Photo from simulated result in HEC-RAS

Figure 3.09. Land Use Map in Barangay Mambangnan, San Leonardo with Flooding

Photo from simulated result in HEC-RAS

Figure 3.10. Land Use Map in Barangay Nieves, San Leonardo with Flooding

Furthermore, the affected area in Barangay Mambangnan (see in Figure 3.09) and

Barangay Nieves (see in Figure 3.10) are also only annual crops. Therefore, the residential or

commercial area are unlikely to be affected by the overflowing of Peñaranda River in this current

44
year of 2024. Unless, new structures are built near the Peñaranda River, then further study is

necessary to conduct in this area. Hence, the use of this flood hazard map may give an importance

to the residents to show on which area is safe when constructing a house or a commercial space.

Since most of the barangays are affected by the overflowing of river through the damage

of crops, then the losses in the municipality of San Leonardo are mostly involved with the farmers

or the agricultural sector. In this case, as stated by Galvez (2013), that when Typhoon Santi hits in

the Philippines, the fourth congressional district had received the biggest crop damage in Nueva

Ecija. This fourth congressional district includes Gapan City, towns in Cabiao, General Tinio,

Jaen, Peñaranda, San Antonio, San Isidro, and San Leonardo accounting to a crop damage of

P741.8 million. Furthermore, as shown in Figure 3.11, are the annual crops found alongside the

Peñaranda River that are washed away during the river flooding. Moreover, many and various

crops are found in this area which are the sole livelihood of some people in this area. Resulting to

difficult recovery of the residents after the typhoon and overflowing of Peñaranda River.

Figure 3.11. Annual Crops alongside of the Peñaranda River

Therefore, the consideration for the flood control mitigation in Barangay Mallorca,

Barangay San Bartolome, and Barangay Mambangan held an importance for the agricultural

45
sector. However, they are second to the priority as it has no risk in terms of property damage and

human lives.

Photo from simulated result in HEC-RAS

Figure 3.12. Land Use Map in Barangay Castellano, San Leonardo with Flooding

Figure 3.13. Purok 7B in Barangay Castellano, San Leonardo

On the other hand, the residential area, commercial area, and agricultural area are affected

in Barangay Castellano due to the overflowing of Peñaranda River as shown in Figure 3.12.

Moreover, as stated earlier on the interview, that there are many people living near the river (see

46
in Figure 3.13) and are endangered during the typhoon season. This concludes the importance of

the construction of flood mitigation structure in Barangay Castellano.

Photo from simulated result in HEC-RAS

Figure 3.14. Levee along Barangay Castellano

Photo from simulated result in HEC-RAS

Figure 3.15. Levee along Barangay Castellano and some part of Barangay Nieves

47
 Hence, as shown in Figure 3.14 is a flood control structure, specifically a levee is

constructed along the Barangay Castellano, however there are still a few inundations in a few areas

because of the flood water coming from Barangay Nieves. Therefore, the dike is stretched out in

a part of Barangay Nieves (see in Figure 3.15), to stop the overflowing in those area and to

completely stop the flood in the affected residential area and commercial area in the Barangay

Castellano.

Therefore, according to the simulation of flood in a rainfall intensity of 24-hr frequency

storm that is generated in 25-year return period, it shows that the placement of flood control

structure must start in a coordinate of 15°18′52.452″N 120°55′24.858″E, and ends with a

coordinate of 15°20′12.642″N 120°57′47.916″E. Moreover, this flood control structure must have

a total length of 6-km, starting from Barangay Castellano to some part of Barangay Nieves.

Effect of Existing Flood Control Structure in the Municipality of San Leonardo

Flood control structures are constructed to reduce the area of inundation on floodplains,

reduce flood stage, and reduce flood duration. An example of flood control structures are dikes,

levees, drainage canals and floodways. (Benito, 2013)

In addition to that, there is a positive and negative impact on the construction of flood

control structures. In this case, a widely used structure to protect low-lying, coastal and river side

areas from the inundation of this bodies of water under extreme conditions are dikes or levees.

