Stormwater and Drainage Construction in the Boulder Valley

Boulder Creek ran past 5,000 cubic feet per second in September 2013, against a normal range of about 150 to 200, and the buildings along it, Boulder High School among them, were seriously damaged. That is the number we keep in mind when a drainage plan lands on the table. The pipes and basins we build are sized for storms the owner hopes never to see, and they have to work on the day the storm arrives, not the day the engineer drew them.

We build storm sewer, inlets, culverts, swales, detention and water quality basins, outlet structures and the grading that ties them together. In this metro the work is rarely isolated. A detention pond is usually the lowest point of a parcel we are also mass grading, and the pipe network usually shares trenches and conflict points with every other utility on the site development package. We plan those pieces as one system so the water has a clear path from roof drain to outfall.

The people who ask us for this work are developers laying out a new pad, owners fixing a lot that holds water against a loading dock, and property managers responsible for sites where a clogged inlet becomes a tenant complaint within one storm.

Creek country and the drainage it demands

Water is the first design condition on many Boulder Valley sites. In Gunbarrel, flooding is described as the principal natural hazard, and the community maintains a public improvement district that can levy taxes and take on debt, a reminder that drainage in Gunbarrel is a shared, funded concern and not only a lot-by-lot matter. When we build there we expect to tie into existing outfalls that other owners also depend on, and we confirm what the system can accept before we commit to a discharge point.

Erie shows the same story on a different time scale. Major floods reached the Erie area in 1890, 1921 and 1972, and after the 1972 event a dike was built around nearby Coal Creek. Sites in Erie sit in that history, so we look hard at where flows spill out of the creek corridor and how a new building pad changes the path of overland water. A well graded lot can protect a neighbor or flood one, and the reviewer will ask.

Floodplain permits and stormwater review

Inside the City of Boulder, any building with part of its structure in the 100-year floodplain needs a Floodplain Development Permit, and the rule reaches roof overhangs, gutters, footings, decks and balconies. That detail matters for drainage contractors because outfall headwalls, grade changes and retaining elements near a creek can be pulled into the same review. We ask the floodplain administrator early which parts of the work count, and we put that review on the schedule next to the building permit.

In unincorporated Boulder County the floodplain rules in Section 4-400 of the Land Use Code win over conflicting building code provisions, and the building official can ask for a soils or geologic report for flash flooding before a permit. Beyond local review, disturbing an acre or more in Colorado ordinarily brings a state construction stormwater permit and a written plan for erosion and sediment control. We prepare the drainage submittals, install the temporary controls, and keep the inspection log so the file is complete when the inspector asks.

Building the system from the outfall back

We build from the downstream end. The outfall, the outlet structure and the receiving channel go in first, because every pipe upstream depends on their elevations being right. Surveying the invert of the connection point before we order structures saves the argument that happens when a precast manhole arrives two inches off. We then work upstream through the storm sewer, set inlets to the final paving grades, and leave the basin in a state where it can serve as a temporary sediment trap during the rest of the job.

Detention basins get the same attention to detail. The floor slope, the low flow channel, the micropool and the emergency spillway are all small earthwork items with large consequences if they are off, so we stake them tightly and check them again after compaction. Where the plans call for a water quality feature, we protect it from construction silt, because a basin that fills with mud during the building phase starts life as a maintenance problem.

What moves drainage cost

Pipe material, depth and the number of structures set the base price, but the site decides the rest. Shallow groundwater means dewatering, and in creek country that can be the largest unpriced item in a trench. Rock or cobble changes production. Cross-connections with existing lines add potholing and survey time. We list these as separate allowances with stated assumptions, and we recommend a few test pits near the deepest runs before bidding when the geotechnical report leaves doubt.

Schedule exposure is another driver. A drainage system built late forces an owner to run temporary pumps and berms through the wet months, while one built early can serve paving and landscaping crews as the site settles. We also account for the extra cost of working beside active tenants, where flagging, plating and restoration of existing paving add to the quantity. A clear scope and honest unit prices keep the final number close to the first one.

Questions to settle before the first trench

Three questions shape most drainage projects. Where does the water go, who owns the downstream path, and what does the jurisdiction expect to see at final inspection? An industrial warehouse with a large roof and truck court sheds far more runoff than the same acreage in landscape, and the outfall may not have been sized for it. We bring the civil engineer, the owner and the permitting staff to the same conversation so that surprises show up on paper first.

We also talk through maintenance. Inlet types, access for jet trucks, sediment forebays and trash racks decide how much attention the system needs after we leave. Owners who plan for that at the start save money every spring. We deliver an as-built survey and a short operating note with each system so a facility manager knows what to inspect after the first heavy rain.

Send us the drainage plan

Share the civil set and the geotechnical report and we will review outfall points, crossings and the permit path with you before the schedule is set.

Questions Owners Ask

Do we need a floodplain permit for work near a creek?

Inside the City of Boulder, a Floodplain Development Permit is required for any building with part of its structure in the 100-year floodplain, including overhangs, gutters and footings. Drainage structures near the creek can be reviewed under the same process. County land follows its own floodplain rules in Section 4-400 of the Land Use Code. We confirm the mapped limits with the local floodplain staff before pricing.

How big does a detention basin need to be?

The civil engineer sizes it from the contributing area, the percentage of hard surface and the release rate the jurisdiction allows. We do not size basins, but we do check that the constructed volume matches the design by surveying the floor and side slopes. A basin that is even a little shallow loses storage the permit counted on.

Can drainage work continue through winter?

Usually, with adjustments. Frozen soil cannot be compacted to specification and concrete structures need cold weather protection, so we sequence pipe laying and structure setting around the coldest weeks and keep trenches short so backfill is placed before temperatures drop overnight. Chinook warm-ups can reopen working windows, and we watch the forecast.

What happens if groundwater shows up in the trench?

We dewater with sumps or well points, route the discharge through filtration so we meet the stormwater permit, and tell you immediately. Because we list dewatering as an allowance with a stated assumption, the change is a known line item and not a dispute. The geotechnical report usually gives a good hint, and we check it against the borings before we price.

Who is responsible for erosion control during construction?

We install and maintain the temporary controls on our scope, inspect them on the schedule the permit requires and keep the log. If other trades disturb the site, we coordinate so one party owns each control. A written plan with named responsibilities avoids the common situation where everyone assumes someone else will fix the silt fence.

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