PEAK FLOW ATTENUATION USING ECOLOGICAL SWALE AND DRY POND

ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 1 PEAK FLOW ATTENUATION USING ECOLOGICAL SWALE AND DRY POND A. Ainan1, N.A. Zakaria2, A. Ab. Ghani3, R Abdullah4, L.M. Sidek5, M.F. Yusof 6 and L.P. Wong7 ABSTRACT Data collection system to gauge the system effectiveness of BIOECODS is operating from June 2003. The data collection system focuses on the ecological swale, dry pond and ecological pond. The effectiveness of an ecological swale is considered from the aspect of quantity control in term of peak flow attenuation. The operational functional of a dry pond is calculated in terms of its capability to retain and drain storm water. Dry pond is an off-line storage with the function to reduce peak discharge at the downstream. The storm water in the dry pond recedes by infiltrating through the layer of topsoil and river sand to the storage module underneath and then flows downstream along the sub-surface module of the swale. Examples of recent data collected will be presented in this paper. The results on ecological swale show that there is a lag time between rainfall event and the resulting flow from the surface and subsurface ecological swale. The catchment response time to rainfall is about 40 min giving an indication that ecological swale has a capability to delay the flow to the downstream site. The hydrographs for the surface swale appear attenuated where the volume of the storm water is distributed over 3-hour period and a peak discharge at an upper reach of the swale is higher than the lower reach. The results show that ecological swale has the capability to attenuate the peak discharge. Water level that was measured using Ultrasonic Water Level sensor at the dry pond outlet confirms the retention behavior of the dry ponds. The storm water infiltrates and empties the dry pond outlets over the period of 20 hours. The emptying time depends on the capacity of the adjacent ecological swale. 1 Engineer, River Engineering Section, Department of Irrigation and Drainage Malaysia, Jalan Sultan Salahuddin, 50626 Kuala Lumpur, MALAYSIA ([email protected]) 2 Associate Professor and Director, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MALAYSIA ([email protected]) 3 Associate Professor and Deputy Director, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MALAYSIA ([email protected]) 4 Lecturer, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MALAYSIA ([email protected]) 5 Senior Lecturer, Department of Civil Engineering (Water Engineering), College of Engineering, Universiti Tenaga Nasional, KM 7, Jalan Kajang-Puchong, 43009 Kajang, MALAYSIA ([email protected]) 6 Research Officer, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MALAYSIA ([email protected]) 7 Research Officer, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MALAYSIA ([email protected]) ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 1. 2 INTRODUCTION Urban development in Malaysia with the construction of new towns may change the natural hydrology and infiltration characteristics of the previously rural catchments. This phenomenon has produced various problems regarding the stormwater runoff, such as increase in stormwater flow discharges into receiving waters, increase in flood peaks and degradation of the runoff water quality. Adverse impacts including flash flood at downstream, scouring of channel, sedimentation and transportation of pollutant load from upstream to downstream constantly occurs in major cities throughout Malaysia during rain event either on minor or major storm event. As the urban areas continue to develop, these problems become more severe and cost-effective options for treating such problems (Baber et. al. 2003). The launching of New