A portable, water-filled barrier is internally divided into cells and emulates a sandbag dike or wall without requiring sand or intensive labor to install. Rigid steps spans the barrier to provide structural support and enable traversing of the barrier. The steps can be attachable to the barrier. The light, flexible barrier material can include nanofiber. Automatic valves can seal openings between the filled cells, so that a punctured cell will not cause cells below and behind to deflate. Cells can project below the base into a stabilizing trench. Cells can include internal or external rods or plates that support the barrier structure. Some embodiments can be initially filled with air, positioned, and then filled with water while the air escapes through a pressure valve. Other embodiments are filled from above, and inflate and deflate without allowing air into the barrier. Barrier end structures can enable interlocking of adjacent units.

RELATED APPLICATIONS 
     This application is a continuation in part of application Ser. No. 13/663,756, filed on Oct. 30, 2012 and issued as U.S. Pat. No. 8,956,077 on Feb. 17, 2015. Application Ser. No. 13/663,756 claims the benefit of U.S. Provisional Application No. 61/553,403, filed Oct. 31, 2011. Both of these applications are herein incorporated by reference in their entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to temporary barriers, such as dikes used for flood control, and more particularly, to water-filled portable barriers. 
     BACKGROUND OF THE INVENTION 
     Circumstances sometimes arise where a temporary dike, wall, or other barrier is needed to prevent a flood, landslide, or other threat from spreading and threatening lives and property. Often, such a temporary barrier is constructed from sandbags, whereby empty bags and a quantity of dirt or sand is brought to the site, and a crew of workers fills the bags with the dirt or sand and stacks the bags to form the barrier. With reference to  FIG. 1 , the bags are often stacked so as to form a barrier with a “pyramid” cross-section  100  that is widest at the base, and narrower at the top. 
     In some cases, the barrier  100  is constructed on flat ground, and the weight of the sand in the barrier  100  is sufficient to hold the barrier  100  in place during the flood or other threat. With reference to  FIG. 2 , in other cases a shallow trench  200  is prepared first, the trench having a depth that is approximately equal to the thickness of one sandbag. One or two rows of sandbags  202  are laid in the trench  200 , with the remainder of the barrier  100  being constructed on top of the initial one or two rows  202 . In this way, friction between the sandbags in the trench and the remainder of the sandbags further helps to hold the barrier in place. 
     While a sandbag barrier is generally effective and the materials are relatively inexpensive, there can be significant costs and construction time associated with a sandbag dike, due to the requirement to bring the sand or dirt to the construction site, which may weigh many tons, and due to the need to employ significant labor to fill and stack the bags. 
     In addition, after the flood or other threat has subsided, disposal of the sandbags can be time consuming and costly, especially if the sand and bags have become wet and contaminated by flood water and require special disposal procedures to avoid risks to health and to the environment. 
     What is needed, therefore, is a portable dike, wall, or other barrier that functions in a manner similar to a sandbag dike or wall, but does not require delivery of large quantities of heavy materials to the construction site, does not require large amounts of labor to assemble, and is simple and inexpensive to remove when it is no longer needed. 
     SUMMARY OF THE INVENTION 
     A portable, water-inflatable barrier has an internal structure similar to a sandbag dike or wall, and functions in a similar manner, but does not require delivery of large quantities of heavy materials to the construction site, does not require large amounts of labor to assemble, and is simple and inexpensive to remove when no longer needed. The barrier is made of a light, flexible material such as a heavy plastic or nanofiber, and can be transported to the construction site in a deflated state, after which it is positioned and filled with locally available water. 
