Benefits of Rainwater Utilization- The Applications on Flood Mitigation, Water Harvesting and Gutter Snipe Design

In order to achieve the purposes of environmental protection, such as flood control, sustainable development on water utilizat ion and increasing of water demand with the decreasing of conservative water resources, to find new supplementary water resources and effective utilization those resource becomes a very significant topic. Rainwater, which is caught by reservoir at upstream while flowing down into sea through the middle or downstream, a best blessed gift and it has no water right problem without falling down the ground. Considering from the view point of environmental, economic, and social impacts, the utilization on water harvesting and flood mit igation with the benefit-cost ratio analysis and the effective function of gutter snipe design keeping water clean evoke us to study in paying much attention. In this study, high B/C ratio and wide application are presented.


Introduction
The damage fro m rain water flood with its quantity and intensity, [1], [2], [3], [4], and [5] with pictures 1 and 2, and the essential necessity from its effect ive use, [6] with pictures 3, 4, and 5, always make us confuse. The detail ana lysis of a given rainwater cistern-supply system (RWCS), with p ictures 6, 7 and 8, will affect the economic efficiency. The resultant gutter snipe design (RGSD) will determine the water quantity for utilizat ion, and this magnitude depends on the flo w situation, laminar or turbulent, so eventually. In this study, flood routing and analysis, rain water cistern-supply system construction from a g iven catchment system, and characteristics and design criteria of gutter snipe will be the main issues for discussion and application.

Flood Routing and Analysis
In order to have the plan of flood-control for natural hazard mitigation, the fo llowing steps are made: Picture 7. Standpipe first-flush diverter Picture 8. Galvanized sheet metal tanks

Mean Di mensionless Curve (MDC)
First, the MDC is obtained by using the flood-time discharge associated with the simultaneous mean rainfall of a given river catchment. The M DC is shown in Figure 1.

Uni t Hydrograph of Discharge (UHD)
Fro m the MDC in Figure 1, the UHD is formed with a given time lag, said 3.56hrs, and the unit time 1.0hr, and assumed rainfall time duration 4.06hrs (the time lag plus half of the unit time), and is presented in Figure 3.

Anal ysing Maxi mum 48hrs Stormy Rainfall(AMS R)
Collecting the annual maximu m 48hrs rainfall recorded data of a given hydrological station, the rainfall frequency analysis of 48hrs can be constructed by using statistic hydrological events frequency analysis method. The result of this AMSR for the known station is supposed in Table 1.

Fl ood Frequency Curve of Discharge (FFCD)
The FFCD is resulted fro m the co mbination of Figure 2 and Table 1. The frequency analysis for d ifferent return period, T, of this given catch ment system is presented in Table 2.

Fl ood-Damage with Flood-Frequency (FD-FF)
Based on the return period and the real document of flood loss, the FD-FF curve is made and given as Figure 3. As you pick any return, for examp le T=20yr, the corresponding annual cost including construction, operation and maintenance, will be USD 3,961,000. While, the annual benefit directly and indirectly fro m the drainage planning, such as high level utilization of land and more job chances, etc., fro m Figure 3 is USD 5,750,000. The B/C ratio is almost 1.45 for FD-FF.

Rainwater Cistern-Supply System
Analysis (RWCS) on Flood M itigation, Water Harvesting and Gutter Snipe Design A flood-mit igation used to separate the flood and delay the flood peak is to store a certain quantity of flood discharge from a given period, such as T=20yrs, to a planning return period, said T=10yr. Fro m Tab le 1 and Table 2, the corresponding reducing rainfall is 123mm/hr with the storing discharge 60cms. By using rat ional formu la to calculate the area for catching the storage volume, Q= C．I．A (1) with C=0.8, the runoff coefficient, the area A=2,195,200m².
If the duration for the rainfall is 3hrs per day, the storage volume will be: V=Qt=(60c ms)(3hrs/day)(3600s/hr)=648,000m³/day (2) If there are 30days with such reducing rainfall, 123mm/h r , or said 60cms, therefore, the total storage volume per year for utilization on life substituting use becomes, Vy = (648,000m³/day)(30days/year) =19,440,000m³/year (3) The mean annual life water fee is 0.33USD/ m³ with return on investment rate, 0.08 on return interval, n=10years, in Taiwan, it means that the corresponding life water volume is substituted by rainfall to save that corresponding money. That is the annual benefit is, Fro m Figure 3, the reducing damage fro m T=20yr to T=10yr is, 5,750,000USD-4,650,000USD=1,100,000USD (5) Here, the total benefit becomes, 6,415,200USD+1,100,000USD=7,515,200USD (6) Of course, the construction fee for storage the volume, 648,000m³, must be calcu lated.. The cost for construction is 7USD/ m³ with return on investment rate, 0.08 on return interval, n =10years, then the total cost is, (648,000m³)(7USD/ m³)=4,536,000USD (7) Finally, the B/C ratio for RW CS is 1.66. When we combine the effects of FD-FF and RW CS, The benefit-cost ratio becomes, (5,750,000+7,515,200)/(3,961,000+4,536,000)=1.56 (8) For the mere purpose, such as FD-FF, the B/ C ratio is just only 1.45 wh ich is the least one, while RW CS, 1.66. But by considering the environment protection and sustainable development together, the optimu m solution with B/ C rat io, 1.56, will be the best policy.

