Home > Vol. 15, No. 5
| [ Article ] | |
| Journal of the Korea Institute of Ecological Architecture and Environment - Vol. 15, No. 5, pp. 5-12 | |
| Abbreviation: J. Korea Inst. Ecol. Archit. And Environ. | |
| ISSN: 2288-968X (Print) 2288-9698 (Online) | |
| Print publication date Oct 2015 | |
| Received 26 May 2015 Revised 08 Oct 2015 Accepted 13 Oct 2015 | |
| DOI: https://doi.org/10.12813/kieae.2015.15.5.005 | |
| Application of Percentile Rainfall Event for Analysis of Infiltration Facilities used by Prior Consultation for LID (Low Impact Development) | |
Kwon, Kyung-Ho* ; Song, Hye-Jin**
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| *Corresponding author, Urban Water Cycle Research Center, Korea Institute of Safe Drinking Water Research (kwonkh@kisd.re.kr) | |
| **Coauthor, Urban Water Cycle Research Center, Korea Institute of Safe Drinking Water Research (shj@kisd.re.kr) | |
ⓒ Copyright Korea Institute of Ecological Architecture and Environment | |
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Retention and infiltration of small and frequently-occurring rainfall by LID facilities account for a large proportion of the annual precipitation volume. Based on 4 standard facilities such as Porous Pavement, Infiltration Trench, Cylindrical Infiltration Well, Rectangular Infiltration Well by Seoul Metropolitan Handbook of the Prior Consultation for LID. The total retention volume of each facility was calculated according to the type and size. The Purpose of this study is to find out the quantitative relationship between Percentile Rainfall Event and Design Volume of Infiltration Facilities.
For the estimation of Percentile Rainfall Event, Daily Precipitation of Seoul from 2005 to 2014 was sorted ascending and the distribution of percentile was estimated by PERCENTILE spreadsheet function. The managed Rainfall Depth and Percentile of each facility was calculated at the several sizes. In response to the rainwater charge volume of 5.5mm/hr by the Category "Private large site", the 3 types of facilities were planned for example. The calculated Rainfall Depth and Percentile were 54.4mm and 90% by the use of developed Calculation-Module based on the Spreadsheet program.
With this Module the existing Designed Infiltration volume which was introduced from Japan was simply converted to the Percentile-Rainfall-Event used in USA.
| Keywords: Low Impact Development, Rainwater Infiltration Facilities, Design Infiltration Volume, Percentile Rainfall Event |
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The Seoul Metropolitan government established “Seoul Metropolitan Basic Ordinance on Water Circulation Recovery & Low Impact Development” for the purpose of comprehensively and systematically driving the policies for preserving the natural infiltration capacity of rainwater and prohibiting the surface runoff of it(Seoul Metropolitan Government, 2014b).
The city notified the rainwater load and the average saturated permeability coefficient for each kind of land use in order to implement the ordinance(Seoul Metropolitan Government, 2014c) Based on it, the government is running the “Prior Consultation System for LID”. The government induces to establish certain scale of rainwater infiltration facilities for new development projects through the “Prior Consultation System for LID” which has been implemented since 2014. The infiltration volume required for each kind of land use is suggested as “each rainwater load” and the sum of the possible infiltration volumes of all individual facilities is indicated as the amount of reduction. The government induces to establish the rainwater infiltration facilities in order for them to have a value bigger than each load which is defined by each kind of use and area of land. (Seoul Metropolitan Government, 2014a) In this case, the unit indicating the “rainwater load”, “amount of reduction” and design capacity of each infiltration facility is mm/hr. The National Emergency Management Agency accepted the method by Tokyo and the Japan Technology Association for Basin Infiltration of Rainwater that applied the method of deducting a certain volume by cutting the base part of the hydrograph indicating the rainwater volumes which are flowed into the infiltration facilities by a certain amount accounting for the infiltration capacity as a planned unit of rainwater runoff reduction facility and it was adopted by the Seoul Metropolitan Government as the capacity calculating unit. (Seoul Special City, 2013a, Joo-seok Park et al., 2006, Tokyo, 2009) It is conceptually described well that the infiltration facilities contribute to reducing the peak flow of a flood by the designed infiltration volume during the whole period of a rainfall event regardless of the scale of a rainfall. Jong-sang Sung et al. (2004) and Young-hae Han et al. (2012), etc., once suggested the effects of the infiltration facilities by measuring and plotting the designed infiltration volumes and Young-ran Kim et al. (2013) utilized the results as the input values for designed capacities of a simulation program related to flood mitigation effects.
