Application of Bio-pore Infiltration Hole as an Urban Runoff Management

The application of bio-pore infiltration holes (BIH) can be one solution for urban runoff management by reducing surface runoff to the ground. But, the difference in soil types and characteristics could affect the runoff reduction that can be achieved by BIH. This research aims to determine the runoff reduction can be achieved by bio-pore infiltration hole (BIH) from different soil types and conditions. The methods in this study mainly focus on hydraulic conductivity calculations using Porchet method and the implementation of Minister of Environment Regulation Numb. 12/2009 for the BIH installations. Based on the implementation of Minister of Environment Regulation Numb. 12/2009, the required BIHs for the area of 500 m2 are 1,000, both for silt and clay soils. The runoff reductions that can be achieved with the application of BIHs are 38.98 95.73% for silt soils and 20.67 54.28% for clay soils, depends on the soil conditions. Keywords―Bio-Pore Infiltration Hole, Runoff Reduction, Soil Types.

I. INTRODUCTION 1 Urban areas developments that caused by the development of population, can potentially threaten natural dynamics, resource availability and environmental quality [1]. The decreased pervious areas in urban areas result in increased surface runoff and potentially caused flood and inundation [2]. In general, drainage is the most feasible and economical solution for managing surface runoff by diverting it as quickly as possible [3]. However, the conventional drainage concept is less effective to use in the long term because it has to be gradually expanded over time and requires a large amount of money, as well as designs that pay less attention to water quality [4]. Thus, a new concept of the drainage system is introduced called a sustainable urban drainage system (SUDS) [5]. This system is utilizing some portion of urban landscapes like vegetated land surfaces to replicate the natural hydrological cycle process [6]. The purposes of this system is to encourage infiltration of stormwater to the ground, filtering the pollutants from source, and also temporarily storing water [7].
Bio-pore infiltration holes (BIH) is a one of a kind of SUDS that can be one solution for urban runoff management by reducing surface runoff to the ground, and 1  it is widely implemented because the required costs for the installations are affordable [8]. Nevertheless, the soil types and characteristics in an area are not exactly same as other location, even though it is in a city [9], and this means the permeability of soils are also different from one and the other. Thus, the application of BIH may have various results in terms of runoff reduction. The main objective of this study is to determine the runoff reduction can be achieved by BIH application from different soil types and conditions.

A. Sustainable Urban Drainage System
Sustainable urban drainage system (SUDS) is promoted as the main focus in urban development, especially for the water resource management in cities by using urban landscapes to provide spatial amenities and have ecological functions that facilitate hydrological processes [6]. It is because water quality has become an important key for the design of urban drainage, as a result of a wider political recognition of sustainability [5].
The aims of this concept on runoff or stormwater management mainly to reduce the quantity of runoff through source control and to slow the velocity of runoff. It also can be used to improve the quality of stormwater by providing passive treatment, and to enhance amenity and maintain biodiversity [4].

B. Bio-pore Infiltration Hole
Bio-pore infiltration holes (BIH) is one of the concepts for water conservation (rainwater harvesting) by storing some portion of surface runoff or stormwater to the ground, especially in the raining season [10]. This concept can be called as a part of sustainable urban drainage system mainly because it has the same aim, to reduce the quantity of runoff [8].
BIH has some set of criteria, mainly it is preferred to be installed in the settlement, park, parking area, around the tree(s), or in the area where the surface runoff flows through. It is created as a cylindric form in the ground with a diameter of 10 cm and the depth of 100 cm or not overlapping with the groundwater level. The gap between BIHs is also set between 50 -100 cm. Sometimes, it needs to be strengthened by using a casing made from PVC pipe to prevent the collapse inside the hole [10].

A. Preparation Stage
In this stage, it consists initial data collection, field surveys, soil compacting, and creating BIH. The required data for this study were the soil type map, rainfall height and intensity of Surabaya City from their respective city departments. Those data are used as the basis for surveying and determining some locations at Surabaya that will be used as field tests, especially for BIH tests. The chosen locations for this study are Kenjeran Beach Amusement Park and Lempung Urban Forest, Surabaya, that can be seen in Figure 1. Some soil were sampled from those locations to be tested in laboratorium to determine their texture soil class. Each of the locations will be divided into 4 test plots, where the conditioning on each plot described in Table 1.
Four out of eight areas will be compacted by stamper with durations around 30 seconds based on Gregory et al. [11]. Then, BIH will be constructed in every test plots by boring it using auger hand bore with the sizes of the hole about 10 cm of diameter and 100 cm of depth. After that, that hole will be covered using a pored casing pipe with the same size as the hole.

