Effect of Minimum Inter Event Time on Water Quality Capture Volume

Chun Kiat Chang, Aminuddin Ab. Ghani, Keng Yuen Foo, Mazlina Alang Othman

Thursday 2 july 2015

8:45 - 9:00h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 10G. Environment - Impact

In Malaysia, with lands and spaces becomes limited caused by urbanization, size of stormwater facilities should be effective to meet the needs of planning, designing and managing stormwater systems. Water quality captured volume (WQCV) has been introduced and practiced in Urban stormwater Management Manual for Malaysia (MSMA 2ed) to give a balance between the storage size and water quality treatment effectiveness. WQCV is a concept to relate the urban runoff volume with the local rainfall pattern, and the focus of the WQCV is to capture the specified runoff volume of storm event. 75th to 95th percentile runoff volume was used in WQCV estimation from historical rainfall series analysis. As a smaller event dominate long term rainfall records and this event did not produce runoff, it will effected the WQCV estimation. Independent rainfall event need to identify from rainfall series and separation of rainfall event depends on time chosen in minimum inter event time (MIT). MIT is a minimum dry period between two rainfall pulses and increasing of MIT will increase the rainfall depth of an event. This paper aims to study the effect of different MIT in WQCV estimation. Historical rainfall data from selected rainfall station in Malaysia was analyzed with different MIT at 2hr, 4hr, 6hr, 12hr,18hr and 24hr. As it is more accurate to use shorter interval, 10 min interval of rainfall record was used to separate rainfall event. Relation between MIT and rainfall depth with different percentile was analyzed and their probabilistic behaviour was assessed. This relation study has improved the accuracy of the design of stormwater quality control facilities especially in Malaysia. Keywords: Rainfall events, long term rainfall analysis; water quality capture volume; minimum inter event time.