Important Elements in High-Performing, Arid-Adapted GSI
By Tess Houle, Landscape Designer, MRWM Landscape Architects
The contemporary practice of GSI was developed in cities like Portland and New York to decrease the volume of stormwater entering the sewer system. Therefore, most GSI practices were designed for use in wet climates. While many aspects of these temperate GSI practices work well in arid climates, there are some important differences to consider.
Use less asphalt! Cities in the Southwest tend to use too much asphalt. By simply reducing the amount of impermeable surfaces, problems of stormwater quantity and quality can be reduced. Narrower roads and angled parking stalls in parking lots decrease the amount of paving required, and porous paving materials such as permeable pavers, can be used in a variety of situations.
Highland Park Parking Plaza (designed by MRWM 2013) features permeable pavers, flush curbs and crusher fines for an elegant and stormwater-friendly alternative to asphalt.
Pay attention to sediment! The erodible soils of arid climates require special attention to sediment transport and accumulation. Soils must be stabilized with plant roots, mulches, and tackifiers and erosion blankets where needed. GSI features in arid climates will accumulate more sediment than those in wet climates, and so sediment traps are critical. There must also be routine maintenance to clean out and properly dispose of sediment (which will contain pollutants such as polychlorinated biphenyls that attach to sediment). Practices that are prone to failure due to sediment should be used sparingly. For example, an infiltration trench/French drain may work well in Oregon, but in NM, it will soon fill with sediment and require replacement.
Sediment clogs the inlet to a recently constructed basin, preventing the first flush of stormwater from entering the basin.
Disperse stormwater flows! The combination of excessive asphalt coverage, low vegetative cover, hydrophobic soils, and intense storms causes high runoff volumes and velocities. When these volumes and velocities are concentrated at a curb inlet or slope drain, a GSI feature can easily ‘blow-out.’ To avoid concentrated flows and velocities of stormwater that cause erosion, flush curbs should be used instead of curb inlets wherever possible.
Flush curbs in the parking lot at CNM’s Smith Brasher Hall (designed by MRWM 2015) minimize erosion by allowing water to sheet flow into a stormwater tree trench.
If curb inlets are needed, increasing the width and number of curb inlets reduces the volume of stormwater at any one inlet, while also making it easier for stormwater to flow into a basin.
Include plants! Perhaps because vegetation density in arid climates is less than in wet climates, there is a common misconception that GSI features in arid places do not need vegetation. Although it is possible to construct GSI practices without plants, plant-less GSI structures do not offer the wide-ranging benefits and lasting performance that structures with plants offer. For example, while permeable pavement has been shown to filter sediment and heavy metals, it will never process hydrocarbons, fertilizers, or bacteria the way that a bioinfiltration basin does. Only GSI practices with plants provide shade, habitat, and aesthetic value.
A basin without plants, in addition to quickly becoming stained and unattractive, does not provide community benefits such as pollutant treatment, shade, habitat, or aesthetic improvement.
Also, plant roots are critical in maintaining and improving infiltration rates in GSI features. A concrete basin will always have the same performance, but over time a planted and well-maintained bioinfiltration basin will have improved infiltration. Of course, plant selection is very different in arid climates. A grass-lined bioswale, which has high long-term infiltration rates, might be a beneficial practice in Chicago, but would require too much irrigation in New Mexico where water conservation is a higher priority. Plants should be selected that can survive on stormwater alone (after an establishment period for irrigation), while providing shade, pollutant treatment, carbon sequestration, beauty, and habitat. In some situations, this may mean relying on large shrubs instead of trees.
A bioinfiltration basin at Smith Brasher Hall contains trees, shrubs, grasses, and forbs that treat stormwater while providing a beautiful landscape.
Support healthy soils! Unlike soils in temperate climates, arid soils have very little organic content and repel water when dry (hydrophobic). The structure and water-holding capacity of arid soils is further decreased by construction practices that compact and disturb soil. For these reasons, it is important to support the health and water-holding capacity of arid soils by avoiding compaction, and if soils are inadvertently compacted then mechanically ripping soil to a depth of 18” to encourage infiltration. The addition of soil sponges – excavated cylinders filled with compost, wood mulch, and volcanic rock – help to build healthy soil fungi and bacteria. Shredded wood mulch contributes organic material to soil and also supports a healthy micro biome. Maintenance practices that leave leaf litter on the ground allow plant debris to contribute organics to the soil. Although most of the Middle Rio Grande Valley has soils with good infiltration rates, without attention to protecting soils from compaction and promoting a healthy micro biome, infiltration rates in GSI features can be much lower than expected.
Include GSI in flood control! High volumes and velocities of runoff mean that flood control and potential damage from runoff must be considered differently than in climates receiving steady, gentle rains. Landscape architects must work closely with engineers to ensure that GSI features are incorporated into the flood control system and protected from high-velocity flows. Protection against high-velocity flows may require larger aggregate sizes or small areas of concrete that would not be necessary in other climates. Where possible, grey infrastructure can be downsized where green infrastructure plays a reliable role in flood control.