protection

2. Site criteria

a. length and slope

b. debris and siltation control

c. culvert barrel bends

d. ice buildup

3. Design limitations

a. headwater limitations

b. tailwater conditions

c. storage –temporary or permanent

4. Design options

a. culvert inlets

b. inlets with headwalls

c. wingwalls and aprons

d. improved inlets

e. material selection f. culvert skews

g. culvert sizes and shapes

h. twin pipe separations (vertical and horizontal)

i. culvert clearances

5. Related designs

a. weep holes

b. outlet protection

c. erosion and sediment control

d. environmental considerations

Culvert Designing Step

1. Determine the Horizontal Distance

2. Determine the Required Pipe Size

3. Determine the Energy Loss Gradient

• With the full pipe flow impending, the energy loss gradient can be estimated by the Manning equation for open-channel flow

4. Determine the Critical Slope

• For critical flow, the Froude number is equal to unity:

MATERIALS:-

Culverts can be constructed of a variety of materials including cast-in-place or precast concrete (reinforced or non-reinforced), galvanized steel, aluminum, or plastic (typically high-density polyethylene). Two or more materials may be combined to form composite structures. For example, open-bottom corrugated steel structures are often built on concrete footings.

Culverts are like pipes but very large in size. They are made of many materials like

• Concrete
• Steel
• Plastic
• Aluminum
• high density polyethylene

In most cases concrete culverts are preferred. Concrete culverts may be reinforced or non-reinforced. In some cases culverts are constructed in site called cast in situ culverts. Precast culverts are also available. By the combination above materials we can also get composite culvert types.

Inlet Control

If water can flow through and out of the culvert faster than it can enter, the culvert is under Inlet Control. Flow capacity is controlled at the entrance by the headwater depth, cross-sectional area and type of inlet edge. Culverts under inlet control will always flow partially full and are in a state of shallow, high velocity known as Supercritical flow. Any downstream disturbance will not be propagated upstream since the flow of water is too great. The roughness, length and outlet conditions are not factors in determining capacity. Flow is therefore controlled upstream and is limited to what can enter the culvert. Culverts that have a drawdown at the inlet and a perch or hydraulic jump at the outlet are usually inlet control.

Outlet Control

If water can flow into the culvert faster than it can flow through and out, then it is under Outlet Control. Culverts under outlet control can flow either partially full or full. In this case water is relatively deep and slower, known as Subcritical flow and a disturbance propagates upstream. Therefore flow is controlled downstream and limited to what the pipe can carry. In this case friction and roughness in the culvert are significant in the flow through a culvert and the difference in headwater and tailwater depth represents the energy which conveys flow through the culvert.

Inlet and outlet control are set by the slope of the stream, it is not a designed feature. Generally speaking, when culverts are designed, calculations are made assuming both inlet and outlet control and comparing the headwater depth under both conditions. Designs for low headwater depths reduce pipe diameter and fill material, but risk overtopping and often result in undersized culverts when exposed to natural conditions. Conversely designs for higher headwater depths are more conservative and generally govern design.

Culvert Design Considerations

The structural choice of a culvert and corresponding inlet is based on environmental considerations, risk to property, cost of construction and maintenance and also aesthetic considerations. The capacity of an existing culvert can be increased with an improved inlet.

Culvert Inlet Design

An improved inlet serves to funnel the flow into the culvert to remove the point of control from the face of the inlet to a throat located downstream from the face. The normal contraction of flow is included in the transition from the face to the throat. An improved inlet may be economical if the culvert is operating under inlet control, but not if the culvert is operating under outlet control.

An improved inlet may offer the advantage of increasing the capacity of an existing culvert that has become inadequate because of changes in the watershed which have increased the discharge to the culvert. However, improving a culvert inlet is not recommended in the following situations:

• Available design procedures cannot accommodate an improved inlet when the face of the inlet is skewed to the flow entering the inlet.
• Heavy debris loads could pass through the inlet entrance and become lodged further in the culvert due to the restriction at the throat.
• The flow reduction at the throat may cause the culvert to flow as outlet control, which would negate any advantages of the improved inlet.
• Improved inlets are usually costly to construct when compared with standard inlets.

