Drainage and grading when the rain arrives all at once

People are surprised that drainage work is a real category of landscaping in a hot semi-arid city. Laredo averages 19.68 inches of precipitation a year, which is less than a third of what East Texas gets. Yet standing water in back yards, water against slabs, and eroded beds after a storm are all common complaints here. Three things explain it, and none of them appear in the annual rainfall figure.

Why a dry place has wet yards

The rain arrives in bursts. A semi-arid climate does not spread its rainfall evenly. Long dry stretches are broken by convective storms and, in late summer, by moisture from tropical systems tracking up from the Gulf. A large share of the yearly total can fall in a handful of events. A drainage system is not sized against an annual average, it is sized against the worst afternoon.

The ground below stops taking water. Soils across this area carry secondary calcium carbonate at shallow depth. The USDA set up the Brennan series for the Laredo area in 1906 and puts depth to secondary carbonate at 10 to 36 inches, over a fine sandy loam with a sandy clay loam argillic horizon. The Webb series carries carbonates below roughly 18 inches with sandy clay under a very fine sandy loam surface. Where a cemented or clay-rich layer sits within the first two or three feet, water passes quickly through the surface soil and then stops. It perches on the layer and either sits there or runs sideways along it, which is why a yard can be soft for days after a storm that lasted twenty minutes.

The lot was scraped flat. Modern subdivision lots are graded for construction access and then finished close to level. Add a slab, a driveway, a walk and a patio, and a large proportion of the lot no longer absorbs anything at all. The water that used to spread across open ground now arrives at whatever low point exists, in volume, in a few minutes.

Read the lot during the storm

There is no substitute for standing outside in the rain with a raincoat on. Everything a drainage design needs to know is visible in the ten minutes while water is moving and in the hour after it stops.

• Where does water enter the lot? Off a neighbour's slope, off the street, off the roof at a particular downspout.
• What path does it take across the ground? Follow the sheet flow, not the puddles.
• Where does it leave, and does it leave at all?
• Which low spots hold water, and how long do they hold it? A puddle gone in an hour is a nuisance. One still there the next morning is a design problem.
• Is water moving toward the house at any point, or sitting against the slab?
• Where is mulch or soil ending up? A fan of washed material across a walk shows exactly where the flow concentrates.

Photographs during the event and the following morning are worth more to a company pricing the work than any description. Attach them to the quote form and the site visit starts from a known picture rather than a guess.

Fall away from the slab comes first

Every other drainage measure is secondary to getting ground to fall away from the foundation. The convention builders work to is roughly six inches of fall in the first ten feet out from the wall. Over years, beds get topped up with mulch, soil settles, and edging traps material, until the grade next to the house is level or tipping back toward it. At that point the slab is the low point of the yard, which is the one thing a landscape must never allow.

Fixing it is usually soil work rather than pipe work: pulling material back off the wall, re-establishing the fall, resetting bed edges so they do not act as dams, and extending downspouts far enough out that roof water lands beyond the backfill zone. That last item is the cheapest meaningful drainage improvement available on most Laredo houses. A downspout discharging at the corner of a slab is putting a large share of the roof area into one spot on the ground.

Across the open yard, a working figure for lawn and bed areas is around one to two percent fall, which is one to two feet of drop over a hundred feet. It is barely visible and it is enough to keep water moving.

Swales, or pipes

A swale is a shallow, wide, planted depression that carries water across the lot to somewhere it can leave or soak in. It is cheap, it never clogs, it can be mown or planted, and on a lot with any usable fall it is usually the better answer. A well-built swale looks like part of the yard rather than infrastructure, and a bed of native grasses and cenizo sitting in one hides it entirely.

A pipe, typically a perforated line in gravel taking water from surface inlets or a channel drain, is what you use when there is no room for a swale, when the water has to cross under a driveway or a patio, or when the flow is concentrated rather than spread. Pipes need fall to work, they need a place to discharge, and they need cleanouts because they will eventually collect silt and roots. A pipe with no outlet is a very expensive way to store water underground.

Most lots here end up with a combination: surface grading to move sheet flow, a swale along the length of the lot where there is space, and a short piped run where something has to be crossed. On caliche, the trenching for that piped run is the line item that swings the price, because ground that a trencher walks through at one end of a yard can need a rock saw twenty feet away.

Let some of it soak in

Every square foot of impermeable surface adds to the volume that has to go somewhere. Permeable choices reduce the problem instead of relocating it. Decomposed granite paths, gravel courts, permeable pavers on an open-graded base, and simply keeping planted beds rather than paving them all take a share of the rainfall out of the runoff.

There is a limit here that people from wetter regions get wrong. Where the cemented layer sits close to the surface, an infiltration feature only works if the water has somewhere to go once it reaches that layer. A gravel pit dug down onto caliche fills and stays full. So permeable surfacing is a good idea on this ground, and it is not a substitute for getting the surface grade right.

The order the work has to run in

Drainage is first, before anything else is built, and the reason is money. Every stage after it gets torn up if the grade changes.

1. Survey the fall and confirm where water can legally and physically discharge.
2. Correct the grade around the slab and across the open yard.
3. Cut swales and lay any pipe, including sleeves under anything that will later be paved.
4. Build hardscape onto the corrected grade.
5. Prepare soil and plant.
6. Mulch, chosen coarse enough that it does not float away in the first storm.

One legal point worth knowing early: what you do with water at the property line matters. Concentrating flow and discharging it onto a neighbour is the kind of change that causes disputes, and where a wall, a structure or an alteration to drainage is involved, the city may have a permit requirement. The contractor doing the work is the right person to settle that with the city before anything is cut. Design and installation work normally opens with this stage, and the main Laredo guide sets it in the wider sequence.

Short version

• The annual rainfall total tells you nothing; the worst afternoon does.
• Water perches on the cemented layer, so it lingers on ground that looks dry.
• Watch the yard during a storm and photograph it the next morning.
• Ground must fall away from the slab, roughly six inches in the first ten feet.
• Extend the downspouts before you price anything else.
• Swales where there is room, pipe where there is not, and a cleanout on every run.
• Grade first, hardscape second, planting last.