Infiltration well and urban drainage concept

Transcription

1 Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHSPubl.no. 222, Infiltration well and urban drainage concept S. SUNJOTO Faculty of Engineering, Gadjah Mada University, Yogyakarta 55198, Indonesia Abstract Generally speaking, what is meant by urban drainage is to allow runoff water flowing from drainage areas to a trench or a canal, then to a river and finally to the sea. Consequently the decrease of groundwater recharge in the urban areas is proportional to the increase of the roof and pavement area. In this paper the infiltration idea will be introduced. It is designed to facilitate the temporary storage and infiltration of precipitation which falls on the roof or pavement. A method of analysis to determine the minimum dimension of infiltration well and to get a maximum quantity of water infiltrated is proposed, using a new formula of calculation derived mathematically. This formula is based on Darcy's law concerning the unsteady state radial flow and that the measurement of the infiltration well is related to the roof or pavement area, intensity of rainfall for certain period, hydraulic conductivity and shape factor of the bottom of the well. INTRODUCTION Existing drainage concept The existing drainage concept applied so far can be called "pull and push" which means palling water from the surface to prevent ponding water, by using drainage networks which pushes water to the river and eventually to the sea. The concept is applied to both problems: surface drainage and subsurface drainage, and it results in the abandonment of water resources and is against environmental principles. Water balance in Indonesia According to the findings by many researchers, the water table level of most big cities is decreasing. In Jakarta, for instance, the capital of Republic of Indonesia, the surface of confined groundwater decreases between m during the last century, and the élévation of unconfined groundwater surface decreases about 0.50 m a" 1. The rate of population growth in Java and Madura islands (both are in Indonesia) is so high, that Department of Public Works (1984) predicted that both islands will have a deficit of 50% in the year of 2000 because the water necessity is one and half times the available water. According to Time Magazine (November, 1990), the precious resource of world water is becoming scarcer due to the population growth and development. Both have depleted and polluted the world's water supply, raising the risk of starvation, epidemic amd even war.

2 528 S. Sunjoto Prevention To prevent the above problem, general disciplines intend to improve water balance deficit by increasing dependable flow, reutilisation, terracing and other technical related means. There is in Indonesia a traditional technique to conserve surface water which that becomes groundwater by using the concept of an infiltration well. Sunjoto (1988) proposed a formula to calculate an optimum dimension of the infiltration well. INFILTRATION WELL The idea is based on the concept of natural water balance as apart of hydrological cycle. When the ground surface is covered fully with vegetation and is permeable, some precipitation will then infiltrate naturally and the rest as runoff flows along the ground surfaces. But now that the world's population is growing which results in the increasing needs of housing, manufacturing, transportation facilities etc., much of ground area is covered by impervious layers such a roofs and pavements. This layers prohibit water infiltration process and consequently there will be more runoff water than that infiltrated and stored as groundwater. It is the infiltration well that will infiltrate precipitation in spite of the impervious layers over the ground surface. This technique allows water to get in to the well which functions as a temporary storage facility, as well as to infiltrate, to become groundwater. PARAMETERS OF CALCULATION An appropriate method of calculation using relevant parameters is needed in order to get a suitable design. In other words, the dimension of an infiltration well depends on: (a) Surface coverage area: a surface coverage area is an area where precipitation falls and flows to the infiltration well. Usually this coverage is roof or pavement. (b) Intensity: intensity is a value of the depth of precipitation which is calculated statistically, and gives different value for different geography, duration, and frequency of precipitation. (c) Hydraulic conductivity: Sunjoto's formula based on the Darcy's law, that is the discharge of the infiltration flow depends on coefficient hydraulic conductivity. (d) Dominant duration: dominant duration of precipitation is the most frequent occurrence of precipitation in certain region. It is used to determine the value of intensity by a curve of duration against intensity. The formula proposed is an unsteady state radial flow, it means that the height of water calculated is function of time, and in this case time is equal to dominant duration. (e) Shape factor: shape factor is a value of capability of the well in relation with the layer of soil to transmit a water from the well to the ground. The porous wall of the well will give a bigger value of shape factor. (f) Others: the others parameters to check the final result are time lag of precipitation, groundwater surface and existing wells distribution (infiltration and dug wells).

