WRENTHAM BOARD OF HEALTH

SUBMITTAL GUIDELINES FOR SUBDIVISION PLANS, SITE PLANS OR OTHER TYPES OF PROJECT PLANS

Prepared by: William R. Dorney, P.E. , Agent

Any applicant who seeks review comments for a subdivision plan, site plan, or other project plan submitted to the Wrentham Board of Health for review and approval, shall have the project designer complete the checklist below and follow the guidelines that are herein described. The project designer is also referred to the actual applicable Board of Health regulations that are available at the Board of Health office, and which contain the complete requirements.

Any plan and related documents being submitted for review by the Board of Health and/or its agent, regardless of whether such information is being referred as part of a subdivision, site plan, or special permit process, shall be signed and stamped by a Professional Engineer, Registered in the Commonwealth of Massachusetts.

No plan shall be deemed to be "SUBMITTED" tmder Board of Health regulations, until (1) an application has been completely executed, (2) two copies all of the required plans, calculations, and other required documents, have been submitted, (3) the required fee has been paid, and (4) a copy of this executed guidelines checklist has been submitted.

All submittal items required by the Planning Board shall be included in the submittal to the Board of Health.

The Plan Content shall include all items required by the Planning Board Regulations as well as those required by the Board of Health.

STORMWATER MANAGEMENT - While the following checklist is provided as a guideline for review purposes, the designer must refer to the Board of Health regulations and is responsible for full compliance with the performance and other standards therein.

Designer has a copy of the Board of Health Stormwater and Runoff Management Regulations.

Hydrologic Report has been prepared which is stamped and signed by a Professional Engineer, Registered in the Commonwealth of Massachusetts, and includes a Table of Contents and has sequentially numbered pages throughout, and is based upon the methodology of the United States Department of Agriculture (USDA), Natural Resources Soil conservation Service (NRSCS).

Any Zone II of the public water supply or other nitrogen sensitive or limiting area is clearly designated and defined. Proposed system has been analyzed for the 2-inch storm, and 1,10,50, and 100-year storm events as established from data of the Northeast Regional Climate Center.

Both volume and rate of runoff amounts have been calculated. A tabular summary of results has been prepared.

Separate overlays have been included of pre- and post- development watershed catchment areas, including the soil types, hydrologic categories, CN values of the NRSCS, and the Time of Concentration flow paths and design points delineated.

Best Management Practices have been provided for removal of contaminants from the peak runoff from the 2-inch storm. Specific calculations have been prepared.

High groundwater determinations have been made in the areas of any detention or infiltration basins based upon soil morphology or by use of an adjustment satisfactory to the Board of Health based upon the methodology of Frimpter. The location of all test holes and monitor wells shall be shown, including elevation of top of monitor well, elevation of ground, date of water level readings (should usually be taken between the 22nd and 29th of the month), and groundwater adjustment used with supporting data, where applicable.

Hydrology Calculations

The methodology of the NRSCS has been used.

Overall watershed contour map at a scale of 1" = 500' or larger. This typically may extend outside the boundary of the project. Show Tc, CN, and Drainage Area for each subarea on the map. Indicate relevant structures.

Large-scale map at a scale of 1" = 1 00 feet or larger, showing different soils within each sub-area boundary, which may also be used to delineate drainage areas. Show Tc calculation and path used for each sub-area.

CN value calculations and work sheets included.

Times of Concentration calculations and work sheets included. Note that sheet flow components should not exceed 50 feet and are usually less .

Hydrographs printed out and show data and graphical representation for pre- and post­development conditions.

Tabular sheet showing stage-discharge-storage volumes for detention/retention facilities, along with supporting calculations. Include drawings of structures and cross sections showing elevations and dimensions used in the calculations.

Tabular sheet showing stormwater flow rates and volumes generated prior to development, for the development without attenuation, and the final discharge.

Basin Design

Plan of basin at scale of 1 inch = 20 feet provided.

20-scale Cross-Section view of basin showing detail of design features and underlying profiles of high groundwater, existing grade, proposed grade, soil strata, and impervious/bedrock layers. All test holes and borings also shown in appropriate perspective.

Geometric Design follows both Board of Health requirements and DEP Stormwater Handbook. Note that 4:1 side slopes and 10' safety bench is required. The width of the top of the containment berm must be at least 10' wide.

Water depth shall not exceed 3 feet.

Emergency spillway provided.

Maintenance Plan submitted.

Detention Basin

24-hour average detention provided for 2-inch storm.

Inlet and outlet separation has been maximized.

Inlet energy dissipater and forebay is provided.

Maintenance access has been provided.

Multi-stage outlet provided as required.

Ten-year storm will empty in 24 hours maximum.

100-year storm will empty in 72 hours maximum.

Infiltration Structure

The Wrentham Board of Health, Commonwealth of Massachusetts, acting in accordance with Chapter 111, Section 31 of the Massachusetts General Laws and by any other power thereto, and by any other power thereto enabling, and acting thereunder have adopted STORMW ATER AND RUNOFF MANAGEMENT REGULATIONS for the preservation of the public and environmental health.

These regulations for storm water management are intended to protect the public and environmental health by providing adequate protection against pollutants, flooding, siltation, and other drainage problems.

The storm water management design shall include a control strategy and plan for Source Control and Best Management Practice (BMP) for any particular development or project and shall accomplish the following goals.

A. Reproduce, as nearly as possible, the hydrological conditions in the ground and surface waters prior to development.

B. Reduce storm water pollution to the "Maximum Extent Possible" (MEP) Llsing Best Management Practices (BMPs).

C. Have an acceptable future maintenance burden.

D. Have a neutral effect on the natural and human environment.

E. Be appropriate for the site, given physical restraints .

F. Provide a sufficient level of health and environmental protection during the construction phase.

An acceptable storm water management plan shall

1. Capture and treat the "FIRST FLUSH" of storm, usually the runoff from the first 2 inches of precipitation for a small land area or other value as may be designated by the Board of Health.

2. Not cause an increase or decrease in either the total volume of runoff discharged offsite, or total rate of runoff discharged offsite, as compared with the respective discharge offsite prior to the development. Such condition shall be required for storms of 1, 10, 50, and 100 year frequency events.