Careful consideration should be given before selecting and using an improved inlet design. Guidelines for design can be found in FHWA publication, Hydraulic Design of Highway Culverts, HDS-5.

The recommended types of improved inlets are top-tapered inlets, side-tapered inlets, and slope-tapered inlets.

Top-Tapered Culvert Inlet

A simple transition of depth in a rectangular box culvert may improve the hydraulic efficiency. If the box culvert is operating under inlet control, the barrel of the culvert is more hydraulically efficient than the entrance geometry. The designer may reduce the barrel depth in transition from the original depth to a minimum of 1.0 feet (0.3 m) greater than the uniform depth of flow. The transition length should be a minimum of 20 feet (6 m) as shown in Figure 1, below. This method is arbitrary, and care should be exercised when the culvert is operating in inlet control.

. Top-tapered box culvert

In terms of design and construction, top-tapered transition inlet is effective, economical, and simple to construct. This type inlet improvement is desirable when designing a multiple barrel box culvert. Other inlet improvement types are not feasible for multiple barrel box culverts because of the need to taper or flare the sidewalls of the barrels.

Side-Tapered Culvert Inlet

Side-tapered inlets involve a widening of the face area of the culvert by tapering the sidewalls. Such inlets have two possible control sections as shown in : the face and the throat as shown in Figure 2. Maintain control at the throat for the design discharge in order to realize significant cost savings in the culvert barrel. This type of improvement is similar in operation to the flared inlet for pipes.

Side-tapered inlet

Slope-Tapered Culvert Inlet

The slope-tapered inlet incorporates the efficient flow characteristics of side-tapered inlets with a concentration of more of the total available culvert fall at the throat control section. Figure 3 shows a slope-tapered inlet. Generally, slope-tapered improvements are not practical for pipe culverts because of their complexity.

Some of the drawbacks of slope-tapered inlets are as follows:

• Slope-tapered inlets have a tendency to allow sediment deposition; this can result in maintenance problems.
• The degree of the slope taper is limited by how flat the remaining portion of pipe can be made without resulting in a mild slope.
• The use of slope-tapered inlets can increase costs of structural excavation because of the lowering of the upstream end of the culvert.

. Slope-tapered inlet

CULVERT HYDRAULICS:-

A culvert is a relatively short segment of conduit that is typically used to transport water underneath a roadway or other type of earthen embankment. There is some common terminology that is used in culvert hydraulics that can best be presented by referring to Figure 1. The culvert itself consists of an entrance, an outlet, and a culvert barrel. Common culvert shapes include circular pipes, rectangular boxes, ellipses, and arches. Noncircular culverts are generally described by their size in terms of a culvert rise (D) and a culvert span (B). The size of a circular culvert is usually expressed in terms of the culvert diameter (D).

There is a wide variety of entrance conditions found at culverts, including square edge, angled wingwalls, beveled edges, entrance mitered to slope, et cetera. Some of these common culvert end treatments are shown in Figure 2. It is not uncommon for the opening of a culvert to be smaller than the original channel cross-section prior to the culvert installation. All else being equal, a smaller waterway opening will result in a lower channel conveyance, that is, a lower carrying capacity of the channel. For the same flow, a lower conveyance will, in turn, result in a higher depth of water upstream of the structure, called the headwater.

In today's environment of floodplain management and regulations, the increase in water surface upstream of culverts is often limited. Therefore, culvert designs that convey water under roadways with minimal headwater buildup are becoming more common. The hydraulic solution to minimize the head loss would be to not constrict the flow by spanning the entire conveyance channel. However, economic considerations many times prohibit this approach. While some increase in water level upstream of the culvert may be tolerated, the basic principle behind culvert design is to ensure that the water level increase is not unacceptably high. The headwater can be estimated using well-established design methodologies.

Many installations use three-sided culverts, where the bottom of the culvert is typically the natural channel bottom.

Historically, most culverts were closed conduits, where the same material is found on the top, bottom, and sides of the culvert, for example, a corrugated metal pipe culvert. With environmental regulations becoming more stringent, many culvert installations utilize three-sided culverts. A three-sided culvert is a structure that has the same material on the top and sides of the structure. The bottom of the culvert is typically the natural channel bottom. The most commonly used culvert materials are concrete, corrugated metal, and plastic. Usually, the internal roughness of a culvert is a function of the culvert material. However, for a three-sided culvert, where the bottom of the installation is the natural channel, the internal roughness is a function of the culvert material and the roughness of the channel itself.

