The Effectiveness of Rainwater Catchment on Flood Control in Slope Land Area Introduction Runoff Retardation Facilities

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1 The Effectiveness of Rainwater Catchment on Flood Control in Slope Land Area K.F. Andrew Lo and Chao-Chun Huang Department of Natural Resources, Chinese Culture University, Taipei, Taiwan. Introduction Taiwan is a mountainous island. The total area is about 36,026 km 2. The Central Mountain Range runs north to south, dissecting the island into two halves. Flatland with less than 100 m elevation comprises 26.4% of the total area. The remaining 73.6% is slopeland. Forest cover in higher elevation is about 46.6%. Along the Central Mountain Range, there are about 20 peaks exceeding 3,000 m. Rivers originate from mountain tops flow east and west to the Pacific Ocean and the Taiwan Strait, forming narrow river valleys as a result of torrential river flow. Suitable reservoir sites are limited. The average annual rainfall is about 2,500 mm. However, rainfall is unevenly distributed both temporally and spatially. The north, central, south and east receive about 62%, 78%, 90%, and 79% of the total rainfall during the month from May to September, respectively. Wet and dry seasons are very pronounced. The available water resources are limited. Due to increases in population, economic development, housing areas, and agricultural land, slopeland ecology has been damaged, resulting in severe soil erosion and runoff, ruined water retention capacity, accelerated water pollution rate, and deteriorating sedimentation and water pollution problems in reservoirs. Torrential typhoon rains often lead to massive soil movement and landslides; concentrated runoff and large peak flow, causing significant damages downstream in the Hsichih area, Taipei. The main objective of this study is to determine the effectiveness of storm water collection using small infiltration enhancement structures and retention ponds on storm runoff reduction and flood control. Runoff Retardation Facilities There are many different types of runoff retardation facilities. According to their usage, they can be classified as: (1) Detention pond (2) Retention pond (3) Wetland (4) Bio-retention (5) Gravel-pore storage (6) Rooftop rainwater catchment (7) Underground storage and (8) Infiltration enhancement. Most runoff retardation facilities may be used for storm detention, storm retention, infiltration, sustaining eco-environment, water quality improvement, and recreational purposes. Table 1 categorizes the different usage of these facilities. Various runoff retardation facilities should be combined in actual implementation of flood control work. The selection

2 should be able to adapt to different local conditions in order to achieve optimum effectiveness. Table 1. Different usage of runoff retardation facilities. Facility type Detentio Retention Infiltration Eco-environment Water quality Recreatio n sustainability improvement n Detention pond! Retention pond!!! Wetland!!!!! Bioretention!!!! Gravel-pore storage!! Rooftop rainwater catchment! Underground storage! Infiltration enhancement!!!!! Simulation of Flood control Effect A simplified single-peak hydrograph was chosen to assess the effect of detention ponds, retention ponds and other small-scale infiltration enhancement facilities built at distributed locations throughout the watershed for the purpose of runoff reduction at the watershed outlet. Assuming the watershed area of about 3.75 km 2 and the following water hydrograph characteristics: T b = 2.76 T p [1] T p = T lag + T d /2 [2] T lag = 0.6 T c [3] Where, T d = effective rainfall duration; T c = Time of concentration; T lag = time lag; T b = time base of the hydrograph; T p = time to peak. Figure 1 displays the unit hydrograph (1 hr, 1cm) resulting from a discharge of 5.2 cm, T p = 1.5 hr, and T b = hr. For a simulated storm of a characteristics listed in Table 2, and a 10 mm rainfall loss, various flood discharges were obtained with the following runoff retardation treatments: 1. No treatment 2. Installation of small-scale distributed infiltration enhancement facilities of 0.3 m deep, 200 m x 500 m area (comprises 2.67% total watershed area) 3. Installation of small-scale distributed infiltration enhancement facilities of 0.3 m deep, 1000 m x 1000 m area (comprises 26.7% total watershed area) 4. Installation of 2.5 m deep, 200 m x 500 m area detention ponds 5. Installation of 2.5 m deep, 200 m x 500 m area retention ponds 2

3 Figure 1. Unit hydrograph of the simulated storm. Table 2. Simulated rainfall characteristics. Time (hr) Rainfall amount (mm) The simulated results are listed in Table 3 and displayed in Figures 2 and 3. As shown in Figure 3, after reaching a certain level of coverage the infiltration enhancement facilities are capable of reducing the flood peak significantly. The reduction in runoff volume, the flood peak depression, and flood peak time lag amount to 66.7%, 40.3% and 1 hr, respectively with 0.3 m deep infiltration enhancement facilities built at strategic locations covering 26.7% of the total watershed area. Comparing the effect between detention and retention ponds, the retention ponds may reduce 55.6% of the total runoff volume; whereas the detention ponds due to their open outlet design have no effect on runoff reduction. However, the detention ponds are more efficient in depressing the flood peak and flood peak time lag (32.6%, 1.5 hr) as compared to the retention ponds (20%, 1.0 hr). Table 3. Effect of different runoff retardation facilities. Low-impact runoff retardation facilities Small-scale infiltration enhancement facility (200x500x0.3m) comprises 2.67% total area Small-scale infiltration enhancement facility (1000x1000x0.3m) comprises 26.7% total area Runoff volume reduction (%) Flood peak depression (%) Flood peak time lag (hr) Detention pond (200x500x2.5m) Retention pond (200x500x2.5m)

4 Discharge (cms) No treatment 2.67% total area 26.7% total area Time (hr) Figure 2. Effect of small infiltration enhancement structures on storm runoff. 50 Discharge (cms) No treatment Retention pond Detention pond Time (hr) Figure 3. Effect of retention and detention ponds on storm runoff. Conclusions Up till now, the most popular flood control strategy always focuses on the end treatment method using engineering structures. So far, no major breakthrough has been achieved. It is, therefore, the purpose of this study to control floodwater using low-impact runoff retardation technology. Storm water is controlled in the upstream of the watershed. Although the effectiveness of this technology is only limited to a certain size of storm events, it can easily control ordinary flood with satisfactory results. Usually small-scale runoff retardation facilities may not be able to prevent large storm runoff events. However, they may mitigate the problem to a certain extent. Besides flood control benefits, the runoff storage should increase the availability of water resources. Water stored in ponds may be used for non-potable uses such as irrigation, toilet flushing, landscaping, and groundwater recharge. 4

5 References Chen, W.T Hydrologic design of slopeland retention pond development. MS Thesis, Dept. of Agric. Engin., National Taiwan University. (in Chinese). Chiung, L.C Estimation of runoff hydrograph in small watershed. Chung Hsing Engineering 23: (in Chinese). Chow, C.H A feasible pilot study of flood control detention pond. MS Thesis, Dept, of Soil and Water Conservation, National Chung Hsing University. (in Chinese). Huang, H.B A study of the design of regulation ponds. Chinese J. Soil and Water Conservation 27(1): (in Chinese). Li, H.J. and C.F. Chow Impact of drainage basin development on runoff hydrograph. Hydraulics 9: (in Chinese). Wang, M.H A study of integrated planning of retention ponds in slopeland watersheds. Taiwan Hydraulics 41(1): (in Chinese). Wang, W.H A study of the important design of rainwater retention ponds. Modern construction Management 98: (in Chinese). Wu, J.H. and C. Yu Comparison study of retention pond capacity computation methods on slopeland areas in Taiwan. Taiwan Hydraulics 44(1): (in Chinese). 5