3. Include source controls and design ofBMPs and Infiltration and Detention structures in accordance with procedures acceptable to the Board of Health such as are described in the following publications.

a. "Controlling Urban Runoff - A Practical Manual for Planning and Designing urban BMP's - Department of Environmental Programs - Metropolitan Washington Council of Governments"

Soil hydraulic conductivity based upon borehole permeability tests.

Complete Boring Logs and Details of Calculations submitted.

Elevation of high ground water, elevation of underlying impervious layer (ledge or clay), and saturated thickness of underlying aquifer has been determined.

Mounding of Groundwater shall be considered in the design.

An infiltration structure for a 2-year storm will have a minimum of 2 feet of vertical clearance (preferably 4 feet) to the high ground water with consideration of the groundwater mound.

Ten-year storm will empty (infiltrate) in 24 hours maximum.

lOO-year storm will empty (infiltrate) in 72 hours maximum.

Underground Infiltration Facilities to be preceded by an Innovative/Alternative stonnwater quality enhancement system that has had its perfonnance verified by the Massachusetts Strategic Envirotechnology Partnership (STEP).

OPERATION AND MAINTENANCE PLAN

Stormwater management system has an operation and maintenance plan satisfactory to the Board of Health in accordance with Mass DEP guidelines and good engineering practice to ensure that systems function as designed.

WATER SUPPLY AND SEWAGE DISPOSAL

Source of water supply is identified. If an on-site well water supply is proposed, evidence is provided that the proposed source will provide a quantity and quality in accordance with town, state and federal standards for the proposed use

Soil data, including percolation rates and high groundwater data, is provided to demonstrate that all building sites are suitable for the subsurface disposal of sanitary sewage where applicable.

Name of person completing this guideline: (print) ______________ _

Signature: Date: - ----- - ---- ------

II

STORMWATER GUIDANCE SERIES No.1.

24 HOUR RAINFALL

Prepared By William R. Dorney, P.E.

An updated Atlas of Precipitation has been published by the Northeast Regional Climate Center at Cornell University that provides more accurate data for the 24 Hour Rainfall and precipitation of other storm events than the National Weather Service TP40 - Rainfall Frequency of the United States (Hershfield 1961) which has been used widely to calculate stormwater runoff rates and volumes in Massachusetts. The updated atlas should be used instead since it is scientifically sound and up to date. Otherwise, structures for stormwater infiltration, retention, detention, and other BMP's may be incorrectly and/or undersized for real storm events.

The new Atlas:

Utilizes the advances in statistics methodology and computing power since 1961.

Provides results determined from data of stations having an average length of record of 51.3 years as compared to the data ofTP40, which had an average length of record of 22.6 years.

Recognizes that the frequency of heavy rain events has increased since 1961. TP40 encompasses a relatively dry period compared to the past 40 years.

Provides empirical adjustment factors to transform precipitation amounts pertaining to calendar day observations to maximum precipitation regardless of time of observation.

Analysis of the 1993 Northeast Regional Climate Center Atlas for Southwest Middlesex and Western Norfolk Counties, corrected for the 24-Hour Storm, results in the following rainfall values.

24-Hour Storm

1 2 5

10 25 50

100

Rainfall Cinches)

2.6 3.25 4.1 4.9 6.1 7.3 8.5

The title of the new atlas is Atlas of Precipitation Extremes for the Northeastern United States and Southeastern Canada , Cornell University, Ithaca, New York, Publication No. RR 93-5, September 1993. Telephone (607) 255-1751. A second publication entitled Atlas of Short­Duration Precipitation Extremes for the Northeastern United States and Southwestern Canada, Publication No. RR 95-1 , March 1995, is also available.

II

INFILTRATION POND DESIGN LATERAL FLOW CONDITIONS

By William R. Dorney, P.E. - July 18, 1989 (Revised 2002)

In certain cases, the groundwater is at or close to the elevation of the proposed infiltration pond. When this is the condition, the immediate area becomes saturated and usually a groundwater mound will develop up to and possibly into the basin. In this situation, the infiltration flow is more horizontal and the hydraulic gradient is fairly flat.

The use of flow nets or Darcy's Law can solve this problem. To assist in the use of these methods, attached are excerpts from two sources.

1 U. S. Department of Transportation, Federal Highway Administration, "Underground Disposal of Storm Water runoff, Design Guidelines Manual, February 1980".

2 Cedergren, Harry R., Seepage, Drainage, and flow Nets, Wiley Interscience, 3rd

Edition, 1989

This material should be self-explanatory. Note that both references stress the need for conservative design and careful investigation of site conditions because of the uncertainty of many of the parameters.

Additional Reference (Attached)

3 Dorney, William R., 2002, "Effect of the Underlying Groundwater System on the Rate of Infiltration of Stormwater Infiltration Structures", SNEC-SWCS Conference, May 2002.

Wrentham Board of Health

Effect of the Underlying Groundwater System on the Rate of Infiltration of Stormwater Infiltration Structures.

Presented at: Storm Water Infiltration & Groundwater Recharge A Conference on Reducing Runoff while Maintaining Water Quality

Southern New England Chapter of the Soil and Water Conservation Society May 21, 2002 (Edited April 2006)

By: William R. Dorney, P.E. , Consulting Engineer, 1 Brush Hill Road, Sherborn, MA 01770 Email wrd1 @verizon.net

INTRODUCTION

It has often been reported that stormwater infiltration facilities do not have a good record for functioning properly with time. Usually this is attributed to siltation over the infiltrative surface. However, it could also be because of an often-overlooked aspect of the design of an infiltration system. That is, the effect of the hydraulic capacity of the underlying groundwater system to accept water. This could be limited because of a shallow high groundwater table, a shallow depth to impervious soil or ledge beneath the infiltrative layer, or an inadequate thickness of the underlying saturated zone.