Culvert geometry

Culverts are usually laid on a slope, which can be found by dividing the elevation difference between the upstream and downstream ends of the culvert (?Z) by the culvert length (L ). Typically, the slope is downward such that the outlet elevation is lower than the inlet elevation. In some cases, culverts may be laid horizontal or on an adverse slope where the downstream elevation is higher than the upstream elevation.

The tailwater at a culvert is the depth of water at the downstream end of the culvert, as measured from the downstream invert of the culvert. The tailwater must be known or estimated prior to performing the culvert hydraulic calculations. There are various methods to estimate the tailwater at a culvert. One method is to estimate a downstream channel shape and use Manning's equation to calculate a tailwater depth. Another method is to conduct a water surface profile analysis of the steam reach downstream of the culvert.

For a given design discharge (Q), there will be a corresponding headwater depth (HW) upstream of the culvert entrance. In fact, it is the headwater depth that pushes or forces the design discharge through the culvert opening. For a given culvert opening, a higher discharge will typically result in a higher headwater depth since more energy is needed to force the flow through the culvert. In open-channel hydraulics, energy is synonymous with water depth as shown in Equation 1.

Equation 1:

Where:
E is specific energy (feet);
Y is depth of water (feet);
V is mean water velocity (feet per second);
g is acceleration due to gravity (feet per second per second).

Culverts are frequently designed to pass some specified design discharge without creating an unacceptably high headwater depth. Thus, for an engineer to design a culvert successfully, the headwater depth for the design discharge must be reliably predicted. For many applications, the culvert design discharge is frequently associated with the 1-percent, 2-percent, or 4-percent annual chance storm event. Knowledge of the headwater depth associated with a particular flow condition will reveal to the engineer whether or not the culvert will pass the design flow safely — without overtopping the embankment or violating applicable regulations. The definition of an unacceptable headwater depth may vary among sites, but typically, the maximum headwater elevation should be about 1 or 2 feet lower than the roadway shoulder elevation to minimize the potential for roadway flooding. Of course, other factors, including site conditions and construction schedules, contribute to the final culvert design specifications. Nonetheless, it is important for engineers and others involved with culverts to be able to predict the hydraulic performance of these structures accurately so that they operate without any undesirable effects.

Figure 2: Common culvert end treatments

Standard FHWA culvert design approach

According to research sponsored by the Federal Highway Administration (FHWA), culvert operation is governed at all times by one of two conditions: inlet control or outlet control (Normann, et al, 1985). Inlet control is a common governing situation for culvert design, characterized by the fact that the tailwater or culvert barrel conditions allow more flow to be passed through the culvert than the inlet can accept. The inlet itself acts as a controlling or governing section of the culvert, restricting the passage of water into the main barrel.

Outlet control is different from inlet control in that the barrel or tailwater cannot accept as high a flow as the inlet may allow. This may occur with a high tailwater or a long culvert with a rough interior. Outlet control may be mathematically modeled using water surface profile methods or by an energy balance. Because outlet control conditions in culverts can be calculated with open-channel hydraulic principles, there is no need for empirical testing and regression formulas to describe the relationship between the flow through the culvert and the headwater. However, testing on scale models can provide valuable information about the head loss coefficients associated with the culvert entrance. Once the outlet control situation has been modeled as accurately as possible based on known information, the headwater may be calculated to evaluate the culvert design.

The FHWA has standardized the manner by which culverts are examined and designed. The design approach involves first computing the headwater elevation upstream of the culvert assuming that inlet control governs. The headwater elevation is then also found assuming that outlet control governs. The two headwater values are compared with one another and the higher of the two is selected as the basis of the culvert design.

Generally speaking, the procedure described above is repeated for different types of culvert shapes, sizes, and entrance conditions. The least expensive culvert that produces an acceptable headwater elevation is typically chosen for the final design. Of course, site conditions, structural considerations, permit requirements, or aesthetic appeal may also influence the choice of culvert design.