On the other hand, Americans indicate the designed capacity of each random rainwater infiltration facility as a rainfall depth(mm) (Schueler and Claytor, 2000) or as the percentile that the relevant rainfall depth occupies the whole rainwater event. (Comstock & Wallis, 2003; US EPA, 2009) This method has the merit that can indicate the functions of an infiltration facility that make large amount of runoff rainwater be infiltrated and evaporated after being temporarily stored at the local site while relating them to the catchment area through the management of a small scale of rainfall after paying attention to the fact that the ratio of a small scale of rainfall event occupying the total annual rainfall is considerable.
In this study, the researchers developed a calculation module converting the design capacity which is indicated as the sizes and designed infiltration unit volumes of rainwater infiltration facilities suggested in the Seoul Metropolitan Handbook of the Prior Consultation for LID into the rainfall depth of a catchment area and the rainfall event percentiles by using the method designed by EPA in the USA. It is intended to relate the merits which can estimate and suggest the ratio of the annual rainfall events controlled by the infiltration facilities more directly than the existing methods to the calculation table for the facility capacity targeted for the Prior Consultation for LID based on the method mentioned above.
The unit storage volume of each facility is calculated while targeting 25 types of infiltration facilities divided into the following 4 kinds, such as Porous Pavement, Infiltration Trench, Cylindrical Infiltration Well and Rectangular Infiltration Well which are specified in the Seoul Metropolitan Handbook of the Prior Consultation for LID. Based on the daily rainfall data from the recent 10 years in Seoul Region, the distribution chart of the percentile rainfall events is made. Then, the changes in the percentiles of rainfall events which can be controlled according to the size and quantity of each individual facility are investigated. Regarding to random use of a land, the percentiles of rainfall events that the rainwater management facilities satisfying the given rainwater loads can control are calculated and suggested for example. The method and procedure of this study are shown in Fig. 1.
Fig. 1.
Flow chart of study
The diagrams and the sizes for each rainwater infiltration facility which is suggested in the Seoul Metropolitan Handbook of the Prior Consultation for LID are shown in Fig.2 and Table 1.
Fig. 2.
Illustration of infiltration facilities
Sizes of infiltration facilities
| Name | Type | Design Head(H:m) |
Breadth (B:m) |
Diameter (d:m) |
Distance of Left-Right (Lt:m) |
Upper Distance(Ot:m) |
Under Distance (Ut:m) |
Unit Infiltration Volume (m3/hr.m2,m,EA) |
|---|---|---|---|---|---|---|---|---|
| Porous Pavement | T0.4 | - | - | - | - | - | - | 0.017 |
| T0.6 | - | - | - | - | - | - | 0.017 | |
| T1.0 | - | - | - | - | - | - | 0.017 | |
| Infiltration Trench | W150 | 0.6 | 0.35 | 0.15 | 0.1 | 0.15 | 0.3 | 0.04 |
