Evaluating the Efficiency of Household Stormwater Detention System

Abstract

This paper describes the evaluation of water storing capacity of a household stormwater detention system based on field data. Collection of field data is often sidelined due to the cost and human capital incurred. However, the true value of field data is demonstrated here by comparing the observed and design data. A field test is completed in a real-life terrace house, utilizing the house’s 95m2 side canopy as roof catchment and 4.40m x 4.70m car porch area to station a detention tank. Precast concrete modular units with 3.9m3 effective storage volume are assembled within the tank. Downpipe with 0.1m diameter is installed to connect the roof gutter to the detention tank; while pipeline with 0.05m diameter is installed as the outlet from tank to the house perimeter drain. The mentioned setup is subjected to actual rainfalls from December 2019 till February 2020 that corresponded with the peak of Northeast Monsoon season. Ten observed storm events with peak hourly total rainfall readings ranging from 22 to 48mm are selected for analysis. Rainfall and water level readings from the field test allow the derivation of roof runoff volume and detained water volume in the tank. It is found that the household stormwater detention system is able to capture about 50% of the roof runoff. However, the current setup is found to cause flooding for rainfall over 40mm. The flooding issue, however, is undetected by the design data that underestimated the water storing capacity. This is due to the use of uncommon precast concrete modular units that may not have its flow characteristics represented by existing formula and model. No matter how uncommon the modular units be, various types and forms of stormwater detention system are becoming the new normal in the industry and field test is the best tool to validate their performances.

Detention Properties of Subsurface Stormwater Modules Under Tropical Climate

“…The percentage volume of water detained for total rainfall between 11.1 mm to 56.8 mm ranged between 35.2% to 95.6% with a mean value of 73.9%. Comparing this findings to the percentage of volume detained (between 30% to 74%) from the study by Ngu et al [4] for modular pre-cast concrete stormwater detention system; the current subsurface stormwater modules performed slightly better. This could be due to the top soil layer used in the current study that allows runoff to infiltrate and store in the layer before reaching the subsurface modules.…”

“…In terms of volume attenuation, again, modules with infiltration system performed better than the other two systems with 27% as compared to 19% for loose rock infiltration system and 3.45% for modules storage tank system for 2-year storm [3]. As comparison, a more recent study using modular pre-cast concrete stormwater detention system has shown the effectiveness to detain between 30% to 74% of runoff for total rainfall depth of 42.5 mm to 117.5 mm [4]. One of the factors that determine the water volume that can be detain by the stormwater modules is the porosity/cavity/void ratio of the modules.…”

Case Study of Stormwater Control by Permeable Road in Commercial Centre under Equatorial Climate

“…According to a local study [3] conducted from December 2019 to February 2020 in conjunction with the Northeast Monsoon, it was reported that 90% of the recorded rainfall were below 20 mm, 5% between 20-30 mm, 4% between 30-40 mm and 1% between 40-50 mm. The values of 10-year ARI design storms for 5-, 10-and 15-min are estimated as 23, 36 and 46 mm, as intense rainfalls.…”