Urban Drainage Manual known as Urban Storm Water Management Manual for Malaysia (Manual Saliran Mesra Alam or MSMA) emphasizes the implementation of the concept of Best Management Practices (BMPs) e.g., wet ponds, grass swales, wetlands, sand filter dry pond, etc. (MSMA, 2000). Effective 1st January 2001 new development in Malaysia must comply with MSMA to control stormwater from the aspect of quantity and quality runoff to achieve zero development impact contribution to overcome the problems of inundation and increase of stormwater runoff pollution. Effectiveness of a BMPs component is highly dependent on its design characteristics which influence the detention time and hence treatment efficiency (Yu et. al. 2001). Generally, there are many studies that have been completed that assess the ability of storm water treatment BMPs, to reduce pollutant concentrations and loadings in stormwater system discharges (Strecker et. al. 2001). It is essential to identify the effectiveness of the BMPs from the aspect of quantity as well. Consequently the hydraulic effectiveness from the aspect of quantity for ecological swale and dry pond is emphasized in this paper. It is deemed that the ecological swale can provide control of certain peak runoff rates by retarding and impounding stormwater and conveying it downstream at velocities low enough to protect against channel and streambank erosion. Roesner et al. (2001) found that the higher frequency of the peak flow causes the stream to cut a deeper and wider channel. Meanwhile the dry pond is capable to retain and store the stormwater runoff in an average typical duration of 24 hour before draining into other stormwater systems. The objectives of the study are to identify percentage reduction of flow volume and peak flow, and lag time between the inflow and outflow for an ecological swale, and to determine the emptying time for a dry pond on typical rainfall events. 2. BIOECODS, USM ENGINEERING CAMPUS The site of the study is located in USM Engineering Campus Perai Selatan District, Pulau Pinang, Malaysia. The campus covers about 320 acres and is made up of mainly oil palm plantation land, which is fairly flat. The USM Engineering Campus project has taken a series of measures to reduce runoff rates, runoff volumes and pollutant loads by implementing a source control approach for stormwater management as suggested in MSMA. This include a series of components namely ecological swale, on-line underground storage, and dry ponds as part of the Bio-ecological drainage systems (BIOECODS) that contribute to the treatment of the stormwater before it leaves the campus. ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 3 Figure 1 Layout Plan for USM Engineering Campus. 3. DATA COLLECTION 3.1 Ecological Swale Ecological swale is the main component of BIOECODS, which consists of a grass-earth channel as the surface channel and combined with a subsurface module enclosed within a permeable geotextile. The effectiveness of ecological swale is highlighted in this paper from the aspect of quantity control. Flow data for inlet and outlet of an ecological swale are measured using the Area Velocity Module. The cross section of the ecological swale and monitoring station is shown in the Figure 2. 