The detention basin decides how much of the parcel you can build on. On an industrial site with large impervious areas, such as roofs, truck courts and trailer parking, the volume of runoff that must be held and released slowly is substantial, and the basin, its outlet works and its access consume acreage that the owner would otherwise use for buildings or yard. Settling the drainage concept early is one of the largest levers on site yield.
Our scope covers the detention and water quality facilities and the conveyance that feeds them: storm sewer, inlets, culverts, swales, outlet structures, spillways and erosion protection. We build to the drainage report and the approved plans, and we coordinate closely with the civil engineer because small grade changes in the field can alter storage volume or flow paths.
We also build the temporary stormwater measures that keep the project legal while it is under construction. Sediment basins, perimeter controls, stabilized entrances and inlet protection are installed before earthwork, inspected on a routine, and repaired after storms. An inspector who finds failed controls can stop work, so we treat them as production items.
Floods in the record
Colorado's flood history is a design input. In the September 2013 floods Boulder County recorded 9.08 inches of rain on September 12 and up to 18 inches by September 15, Boulder Creek flow exceeded 5,000 cubic feet per second against a normal range of about 150 to 200, and buildings along the creek were seriously damaged. In Erie, major floods hit in 1890, 1921 and 1972, and a dike was built around nearby Coal Creek after the 1972 flood.
Mountain-edge communities reacted with stricter standards. Estes Park now has a required freeboard standard of one foot, and a post-2013 flood report noted that every building with structural damage, including one that was destroyed, was outside the mapped floodplain. We take two lessons from that record. Mapped floodplains are a minimum, and the grade around a building should direct water away from it even when a map says the parcel is dry.
Criteria, floodplain and permitting
Detention and drainage design follows local criteria, and release rates, storage volumes and water quality requirements vary by jurisdiction. Commerce City, for example, adopts a flood hazard map based on a FEMA study dated March 5, 2007 as part of its construction code. In unincorporated Weld County around Platteville, floodplain review is required before construction in designated flood hazard areas. Each of these steps can add weeks to a schedule if it is discovered late.
We read the drainage report before bidding, compare it to the grading plan, and flag mismatches. We also confirm the maintenance responsibility and access easements the jurisdiction will require, because a basin that cannot be reached by a maintenance vehicle will draw a review comment. Final as-built surveys of the basin volume and outlet elevations are prepared for acceptance.
What we build
We construct dry detention basins with forebays and low-flow channels, extended detention basins for water quality, underground vaults where land is too valuable for a surface basin, bioswales and rain gardens when required, and the outlet structures and emergency spillways that control release. Storm sewer is installed with the grades and structures shown, pressure tested or inspected as required, and backfilled in controlled lifts.
Construction sequence matters as much as the details. We build the downstream outlet and the basin first, so that runoff from the disturbed site has somewhere to go on day one. Storm sewer is then laid from the low end up, which keeps trenches from collecting water and lets us test each run as it is completed. Finally the inlets are brought to final elevation after paving subgrade is set, so that frames and grates sit flush with the pavement and are not disturbed by the paving equipment. The surrounding paving drains to the system. The stabilized parking lots and truck court crews set inlets and cross slopes to the drainage plan, and we check subgrade elevations before base is placed. Trailer parking and fueling areas may require oil and grit separation or containment, and we build those to the plans as well.
Building to the as-designed volume
The volume of a detention basin is a number the jurisdiction checks. We stake the basin from the grading plan, excavate to subgrade, survey bottom elevations, and verify the stage-storage relationship against the design. If compaction or topsoil placement changes the volume, we report it and propose a correction before the final surveys.
Outlet structures are cast with tolerances that matter. An orifice plate set a few inches too high changes the release rate. We verify elevations at each pour, set reinforcement per the details and test the outlet before it is covered. Pipe joints and manholes are tested, and we photograph and log each structure so that the record is complete.
Drainage and the rest of the site
Drainage touches other work. Site clearing and mass grading creates the grades the system depends on, and truck courts and trailer yards depend on correct slopes to avoid ponding and ice. Owners in food and beverage and cold storage also manage process wash-down water, which may need separate treatment or sanitary discharge.
Buildings that handle many trucks, such as a truck terminal and cross-dock, benefit from a detailed drainage plan for the yard because water that stands on concrete near the dock face freezes and creates a hazard. We design joints and slopes to shed water and we plan snow storage areas that do not drain over walkways.
Bring the drainage report
Send the drainage report and grading plan for your parcel. We will review them and tell you what the basin and conveyance will take to build.
Common Questions
How much land does detention take?
It depends on the impervious area, the local release rate and the soil. On many industrial sites detention takes a meaningful share of the parcel, so we evaluate underground storage and shared regional facilities during preconstruction when land is tight.
Can detention be located in the truck court or parking area?
Sometimes, as shallow ponding areas or underground vaults, if the jurisdiction allows it. Those options cost more to build and maintain, so we compare them with a surface basin.
Who maintains the basin after turnover?
Usually the owner, under a maintenance agreement with the jurisdiction. We provide the as-built drawings, an inspection checklist and a plan for sediment removal.
Does a site outside the mapped floodplain still need flood protection?
It can. Mapped floodplains are not a guarantee, and local rules may require freeboard above base flood elevation. We set finished floor and pad elevations with the engineer to meet the requirement and add a margin where drainage paths are uncertain.
What controls are required during construction?
Sediment controls, stabilized entrances, inlet protection, concrete washout and stockpile management, plus inspections after storms. We maintain a written log of inspections and repairs.