These structures have a high volume which helps to resists water pressure, sloping sides to reduce

wave loadings and crest heights sufficient to prevent overtopping by flood waters. (Linham and

Nicholls, 2010) This way, the dikes or levees have been extensively utilized as flood defenses to

protect the low-lying areas against inundation.

48
 However, a study is conducted assessing the construction of dike in Vietnamese Mekong

Delta showing that there is an increased flood risk downstream in the Vietnamese Mekong Delta.

Since, the expanded high dike construction in the upper Mekong Delta had a large hydraulic impact

of an increase of floodwater level for up to +68 cm, and further high dike construction can increase

an additional of +100 cm of floodwater level. Hence, the construction of dike has affected the

flood levels and distribution thus, affecting the downstream region. (Duc Tran, 2018)

Which may be true in the case on the construction of flood mitigation structure along the

Peñaranda River at the side of Gapan City that may redirect the water in the Municipality of San

Leonardo instead. This flood mitigation structure (see in Figure 3.16) is a levee with a two-berm,

concrete slope protection structure on steel sheet pile foundations, which are engineering

interventions that are designed to mitigate the risks of riverbank collapse and water overflow,

preventing threats and damage to properties. (PIAIII, 2023)

Photo from Punto

Figure 3.16. Flood Control Structure in Gapan City

Moreover, in comparison of the overflowing of Peñaranda River, as shown in Figure 3.19;

which is the overlapping simulated flood result of Hydrologic Engineering Center's River Analysis

49
System (HEC-RAS) of the overflowing of Peñaranda River without flood control structure (see in

Figure 3.17) and wih flood control structure (see in Figure 3.18). It shows that there is a difference

in the covered flooded area, which proves that there is an effect on the construction of flood control

structure in San Nicholas to Pambuan, Gapan City.

Photo from simulated result in HEC-RAS

Figure 3.17. Overflowing of Peñaranda River without Flood Control Structure

Photo from simulated result in HEC-RAS

Figure 3.18. Overflowing of Peñaranda River with Flood Control Structure

50
 Photo from simulated result in HEC-RAS

Figure 3.19. Overflowing of Peñaranda River with and without Flood Control Structure

5
Depth (m)

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Duration (mins)

With Dike Without Dike

Figure 3.20. Overflowing Water Depth Comparison in Peñaranda River of With and Without

Flood Control Structure in Gapan City

51
 Furthermore, according to the simulation of flood in HEC-RAS, it shows that there is a

different level of depth of the overflowing water in the Peñaranda River. As shown in Figure 3.20,

we can see that the Peñaranda River with levee have started overflowing after 17 minutes and 26

seconds, in contrast to the Peñaranda River without levee that had only started overflowing after

17 minutes and 32 seconds. Moreover, the maximum flood depth in the Peñaranda River with

levee is 5.782 m at 18 minutes and 54 seconds, while the Peñaranda River without levee is 5.65 m

at 19 minutes. Hence, this proves that there is an increase in the flood depth and the time it takes

for flood to occur.

Therefore, with the construction of flood mitigation structure in Gapan City, there was a

redirection of water that creates a larger flood risk area in the Municipality of San Leonardo.

Moreover, due to the uneven distribution of water, there was an increase of flood depth by 0.132

m which increases the flood risk in the Municipality of San Leonardo. Hence, the proposal of new

flood mitigation measure at the flood risk area is recommended in this study.

Proposal of New Flood Mitigation Measures

There are various ways to optimize flood protection and reduce flood damages along the

river. This includes dike/levee, widening of river, dredging, dam, retarding basin, dam, revetment,

spur dike, groundsill, and sabo works. This flood control measures are based on it purpose of

construction, as shown in the category found in the Table 3.02 of the “Manual on Flood Control

Planning” by the Japan International Cooperation Agency (JICA).

According the Manual on Flood Control Planning, that there are two categories related to

design the safety level against flood and the design flood frequency. This involves, the increase in

river flow capacity and the reduction/control of the peak discharge of flood. Wherein, as shown in

the Table 3.2, it includes the construction of dike/levee, widening of waterway/rive,

52
dredging/excavation, dam, retarding basin, and floodway. However, which of this following stated

is more convenient and efficient to construct?

