Study on the Effectiveness of Infiltration Wells to Reduce Excess Surface Run Off In ITB

Institut Teknologi Bandung (ITB), Ganesha Campus, Indonesia, has an area of 28.86 hectares. The campus is located in Bandung. Starting from 2012, new buildings were constructed within the area, reducing the area of permeable surface significantly. In the past few years, there were several excess run off incidents in the campus. The insufficient area of permeable surface as well as the inadequate capacity of the drainage system contributes to the excess surface run off. The drainage system has only two outlets. Moreover, in some areas, the drainage systems are disconnected. Thus, most the surface run off are stored within the drainage system. The purpose of this study is to evaluate the effectiveness of infiltration wells for reducing the local excess run off in ITB. Precipitation data and drained service area are used to estimate the design discharge from each building in ITB. In order to avoid the excess surface run off of certain locations in ITB, then the infiltration wells are proposed to balance the area of impermeable surface. The effectiveness of the infiltration wells are evaluated by assessing their number to their contribution in reducing the excess surface runs off.


Introduction
Precipitation in Bandung area has increased in the past few years. The rainfall intensity in the last 25 years ascends by 10-20 mm every year [1].Based on Regional Regulation No. 26 of 2007 on Green Area (Ruang Terbuka Hijau/RTH),  from 661.4 thousands hectare area of Citarum River Basin, 198 thousands is supposed to be reserved area. In accordance with Regional Regulation No. 2 of 2016 on controlling the utilization of North Bandung Area (Kawasan Bandung Utara/KBU), the local government -tries to protect KBU by limiting physical development to only 20% of land area. Area of KBU Bandung is 38 thousands, but 80% have been intervened by constructions. The increase in industrial activities eventually increases the degree of groundwater withdrawal from the aquifers in Bandung [2]. Thus, most the surface run off are stored within the drainage system. Furthermore, land use and land cover change affect hydrological components such as interception, infiltration and evaporation. They influence the soil moisture content, runoff generation (both process and volume) and stream flow regimes [3]. Previous surfaces decrease the amount of runoff as precipitation infiltrates [4]. These also affect the groundwater table [5].
As well as the city of Bandung, new buildings have been constructed within the area of ITB. These building significantly reduce the permeable area. Soil type in the area is mostly clay up to 6 meters depth, which has a slow infiltration rate. The construction was not followed by increasing the capacity of the drainage system. Thus, excess runoff occurs quite frequently during the last few years in a few internodes such as Labtek V,VI,VII,VIII, Aula Timur and Lapangan Seni Rupa.
One of the solutions to reduce excess surface run off is infiltration wells. Infiltration wells are structures that highly allow surface runoff to drain through underground [6]. Infiltration well was initiated in Indonesia by Sunjoto [7] and developed by some other researchers [6,[8][9][10]. The benefits of infiltration wells are a quantity of unconfined groundwater can be conserved, the surface level of unconfined ground water stays stable, the area of ponding water is minimized, and the dimension of drainage networks is minimized. In addition, the groundwater quality is improved, land subsidence is prevented, and salt water intrusion in coastal area is avoided [11]. The disadvantage of infiltration wells is the infiltration takes time depending on the permeability. The purpose of this study is to evaluate the affectivity of infiltration wells in order to reduce the excess surface run off in ITB.

Methodology
The results in this paper are based on two well-

Hydrology
Hydrology analysis was conducted based on the daily maximum precipitation. The maximum precipitation in a return period can be estimated through 4 distribution methods such as normal, log normal, log pearson III and gumbel 12 . The methods concepts are fitted to probability concepts. SNI T-02-02-2006 states that the local drainage system design is based on a 5 years return period.

Standard Design for Infiltration Wells
SNI 03-2453-2002 states the general requirements of infiltration wells are as follows: 1. Infiltration wells are placed relatively in flat contour; 2. The water entering the catchment is unpolluted rainwater; 3. Decision of infiltration wells shall take into safety building; 4. It must pay attention to local regulations; 5. Things that do not meet these requirements shall be approved by authorized agencies. Technical requirements that must be included are in the following: 1. To ground water minimum 1.5 meter in the rainy season; 2. The usable soil structure should have soil permeability ≥ 2.0 cm / hour. The shape and type of the infiltration well can be either an infiltration wells made in quadrilateral or cylindrical with a certain depth and the bottom of the well is located above the groundwater level. Based on the Regional Disaster Management Agency of Jakarta, types of infiltration wells construction are: 1. Wells without couple walls, bottom without filled stone or fibers (empty). 2. Wells without couple walls, bottom filled with stones and fibers 3. Wells with brick, stone or stone compartments on the wells wall, bottom filled with stones and fibers or empty 4. Wells use blawong (rock formed specifically for wells wall).

