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DESIGN OF EXFILTRATION TRENCHES

TO MEET

SFWMD REGULATORY CRITERIA

October 1982

Charles A. Hall, P.E., Director Water Management Division Resource Control Department

South Florida Water Management District

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lIST OF FIGURES

Figure 1 - SFWMD Jurisdictional Boundaries ------------------- 2

Figure 2 - Ideal Condition Test ------------------------------ 10

Figure 3 - Usual Open-hole Test ------------------------------ 12

Figure 4 - 0.0.1. Standard Test ------------------------------ 14

Figure 5 - Falling-head Open-hole Test ----------------------- 16

Figure 6 - Typical Exfiltration Trench ----------------------- 21

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TABLE OF CONTENTS

I. Introduction --------------------------------------------- 1

U. SFWMD Trench Design Requirements ------------------------- 3

III. Field Test Procedures ------------------------------------ 6

A. Ideal Condition Test ---------------------------------- 8

B. Usual Condition. Test --------------------------------- g

C. D.O.T. Standard Test --------------------------------- 11

0. Falling-head Test ------------------------------------ 15

IV. Analysis of Test Data ------------------------------------ 17

V. Design of Trenches --------------------------------------- 19

Appendix - Derivation of Equations --------------------------- 23

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I. INTRODUCTION

The South Florida Water Management District is responsible for

the permitting of surface water management systems within its

boundaries (see Figure 1) under Part IV of Chapter 373, Florida

Statutes, and Rule Chapter 40E-4. The objective of the permitting

program is to insure that proposed systems will not be harmful

to the water resources of the District and are consistent with

the public interest. Since the deterioration of water quality

is harmful to the water resource, specific Best Management Practices

(BMP) have been.developed to provide for the "treatment" of

storm water prior to discharge.

It is generally felt that total retention of a specific runoff

volume, known as the "first flush", provides, the most desireable

pollutant removal efficiencies. Retention is defined in the

District's "Basis of Review for Surface Water Management Permit

Applications Within the South Florida Water Management

District" as:

"RETENTION" - the prevention of storm runoff from direct

discharge into receiving waters; included as

examples are systems which discharge through

percolation, exfiltration, and evaporation

processes.

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BEST AVAILABLE COPY

South FIorIda Water

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Since the inception of the District's surface water management

permitting program the most popular retention design technique

utilized is the exfiltratlon trench, which is also referred to

as french drain or seepage trench. Unfortunately, with the wide

usage of the concept also came a wide range of interpretations

and misinterpretations of how to design such systems.

It is felt that a more rigorous treatment of exfiltration

system design is now needed. The purpose of this report is to

present alternative field testing methods and application of the

test results to the final trench design.

II. SFWMD TRENCH DESIGN REQUIREMENTS

The District's "Basis of Review ." document specifies criteria

for the design of exfiltration trench retention systems as follows:

"3.2.2. WATER QUALITY

3.2.2.2. Retention/Detention Design Criteria - retentionand/or

detention in the overall system, including swales, lakes,

canals, greenways, etc., shall be provided for one of the

three following criteria or equivalent combinations

thereof:

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a. Wet detention volume shall be provided for the first

Iinch of runoff from the developed project, or the

total runoff from a 3-year, 1-hour rainfall event

Iwhichever is greater. lb. Dry detention volume shall be provided equal to

75 percent of the above amounts computed for wet

detention.

c. Retention volume shall be provided equal to 50

1 percent of the above amounts computed for wet

detention."

I and Section 3.2.2.5:

I"Underground Exflltration Systems

a. Systems shall be designed for the retention volumes

specified in Section 3.2.2.2 above for retention systems,

exfiltrated over one hour for retention purposes,

prior to overflow, and based on test data for the site.

b. Safety factor - 2 minimum

To aid engineers in the design of exfiltration systems an

Iearly "Basis of Review . ." provided some "quick n' dirty"

I

design guides:

"The following procedure nay be used in the design of surface

Iwater management systems utilizing underground exfiltration

systems such as French Drains, etc.

