The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila 4.4 Design Flood Discharge With examining the alternatives of flood management facility plan, optimal design flood discharge allocation and its flood management plan is proposed. Since flood control works for 1/30 years flood have been conducted in Pasig and Lower Marikina River, design flood discharge of these sections shall be set considering the previous works. Design flood water levels will be checked with the design water level of the JICA Study. 4.4.1 Evaluation of Conditions for Pasig-Marikina River Channel Improvement Project (PMRCIP) (1) Design Flood Discharge for PMRCIP PMRCIP can be divided as the following 4 phases. Phase I : Detailed Design Works for Whole Section Phase II : River Improvement Works from River Mouth to Confluence of Napindan Channel Phase III : River Improvement Works from Confluence of Napindan Channel to Diversion of Manggahan Floodway without MCGS Phase IV : River Improvement Works from Diversion of Manggahan Floodway to Marikina Bridge with MCGS In the detailed design conducted in 2002, the design flood discharge allocation of 1/30 years flood was formulated as shown in Figure 4.29, and river improvement works and construction of MCGS was planned. Source: Preparatory Study for Pasig-Marikina Channel Improvement Project (Phase III) Figure 4.27 H-Q Curve Formulated in JICA M/P Study (2) Operation of NHCS The JICA Study planned that NHCS should be closed during flood to block inflow from Laguna Lake to Pasig River while the WB Study planned that NHCS should be open during flood to expect reverse flow from Pasig River to Laguna Lake. Advantages and disadvantages of operation of NHCS are summarized in Table 4.15. With the following reasons, it is judged as appropriate that NHCS shall be closed during flood to avoid uncertain phenomena in flood management plan. Final Report 4-30 The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila Diversion through Napindan Channel to Laguna Lake is uncertain since reverse flow will happen depending on water level in Laguna Lake. If NHCS would open during flood, channel improvement of Napindan Channel is inevitable to protect surrounding dense urbanized area resulting difficulty of land acquisitions. Table 4.15 Comparison of NHCS Operation Open during Flood Close during Flood Inflow from Laguna Lake to Pasig River can be Advantage Gate operation is not required. prevented. In case WL of Laguna Lake is higher than Pasig River, flood risk in the center of Metro Manila increases due to inflow from Laguna Lake to Pasig River. Diversion of flood discharge to Laguna Lake through Napindan Channel is uncertain depending of water levels of Laguna Lake and Pasig River. Disadvantage If diversion to Laguna Lake through Gate operation is required. Napindan Channel is included in the discharge allocation plan, discharge of Lower Marikina becomes large resulting increase of flood risk and heightening of dyke is required. Napindan Channel also requires improvement works such as dyke heightening. Evaluation Not Favorable Favorable Source: JICA Study Team (3) Necessity of MCGS The JICA Study planned to fully utilize Laguna Lake with sure diversion through Manggahan Floodway controlled by MCGS. On the other hand, the WB Study recommended that MCGS is not necessary and flood control is planned with diversion through Napindan Channel. Advantages and disadvantages of MCGS are summarized in Table 4.16. MCGS is necessary for sure diversion of design discharge to Laguna Lake. Besides, excess flood also can be diverted to Manggahan Floodway by MCGS resulting mitigation of flood risk at the center of Metro Manila. Without MCGS, re-improvement of channel downstream of Rosario Weir since HWL increases. Rise of HWL leads to increase of flood disaster potentials. Table 4.16 Comparison of With/Without MCGS Without MCGS With MCGS Sure diversion is available. Excess flood also can be diverted resulting Advantage WL of Upper Marikina decrease. mitigation of flood risk at the center of Metro Manila. Flood risk for excess flood increase. HWL in Pasig-Lower Marikina increases Gate operation is required. Disadvantage leading to increase of flood disaster WL in Upper Marikina rise. potentials Evaluation Not Favorable Favorable Source: JICA Study Team The basic specifications and operation rule of MCGS designed in the detailed design in 2002 are summarized in Table 4.17. Final Report 4-31 The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila Table 4.17 Comparison of With/Without MCGS Item Specifications Design Discharge 500m3/s (Lower Marikina River) Design HWL Upstream Side: EL. 17.40m Downstream Side: EL. 14.74m Design Section Riverbed Width: 43.5m Riverbed Elevation: EL.8.0ｍ Gate Dimensions: W20ｍ x H11ｍ x 2 units Type: Roller Gate Operation Rule MCGS Gate shall be operated together with Rosario Weir with monitoring of levels