B. Research Stage
This stage comprising the infiltration rate and hydraulic conductivity measurements along with calculations of hydraulic conductivity, BIH flowrate, rainfall heights and durations, and runoff coefficients from plot tests. Infiltration rate measurement conducted at every test plots by using single-ring infiltrometer. Hydraulic conductivity measured by using inverse auger hole method [12], [13], with some small modification, which single-ring infiltrometer and ruler are used to measure surface water level changes instead of using measurement tape. Water will be added to the system and maintained in the same water level after it changes in several minutes. Measurement will be stopped if changes in the surface water level of the system remain the same after the last three tries.
The calculation of hydraulic conductivity conducted by using Porchet method from all gathered data measurements in the field. The equation for calculating hydraulic conductivity is as follows: can be modified to determining the water flowrate into the hole as follows: where Q is the BIH's water flowrate; K is hydraulic conductivity; hBIH is the depth of BIH; h0 is the initial surface water level; ht is the constant surface water level; t is time; and D is the diameter of BIH.
From the calculations of hydraulic conductivity, it will be compared to the permeability classes from Natural Resource Conservation Service (NRCS), USA [14]. From these comparisons, the ability of the bio-pore infiltration holes to infiltrate and percolate the water from surface can be determined.
Meteorological data such as rainfall height in last 10 years and rainfall intensity in last 13 months gathered from nearest weather station, are used to determine the average value of each data and to calculate runoff coefficient from each test plot along with infiltration rate measurement on the field of study and depression loss values from UDFCD (Urban Drainage Flood Control District, USA) [15]. Based on Guo and Urbonas, the calculation of runoff coefficient is done by using some equations [16] which can be reviewed below: 1. The equation for calculating the rainfall volume is as follows: 2. The equation for calculating the runoff volume is as follows: 3. The equation for calculating the runoff coefficient is as follows: 4. The Equation 5 above can be modified to calculate the runoff volume as follows: where VP is rainfall volume; VR is runoff volume; P is the rainfall height; DL is the depression losses; F is the infiltration height; A is the area of assumed watershed; and C is the runoff coefficient.

C. Analysis Stage
In this analysis, the analysis of runoff reduction from different soil types and conditions is conducted with the calculation of BIH application in the assumed area of 2,000 m 2 which consists of 75% impervious area and the rest is the pervious area. The impervious area is assumed to have a runoff coefficient value of 0.83 [17], and it has a sloppy surface that lean towards to the pervious area. Also, the pervious areas will be adjusted to the characteristics of plot tests. This analysis also includes the implementation of BIH installation based on Minister of Environment Regulation Number 12/2009. In this regulation, it states about the distance required between bio-pores, so it is assumed that the maximum number of BIH can be installed every 1 m 2 area is two pieces.
The equation that can be used to calculate the remaining surface runoff after the application of BIHs is as follows: As for the calculation of the percentage of runoff reduction that can be achived with the application of BIHs, it can be done with using the Equation 8 as follows: where VR rem is the volume of remaining surface runoff; VR in is the volume of initial surface runoff; VR BIH is the volume of bio-pore infiltration hole; and A is the area of the assumed watershed.

A. Soil Texture of Test Fields
In this study, there are two sites to be tested, namely Kenjeran Beach Amusement Park and Lempung Urban Forest. These two sites are located on different types of soil based on the map of Surabaya's soil type, where the Kenjeran Beach Amusement Park belongs to the hydromorphic alluvial type, while in the Lempung Urban Forest area it belongs to the type of gray alluvial soil.
In reality, the soil conditions in both locations are landfilled soil, where the landfill covers the original soil surface. The soil in the Kenjeran Beach Amusement Park is a combination of silt loam-textured soil on its surface with padas pile at the bottom, while in the Lempung Urban Forest the clay is in the form of native land originating from the excavation of boozem (detention pond) around the forest. For the particle composition of the soil sampled can be seen in Table 2.
Based on Table 2, from the results of laboratory analysis, the soil sampled from the study site was dominated by fine-sized particles, where the soil in Kenjeran Beach Amusement Park was dominated by silt particles with the range around 64.026 -81.456%, while Lempung Urban Forest was dominated by clay particles with the range around 56.643 -61.362%. For its soil texture, the soil in Kenjeran Beach Amusement Park was classified as silt loam texture, while Lempung Urban Forest was classified as clay texture.