1 a. Systems should preferably have an overflow to a

positive system with a control device, if necessary,

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between the exfiltration system and the outfall pipe.

The overflow or control device should be sized for the

allowable discharge. Based on the geometric properties

of an exfiltration system, the length of the system may

be determined as follows, unless other methods having

scientific validity or local jurisdictional approval

are utilized:

L = 10 150 C A

wtfü h) where, L = Length of system (feet)

C = Runoff coefficient (rational method) A = Contributing area (acres) W = Trench width (feet) h = Average drop per minute in open hole

exfiltratlon test data (Inches) H = Non-saturated trench depth (feet)

b. Based on the geometric properties, the length of

exfiltration systems without overflows may be determined

as follows, unless other methods having scientific

validity or local jurisdictional approval are utilized:

L = 7260CAR W(l0 h + H)

where all terms are the same as in item "a" plus:

H = one hour rainfall to meet local jurisdictional frequency criteria (inches)."

These simple short-cut design tecnniques have been so extensively

utilized, it is felt that more reliable procedures should be developed.

Much reliance has been placed on these formulae with only a light

treatment of the subject of the type of percolation test or use

of test data.

This report will present both field test procedures and application

of test results to trench design in order to meet the above stated

cr1 teria.

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III. FIELD TEST PROCEDURES

IThe District's criteria states that, "...tests shall he consistent

as to elevation, location, soils, etc. with the system desiqnto

Iwhich the test data will be applied... ." There has been a sub-

stantial amount of confusion in the design community as to what

Ithe District staff intended by this statement:

IThe field test utilized should be of a type which will yield the

desired Information, i.e. percolation ability of the applicable

1 soil stratum.

IPerhaps the most common test utilized was the Health Department

standard septic tank test for obtaining the design percolation rate.

There Is a basic problem with use of the septic tank test. The

problem is that the test is usually run with a hole eighteen (18)

inches deep, whereas the design depth of the exfiltration trench

Iis usually at least five (5) feet deep, and the data generated

may not at all be representative of the hydraulic characteristics

of the deeper strata. Most soil engineering texts recommend that

the permeability of the appropriate soil layers be determined.

Four field test procedures for determining hydraulic conductivity

Iwill be described in the following, all of which should yield the

needed design information. Three of the tests are known ; constant

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head tests which should be representative of the design condition of

water ponded at the elevation of the inlet. The fourth test is a

falling-head test which should be used in areas of known good

percolation and when difficulty "filling the hole" is encountered.

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A. Ideal Condition Test

IThe "ideal" test be duplicated as procedure would one which

I. many of thedesign conditions as practical. This procedure

would have a test hole "drilled" such'that the bottom of

Ithe hole was at the proposed invert of the exflltration

trench. It would have a perforated casing which would

Iextend from the bottom of the hole up to the proposed top

elevation of the trench, and would have an impermeable

from casing there up to natural ground level (or estimated

I. finished grade level would be better). The test would be

run until in the opinion of the engineer the pumpinn rate stabil-

I izes and remains essentially constant. The field procedure would

be described as follows:

1. Auger a 4 to 9 inch diameter hole to a depth below the

ground surface equivalent to the design depth of trench

I(usually 4 to 6 feet).

I2. Record distance from ground surface to water table

prior to addition of test water.

3. Lower a casing into the hole with perforatiOns in the

Ibottom portion equivalent to the design trench height.

The perforations should be 3/8 inch in diameter and

Ishould be uniformly spaced with not less than 30 perfora-

tions per square foot of pipe surface (RE: FHWA-TS-80-218).

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4. Fill hole with water and maintain water level at ground

surface. Record rate of pumping in g.p.m. giving

direct readings from water meter at fixed intervals.

Use one minute intervals or greater, and continue

recording rate of pumping for approximately 10 minutes

following the stabilization of the recorded pumping rate.

Figure 2 shows a cross-section of the test hole with the

appropriate dimensions shown. Also shown is a formula for

relating the field information to the soil hydraulic conductivity.