| W200 | 0.7 | 0.4 | 0.2 | 0.1 | 0.2 | 0.3 | 0.045 | |
| W250 | 0.8 | 0.55 | 0.25 | 0.15 | 0.2 | 0.35 | 0.052 | |
| W300 | 0.9 | 0.6 | 0.3 | 0.15 | 0.2 | 0.4 | 0.057 | |
| W400 | 1.1 | 0.8 | 0.4 | 0.2 | 0.2 | 0.4 | 0.069 | |
| W500 | 1.3 | 1 | 0.5 | 0.25 | 0.25 | 0.45 | 0.08 | |
| Cylindrical Infiltration Well | D200A | 0.8 | - | 0.2 | 0.2 | - | 0.3 | 0.078 |
| D300A | 0.9 | - | 0.3 | 0.2 | - | 0.3 | 0.098 | |
| D450A | 1.0 | - | 0.45 | 0.2 | - | 0.3 | 0.127 | |
| D600A | 1.15 | - | 0.6 | 0.2 | - | 0.3 | 0.163 | |
| D800A | 1.3 | - | 0.8 | 0.25 | - | 0.4 | 0.228 | |
| D1000A | 1.45 | - | 1.0 | 0.3 | - | 0.4 | 0.303 | |
| D1200A | 1.6 | - | 1.2 | 0.35 | - | 0.4 | 0.388 | |
| D1500A | 1.85 | - | 1.5 | 0.4 | - | 0.5 | 0.528 | |
| Rectangular Infiltration Well | D200A | 0.8 | 0.6 | 0.2 | 0.2 | - | 0.3 | 0.087 |
| D300A | 0.9 | 0.7 | 0.3 | 0.2 | - | 0.3 | 0.11 | |
| D450A | 1.0 | 0.85 | 0.45 | 0.2 | - | 0.3 | 0.143 | |
| D600A | 1.15 | 1.0 | 0.6 | 0.2 | - | 0.3 | 0.173 | |
| D800A | 1.3 | 1.3 | 0.8 | 0.25 | - | 0.4 | 0.246 | |
| D1000A | 1.45 | 1.6 | 1.0 | 0.3 | - | 0.4 | 0.33 | |
| D1200A | 1.6 | 1.9 | 1.2 | 0.35 | - | 0.4 | 0.424 | |
| D1500A | 1.85 | 2.3 | 1.5 | 0.4 | - | 0.5 | 0.575 |
The calculation formula of retention spaces and the resulting values while using the randomly chosen unit areas, lengths and quantities based on the facility sizes (Table 1) and the designed unit infiltration volume (Q: m3/hr) which are suggested in the Seoul Metropolitan Handbook of the Prior Consultation for LID are as follows. Here, the porocity of crushed stones(ng) and sands(ns) were designated as 35% and 25% respectively (Seoul Metropolitan Government, 2013b). The retention volume which is secured by the infiltration volume is calculated based on the saturated infiltration coefficient and it means the final infiltration capacity(fc) under the saturated state of a land through antecedent rainfalls or consecutive ones.
If it rains continuously for 24 hours, that is, one day as the basic time unit of the rainfall event percentile method, the possible virtual retention volume which is secured by infiltration is calculated by multiplying the designed unit infiltration volume (m3/hr) by 24(hr) but considering that some rainfalls are not secured in that way, the infiltration volume for 1 hour at minimum was used.
The calculation formula of the unit retention volumes of porous pavements and the results are shown in Table 2.
Unit Retention Volume and calculation formula of Porous Pavements
| Type | Porous V. of Gravel [m3] |
Porous V. of Sand[m3] |
V. of Porous Pipe [m3] |
V. of Infiltration [m3] |
Unit Retention Volume [m3] |
|---|---|---|---|---|---|
| T0.4 | 0.14 | - | - | 0.017 | 0.157 |
| T0.6 | 0.21 | - | - | 0.017 | 0.227 |
| T1.0 | 0.35 | - | - | 0.017 | 0.367 |
| Porous V. of Gravel (V1) | S×H×ηg |
| V. of Infiltration(V4) | Q×t |
| Unit Retention Volume(V) | V1+V4 |
| S: Area(m2) Q: Design Infiltration Volume(m3/hr), t: Min. Duration(hr) | |
The calculation formula of the unit retention volumes of Infiltration Trenches and the results are shown in Table 3.
