By Lawrence R. Zamoras, Ph.D.
A Regional Special Order was issued on March 15, 2018 by DENR Regional Director Carlito M. Tuballa creating a Team to conduct investigation and assessment on the alleged quarrying activities along Salug Daku River in Mahayag, Zamboanga del Sur. The team is composed of representatives from the DENR-IX Enforcement Division, MGB-IX, EMB-IX, PENRO and CENRO. The conduct of this activity aims to address the Resolution of the Sangguniang Bayan of Mahayag Zamboanga del Sur on February 7, 2018 requesting the DENR-IX to cease the sand and gravel concessionaires operating along the Salug Daku River, which allegedly caused (a) erosion along riverbanks and (b) heavy siltation along downstream channel. This rapid field assessment was conducted on March 21, 2018 along Salug Daku River in Mahayag Zamboanga del Sur observing the conditions on both upstream and downstream sections using Drones and other field instruments. This area of interest is located at the eastern end of the Zamboanga Peninsula, along its boundary with the Misamis Occidental Province (Figure 1).
The following are the initial findings:
1. SALUG DAKU WATERSHED. The Salug Daku River has an extensive watershed covering the southwestern side of Mt Malindang and the southern side of Mt. Dapiak. It captures much of southern tributaries of the Municipality of Sergio Osmena (Zamboanga del Norte), northern drainages of Molave (Zamboanga del Sur), as well as most drainages of Don Victoriano Chiongbian (Misamis Occidental), Josefina (Zamboanga del Sur) and Mahayag (Zamboanga del Sur) (Figure 2). It has a dendritic drainage pattern with an area of around 36,000 hectares i.e., covering only the specific portion from the Mahayag Bridge up to its tributary headwaters. Its upstream tributaries in the Mt Malindang side represent the southwest section of the overall radial drainage pattern. Its eastern headwater is the 2,404-meter high Mt. Malindang, an inactive volcano whose slope is one of the two main contributors of the aggregate materials found along the Salug Daku River. The northwestern portion of the headwater covers the slopes of the 900-meter high Mt Dapiak which also includes the western side of Sergio Osmena Municipality. Their steep slopes with abundant sediments generated the high sedimentation rate along the Salug Daku River. And at the onset of this fast-developing Climate Change characterized by increasing rainfall, and typhoon paths moving down south to Mindanao, this huge watershed will generate bigger floods and trigger more erosion in the mountain slopes bringing more sediments or aggregate materials along river channels. The influx of these river materials will potentially enhance the geohazard susceptibility ratings for the Municipality of Mahayag.
2. NIA DAM CONSTRUCTION. The NIA Dam was constructed in 1967 along the Salug Daku River under the Salug Daku River Irrigation System (SARIS) project and was completed in 1970. The dam was built in Mahayag and intended to serve the municipalities of Mahayag, Molave, Tambulig, Ramon Magsaysay, Sominot and Bonifacio (Mis. Occ). It spans 110 meters across the river channel and has a spillway where water drops 3-meter high into the Downstream Apron and then passes through the Concrete Block before entering back into the river channel. Since the construction of the dam, the Salug Daku River has been dissected into two sections namely the upstream and the downstream sections. The dam was particularly built at the river bend such that it is oriented oblique with respect to the upstream river channel by an angle of about 40° from the perpendicular (Figure 3).
3. EFFECTS OF THE DAM AFTER 50 YEARS. The emplacement of the dam has significantly changed the dynamics of flowing water along Salug Daku River and has obstructed the transport of sediments from upstream to downstream, which leads to the following consequences:
a. Sand and gravel aggregates have accumulated especially in the first 3-kilometer upstream starting from the dam (Figures 4 & 5). Since most of these sediments cannot cross the dam, the upstream section will pile up and will eventually elevate the river channel to the level of the floodplain. Dredging of the dam water storage has been done in order to maintain the volume capacity of the dam. Once the capacity of the channel to hold water is reduced, flooding will become more frequent in the upstream section of the Salug Daku River. Furthermore, aggregates may pile up in the upstream section to dangerous levels which may eventually cause debris flow event during a typhoon and may potentially bury parts of Poblacion Mahayag.
b. Deposition of river aggregates stretches even as far as 20 kilometers upstream from the NIA Dam. Coarse-grained aggregate materials particularly cobbles and boulder size have accumulated along the upstream river channel for a stretch of roughly 14 kilometers from the NIA Dam to the river confluence in Brgy New Tangub (Sergio Osmena); after which, Salug Daku River splits into two upstream river tributaries similarly with abundant aggregate materials. The western tributary channel is roughly 7-kilometer long from Brgy New Tangub up to Brgy San Isidro (Sergio Osmena), while the eastern tributary is roughly 5-kilometer long from Brgy New Tangub up to Brgy San Juan (Sergio Osmena). River channels that are filled with aggregates have lower capacity to store water and have tendency to easily overflow during heavy rains and submerge surrounding farmlands.