B. Hydraulic Conductivity and BIH Water Flowrate of Test Fields
Based on the field measurements of the decreased surface water level in the BIHs on test fields, the value of the hydraulic conductivity are determined using Porchet method using Equation 1. The results of the hydraulic conductivity calculation can be seen on the Table 3.
From the Table 3 above, it can be seen that the plots with silt soil have a higher hydraulic conductivity with the plots with clay soils. In terms of the soil conditioning, the highest value of hydraulic conductivity are found in vegetated plots (K1 and L1). Then, followed by unconditioned plots (K3 and L3), compacted vegetation plots (K2 and L2), and only compacted plots (K4 and L4).
For the plots located in Kenjeran Beach Amusement Park or the silt soils, the highest hydraulic conductivity value is in the K1 plot with vegetated soil about 20.89 cm/hr. Then, it followed by the K3 plot with no conditioning on the soil about 16.88 cm/hr, the K2 plot with vegetated dan compacted soil about 10.91 cm/hr, and the K4 plot with compacted soil about 8.88 cm/hr. As for the plots located in Lempung Urban Forest or the clay soils, it followed the same trends as in previous location. The highest hydraulic conductivity value is in the L1 plot with vegetated soil about 12.39 cm/hr. After that, the hydraulic conductivity values are classified based on permeability class from from Natural Resource Conservation Service (NRCS), USA. The classification of hydraulic conductivity from the eight test plots based on permeability class can be seen in Table 4.  Table 4, by reviewing the hydraulic conductivity of the eight test plots based on the permeability class, it can be seen that the four plots found in the Kenjeran Beach Amusement Park (K1, K2, K3, and K4) are in the "moderately fast" class range. As for test plots in Lempung Urban Forest, it is included in the class range from moderately fast to moderate. L1, L2, and L3 plots belong to the "moderately fast" class, while the L4 plot is classified in the moderate class.
Based on the classification, it can be concluded that the soils in those locations have a quite good permeability. It can be used or utilized for rainwater harvesting with the application of bio-pore infiltration hole, and can become a decent urban runoff management.
After that, the hydraulic conductivity value is used to calculate the water flow rate of bio-pore infiltration hole. As for the results of the water flowrate of bio-pore infiltration hole calculation can be seen on the Table 5. From the Table 5 above, it can be seen that the plots with silt soil have a higher flowrate compared with the plots with clay soils. In terms of the soil conditioning, the highest value of the flowrate of BIH are found in vegetated plots (K1 and L1). Then, followed by unconditioned plots (K3 and L3), compacted vegetation plots (K2 and L2), and only compacted plots (K4 and L4).
For the plots located in Kenjeran Beach Amusement Park or the silt soils, the highest flowrate of BIH is found in the K1 plot with vegetated soil about 67,285.21 cm 3 /hr. Then, it followed by the K3 plot with no conditioning on the soil about 54,392.61 cm 3 /hr, the K2 plot with vegetated dan compacted soil about 35,145.91 cm 3 /hr, and the K4 plot with compacted soil about 28,591.34 cm 3 /hr. As for the plots located in Lempung Urban Forest or the clay soils, it followed the same trends as in previous location..

C. Runoff Coefficients of Test Plots
The average of rainfall height in the last 10 years and rainfall duration in the last 13 month calculated by data from nearest weather stations at Surabaya City are 99.61 mm and 2.28 hours, respectively.
Infiltration rate measurement is conducted to determine the constant infiltration rate from each test plot. After that, those data will be multiplied with average rainfall duration at Surabaya City to find the constant infiltration height. The result of measurements and infiltration height calculations can be seen at Table 6. From Table 6, it can be seen that test plot with the highest constant infiltration rate is K1 plot with silty and vegetated soil about 8.000 mm/hr. Meanwhile the smallest one is on the L4 plot with clayey and compacted soil about 0.0625 mm/hr. It also correlates with the infiltration height from each plot test. Using the average rainfall duration data, the highest infiltration height with the rainfall duration around 2.28 hr is plot K1 about 18.24 mm/hr and the smallest one is plot L4 about 0.1425 mm/hr. Generally, the plots with silt soil have a higher constant infiltration rate and infiltration height compared with the plots with clay soils.
In terms of the soil conditioning, the highest value of constant infiltration rate and infiltration height are found in vegetated plots (K1 and L1). Then, followed by unconditioned plots (K3 and L3), compacted vegetation plots (K2 and L2), and only compacted plots (K4 and L4).
Based on test plot conditions, using UDFCD guideline book, the depression losses value for every vegetated soil are 0.35 in or 8.89 mm and 0.4 in or 10.16 mm for bare soils [15]. The runoff coefficient of each test plot can be calculated together from all those data above, along with the calculation of infiltration height, rainfall height and rainfall duration. For the result of runoff coefficient calculation can be seen in Table 7.  Based on Table 7 data, the range of runoff coefficient (C) from each soil type are quite different, and the runoff coefficient of silt soils are lower compared to the clay soils. For the plots with silt soil, the value of C ranges from 0.728 -0.867. Meanwhile, at the plots with clay soil, the range is a bit closer compared to the silty ones. The value of C in those location ranges from 0.876 -0.908. This may related to the infiltration height from each sites. At Kenjeran Beach Amusement Park with silt soils, the infiltration height range between its plots in there (K1, K2, K3 and K4) are quite wide compared to those in Lempung Urban Forest with clay soil. The infiltration height on silt soil ranges from 3.039 -18.24 mm, while on the clay soil it ranges from 0.143 -3.42 mm.
The plot that generates the highest runoff is the L2 plot with clay-textured, vegetated and compacted soil at 0.090435 m 3 , while the plot with the lowest runoff volume is on K1 plots with silt-textured and vegetated soil by 0.07248 m 3 . The same trends also happened with the runoff coefficient. The plot that has the biggest runoff coefficient value is the L2 plot with 0.907891, while the plot with the lowest runoff coefficient value is on K1 plot with 0.727638.