Further discussion of the interpretation of test data will

be contained in Part IV of this report. Obviously to perform

this test the designer must, to some degree, have already

designed the trench since special casings must be fabricated

in advance which have the proper lengths of perforated and

unperforated sections. Since this is usually n.qt. the case

the next test procedure is the type most commonly performed

and. is hence entitled the 'usual" test.

B. Usual Condition Test

The usual test performed is an open-hole test which is

either un-cased or cased with fully-perforated casing.

The procedure is described as follows:

1. Auguer a 6 to 9 Inch diameter hole to a depth below

the ground surface equivalent to the design depth of

trench (usually 4 to 6 feet).

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I. IDEAL CONDITION TEST

NG.

i d 1. IMPERVIOUS CASING

H,

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WATER TABLE Du

PERFORATED CASING

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"A" - ELEV.

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4Q IK lTd(4H2Du2Du2+4H2Ds+H2d)

K HYDRAULIC CONDUCTIVITY (CFS/FT.2- FT. HEAD) Q AVERAGE FLOW RATE (CFS)

I d DIAMETER OF TEST HOLE (FEET) H2a DEPTH TO WATER TABLE (FEET)

I Dua UNSATURATED HOLE DEPTH (FEET) D8 SATURATED HOLE DEPTH (FEET)

IELEV. "A" a PROPOSED TRENCH BOTTOM ELEV. H, a AVERAGE HEAD ON UNSATURATED HOLE SURFACE (FT. HEAD)

ID a UNPERFORATED CASING DEPTH (FEET)

10- FIGURE 2

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2. Recor1 the distance from the ground surface to the water I table prior to the addition of test water.

I 3. If holewalls are, unstable lower screen or fully-

perforated casing into the hole. I

4. Fill hole with water and maintain water level at ground I

surface. Record rate of pumping in g.p.m. giving direct

readings from water meter at fixed intervals of one I minute or greater. Continue recording rate of pumping

for 10 minutes followng the stabilization of the I

recorded pumping rate.

Figure 3 shows a cross-section of the test hole with a

formula relating the hydraulic conductivity to the field

information. The hydraulic conductivity obtained by this method

may be either greater or less than the effective trench hydraulic

conductivity depending upon the relative hydraulic conductivity I of the surface layers. Another test which is quite often used

'is the D.0.T. standard test. I

C. 0.0.1. Standard Test I The Florida Department of Transportation utilizes a standard

test for design of seepage trenches in conjunction with I highway projects. The 0.0.1. test procedure is as follows:

1. Auger a 7 Inch diameter hole to a depth of 10 feet

below normal ground surface.

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USUAL OPEN HOLE TEST

Q

4Q K 1-rd(2H22+ 4H2Ds+ H2d)

K' HYDRAULIC CONDUCTIVITY (CFS/FT.1- FT. HEAD) Q'°STABILIZED" FLOW RATE (CFS) d DIAMETER OF TEST HOLE (FEET) P12 = DEPTH TO WATER TABLE (FEET) Ds SATURATED HOLE DEPTH (FEET) ELEV. 'A" PROPOSED TRENCH BOTTOM ELEV.

AVERAGE HEAD ON UNSATURATED HOLE SURFACE(FT.HEAD)

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FIGURE 3

2. Record distance from ground surface to water table

prior to addition of test water.

3. Pour 1/8 cubic foot of 1/2 inch diameter gravel in

hole to prevent scouring.

4. Lower a 6 inch diameter perforated 10 gauge aluminum

casing into hole. Casing to be 9 feet in length with

perforations in the bottom 6 feet of the casing.

5. Fill hole with water and maintain water level at ground

surface. Record rate of pumping in g.p.m. giving direct

readings from water meter at fixed intervals. Use one

minute intervals or greater, depending on the hydraulic

conductivity of the soil. Continue recording rate of

pumping for 10 minutes following the stabilization of

the recorded pumping rate.

A schematic cross-section of the D.0.T. test hole is shown in

Figure 4 with a formula which relates the hydraulic conductivity

to the field data. The 0.0.1. does not recommend utilization

of seepage trenches in. areas where this test yields less than

6 g.p.m.

