Abstract:
An application for a flow control system includes a movable riser in fluid communication and slideably engaged with a stationary riser, the stationary riser being in fluid communication with a drainage system. The movable riser is made buoyant by one or more attached floats such that, when the liquid level around the flow control system increases to a pre-determined level, the movable riser lifts due to the buoyancy of the float(s), thereby maintaining the pre-determined displacement as the water level continues to rise, yielding either a constant flow rate or a variable, predictable flow rate through the drainage system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-part of U.S. patent application Ser. No. 12/463,614, filed May 11, 2009, and inventor Jonathan D. Moody. This application is related to U.S. patent application Ser. No. 12/570,734, filed Sep. 30, 2009, and inventor Jonathan D. Moody. This application is also related to U.S. patent application Ser. No. 12/570,756, filed Sep. 30, 2009, and inventor Jonathan D. Moody. 
    
    
     FIELD OF THE INVENTION 
     The disclosure relates to the field of flow control devices and more particularly to a flow control device for a detention pond or surge tank. 
     BACKGROUND 
     Detention ponds and surge tanks are deployed to temporarily store a fluid and limit the rate of fluid discharge to a downstream system when the inflow rate of the fluid is variable at times exceeds the functional capacity of the downstream system. In the case of a storm water detention pond, the pond receives increased rates of storm water runoff generated by the development of upstream lands, temporarily stores the runoff and limits the rate of discharge of the runoff to a receiving system of water conveyance such as a river, stream or storm sewer such that the capacity of the receiving system is not exceeded thereby causing flooding, harmful erosion or other environmental damage. Similarly, a surge tank temporarily stores a process fluid of varying inflow rate and limits the rate of discharge of the fluid to that which will not exceed the capacity of a downstream process. In the field of wastewater treatment, a surge tank may be deployed to receive wastewater flows during peak periods of water use, temporarily store the wastewater and limit the release of the wastewater flow to the treatment plant to a rate not exceeding the design capacity of the plant. 
     The temporary storage volume required for a detention pond or surge tank is dependent on the rate and duration of fluid inflow and the allowable rate and duration of fluid outflow. The larger the difference between the peak rate of inflow and the allowable rate outflow, the greater the volume is required for temporary storage. Whereas providing large storage volumes can be costly such as the expense incurred for land acquisition and excavation required to construct a large detention pond or the expense of fabrication and installation of a very large tank it is therefore advantageous to minimize the amount of temporary storage volume required for safe operation of the system. Minimization of the temporary storage volume required can be accomplished by minimizing the difference between the duration and rate of inflow and the duration and rate of outflow. Since the rate inflow is variable and cannot be controlled, minimization of the required temporary storage volume is achieved when the maximum allowable rate of discharge is sustained for the longest possible duration of time. 
     The prior art is generally concerned with limiting the maximum outflow rates, at which damage can occur, by employing discharge control mechanisms such as fixed weirs, orifices, nozzles and riser structures whereby the maximum discharge rates of such mechanisms are determined by the geometric configuration of the mechanisms and the height of the fluid or static head acting on the mechanisms. In each case, the maximum flow rate is achieved only at the single point in time at which the static head acting on the mechanism is at its maximum level. Therefore, all discharges occurring when fluid levels are not at their maximums are less than optimum. 
     One solution to this problem is described in U.S. Pat. No. 7,125,200 to Fulton, which is hereby incorporated by reference. This patent describes a flow control device that consists of a buoyant flow control module housing an orifice within an interior chamber that is maintained at a predetermined depth below the water surface. This flow control device neglects the use of other traditional flow control mechanisms such as weirs, risers and nozzles, has limited adjustability, and utilizes flexible moving parts subject to collapse by excess hydrostatic pressure or failure resulting from material fatigue caused by repeated cyclical motion. 