Table 3.02. Flood Control Measures (Table 5.1, Manual on Flood Control Planning – Japan
International Cooperation Agency)
No. Category Facility/Measure
1 Increase of river flow capacity Dike/Levee
Widening of waterway/river
Dredging/Excavation
Combination of above
2 Reduction/control of the peak discharge of Dam
flood Retarding Basin
Floodway
3 Prevention of bank collapse Revetment
Spur dike
Change of waterway/cut-off
channel
4 Prevention of riverbed degradation Groundsill
5 Prevention of obstruction against river flow Sabo works (for sediment control)
and/or maintain/conserve the good condition Regular maintenance (channel
of the river to keep the flow uninterrupted excavation/ dredging)

Table 3.03. Dam and Reservoir Potential Hazard & Risk Classifications (PHRC) Designation
System (Table 3.1.2, General Guidelines and Criteria for Planning, Design, Construction,
Operation, and Maintenance of Reservoir Dams – National Irrigation Administration)
PHRC DESIGNATION & RATING
Risk PHRC-1 PHRC-2 PHRC-3
(Low) (Significant/Moderate) (High/Extreme)
*(H2.V1/2) (H2.V1/2) < 25.0 25.0<(H2.V1/2)< 275.0 (H2.V1/2)>275.0
Life Safety (Lives in Loss of human life is
None/Loss of human Loss of human life is
Jeopardy) inevitable. (More
life is unlikely and possible (1-Family or
Risk/Loss of Life than 5-Persons or >1-
not expected 5-Persons)
Family)
Economic Risk Low Moderate/Significant High/Extreme
Environmental Risk Low Moderate/Significant High/Extreme
Social Disruption Low (Local/Rural
Regional National
Risk Area)
*(H2.V1/2) - Shall be one of the Criterion for Potential Hazard & Risk Classification (Ref.#22)

53
 When it comes to controlling the river flow, dams have the capacity to hold different

amounts of floodwater and precisely regulate when the water releases. This lessens how big of an

impact flood will have (International Hydropower Association, 2023). However, dams can also

have socio-economic impacts, including displacement of communities, loss of livelihoods, and

changes to local cultures and traditions. Dams on the other hand is prone to water stagnation, which

can result in issues like the growth of dangerous algae and the deterioration of water quality, is

avoided by constant flow (WWF, n.d.). Given the elevation of the natural ground line along the

river banks of the Peñaranda River, building a dam will require a much larger dike and levee

around it which may result to a much expensive infrastructure project.

Furthermore, based on the Dam and Reservoir Potential Hazard & Risk Classifications

(PHRC) Designation System of the National Irrigation Administration as shown in the Table 3.03,

considering the number of population and commercial establishments in the nearby Barangays,

building a dam in Peñaranda River along the communities of Castellano, Pambuan, and San

Nicolas can be qualified as Potential Hazard and Risk Classification or PHRC-3 (National

Irrigation Administration, 2019). This shows that building a dam in the said location possess a

high risk for the communities nearby.

For several reasons, constructing dikes and levees is a better choice than creating a

retarding basin. First, a river's flow capacity can be increased by using dikes and levees, enabling

it to carry more water and lowering the danger of flooding. Retarding basin takes time to fill and

might not be able to stop unexpected flash floods. A retarding basin only offers restricted

protection while dikes and levees can be constructed all the way up a river, protecting a wider area.

As shown on the map, the Municipality of San Leonardo, specifically the surrounding of

Peñaranda River is surrounded by farmlands which therefore, building dikes and levees can have

54
less impact on the surrounding ecosystem than building a retarding basin. Keeping an eye out for

clogged or blocked openings is one of the most crucial maintenance tasks for a retarding basin.

The basins will operate correctly only if the pipelines are kept free of debris. It can be more

expensive to maintain than to maintain dikes and levees (Leber, 2015).