Inlet control

Inlet control represents a much more complex hydraulic environment than outlet control, and it cannot be strictly mathematically modeled to obtain headwater depths. Under inlet control, the flow patterns at the entrance to the culvert may be three dimensional with vortices or other unpredictable features. These patterns are influenced by a number of factors, the most important of which are inlet geometry, wingwall configuration, culvert shape, and degree of beveling. Fortunately, culverts operating under inlet control can be modeled using regression equations.

For many years, inlet control culverts modeled using the methodology outlined in the FHWA Hydraulic Design Series No. 5 (HDS-5) – Hydraulic Design of Highway Culverts have successfully withstood both extensive laboratory tests as well as the test of time in field installations. Empirical measurements on small-scale models of varying inlet geometries and wingwall configurations led to derivation of unique regression coefficients for each case. These models possess remarkably similar hydraulic characteristics to their full-size counterparts and provide the best approximation of how a particular culvert shape will perform in the field (Normann, et al, 1985).

Because inlet control represents the case where the culvert barrel will convey more flow than the inlet will accept, the culvert normally will not flow full for its full length, thereby resulting in a free water surface that exists along the length of the structure. Under inlet control, the culvert entrance may either be unsubmerged or submerged. Figure 3 shows the latter case.

Figure 3: Example of submerged inlet control

At low flows, the culvert entrance is unsubmerged and the discharge through the culvert entrance behaves like weir flow. A weir is a flow control cross-section where the discharge and depth of water are related to one another through some predictable relationship. At much higher flows, the culvert entrance is submerged and the flow through the entrance acts like orifice flow. Orifice flow represents the case where an opening is submerged and the discharge through the opening increases as the depth or head above the opening increases.

One example of where inlet control occurs is when there is a mild channel slope upstream of the culvert that transitions to a steep culvert slope (Norman, et al, 1985). The transition from a mild slope to a steep slope causes a change in the flow regime from subcritical to supercritical flow. The change from subcritical to supercritical flow results in critical depth occurring at or near the entrance to the culvert. In some cases, such as short, smooth culverts, the nature of the culvert entrance can cause inlet control to occur even if the culvert slope is mild or flat.

While the behavior of flow at the entrance to a culvert is extremely complex, the primary influencing factors for headwater depths are the type of opening (pipe, box, arch, et cetera), the size or area of the culvert opening, and the entrance conditions. Commonly found entrance conditions include square edge with headwall, end mitered to the slope, projecting barrel, and beveled entrance. Culvert inlets may also utilize wingwalls placed at an angle from the culvert barrel. Not only do wingwalls provide structural stability to the culvert and act as retaining walls for fill slopes, they can also perform a hydraulic function by funneling flow into the culvert opening.

Recall that the complexity of the hydraulics associated with inlet control, when combined with the large number of different shapes, sizes, and entrance conditions available for culverts, make it nearly impossible to develop a single formula capable of describing the hydraulic behavior of culverts operating under inlet control. As a result, empirical methods are typically used to evaluate inlet control.

Inlet control equations are presented in HDS-5 that describe unsubmerged and submerged inlet control (Normann, et al, 1985). For the unsubmerged case, two expressions can be used as shown in Equations 2 and 3. While both expressions provide acceptable results, Equation 2 is theoretically more accurate, while Equation 3 is easier to apply. Additionally, the latter equation is easier to use when developing regression coefficients from observed headwater depths and discharges or when the critical depth through a structure is not easily determined.

Equation 2:
Equation 3:

Where:
HW is headwater depth at the culvert entrance (feet);
HC is specific energy at critical depth (feet);
Q is discharge through the culvert (cubic feet/ second (ft3/s));
A is full open area of the culvert (square feet);
D is culvert rise (feet);
S is the slope of the culvert barrel (feet/foot);
K and M are inlet control regression coefficients for unsubmerged conditions.

Either form of the two equations above will produce acceptable results (Normann, et al,1985). For model studies, quantities measured in the lab are typically the headwater (HW) and the discharge (Q). Other known quantities include the area of the model (A), the model rise (D) and the slope of the channel. When developing regression coefficients using Equation 2, the specific energy at critical depth must be computed and used in the regression analysis. Use of Equation 3 avoids the need to make these additional calculations.

