3.6 Rainfall-Storage-Infiltration-Discharge (R-S-I-D)
3.6.1 Modeling of R-S-I-D system
The R-S-I-D system consists of a rooftop, downpipe, storage tank, infiltration box, low and high water levels and the overflow from the rainwater tank to the nearby sewer system (Figure 3.21). Figure 3.22 shows a typical polyethylene infiltration box (0.5 m (Width) × 1.0 m (Length) × 0.4 m (Height), overall void ratio = 0.95). It can be wrapped with permeable sheet so that water can flow out, while soil particles are kept outside. One or more boxes can be connected each other depending on the required infiltration flowrate.
3.6.1.2 Equations
To simulate water flow in an R-S-I-D system, the water balance can be set up with respect to the conditions of rainwater tank and infiltration box (Figure 3.21). The water balance for the rainwater tank is described as follows:
Vt = Vt−1 + (Qin,t − QI,t − Qout,t)Δt (3.11)
Figure 3.21 Notation and water flow of an R-S-I-D system.
Figure 3.22 A typical infiltration box.
QI,t is the inflow rate from the rainwater tank to the infiltration box (m3/h) at time t. In the system as shown in Figure 3.21, infiltration valve and pipe are designed so that QI,t is controlled so that it is not larger than the infiltration rate. Therefore, QI,t can
be determined based on the water level in the rainwater tank with respect to the L.W.L and the water level in the infiltration box. It can be mathematically described as follows:
If Vt < VES, QI,t = 0 (3.12) VES is the water volume stored below the L.W.L (m3)
If Vt = VES, rainwater in the tank flows to the infiltration box.
The water balance in the infiltration box is described as follows:
VI,t = VI,t−1 + QI,tΔt − QF,tΔt (3.13)
VI,t is the volume of water remaining in the infiltration box (m3) at time t. VI,t−1 is the volume of water stored in the infiltration box (m3) at time t−1. QF,t is the infiltration rate out from the infiltration box (m3/h). Several equations have been developed to estimate the infiltration rate depending on the soil properties, among which Horton’s equation is used in this analysis. In this section, infiltration rate of 1 L/min per one infiltration box is assumed as an example, which is reasonable for general soil condition. For different soil conditions, proper infiltration rate should be selected based on the field measurement of infiltration rate. Infiltration boxes are used in combination up to 20 units.
When the infiltration box is not full, the inflow rate to the infiltration box is the same as inflow rate to the rainwater tank.
If VI,t ≤ VI, QI,t = min (Qin,t; QInfiltration) (3.14) The infiltration rate is determined by the water remaining in the tank and the inflow quantity in the infiltration box.
VI,t−1 + QI,tΔt < QInfiltrationΔt → QF,tΔt = VI,t−1 + QI,tΔt (3.15) VI,t−1 + QI,tΔt ≥ QInfiltrationΔt → QF,t = QInfiltration (3.16) Or
QF,t Δt = min (VI,t−1 + QI,t Δt; QInfiltrationΔt) (3.17)
VI is the volume of the infiltration box (m3). QInfiltration is the infiltration rate (m3/h).
If VI,t > VI, the infiltration is full
QI,t = QF,t = QInfiltration (3.18)
Qout,t can be determined based on the water level in the
rainwater tank with respect to the maximum water level.
If Vt > V, Qout,tΔt = Vt−1 − V + (Qin,t − QI,t)Δt (3.19) V is the volume of the rainwater tank (m3).
3.6.1.3 Flow chart
The outflows from the rainwater tank under various conditions are calculated by the simulation based on the flow chart (Figure 3.23). The R-S-I-D model requires inputs of design parameters including runoff coefficient (C), catchment area (A) (m2), rainwater tank volume (V) (m3), infiltration box volume (VI) (m3), emergency storage (VES) (m3), infiltration rate of the infiltration box (QInfiltration) (m3/h), and design rainfall
(ip,d,t) (mm/h). Outputs are the outflow presented by TP (Tank
volume – Peak runoff) curves and TD (Tank volume – Design return period) curves, Groundwater Utilization Ratio (GRR) and Emergency Storage (ES).