3 Infiltration well and urban drainage concept 529 FORMULA Sunjoto (1988) had derived a formula of calculation of infiltration well which based on Darcy's law. This formula is an unsteady state radial flow which had derived mathematically from the dynamic equilibrium of hydraulic flow, as follows: where: H = F = Q = T = K = R = Water # ^ _ _ 1 - exp( ) water depth in the well (m), the shape factor (m), (Table 1), water discharge from the roof or pavement (m 3 s" 1 ), the flow duration (dominant duration) (s), hydraulic conductivity (m s" 1 ), the radius of the well (m). discharge Q is calculated by using the Rational Formula: Q = CIA where: C = runoff coefficient of the roof or pavement, / = intensity of precipitation (ms" 1 ), A = area of the roof or pavement (m 2 ). Before to determine the final dimension of the well, the calculated water depth in the well has to be controlled by using time lag data of precipitation, the unconfined groundwater table level and finally multi well systems if there are many wells. CONSTRUCTION Ijrfiltration wells is like a dug well only of different function. A dug well keeps a water to be drawn but an infiltration well is to conserve water. The well should have a brick wall or be of concrete pipe, or as such. The inner space of the well should be left empty as to be able to store water before it infiltrates. The bottom of the well is covered with broken rocks and gravels to prevent the layer from being eroded by the water fall. The bottom should have a porous layer in order to have minimum dimension. The upper part of the well is covered with concrete slab and soil put above the slab. Here one can grow flowers for better landscaping and sturdier construction (Fig. 1). ADVANTAGES The advantages of this system are: Ca) a quantity of unconfined groundwater can be conserved; Cb) the surface level of unconfined groundwater stays stable; (c) the area of ponding water is minimized; C d) the dimension of drainage networks is minimized;

4 530 S. Sunjoto Table 1 Value of shape factors. mm Shape factor, F ( m ) References Sam3ioe (1931)' Aravin (1965) 3&. fi^ll )z;;uc<=r;:ïir 2R -# Samsioe (1931)* Aravin (1965) Forchheimer (1930)" Aravin (1965! Harza (1935)* Taylor (1948)* Hvorslev (1951)* lui m ln( L/R + /(L/R! T jl t 2/3 B) ln((l+2r)/r t * /(L/R)' + 1 3*-2R->l& 1! $1 ft* f?^!*-2r-dl :h lia jik* ln(l/2r */; /(L/2R)' + 1) 2 IT (L 2/3 R) ln((l+2r)/2r,- ^ /(L/2R)' + 1) TT (h + h ) O w + /(h.'/a) 2 r l. I m Source : " Olson 4 Daniel (1931) ln(2(d+2r)/r + /(2D/R) 2 + 1) (e) groundwater quality is improved; (f) land subsidence is prevented; (g) salted water intrusion in coast areas is avoided.

5 Infiltration well and urban drainage concept 531 House with gutter at the eaves House with gutter at the ground Fig. 1 Schema of infiltration well construction. CONCLUSION TTiere are a lot of techniques to conserve water. But using an infiltration well for urban djrainage concept is considered appropriate, because has much advantages. The dimension of the well can be calculated by Sunjoto's formula (1988) with the relevant stiape factor.

6 532 S. Sunjoto REFERENCES Aravin, V. I. & Numerov, S. N. (1965) Theory of Fluid Flow in Undeformable Porous Media. Translated from Russian, Jerusalem. Dachler, R. (1936) Grundwasserstromung. Julius Springier, Vienna. Darcy, H. (1856) Histoire des Fontaines Publiques de Dijon. Dalmont, Paris. Department of Public Works of Republic of Indonesia (1984) Available water and housing in the year of 2000 (Proc. Symp. PSLH-ITB, Bandung). Forchheimer, P. (1930) Hydraulik, 3rd edn. Teubner, Leipzig. Harza, L. F. (1935) Trans. ASCE100, Hvorslev, M. J. (1951) Time lag and soil permeability. Groundwater Observation. Bull. no. 36, Waterways Experiment Station, Vicksburg, Mississippi. Olson, R. E. & Daniel, D. E. (1981) Measurement of hydraulic conductivity of fine grained soils, Permeability and groundwater contaminant transport. ASTM, STP 746. Samsioe, A. F. (1931) Z. fur angewandtemathematik undmechanik 11, Sunjoto, S. (1988) Optimization of infiltration well to avoid salted water intrusion in coast areas. Proc. Sent. IUC-UGM, Yogyakarta. Sunjoto, S. (1989) The development of groundwater hydraulic model. IUC-UGM, Yogyakarta. Sunjoto, S. (1993)Sustainableurbandrainage.Proc./nrf. Conf. onenvironmentalmanagement, Geo-Water and Engineering Aspects (Wollongong, 8-11 February Taylor, D. W. (1948) Fundamentals of Soil Mechanics. New York.