Patent Publication Number: US-6991734-B1

Title: Solids retention in stormwater system

Description:
This application claims the benefit of application Ser. No. 60/459,478 filed Apr. 1, 2003. 
    
    
     TECHNICAL FIELD 
     The present invention relates to mechanical systems that receive and temporarily store stormwater beneath the surface of the earth, so the water can subsequently be dispersed within the earth, or discharged to a water course or other receiving point. 
     BACKGROUND 
     One way of handling stormwater runoff, such as from streets or a large shopping center parking lot, is to collect the water by means of surface drains, and send it to an underground structure, where it can be detained and subsequently released in a controlled manner to a water course or municipal storm sewer, and or so it percolates over time into the surrounding soil. Underground structures which have been used heretofore for the purpose comprise corrugated pipes, arch shape cross section molded chambers, concrete galleries and the like. Commonly, they are buried in a bed of stone or gravel, and with the stone interstices they provide void space within the earth, for receiving stormwater. For examples of apparatus used for receiving stormwater see Moore et al. Pat. No. 5,890,838, Maestro Pat. No. 6,361,248. 
     Stormwater typically carries with it debris, such as sand, paper and plastic things, and other miscellaneous matter, which had accumulated on the surface being drained. Catch basins and other devices are commonly used to trap or settle out such debris, to prevent large amounts from entering the subterranean system. For the debris that nonetheless enters and accumulates in the underground stormwater systems, access ports and manholes provide access for cleaning. 
     Increasingly, there are regulatory limits on the total suspended solids TSS content of stormwater which is discharged to streams or municipal storm sewer systems from parking lots and the like, and from stormwater detention systems used with such. But it is not so easy to prevent finer solids from entering the underground systems, or to retain them in the system in a way which enables them to be removed, so the system does not become gradually clogged. The prior art comprises various devices and methods, including Stever Pat. No. 6,350,374, which describes apparatus comprising a series of baffles; and, Stewart Pat. No. 5,322,629 which describes flowing the storm water through a layer of leaf compost material. Applicants are unaware of widespread use of such. So, there is a need for improvement in stormwater systems, to isolate or contain within a system debris which enters with stormwater, so it can be later largely removed, and so it is prevented from discharging to a watercourse. 
     SUMMARY 
     An object of the invention is to improve the handling of stormwater which is flowed into underground chambers or cavities, by capturing, and holding for convenient subsequent removal, solids which entrained in the stormwater. Another object is to enhance the long term performance of stormwater systems by avoiding progressive clogging of the systems. 
     In accord with the invention, stormwater handling apparatus is comprised of an array of chambers buried within permeable media, such as crushed stone. The chambers may be any of a variety of cross sectional shapes. Stormwater from surface drains flows into a solids retention subsystem (SRS), which is buried beneath the surface of the earth within the permeable media, and then to the array of chambers. The SRS settles and filters the stormwater, and discharges partly clarified water, so it flows through a quantity of the permeable media, to one or more chambers of the spaced apart array. The chambers which comprise the SRS may be like those used in the array. Preferably, the SRS comprises a string of arch shape cross section perforated wall chambers which have geotextile running along the exterior sidewalls and across the crushed stone at the bottom of the arch. Preferably, a combination of woven and non-woven geotextiles is used. Substances that capture hydrocarbons or metal ions may be contained within the SRS. Water which flows to the array is detained in the chambers and media and percolated over time into the surrounding earth, and or it is controllably released to a storm sewer, water course, or other place. 
     In further accord with the invention, the stormwater first flows through a diverter, which is upstream of the solids retention subsystem (SRS). In the diverter, some of the coarser solids in the water settle out, and the water flows through a first outlet to the SRS. If the flow capacity of the SRS is exceeded such as when stormwater flow is very large, the diverter channels the excess water through a second outlet, so it runs down a bypass line which is connected directly to the chambers of the array. Thus, from the user standpoint, the surface draining function is not impeded. 
     The solids which are retained in the SRS can be conveniently removed during maintenance operations, by use of access points at the end of the chamber row which comprises the unit, when there is no storm. The SRS lessens the amount of solids which flows into the chambers of the array, and minimizes degradation of their capacity over time, which could other wise occur. At the same time, maintenance is made easier, because the solids are concentrated in one spot, rather than dispersed within the array. 