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D.O.T. STANDARD TEST

FOR H3.O FEET:

K ir(2O.25H2-H 9) K HYDRAULIC CONDUCTIVITY (CF/FT.Z FT. HEAD) Q "STABILIZED" FLOW RATE (CFS) d DIAMETER OF TEST HOLE (FEET) Du UNSATURATED HOLE DEPTH (FEET) D5 SATURATED HOLE DEPTH (FEET) H1 AVERAGE HEAD ON UNSATURATED HOLE SURFACE (FT. HEAD) H2 DEPTH TO WATER TABLE (FEET)

FOR H23.O FEET: 0

K II.192H2

- 14 FIGURE 4

D. Falling-head Test

The falling-héad test is an open-hole test which is either

un-cased or cased with fully-perforated casing. The procedure

is described as follows:

1. Auger a 6 to 9 inch diameter hole to a depth below the

ground surface equivalent to the design depth of the

trench (usually 4 to 6 feet).

2. Record the distance from the ground surface to the water

table prior to the addition of test water.

3. If hole walls are unstable lower screen or fully-perforated

casing into the hole.

4. Fill hole with water and maintain water level at ground

surface. Cease adding water and measure the water level

versus elapsed time in equal time increments, usually

in 15-second increments. Continue measuring water level

until it has dropped at least half the distance to the

water table.

Figure 5 shows a cross-section of the test hole with a formula

relating the hydraulic conductivity to the field information.

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WAT

FALLING - HEAD OPEN - HOLE TEST

N. G.

dln(Hi/H2) K

(2H1+2H2+4Ds+d)(t2- t1)

K HYDRAULIC CONDUCTIVITY (CFS/FT.2-FT. HEAD) c1DIAMETER OF TEST HOLE (FEET) Hi HEIGHT OF WATER IN HOLE ABOVE WATER TABLE AT TIME,ti H2 HEIGHT OF WATER IN HOLE ABOVE WATER TABLE AT TIME,t2 D3SATURATED HOLE DEPTH (FEET) ELEV. UAU:PROPOSED TRENCH BOTTOM .ELEV. (FT.-NGVD) til f2 TIME, SECONDS

16 FIGLFcE 5 -c-g1-

IV. ANALYSIS OF TEST DATA

In this section actual test data which was compiled during a field

test of the "usual" case will be described and the soil permeability

calculated. The test was performed on a piece of property in

Broward County, Florida. The test hole was 9 Inches In diameter

augered to a depth of 6 feet. A 9 inch diameter by 72 inch long

perforated casing was set In the hole. The depth to the water

table prior, to introduction of test water was 5.3 feet below the

ground. The field data collected during the test Is shown ln

Table 1.

Taking the total flow Into the test hole during the 75 minute

test period and dividing by 75 minutes, since there was no

significant variation in flow during the test, yields an average

flow rate, Q, of 3.46 g.p.m., which is equivalent to 7.71 x 1O cfs.

The diameter of the test hole, 0, was 0.75 feet. The saturated

hole depth, Ds, was equal to the depth of the hole, 6 feet,

minus the depth to the water table, 5.3 feet, which is equal to

0.7 feet.

Utilizing' the formula from Figure 3:

K = 4Q

TTd(2E1 + 4H2D5 + H2d)

K 4ç7.7lx io) '

r(O.75)(2(5.3)+4(5.3)(o.7) + (5.3)(0.75))

K = 1.75 x IO4cfs/ft?ft.head

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TABLE 1

BROWARD COUNTY - USUAL OPEN-HOLE TEST

Elapsed Time Begin Meter End Meter Flow Q (Minutes) Reading Reading Gallons (G.P.M.)