What does "functioning properly with time" mean? First of all, it is important for an infiltration facility to be designed to fully drain or percolate within an acceptable time period. One of the reasons is for public acceptance. When an open infiltration basin holds runoff for many days or even weeks without dissipating, there is bound to be complaints and outcries from the neighbors. Secondly, and technically more important, the infiltration facility must drain sufficiently fast enough to provide capacity for a subsequent rainfall event. Also, it is important from a standpoint of long-term viability, that such facilities rest between events to prevent sealing of the soil pores and to maintain an adequate unsaturated soil zone between the bottom of the infiltration facility and the high groundwater for contaminant attenuation. Most authorities agree that infiltration facilities must be designed conservatively to do SO.I

COMMON DESIGN PROCEDURES

To prepare a design to address these issues, one must detennine (1) the ability of the bottom of the basin to percolate water or downward infiltration, (2) the ability of the underlying soil to transport the water to the surrounding groundwater system, and (3) the ability of the groundwater system to accept the water. This will require a detailed investigation of the site.2

A designer must properly address these criteria. Unfortunately, it is not uncommon to find a design proposed where the designer simply calculates the volume of runoff from the design storm event and creates a basin to hold that volume. No consideration is given to the length of time that the basin will require to completely drain. There is only the hopeful expectation that it will drain eventually. This, of course, results in an unacceptable design.

Most designers will determine the size of the infiltration basin by the hydrologic routing of the design storm into the basin, using a computer program, which uses the methodology ofTR-20. The outflow rate of the basin is the value of downward infiltration which is calculated by using Darcy's Law (Q = kiA), where k is the Soil Conductivity, (determined by a permeability test), i is

the Hydraulic Gradient (normally 1), and A is the plan area of the facility. The vertical separation to groundwater may be taken from a government agency regulation or guideline. The time to drain will be calculated from the results.

However, this design approach does not take into account the volumetric capacity of the soil in the unsaturated zone or the saturated thickness of the underlying groundwater regime as determined by impervious soil strata. As the column of dry soil below the recharge area becomes filled during a rainstorm, depending upon the amount of vertical clearance, the groundwater may mound up to, into, and around the basin. The amount of the mounding is dependent upon the overall area of infiltration, the geometry of the infiltrative surface, the hydraulic conductivity of the soil, the fillable porosity of the soil, and the saturated thickness ofthe soil above bedrock or other impervious layer. Should the mounding of the groundwater reach the infiltration facility, this mounding can change the outward flow from vertically downward to horizontal. Once this happens, this causes both the hydraulic gradient and the infiltrative area to be drastically changed. As a result, the basin outflow rate can becomes only a small percentage of that calculated, and the unsaturated zone for contaminant removal is effectively eliminated. The basin may not drain quickly enough to have capacity for subsequent rainfall events. (SEE FIGURES lA and IB.)

ADDITIONAL CONCEPTS TO BE CONSIDERED

Once preliminary estimates of the basin shape, size, and general geometry have been made using the previously indicated procedures, a mathematical solution can be performed using one of several analytical methods that have been developed for determining mound height and shape, which are based upon the work of Han tush (1967) as well as others. While in the past, this type of a calculation was too cumbersome and unwieldy for most designers; it is now a relatively simple task with the use of a solution by microcomputer. From this solution, the designer can determine (1) whether or not mounding of the groundwater is indeed a significant factor in the rate of outflow time from the facility, (2) and whether or not there is sufficient unsaturated zone for contaminant removal.

If it is shown that the mound intercepts the elevation of the bottom of the infiltration facility, then the outflow area is only that of the sidewalls and then only at the downgradient location. The solution will suggest the actual hydraulic gradient that must be used, which will be only a fraction of the original estimate. Use of flow nets can also be considered.

The following hydrological and hydrogeologic parameters are required for all of the microcomputer solutions for the groundwater mound and must be determined by the designer.

1. Recharge Rate. (ft/day) during the recharge time. This must be simplified to be a constant value for the available Hantush method computer programs.

2. Depth to High Groundwater. (ft) 3. Transmissivity (T) of the underlying saturated zone. (sq. ft. per day)

This is the product of the soil conductivity (k) (ft/day) times the thickness of the saturated zone (ft). The soil conductivity is best determined by a standard borehole permeability test. While many designers attempt to use a simple percolation test for this parameter, it should be recognized that the same deficiencies of this test with respect to the design of septic systems

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exist in this application. While some investigators have attempted to correlate the septic system percolation test to permeability, they may only be site specific.

4. Specific Yield or Drainable Porosity (dimensionless) which can range from 0.15 to 0.25 in most soils . This can be estimated from the available literature.

5. Length of Basin (ft) 6. Width of Basin (ft) 7. Depth to High Groundwater. (ft)

RECHARGE RATE:

One of the first steps in the design is to determine the runoff loading-rate to the infiltration faci lity. First it should be understood that this value is not the average rate of inflow over a 24-hour period. Most designers in southern New England today use the NRSCS Type III rainfall distribution. This model does not flow uniformly into the basin over a 24-hour period. To illustrate this, Figure 2a is an example of the runoff hydro graph of a Type III - NRSCS storm rainfall distribution over a 2.6 acres drainage catchment area with a time of concentration of about 15 minutes which can be typical of a developed area with significant impervious area. The volume of runoff from this storm is 27,550 cubic feet. Note that there is a peak flow over a relatively short period of time. Obviously, during that time period, the inflow to the basin surges and is much greater than the 24-hour average value. Figure 2b is a plot of the decimal fraction of the volume of runoff with respect to time. This plot shows that 80% of the storm volume occurs in about 5 hours between hour 11 and hour 16. It also shows that 60% of the storm volume occurs in a period of about 3 hours. Average recharge rate over either of these time periods is a more accurate representation of the design storm.

For this example, using the 80% scenario, the initial recharge area design has a plan area of 4588 square feet. The 5-hour average recharge rate will then be:

0.8 x 27, 550 cubic feet 24 x ----------------

4588 sq. feet 5

23 feet per day over a 5 Hour Time Period.

It should be understood that the recharge rate into the soil can not exceed the hydraulic conductivity

CALCULATION OF THE MOUND

An example of a computer model input and output is shown in Figures 3A and 3B for the example in Figure 2. Figure 3A calculates the height of the mound with respect to time. A mound of7.3 feet is calculated by the time the recharge period ends. However, Figure 3B shows that this is a marginal situation and indicates that, while there will be little unsaturated saturated zone, the use of Darcy's formula will be valid for most of the recharge time as in Figure lA. Should the mounding have reached the recharge basin sooner, the outflow rate to determine drain time would have to be re-calculated using the criteria as shown in Figure IB.