     The foregoing and other objects, features and advantages of the invention will become more apparent from the following description of preferred embodiments and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a stormwater handling system. 
         FIG. 2  is an elevation cross section view through a portion of the system shown in  FIG. 1 . 
         FIG. 3  is an elevation cross section view of a diverter. 
         FIG. 4  is a plan view of another embodiment of stormwater handling system. 
         FIG. 5  is an elevation cross section view of a solids retention subsystem comprising a round pipe. 
     
    
    
     DESCRIPTION 
     The present invention is described in terms of use of arch shape cross section plastic molded open-bottom chambers having perforated sidewalls, such as Stormtech® Model SC310 or Model 740 chambers (Stormtech LLC, Wethersfield, Conn.). Chambers like those of Stormtech are described in patent application Ser. No. 09/849,768 of Krueger et al., filed May 24, 2001, the description and drawings of which are hereby incorporated by reference. Other commercial arch shape molded plastic chambers may be used in substitution. And other kinds of void-creating structures may be used in substitution of arch shape cross section chambers. For instance, use may be made of commercial corrugated round (circular, oblong, and other closed cross section shape) perforated non-metal or metal pipe; as well as arch shape structures, galleries, and other devices of concrete or metal, and so forth, as known in the field, or still to be introduced. The term “chamber” as used in claiming the invention here is intended to encompass the generality of such devices. 
       FIG. 1  is a semi-schematic plan view of a subterranean stormwater system.  FIG. 2  is a vertical elevation cross section view through a portion of the system. An array  25  of parallel rows of Stormtech perforated wall chambers  20  is buried beneath the surface  24  of the earth, i.e., under an automobile parking lot, within angular crushed stone  22 , or other suitable water permeable media, such as gravel, round stone, slag, etc. Depending on the inflow rate and particular stormwater system design, water is temporarily stored in the chambers  20  and surrounding crushed rock interstices, and then dispersed over time. Typically, there is always some dispersion by percolation into the earth surrounding the crushed stone; and, often there is discharge through an outflow pipe at a predetermined maximum rate, to a municipal storm sewer, pond, watercourse or other receiving point. 
     In use, stormwater which falls onto surface  24  flows into conventional surface drains, such as a familiar combination of spaced part catch basins, interconnected by buried pipes. Handling stormwater collected by other means, and in other applications, is within the scope of the invention. Collection basin  32  in  FIG. 1  is suggestive of such a system. Water from catch basins may also flow through one or more commercial settling devices, before entering the apparatus of the invention, to settle out the most coarse and heavy solids and floating matter. 
     The stormwater carrying the remaining solids flows from the collection basin  32  to optional diverter  40 , where some more coarse debris settles out. In this description, the term solids is used to refer to any of various undissolved materials that are entrained by stormwater, including those variously referred to as suspended solids and settlable solids. The stormwater from the diverter flows to solids retention subsystem (SRS)  30 , for example a string of perforated wall chambers  33  like those used in array  25 . The SRS removes a substantial fraction of solids from the stormwater. The fraction will depend on the character of the solids, the fine-ness of the filtering function of the SRS, and the water flow rate. Resultant partially clarified water then flows generally laterally through a mass of permeable media, toward the nearest chamber  50  of a parallel array  25  of chambers  20 . The water flowing from the SRS is referred to here as being partly clarified, or more simply, clarified. The meaning intended for either term, without distinction, is that the solids load in the stormwater has been decreased. Particularly when the solids are very fine, e.g. silt, commercial geotextiles used in the invention will be insufficient to remove all solids. 
     If the stormwater apparatus design and use intends that water peroclation over time within the soil, the water accumulates in the interstices of the gravel and within the chambers  20 , until that occurs. If the system design intends that some portion of the water be delivered to a municipal storm sewer watercourse or other receiving point, a perforated collection pipe  60  is buried in the crushed stone, preferably spaced apart from the row of chambers  20  which is farthest from the SRS. See  FIG. 2 . 
     Outflow from the collection pipe  60  may be regulated by pipe diameter, orifice plate of other means, to meet regulatory discharge rate requirements. The last row of chambers  20  may function as the collection pipe. Alternately, a plenum, connected to chambers  20  by pipes, may be used. 