3.2 Dry Pond The operational functional of a dry pond is evaluated in particular the capability of the dry pond to retain and drain storm water. Dry pond is an offline storage function to reduce peak discharge at the downstream. The storm water in the dry pond recedes by infiltrating through the layer of topsoil and river sand to the storage module underneath and then flows downstream along the sub- surface module of the swale. The cross section of a dry pond is illustrated in Figure 3. The flow from the dry pond drains into the adjacent ecological swales when the water in the surface swale is completely drained into the ecopond. Consequently, the water level data from dry pond can be used ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 4 to access the performance and effectiveness of the pond outlet, which infiltrates the water through sand layer to the modular sub-surface storage. Water levels of five selected dry ponds have been measured using Ultrasonic Water Level sensors (Figure 4). The five selected dry ponds are labeled UWL 1 to UWL 5. (a) (b) Figure 2. Ecological Swale Type B (a) Cross Section and (b) Monitoring Station (a) (b) Figure 3 Typical Cross Section of A Dry Pond: (a) Cross Section and (b) Monitoring Station 4. RESULTS AND DISCUSSION 4.1 FLOW ATTENUATION OF ECOLOGICAL SWALE The following presents the results of flow attenuation for an ecological swale. The effectiveness of the ecological swale is identified based on the percentage of reduction in volume or peak flow as summarized in the Table 1. The percentage reduction of volume or peak flow in Table 1 is calculated based on the differences between the inflow and outflow over inflow. It has been observed that the percentage of volume reduction for surface channel is between 19.4% and 69.8% meanwhile the percentage of volume reduction for subsurface channel is between 23.7% and 89.2%. The reduction in peak flow ranges from 28.9% to 55.9% for surface swale while for subsurface channel from 0% to 59.5%. Besides, the catchments response time to rainfall is about 40 minutes giving an indication that ecological swale has a capability to delay the flow to the downstream site as shown on the inflow and outflow hydrograph in Figure 4. ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 5 Table 1 Flow Attenuation for Ecological Swale (June – November 2003) Rainfall Intensity (mm/hr) Rain Event Location Channel ARI Peak flow (l/s) (2003) (Inlet) 24/6 11 3 month 26/6 31.6 6 month 30/8 14.5 3 month 8/9 13.8 5 year 4/10 10/10 3/11 6.18 33.6 44.2 2 year 2 year 1 year 8/11 9.3 6 month Surface Subsurface Surface Subsurface Surface Subsurface Surface Subsurface Surface Surface Surface Subsurface Surface Volume (m3) (Outlet) 128 79 45 53 59 41 59 70 201 226 172 41 115 91 32 22 53 26 50 26 51 167 168 120 23 75 Surface Channel: Flow s and Rainfall Event on 24th June 2003 (Inlet) (Outlet) 418.5 134.1 105.6 53.1 388.8 119.1 4043.1 160.2 2202.9 1711.8 1134.6 108.9 607.8 246.6 16.2 31.2 31.2 123.6 90.9 3043.2 83.1 1560 1380.3 599.4 11.7 357.9 Subsurface Channel: Flow s and Rainfall Event on 24th June 2003 Time (minit) Time (minit) 0 50 100 150 200 250 300 Percentage Reduction (%) Peak Volume Flow 28.9 41.1 59.5 87.9 51.1 70.5 0 41.2 55.9 66.6 0 23.7 55.9 24.1 27.1 48.1 16.9 29.2 25.7 19.4 30.2 47.2 43.9 89.2 34.8 41.1 350 400 450 500 550 0 600 0 160 50 100 150 200 250 300 150 0 125 2 100 4 75 6 50 8 25 10 1 140 2 4 5 80 6 60 7 Rainfall Depth (mm) 100 Flow (l/s) Flow (l/s) 3 Rainfall Depth (mm) 120 40 8 20 9 0 0 50 100 150 200 250 300 350 400 450 500 550 12 0 10 0 600 50 100 150 Rainfall Inlet Outlet Rainfall Surface Channel: Flow s and Rainfall Event on 26th June 2003 50 100 250 Inlet Outlet Subsurface Channel: Flow s and Rainfall Event on 26th June 2003 Time (minit) 0 200 Time (minit) Time (minit) Time (minit) 150 200 250 0 0 80 25 50 75 100 125 150 175 0 100 2 2 80 4 60 6 40 8 8 20 10 12 0 0 50 100 150 200 Inlet 10 12 0 0 25 50 75 100 125 Time (minit) Time (minit) Rainfall 250 20 Outlet Rainfall Inlet Outlet Figure 4 Inflow and Outflow Hydrograph for typical rainfall events 150 175 Rainfall Depth (mm) 6 Flow (l/s) Flow (l/s) 4 40 Rainfall Depth (mm) 60 ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 6 Subsurface Channel: Flow s and R ainfall