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LL

8.0

7.0

6.0

5.0

Vl u.. 4.0 u

3.0

2.0

1.0

0.0

10.0

1.200

1.000

0.800

0.600

0.400

0.200

0.000

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20.0

Groundwater Mounding Analysis (Hantush's Method using Glover's Solution) Mound on AXIS

INPUT PARAMETERS Application rate : 23.0000 cFeeUday/sFeet

Duration of application: 0 days Fillable porosity: 0.20

Hydraulic conductivity: 91 .00 Initial saturated thickness : 30.0 Length of application area: 95 Width of application area: 48

FeeUday Feet Feet Feet

No constant head boundary used Plotting axis from Y-Axis: 90

Edge of recharge area: positive X: 24 positive Y: 0

degrees

Feet

Total volume applied: Feet

209.80+02 cFeet

MODEL RESULTS Plot Mound

X Y Axis Height (Feet) (Feet) (Feet) (Feet)

-250 -0 -250 0.00 -210 -0 -210 0.01 -170 -0 -170 0.06 -131 -0 -131 0.27

-99 -0 -99 0.76 -75 -0 -75 1.53 -55 -0 -55 2.61 -39 -0 -39 3.98 -24 -0 -24 5.65 -15 -0 -15 6.68

-8 -0 -8 7.08 0 0 0 7.25 5 0 5 7.19 9 0 9 7.04

15 0 15 6.68 23 0 23 5.78 33 0 33 4.55 45 0 45 3.39 60 0 60 2.33 78 0 78 1.40

102 0 102 0.69 126 0 126 0.32 150 0 150 0.14

9

h

PREDICTED QHOUHDJ04ATER HOUNDINQ

HOUHD

D ....... iP.-tiOTl o d a y .

Groundwater Mounding Analysis (Hantush's Method Using Glover's Solution)

MOUND vs TIME

INPUT PARAMETERS

Application rate: 23.0000 cfeet/day/sfeet Duration of application: 0.2 days Total simulation time: 0.8 days

Fillable porosity: 0.20 Hydraulic conductivity: 91.00 feet/day

Initial saturated thickness: 30.0 feet Width of application area: 48 feet

Length of application area: 95 feet No constant head boundary used

Groundwater mounding @ X coordinate: 0 feet Y coordinate: 0 feet

Total volume applied: 211.10+02 cfeet

MODEL RESULTS

Mound Time Height Time (day) (feet) (day)

0.0 0.00 0.2 0.0 0.30 0.2 0.0 1.00 0.3 0.0 1.91 0.3 0.0 2.71 0.3 0.0 3.44 0.4 0.1 4 .13 0.5 0.1 4.81 0.6 0.1 5.51 0.8 0.1 6.29 0.2 7.28

Height (feet)

6.51 5.16 3.96 3.17 2.59 1.77 1.45 1.15 0.86

PREDICTED GROUNDHATER MOUNDING MOUND vs TIME

B

7

6

H :5 ..

9 4

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t 3

( Cee-t)

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REDUCING THE GROUNDWATER MOUND

If the groundwater mound penetrates the recharge basin and causes outflow difficulties, there are ways to prevent it as follows.

1. Change the Basin Geometry - In general, there is a higher groundwater mound underneath a recharge basin with square, circular, hexagonal, and triangular shapes than when compared with a rectangular shape basin. In other words, a long narrow recharge basin will have a smaller mound than a square one. For example, for this example, if this 4560 square feet recharge basin had been 10 feet by 456 feet, the mound would be 2.4 feet. If it were 20 feet by 228 feet, the mound would have been 4.5 feet.

2. Increase the Plan Area. For example, if this basin plan area were increased 50%, the mound would be reduced from 7.3 feet to 6.2 feet.

3. Reduce the Depth of the Recharge Facility - Underground recharge are often designed to be up to 5 to 6 feet deep using various configurations of plastic and concrete chambers. A shallower depth should be considered to raise the elevation above the elevation of the mounded water table.

ALLOW ABLE DRAIN TIME

Also to be considered in the design procedure, is the allowable time required to drain so that a basin will be able to handle consecutive-day storm events. It is reasonable to expect that where complete infiltration of runoff must be performed and no other outlet is available, the system should be designed to completely drain in 24 hours for the 10-year event or smaller and 72 hours for the IOO-year storm.

CONCLUSIONS

A straightforward method of design for an infiltration facility is available using an analytical model microcomputer solution. This can provide greater assurance that the soil and groundwater conditions are amenable to the infiltration of stormwater. The effect of the groundwater mound on outflow rate and contaminant removal can be easily evaluated. By placing the bottom infiltrative soil interface sufficiently above the high groundwater when mounded, optimal contaminant removal can be provided. The determination of the outflow rate will be more reliable, so that the time to drain will be able to accommodate subsequent rainfall events, and will also not cause a safety hazard from too lengthy a detention time in residential subdivisions. The designer can readily evaluate and compare the use of alternative shapes and geometry to minimize the adverse effects that the groundwater mound will impact.

REFERENCES

1. Stahre, P., and Urbonas, B., 1990, Stormwater Detention for Drainage, Water Quality, and CSO Management, Prentice Hall.

2. Cedergren, Harry R., 1989, Seepage, Drainage, and Flow Nets, 3rd Edition, John Wiley & Sons.

3. Hantush, M. S., 1967. Growth and decay of groundwater mounds in response to uniform percolation, Water Resources Research. V. 3. pp 227-234.

4. Molden, D., Sunada, D. K., and Warner, J. W., 1984, Microcomputer Model of Artificial Recharge Using Glover's Solution, Ground Water, v. 22. No.1, pp 73-79.

5. Smith, Stephen W., 1991. Hantau and Hantaxis computer programs for groundwater mound development over time and space.

6. Sunada, Daniel K., 1985. Flow from wells and Recharge Pits computer program, Colorado State University.

7. Finnemore, E. John, 1993. Estimation of Ground-Water Mounding Beneath Septic Drain Fields, Ground Water, v. 3l. No.6, pp 884-889.

8. Rastogi, A.K. and Pandey, S.N., 1998. Modeling of Artificial Recharge Basins of Different Shapes and Effect on Underlying Aquifer System, Journal of Hydrologic Engineering, pp 62-68 .