     In one general aspect of the present invention, the barrier is a single unit that includes shaping and internal partitions which create an overall structure similar to a sandbag wall. The interior of the barrier is divided into a plurality of approximately rectangular cells. Passages between the tops and bottoms of the cells allow the entire barrier to be filled from a single water inlet. In some embodiments, the cells include passive automatic valves that seal the passages after the cells are filled with water, so that deflation of one cell due to a puncture or some other cause will not cause the cells beneath it to deflate. In various embodiments, the outer shell of the barrier is made of a thicker material, such as thick plastic, a synthetic rubber, or a thick layer of nanofiber, so as to better resist puncture by an external threat. In similar embodiments, the outer shell is double-walled, so that puncture of the outer wall does not affect the internal cells, so long as the inner wall remains intact. In certain embodiments the walls are coated with a protective material such as tyvec or liquid rubber that will seal punctures if they occur. 
     The unitary nature of the barrier in these embodiments eliminates any concern about interlocking and potential separation of individual units. The internal structure of the barrier enables it to maintain its shape when the barrier is subjected to externally applied horizontal forces, such as pressure from flood waters. In some embodiments, the shape of the structure is made even more rigid by the inclusion within the cells of stiff, lightweight rods or plates made of plastic, bamboo, or a similar material. 
     In further embodiments, additional rows of cells extend below the base of the inflatable barrier so that they can be placed in a trench prepared at the construction site; thereby further resisting dislodgement of the barrier by flood waters or other forces. 
     In various embodiments, the barrier can be initially inflated with air, so that the barrier can be easily positioned while it is in its filled configuration. The barrier can then be filled with water, while the displaced air is released through a pressure valve at the top of the barrier. 
     In circumstances where a long dyke or other barrier is required, a plurality of barriers of the present invention can be placed side-by-side. In some embodiments, the barriers have interlocking ends that provide structural cooperation and a water-tight seal between adjacent barriers. In some of these embodiments, pre-inflation of the barriers with air allows them to be easily placed in their interlocking configuration before the air within the barriers is replaced by water. 
     In a second general aspect of the present invention, the barrier is assembled from individual, water-inflatable modules that interconnect with each other, by ties, hook-and-loop, or by any other attachment mechanism known in the art. In some of these embodiments, the individual modules are triangular or wedge-shaped in cross section, thereby allowing the modules to be assembled so as to create an overall shape that is optimal for a specific circumstance. 
     Embodiments of the present invention include an anchoring sheet that surrounds part or all of the barrier, or is otherwise attached to the barrier, and extends flat against the ground in front of the barrier, so that the weight of the water in front of the barrier presses the anchoring sheet against the ground and creates a high frictional resistance to movement, thereby anchoring the barrier in place. In some embodiments, the anchoring sheet covers a water-facing surface of the barrier, and is sufficiently flexible to allow it to conform closely with the underlying shape of the water-facing surface. And in some of these embodiments, the anchoring sheet is made from a material that naturally clings to the water-facing surface of the barrier due to static electrical attraction. 
     Other embodiments include a flexible underlying sheet that further resists puncture from beneath, and which seals to the ground so as to resist penetration of water beneath the barrier. In some of these embodiments, the underlying sheet includes a cushioning layer. In other of these embodiments, the underlying sheet is filled with dry sand, foam or some other compliant material that will not get wet from the flood water. 
     In various embodiments, a base width of the barrier is at least six times as large as a height of the barrier. 
     Some embodiments include steps that are configured to be free-standing, but to conform somewhat closely to the outer profile of the barrier. The steps allow for a convenient means for crossing the barrier, and provides additional structural support to the barrier by inhibiting distortion of the shape of the barrier. In embodiments, the steps further provide horizontal and/or vertical support to the barrier by including coupling features on the steps that can be attached to complementary coupling features provided on the top of the barrier. 
