A Rapid Assessment along the Kabasalan River, Barangay Sanghanan, Municipality of Kabasalan, Zamboanga Sibugay

By Lawrence R. Zamoras, PhD.

A multi-sectoral team was created to conduct investigation and assessment of Kabasalan River and its tributaries through a Regional Special Order from the DENR OIC-Regional Director Carlito Tuballa. It was attended by representatives from DENR-IX Office, MGB-IX, EMB-IX, PENRO, CENRO, Provincial LGU, Municipal LGU, PNP, Civil Society and Religious Group. This activity began with a brief meeting on Feb 20, 2018 at the Sangguniang Bayan Hall of the Kabasalan Municipal Hall. The discussion focused on the issues concerning the Kabasalan River watershed such as the alleged illegal logging in the watershed area and quarrying activities along the Kabasalan River. The short meeting was followed by a short field visit in Purok 4, Barangay Sanghanan. The following are our field findings:

Climate Classification.The Kabasalan River catchment basin (watershed) covers an area of at least 22,000 hectares including most of northern section of the municipality up to its boundary with Godod, according to Forester German M. Romano of the Watershed Management Unit – DENR IX. The area lies near the boundary between Type 3 and Type 4 of the Climate Classification of the Philippines (PAGASA, 1992). Type 3 is characterized by “No very pronounced rain period with a dry season lasting only from one to three months, either during the period from March to May”, whereas Type 4 has “rainfall that is more or less evenly distributed throughout the year but without dry season”. This implies that this area may experience periods of either Type 3 or Type 4 conditions. However, since the mountainous topography of the Kabasalan River watershed is contiguous with the mountainous terrain of the Type 4 climate in the Zamboanga Peninsula, the Kabasalan River watershed may tend to lean towards the Type 4. In such case, the rainwater recharge is generally continuous, and the groundwater flow is presumed dynamic all year round.

Geology. The watershed area is generally underlain by a thick volcaniclastic sequence which has moderate to good permeability. Being under the Type 4 Climate zone, such underlying good aquifer has high capacity in storing groundwater.

Sand and Gravel Deposit. Along the downstream Kabasalan River there is abundant supply of sand, gravel and boulder which originated from the northern mountains and transported downstream by its normal flow, and in larger volumes by periodic flooding events. These alluvial materials accumulate along the riverbed and form floodplains beside the river channel. The Purok 4 grounds, in particular, is a floodplain area underlain by unconsolidated alluvial deposits, which are prone to disintegration and erosion. Its nearby riverbank may easily topple and erode upon impact of floodwater resulting in the natural widening of the river channel. Thus, widening of the channel may be both due to quarrying and natural erosion.

Figure 1. Modified Coronas Climate classification map of the Philippines with the Kabasalan area indicated in red box (PAGASA, 1992).

Quarry Operators. There are two (2) Industrial Sand and Gravel (ISAG) and six (6) Commercial Sand and Gravel (CSAG) operators duly permitted by the Provincial Governor’s Office, Province of Zamboanga Sibugay. The use of heavy equipment such as trucks and backhoe has been observed along the Kabasalan River channel by the ISAG operators, which is allowed under the ISAG permit.

Widening of the River Channel. The river channel has widened from about 25 meters wide in 2003, to about 40 meters in 2015 based on Google Earth Satellite Image, and to about 60 meters wide as per recent visit; such rate of increase is allegedly due to excessive sand and gravel extraction. No data are available regarding increase in depth to date. The Kabasalan River quarry area is reportedly a major source of sand and gravel needed in the ongoing widening and concreting of the Maharlika Highway (AH-26).

Photo 1. Widened Kabasalan River channel from around 25 meters in 2003 to about 60 meters at present. Its bared embankment shows unconsolidated sand and gravel containing occasional tree trunks, indicating that these were deposited during its past flooding events; the adjacent coconut farm across the river is underlain by alluvial deposit, which also implies that it is integral part of the river system. [Looking southwest/ downstream direction] (7°50’ 51.73”, 122°47’54.5”)

Flood Susceptibility. The communities of Puroks 3 and 4, Brgy Sanghanan are situated along the narrow floodplain of Kabasalan River, which are categorized under High Susceptibility to Flooding. During the STS Vinta, Brgy Sanghanan experienced a relatively lower flood level than those areas in and around Poblacion Kabasalan. The widening and deepening of the Kabasalan River channel in Brgy Sanghanan has increased its volume capacity to contain floodwater, thus lessened the impact of flooding in the area during the STS Vinta. In contrast, the downstream section of Kabasalan River with generally silted riverbed and relatively narrower width, maintains relatively smaller volume capacity to contain floodwater, causing it to overflow during STS Vinta and submerged Brgy Banker and other barangays. The flood level was high causing numerous damages of properties and some casualties.