     What is needed is a flow control device that provides for deployment of a variety of discharge control mechanisms in singular or in combination, is readily adjustable to accommodate for deviations incurred during installation, settlement, or by variability in the weights and densities of the materials of which it is comprised and does not rely on parts subject to failure by excess hydrostatic force or repeated cyclical motion while maintaining a nearly constant rate of discharge at varying fluid levels. 
     SUMMARY OF THE INVENTION 
     A flow control system of the present invention includes a movable riser slideably engaged with a stationary riser. The stationary riser is interfaced to a downstream drainage system. The movable riser is made buoyant by one or more floats attached to the movable riser such that, when the water level around the flow control system increases to a pre-determined level above a top rim of the movable riser, the movable riser lifts due to the buoyancy of the float(s), thereby maintaining the pre-determined level, even as the water level continues to rise. 
     In one embodiment, a flow control system for integration into a detention pond or surge tank is disclosed including a stationary riser having a hollow core, an axis of which is vertical. The hollow core of the stationary riser is fluidly connected to a downstream drainage system. A movable riser is slideably interfaced with the stationary riser and also has a hollow core, an axis of which is also vertical. A rim is at the top surface of the movable riser. The hollow core of the movable riser is fluidly connected to the hollow core of the stationary riser so that water from the detention pond or liquids from the surge tank flow over the rim, through the hollow core of the movable riser through the hollow core of the stationary riser and into the downstream drainage system. At least one float is interfaced to the movable riser, providing buoyancy to the movable riser and maintaining the rim at fixed distance below the fluid surface. 
     In another embodiment, a flow control system for integration into a detention pond or surge tank is disclosed including a stationary riser having a hollow core, an axis of which is vertical. The hollow core is fluidly connected to a downstream drainage system. A movable riser is slideably interfaced with the stationary riser and also has a hollow core with an axis that is also vertical. A single nozzle or combination of nozzles or similar or differing geometries, an axis of which is vertical and fashioned to fit over the rim of the movable riser, is fluidly connected to the hollow core of the movable riser and the hollow core of the movable riser is fluidly connected to the hollow core of the stationary riser whereas water from the detention pond or liquid from the surge tank flows through the nozzle, through the hollow core of the movable riser through the hollow core of the stationary riser and out of hollow core of the stationary riser and into the downstream drainage system. At least one float is interfaced to the movable riser, providing buoyancy and maintaining the nozzle at a fixed distance below the fluid surface. 
     In another embodiment, a flow control system for integration into a detention pond or surge tank is disclosed including a stationary riser having a hollow core, an axis of which is vertical. The hollow core is fluidly connected to a downstream drainage system. A movable riser is slideably interfaced with the stationary riser and also has a hollow core with an axis that is also vertical. A single nozzle or combination of nozzles of similar or differing geometries, an axis of which is horizontal and penetrate the vertical surface of the movable riser, is fluidly connected to the hollow core of the movable riser and the hollow core of the movable riser is fluidly connected to the hollow core of the stationary riser whereas water from the detention pond or liquid from the surge tank flows through the nozzle, through the hollow core of the movable riser through the hollow core of the stationary riser and out of hollow core of the stationary riser and into the downstream drainage system. At least one float is interfaced to the movable riser, providing buoyancy and maintaining the nozzle at a fixed distance below the fluid surface. 
     In another embodiment, a flow control system for integration into a detention pond or surge tank is disclosed including a stationary riser having a hollow core, an axis of which is vertical. The hollow core is fluidly connected to a downstream drainage system. A movable riser is slideably interfaced with the stationary riser and also has a hollow core with an axis that is also vertical. A notch or combination of notches with similar or differing geometries fashioned below the rim and through the vertical surface of the movable riser, is fluidly connected to the hollow core of the movable riser and the hollow core of the movable riser is fluidly connected to the hollow core of the stationary riser whereas water from the detention pond or liquid from the surge tank flows through the notch, through the hollow core of the movable riser through the hollow core of the stationary riser and out of hollow core of the stationary riser and into the downstream drainage system. At least one float is interfaced to the movable riser, providing buoyancy and maintaining the notch at a fixed distance below the fluid surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a schematic view of a system of the present invention. 
