Commercial Site Floods After Storms: Who Fixes the Drainage?

The short answer

A commercial site that floods after storms is diagnosed by a civil engineer whose practice includes stormwater and drainage design, and repaired by a site or drainage contractor working from that engineer's plans. The engineer finds out why the water is where it is and designs the fix; the contractor builds it; the local drainage authority or water management district reviews and permits the change where a permit is required. Calling the contractor first is the common mistake, because a contractor can only build what someone has designed.

Why sites flood

Most commercial drainage failures come down to one of a handful of causes, and the assessment is there to tell them apart:

• The system was designed for less water than it now receives. Impervious area has grown — a paved yard, an addition, a neighbor's development upstream — and the pipes, inlets and pond that were adequate at design are not any more.
• The system is blocked or has failed. Sediment in the pond, a collapsed or root-bound pipe, a clogged inlet, an outfall that is silted or has been buried.
• The outlet is the problem, not the site. The receiving ditch, canal or storm sewer backs up in a storm, so the site's system cannot discharge no matter how well it is built. This is a tailwater problem and it shows up as flooding that arrives late and drains slowly.
• Grading has changed. Repaving, landscaping or settlement has created a low point with no path to an inlet, or reversed a slope that used to carry water away.
• The storm was larger than the design storm. Every system has a limit. If flooding happens only in rare, very large events, the question is whether the design standard should change, not whether the system has failed.

What a stormwater assessment involves

• Records and history. The original drainage plans and permits, the flooding history (when, where, how deep, how long), and any changes to the site or its surroundings since construction.
• A site inspection. Inlets, pipes, ponds, control structures and the outfall, usually with a camera inspection of the pipe network and a survey of key elevations.
• Hydrologic and hydraulic analysis. How much water reaches each part of the system in the design storm, and whether the system can convey and store it. This is modeled, because the problem is often not where the water is seen.
• A tailwater check. Whether the receiving system backs up, which changes the whole diagnosis.
• A findings-and-options report. The cause, the options (cleaning and repair, adding capacity, regrading, new storage, changing the outfall), a cost range for each, and what permits each option needs.

Who does what

Questions to ask before hiring anyone

• Does the firm do the analysis and design, or only construction? You need the design first.
• Will they model the system or only inspect it? Flooding that comes from capacity or tailwater cannot be diagnosed by inspection alone.
• Have they worked with the drainage authority that has jurisdiction over your site?
• Will they give you options with costs, or one recommendation?
• Can they handle the permit and, if required, the recertification?

Where RSP Engineers fits

RSP Engineers provides the engineering side of this: storm drainage design, including hydrologic and hydraulic modeling, floodplain analysis, retention and detention design, exfiltration and infiltration systems, stormwater permitting and the inspection and recertification of existing systems; sanitary and storm sewer camera inspection for the pipe network; geotechnical engineering where soil permeability matters; and civil construction administration once the fix is being built. The construction itself is the contractor's work.

If your site floods, tell us where it is and what happens when it rains and we will come back to you about an assessment.