Considering floodways on the other hand, just like retarding basins, dikes have lower

maintenance costs compared to floodways. This is because floodways may require regular

dredging to remove sediment and debris, while dikes mainly require inspection and repair of any

damage (Department of Public Works and Highways; Japan International Cooperation Agency,

2002). Water displaced by floodways could result in more flooding in regions close to the protected

area. In a study conducted by the University of Bristo (2017) concluded that the economic and

long-term benefits of building dikes to reduce flood damage far outweigh their initial cost.

Moreover, the cost of building a dike or a dam can vary significantly based on the data,

including the project's size and location, the surrounding environment, and the materials being

used. A dam's construction costs include several elements, including labor, materials, design, and

other associated costs in addition to the cost of obtaining the property. A dam can be built for

approximately $0.27 billion (₱15 billion) to $3.6 billion (₱200 billion) per kilometer, and its annual

operating and maintenance expenditures can vary from $0.6 million (₱34 million) to $22 million

(₱1.2 billion) (Aerts, 2018). Levees and Dikes on the other hand can be less expensive than

building a dam, especially when taking the structure's length into account. Most dams are shorter

than 0.5 miles (804 meters), while building levees and dikes are frequently done simultaneously

over many meters (Pile Buck International, 2019).

Therefore, the construction of dike/levee is chosen as a flood mitigation measure to be

constructed in Barangay Castellano to some part of Barangay Nieves in the Municipality of San

55
Leonardo. Furthermore, by the definition of Britannica, the dikes protect the land that would

naturally be underwater most of the time. Meanwhile, Levee protect the land that is normally dry

but may be flooded when rain raises the water level in the body of water, such as a river. Hence,

the proposed flood mitigation structure to protect the residential and commercial areas of Barangay

Castellano is the construction of Levees.

Procedure of Designing Levees

The location of the dike is determined by factors such as proximity to the area requiring

flood protection, land availability, and foundation conditions. The land surrounding the Peñaranda

River around the Brgy. Castellano has 500 meters to 1 kilometer offset from the riverbank to the

residential areas. Thus, having this enough space in constructing a levee is enough without being

an inconvenience to the residence of Brgy, Castellano. The height and geometric configuration of

the dike are generally dictated by the containment requirements, availability of construction

materials, and prevailing foundation conditions.

The material selection for the dike is crucial. The dike is typically made of compacted earth

and can be protected by reinforced concrete. Embankment and backfill must not have muck, peat,

sod, roots, or other deleterious matter. Steel sheet pile shall meet the requirements of American

Association of State Highway and Transportation Officials (AASHTO) M 202. The steel sheet pile

used in this plan is an SKSP-II. SKSP sheet piles are suitable for temperate climate. SKSP type

ⅠA, Ⅱ, IIA, Ⅲ, ⅢA, Ⅳ, ⅤL, ⅥL sheet piling is most cost effective in situations when a piled

foundation is required to support the dike or where speed of construction is critical. Supports

formed from SKSP sheet piling can act as both foundation and support and can be driven in a

single operation, requiring a minimum of space and time for construction. Shown in Table 3.4 is

the characteristic of SKSP-II.

56
 Table 3.04. Characteristic of SKSP-II steel sheet pile

Dimensions Per Pile Per Linear Meter of Wall

Section Moment Moment
Sectional Section Sectional Section
Type Width Height Thickness
Area
Weight of
Modulus Area
Weight of
Modulus
Inertia Inertia
Mm Mm Mm Cm2 Kg/m 4cm 3cm Cm2/m Kg/m 4cm/m 3cm/m
874
SKSP-II 400 100 10.5 61.18 48 1240 152 153 120 8740

Stable excavation below the existing ground surface is highly dependent on foundation soil

and groundwater conditions. For most soil conditions with limited seepage, unsupported

excavation side slopes can typically be developed at 1.5 Horizontal to 1 Vertical (1.5H:1V) or

flatter. Shown below is the computation of side slope. Average elevations are based from the

DPWH dike plan on Brgy. Pambuan, San Lorenzo, and San Nicolas where Ave. Natural Grade

Line is 45,000mm, deepest river bed is 40,000mm, maximum water level is 53,640mm, adding a

1-meter freeboard providing a design elevation of 54,640mm.