         FIG. 2  illustrates a perspective view of the movable riser of a first embodiment of the present invention. 
         FIG. 3  illustrates a perspective view of the movable riser of a second embodiment of the present invention. 
         FIG. 4  illustrates a perspective view of the movable riser of a third embodiment of the present invention. 
         FIG. 5  illustrates a perspective view of the movable riser of a fourth embodiment of the present invention. 
         FIG. 6  illustrates a top plan view of a float system of the present invention. 
         FIG. 7  illustrates a top plan view of an alternate float system of the present invention. 
         FIG. 8  illustrates a perspective view of another alternate float system of the present invention. 
         FIG. 9  illustrates a perspective view of another alternate float system of the present invention. 
         FIG. 10  illustrates a perspective view of an alternate embodiment of the present invention. 
         FIG. 11  illustrates a perspective view of another alternate embodiment of the present invention. 
         FIG. 12  illustrates a perspective view of an alternate embodiment of the present invention. 
         FIG. 13  illustrates a perspective view of an alternate embodiment of the present invention. 
         FIG. 14  illustrates a perspective view of an alternate embodiment of the present invention. 
         FIG. 15  illustrates a perspective view of an alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. Throughout the following description, the term detention pond and surge tank represent any such structure and are equivalent structure for detaining liquids. 
     The flow control system described provides for an initial discharge rate starting as soon as the detention pond or surge tank reaches a pre-determined liquid level, then, as the liquid level increases, the discharge rate and the down-stream water pressure remain relatively constant until a high-water level is reached, at which level the flow control system provides for an increased discharge rate to reduce the possibility of exceeding the volumetric capacity of the detention pond or surge tank. 
     Prior to more advanced flow control systems, limiting the maximum outflow rates, at which damage can occur, was accomplished by deploying discharge control mechanisms such as fixed weirs, orifices, nozzles and riser structures whereby the maximum discharge rates of such mechanisms are determined by the geometric configuration of the mechanisms and the height of the fluid or static head acting on the mechanisms. In each case, the maximum flow rate is achieved only at the single point in time at which the static head acting on the mechanism is at its maximum level. Therefore, all discharges occurring when fluid levels are not at their maximums are less than optimum and require provision of greater temporary storage capacities. The present invention solves these and other problems as is evident in the following description. 
     Referring to  FIG. 1 , a schematic view of a system of the present invention will be described. The detention pond or surge tank flow control system  20  has two primary components, a holding box  26 / 28 / 30  and the actual flow control device  40 . 
     The holding box  26 / 28 / 30  consists of a holding box  26 , typically made of concrete and having a lid  28 , typically made of concrete or metal. A debris shield  30  partially covers an opening  32  in the side of the box  26 . The holding box  26 / 28 / 30  is positioned part way into the bed  12  of the detention pond or bottom of the surge tank  10 . As the liquid level  9  in the detention pond or surge tank  10  rises, it is skimmed by the debris shield  30 , holding back some or all of any floating debris, oil, etc, and allowing liquid from the detention pond or surge tank to spill over into the holding box  26 . 
     The flow control device  40  consists of a stationary riser  42  and a movable riser  46 . The movable riser  46  is supported by floats  50 / 52  such that, as liquid begins to rise within the holding box  26 , the floats become buoyant and lift the movable riser  46 , maintaining a constant water depth over the top rim  48  of the movable riser  46 . Once the liquid level  11  within the holding box  26  rises above the top rim  48 , liquid flows over the top rim  48  at a constant rate independent of the liquid level of the detention pond or surge tank  10  because the top rim  48  is held at approximately the same depth beneath the liquid surface  11  within the holding box  26 . The liquid flows through the stationary riser  42  and out the drain pipe  24  to the drainage system, streams, rivers, etc. in the case of a storm water detention pond or downstream process in the case of a surge tank. 