     The present invention is a water inflatable barrier that includes a flexible shell configured to contain water in its interior, the shell having a front, a rear, a length, a width, and a substantially uniform cross-section along its length, the cross section being wider at a base of the shell than at a top of the shell. The barrier further includes a plurality of water-tight partitions bounded by substantially horizontal and substantially vertical partition walls, each of said vertical partition walls being oriented either substantially parallel to the front of the flexible shell or substantially perpendicular to the front of the flexible shell, said water-tight partitions dividing the interior of the shell into a plurality of adjacent cells having substantially identical dimensions, the cells being shaped approximately as rectangular parallelepipeds arranged in a plurality of horizontal layers that are vertically stacked on top of each other, each of widths and lengths of the cells being at least twice as large as heights of the cells, the cross-section of the flexible shell being thereby shaped by a step-wise ascending and descending series of the horizontal and vertical partition walls. The barrier further includes a plurality of vertical openings configured to allow water to flow vertically between the cells, a plurality of horizontal openings configured to allow water to flow horizontally between at least some of the cells, a water inlet proximal to the top of the flexible shell and configured to allow filling of the flexible shell with water, and rigid steps spanning the width of the flexible shell in substantial conformance with the step-wise cross-sectional shape of the flexible shell, the steps being configured to enable an individual to traverse the flexible shell. 
     In embodiments, the horizontal layers that are vertically stacked on top of each other are offset from each other such that none of the vertical partition walls that are parallel to the front of the flexible shell aligns with a vertical partition wall in a vertically adjacent layer. Some embodiments further include a water outlet proximal to the base of the flexible shell and configured to allow draining of water from the flexible shell without collapsing the flexible shell. 
     Certain embodiments further include a first coupling mechanism attached to the steps and a second coupling mechanism attached to the flexible shell, the coupling mechanisms being configured for attachment of the steps to the flexible shell. In some of these embodiments, the coupling mechanisms are configured to enable the steps to provide vertical support to the flexible shell. In other of these embodiments, the coupling mechanisms include a loop extending from one of the steps and the flexible shell, and a strap and buckle extending from the other of the steps and the flexible shell. 
     Some embodiments further include an automatic valve cooperative with a vertical opening and configured to automatically seal the vertical opening when the cell below the vertical opening is filled with water. Other embodiments further include an automatic valve cooperative with a horizontal opening and configured to automatically seal the horizontal opening when the cell located to the rear of the horizontal opening is filled with water. 
     Various embodiments further include a structure reinforcing element contained within a cell and configured to maintain at least one of a length and a width of the cell in which it is contained. In some of these embodiments, the structure reinforcing element is a rod or a plate. In other of these embodiments, the structure reinforcing element is made of plastic, wood, or bamboo. 
     Certain embodiments further include a structure reinforcing element that is external to the shell. In some embodiments, the barrier includes an interlocking end structure configured to interlock with a second barrier having a compatible end structure. 
     In exemplary embodiments, the barrier is inflatable with air. And some of these embodiments further include an air pressure relief valve configured to permit air to escape from the shell as the barrier is filled with water. 
     In some embodiments, the base of the barrier is flat. In other embodiments, the base of the barrier includes at least one row of cells extending below other rows in the base, the extended rows being configured for placement in a trench prepared at a site where the barrier is to be installed. 
     In various embodiments, at least the front of the flexible shell is reinforced as compared to the internal partitions. In some of these embodiments, the front of the flexible shell is reinforced due to an increased thickness of material relative to the internal partitions. In other of these embodiments, the front of the flexible shell is reinforced due to inclusion of a material not included in the internal partitions. In certain of these embodiments, the front of the flexible shell is reinforced due to inclusion of nanofiber in the flexible shell. And in various of these embodiments, the front of the flexible shell is reinforced due to double-walled construction. 
     In some embodiments, the outer shell includes a coating of a protective material that tends to seal punctures in the outer shell. And in some of these embodiments, the protective material is tyvec or liquid rubber. 
