Archive for May 2023

Be a beaver

May 13, 2023

I recently read the book Water Always Wins by Erica Gies. A major theme is that to best manage water resources as they flow through our environment, in particular to promote and enhance sustainable water, we need to be imparting a “Slow Water” regime. We need to use management methods that blunt the rushing off of runoff and instead install means and methods that slow the flow, causing it to spread so that more of the water soaks in instead of running off. By these means, Gies argues, we can retain, and restore, the hydrologic integrity of sites, and by so treating a multiplicity of sites, of the watershed, so it would deliver more water supply.

A chapter in the book is about beaver, the undisputed champion in the animal kingdom at modifying and improving the hydrology of watersheds. The instinctive dam-building activities of beaver indeed create a Slow Water regime. They dam the flow, causing water to spread across the floodplain, filling up the “sponge” that landform creates, so storing much more water in the watershed than would occur otherwise. This slowing and retention of the water decreases downstream flooding, as more rainfall is detained/retained rather than zooming downstream. A good bit of this retained water would over time augment downstream baseflow, so improving the riparian ecology. The dams also retain sediment and so improve downstream water quality. All of this also stores carbon, and so helps to blunt climate change.

It struck me while reading that chapter that being like a beaver and creating a Slow Water regime is essentially what we did in the Villa Court project. The 3505 Villa Court project lies between Garden Villa and South 5th Street, in the Bouldin Creek watershed in South Austin. It’s a 13-unit townhome project that was installed on 1.43 acres of formerly “vacant” land. The final layout of the project imparted 67% impervious cover of the land.

In 2010, I was approached by PSW, the developer of many townhome projects in Austin, to help them obtain the water quality permit they would need to execute this development. They related that a senior level person in the City of Austin Watershed Protection Department had told them, “You need a retention-irrigation system.” Won’t bore you with the details of that method, except to say it would have entailed encumbering the back yards of all the townhomes along the downslope edge of the property with tanks to store the runoff, and taken up just about every square inch of greenspace in the development plan to “irrigate” the water gathered into those tanks. I looked at that for, oh, about 2 or 3 nanoseconds, then said, “This is insane.”

I told them, we could provide the water quality treatment for this project with rain gardens, and we could do that without needing any variances. This surprised them, and the folks at Watershed Protection too, as no one had run this idea at them before that. But I showed them that the rules did indeed support the rain garden scheme. Besides meeting the formal water quality management requirements, the rain gardens would capture and insoak the “excess” runoff created by development and keep about as much water on the land as had been soaked up by the land in its “natural” state. This despite a rather high percentage of the land having impervious cover. So we proceeded to be like a beaver.

“Rain garden”, as a formal water quality device, is a term often used for a full infiltration bioretention bed. This device is a vegetated bed of a specialized soil mix that intercepts runoff from its drainage area. Under Austin rules, the volume of water to be captured is termed the “water quality volume”. This is the volume of runoff from the drainage area generated by the “water quality depth” of runoff from the area. The water quality depth required to be captured under Austin rules increases with the percentage of impervious cover over the drainage area tributary to each rain garden. The idea is that the more impervious cover, the more the balance would be shifted from infiltration to runoff of rainfall, so a larger volume of water would need to be captured in order blunt that shift.

The captured water volume would be stored in the bed until it is infiltrated into the soil under and surrounding the rain garden. This process both intercepts pollution entrained in the runoff – “treating” it as it flows down through the biofiltration bed root zone – and retains the water quality volume on site, rather than allowing it to run directly off, so helping to control and mitigate downstream flooding and channel erosion.

The 3505 Villa Court water quality management plan entailed several rain gardens, distributed around the site. Five of them intercepted runoff from rooftops, pavement and green spaces. Another seven captured runoff from a rooftop that could not be routed to one of the other five rain gardens, so that these areas of impervious cover would be “treated”. The project layout is shown in the figure below.

This shows how indeed we intercepted, spread and infiltrated runoff flows. As noted, this restores to some degree the rainfall-runoff response of the “natural” site, as it blunts the increase in runoff immediately leaving the site that installing impervious cover on the site would otherwise impart. This indeed mimics to an extent the impacts that beaver have on a stream, only this is applied in the uplands rather than in a streambed, intercepting and insoaking the increased runoff on its way to a stream. But the ultimate impact is largely the same, damming up the flows, holding – storing – water on the land. And reaping the benefits of that.

Some of this water would infiltrate and ultimately migrate to a stream, to impart baseflow, just as water seeping out of the floodplain “sponge” that beaver create would impart baseflow downstream. As noted, the rain gardens also intercept and store sediment entrained in runoff, just as beaver ponds do, and also intercept and “treat” pollutants that developing land causes to be entrained in runoff. Then too, there would be carbon sequestration in the rain garden beds. So in imparting the water quality management scheme at Villa Court, we were being like a beaver, mimicking the ecological wisdom they evolved to deliver to the landscape.

Now of course the retention-irrigation scheme, entailing interception of runoff and subsequently spraying it over landscape to largely infiltrate the water that would have otherwise run off, would have also helped to maintain the hydrologic integrity of the site. But that would have been acting like a human, using a failure-prone mechanical system, requiring assured power supply, to redistribute the water over the site. Noting that little actual irrigation benefit would be obtained by spraying the captured runoff over the green spaces, because the storage tanks have to be evacuated only a couple days after the rain fell, which had just “irrigated” those spaces. Any sediment removal would have to be accomplished by “actively” cleaning the storage tanks, into which the “raw” runoff would flow. Rather than incorporating the sediment into the plant-soil system on the site just as a matter of course, this would create a “waste” stream that would have to be “disposed of”. Then too, the shallow-rooted turf that would be “irrigated” would not sequester anywhere near the level of carbon that the deep, more biologically complex rain garden root zone would.

All this illustrates we would be well served to be like a beaver in the way we design sites for water quality management, in the manner we did at Villa Court. Damming up water flows to create a Slow Water regime at the site level, holding up and spreading out the flows, infiltrating on the site “excess” runoff imparted by development, rather than it rushing away, so retaining/restoring the hydrologic integrity of the site. And by a multiplicity of such sites in a watershed, we can retain/restore the hydrologic integrity of the watershed. It could fairly be stated that if every site in the Bouldin Creek watershed had been treated like Villa Court was, we would still have baseflow in Bouldin Creek. And that would enhance the ecology of the whole watershed.

Almost like re-introducing beaver to the watershed would. So when considering how to manage storm water as a site is being developed, be a beaver.