     The movable riser  46  and the stationary riser  42  have hollow cores and the hollow cores run vertically to accept liquid from the detention pond or surge tank  10  and transfer the liquid from the holding pond  10  to a down-stream drainage system  24 . The movable riser  46  hollow core accepts liquid flowing over the rim  48  from the detention pond or surge tank and passes it into the stationary riser  42  hollow core. The stationary riser  42  hollow core passes the liquid to the drain pipe  24  and out to the drainage system, streams, rivers, etc. in the case of a storm water detention pond or downstream process in the case of a surge tank. 
     In some embodiments, the floats  50 / 52  are mounted on float shafts  54 / 56 . In such embodiments, optionally, the float shafts  54 / 56  extend upward beyond the floats  50 / 52  to provide a maximum lift height for the movable riser  46 . In this, as the liquid level  11  rises within the holding box  26  to a high point, the tops of the float shafts  54 / 56  hit the cover  28 , thereby preventing further lifting of the movable riser  46 . This accomplishes at least two functions: it prevents the movable riser  46  from disengaging with the stationary riser  42  and it allows a greater flow rate during emergency situations—when the detention pond or surge tank  10  over-fills. In addition, also anticipated is a bypass drain  22 , which begins bypassing water when the liquid in the detention pond or surge tank  10  reaches a certain height. 
     Although there are many ways to interface the floats  52 / 54  with the movable riser  48 , shown is a pair of float shafts  54 / 56 . In one embodiment, the float shafts  54 / 56  are threaded shafts with nuts  51  holding the floats  50 / 52  at an adjustable height on the float shafts  54 / 56 . In this way, with a simple tool, the operating depth (depth of the top rim  48  with respect to the liquid level  11  within the holding box  26 ) is easily adjusted. As shown, the float shafts  54 / 56  are interfaced with the movable riser  46  by two float cross members  60 / 62 , although any number of cross members  60 / 62  are anticipated, including one. It is also anticipated that the floats  50 / 52  are also adjusted by bending of the float shafts  54 / 56  and or the float cross members  60 / 62 . 
     Although the flow control system  40  is capable of supporting itself within the holding box  26 , it is anticipated that one or more optional struts  44  are provided to secure the flow control system  20  to the holding box  26 . 
     In some embodiments, a lock (not shown) is provided to lock the cover  28  on top of the holding box  26 . 
     Referring to  FIG. 2 , a perspective view of the movable riser  46  of a first embodiment of the present invention will be described. For simplicity, the floats  50 / 52  are shown affixed to float shafts  54 / 56  and a single cross member  62 , the cross member  62  holding the float shafts  54 / 56  to the movable riser  46 . In such embodiments, the floats  50 / 52  are adjustable by bending of the float shafts  54 / 56  and/or the cross member  62  or by adjusting the vertical position of the floats  50 / 52  on the float shafts  54 / 56 . Any number and/or shape of floats  50 / 52  are anticipated. Although shown throughout this description as spherical, other shapes of floats  50 / 52  are anticipated including square or rectangular boxes, etc. 
     There are many shapes and configurations for the top opening of the movable riser  46 , one example of which is shown in  FIG. 2 . In this example, a movable riser top cover  61  has a nozzle  63 . The nozzle  63  is smaller than the diameter of the movable riser  46 , therefore, restricting the flow of water from the holding box  26  into the movable riser  46  and, hence, out of the drain pipe  24 . Although shown as being circular in shape, any shape nozzle  63  is anticipated. 
