Stormwater Detention for Tilt Wall Sites in Texas

The detention pond is the first thing we dig and the last thing most owners ask about. On a tilt wall site it does three jobs at once. It holds back storm runoff so the neighbors downstream stay dry, it supplies the fill that raises the building pad, and it gives the crew somewhere to send water while panels are being cast. Get the order wrong and a casting bed turns into a wading pool the week before lift day.

Texas rain does not behave. A site can sit dry for six weeks and then take a year's worth of water in a weekend, and a flat slab-on-grade building with a large roof sheds all of it fast. We build detention as part of the civil package rather than as a leftover, with the grading, the storm pipe, the outlet and the pad all planned from one set of elevations.

Below we walk through how we sequence pond work against the pour-cast-lift schedule, what drives the cost, and what the city or county will want to see before they sign off.

Flood history shapes how reviewers think. After heavy rain following a hurricane in October 2018, four floodgates at Mansfield Dam were opened and Austin issued a boil-water advisory from October 22 to 29, 2018. In San Angelo, the National Weather Service records 11.75 inches of rain on September 15, 1936 during the Concho River flood, with the South Concho peaking at 111,000 cubic feet per second and about 300 houses destroyed. Nobody designs a warehouse pond for a storm like that, but plan reviewers carry those numbers in their heads, and they ask hard questions about outfalls.

On the Gulf side the problem is different. Cypress sits inside the Cypress Creek and Little Cypress Creek watershed, which means a site there drains into a creek system that a lot of other owners also drain into. Detention on those sites is not a nicety, it is the price of being allowed to add roof and truck court to a watershed that already runs full. We read the drainage criteria for the specific city, county or district before we price a pond, because the release rates, storm events and freeboard rules differ from one jurisdiction to the next.

Our scope starts at the drainage report and runs through the final stabilization of the pond slopes. That means mass excavation, shaping the basin to the designed volume, building the outlet structure with its low-flow orifice and overflow weir, setting the storm pipe that feeds the basin from the truck court and roof leaders, and installing the emergency spillway. We also place the erosion controls, the construction entrance and the inlet protection that keep silt out of the system during the build.

Where land is tight, we price underground storage as an alternative. A large-diameter pipe system or a vault under the trailer parking lot trades a lower land requirement for a higher cost per cubic foot, and it needs a hard look at the water table and the loads from loaded trailers. We lay out both options early so the developer can see what the land is worth against the storage cost.

Drainage is reviewed locally. The city or county engineer approves the drainage study and the grading plan, and in some areas a special district or flood control agency has its own say on the outfall. A site that disturbs enough ground also needs coverage under the state construction general permit and a pollution prevention plan, and the inspections that go with them run weekly and after storms. We keep the inspection log on site and current, because a missing entry can stop a pour.

Outfall easements and downstream capacity are where schedules slip. If the receiving channel or storm sewer is already at capacity, the review comes back asking for a lower release rate and a bigger pond, which is a redesign. We ask for a pre-application meeting before the engineer finishes the report, so the target release rate is known before the pond is sized.

Pour then lift then roof only works if the site is dry at each step. We excavate the pond first, because that dirt becomes the pad fill and the base for the truck court. We also grade the pad to shed water to the pond with a positive slope, put the outlet in early so the basin drains rather than stores, and keep a pump on standby until the permanent outfall is live. Casting beds cannot sit in standing water; a wet bed ruins the bond breaker and leaves a rough face on the panel.

Pond slopes are the next trap. Steep, unstabilized slopes wash out in the first heavy rain, silt runs to the outlet and the inspector shuts the job down. We seed or blanket the slopes as soon as they are shaped, and we line the low-flow channel so the first storm does not cut a gully through the pond floor. Crane mats and the lifting route have to stay clear of the pond edge, so we lay out the crane path before the pond is dug and keep the outer berm well away from it.

Haul distance and soil quality drive most of it. A pond that makes more fill than the pad needs forces us to export dirt, and a pond in poor clay forces us to import select fill or treat the subgrade. The depth of the basin matters too, since a deep pond in a high water table needs dewatering, and rock or caliche can turn a simple dig into a blasting or ripping job. We quote with soil borings in hand and put allowances in writing so the owner is not surprised later.

The outlet structure and the pipe are the other big lines. Cast-in-place outlet boxes cost more than precast, and long runs of large reinforced pipe add up quickly. We bid alternates where the engineer will accept them. For related earth and utility work, see our site grading and building pads and site utilities and fire lines scopes, which are usually priced with the pond.

Industrial developers carrying a speculative shell need a pond that satisfies the city and leaves the pad ready for a future tenant, and industrial developers and REITs usually want the lowest cost that still passes review. Logistics owners care about trailer parking and truck-court drainage, so we coordinate with the truck courts and dock aprons design so the pavement and the pond work as one system.

If your site is in a floodplain or next to a creek, bring us in before the land closes. We can walk the site, read the drainage and floodplain maps and give you a realistic view of how much of the acreage goes to storage.

Send us the survey, the geotechnical report and your target building size. We will tell you what the drainage package should look like, what it costs, and where it can slip the schedule. See also our preconstruction and estimating work for a full budget before you commit.

How big does a detention pond need to be for a tilt wall warehouse?

It depends on the impervious area and the local release-rate criteria. A large roof and a paved truck court create a lot of runoff, so ponds on distribution sites often take a meaningful share of the acreage. The engineer sizes it from the drainage study, and we price it from the volume and the soils.

Can we use the pond as a borrow pit for the building pad?

Yes, and we usually do. The excavated clay or select material becomes the fill under the slab and the truck court if it meets the geotechnical requirements. If it does not, we stabilize it or bring in other fill, and we tell you the cost difference before we dig.

What happens if it rains during construction?

The construction drainage plan matters more than the permanent one. We keep silt fence, inlet protection and a pump ready, and we grade so water leaves the casting area. After a storm we inspect, clean out sediment and log it, because the permit requires it.

Do we need an engineer or can you design it?

The site civil engineer of record designs the pond and signs the drainage report. We build to those drawings and can offer pricing advice early, which is the main reason owners bring us in during design rather than after bidding.

Is underground detention worth considering?

On tight infill sites it can be, because it frees land for building or parking. It costs more per cubic foot and needs good access for maintenance, so we price it against a surface pond and let the land value decide.