9. Todd, David Keith, 1980. Groundwater Hydrology, 2nd Edition, John Wiley & Sons. 10. U. S. Department of Transportation, Federal Highway Administration, February 1980.

Underground Disposal of Storm Water Runoff, Design Guidelines Manual. 11. Allen, Dan H., January 1980. Hydraulic Mounding of Groundwater under Axisymmetric

Recharge, New Hampshire Water Supply Pollution Control Commission. 12. Finnemore, E. John, 1995. A Program to Calculate Ground-Water Mound Heights, Ground

Water, v. 33, No.1, pp139-143 .

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UNDERGROUND DISPOSAL OF STORM WATER RUNOFF - DESIGN GUIDELINES MANUAL

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FEB 80

U.S . DEPARTMENT OF COMMEttCE National Technical Information Service

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APPENDIX 0-8

METHODS FOR ESTIMATING INFILTRATION RATES

As noted in Chapter IV-A-3 of the text, Darcy's law and flow nets dre two useful methods for analyzing potential

infiltration rates when the permeabilities of the for ma tions are reasonably well known. Some theoretical solutions to seepage problems mdY be too rigorous for the assumpt ions

that are made in the derivations and may fit actual cases only approximately. Most engineers experienced in seepage calculation feel that it is far better to make use of an approximate method than to rely on a rigorous theoretical formula that is not easily under st ood and which represent only a crude approximation of true conditions due to questionab l e assumptions. Some simp l ified procedures are shown in Fig. 0-8-1.

a. Vertical Flow Case

In Fig. D-8-la an infiltration basin is constructed in soil formations having a vertical un satura ted permeability of I ftlday (3.5xIO- 4 em/sec), an impervious stratum at 20 ft depth ( 6 .1 m), and a natural water table at 10 ft

(3.05 m). What is the capability of th e site for vertical discharge of water? From Darcy's law, g " !J.A, the flow can be estimated from the vertical permeability of 1 ft/day (3.5xlO- 4 em/sec), and a downward hydraulic gradient of

1.0 (conservative assumption); hence q a 1.0 ft/day (1 .0) " 1.0 ft/day/sq ft (3.28 m/day/m 2)

For a JOO - i t ( 91.5 m) wide basin, the possible Y is 308 cu ft/day/1 ineal- foot (27.54 mJ/day/m).

0-8-1

{el An txnmple of dowr.word fl ow f,om infillralion bo~jn in on area wilh a 10 .... wo ler loble_

i " (T ·,'-.j I " . .. .' - -.:::--, "": ~ ~:-c"~i ·- - 20·v':o-. "l _ . .L I.=--=-c:r - ,~­

I . -- - . -.,{ LJ -LJ J_.d,. ', ---.:- . •. -,..._.. ....-c - - : ---- , - -. -'_. ---- ---, -- -- '--·- 1--'· _ ··_--, ---~-l----'-'-'-,-

o 50' 100' 150' 200'

(b) An e,.cmple of lalual fl ow. Shvws lower side of inlillrol ion bo,J n on area wilh high waler Ic~le .

FIGUi([ 0- 6- 1 ESTIII,HIIIG IIi FI LT RATID II HATES IIITIl DARCY' S LAI I Alal FLOII II ETS (,~ PPROX l I I ATE I;E TII00 5) (a) 001l1lllAI10 ptRCO~,\TIOII (Il) LI\TE HAL ( II ORILOIIT I\ L) SE UflGE

b. Late r a l f10\'l Case

l1 a ny si te ,; th at arc capdblL of relativel y l arge disposal

rates wh e n the und e rlying wa t e r table i s low, become

rclatlvely valuel ess if th e water table i ~ high and flow

is lilt e ral. fig. 0-8-1b illustrat es th e flo,·/ conditions

at th e ed gc o ¥ a basin whcre thc groundwa t er mound ha s

ris e n to the bettom of the bas in. Th e flow net gives

thc shape fa ctor, nf/n ,.\' which is us cd in thl' f oll o,;i ng expl'csS I on ~o es ti/;;dtel a tera 1 seepage:

q (0- 8 .1 )

0- 8- 2

Equation D-8.1 is the conventional formula for estimating seepage quantities with flow nets. The seepage quant{ty per linear foot is S. under net head ~, while nf is the number of flow channels in the flow net, and n~is the number of equipotential drops. In this example, assuming a horizontal l = 5 ft/day (1.53 m/day), and a head of 20 ft (6.1 m), with the shape factor of nf/n d = 1.1/20, the seepage quantity is:

q = kh(nf/n d) = (S ft/day){20 ft)(1.1/20) =

5.S cu ft/day/foot (0.50 m3/day/m)

If the flow is limited to only tha low side of the infiltra­tion basin (as assumed here) the potential for lateral flow of this basin is only about 2% of the vertical flow capabi1-ity of the basin in Fig. D-S-la; even though the assumed lateral ~ is 5 times the vertical ~ in that example. When estimates of infiltration are made by the methods suggested here, designurs should recognize they are approximate only and should try to use conservative assumptions so as not to overestimate the capabilities of infiltrat~~n systems.

