Published
A new road’s drainage: a 10-year 6-hour storm, 24-inch ditches, and culvert floors nobody argues their way past.
A road is a drainage decision before it is a driving surface
Appendix B’s drainage section opens with the sentence that shapes everything after it: every drainage facility for new roadway construction must be designed by an Idaho registered professional engineer and approved by the Public Works Director before construction starts. And it names the storm the whole system is sized for:
“All drainage facilities for new roadway construction shall be designed by an Idaho registered professional engineer and approved by the Fremont County Public Works Director in conjunction with the roadway plans, prior to commencing construction. The design storm return period shall be a ten (10) year, 6-hour storm and meet the winter and spring runoff needs.”
— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), Appendix B, section 1.13.010. Consolidated through Ordinance 2026-03. Retrieved 7 September 2026
A ten-year storm, delivered over six hours, plus spring melt. That is the event the ditches and pipes under a county road have to survive, and the “winter and spring runoff needs” clause is the part that matters at this elevation — a system that passes a summer cloudburst and fails in May has still failed the standard.
Ditches: two feet deep, and reseeded
The roadside ditch has two written requirements:
- All roadside drainage ditches must be a minimum depth of 24 inches, measured from the edge of the road surface shoulder.
- Ditches must be reseeded after construction with a seed mixture appropriate for the area, as recommended by the local Natural Resources Conservation Service office.
The reseeding clause is not decoration. An unprotected ditch guts itself in one runoff season; a seeded one holds. It is also a line item that vanishes from low bids, and the NRCS office is the county’s named source for what to plant.
The culvert rules, floor by floor
The section sets minimums by what the culvert is under:
| Where the culvert sits | The written minimum |
|---|---|
| Across the roadway | 18 inches diameter, or the size needed to convey the design volume — whichever is greater |
| Under an approach road or driveway | 12 inches diameter (or the size necessary) and a minimum length of 30 feet |
Verbatim, because the pairing is the point:
“Culverts across the roadways shall be a minimum of eighteen inches (18") diameter or the size necessary to convey the design volume of water, whichever is greater. Culverts under approach roads or driveways shall have a minimum diameter of 12" (or the size necessary), a minimum length of thirty feet (30'), and shall meet the requirements of 1.13.030.”
— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), Appendix B, section 1.13.040. Consolidated through Ordinance 2026-03. Retrieved 7 September 2026
Note how the two floors relate to the county’s access standard: that document sets its own, higher floor for culverts required at an access onto a county road. The code does not spell out which floor controls when both apply — that is a question for Public Works, and it is worth asking before pipe is bought. The question of whether a culvert is required at all is still answered inside the permit, not at the gravel counter.
A third floor appears in the code’s off-street parking standards, of all places, in a clause about access driveways that cross a ditch:
- Where required for drainage, access driveways shall be constructed over a minimum 12 inch culvert capable of supporting a load of 40,000 pounds.
That 40,000-pound rating is the figure no other page in this county prints, and it is the reason a culvert is not a length of whatever pipe the supplier has: it is a structural element under a loaded truck. Two sections, two different tests — diameter for water, rating for weight — and a pipe has to pass both.
The materials table, and the cover over the pipe
Section 1.13.030 limits culvert materials to corrugated steel, aluminum, or concrete — anything else needs Public Works approval — and sets thickness and cover by a short table:
| Diameter | Steel thickness | Aluminum thickness | Concrete class | Cover required |
|---|---|---|---|---|
| 12” through 36” | 0.064” | 0.060” | V | 12” minimum |
The footnotes carry real conditions:
- Other classes of concrete pipe may be used with proper cover per the manufacturer’s recommendations.
- The steel figure assumes corrugated metal pipe with 2 2/3" x 1/2" corrugations.
- Anything larger than 36 inches, any pipe-arch, or any culvert under extreme fill requires special consideration for cover and other parameters.
The 12 inches of cover over the top of the pipe is also why driveway gravel depths and culvert placement get argued together — the pipe, its bedding, and the fill above it are one build.
Three rules people learn the expensive way
- Road ditches are not irrigation ditches. Disrupting natural drainage ditches to run the roadway ditch as their substitute is prohibited, and highway drainage ditches shall not be used for conveying irrigation water of any type. New irrigation facilities are built and maintained outside the public right-of-way.