Event on 30th August 2003 Surface Channel: Flow s and Rainfall Event on 30th August 2003 25 50 75 100 125 Time (minit) Tim e (minit) 150 175 200 225 250 275 300 325 0 0 100 10 20 30 40 50 60 70 80 90 100 0 100 2 2 75 8 Flow (l/s) 50 6 Rainfall Depth (mm) Flow (l/s) 6 4 75 4 Rainfall Depth (mm) 0 8 50 10 12 10 25 25 14 12 16 14 0 0 25 50 75 100 125 150 175 200 225 250 275 300 18 0 325 0 10 20 30 40 50 Time (m init) Rainfall Inlet Outlet Rainfall S u rface C h ann el: F low s an d R ainfall E ven t on 4th O ctob er 2003 50 1 00 15 0 2 00 25 0 3 00 35 0 4 00 450 80 90 100 Inlet Outlet T im e (m in it) 500 55 0 600 65 0 700 75 0 0 800 35 0 0 30 0 50 1 00 15 0 2 00 250 30 0 40 0 0 35 0 2 30 0 4 6 15 0 6 25 0 8 20 0 10 15 0 Rainfall Depth (mm) 20 0 Flow (l/s) 4 Rainfall Depth (mm) 2 25 0 Flow (l/s) 70 S urface C han n el: F low s an d R ain fall E ven t on 10th O ctob er 2003 T im e (m in it) 0 60 Time (minit) 8 12 10 0 10 0 14 10 50 50 16 12 0 0 50 100 150 200 250 300 3 50 4 00 450 5 00 5 50 60 0 6 50 7 00 75 0 0 80 0 0 50 100 T im e (m in it) R ainfall In let O utlet R ainfall S u rface C han n el: F low s an d R ain fall E vent on 3rd N ovem ber 2003 25 50 75 100 1 25 150 In let 200 250 300 O utlet S u bsu rface C h an n el: F low s an d R ain fall E vent on 3rd N ovem ber 2003 T im e (m in it) 0 150 T im e (m in it) T im e (m in it) 1 75 200 225 25 0 0 0 250 10 20 30 40 50 60 70 80 90 0 50 2 2 4 200 40 10 100 12 Flow (l/s) Flow (l/s) 8 Rainfall Depth (mm) 150 6 30 8 20 10 14 50 12 10 16 14 18 16 0 0 20 50 75 1 00 125 150 T im e (m in it) R ainfall In let 175 200 225 0 10 20 30 250 40 50 60 70 T im e (m in it) R ainfall O utlet Inlet O utlet Surface Channel: Flows and Rainfall Event on 8th November 2003 Time (minit) 0 50 100 150 200 250 300 350 400 450 500 0 150 2 125 4 100 6 75 8 Rainfall Depth (mm) 25 Flow (l/s) 0 50 10 25 12 14 0 0 50 100 150 200 250 300 350 400 450 500 Time (minit) Rainfall Inlet Outlet Figure 4 Inflow and Outflow Hydrograph for typical rainfall events (Continued) 80 90 Rainfall depth (mm) 4 6 ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 4.2 7 RETENTION BEHAVIOR OF DRY POND The water levels of five selected dry ponds, which are labeled as UWL 1 to UWL 5, on six typical rainfall events, are presented in the Figure 5. The water levels were measured using Ultrasonic Water Level sensors located at each dry pond outlet, to represent the retention behavior of BIOECODS’ dry pond. The water level data are summarized and tabulated in Table 2. Table 2 shows the emptying time of a dry pond depending on the Average Recurrence Interval (ARI). The higher ARI of a rainfall event, the longer time is needed for emptying the dry pond. For example, emptying time for dry pond UWL1 for the rainfall event on 8th of September 2003 with 5 year ARI is 48 hours, whilst 26 hours is needed to empty the dry pond for rainfall event with 3 month ARI, which fell on 30th of August 2003. Table 2 Performance of Dry Pond on Typical Rainfall Event (June – November 2003) Rainfall Event 17/6/2003 Rainfall Intensity (mm/hr) 35.7 Average Recurrence Interval (ARI) 6 month 30/8/2003 14.5 3 month 8/9/2003 13.8 5 year 10/10/2003 33.6 2 year 3/11/2003 44.2 1 year 8/11/2003 8.3 6 month Location Dry Pond UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 Maximum Water Level at outlet (mm) 131 560 73 429 203 210 388 144 476 268 357 669 266 511 373 321 661 242 526 356 247 505 164 503 322 240 531 169 497 300 Emptying Time (hour) 6 16 5 19 7 26 17 20 31 15 48 36 34 44 28 45 31 24 40 21 27 25 25 36 21 33 37 30 43 22 ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 8 Dry Pond:Water Level for Rainfall Event on 17th June 2003 Dry Pond:Water Level for Rainfall Event on 30th August 2003 Time (min) Time (min) 0 125 250 375 500 