0-8-3

11.2 INP1LTRt..TION ?Of-lCS

::;zsic Concepts

~~~~~~~~~~g~~~i~~r~'g~~~:;'~~~ ~~o~~:.,:~~~' ::/~~~ [~~~~~i:d~!~:~!~:~~:~":; '.II'l::. 1:1I1!:: .a:..ion pC:1Cs ar: ce::.& bt:I:c i:: r=2..-:~' =':::2..5 to i!.d': ~ o :~.: ::r'l :=.!!h­men( oi s:"ct.!nd·,;Iat!'i and tD pu~:y t~e S':J.~~ i}' ~i:a~ !S re:uril!= : 0 a-:jac:::t s:r :::ar.5. In C.1.IUo;:1iil'S C::.l~;,,:ti Val!::; r.~: : :::.s i\''! ?cn~ S:','5:~=S C.r~ be:~~ u.se': for the.:;: pur;::oses, St;)rr::w.1: e~ runoff:s b!:ng ~:i,~rut!d by ir:~l::a{ion f iiiC:U­ties de!:;~!:lcd i!.S p::.rt 0: rna:,:)' hi~hwtiy and .al:-;'3c::! p2.ve:nem sy$~!::1S . Fiooc fi o\l.'s in rive::; i:: d:-! \'all-:ys u:: :c:::ng pO:Jdec ;,y s:7l.:.!1 :e ~aiC".in g c :l:!s 0 : cG...::::.s :.! ~dl t!~= \"a!~: C"'.:..n so~k in:;) 'h:: &:ou~d .

wnen W~{:: is b:ing r:tu rne:f to g:ol!ndw~:::: syz;:e::u , it is L-n?or~2...'"lt :..:iar con:ami:lation be zvoiC!~. )'Jso 1 in sy~~~i.l.S desig:t it !s i:n~or.z.nt [Q b! S'i.!: ~

!:~3.r no k;;:,1 wa!~r righ~ z:-e :;c:ng vio/ar:=. Designing h fHc:2tioo pond sys:c!':"..s :- eqt!J:~ ca:-::ft.:! esti:::2tiOr: o f :;:: ~1.!'-1·

(id~ o( W;UCi that will be put :r:no a g-:\'en S'ys,~:7l a.'1d the a;:pEC2!Cn of s.:.:?ag: ;;rinciplc:i [ 0 d~t!:-mine the siz~ :md d:: .:tils of 2 fac:Jry ne::::!ed for cfspcsbg or ,h~ W:i.ce:-, Prev::~don oj cbggL1g of ,;;e SU:-j'l~ by 511i. mud. iL,d o,j{h~; ii:::':: !'f ::~::icd by '.lotte::- is z m:;jo :' problc:r. in a.r.y .!i:or::: wa:::- o r cr::i:e:: jcw:::-.. g: wat.:r collc::rion .!nd inflh:-adon syst!:n anc goo':: r.:J.:nt:n2..."'lc:: prog::a~s z:e C!.s c:nual to th:~r condnaed SUCC!ss . . Also, ~ood moru:~r:ng progr2!I'''s (or : h: :l:ing grol.!:1cw:u..!r Ic"'e!s a..'ld qt:aficy a.r:n.:.n: ponds a:e esser:t:zl to tJ!~:­: fr::=:::ve, ~2.fe opei:!tior. .

Infil tration ponds C:lnnoc be e:[::~\,tlr l!!je~ l!r:lc.:s soli wd g:cundw:H::: concitions nre filvorab!e to c,:,ci: I..! !i ~ . D~;; i pe:-~ .:: ~ (')uid ::r.aJ:: de::!..ik.:::' j r:'1e.5~i~

;;a:ior:s at ~i~t!5 znd t::;,! J2.rcy 's l2.w .:nd at:l !!:" 5!e~<: a; : :;::i:nd;::i:...: rc 't'e:-:fy LI.: .::

follow!:lC':

1. Tn! c2pabili~y of the bocwrTls of penes ~c i:1n.l rer- W2.u:; on a long<=r::1 operadon21 basis.

2. The c.apability of th~ unc!::-!::ir:g soil to cisch.uE::: the inf1vwti1g Wi!:~: inw t:1:: sUiTound:ng g:ou:.td· ... ·zter syste::;·!.

j. Th~ ::apa.biiic:1 of (he sur,ouncing ground',vJ(:!r sys~e:il to ~c~:;P' Wac!:.

Accumula:ion.s or colloids :.L'1d oc~~ !' fin~ 1:1:lteriais :cat se~Ll :: out of t.h~ water in i:tfJl!raLion ponds can grJ.uuaily r:duc:: the r:!t~ of lnfU:;-at ioo. On! v::y successful sys;:.::n in Cal[fomi2. had apprO,tifLately 1,200 acr~s of ponds. About one~ t rjrd of the ponds we:e b~:ng illcwed to d:-y oat on a regula: Dasl.s. so chac the bonDm coule:! be scarifid or s':::-~;:l~d 2.S ne::essa~f to inc:easc t..t-;~

p(::ocn:,iiicy 2.nd bring tIH: infil:...-acicn :2.t~ up to satisfilctory Ieves, ',.Vien tl'!i3 mz.ln ter:anc: prlC"Jc.:, L11!Se pones W~:' ': k.:;::! in Y::-y :f::::dve ope:2-d . .''lg ccmci~ lion . A nUCloer of !!n:s of moni toring w(:ils w~;"~ ir:.5 .. a.ll!:d ?....round this in:-ut:~~ tion systc;n snoiJy after it WIl.S !1!"-.; .. put imo o;:~ ;'":!::ont ai::.c, Wi!t ~:' :ev ~! !'c<ld~ lngs we=-= lJl.2ci:: r::guluiy to dc~.:rmja: if '::""1)' r~c;bg cn:nc:; could i.r.:p i!.ir th~ di!.;:harg~ i~t:'!:.i imo sUITQUndi..1g J.::~. Aft~: ::!. .se~~ of un l..:s~::illy w ·'!t Y~:l:S,

tbe waC.:":! /':v~L .. \;nd::r [be ponds :-05': (C l~v:!.s L~.:!: 1,1,':::: se~!J 3.5 approa~~:n;

conditioo5 C!I.<:!.t could g:eady rceue!: .. h: ei.-:::ti'le ~l ~:; s or :he sysc:::n, Ac::arci ­i.."gly. \1 number of deep, pumped wells wee imt2.U!:d, and pUI':1pcd for il few mon:.hs to lewe:- me wa[~r k'/!!ts t.1 lC:!~l!a{ ! d-:prns. This W2. ~:' W?..5 piped ;lW2Y fiom ~h!: sit~ to m othc: dillip0!;J...! ar::=., T.1:Je te::l porary ITi .. : a.su.: ::s h~:p~d to k::;> t.his EYS~~=- ~"! good conci!tion.