1 0.0 5.5 5.5 5.5 2 5.5 11.0 5.5 5.5 3 11.0 16.0 5.0 5.0 4 16.0 19.0 3.0 3.0 5 19.0 22.5 3.5 3.5 6 22.5 26.5 4.0 4.0 7 26.5 30.0 3.5 3.5 8 30.0 33.5 3.5 3.5 9 33.5 37.5 4.0 4.0

10 37.5 40.5 3.0 3.0 11 40.5 44.5 4.0 4.0 12 44.5 48.5 4.0 4.0 13 48.5 51.5 3.0 3.0 14 51.5 55.5 4.0 4.0 15 55.5 59.5 4.0 4.0 16 59.5 63.0 3.5 3.5 17 63.0 67.0 4.0 4.0 18 67.0 70.0 3.0 3.0 19 70.0 73.5 3.5 3.5 20 73.5 77.5 4.0 4.0 25 77.5 96.0 18.5 3.7 30 96.0 114.5 18.5 3.7 35 114.5 132.0 17.5 3.5 40 132.0 154.0 22.0 4.4 45 154.0 172.5 18.5 3.7 50 172.5 190.5 18.0 3.6 55 190.5 208.5 18.0 3.6 60 208.5 220.0 11.5 2.3 65 220.0 235.0 15.0 3.0 70 235.0 247.0 12.0 2.4 75 247.0 259.5 12.5 2.5

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V. DESIGN OF TRENCHES

Since the first publ1ction of Volumej, Permit Information Manual

additional consideration has been given to the derivation of an

acceptable exflltration trench design formula. The latest

development is shown on Figure 6 along with the description of

the appropriate parameters. The derivation of this trench sizing

formula Is given in the Appendix along with the derivations of

the formulae used for use with the field testing procedures.

An example of the use of this formula with the data from the

Broward County test site follows:

L= CAR

(1.39 x 1 o)wD

C=O.60

A = 10.0 Acres

R = 2.5 Inches

K = 1.75x 10 CFS/FT.2-FT.HEAD

112= 5.0 Feet (Design Condition)

W = 4.0 Feet

2.5 Feet

b 1.5 Feet

H = + 4.0 Feet

Solving for L gives,

L 1389 feet of 4' x 4' exfiltration trench.

This formula can be used for sizing exfiltration trenches to meet

SFWMD criteria as it is since it already takes into consideration

both a Safety Factor of 2 and the 50% credIt for retention systems

as opposed to detention systems.

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For those situations when either: (1) the saturated depth of trench

greater than the non-saturated depth of trench; or (2) the trench

width is greater than two times the total trench depth, the proportional

assumptions for flow out the trench bottom are probably not valid. A

conservative design formula for use in these cases would be: L CAR

IK(2H2Du - Du2 + 2H2Ds) + (1.39 x 104)WDu

As with any design method a good amount of engineering judgement must

Ibe applied for use on site-specific cases.

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TYPICAL EXFILTRATION TRENCH

, 1.*..::.: ': I9-I 12 INCHES tBACKFILL

6INCHES PIPE COVER H2 TRENCH UNSATURATED

MINIMUM

12 INCHES PERFORATED Du DEPTH

MINIMUM PIPE DIAMETER

12 INCHES OARSE RO MINIMUM PIPE BED

TRENCH WIDTH

CAR

L: .K(H2w$-2H2Duu2+2H2Ds)(I.39xIo4)wDu

La LENGTH OF TRENCH REQUIRED (FEET)

Ca RUNOFF COEFFICIENT (RATIONAL RUNOFF METHO .Aa CONTRIBUTING AREA (ACRES)

R: ONE-HOUR DESIGN RAINFALL (INCHES)

W=TRENCH WIDTH (FEET) HYDRAULIC CONDUCTIVITY (CFS/FT. - FT.HEAD)

H2 DEPTH TO WATER TABLE(FEET)

Du: NON-SATURATED TRENCH DEPTH (FEET)

Ds* SATURATED TRENCH DEPTH (FEET)

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REFERENCES

.1. Bouwer, Herman, Groundwater Hydrology, McGraw-Hill Book Company, I 1978.

2. Harr, M.E., Groundwater and Seepage, McGraw-Hill Book Company,

I 1962.

3. Hwang, Ned H.C. , Fundamentals of Hydraulic Eninj, Prentice- i Hall, Inc., 1981.

U 4. Raudkivi, A.H., 8nd Callander, R.A., Analysis of Groundwater Flow,

John Wiley & Sons, 1976.