625 750 875 1000 0 1125 800 250 500 750 1000 1250 1500 1750 2000 2250 2500 800 0 0 1 700 700 2 2 600 600 400 8 300 3 500 4 400 5 6 300 Rainfall Depth (mm) 6 Water Level (mm) 500 Rainfall Depth (mm) Water Level (mm) 4 7 10 200 200 8 100 0 0 180 360 540 720 900 12 100 14 0 9 0 1080 300 600 900 1200 Rainfall UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 Rainfall Dry Pond:Water Level for Rainfall Event on 8th September 2003 UWL 1 250 500 750 1000 1250 1500 1800 2100 10 2700 2400 UWL 2 UWL 3 UWL 4 UWL 5 Dry Pond:Water Level for Rainfall Event on 3rd November 2003 Time (min) 0 1500 Time (mm) Time (min) Time (min) 1750 2000 2250 2500 2750 3000 0 1000 0 900 2 250 500 750 1000 1250 1500 1750 2000 800 0 700 800 2 4 8 500 10 400 12 300 14 200 16 100 18 4 500 6 400 8 300 Rainfall Depth (mm) 6 600 Water Level (mm) 700 Rainfall Depth (mm) Water Level (mm) 600 10 200 12 100 0 0 20 0 300 600 900 1200 1500 1800 2100 2400 2700 14 0 300 600 900 1200 1500 1800 2100 3000 Time (min) Rainfall UWL 1 Time (min) UWL 2 UWL 3 UWL 4 UWL 5 Rainfall Dry Pond:Water Level for Rainfall Event on 10th October 2003 UWL 1 250 500 750 1000 1250 1500 UWL 4 UWL 5 Time (min) 1750 2000 2250 2500 2750 1000 3000 0 900 2 800 4 700 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 900 0 800 2 700 4 600 6 500 8 400 10 300 12 8 500 10 400 12 300 14 200 16 200 14 100 18 100 16 0 20 3000 0 0 300 600 900 1200 1500 1800 2100 2400 2700 18 0 300 600 900 1200 Time (min) Rainfall UWL 1 UWL 2 1500 1800 2100 Time (min) UWL 3 UWL 4 UWL 5 Rainfall UWL 1 UWL 2 UWL 3 UWL 4 UWL 5 Figure 5 Water Level at outlets of UWL 1 to UWL5 on Typical Rainfall Events. 2400 Rainfall Depth (mm) 600 Water Level (mm) 6 Rainfall Depth (mm) WaterLevel (mm) UWL 3 Dry Pond:Water Level for Rainfall Event on 8th November 2003 Time (min) 0 UWL 2 ADVANCES IN HYDRO-SCIENCE AND –ENGINEERING, VOLUME VI 9 5. CONCLUSION Nine different rainfall events were presented in this paper to show the ecological swale performance in attenuating flows. The results show that the volume reduction can be achieved at a minimum of 19.4% for surface channel and 23.7% for subsurface. The retention behavior of the dry ponds has shown significant relationship between ARI and the emptying time. The higher ARI of a rainfall event, the longer time is needed for emptying the dry pond. It is shown that the five selected dry ponds perform very well in retention of the stormwater runoff for a typical duration before draining into the stormwater system, therefore avoiding the occurrence of inundation at the downstream end. ACKNOWLEDGEMENT The authors would like to thank the Department of Irrigation and Drainage, Malaysia for the support in providing the research grant for this pilot project. The authors also would like to gratefully acknowledge the full support given by the Vice Chancellor of University Science Malaysia for giving them the opportunity to construct the BIOECODS at the new USM Engineering Campus. They are also grateful to His Excellency the Governor of Penang for officially launching the BIOECODS at the national level on 4th February 2003. REFERENCE Barber, M. E., King, S. G., Yonge, D. R. & Hathhorn, W. E. (2003). “Ecology Ditch: A Best Management Practice for Storm Water Runoff Mitigation”. Journal of Hydrology Engineering, Vol. 8, No. 3, pp. 111-122. “Urban Storm Water Management Manual For Malaysia” (2000) Department of Irrigation and Drainage Malaysia. Percetakan Nasional Malaysia Berhad. Roesner, L. A., Bledsoe. B. P., & Brasher, R. W. (2001). “Are Best Management Practice Criteria Really Environmentally Friendly?” Journal of Water Resources Planning and Management, Vol. 127, No. 3, pp. 150-154. Strecker, E. W., Quigley, M. M., Urbonas, B. 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