Illust rative E.~amp l e5

Fi~ ~:-e 11.6 is J simpli rl c:: i!!usr:-3d0n c( ~ow fref':". ::::-l!ciJ.tion pone5. A :00-acre plot (Fig . 11.6a) had b(.:".:n prcp0:id for an 1n1-Ucra.t!on pond in a cry r!::tt.:i:ed {Q dispuse of 20 miilion g21j.,~i15 ::l C2!, of cre:m:d seW!r2.g~. The ~j:::

W:!S covc:--:d \;.;th 10 it of sa..'1dy soil with 2. '1e:-JcJ.l perrn:''l bm~j k. = 1 f:leay ::l:1d :l hor.;:ol1!.al p:rme:loiiiry k lo ;;; 5 ft/d2.Y. Th e ·,I.·c:.~~r mbk swod at C!. lO-f{ ci ~ p{h, a.1G an i::l pe;.nc:! bl!' clay I:!y~: J.y ~ e~:~d :u :::0 fL

Toe capabiEry of the site [or dO'.l.llw<:.:c per'<':'oiation WJ.5 c:?"'!c!.!bred by Da;· cyls bw (Q = kiA) by L!Sina the I? "!; irl! plct: ::r.~:J il.nd il co ' .... nw ?.rd hydr:luiic g:::;di'!m of 1.0 (Fg. !! .6b) :as

Q. = ki .'1 = ),0 .ft/dayiJ.O) (2640 x 1650) "" 4.~OO . OOO cu fl/d~y (Jr' 33,WO,OGO g:o.l/coy

Th!s was ;r~o t! tn?..:, a:i~~uatc tc: rn~:::t the d:y/ s n:;t:is; however, (his rate of infLl::;!!.ion (LO cu ft lsq ft / c2Y) woda f:U che (O· f( colum n of .soH <!obove the wau:t :a:,ie !:13.5 d:!.)'s (assuminl! aporc.s::y "fO.35). The fio',v wQuld the:J. sudd~n1y ch:u:ge f!'or.1 \'e~ka1ly dow!'! ';.·uC:o hor.:cn ~ a.! (Fig . lLoc) L!.."ld t:.':::: aoi!:{: ' of {ht si'~ te· d.is::-:arg: '''''3.:'::: wcw.id b~or.i:: Q" = k:·..-!. i:1 which hed]

i-----·----··---- . 20'0' - -----------'

I I/I ! I I '

--~ ----)I

~ II I I

===,==,:;== II ,Cil:.e Wmr :.u~¥.:!: '\ $.i['.H'd:icn mavin']) Oik~',

---,," ,~~. ~_. d:wnWllrJ . . w • . ' ---'/~; O' (l:J i. I. e- O;j<j;fI~i :'r.mf ::;'Olt;:::r --.::,.irl

U--TiO• , ... ,.~ .. "" .... . ...... , ... .. ~Q, \ · ' .. , . ..... " " .... ,(.~·' .. ·· ·C! ,y .-«"" ....... , ....... ~ ....... ''''_<,."' .. ,." .. ~~:::::,:7Z:::\~

FlC .. 11.5 Wust .. .:don t'Jf flew from infihr.:dcn ;::::m6:. (a) ?l~n of JCC .. tl~Te plOL (b) Cr:::::s s~_-::ion ~howing initi:ti condi:.iol1 wi:h downward fio w. (c-) :::::dg::: s~::::jon 5.1.0Wl!lg pe:-c:lJ!l~nr condic!on with hor:::omt:! (1a:e:-J.!) flow. (c) A~ot!:te; 5;(·: wi ... i u.:l de~~yir.;

pc:-:ni::JO!c lay·::s anc much be!:!:: dr 2.ioage .

i 2nd. .. 4 wcuJd b~ sba ... ·''PJy : edu:!c. A rr.UC.1 small:: : hydra ulic g:-acii(!nt would 2.ppiy, and ~ 'l wouid becom~ -::he perir:1c::: :" ~cng:h r.i:-n::.5" L1e cie~ c h of s<::.:t.!ra! ~d soil c Ls::harg.:ng ',\'m::: olHward. By usi::g the v:!.JL:e.s for i <".no A shown i:1 F:g~ urI! 11.6c

Q.~ =: KIA .;;; 5.0 ftldai'(O.02) (l5·1 ,COO 5::; f t)

::= 15,400 C:.J ft / dr:.y :); ! 15,000 g2.'/d2.Y (1~$S than I ar~ (,If the rl.:ol!i red ra te )

Even chQug..fJ the ciesign:!f had originally rc::oln..r.Iend~d the site (he had c.1lc~~ !ate::! o rJy Q~j , i: had to be dis::arded.

Fer .seycnu ye1..:s .a n~a:by cilY O.1d be!:1 dispcsir;g of 5 r:-illio;: [;:aJ/ c.zy oi tr:!'a~::d se ',,:c:ag:: on a 20¥ac!:! pier, whic r. h2.d !ed ~t~ ci.'!sigr • .::- to (h i:;.k tha~ t~e lOO-ac:-e si te ::ouid hwdk 20 !TIUuo r! g~J/c.<!y re:lcily. T:t~ ~O-ac:-: piot ,

howcv~:;- , is n:ar a river b~r1.~ a.nd is url'':cr!aid. by higtly pt:rm:abk g:-zv!:~.3

(Fi~. j; .&1)1 wrjch provide r2s= :.md~;dr2.1n.16!! a.=lQ a.11cw p"!~ .... '1a"''!~:1t dG~\'::"v

Obviou:;ly the ca?a::iii(i::-~ c·r si t!S co r:~o~'c inf:.l rr~diJ J can \'~}' sub:z:;­tiaE)' 2..:".iC depend nor or..iy on tn~ c.iep:r. to wa:.:t bur .also or:. sUDsur:a::e ~:.i~Civ tians. Tllcrocgh s:uci!s 3.re nedd if r::.a.;;;:JGabl: !stirr:a:::s 2.:: to be r:1a~:! c;: possible cii.s;:harg:: ;-.:ues fo r indvi eiu J.1 sit!::;.

Figur: 1 1.7 iilu:irr<!tes the \v <!~ re:;uhr r:::ndings of w;:ncr ]eve!s in r.:oaj tori:ug

c . I rtC5:oe!":o:!! ;u~::;' ~:m .. r:(!r:;! .. 1 deve:c-;r.;el1:":;

I r lS.!O · .... ate' level> ---=::-::::-:::..