𝑆𝑙𝑜𝑝𝑒 𝑐𝑜𝑚𝑝𝑢𝑡𝑎𝑡𝑖𝑜𝑛:

54,640𝑚𝑚 − 45,000𝑚𝑚 = 9,640𝑚𝑚

9,640𝑚𝑚
= 4,820𝑚𝑚
2

𝑎2 + 𝑏 2 = 𝑐 2

4,8202 + 1.5(4,820)2 = 𝑐 2

𝐶 = 8,589𝑚𝑚

Therefore, the height of the dike from the natural grade line is 9.64 meters and the slope

length is 8.59 meters. These are key parameters in the structural design of the dike. Hence, these

measurements ensure that the dike can effectively protect the area from flooding while maintaining

its stability.

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 CHAPTER IV

Summary, Conclusion and Recommendation

Summary

Flood occurs due to several reasons, this includes heavy rainfall, overflowing of river,

inefficient flood mitigation control, etc. These flood mitigation controls are either structural or

non-structural that reduce the area of inundation on the floodplains.

One example of a non-structural flood mitigation control is a flood hazard map. This map

indicates the intensity of flood hazard in its covered area. Thus, through this, it helps the residents

where they can safely construct a structure. Moreover, the flood hazard map usually shows that

there is a high intensity of flood risk along the river. This is due to the occurrence of river flooding,

or the overflowing of river in the surrounding land.

Whereas, this is the case in the Municipality of San Leonardo due to the overflowing of

Peñaranda River. According to the interviews on Barangay Officials on the Municipality of San

Leonardo, we found out that the barangays near the river includes Barangay Mallorca, Barangay

San Bartolome, Barangay Mambangnan, Barangay Nieves, and Barangay Castellano. Moreover,

they stated that the community of Mallorca, San Bartolome, Mambangnan, and Nieves are not

affected by the overflowing of river. However, on the interview with the Barangay Secretary of

Castellano, she stated that the Purok 1, Purok 5, Purok 7A and Purok 7B in Barangay Castellano

are all inundated by the river during the typhoon season. In addition to that, the researchers also

conducted an interview to the residents on this area, and they found out that most residents in the

low-lying areas experience a one-storey depth of flood. Hence, proven the intense flood hazard

risk in the residents of Barangay Castellano.

63
 Therefore, the researchers also conducted an assessment on the behavior of the river

flooding, especially in Barangay Castellano to identify how flood affects the residents. In this case,

a software program was used to simulate flood which is known as the Hydrologic Engineering

Center River’s Analysis System (HEC-RAS). To use the software, it needed to input the frequency

storm which is derived from the rain intensity duration frequency (RIDF). With the use of

frequency storm, the researchers can identify the intensity of rainfall in the covered area and the

probable amount of rain that can be collected per return period.

However, in this study the researchers used the 24-hr hypothetical storm with a one minute

computation interval for a 4% Annual Exceedance Probability (AEP). With this, they simulated

the flood and it shows that the residents living in the low-lying areas of Barangay Castellano are

indeed inundated by the river.

Moreover, there are two conditions made to identify the cause of the river's overflowing.

First, is the runoff water from the mountains of General Tinio, and then the runoff water with the

occurrence of rain in San Leonardo. And as a result, that even though it did not include the rainfall

in San Leonardo, the overflowing of river still occurs. Hence, the cause of the river flooding is the

runoff water from the mountainous region of General Tinio.

Furthermore, the researchers identify the losses per barangay during the inundation of

Peñaranda River. And the result shows that 4 out 5 barangay that are beside the river are only

affected by the damage of crops; this includes the barangay of Mallorca, San Bartolome,

Mambangnan, and Nieves, whereas, the residents living in this area have a wide setbacks from the

river, thus the probability of inundation due to the river are low. However, there are some residents

in Barangay Castellano that are living in low-lying areas near the river, or in the high intensity

flood risk areas.

64
 Thus, in this study the researchers recommend the placement of flood mitigation structure

along Barangay Castellano and some part of Barangay Nieves. They also extended the flood

mitigation structure to Barangay Nieves, because the simulation shows that there are still

inundation in Castellano due to the runoff from Nieves.