     Referring to  FIG. 3 , a perspective view of the movable riser  46  of a second embodiment of the present invention will be described. For simplicity, the floats  50 / 52  are again shown affixed to float shafts  54 / 56  and a single cross member  62 , the cross member  62  holding the float shafts  54 / 56  to the movable riser  46 . In such embodiments, the floats  50 / 52  are adjustable by bending of the float shafts  54 / 56  and/or the cross member  62  or by adjusting the vertical position of the floats  50 / 52  on the float shafts  54 / 56 . There are many edge shapes and configurations for the top rim of the movable riser  46 , one example of which is shown in  FIG. 3 . In this example, a rectangular notch  70  is cut or formed on the rim  48  of the movable riser  46 . The notch  70  provides a first flow of water from the holding box  26  into the movable riser  46  at a point at which the water level  11  rises above the bottom surface of the notch  70  and a second, greater flow of water from the holding box  26  into the movable riser  46  at a point at which the water level rises above the rim  48  of the movable riser  46 . Although a single notch  70 , rectangular in shape is shown, any number of notches  70  or any shape opening  70  is anticipated. 
     Referring to  FIG. 4 , a perspective view of the movable riser  46  of a third embodiment of the present invention will be described. For simplicity, the floats  50 / 52  are again shown affixed to float shafts  54 / 56  and a single cross member  62 , the cross member  62  holding the float shafts  54 / 56  to the movable riser  46 . In such embodiments, the floats  50 / 52  are adjustable by bending of the float shafts  54 / 56  and/or the cross member  62  or by adjusting the vertical position of the floats  50 / 52  on the float shafts  54 / 56 . There are many edge shapes and configurations for the top rim of the movable riser  46 , one example of which is shown in  FIG. 4 . In this example, a triangular notch  80  is cut or formed on the rim  48  of the movable riser  46 . The notch  80  provides a gradually increased rate of flow of water from the holding box  26  into the movable riser  46  starting at a point at which the water level  11  rises above the bottom corner of the triangular notch  80  and increasing as the water level rises to a point equal to the rim  48  of the movable riser  46  at which point the water flow further increases as the water rises above the rim  48 . Although shown as being triangular in shape, other opening shapes  80  are anticipated. Also, any number of notches  80  and/or notch  80  shapes is anticipated 
     Referring to  FIG. 5 , a perspective view of the movable riser of a fourth embodiment of the present invention will be described. Again, for simplicity, the floats  50 / 52  are shown affixed to float shafts  54 / 56  and a single cross member  62 , the cross member  62  holding the float shafts  54 / 56  to the movable riser  46 . In such embodiments, the floats  50 / 52  are adjustable by bending of the float shafts  54 / 56  and/or the cross member  62  or by adjusting the vertical position of the floats  50 / 52  on the float shafts  54 / 56 . There are many edge or rim  48  shapes and configurations for the top rim  48  of the movable riser  46 , one example of which is shown in  FIG. 5 . In this example, the rim  48  of the movable riser  46  is sloped  90 / 92 . The slope  90 / 92  provides a gradual and linear increased rate of water flow starting at a point at which the water level  11  rises above the lower point  90  of the rim  48 , increasing until the water level rises to the top point  92  of the rim  48 . Although shown as being a linear increase between the lower point  90  and the top point  92 , any other slope and or stepping is anticipated. For example, the increase between the lower point  90  and the top point  92  is stepped at equal steps or is asymptotic. 
     Referring to  FIG. 6 , a top plan view of a float system of the present invention will be described. In this example, two floats  50 / 52  are attached to the movable riser  46  by cross members  62 . It is anticipated that the cross member  62  is either affixed to the surface of the movable riser  46 , passes through the movable riser  46  or is held by a bracket passing all or part way around the movable riser  46 , as known in the industry. 
     Referring to  FIG. 7 , a top plan view of an alternate float system of the present invention will be described. In this example, three floats  50 / 51 / 52  are attached to the movable riser  46  by cross members  62 . It is anticipated that the cross member  62  is either affixed to the surface of the movable riser  46 , passes through or part-way the movable riser  46  or is held by a bracket passing all or part way around the movable riser  46 , as known in the industry. Although any number of floats  50 / 51 / 52  is anticipated, two or three floats  50 / 51 / 52  are preferred. 