D. Analysis of The Runoff Reduction from Bio-pore Infiltration Hole Application
Based on the implementation of Minister of Environment Regulation Numb. 12/2009, maximum number of BIH installed in the area of 1 m 2 is two pieces. From the assumed area of 2,000 m 2 and only 25% from it or 500 m 2 , that can be used for the application of BIH. Thus, the BIHs required in an area of 500 m 2 are 1,000, both for silt and clay soils.
With including all calculations of BIH's flowrate, rainfall height, rainfall duration, runoff coefficient, along with other assumptions mentioned in the methods sections, the number of runoff reduction can be achieved from assumed areas can be seen in Table 8.
From Table 8, with the implementation of BIHs based on the Minister of Environment Regulation Numb. 12/2009, silt-type soils can reduce surface runoff greater than clay type soils. The percentage of surface runoff reduction for silt-type soils ranges from 38.98 -95.73%, depending on the treatment of the soil, both the influence of soil compaction and or vegetation cover. In clay-type soils, the percentage of surface runoff reduction ranged from 20.67 -54.28%.
The soil conditioning that has the highest runoff reduction is found in the vegetated soil (plots K1 and L1), then followed by unconditioning soil (plots K3 and L3), soils with variations in combinations of compaction and vegetation (plot K2 and L2), and only compacted land (plots K4 and L4). This is related to the ability of recharge by the BIH of each land condition, where land whose soil is compacted tends to reduce the absorption of water [11] and land that has vegetation cover tends to increase water absorption [18]. For the effect of soil compaction, this confirmed with the previous study by Gregory et al. Compaction affects the physical properties of the soil while reducing the porosity and pore distribution in the soil [11]. As for the influence of vegetation cover, this was confirmed by Gadi et al. that higher vegetation density in the soil results in higher hydraulic conductivity value [18].
The variable combination of vegetation cover with soil compaction (K2 and L2) has a greater water absorption than the soil which is only compacted (K4 & L4), but not greater than the land that is not given any treatment (K2 and L2). Thus, the greater the absorbency of the water, the more runoff can be reduced.
The type of land that can reduce the biggest runoff after the BIH installation is the land with K1 plot characteristics with silt textured soil conditions, with vegetation cover and not compacted at 95.73%. While the land that can reduce runoff or lowest runoff after the BIH installation is the land with L4 plot characteristics with clay, compacted and nonvegetated soil conditions of 20.67%. This also correlates with the remaining surface runoff voulme (VR rem) from each soil characteristics, where the value of VR rem has a negative correlation with the runoff reduction, which can be seen in Figure 4. The higher the runoff reduction, the value of VR rem becomes more lower, and vice versa. The lowest VR rem is happened in the land with K1 plot characteristics about 6.84 m 3 , while the highest VR rem is happened in the land with L4 plot characteristics about 133.80 m 3 .
Because the compaction was only based on the duration and only have two variations between compacted and noncompacted soils, potentially the soils are not compacted enough or have a low degree of compaction. Thus, the area needed for BIH installation and the amount of BIH may be higher on the land or soil with higher degree of compaction for reducing the surface runoff or stormwater, especially in big cities with high land uses and in the tropical climate.

V. CONCLUSION
The result of the study shows that with the implementation of Minister of Environment Regulation Numb. 12/2009, silt soils have higher runoff reductions compared to the clay soils, if it is compared with the same soil conditions. Overall, runoff reductions that can be achieved with the application of this regulation are 38.98 -95.73% for silt soils and 20.67 -54.28% for clay soils. As for the soil conditioning, the highest runoff reduction is achieved in the vegetated soil, then followed by unconditioning soil, combination of compaction and vegetation on soil, and only compacted land. However, this may be only applicable to the land or area that not have a high degree of compaction because the compaction variables were not on the wide range. And, that possibility requires further research.