c. The downstream section of Salug Daku River after the dam is deprived of new sediments specifically larger clast-size such as cobble and boulder due to the obstructive NIA Dam constructed in the 1970’s. Without this dam, the loss of aggregates that were naturally taken away by floodwater would have been replenished by new sediments arriving from upstream. Despite the fact that no quarrying has been allowed between the NIA dam and the Mahayag Bridge, and within the 1-kilometer downstream stretch of the river from the bridge as per Section 79 (a) of the DENR Administrative Order (DAO) No. 2010-21, the river channel continues to deepen. On the other hand, finer sediments such as pebble, granules, sand, silt and clay are carried by floodwater in suspension during heavy rainfall, thus, able to pass through the dam and cross into the downstream section of the river, leaving the cobble to boulder size aggregates behind trapped along the upstream.
d. The water flow crosses the dam with sudden drop in elevation of at least 5 meters from the crest spillway to the downstream riverbed. Such drop adds more energy to the floodwater that it scours the riverbed deeper and erodes the riverbank at the downstream side of the dam. This also results in undermining the Apron structure of the dam and foundation, which eventually leads to collapses around the edges of the dam structure (Figure 6). During flood events, the floodwater about 3 to 5 meters above normal water level will simply glide across this 5-meter drop across the dam. This entails a tremendous amount of energy hitting the riverbed below the DAM and dragging all the aggregates large and small downstream. Such process has been prevailing for about 50 years resulting in lowering of the riverbed by 2 meters and in some sections 3 meters. The overall result is gradual deepening of the river channel in the downstream section of the dam, while the shallowing prevails in the upstream section.
e. It is revealed that the riverbanks are also composed of unconsolidated well-rounded aggregates of sizes ranging from sand to boulder (Figure 7). These were deposited during the past thousands of years through flooding which transported eroded materials downstream and created the vast plains of Salug Valley. The surrounding rice fields and other agricultural lands are also likely underlain by such aggregate materials. If farmlands manifest presence of cobbles or boulders among the cultivated soil, it is likely because there is a thick layer of aggregate materials below the soil. The scouring of the riverbank occurs during flood events. When the riverbank is scoured, the cobbles and boulders retain in the area as aggregates, and partially replenish the lost aggregates that were taken by floodwaters.
f. Since the NIA dam was built at the bend (meander) of the Salug Daku River, the surge of floodwater is directed toward the outer side of the bend. This means that the tremendous energy of the floodwater that drops 5 meters down across the dam strikes toward the east embankment, which is the side of the Poblacion (Figure 8). This particular embankment will experience very high rate of erosion, whereas the opposite embankment will have deposition. During the STS Vinta, the flood level was about 5 meters above the normal water level of the river. Such huge volume of floodwater smashed against the east embankment and eroded about 2 meters width of ground from the embankment (Figures 9 & 10), and collapse of the Mahayag Bridge abutment and the bridge approach segment (Figure 11).
4. EFFECTS OF MEANDERING RIVER CHANNEL. The meandering channel of Salug Daku may be described as having the normal characteristics of a meandering river; riverbank erosion in the outer side of the meander and deposition in the inner side. Such character prevails when a large river channel crosses a vast flat. This is the reason why several houses in Brgy Tumapic were engulfed by the river when the riverbank collapsed during the STS Vinta flood. The overall effect to this is gradual migration of the channel from one point to another. For this reason, it is not safe to establish schools near a river channel, unless proper riverbank protection is emplaced. An example is the Tumapic Elementary School wherein several school classrooms fell together with the collapsed riverbank.
5. EFFECTS OF QUARRYING. In general, a well-managed sand and gravel quarry along a riverbed can help the river restore its capacity to accommodate flowing water. The Salug Daku River has high competent system that can carry cobble to boulder size in large amounts. Its headwater stretches up to the 2,400-meter high Mt Malindang and the 900-meter high Mt Dapiak, giving it ample supply of sediments to be carried by high volume of water. Since the emplacement of the NIA Dam however, the dynamics of the river have significantly changed in 50 years.
a. The potential effects of quarrying in the upstream side of the dam would be increasing the capacity of the dam to store water for the dry season, as well as increasing its capacity to contain floodwater during typhoons. Since the river itself has a high rate of sedimentation, then dredging, which is costly, is important for the dam to be remain functional. With a well-managed quarrying operation, the NIA can keep the dam desilted for free and the flood disaster mitigation of the municipality is implemented for free.
b. The potential consequence of not quarrying the upstream is accumulation of sediments along the river channel that it becomes too shallow to contain floodwater during heavy rainfall. Flooding would eventually become widespread in the upstream section of the river, and the NIA Dam will be filled up with aggregates.
c. The immediate downstream section of the NIA dam, within 1-kilometer downstream, is a high-energy environment wherein erosional processes prevail; therefore, any quarrying activity should be prohibited along this section. The obstruction of the coarser sediments (cobble to boulder) by the NIA Dam further aggravates the loss of aggregates in the area, resulting in deepening of this particular section of the river channel. Finer sediments such as pebbles, sand, silt and mud can still cross the NIA dam as these are transported by suspension during flood events. However, these finer materials may only accumulate along the river around 3 kilometers away downstream from the dam wherein the river starts to meander. In such case, aggregate quarrying in such section of the river may be operated. Whether or not there is quarrying in the downstream these river materials are bound to be carried away into the sea either by flood or by normal river flow.