     Referring to  FIG. 8 , a perspective view of another alternate float system of the present invention will be described. In this example, two floats  50 / 52  are attached to the movable riser  46  by the float shafts  55 / 57 . It is anticipated that the float shafts  55 / 57  are either affixed to a surface of the movable riser  46  or are tapped/threaded into the movable riser  46 , as known in the industry. Again, any number of floats  50 / 52  of any shape is anticipated. 
     Referring to  FIG. 9 , a perspective view of another alternate float system of the present invention will be described. In this example, the float  100  surrounds or is directly affixed to the outside of the movable riser  46 . Although shown as a single float  100  affixed to the entire circumference of the movable riser  46 , it is also anticipated that the float  100  is in sections, each affixed to the outer circumference of the movable riser  46 . In this embodiment, the float is, for example, a Styrofoam ring or balloon filled with a gas that has a specific gravity of less than 1. It is anticipated that, in some embodiments, the float  100  is slideably affixed to the movable riser  46 , such that, the float  100  is repositionable either closer to or further away from the rim  48 , thereby adjusting the average liquid height above the rim  48 . It is also anticipated that, in embodiments in which the float  100  is a balloon filled with a gas, the inflation volume is adjustable, also adjusting the average liquid height above the rim  48 . 
     Referring to  FIG. 10 , a perspective view of an alternate embodiment of the present invention will be described. In this example, a pointer or scribe  110  is affixed to the movable riser  46  and set to aim at a gradient  112 , providing a means for helping the site engineer to properly adjust the floats  50 / 51 / 52 / 100  based upon the desired discharge rate. 
     Referring to  FIG. 11 , a perspective view of another alternate embodiment of the present invention will be described. This shows an exemplary way to restrict the rise of the movable riser  46  when there is no surface above the float rods  54 / 56  to restrict the height of travel of the movable riser  46 . In this, one or more arms  120  are affixed to the cross members  62  by, for example, by loop(s)  122 . The arm(s)  120  freely pass within an eye  124  or eyes  124  or other similar structures and there is a stop  126  at the bottom end of the arm(s)  120  such that, as the movable riser  46  lifts to a predetermined limit, the stop(s)  126  prevent the movable riser  46  from raising any further than allowed by the stop(s)  126  and the length of the arm(s)  120 . It is anticipated that the stop(s)  126  are adjustable along the length of the arm(s)  120 , providing an adjustable maximum height of travel for the movable riser  46 . 
     Referring to  FIG. 12 , a perspective view of an alternate embodiment of the present invention will be described. In this embodiment, the top rim  48  of the movable riser  46  is below the surface of the liquid  9 , held by floats  50 / 52  on supports  54 / 56 / 62 . In this example, there is also a noticeable interstitial space  102  between the stationary riser  42  and the movable riser  46 . The liquid flows over the top rim  48  of the movable riser  46  and eventually out through the drainage system  24  (see  FIG. 1 ). The liquid also flows out through the interstitial space or gap  102  between the movable riser  46  and the stationary riser  42 . Since the movable riser  46  rises in response to the fluid level  9 , and the top rim  48  of the movable riser  46  is maintained at a constant depth with respect to the fluid level  9 , the flow rate through the movable riser  46  is constant as long as air is allowed to enter the movable riser  46  through one or more air vent tubes  100  when the drainage system  24  (see  FIG. 1 ) is surcharged and not otherwise operating under open channel flow conditions. In some embodiments, instead of independent air vent tubes  100 , the supports  54 / 56 / 62  are hollow, venting air into the movable riser  46 . Since the restriction to flow through the interstitial space or gap  102  is fixed at the top edge of the stationary riser  42 , the flow rate through the interstitial space  102  is variable with respect to the fluid level  9 ; where the degree of variability in the flow rate is a function of the cross sectional area of the interstitial space or gap  102 . The liquid level  115  in the drainage system  24  and stationary riser  42  is lower than the bottom of the movable riser  46 . 