~~~:~:,- ;;r-j:_i:L~Bi:J <~5~-:~~ - r F. !930 Ii 1 I Ii! I I I , ! I I i --. - !-l "7 m~u~' ; i :.1 l M i j >-.'-... w~ n~ ~CO

!~ t 61's·'lcj'l2 . .. !'5

0

I J Mile:

(0)

F1G. 11.i m:...:s: :mior: o{ :':Ior.it::J:i ::g of r:,:o::::c ,,""a,:: SYS! C.T..s a:ocr.d ;:~ ;l it:~:.:cc

penes. (a) Plan s~:::>wing gi'OU:icw~:,:~ ;on~c'J!,S o!::we::::--, $0;:1 : pO:1C~ <L""lC en ad)ac: :l: c: ry. (b ) S:;~icl~aJ view ~.h~wi.ng g:-·::t.!f,: !:; 'Je! lnd s=v:::~a: w:.~!!:' !:- ... .:!..$ :;\"::~ 4 ~'';..:'' C2...

oC :- lod. ($!'':. A·A.),

wc!l.s arou:ld infiltration ponds and ~~c d:vdof;me~t of gro12no''''''2.!:!r como!.:::­:"P.2.;:'~ ~ 1 ~ b:::~ i:1~!:;2~i'J:1 syst::r: !7:2.!'!2.g=r<:: £1,b~e.2.;~ or" (he beh;:.vior of t':"!e .grouncwa:l:r s ysi~rn5 being used for eh:: cispos:l1 nf their water. ,6.. cicy locate~ in (he e2..Stc:-n half of rht! area shG·.~.:r. in Fig~rc l L i a is piping its trt.:3.[C~ ·.I.'2...:S ~e·

W:2ter through la:g! trunk sewe:-s t:.1 a.n 800·aCie pond sySt~:::n, w·=:r.t of t.~! d:· ve!op::d a.re~. Th:: !2.nds 5urrcur.ding the pond.,; ar~ devot~d to ag:icultura.l uses. Sillce the proje-.:( i.s in a \\'a::n 1 sunny arc.2, par: of the W;!(er is eriminated by evaporadon , bur a Jarge porrion 5.~aks t!1rough (he OOCtOf;'tS of (he pones

to reach the g:oundw:uer below. nii5 \Va:!:!"' buijds up a snruration mound of sufficient heighc arid see co dispe!"'se th: ',vat:; into the surround:ng g::-oundwa­tef aquir1ers. The system W2..5 first put in ope:-adcn ~b()U! 1900, In re::::!= y~ars, heavy well pumping throughout the irj~nd V:l.;i::=y in which this city is ioeated 1:2.5 been progr:~5iy::!y lcwe:ing the w;;,t: r in the :::'irire ar:J., Dispos:d 0 f tt~m~:i

s,:'.vJ.~e inw the grOt::~r. r.d~s :0 ,slo',.\" down t!:::! :"21;' o f the gTot.mC wz. :er b. en:!i ar:3, OCt it is sri1l going dO\o\.'"jJ .lC a :·~~iriy r.a r:ic :"a~c, 2....'i i11.Z)' be .se::a by en'! war::r leve! profik:.i in S:!c , A·.A., F!gur·~ 11. 7b, fo r 194 0, ! 960, 2.....'1d : 980. Pun:ping f:-om d ry wells is ::cnrric.:!.luI!g h~a\'ily .0 :he lowe:-ing of ~h.~ W:'::1'

t;~bi:! to tbe east of the s~wage pOJCs. It:: 1980, th e LOp of c h~ groundwat~!"' n:o~.:1d u:-h:e: the ponds ';:"'2.E at 2JOt.;::

~hc same eic-;z..r.icn 2.S the gro t: "jJd·"",Q.~: :," le .... d t:i"J.':~::- :h:: C:~", nine ;:-;.il::s :0 t:le !:'2.St of the ponds, but th: ',\-'2.r::r t::!b!~ ced a positive slope from tbe ciry toW2.:d th= wc.:it. wh.ich c.ade it impossible for w;u-:r fro!:1 tbe pc nds to reach L'lC d:y)') water supply weBs, Sjnc~ :r..e !:C'Nlg·: ;:>12..:1: ef!1ue":"!t :5 ~;-cu[ed, it is hLgzly CD­

likely th 2t it could ha::.n the dry's W"2.~er ~<.!ppl}' ~'I.'!n if ic co u.td r:uc~ some o f the :::ry's w~H.", Neyc~he1ej:s, m COC3JJy e.limin:!..u: :::.(1)' possibilicy oi degr2.ca­JOG of d:·: cilis ";::l~er S:!;Jpty by the {c: .. e:.i 'N(Ue:-. L1C p!~t ru~2..g!rs have i1"...s~a..lJcd wells aro und and LIne:!' the ponds . at:c have: periodic..:tUy op=;-at~d

che we!Is to pre'/~r.r sprl!2ciing of Lh~ in.f!lccrcd W,H;::: toward r!1e ciry. Thus, with good monitoring :md. sound mz..r:2.;e:::.e":".c p ;-:!: :':~!:$, sub~'tC....:"1~:<:...i a.:I!O UntS

of \,.:at::r ar~ bei~g returned to [he rei-on)s aqu.ifk:-s fo r a pa:'"'ri?J ::'/~rsaJ of L.1e dcwnw",-rc trc:ld in the grDunCwat::-.

SOARCE LIST FOR STORMWATER MANAGEMENTPABLICATIONS

1. Control Urban Runoff - A practical manual for planning & designing urban BMP'SDepartment of Environmental ProgramsMetropolitan Washington Council of Governments7777 North Capital Street, NortheastSuite 300Washington, D.C. 2002-42226

2. Storm water Detention for Drainage. Water Ouality & CSO ManagementBy Peter Stahre & Ben Urbonas

Prentice Hall - 1990College Mail Order Sales DepartmentEnglewood Cliffs, NJ 07632

3. American Societlz of Civil Engineers Publications

(a) Desisn of Urban Runoff Quality Controls - 1988rsBN 0-87262-695-4

(b) Urban Runoff Ouality - Impact & OualitvEnhancement Technology - 1 986ISBN 0-87262-577-X

4. Urban Surface Water Management - by Stuart G. WaleshJohn Wiley & Sons Inc., Publisher1 Wiley DriveSomerset NJ 08875

5. Underground Disposal of Storm water RunoffDesign Guidelines Manual PB83- 1 80257

National Technical Information ServiceU.S. Department of Commerce

6. Dept. of Environmental Protection Policy Handbook - 1997

7 . Dept. of Environmental Protection Technical Handbook - 1997