Design and performance of exfiltration systems

Abstract

In urban areas and for other watersheds with a high percentage of impervious area, the only option for stormwater management may be underground storage and exfiltration. the use of surface ponds may be very expensive because of land cost, and other stormwater systems may not be available. Therefore storage in underground trenches from which water will exfiltrate may the only cost effective method available. An exfiltration system is a stormwater management practice used to reduce runoff water and its associated pollutant mass, from direct discharge into adjacent surface waters. The exfiltration system, commonly referred to as exfiltration trench, is typically constructed of a perforated or slotted pipe surrounded with a bed of aggregate. The aggregate is wrapped with a geotextile fabric used to physically separate the system from the surrounding soil. Runoff is diverted into the trench where it is stored and eventually released, or exfiltrated into the surrounding soil. Construction practices for exfiltration systems were observed. During construction, care must be taken to minimize soil and debris entering the system. Tearing of the fabric during installation can be a problem, as soil can wash into the trench after it has been back filled. Use of arr aggregate with minimal fines is also recommended. Any method used to reduce the amount of fines entering the system during construction will extend the life of the system. Four design variables were examined for their effect on performance of the exfiltration systems. These were concentration of solids in runoff, soil type

surrounding the system (silty or sandy), fabric type (woven or non-woven) and location of ground water table ( systems were either kept wet or allowed to dry between runoff events). Sixteen experimental models were constructed, each representing a different combination of the above variables. These models, which were designed to store one inch of runoff, had a storage volume of 1.51 ft3 and were dosed with approximately 2.5 years of stormwater runoff, assuming 50 inches/year of rainfall. Results from the experimental models indicate that an approximate 20 percent reduction of suspended solids (SS) in the runoff did not increase the system exfiltration rates relative to those systems that had no reduction in SS. Soil type, measured by the permeability of the soil, was not the controlling factor for the exfiltration rate from the systems. This rate is limited by sediment buildup on the fabric, not a function of the surrounding, or parent soil.Models were constructed using either a woven fabric (Mirafi 700XG) or a non-woven fabric (Mirafi 140N). The exfiltration rates through both soils, given the same parent soil type, were not statistically different. But the models with 700XG built in silty soil had a higher limiting exfiltration rate than those in sandy soil, and models with 140N built in sandy soil had higher rates than those in silty soil. The systems built in a high ground water table had less storage volume than those in a low ground water condition, but the level of the ground water had no effect on the exfiltration rates. Examination of the e:xfiltration rates through the bottom half of the e:xfiltration system showed that over time the rates decrease, but level off at a rate around one half to one inch per hour.

These are limiting exfiltration rates and should be used in the design of exfiltration systems. Long dry periods and introduction of hydrochloric acid into the trench were examined for possibly increasing the performance of a trench. Exfiltration rates increased after an extended dry period, but this increase was a short term phenomena. Acid was introduced into the models with pH ranging from 2-5. No significant increase of exfiltration performance was noted The timing and volume of runoff into an exfiltration system affect the percentage of runoff water that is exfiltrated into the ground Storm events separated by only a few hours of no rainfall have a cumulative effect on the storage within the exfiltration system. When a storm event occurs at a time when the exfiltration system has a substantial volume of water stored in it, much of the runoff from that storm event may bypass the exfiltration system because it is filled. To account for the effect of volume and distribution on the amount of runoff exfiltrated, a long term mass balance based on local rainfall conditions was developed. The hourly runoff inputs for the mass balance were based on 15 years of precipitation data. From these simulations, design curves for exfiltration systems were developed These curves relate the rate at which stored runoff is removed from the trench to the volume of storage within the trench. Each curve represents a given diversion efficiency which is defined as the percentage of runoff diverted into the system, on a long term basis. These curves, called RED curves for Exfiltration Rate, Efficiency and Design volume, were developed for various regions across Florida using a mass balance on runoff entering

and leaving an exfiltration system. These curves can be used, along with the limiting exfiltration rate, to design an exfiltration trench based on diversion efficiencies of 50%, 60%, 70%, 80%, 85%, 90% and 95%. These efficiencies were selected to be consistent with the requirements of Chapter 17.40, Florida Administrative Code., which specifies average annual percentage of i;emoval of pollutants for various classes of receiving waters.

Notes

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Graduation Date

1991

Semester

Summer

Advisor

Wanielista, Martin P.

Degree

Master of Science (M.S.)

College

College of Engineering

Department

Computer Engineering

Degree Program

Computer Engineering

Format

PDF

Pages

213 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0028128

Subjects

Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic

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