3.6.2 Results and discussion
3.6.2.1 TP (Tank volume – Peak runoff) curve
If the rainwater tank is emptied slowly by percolating the stored rainwater for groundwater recharge, the runoff can be reduced, and flood mitigation capacity will increase. Figure 3.24 shows TP curves under different number of infiltration boxes for a 100- year design period in Seoul. In this figure, the
Figure 3.23 Flow chart figure for R-S-I-D model, P (Design period), D (Rainfall duration), T (Simulation period).
‘No infiltration’ line represents the R-S-D model. The larger the rainwater tank is, and the more infiltration box is used, the less will discharge flow be. The solid horizontal line represents the peak runoff flow (11 m3/h) for the 2-year design rainfall.
The minimum tank volume without infiltration to control a 100-year peak runoff for 2 years is 11 m3/100 m2 (Point A).
The tank volume can be reduced to 9 (Point B), 8 (Point C), 6 (Point D) and 5 (Point E) m3/100 m2 with infiltration box numbers of 5, 10, 15 and 20 boxes, respectively.
Figure 3.24 TP (Tank volume – Peak runoff) curves for R-S-I-D system (using 100-year design return period rainfall in Seoul, normalized for 100 m2 catchment area).
Table 3.5 shows the comparison between the peak runoff of R-D and R-S-I-D system, and peak reduction ratio. By installing a combination of rainwater tank of 10 m3/100 m2 and 10 infiltration boxes, the sewer system can be ensured for a heavier rainfall of
100-year return period. It is also found that with the R-S-I-D system, the peak runoff can be reduced by 68.5–100% for a combination of 10 m3/100 m2 rainwater tank and 10 infiltration boxes.
Table 3.5 Peak reduction of R-S-I-D system (10 m3/100 m2 of rainwater tank and 10 boxes of infiltration).
Return Period
R-D System R-S-I-D System (S: 10 m3/100 m2
3.6.2.2 TD (Tank volume – Design return period) curve Figure 3.25 shows TP (Tank volume – Design return period) curves. If a sewer system is designed for a 2-year return period rainfall, a rainwater tank of 9 m3/100 m2 (Point A) is required to control a 100-year return period rainfall. Similarly, the sewer system can be safe even with a smaller tank volume of 7 m3/100 m2 (Point B) or 3 m3/100 m2 (Point C) if 5 or 20 infiltration boxes are integrated, respectively.
3.6.2.3 Groundwater recharge ratio
R-S-I-D systems can contribute to both flood mitigation and groundwater recharge. Groundwater recharge ratio (GRR) is
defined as the ratio of the total amount of water recharged to the ground to the total annual rainwater volume from the roof. Figure 3.26 shows the annual GRR from the R-S-I-D system in Seoul rainfall and assumed soil conditions and under different number of infiltration boxes. At the larger rainwater tank volume and higher infiltration volume, more groundwater will be recharged.
With the rainwater tank volume of 5 m3, the annual GRR of an R-S-I-D system is 77% (Point A) and 88% (Point B) (107.8 m3 and 122.5 m3 annual groundwater recharge) with the infiltration box of 5 and 20 boxes, respectively.
Figure 3.25 TD (Tank volume – Design period) curves for R-S-I-D system (using Seoul rainfall data and Huff method, normalized for 100 m2 catchment area).
3.6.2.4 ES (Emergency Storage)
Similar to previous systems, the R-S-I-D system can also provide ES to save rainwater for emergency cases. The water volume stored below the L.W.L can be used for emergency at any time.
Figure 3.26 Groundwater Recharge Ratio for R-S-I-D system (using Seoul daily rainfall data, normalized for 100 m2 catchment area).