6. EFFECTS OF STS VINTA. The volume of the floodwater brought by STS Vinta was so immense that it brought deadlier flooding in the municipalities of Zamboanga del Norte such as Salug, Labason, Gutalac, Baliguian, Siocon, Sirawai and Sibuco. Human casualties from these mentioned towns reached 101, with flood heights reaching up to 15 meters in Brgy Anonang, Sibuco, and flood duration for more than 12 hours particularly Siocon. In the Municipality of Mahayag, STS Vinta incurred the following effects:
a. Flooding in the floodplains of downstream Salug Daku River at about 1.5 meters high
b. The flood eroded as much as 5 meters wide strips of embankment along Salug River devouring several houses in Brgy San Isidro.
c. Destruction of Mahayag Bridge abutment and cutting-off of its bridge approach
d. Damages in some sections of the NIA Dam particularly in the Apron portion.
e. Lateral shifting and migration of the Salug River channel resulting in cutting-off of a meander segment in Barangay Tumapic area
- The Salug Daku River is a huge watershed covering an area of around 36,000 hectares with upstream tributaries characterized by high sediment loads. The transport of its aggregate materials downstream had been disrupted since the obstructive NIA dam structure was built in 1970. With the naturally huge volume of floodwater during heavy rainfall, the downstream side of the dam becomes highly erosional which will cause further deepening of that section of the river channel.
- Coarse-grained aggregate materials particularly cobbles and boulder size have accumulated along the upstream Salug River for a stretch of roughly 14 kilometers from the NIA Dam to the river confluence in Brgy New Tangub (Sergio Osmena); after which, Salug Daku River splits into two upstream river tributaries with combined lengths of 12 kilometers long, giving a total length of 26 kilometers of river channels having high amounts of sediments. River channels that are filled with aggregates have lower capacity to store water and have tendency to easily overflow during heavy rains and submerge surrounding farmlands.
- Fine-grained aggregate materials such as pebble, sand, silt and mud have accumulated along downstream Salug River and have similarly lowered the capacity of the channel to contain floodwater during heavy rainfall to cause widespread flooding in barangays Tumapic, Upper Sto Nino and Lower Sto Nino.
- The downstream Salug Daku River has a meandering channel causing a natural tendency to laterally shift its course through gradual riverbank erosion, making the areas near the channel susceptible to mass wasting (landslide).
- The Resolution No. 255-2018 issued by the Sangguniang Bayan of Mahayag to cease all quarrying activities along Salug Daku River can result in further accumulation of aggregate material upstream as well as downstream. In this fast-developing Climate Change where rainfalls are becoming heavier, and typhoon paths shift south down to Zamboanga Peninsula, more sediments will be mobilized from the slopes of Mt Malindang and Mt. Dapiak, and get transported down to the segment approaching the NIA Dam (upstream side). The volume of accumulated aggregate materials may reach a certain level wherein a stronger flood can trigger a debris flow (like lahar) that may potentially bury some areas of the Poblacion. In the downstream section, the effect of the same Resolution can cause the lower Salug River to become progressively shallower, which will result in frequent flooding in the low-lying barangays incurring losses in agriculture.
- Construction of a concrete levee along the east embankment connecting the NIA Dam and the Mahayag Bridge is needed in order to prevent the river channel from shifting course toward the Poblacion, and to protect the approach section of the Mahayag Bridge. Its design must be strong enough to withstand the tremendous impact of the floodwater descending directly from the NIA Dam.
- Concrete levee should also be constructed in other areas with schools and built-up structures particularly along the eastern embankment. Such infrastructure prevents a particular segment of Salug Daku River from migrating through riverbank scouring toward populated areas.
- Dredging of the Salug Daku River in both upstream and downstream sections should be implemented to mitigate the effects of flooding during typhoons; this should be integrated in the Local Climate Change Action Plan (LCCAP) of Mahayag. However, to implement this disaster prevention measure, it needs a huge budget which may be too costly for the Municipality of Mahayag. The other option is resume quarrying of the sand and gravel in areas along the river channel following the terms and conditions stipulated in Section 79 of the DENR Administrative Order (DAO) No. 2010-21, the Consolidated Implementing Rules and Regulations of RA 7942, the Philippine Mining Act of 1995. The availability of this aggregate deposit will be very helpful in the addressing the needs of the local residents of Mahayag and surrounding municipalities for their construction purposes. This abundant resource of aggregate materials will also be needed in government projects such as the widening of the National Highways into four concrete lanes and the construction of school buildings. Lifting of the quarry ban along the Salug Daku River will help address the shortage of aggregate materials and will certainly harmonize with the Build Build Build program of the government.