Harnessing the Storms: Massive Suburban Rainwater Sheetflow Control

(video coming soon!)

Many years ago, Liane and David bought a gorgeous property in North Scottsdale with an expansive landscape full of Sonoran Desert native trees and shrubs… and, they unintentionally inherited one of the largest residential watersheds in the Valley.

Their property receives over 234,000 ft² of catchment from the road north & east of it, and all the front yards / front roof catchments of those houses.

That’s a whopping 145,746 gallons of rainwater per inch of rain, and would take somewhere around a 1-acre basin to hold it all for infiltration.

A massive section of neighborhood flows directly into 3 inlet channels at the north end of Liane & David’s property.

Where most see liability, we see opportunity, and in 2024 we got to work designing & building a stronger, more optimized system that will sink millions of gallons of precious rainwater into their surrounding landscapes.

The backyard watershed

Taming the inflow

A watershed like this has to be managed as high up as possible — and since we could only work within the property limits, we had to start right at the 3 successive inlets built into the low block wall.

To mitigate the “carving out” effect of water crashing at high velocity into the soil underneath the inlets, we built large ramps — combining the largest river rock and rip-rap we had with the cinder blocks they had attempted to use prior to working with us. These structures create a landing pad for the heavy water to flow down, absorbing the impact and channel the rainwater intentionally the way we want it.

We are not trying to stop the water — we simply aim to slow it down and spread it out.

Slowing the mid-flow

At various intervals throughout each natural wash, we built check dams out of small boulders and large rip-rap to slow water down before it picks up too much speed, allow it to infiltrate while it’s pooling up, and — most importantly — let the excess slowly & steadily continue down its path.

Again the goal here is not to prevent water from flowing — flowing water will always be stronger than anything in its way — so instead we simply slow it down & spread it out in order to reduce its erosive impact on the landscape around it.

Releasing the end-flow

County regulations require that all watersheds enter & exit a property in the exact same places after we are done with our landscape improvements. This is to protect other nearby properties from unfortunate surprises.

Therefore, we created one final massive check dam at the end of Liane & David’s backyard system in an attempt to capture as much as possible within their property limits, and the rest continues to flow in exactly the same wash onto their west neighbor’s property (which we also improved as part of a subsequent project — more details later in this post).

The front yard watershed

This Scottsdale property’s front yard had a whole different challenge for us to solve. On large storms their culvert (the pipe underneath the driveway that is meant to convey water safely to the other side) would overfill due to debris clogging it — which then resulted in the storm water rushing across the top of the driveway and ripping it apart:

In the photo you can see where they attempted to hold water back with cinderblocks — but it is just way, way too much water.

Because they also plan to build an addition to the front of their house, they asked us to reroute the entire arroyo further away from the house. Again, county regulations require us to ensure the water leaves the property in the same place, so we swooped the arroyo back around towards the driveway in order to send it onward to the west in the same place.

We tore open the driveway installed a new 12” culvert pipe, and completely sealed off the old one rather than trying to dig it out and having to backfill with more dirt. Then we replaced the driveway quarter minus (decomposed granite), re-compacted it to avoid any unexpected settling, and hand-placed heavy rock on both sides to hold the dirt in place permanently without cement.

We took this opportunity to expand their front yard arroyo to hold even more rainwater in their land. Now, not only is it now longer, it is also wider and deeper in several places, effectively doubling its ability to infiltrate that precious rain water into the surrounding landscape.

Phase 2 fixes & plantings

The entire time we worked with Liane & David, we set clear expectations — we believed our first round of boulder check dams would be fairly successful, but would still require some “beefing up” after the first big daddy storm came through.

Sure enough, they reached out in 2025 to show us which parts had failed, and we got to work improving the system. The good news? Almost everything held up perfectly! There was just one area where the water managed to get up and around one of the check dams.

We brought in some additional large rip-rap and almost doubled that check dam in size, and we’re feeling even more confident now — but we will continue to monitor and update it as needed to ensure their investment is not wasted.

We also took this opportunity to plant several Sonoran Desert-native and arid-adapted shade trees throughout their backyard’s only remaining open area to establish a cooler, more humid microclimate. Two desert willows, two southern live oaks, and one desert fern will now grow in to fill that space and offer wildlife additional habitat!

Thanks for reading along. If you’d like to do something similar in your yard, schedule a time at the bottom of our website or DM us on social media @permascaping.