     Various embodiments further include an underlying sheet that further resists punctures of the barrier from beneath, and which seals to the barrier and to the ground beneath the barrier so as to inhibit penetration of water beneath the barrier. And in some of these embodiments, the underlying sheet is a cushioning layer. And in other of these embodiments, the underlying sheet is filled with dry sand or foam. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is perspective view of a sandbag barrier of the prior art having a flat base; 
         FIG. 2  is perspective view of a sandbag barrier of the prior art having two rows of sandbags at its base that are placed in a trench prepared at the construction site; 
         FIG. 3  is a perspective view of an embodiment of the present invention; 
         FIG. 4A  is a cross sectional view of an embodiment having a water inlet on top, a water outlet near the bottom, and simple passages between tops and bottoms of cells; 
         FIG. 4B  is a cross sectional view of an embodiment similar to  FIG. 4A , but including only a water port at the top through which the barrier is both filled and emptied with water; 
         FIG. 5  is a partial cross sectional view of an embodiment having passages between tops and bottoms of cells that are closable by passive valves; 
         FIG. 6  is a cross sectional view of an embodiment that includes stiffening rods within the cells; 
         FIG. 7  is a perspective view of an embodiment that has two additional rows of cells at its base that are placed in a trench prepared at the construction site; 
         FIG. 8  is a perspective view of an embodiment that has interlocking ends; 
         FIG. 9A  is a perspective view of an individual, inflatable module having a triangular cross section that can be combined with similar modules to form a barrier in embodiments of the present invention; 
         FIG. 9B  is a cross-sectional view of a barrier constructed using the modules of  FIG. 9A , and further including an anchoring sheet and an underlying sheet; 
         FIG. 10  is a perspective view of an embodiment of the present invention which includes steps that provides a means for crossing the barrier and also provides vertical support to the barrier; 
         FIG. 11  is a cross-sectional view of the embodiment of  FIG. 10 ; 
         FIG. 12  is a close-up view of the top of  FIG. 11 ; and 
         FIG. 13  is a cross-sectional view of an embodiment similar to  FIG. 11 , but wherein the steps do not provide vertical support to the barrier, but is optimized to inhibit distortion of the shape of the barrier. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 3 , the present invention is a portable, water-inflatable barrier  300  that has a structure similar to a sandbag dike or wall  100  and functions in a similar manner, but does not require delivery of large quantities of heavy materials to the construction site, does not require large amounts of labor to assemble, and is simple and inexpensive to remove when no longer needed. The barrier  300  is made of a light, flexible material, such as a heavy plastic for nanofiber, and can be transported to the construction site in a deflated state, after which it is positioned and filled with locally available water. In embodiments, the barrier material is coated with a material such as tyvec or liquid rubber that will tend to seal any puncture of the material that may occur. 
       FIG. 3  illustrates an embodiment of a first general aspect of the present invention in which the barrier is a single unit  300  that includes shaping and internal partitions which create an overall structure similar to a sandbag wall. The interior of the barrier is divided into a plurality of approximately rectangular cells  302 . With reference to  FIG. 4A , passages  400  between the tops and bottoms of the cells  302  allow the entire barrier  300  to be filled from a single water inlet  402 . A separate water outlet  404  is provided at the base of the structure  300 . 
     With reference to  FIG. 4B , in some embodiments a separate water outlet  404  is not included, and instead water is both added and removed through a common port  406  at or near the top of the barrier. This allows water to be removed from the barrier without introducing air, so that removing the water causes the barrier to be collapsed in preparation for packing and transport. 
     In various embodiments, lateral passages (not shown) are provided at least between adjoining cells in the bottom rear row, so that a single outlet can drain all of the cells  302  in the barrier  300 . 
     With reference to  FIG. 5 , in some embodiments  500  the cells  302  include passive automatic valves  500  that seal the passages  400  after the cells  302  are filled with water, so that deflation of one cell due to a puncture or some other cause will not cause the cells beneath it to deflate. In the embodiment  500  of  FIG. 5 , the valves  502  are flaps of elastic material joined to the upper surfaces of the cells  302  by living hinges  504 . A small air bladder  506  is included in the region of the valve  502  that is positioned to cover the passage  400 . When the cell  302  is empty, gravity causes the valve  502  to fall away from the passage  400 , so that the cell  302  can fill with water. However, once the cell  302  is full of water, the air bladder  506  lifts the valve  502  into place and closes the passage  400 . Once the valves  502  are closed, if a cell should develop a leak and deflate, only the cells directly above it will be affected. 