Design of Gutter Snipe
In order to keep leaves, bugs, dead birds, or other debris out of the collecting water tank, the characteristics of gutter snipe and its design criteria with experimental data are discussed here. Water travels down the roof and along the gutter to the downspout where the gutter snipe is installed. After water going through the slot in the screen as in Figure  4 and stored in the tank, wh ile removing the debris, the clean water can be obtained by the water system and its decomposes.
In order to make sure how much the rain water will be stored with the needed optimu m diameter of pipe for depending the flow situation, such as laminar or turbulent inflow, the theoretical consideration of design based on free-body diagram of fluid acting forces system, in Figure 5; free-body diagram o f fluid and its objectives acting forces system in Figure 6; and effect ive discharge and velocity after screens, in Figure 7, and the theoretical comparisons between the different flo w situations are presented . Finally, the discussion and conclusions are made . Three factors, such as diameter of pipe, D, the frict ion factor, f, and the inclined angel of the screen, θ, will be used to decide the discharge, Q0, and the acting force, Fx.

Free-body Diagram of Flui d and Its Objecti ves Acting Forces System
In Figure 5, the discharges of Q0, Q1, and Q2, and the velocities of Ū0, Ū1 and Ū2, and the acting forces of Fx and Fy must be decided first with the given diameter, D, screen inclined slope, θ, and the friction factor in Moody Diag ram , Fro m Eq. (13), Q2 will be as large as possible for the purpose of collecting rain water, meanwhile the discharge Q1 in Eq. (14) is only just enough for flushing out the solid matter, or say, as small as possible.

Free-body Diagram of Flui d Acting Forces System
In Figure 6, the force for the hinge to hold on the screen can be obtained by Fx+(WsinΦ)-F-(μNWcosΦ)=(W/g)(ax) (17) Assuming ax =0, the Fx=F , when it is (μNcosΦ)=(WsinΦ) (18) and Φ=26.57˚ with μN=0.5 The Ф-value will be the crit ical-angle for body equilibriu m itself without any other acting force. By given any as small as possible force with water flushing, the solid be moved away easily.

Effecti ve Discharge and Vel ocity After Screen
The energy loss in Figure 7 after screen will be solved in order to calculate the effective velocity,U2', and discharge, Q2'. The formu la used to have the solution is

Discussion of the Fricti on Factor for the Situation of Fl ow
The value of f for turbulent flow with any angle θ can be neglected when it is less than 0.02 due to the consideration of material strength and economic views. And the U2' is calculated fro m U2'²/2g= Ū2²/2g-Δh (20) For f= 0.10, the ratio of discharge and force between laminar (L)and turbulent(T) flo w are Q0L/ Q0T=6.4; and FxL/FxT=40, (21) For f= 0.05, the ratio of discharge and force between laminar (L)and turbulent(T) flo w are Q0L/ Q0T= 0.8 and FxL/ FxT= 0.63.

Conclusions
1. While considering the impo rtance of environmental protection and sustainable development, the FD-FF combin ing with RW CS model will be the optimu m solution.
2. Generally speaking, the flow situation is always turbulent one, the criteria for the design of gutter snipe with Eq. (18) and Eq. (22) is f=0.05 and Φ=26.57˚ with μN=0.5.