     As shown in  FIGS. 1 and 2 , stormwater flows by means of pipe  34  to optional diverter  40 , as shown by arrow A, and, then by means of first outlet pipe  42  into solids retention subsystem SRS  30 , as shown by arrow AA.  FIG. 3 , discussed further below, shows in vertical cross section a diverter  40 . Coarse debris  70  that settles to the bottom of the diverter is removable during maintenance activities. Other prior art sedimentation devices may be used upstream of diverter  40  or in substitution thereof. Diverter  40  may incorporate other features than those shown; for instance, baffles and or swirling flow motion. 
     The size and shape of SRS  30  compared to diverter  40  is preferably such that the flow velocity in the SRS is lower than in the diverter. So, the finer suspended solids will tend to flow into, and settle out in the SRS, particularly in the portion which is distant from inflow pipe  42 . As water flows from the SRS into the surrounding media, finer debris which is still suspended is filtered out by filter fabric  36  or substitutional permeable media placed next to the chamber which media is finer than the preponderant crushed stone  22 . 
     As indicated by  FIG. 2 , SRS  30  is preferably comprised of a string, or length, of interconnected arch shape cross section perforated wall, open bottom, chambers  33 . A layer  36  of non-woven plastic filter fabric or geotextile runs along the perforated sidewalls of chambers  33 . A layer  36 A of geotextile runs across the crushed stone at the base or open bottom of the chambers  33  of the SRS. The material  36 A is in part chosen for its toughness, to resist cleaning processes, as described below. A Type 600X woven polypropylene geotextile may be used for layer  36 A, from T.C. Mirafi Co., Pendergrass, Ga. It will have a filtering rating nominally consistent with US Sieve Size 40–70 (0.2–0.43 mm opening dimension) and will meet AASHTO M288 Class 2 standard. Mirafi Type 160 non-woven polypropylene geotextile may be used for the sidewall layer  36 . It filters finer particles than the woven bottom layer material; and, preferably conforms with AASHTO Class 1. Of course, to the extent the end caps at the ends of a chamber string of the SRS allow significant stormwater outflow, they also can be layered externally with geotextile like that used for the sidewalls. The SRS may also be configured to function without filter media surrounding the chambers. For example, particularly with reference to the round pipe embodiment shown in  FIG. 5 , the SRS may comprise one or more chambers which have walls with very fine slots or other perforations, or integral screening, so that the walls themselves perform the filtering function. 
     In an alternate embodiment, a low permeablility or impermeable plastic membrane may be used across the bottom of SRS, so that essentially all the water is made to flow out the sidewalls, and to reduce the possibility of scouring due to stormwater flow or cleaning operations. In another embodiment of the invention, the geotextile at the sidewall and or the base may be replaced by or supplemented with a layer of specially tailored grain size of media, e.g. fine sand, chosen for its ability to function as a filter. In still another embodiment, the SRS may be configured to capture dissolved pollutants, for instance to capture hydrocarbons or metal ions. For example, granular polymer material, such as the kind used in Model 3100 Streamguard (Bowhead Manufacturing Co. LLC, Seattle, Wash.), may be placed within the chambers. If the dissolved-pollutant capturing materials are particulates they may be flowed with water into a newly installed, or newly cleaned, chamber, so they accumulate on the layers of geotextile, for example in a manner similar to that use to pre-coat fibrous filters in industrial operations. Likewise, other materials such as cation or anion exchange agents, such as familiar resins and zeolites, in granular or mat forms, may be similarly placed, for capturing certain metal ions. In carrying out this embodiment, it will be preferred to use an impermeable membrane across the bottom of an arch shape chambers, or to use round cross section chambers, in the SRS. 
     In a SRS which has a geotextile layer  36 A across the bottom of the arch, some water will flow downwardly into the stone and earth below. Depending on the water volume, another portion of the water flows through the sidewall perforations and associated geotextile. Water from the SRS flows diffilusedly through the crushed stone to the nearest spaced apart row of chambers  20 , as indicated by arrows AB. A substantial amount of solids in the water is left behind in the SRS. Water AB which enters or goes around the first chamber row  50  may continue flowing through crushed stone  22  to other chambers  20 , as indicated by arrows AC. 