As defined, flood mitigation structures are the structural form of flood mitigation control.

This includes the construction of dikes, levees, spillways, dams, etc. In this case, the existing flood

mitigation structure in the Peñaranda River is along the Gapan City, which is constructed from

Barangay San Nicholas to Barangay Pambuan. And it is found out in the simulation, that the flood

mitigation structure, also known as levee, had affected the Municipality of San Leonardo due to

the increase of flood depth and the increase on the time it takes for overflowing to occur.

Therefore, due to the high risk of the residents living in the low-lying areas of San

Leonardo, this study proposes a construction of levee from Barangay Castellano and to some part

of Barangay Nieves.

In this case, constructing dikes and levees is a better choice than creating dams for several

reasons. Dams can control river flow by holding and regulating the release of floodwater, reducing

the impact of floods. However, dams can have socio-economic impacts, including community

displacement, livelihood loss, and cultural changes. Building a dam along the Peñaranda River

would require a larger dike and levee due to the natural ground line elevation, leading to a more

expensive project. The dam could pose a high risk to nearby communities of Castellano, Pambuan,

and San Nicolas.

Moreover, constructing dikes and levees can increase a river's flow capacity, reducing

flood risk. They offer wider area protection compared to the localized protection of a retarding

65
basin. Building dikes and levees around the Peñaranda River, surrounded by farmlands, would

have less impact on the ecosystem. They also have lower maintenance costs compared to retarding

basins, which require regular unclogging of orifices and can be more expensive to maintain.

Floodways on the other hand could cause more flooding in nearby areas due to water displacement.

The land surrounding the Peñaranda River around Brgy. Castellano has a 500-meters to 1

kilometer offset from the riverbank to the residential areas, providing ample space for dike

construction without inconveniencing the residents. The steel sheet pile used in this plan is an

SKSP-II, which meets the requirements of Association of State Highway and Transportation

Officials (AASHTO) M 202. Supports formed from SKSP sheet piling can act as both foundation

and support and can be driven in a single operation, requiring minimal space and time for

construction. Side slopes are developed at 1.5 Horizontal to 1 Vertical (1.5H:1V) or flatter. A 1-

meter freeboard is added, providing a design elevation of 54,640mm. In conclusion, the height of

the dike from the natural grade line (9.64 meters) and the slope length (8.59 meters) are key

parameters in the structural design of the dike. These measurements ensure that the dike can

effectively protect the area from flooding while maintaining its stability.

Conclusion

The occurrence of the overflowing Peñaranda River had raised concerns to the safety of

the residents living in the low-lying areas. Hence, in this study the researchers conducted a survey

interview to the residents of the Municipality of San Leonardo, and simulated different conditions

of the overflowing of Peñaranda River. Thus, our result shows that:

1. Through field inspections and interviews, the researchers were able to prove that there are

residents living in the high-risk flood hazard area, and they commonly experience a one-

storey depth of flood.

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 2. The cause of the overflowing of Peñaranda River is due to the large amount of runoff from

the mountainous region of General Tinio.

3. Barangay Castellano is the most at risk area due to the people living near the river, while

other barangays, including Mallorca, San Bartolome, Mambangnan and Nieves

experiences damage of crops.

4. The construction of Levee along the Barangay San Nicholas to Barangay Pambuan, has a

significant effect of increase in flood depth and in the time, it takes for overflowing to

occur. Thus, increasing the risk of inundation in the Municipality of San Leonardo.

5. Construction of Levee from Barangay Castellano to some part of Barangay Nieves is

recommended according to the study we had conducted.

Recommendation

Continuous development of the Municipality of San Leonardo is inevitable, due to this

some data may be invalid and may require another study. In addition to that is for the safety of the

residents in the Municipality of San Leonardo, the researchers recommend further studies for the

following:

1. An Assessment on the Impact of the Construction of Levee in Gapan City, if further

development continuous.

2. An Analysis on the Proposed Construction of Levee in the Municipality of San Leonardo,

Nueva Ecija.

3. An Assessment on the Impact of the Construction of Levee in the Municipality of San

Leonardo, Nueva Ecija.

67