Riverbank Erosion. The Purok 4 is situated in a confluence area of two creeks, namely Sim Dako and Sim Gamay, and with the Kabasalan River, thus, considered under highly susceptible to flood-related riverbank erosion. During intense rainfall event, the area faces surging floodwaters from three drainage systems, which implies convergence of erosional forces. The most vulnerable infrastructure is the Hanging Bridge across the converged Sim creeks as the northern base/ abutment is now surrounded by unstable embankments (Photo 3, 4; Figure 1). Its chapel area in upper Purok 4 is situated on an unstable ground bounded by the Sim Gamay creek in the east, and the Kabasalan River in the west (Figure1). The west-advancing meander of the Sim Gamay will erode the creek’s west embankment to eventually cut a new direct entry channel into Kabasalan River, and in the process, will transform the grounds surrounding the chapel into a river channel. It is therefore not advisable to use the chapel as evacuation area. Instead, the area should be evacuated upon the onset of an intense flooding event. The agricultural land of Purok 4 across the Kabasalan River is also composed of unconsolidated alluvial deposits, which may be erodible during intense flooding or as the river changes its course.

Figure 2. A Google Satellite Image 2015 showing Purok 4 Barangay Sanghanan: (a) Section of Kabasalan River that has widened and deepened due to quarrying, (b) the two tributary creeks [Sim Gamay and Sim Dako] that converges at Confluence 2 [encircled orange] in front of the hanging bridge; (c) Upper Purok 4 that is bounded by advancing erosion of the meandering Sim Gamay Creek on the right side and by the more erosive Kabasalan River on the left side.
Photo 2. Kabasalan River channel with bared solid riverbed exposed after sand and gravel extraction. [Looking northeast/ upstream direction] (7°51’ 0.73”, 122°48’ 3.06”)
Photo 3. Confluence area between Sim Gamay and Sim Dako, just beside the 1- year old pedestrian hanging bridge. The erosion of the riverbank threatens the foundations of this infrastructure. [Looking northeast] (7°50’ 47”, 122°47’54”)
Photo 4. Confluence area between Kabasalan River and Sim creeks, near the 1- year old pedestrian hanging bridge. The erosion of the riverbank threatens the foundations of this infrastructure. The other side of the bridge is referred to as upper Purok 4. [Looking northwest] (7°50’ 47”, 122°47’54”)
Photo 5. Along the meander section of Sim Gamay creek, the flow of floodwater tends to flow straight (as indicated by red arrows) and erode the bounding embankment. Such trend will prevail in each flooding event until riverbank erosion advances toward the cluster of houses of upper Purok 4, Barangay Sanghanan. [Looking northeast] (7°50’ 50.23”, 122°47’55.46”)
Photo 6. The advancing riverbank erosion along Sim Gamay creek exposes the Upper Purok 4 subsurface materials being composed of unconsolidated sand and gravel. Its embankment at the right side shows active erosion which intensifies during flood and advances toward the cluster of houses in upper Purok 4. This erosion is also directed toward the base of the hanging bridge. [Looking south] (7°50’ 50.23”, 122°47’55.46”)

RECOMMENDATIONS:

  1. Monitoring of the ISAG and CSAG quarrying activities along the Kabasalan River should be closely maintained by the Provincial Government which issued the permits to address the environmental concerns of the local residents of Kabasalan and the Social Action Ministry of the Catholic Church.
  2. The Hanging Bridge in Purok 4, Barangay Sanghanan is situated on a confluence area characterized by high rate of erosion particularly the riverbanks of the two Sim tributaries. Riverbank erosion is progressing toward the base/ foundation of the Hanging Bridge, thus, close monitoring should be maintained to ensure safety of its passers-by. Since it was built in a highly unstable area, any structural/engineering protective measures will be too costly than the pedestrian hanging bridge, thus, NOT recommended.
  3. The upper Purok 4 of Barangay Sanghanan is situated on a narrow area sandwiched between two river systems. The area itself is already highly susceptible to flooding and yet its advancing riverbank erosion is further directed toward the upper Purok 4 area. With these observation, close monitoring is advised to the local residents. Any alarming developments on the riverbank erosion should be reported directly to the LDRRMC of Kabasalan. In such condition, preemptive evacuation must be properly implemented for upcoming weather events.
Figure 2. Geohazard map for Barangay Sanghanan, Kabasalan showing High Susceptibility to Flooding along areas beside the Kabasalan River.

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