Deferring waterlogging through stormwater control and channelling of runoff

Abstract

Stormwater control measures (SCMs) in the form of highly permeable tree pits can be useful in two ways. On one hand, they can help reduce surface runoff and on the other hand, the presence of trees in these pits can potentially help mitigate the urban heat island effect. Therefore, it is important to understand the pit designs that underpin tree health and growth performances. Here in this study, four pit designs namely, a control which did not receive runoff water, another that receive runoff into a sandy clay loam soil, yet another that was filled with structural soil, and a third that received runoff into structural soil with an underdrain system, used to test and identify which designs were best able to maintain hydrological balance and growth of Calophyllum inophyllum in an urban area dominated by sandy clay loam soil. Trees in pits which had structural soil and drainage installed had 6-fold more growth than the controls (that received no stormwater runoff), and 3-fold more growth when compared against street trees grown in pits that had sandy clay loam soil (that had run off channelled towards these pits). Treatments with structural soils also experienced more efficient exfiltration rates which helped avoid waterlogging which in turn, benefitted growth. The lower exfiltration rates of the poorly drained sandy clay loam soils resulted in periods of saturation and subsequent slower tree growth. Treatments with structural soil together with drainage and those that had sandy clay loam soils were found to have saturated water levels at a depth greater than 400 mm and between 100−200 mm (from the surface), respectively. The outcomes from this study indicated that tree growth can be improved by channelling stormwater into tree pits with structural soil and especially those that also have drainage installed. Noteworthy however, is that while waterlogging may be avoided through the application of structural soil and a drainage system, the high exfiltration rates may result in water stress when rainfall is scarce though this did not happen here. The need for hydrological balance will be the key to successful urban street tree planting.