I. 5. Measuring Saturated Hydraulic Conductivity of Soils, Special PublTcation SP-SW-0262, American Sodety of Agricultural EngIneers, 1962.

6. 0rainage Manual, A Water Resources Technical Publication, U.S. Department of the Interior, Bureau of Reclamation, 1978.

I 7. Underground Disposal of Storm Water Runoff, Design Guidelines Manual, U.S. Department ofTransportation, Federal Highway Administration, FHWA-TS-80-218, 1980.

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APPENDIX

DERIVATION OF EQUATIONS

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I. IDEAL CONDITION TEST

K S1H1+S2H2 (EQ. I)

WHERE, K HYDRAULIC CONDUCTIVITY (CFS/FT.2-FT. HEAD) Q AVERAGE FLOW RATE (CFS) S1 UNSATURATED SURFACE AREA OF HOLE (FT,2) S2 SATURATED SURFACE AREA OF HOLE .(FT.2) H1 AVERAGE HEAD ON UNSATURATED SURFACE AREA (FT. HEAD) I12 HEAD ON SATURATED SURFACE AREA (FT. HEAD)

FROM FIGURE 2. D1d

S2 Ds1Td+Td2 H1 H2Du

SUBSTITUTING INTO EQUATION I.

K [(Dulld)(H2 Du)+(D51Td+Ttd2) (H2)I

40 K

11d(4H2Du 2Du244H2Ds4H2d)

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(EQ. 2) (EQ. 3) (EQ. 4)

(EQ. 5)

Q K (EQ I)

S1 H1+S2H2

FROM FIGURE 3:

H2 DEPTH TO WATER TABLE (FT.) Os SATURATED DEPTH OF HOLE (FT.) d DIAMETER OF HOLE (FT.) S1 H2lrd (EQ. 2) S2 D1rd f lrd2 (EQ. 3)

H, H2 (EQ. 4)

SUBSTITUTING INTO EQUATION Q

K ((Hjnd)(H2)+(D1Td+4-,d2)(H2)j 4Q

K 1Td(2H*4H2Ds+H2d)

(EQ. 5)

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.. D.OT. STANDARD TESL

EQ. I) 1 K

S1H1+S2H2

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FROM FIGURE I DUZ

d I H2

Id:=

FOR H2 > 3.0

4:

UNSATURATED DEPTH, OF HOLE (FT.) SATURATED DEPTH OF HOLE (FT.) DIAMETER OF HOLE (FT.) DEPTH TO WATER TABLE (FT.) H2 - 3.0 0.0-H2 0.50

FEET:

H1 H2- D= H2+ 1.5 (EQ. 2) S1 D1Td(H2_3.0)11d (EQ. 3) I S2 : Dsird+rd2: (IO.0-H2),id*r1d2 (EQ. 4)

SUBSTITUTING INTO EQUATION I: K [(H2- 3.0),id( H2+I,5)1 ((JO. 0.- H2)TTdt4rd2)H2 J

I K 40

i,2O.25 H2- H- 9) (EQ. 5)

FOR H23.0 FEET

1 SH

IWHERE ; S 7lpd + 1rd2 d: 0.50 FT.

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Q

(71.rd+1,d2)H2

K (EQ. 6) 11.192 H2

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Q KSH (EQ. I)

WHERE 0 AVERAGE FLOW RATE (CFS) K HYDRAULIC CONDUCTIVITY (CFS/FT.2-FT. HEAD) S SURFACE AREA OF HOLE (FT.2) H HEAD ON SURFACE AREA OF HOLE (FT,HEAD)

A P

1rd2 dH 0 4dt

Q AVERAGE FLOW RATE (CFS) d DIAMETER OF HOLE (FEET) dH CHANGE IN HEAD (FT. HEAD) dt = CHANGE IN TIME (SECONDS)

EQUATING EQ. I AND EQ. 2 2 dH

KSH

4KS dH dt= lid H

H2 2 4KS dt - C

5 'rrd

H1

4KS - t ) InCH /H

rrd2 2 I I 2

K -rrd2ln(H1/H2) 4S(t2- t1)

