Water, an essential resource for all life on earth, is the prime mover and is essentially an integral part of every development. Water resources planning and development covers protection, storage, and water distribution. Therefore, events that alter and suppress water supply are of great concern to the water resource system and should be addressed overtime. Natural disasters like earthquakes and volcanic eruptions, though indirectly concerned on climatological variables of the water cycle, may pose an integral damage in the water resource system. Notwithstanding the destruction caused by a volcanic eruption in particular, the deposition of volcanic materials on the river systems and watersheds becomes indispensable and is a great challenge to the water resources engineers.
The devastating eruption of Mt. Pinatubo in 1991 that impacts the country’s society, natural resources, and the global environment continued after more than two decades burying the two irrigation systems of Tarlac province, the Tarlac River Irrigation System (TARRIS) and the San Miguel-O’Donnell River Irrigation System (SMORIS). The head water basin of the O’Donnell River was not spared from such disaster, depositing voluminous lahar in the channel bed of the major stream. The low impounding reservoir of the Armenia Dam located at the outlet of the O’Donnell River basin serving for irrigation and flood mitigation purposes becomes totally buried in lahar.
In parallel with the national thrust on resiliency, the National Irrigation Administration aimed to continue and improve irrigation services amidst the destruction. The developments on the appurtenant structures of the dam were aimed at improving the serviced area for irrigation despite the loss of the impounding reservoir. One of the NIA’s rehabilitation project on SMORIS is the construction of an infiltration gallery to collect water on the buried small impounding reservoir through horizontal perforated pipes leading to a collecting gallery passing the dam intake.
Infiltration galleries have a wide variety of applications in water resources management such as recharging confined and unconfined aquifers, a water-diversion option for surface water, a water treatment option for improving the potability of water, reduction of generated surface runoff and flooding. Studies on the analysis of the infiltration gallery for its effectiveness on these specified applications were based on field investigations using test pits. This study, therefore, focuses on developing a procedure for analyzing the infiltration gallery using groundwater flow (GWF) model in consideration to the draining surface water using flow duration curves (FDC).
The major objective of this study is to determine the reliability of available water that can be extracted through a horizontal infiltration gallery at the Armenia Dam along O’Donnell River. The reliability analysis is based on dependable flow analysis (i.e., flow duration curves) of surface water inflows to the infiltration gallery and subsequently the extracted water. Essentially, this study is to make analytical and numerical GWF model of the system under the present conditions of the riverbed and the horizontal infiltration gallery. This research will prove the following hypotheses: - The reliability of infiltration galleries for water supply can be effectively analyzed through ground water flow models using flow duration curves of the draining surface water ; - The estimated flux at the horizontal infiltration gallery using analytical and numerical modeling does not vary significantly.
The FDC has gained acceptance and credibility by researchers and hydrologists in various water resources problems and applications. This paper provided a different perspective in constructing the FDC for ungauged and partially gauged basins as employed in most of the referenced studies using techniques of synthesizing FDC and regionalizing it for small and large catchments. In this study, the historical streamflow data required to construct the FDC was derived through rainfall-runoff modelling in HEC-HMS using 39 years of rainfall data. The GIS data and ArcMap software geoprocessing provided greater accuracy in the loss and transform parameter input of the model which was calibrated using partial streamflow data that is within the rainfall record. The calibrated model was then used to simulate the 39 years of streamflow describing the full range of runoff response characteristics at the basin outlet.
The continuous simulated streamflow used in FDC offers a better diagnostic tool for assessing the flow on the infiltration gallery. The flow at 60%, 80% and 90% exceedance probability were used in defining the boundary conditions of the GWF model to analyze the flow conditions at the horizontal infiltration gallery. The analytical solution is a two dimensional steady-state flux distribution at the horizontal perforated pipes using Fourier integral as presented by Liongson, 2007. The numerical analysis for the head and flux distribution is a finite difference method as implemented in MODFLOW 6.0 of the USGS. The graphical user interface of MODFLOW, Model Muse, was used in discretization and flow simulation within the infiltration gallery. The validity of the results was verified from the comparative analysis of the analytical and numerical solutions.
The rainfall-runoff model calibration indicates a good fit of the observed and simulated flow data with an index quantified by PWRMSE of 57.49. The model overestimates the observed flow by only 0.06% and underestimates the observed peak flow by 23.3%. This indicates that, relative to the magnitude of flow, the model is unbiased at 0.91% and the random errors in the prediction are small as quantified by the RMSE standard deviation of 0.3. Results revealed that O’Donnell River is capable of sustaining a flow of approximately 6.7 m3/sec seventy percent (60%) of the time, 3.2 m3/sec eighty percent (80%) of the time and 2.1 m3/sec ninety percent (90%) of the time.