- Drainage easements get recorded. Easements for drainage structures and for moving storm water across private property are shown on the plans or final plat and recorded with the county recorder — or, where there is no plat, by a recorded letter from the applicant describing the areas.
- Curbs move the problem, and the bill. Where a curb-and-gutter section is proposed, a complete storm sewer system must be designed and built — and its disposal and maintenance are the applicant’s or a homeowner’s association’s responsibility, not the county’s.
The irrigation prohibition bites hard in a county full of ditches and water rights, and it is stated twice over: no hijacking natural drainage, and no irrigation water in the road’s system. Runoff that leaves your property is governed by its own erosion-control line of rules once ground is disturbed.
Steep ground: the culvert-spacing rule
Where a road runs a centerline grade greater than 6% through soils rated moderate to very severe for erosion in the National Cooperative Soil Survey, the county requires either a hydraulic analysis of erosion potential in the ditches, or additional culvert crossings in the range of one every 300 to 500 feet to shorten the distance overland water travels in the ditch. On climbing roads through erosive ground, that spacing rule quietly adds pipe, headwalls, and grade control to a bid — a cousin of the slope thresholds that decide where building is allowed at all.
Checking a drainage scope against the code
- Confirm the design storm is stated in the plans: a ten-year, 6-hour event, meeting winter and spring runoff needs, engineered and approved before construction.
- Measure ditch depth from the edge of the shoulder — 24 inches is the floor — and put NRCS-recommended reseeding in the contract.
- Match each culvert to its floor: 18 inches under the roadway, 12 inches and 30 feet under approaches, and the stricter access-standard figure where it applies.
- Buy pipe to the materials table — steel 0.064, aluminum 0.060, concrete class V — and bury it with 12 inches of cover minimum.
- Verify the load rating where a driveway crosses a ditch: 40,000 pounds is the written test.
- Keep irrigation water out of the road system entirely, and record every drainage easement the water crosses.
- On grades over 6% in erosive soils, expect either a hydraulic analysis or culverts every 300 to 500 feet.
Drainage is where road bids go wrong twice: once in the pipe count, and once in the water law. The code’s own answer to both is to write the numbers down — which is exactly what the cross-section standard does for gravel, and this section does for water.
Common questions
What storm must a new county road's drainage be designed for?
A ten-year, 6-hour storm, and the system must also meet winter and spring runoff needs. The design must be prepared by an Idaho registered professional engineer and approved by the Public Works Director before construction begins.
How deep must roadside drainage ditches be?
A minimum of 24 inches, measured from the edge of the road surface shoulder. Ditches must also be reseeded after construction with a mixture recommended by the local NRCS office.
What is the minimum culvert size under a driveway approach?
Appendix B sets a minimum diameter of 12 inches, or the size necessary to convey the water, and a minimum length of 30 feet. The county's separate access standard sets a higher floor for culverts at accesses onto county roads, and Public Works settles which applies.
What is the minimum culvert size across a roadway?
Eighteen inches in diameter, or the size necessary to convey the design volume of water, whichever is greater.
What load must a driveway culvert be able to support?
The code's off-street parking standards require that where a culvert is needed for drainage under an access driveway, it be a minimum 12 inch culvert capable of supporting a load of 40,000 pounds.
What materials can a culvert be made of?
Corrugated steel, aluminum, or concrete. For diameters from 12 through 36 inches: steel at 0.064 inches thickness, aluminum at 0.060 inches, concrete class V, with 12 inches of cover minimum over the top of the pipe. Other materials need Public Works approval.
Can irrigation water run in a road's drainage ditch?
No. Highway drainage ditches shall not be used for conveying irrigation water of any type, and new irrigation facilities must be constructed and maintained outside the public road right-of-way or private road easement.
What happens on steep roads with erosive soils?
Where the centerline grade exceeds 6% and soils rate moderate to very severe for erosion in the National Cooperative Soil Survey, the county requires a hydraulic analysis of ditch erosion potential, or additional culvert crossings roughly every 300 to 500 feet.
Who maintains a storm sewer under a curb-and-gutter road?
The applicant or a homeowner's association, not Fremont County. Where curb and gutter is proposed, a complete engineered storm sewer system must be designed and built, and its disposal and maintenance stay private.