Topography and Impermeable Surface
Topography map data with a 50 cm contour interval is provided to continue the study. ITB has a relatively mild slope, almost flat, with the north is higher than the south. The map of constructed area in ITB is also given to by Infrastructure Facilities Division of ITB. Most of areas in ITB are impermeable surface, such as building, pavement area (parking, plaza, and corridor) and roadway thus ITB has more impermeable surface. The total building area in ITB is 99,733 m 2 . Open space are garden, median, and island. ITB has less of infiltration area and the excess surface run off would drastically increase at the time of heavy precipitation.

Drainage Systems
The existing drainage systems were analyzed based on the drainage network data as shown in figure 1. In addition, field survey was conducted to assess the flow direction. In general, the condition of existing drainage in ITB: (1) The drainage systems are disconnected in some places, (2) The drainage system has 2 outlet points but the remaining functional outlet is Dago. Furthermore, the drainages in several points were clogged as shown in figure 1. In some places, the channels dimensions were relatively small and might be in insufficient during a heavy rain. Additionally, the flow directions in some sections were not heading to any outlet causing excess surface run off within the area.

Fig. 2. Soil Characteristics in ITB
The characteristic of soil in ITB depend on the depth of the soil (Soil investigation FTTM Building, 2012). Between 0-1.5 meter depth is silty clay with dark brownish, medium stiff and high plasticity. This kind of soil takes time to infiltrate water [12]. The ground water table reached >20 m. The coefficients permeability of silty clay is 0.48 (m 3 /m 2 /day) or 1.3 x 10-4 m/sec.

Hydrology Analysis
The data are ITB station and Dago Pakar Station in 06ͦ 52' 51'' South Latitude and 107ͦ 36' 52'' East Longitude. Maximum daily precipitation design for the area is calculated using arithmetic mean method. The average maximum daily precipitation data of both stations are shown in the table 1.  The design precipitation was statistically analyzed using the Normal, Log Normal, Log Pearson III, and Gumbel methods. The area design is the area of ITB which is 28.865 ha. According to SNI T-02-2006, the design precipitation return periods are 5, 10, 15, 20 year. In this paper, the return period used is 5 years because the scope of the area is university. Based on the results, the smallest error is Log Normal Method with a 5 years return period daily precipitation of 98.15 mm.

Infiltration Wells Capacity
The design of infiltration wells has 1 meter diameter with the depth of 10 meter and infiltration efficiency is 100%. Soil clay has coefficient permeability 0.48 m3/m2/day and course sand has 0.864 m 3 /m 2 /day.

Surface run off volume (VAB)
For every 500 m 2 of service area, the surface run off volume is given as.

Storage volume after precipitation (VSTO)
The infiltration wells have 1 meter diameter and 10 meter depth along the vertical permeability (Kv) is calculated as below:

Kv= 3.57 m 3 /m 2 hr
The infiltration wells is not impermeable, thus the water could flow as an infiltration or percolation (flowing through 2 axes). The horizontal permeability (Kh) is twice the vertical permeability, so the value is 7.13 m 3 /m 2 /day.
The precipitation duration (Tc) is the time for the precipitation to prolong. 3

. Infiltration Wells Total
The total of infiltration wells is depends on storage volume.
For 10 meter depth of infiltration wells, with a service area of 500 m2, the number of infiltration wells needed is: Total wells = H/Hdesign=27.79/10 ≈ 3 wells (16)

Infiltration Wells Application for ITB
If the totals of infiltration wells are designed according to roof area per building, with the total roof area 99,733 m 2 , ITB is supposed to have 770 wells with the total area of wells is 604.757 m 2 . The number of infiltration wells per building is given in table 2.

Table 2. Number of infiltration wells per building
Nb. n1 is number of wells and n2 is number of wells round up.
Infiltration wells are expected to reduce the amount of water that flow from the outlets to the roadside public drainage. This means that ITB run off contribution to the city drainage system will be reduced significantly.
Infiltration wells are prioritized in the area with known history of excess surface run off such as Aula Timur, Lapangan Seni Rupa and Aula Barat. The wells can either be an infiltration well, connected to the drainage system (drainage well), or not (RTH well). The locations are shown in figure 3.

Conclusions
In this study, the effectiveness of infiltration wells to reduce the excess local run off in ITB has been assessed. The ground water table is located 20 meter depth and in general, the soil type is clay thus the infiltration wells may not be the best answer to the excess run off problem for ITB.
Dimension of the infiltration well is 1 meter diameter and 10 meter depth. It was found that for a service area of 500 m 2 , 3 wells are needed. Therefore, to retain 100%

No Building
Area (m 2 ) n1 n2 Due to the large number of wells needed, it is recommended to construct these wells only in several locations, at internodes that have high excess run off. In addition, it is recommended to develop a new master plaster plan for the ITB drainage system.