When the culvert entrance is submerged, a different equation must be applied to find the headwater depth under inlet control (see Equation 4). As with the case of unsubmerged inlet control, model studies are typically used to develop the inlet control regression coefficients.

Equation 4:

Where:
HW, Q, A, D, and S are as previously defined;
c and Y are inlet control regression coefficients for submerged conditions.

Outlet control — culvert flowing full

In HDS-5 design methodology, outlet control is determined assuming that the culvert is flowing full. The headwater due to outlet control is found from Equation 5, which is an energy balance between the upstream and downstream ends of the culvert.

Equation 5:

Where:
HW is headwater depth above the inlet invert (feet);
EL0 is the elevation of the culvert invert at the outlet;
H0 is the governing tailwater (feet);
hL is head loss through the culvert (feet).

To find the governing tailwater, H0, the critical depth in the culvert must first be determined. The critical depth is then used with the culvert size and compared to the specified tailwater as shown in Equation 6.

Equation 6:

Where:
TW is the tailwater at the downstream end of the culvert (feet);
DC is critical depth in the culvert (feet);
D is culvert diameter or rise (feet).

The head loss through the culvert, hL , is found by considering all losses, including entrance losses, exit losses, and friction losses. Manning's equation is rearranged to quantify friction losses. Equation 7 can be used to determine the head loss through a culvert. If bends occur along the length of the culvert, then these losses must also be included in Equation 7.

Equation 7:

Where:
Kx is an exit loss coefficient;
n is Manning's roughness coefficient;
L is the length of the culvert (feet);
R is the hydraulic radius of the culvert (feet);
Ke is an entrance loss coefficient;
V is velocity in the culvert (feet per second);
g is the gravitational constant (feet per second per second).

Values for the entrance loss coefficient, Ke, are available in various hydraulic texts including HDS-5, and values range from 0.20 to 0.80, depending on the inlet type and configuration. Values for exit loss coefficients, Kx, can vary between 0.3 and 1.0. For a sudden expansion of flow, the exit loss coefficient is set to 1.0. The exit loss coefficient should be reduced as the transition becomes less abrupt (HEC-RAS Hydraulic Reference Manual, 2002).

For culvert applications where a natural bottom is used, a composite Manning's roughness coefficient must be computed. There are several assumptions that can be used to determine a composite roughness value. One common assumption is that each part of the area has the same average velocity, which is equal to the average velocity of the whole section (Chow, 1959). With this assumption, the composite Manning's roughness, nc , may be obtained by Equation 8:

Equation 8:

where:
Ps&t is the wetted perimeter of culvert sides and top(feet);
Pch is the wetted perimeter of the natural channel(feet);
ns&t is Manning's roughness for the culvert sides and top culvert; and
nch is Manning's roughness for the natural channel.

Outlet control — culvert flowing partially full

The methodology in HDS-5 using the equations from the procedure outlined above assumes that the culvert is flowing full along the entire length. A common design case occurs when it is necessary to minimize the head loss through a culvert. The minimal headwater rise, small slope, and high relative tailwaters associated with these conditions usually result in outlet control. In this outlet control case, the culvert is most likely to be flowing partially full. In this case, a water surface profile analysis is necessary to determine the losses through the culvert accurately. The profile analysis is conducted from the downstream end to the upstream end of the culvert.

For culverts flowing partially full, the most efficient method to compute the water surface profile in the culvert is the direct step method. The direct step method computes the water surface profile at increments of known depths. The first step is to compute the exit loss and establish a starting water surface inside the culvert at the downstream end. The starting water surface will either be critical depth or the result of an energy balance between the tailwater and a cross section just inside the culvert on the downstream end. Once a water surface is computed inside the culvert at the downstream end, the designer performs the direct step calculations along the length of the culvert. After the depth of water is determined at the upstream end, the entrance loss is added in to compute the headwater depth.

CONCLUSION

Use culverts as wide as stream width

• Use same gradient as stream channel

• Use same alignment as stream channel

• Single large culvert is better for debris passage than several small ones

• Flared ends improve efficiency