S SURFACE SAREA OF HOLE, EFFECTIVE (FT.2)

S i'rdL-I-ird2

L 2)+ Ds (SEE FIGURE 5)

S

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(EQ. 2)

(EQ. 3)

(EQ. 4)

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rYd2 n(H,/H2) K 4[[(HD] j2 ](t2-t1)

-. dlri(H1/H2)

K (2H1 + 2H2+ 4Ds d)(t2-t1)

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(EQ. 5)

V. TRENCH LENGTH EQUATION

A. VOLUME OF RUNOFF

43560 CAR( 363O CAR (El.3) I2

WHERE

C RATIONAL METHOD RUNOFF COEFFICIENT A : DRAINAGE AREA (ACRES) N ONET HOUR RAINFALL. (INCHES)

B. VOLuME OF STORAGE IN TRENCH:

EASED ON 5QO/ VOIDS: 0.50 WDuL (FT. 3)

WHERE

vV TRENCH WIDTH (FT.) Du UNSATURATED TRENCH DEPTH (FT.)

TRENCH LENGTH (FT.)

(.;. VOlUME EXFILTRATED:

(EQ. I)

(EQ. 2)

KH1WL(3600) (EQ. 3)

WHERE

VBOT VOLUME EXFILTRATED OUT BOTTOM IN I HOUR (FT.3) K HYDRAULIC CONDUCTIVITY (CFS/FT.2_ FT. HEAD) H2. HEAD ON SATURATED SURFACE (FT. HEAD)

AND: V..lDE LKS1H + KS2H) 3600 (EQ. 4)

WHERE,

VSIDE VOLUME EXFILTRATED OUT A SIDE IN I HOUR (Fl.3)

S1 UNSATURATED TRENCH SURFACE (Fl.2) S2 SATURATED TRENCH SURFACE (FT.2) H1 AVERAGE HEAD ON UNSATURATED SURFACE (FT. HEAD) H2 . HEAD ON SATURATED SURFACE (FT. HEAD)

-29- -C-i 04-

I IFROM FIGURE 6 WE SEE

S1z DuL I S2

H2-2Du

I THEN;

[KDULHZ- Du)+ KDsLH2]3600 iVsIOE

VSIDE 3600 KL(H2Du ü2 H 2DS) (EQ. 5)

ISETTING THE VOLUME OF RUNOFF EQUAL TO THE VOLUME EXFILTRATED: Q V8 + 2VSIDE

I3OCAR O.5OWDuL + 3600KH2WL t 2{36OOKL(H2Du4Du + H2 Ds )1 - - I SOLVING '. 4 - THIS EQUATION FOR L: -

L I. 00834 CAR (EQ. 6)

IK(H2W+ 2N2Du- 0u2 2H2DS)+

HOWEVER, CONSIDERING THE EFFECT ON THE ANSWER AND THE NORMAL VARIATIONS IN ESTIMA1ION OF C WE HAVE SIMPLIFIED THE EQUATION TO READ:

CAR L (EQ.7) K(H2W 2H2Du-Du2 + 2H2D)+ (I,39x O4)WDU I

WHERE

LENGTH OF TRENCH REQUIRED (FT.) IL C RATIONAL METHOD RUNOFF COEFFICIENT 4 DRAINAGE AREA (ACRES) R ONEHOUR RAINFALL (INCHES) I H2 DEPTH TO WATER TABLE (FT.) W TRENCH WIDTH (FT.) Du UNSATURATED TRENCH DEPTH (FT.)

IDs SATURATED TRENCH DEPTH (FT.)

I I I I

-30- -C-los-

FOR A DISTRICT AVERAGE 3-YEAR, I-HOUR RAINFALL OF

2.5 U4CHES THIS EQUATIOJ BECOMES, SIMPLY:

2.5 CA L

(EQ. 8)

K(H2W+ 2H2Du- Du2+ 2H2D) +(I.39x 104)WDu

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-C-106-