Surface runoffs drain from the rolling slopes and high plains to six
creeks: Maytunas, Buhangin, Maysilo, Mayon, Buayang Bato and Ermitanio,
and eventually flow to San Juan and Pasig Rivers. (Refer to Map 2 & Map 3 Chapter 1).

To collect surface runoffs and carry it off to receiving water
bodies, the city developed a network of Reinforced Concrete Pipes with
sizes varying from 24ӯ, 30ӯ and 60ӯ and covered line canals lining
all city and barangay roads. This network connects to main drainage
facilities lining national roads. Details of the whole drainage network
serving the city are reflected in Map 61.

It is important to note that while all circulation roads are served
with drainage facilities, the map reflects only those with records
updated since 1986. Records of sizes and types of drainage facilities
installed prior to 1986 are not available at the moment, and so are
those along private subdivision roads which are maintained and operated
by the subdivision management.

All national projects including roads and drainage facilities are
implemented and maintained by the Department and Public Works and
Highways. Its counterpart at the local level is the City Engineer’s
Office which is responsible for all local roads and drainage facilities
in Mandaluyong. Both manual labor and mechanical equipment (vactor) are
applied to maintain these facilities functioning and free of
objectionable matters.

Flood prone areas in the city are generally those with lower
elevation serving as catchment areas to the uplands. These areas are at
random parts of the city given its natural topography where some points
are even lower than the Pasig River shoreline. Flash floods in Maysilo
Circle for instance, where the city hall is located, have become a
common sight during rainy seasons. Based on the Slope Map (See Map 2),
a spot height at Maysilo Circle fronting San Francisco Street is 2.0
meters while spot heights along Pasig River are at 3.0 meters. There
are, however, certain areas in higher elevations identified as flood
prone which point to the fact that other factors may have caused
flooding. (Refer to Vulnerability to Geohazards).

While the city may be adequately served with drainage facilities
considering the lengths and sizes involved, flooded areas and heavy
traffic in rainy days point to its inefficiency. For one, rubbish
unconscionably discarded on streets and other public places eventually
find its way to manholes and drainage pipes and canals, thereby clogging
the system. Worse, there are instances wherein worn-out household
appliances and dead animals are found in the system during de-clogging
operations.

Pasig River serves as the main outlet of Laguna Lake, Marikina and
San Juan Rivers, and other small tributaries. Siltation, sunken
derelicts, and other matters thrown into the river considerably lessen
its carrying capacity causing it to overflow especially on heavy rains
and stormy weather. The same goes with creeks which serve as sewer and
garbage dumps to those illegally occupying its shoreline.