Unit Retention Volume and calculation formula of Infiltration Trench
| Type | Porous V. of Gravel [m3] |
Porous V. of Sand [m3] |
Porous V. of Pipe [m3] |
V. of Infiltration [m3] |
Unit Retention Volume [m3] |
|---|---|---|---|---|---|
| W150 | 0.055 | 0.009 | 0.013 | 0.040 | 0.117 |
| W200 | 0.073 | 0.010 | 0.025 | 0.045 | 0.153 |
| W250 | 0.118 | 0.014 | 0.042 | 0.052 | 0.225 |
| W300 | 0.143 | 0.015 | 0.062 | 0.057 | 0.277 |
| W400 | 0.236 | 0.020 | 0.113 | 0.069 | 0.438 |
| W500 | 0.351 | 0.025 | 0.181 | 0.080 | 0.637 |
| Porous V. of Gravel(V1,) | |
| Porous V. of Sand(V2) | B×St×ns |
| V. of Porous Pipe(V3) | |
| V. of Infiltration(V4) | Q×t |
| Unit Retention Volume(V) |
V1+V2+V3+V4 |
The calculation formula of the unit retention volumes of cylindrical infiltration wells and the results are shown in Table 4.
Unit Retention Volume and calculation formula of cylindrical infiltration well
| Type | Porous V. of Gravel [m3] |
Porous V. of Sand [m3] |
Porous V. of Pipe [m3] |
V. of Infiltration [m3] |
Unit Retention Volume [m3] |
|---|---|---|---|---|---|
| D200 | 0.064 | 0.007 | 0.13 | 0.078 | 0.162 |
| D300A | 0.093 | 0.010 | 0.037 | 0.098 | 0.237 |
| D450A | 0.140 | 0.014 | 0.102 | 0.127 | 0.383 |
| D600A | 0.205 | 0.020 | 0.225 | 0.163 | 0.612 |
| D800A | 0.399 | 0.033 | 0.430 | 0.228 | 1.090 |
| D1000A | 0.661 | 0.050 | 0.792 | 0.303 | 1.807 |
| D1200A | 1.014 | 0.071 | 1.312 | 0.338 | 2.785 |
| D1500A | 1.710 | 0.104 | 2.322 | 0.528 | 4.664 |
| Porous V. of Gravel (V1) | |
| Porous V. of Sand (V2) | |
| V. of Porous Pipe (V3) | |
| V. of Infiltration (V4) | Q×t |
| Unit Retention Volume (V) |
V1+V2+V3+V4 |
The calculation formula of the unit retention volumes of rectangular infiltration wells and the results are shown in Table 5.
Unit Retention Volume and calculation formula of infiltration trench
| Type | Porous V. of Gravel [m3] |
Porous V. of Sand [m3] |
Porous V. of Pipe [m3] |
V. of Infiltration [m3] |
Unit Retention Volume [m3] |
|---|---|---|---|---|---|
| D200A | 0.083 | 0.009 | 0.013 | 0.087 | 0.191 |
| D300A | 0.122 | 0.012 | 0.037 | 0.110 | 0.282 |
| D450A | 0.189 | 0.018 | 0.102 | 0.143 | 0.451 |
| D600A | 0.283 | 0.025 | 0.225 | 0.173 | 0.706 |
| D800A | 0.551 | 0.042 | 0.430 | 0.246 | 1.270 |
| D1000A | 0.921 | 0.064 | 0.792 | 0.330 | 2.107 |
| D1200A | 1.420 | 0.090 | 1.312 | 0.424 | 3.247 |
| D1500A | 2.405 | 0.132 | 2.322 | 0.575 | 5.435 |
| Porous V. of Gravel (V1) | |
| Porous V. of Sand (V2) | B2×St×ns |
| V. of Porous Pipe (V3) | |
| V. of Infiltration (V4) | Q×t |
| Unit Retention Volume (V) |
V1+V2+V3+V4 |
The ascending sort of the effective precipitation among daily precipitation for the recent 10 years in Seoul Region (Fig. 3) except for the effective precipitation less than 2.5mm (US EPA, 2009) is shown in Fig. 4.
Fig. 3.
Daily Precipitation of Seoul (2005-2014)
The number of rainy days with the effective precipitation more than 2.5mm during the period was 642 days in total and the date with the biggest precipitation, 301.5mm, was July 27th, 2011.
