Culvert Upgrade

Replacing existing culverts with large diameter ones. May also include armoring of inlet and outlet areas. 

Purpose: Culvert improvements increase the flow capacity which will prevent damage to roads.

Relative Effectiveness: Excellent-0% Good-80% Fair-0% Poor-20% (Replies = 5).  When sized properly and installed correctly, the results were rated “good.” The “poor” rating was from culverts that were still not large enough and failed.

Environmental/Implementation Factors: Upgraded culverts need to be sized properly based on expected increased flows. They should be installed at the proper slope with appropriate approaches and exits. To be effective, upgraded culverts need to be installed before the first damaging rainfall. Flexible down spouts and culvert extensions often are needed to keep exiting water from highly erodible slopes.

Riser Pipe (Snorkels, Drop Inlets) Implementation


To prevent sediment and floating debris from plugging culverts and subsequent fill failure or loss of control of water.
To capture sediment to prevent loss of water quality and reduce sediment bulking of flood flows.


Applicable to 24", diameter culverts or less.
In intensively burned drainages where topography and/or soils indicate a high likelihood of significant soil and debris transport will occur in a runoff event.


At road crossings where access to culvert inlet is limited by conventional equipment (e.g., backhoe).
Where winter/spring access (road patrol) is precluded by snow or soft roadbed.


Riser pipes are sized and designed individually to meet the needs and/or mitigate the risks at a particular site. Riser pipes should function to sieve out debris and allow passage of water. The riser pipe will also allow accumulation of bedload sediments released due to the loss of soil cover and infiltration, that can later be removed and properly disposed.


The "drop inlet" design is typically the most adaptable and therefore most common, because there is no direct connection to the existing pipe. The vertical riser is a corrugated metal pipe with a notch cut into the bottom (to the diameter of the pipe being enclosed) and fits over the culvert effectively sealing off the opening from the surrounding soil. The drop inlet riser should be no smaller than 36 in diameter. The riser has notches or slits cut into the sides vertically, the lowest point of the notch opening being far enough from the bottom to allow backfilling and deposition of soil sufficient to stabilize the structure. This distance may be as much as one third the total riser height. Very tall risers (over 8 feet) in larger drainages may require anchors to keep them from moving with the expected flow velocities, at least initially until the accumulation of soil surrounding the pipe secures it in place. At the top of the riser a steel grate should be attached to keep floatable debris from dropping in. The erosive power of water pouring in from the sides and the top calls for an impact surface to be added to the bottom. This can be done by pouring concrete into the bottom, but also can be achieved by dumping large rocks into the riser followed by dry cement or ready mix. The dry cement can be worked around the rocks to mortar them together into a solid mass. This impact surface must not be higher than the invert of the culvert and should be at least one foot thick. A dewatering feature is recommended for large risers behind very high embankments to relieve any possible buildup of hydrostatic pressures. This feature can be be as simple as a few small holes near the bottom of the pipe covered with some type of poly-fabric or fiberglass filter to allow drainage into the riser and through the culvert.

Advantages of the Drop Inlet Design:

Can be manufactured with rudimentary skills in welding *Utilizes readily available materials, often stockpiled locally *Can be installed with hand labor only, in some cases *Can become a permanent structure and does not need to be disassembled

Disadvantages of the Drop Inlet Design:

Does not increase the flow capacity of the culvert *Can be labor intensive
If they become plugged they can be difficult to clean out *Hydrostatic pressures could cause piping beside the culvert if not properly relieved through some type of dewatering feature.


The "T" design or "elbow" attachment, requires that a collar be attached to the existing culvert that runs beneath the embankment then either 'IT-ed" or elbowed up to allow the expected accumulation of sediment and debris. Because pipe inlets are often damaged, installation of a collar may require additional excavation to expose the pipe to sufficient length to cut off the damaged section. This riser also has slits and a grate over the top, as with the drop inlet design. The angle between the existing pipe and the riser should not be less than 90 degrees for most efficient flows and to facilitate maintenance. This riser design in a large fill where a lot of sediment is expected, also has the potential to back up water behind the embankment and create a hydrostatic condition. This can be dealt with effectively by installing a section of perforated pipe near the bottom or just perforating the pipe you are using and wrapping with filter cloth.

Advantages of the Collared "T" Design

Makes a firm and rigid connection to the existing pipe *Allows sediment deposition and sieving of debris *Can become a permanent enhancement to the drainage structure

Disadvantages of the Collared "T" Design

Installation requires additional study of the site and design and measurement as to where the top of the riser should be *May require special manufacture or modification of standard pipe configurations
May be difficult to clean if they become plugged because of the bend in the pipe at an elbow or "T". TIP, think about this when specifying the slot width and spacing in the grate openings.

COST (1992-97 costs)

Materials and Labor Cost
Equipment Backhoe $18.21/hr
Excavator $100/hr w/operator
Pickup Truck  $9.5/hr
Equipment Operator $54.29/hr
Laborer  $35.31/hr


Depth of fill or embankment
Condition of culvert inlet, (ie. bent or otherwise damaged)
Availability of materials, (on hand or purchase)
Skill pool availability, (force account or commercial source for fabrication)
Type of material, steel, aluminum, or plastic.( Avoid steel to aluminum connections)
Average installation costs, $1053/riser '92 cost guide


Refer to the Health and Safety Code for the following activities:

Welding and cutting torch operation
Operations of hand tools including shovels, digging bars
Equipment operation for backhoe, dozer, boom truck

This work often entails handling of heavy pipe and requires coordinated efforts sometimes of several laborers. Care needs to be exercised at all phases of the installation to protect from strains, cuts, falls, rolling materials, and working around heavy equipment. Supervision of laborers and tailgate safety meetings are imperative to prevent accident or injury