     When stormwater is actually flowing into the invention system, the interior of SRS  30  functions as a sump or stagnation zone, since there is reduced flow velocity and resultant settling and collection of solids  72  along the bottom of the SRS, compared to the upstream settling devices, including diverter  40 . See  FIG. 2 . It is expected that often water may accumulate in SRS  30  as suggested in  FIG. 2 , when the inflow exceeds flow capacity of the SRS or the system as a whole. Thus, for functional flow capacity reasons, it is preferred to have chambers with perforated sidewalls. However, the invention will work even if the arch shape cross section chamber sidewalls are not perforated, and water flow is only downward.  FIG. 5  shows, as another embodiment, how SRS  30 B may comprise corrugated perforated pipe  33 B surrounded by geotextile filtering fabric  36 B in substitution of arch shape cross section chambers. Stormwater flowing from SRS  30 B can be in all vertical plane directions, depending on how much accumulates in the SRS. 
     With reference again to  FIG. 1  and  FIG. 3 , if SRS  30  cannot handle the volume of stormwater flow, the SRS inflow water will back up in pipe line  42 , and thus into diverter  40 . At a certain point the water level  74  in the diverter will rise until it overflows weir  44 , and then the water will flow out the second outlet pipe  46 , to an associated piping system  47 , and directly into chambers  20  of the array  25 . In substitution of the weir, the elevation of the outlet of bypass pipe  46  may simply be higher than the elevation of the primary outlet pipe  42 , running to the SRS. Typically, the loading of solids in stormwater is greatest during the so-called first flush, or the first rush of stormwater at the onset of storm rainfall. Therefore, a premise of the diverter feature is that, should the capacity of SRS  30  be exceeded, that will occur after the first flush; and, that fraction of water that might flow through the bypass system can be expected to have relatively favorable (low) solids content. Other configuration of diverter may be used in the invention. While undesirable from the standpoint of maintenance, a mechanical or electro-optical level sensing system may be used to gage water level in the SRS or diverter, and open or close a gate valve along the path running to bypass system  47 . 
     During ordinary use, when there is no flow through the bypass piping  46 ,  47 , the preponderance of solids which enter diverter  40  will flow into solids retention subsystem SRS  30  and accumulate there, along the bottom and within the filter fabric or other media along the sidewalls. The solids captured by the fabric at the sidewalls may fall to the bottom of the SRS arch shape cross section chamber string, when the storm ends and water flow ceases, depending on the size and agglomerating tendencies of the solids. At times when there is no stormwater flow, the captured debris may be removed using common cleaning equipment, such as vacuums and water jets. Access to the interior of interconnected chambers of SRS  30  for such purpose may be obtained through the top of diverter  40 , or by opposite end access manhole  48 . Inspection ports are not shown, but they will optionally be provided at points along the SRS length, so the interior can be inspected. Less preferably, SRS may be comprised of chambers of the type shown in U.S. Pat. No. 5,087,151 to DiTuillo. Such chambers have end walls, and a string of chambers is made by interconnection using short lengths of pipes. Those types of chambers cannot conveniently be jet-cleaned from the ends, but may be accessed by vertical ports into each chamber. 
     Other arrangements of the array  25  of chambers relative to the length of the SRS  30  can be used. For instance, referring to  FIG. 1 , there may be a second array of chambers to the left of SRS  30 . In another arrangement, the chambers of the array may be perpendicular to the length of the SRS, rather than parallel as shown in  FIG. 1 . In another embodiment, illustrated schematically in  FIG. 4 , SRS  30 A is more spaced apart from the array, but functions in a similar manner to that which has been described. As indicated by the arrows, water flows from diverter  40 A into SRS  30 A, and then sideways through the filter fabric and crushed stone media, to perforated collection pipes  80 , which run to header  84  which distributes the water of an array  25 A of chambers  20 A. In another embodiment, there is no array  25 , but simply a mass of permeable media. In still another embodiment, the array  25 A is replaced by one or more collection pipe which discharges to a municipal system or other receiving point. While a gravity flow system has been described, within the generality of the invention, pumps may be used at one or more points along the flow path. 
     Although this invention has described with respect to one or more preferred embodiments, and by examples, they should not be considered as limiting the claims, since it will be understood by those skilled in the art that there may be made equivalents and various changes in form and detail, without departing from the spirit and scope of the claimed invention.