     Referring to  FIG. 13 , a perspective view of an alternate embodiment of the present invention will be described. In this embodiment, the drainage system  24  (see  FIG. 1 ) is surcharged (i.e. not operating under open channel flow conditions) and the top rim  128  of the movable riser  120  is held above the surface of the liquid  9  by floats  50 / 52  on supports  54 / 56 / 62 . In this example, there is also a noticeable interstitial space  102  between the stationary riser  42  and the movable riser  120 . The liquid flows through the interstitial space or gap  102  between the stationary riser  42  and the movable riser  120 . Since the movable riser  120  rises in response to the fluid level  9 , the bottom edge of the movable riser  120  is maintained at a constant depth with respect to the fluid level  9  and, therefore, the flow rate is constant through the interstitial space  102  since air is allowed to enter the movable riser  120  through a central opening  121 . The diameter of the movable riser  120  gradually decreases towards the top such that the restriction to flow through the interstitial space or gap  102  is maintained at the bottom edge of the movable riser  120 . The liquid level  115  in the drainage system  24  and stationary riser  42  is lower than the bottom of the movable riser  46 . 
     Referring to  FIG. 14 , a perspective view of an alternate embodiment of the present invention will be described. In this embodiment, the drainage system  24  (see  FIG. 1 ) is surcharged (i.e. not operating under open channel flow conditions) and the orifice or opening  131  of the movable riser  130  is held below the surface of the liquid  9 , by floats  50 / 52  on supports  54 / 56 / 62 . In this example, there is also a noticeable interstitial space  102  between the stationary riser  42  and the movable riser  130 . The liquid flows into the orifice or opening  131  of the movable riser  130  and eventually out through the drainage system  24  (see  FIG. 1 ). The liquid also flows out through the interstitial space or gap  102 . Since the movable riser  130  rises in response to the fluid level  9 , the bottom edge of the movable riser  46  is maintained at a constant depth with respect to the fluid level  9  and, therefore, the flow rate is constant, both through the orifice/opening  131  of the movable riser  130  and through the interstitial space  102  since air is allowed to enter the movable riser  130  through one or more air vent tubes  100 . In some embodiments, instead of independent air vent tubes  100 , the supports  54 / 56 / 62  are hollow, venting air into the movable riser  46 . The diameter of the movable riser  130  gradually decreases towards the top such that the restriction to flow through the interstitial space or gap  102  is maintained at the bottom edge of the movable riser  130 . The liquid level  115  in the drainage system  24  and stationary riser  42  is lower than the bottom of the movable riser  130 . 
     Referring to  FIG. 15 , a perspective view of an alternate embodiment of the present invention will be described. In this embodiment, the drainage system  24  (see  FIG. 1 ) is surcharged (i.e. not operating under open channel flow conditions) and the orifice  141  of the movable riser  140  is held below the surface of the liquid  9 , by floats  50 / 52  on supports  54 / 56 / 62 . In this example, there is also a noticeable interstitial space  102  between the stationary riser  42  and the movable riser  140 . The liquid flows into the orifice  141  of the movable riser  140  and eventually out the drainage system  24  (see  FIG. 1 ). The liquid also flows out through the interstitial space or gap  102 . Since the movable riser  140  rises in response to the fluid level  9 , the flow rate is constant both through the orifice  141  of the movable riser  140  and through the interstitial space  102  and because air enters into the movable riser  140 . Since the diameter of the movable riser  140  is constant along its length and the interstitial space or gap  102  has a uniform cross sectional area, the restriction to flow through the interstitial space or gap  102  is fixed at the rim of the stationary riser  42  and the flow rate through the interstitial space or gap  102  is variable with respect to fluid level  9  where the degree of variability is a function of the cross sectional area of the interstitial space or gap  102 . The liquid level  115  in the drainage system  24  and stationary riser  42  is lower than the bottom of the movable riser  140 . 
     Equivalent elements can be substituted for the ones set forth above such that they perform in substantially the same manner in substantially the same way for achieving substantially the same result. 
     It is believed that the system and method of the present invention and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely exemplary and explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.