Fig. 4.
Ascending sort of daily precipitation
The designed capacity of a infiltration facility can be indicated as “Rainfall Depth x Area of Catchment Surface” under the premise that the runoff volume which is calculated by multiplying the accumulative depth of the rainfall on a certain catchment surface by the area of the catchment surface is flowed into the relevant infiltration facility at a time and such an infiltration facility is evenly distributed on the catchment surface.
Especially, the designed capacity emphasizing the non-point pollution treatment functions of the infiltration facilities is called as Water Quality Volume. (Schueler and Claytor, 2000) Such a capacity indicates the capacity of a rainwater control facility with non-point pollution treatment functions as a volumetric discharge coefficient representing the rainfall depth, the area of the catchment surface and the proportion of the impervious area. And the rainfall depth corresponds to the targeted precipitation which was set through the Water Quality Measurement and Modeling.
Fig. 5.
Catchment area and Rainfall depth
At this time, if the rainfall depth corresponding to the targeted precipitation is indicated as the percentiles of the total rainfall events in the relevant region, the facility criteria considering the rainfall characteristics of the region can be suggested. For example, the regions having the annual precipitation 1,000mm and 1,500mm respectively have their rainfall patterns and the precipitation distributions different from each other. Thus it can be said that the method can represent such differences of those regions by indicating the percentiles.
Fig. 6.
Daily Precipitation Percentile of Seoul
It can be found that 90% of the total daily precipitation events is 56.4mm while referring to the Percentile Distribution of Daily Rainfall Events in Seoul shown above. If it is assumed that a facility with the targeted precipitation, 56.4mm is established on a randomly chosen area of the catchment surface, it means that 90% of the daily rainwater will be flowed and infiltrated into the relevant facility. In addition, the total quantity (Q) of the rainwater which is infiltrated at this time is the sum (1177.9mm/year) of the total daily precipitation(Q1: 828.2mm/year) less than the targeted precipitation and the rainwater volume (Q2: 349.6mm/year) flowing into when the precipitation is more than the targeted one. This corresponds to the 77.9% of the average annual precipitation, 1511.5mm.
However, as the premises for applying for such estimated values, the runoff within the scope of the given daily precipitation must be regular enough in order for all to be flowed into the relevant infiltration facility and the relevant infiltration facility must have the soil conditions in order for it to infiltrate all of the flowed rainwater within 24 hours.
In case of calculating the designed flood level, the precision of the input rainfall data which is increased by using the conversion factor(CF) between the fixed time and the random time. (Ministry of Land, Transport and Maritime Affairs, 2012) However, in case of calculating the water circulation quantity of an infiltration facility, it is important to enter some continuous time-series data. Since the precipitation data during a fixed time, that is, a day is used as the basic data, some temporary retention and infiltration phenomena of the continuous rainfalls could not be represented, so it has a root limitation with which it is indicated that the rainwater quantity more than the real precipitation is infiltrated.
After assuming that an infiltration facility is established in the area of 100m2 for establishing countermeasures, the calculated retention volumes, the rainfall depths and the percentiles of each facilities are shown in Fig. 7, Fig.8, Fig. 9 and Fig.10.
Fig. 7.
Percentile of porous pavement
Fig. 8.
Percentile of infiltration trench
Fig. 9.
Percentile of cylindrical infiltration well
Fig. 10.
Percentile of rectangular infiltration well
From the table, the kinds of the facilities which are established within the area, 100m2and the rainfall depth of each size and the difference between percentiles can be checked and it can be found that the percentile increases in proportion to the retention volume of each facility.
In case of porous pavements, if 25m2of T0.6 Type is established, it can cover 91% of the total rainfall events. In case of establishing 25m of infiltration trench(W250), all of the daily precipitation, 56.2mm/day can be infiltrated after being retained and it has the effect that around 90% of total rainfall events can be treated. In case of a cylindrical infiltration well and a rectangular infiltration well, if 3 wells (D1000A and W1000A) for each type are established, similar effect takes place.
