Attenuation performance of runoff storage basins within a moderate to steep slope urban catchment in Cebu, Philippines.

In urban areas, runoff storage basins (RSBs) are common flood control measures. However, space utilization of these basins compete with other land use demands, thus it is necessary to evaluate the attenuation performance of these RSBs to justify their presence. In this study, the flood attenuation performance of RSBs which comprise a series of 3 mini dams (3-m high), a lagoon, and 19 small detention ponds within a small catchment in Cebu, Philippines was evaluated. The 3 mini dams intercept runoff water from developed portion of the catchment and release the water to a lagoon which in turn discharges the water to an open channel terminating to a street drainage culvert that conveys water to the wetland. The 19 small detention ponds are distributed to different locations within a subcatchment which is also tributary to the culvert. The catchment has an area of 3km2. It is urbanized and has moderate to steep slopes; 19% of which has a slope ranging from 15% to 30% and 43% of the catchment has slope greater than 30%. The catchment drains its runoff into a wetland. The HEC-GeoHMS extension of ArcGIS was utilized in delineating the catchment. The hydrologic properties were determined using soil type, land use, and land cover maps. The soil type map was obtained from the Department of Environment and Natural Resources (DENR). The land use and land cover maps were extracted from 0.5-m resolution Orthophoto and 1-m resolution LiDAR data provided by the Philippine Light Detection and Ranging (Phil-LiDAR) project. HEC-HMS was used in the rainfall-runoff simulation incorporating SCS loss, SCS unit hydrograph, and kinematic wave and Muskingum flow routing. Parameters were calibrated and validated using measured rainfall data and corresponding outflow hydrographs estimated from the time-stamped camera-captured changes in water level and depth-volume relation of the wetland. The calibration and validation yielded Nash-Sutcliffe Efficiency of 0.86 and 0.67, respectively. Existing RSBs attenuated 50% of flood peak at inlet of drainage culvert for 100-yr return period of 6-hr duration rainfall. [ABSTRACT FROM AUTHOR]