of Manila Bay, Laguna Lake and Sto.Nino for secure diversion of design discharge. Source: JICA Study Team Discharge allocations to Manggahan Floodway and Lower Marikina, and influence to downstream with/without MCGS is simulated with following conditions. Diversion volumes of the both case, 1/20 years flood discharge allocation, and longitudinal flow profile of it are summarized in Table 4.18, Figure 4.30 and 4.31, respectively. Design discharge at Sto.Nino is 2,900 m3/s which equivalent to 1/20 years flood with natural retarding function is applied. NHCS is closed not to divert to Laguna Lake through Napindan Channel. Manggahan Floodway has designed capacity without illegal residents in the river course. Without MCGS, the following disadvantage occurs. Design diversion discharge to Manggahan Floodway of 2,400 m3/s is not secured. Water levels in Pasig-Lower Marikina River rise higher than the design HWL about 1 m as maximum resulting increase of flood risk. RE-improvement such as heightening of dyke is required. Table 4.18 Diversion Discharge With/Without MCGS Marikina Lower Marikina Manggahan Floodway (Before Diversion) (After Marikina) Without MCGS 2,900 m3/s 1,000 m3/s 1,900 m3/s With MCGS 2,900 m3/s 500 m3/s 2,400 m3/s Source: JICA Study Team 2,900 2,900 Sto.Nino Sto.Nino MCGS San JuanRiver San San JuanRiverSan Marikina River Marikina River MarikinaRiver Pasig River 650 Pasig River (Lower) 2,900 Pasig River 650 Pasig River (Lower) RiverMarikina 2,900 1,600 1,000 1,000 1,000 1,200 600 500 500 Rosario Weir Rosario Weir Manila Bay Manila Manila Bay Manila P P NHCS P P NHCS 0 1,900 0 2,400 Channel Channel Napindan Floodway Mangahan Mangahan Napindan Floodway Without MCGS Mangahan With MCGS Laguna Lake Laguna Lake Source: JICA Study Team Figure 4.28 Discharge Allocation With/Without MCGS (1/20 years, with Natural Retarding Function) Final Report 4-32 The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila Source: JICA Study Team Figure 4.29 Flow Profile With/Without MCGS Final Report 4-33 The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila (4) Consideration of Urban Development along Marikina River At near Rosario Weir, urban development program is on-going by private developers as shown in Figure 4.32. Influence of this development is calculated based on the design drawings of development program. As shown in Figure 4.33, rise of water level due to the urban development is just 20 cm which can be adjusted in the Phase IV Project. Figure 4.30 H-Q Curve Formulated in WB Study Source: JICA Study Team Figure 4.31 Flow Profile of Influence of Urban Development Program (5) Evaluation of Probability of Design Flood Discharge by PMRCIP Based on the review of hydrological analysis referring the observed flood in recent years, probable flood discharge of 1/30 years return period is estimated at 3,100 m3/s which is 200 m3/s larger than the design discharge of 2,900 m3/s. Table 4.19 shows the probable discharges at Sto.Nino with natural retarding function. According to Table 4.19, the design flood discharge of PMRCIP of 2,900 m3/s is reevaluated as 1/20 years flood. Table 4.19 Reviewed Probable Discharge at Sto.Nino Return Period Sto.Nino (With Inundation) Return Period Sto.Nino (With Inundation) 2 1,620 30 3,030 5 2,290 50 3,220 10 2,670 100 3,580 20 2,860 Source: JICA Study Team Final Report 4-34 The Republic of the Philippines Data Collection Survey on Flood Management in Metro Manila 4.4.2 Flood Management Plan for 1/30 Years Flood (1) Conditions for Examination The following conditions were applied to examine the flood management plan for 1/30 years flood. Change of HWL at the section where PMRCIP has been conducted or on-going since the social infrastructures have been developed based on the designed HWL. Therefore, HWL at the section where PMRCIP has conducted including Phase III Section shall remain same. For Phase IV Section, minor change is considered acceptable. Diversion through Napindan Channel to Laguna Lake is uncertain since reverse flow will happen depending on water level in Laguna Lake. As discussed in Section 4.4.1, NHCS is closed during flood and inflow from Pasig River to Laguna Lake through Napindan Channel is not considered. MCGS is to be constructed. Flood management plan is examined for 1/30 years flood with natural retarding function as shown in Figure 4.34. Montalban Natural Retarding Basin Nangka River 3,100 Sto.Nino MCGS San JuanSan River Marikina River Pasig River 700 Pasig River (Lower) MarikinaRiver 3,100 1,300 600 500 500 Rosario Weir P P NHCS 0 2,600 Channel Napindan Floodway Mangahan Laguna Lake Source: JICA Study Team Figure 4.32 Discharge Allocation of 1/30 Years Flood (2) Alternatives of Flood Management Plan for 1/30 Flood Since the discharge allocation downstream of MCGS is already fixed, flood management plan can be considered upstream of MCGS such as following 2 alternatives.