In case of establishing a rainwater infiltration facility satisfying the rainwater load, 5.5mm/hr, designated by Seoul Metropolitan government in a large size of private land, 1,000m2, the calculated rainfall depths and the rainfall event percentiles which are managed within this site are shown in Table 6 as examples.
Site conditions
| Criteria | Condition | |
|---|---|---|
| Share Volume of Rainwater Burden(A) | Private(large scale) | 5.5mm/hr |
| Site Area(B) | 1000m2 | |
| Green area(C) | 200m2 | |
| Measurement needed area(E) | E = B-(C×3/5) = 1000-(200×3/5) | 880m2 |
| Needed measurement V.(F) | F = A×E×1000 | 4.8m3/hr |
It was assumed that the saturated permeability(ks) of the soil on a randomly chosen site is 16.43mm/hr as the average value of Seoul. However, it is preferable to use a detailed soil map, etc., in case of planning a facility to be established in a real site.
Total retention/infiltration volume, 47.9m3/hr corresponds to the rainfall depth of 54.5mm on a 880m2 area for establishing countermeasures and it corresponds to the rainfall event percentile 89.7% (around 90%). The resulting values mean that the rainfall depth to be managed is 54.5mm and it is possible to retain all of the rainwater in case that 54.5mm of single precipitation pours down at a time. It means that around 90% of all of the daily precipitation in Seoul can be infiltrated after being retained. In addition, under the situation of a continuous rainfall, such as a long rainy season, even though the retention space is filled up by the antecedent rainfalls, it means that rainwater is continuously infiltrated at a speed of 5.5mm/hr evenly over the catchment section, 1000m2 , which is covered by all of the facilities. It is the same as plotting a runoff hydrograph after deducting 5.5mm/hr from the effective precipitation which is induced from the precipitation data while modeling a precipitation-runoff model.
Type of infiltration facilities and the estimated rainwater management effect
| Type | Unit Ret. Volume |
EA | Retention Volume |
|---|---|---|---|
| Porous Pavement(T0.61) | 0.367 | 100m2 | 36.7m3/hr |
| Infiltration Trench W150 | 0.117 | 50m | 5.9m3/hr |
| Cylindrical Infiltration Well D300A | 0.226 | 20EA | 5.3m3/hr |
| Sum | 47.9m3/hr | ||
| Total Retention V. Rainfall Depth Percentile |
47.9m3 54.5mm 89.7% |
||
Fig. 11.
Percentile of infiltration facilities in the site
It can be expected that there will be some effects such as non-point pollution reduction, water circulation and flood management/disaster prevention, depending on the sizes and structures of the rainwater infiltration facilities which are connected to the catchment sections. The current rainwater load which is set has some limitations when the flood management/ disaster prevention effects are expected. Therefore, it is necessary to use the temporary retention function using some openings within the filtration facilities and small & medium sizes of retention facilities, etc., more effectively rather than reducing the damages from the floods in the city by utilizing only the infiltration quantity.
A calculation module converting the designed capacity of an infiltration facility to the catchment area, the rainfall depth and percentile was developed by using a method deducting some quantity which is suggested in the Seoul Metropolitan Handbook of Prior Consultation for LID. In case that it is assumed that all types of individual facilities are established within the impervious area of 100m2, the designed capacity of the facilities within the area as well as the rainfall depths and the percentiles corresponding to such capacity were calculated and compared. While doing so, it was easy to quantitatively compare the rainwater quantities which can be managed from each type of facility.
In case of using such a percentile method together with the existing method, it can indicate the rainwater quantity managed by the relevant rainwater infiltration facility more effectively and there are some merits that the kinds of the facilities which can be applied when doing a prior consultation for LID can be expanded.
However, since a percentile method uses on one day as the boundary between the rainfall events, even though the rainfall events are the same, there is a demerit that a day can be considered as another day when the day is changed. It is necessary to study additionally on the comparison and the correlation among the measured infiltration/runoff quantities, the simulated resulting values of water circulation during a small time scale like 5 minutes and the values estimated by the percentiles.
References
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