     In addition, the embodiment  500  of  FIG. 5  includes lateral passages  508  between neighboring cells at the lowest level of the barrier, so that the entire barrier can be emptied through a single water outlet  404  located at the lower rear of the structure  500 . These lateral passages  508  include automatic valves  510  that will allow water to flow toward the rear as the cells empty from back to front, but will prevent water flowing from rear to front if one of the front cells is damaged. 
     Typically, the cells in the front row  302 ,  302 A will be the cells that are directly exposed to threats such as debris carried by flood waters. The front cells  302 ,  302 A are therefore the ones most likely to be damaged or punctured. In the embodiment of  FIG. 5 , if a cell  302 A in the bottom front row is punctured, the lateral valve  510  will prevent water from flowing out of the cell next to it  302 B and into the damaged cell  302 A. However, if the rear cells  302 B are drained first during the normal drainage process, then the lateral valves  510  will open and water from the front cells  302 A will flow out. 
     With reference to  FIG. 6 , in some embodiments the outer shell is made of a much thicker material than the internal cell walls  608 , so as to better resist puncture by exterior threats. In similar embodiments, the outer shell  606  is a double layer of material, so that penetration of the outer layer does not affect the adjacent cell, so long as the inner layer remains intact. In various embodiments, only the portion of the outer shell  606  that will face the flood or other threat is thicker, double-walled, or otherwise reinforced. 
     In embodiments, the internal cell walls enable the barrier  300  to maintain its shape when it is subjected to externally applied lateral forces, such as pressure from flood waters. As illustrated in  FIG. 6 , in some embodiments, the shape of the barrier  600  is made even more rigid by including within the cells  302  stiff, lightweight rods  602  or panels made of plastic, bamboo, or a similar material. 
     In certain embodiments, the shape of the barrier is supported by external reinforcing structures. The embodiment of  FIG. 608  includes a plurality of bent metal rods  608  that can be located at intervals along the rear side of the barrier  600 . The rods  608  include vertical sections  610  that can be placed against the back sides of cells at the rear of the barrier  600  so as to provide further resistance to horizontal forces applied to the front of the barrier. 
     In various embodiments, the barrier  600  can be initially inflated with air, so that the barrier  600  can be easily positioned while it is in its inflated configuration. The barrier  600  can then be filled with water, while the displaced air is released through a pressure valve  604  at the top of the barrier  600 . 
     With reference to  FIG. 7 , in further embodiments, additional rows  702  of cells extend below the base of the inflatable barrier  700  so that they can be placed in a trench  200  prepared at the construction site, thereby further resisting dislodgement of the barrier  700  by flood waters or other forces. 
     In circumstances where a long wall or dike is required, a plurality of barriers of the present invention can be placed side-by-side. With reference to  FIG. 8 , in some embodiments the barriers  800  have interlocking ends that provide structural cooperation and a water-tight seal between adjacent barriers. In the embodiment of  FIG. 8 , alternate rows of cells  802  extend from the end by a length of one cell, while the interleaved rows  804  do not. The opposite pattern is provided on the other end of the barrier  800 . It can be seen that a second barrier of the same configuration can be positioned so that its extended cells fit between the extended cells  802  of the adjacent barrier  800 . In some of these embodiments, as mentioned above, the barrier  800  can be initially filled with air, and then positioned with the ends interlocking, after which the barriers are filled with water while the displaced air is allowed to escape through pressure valves provided at the tops of the barriers  800 . 
     With reference to  FIGS. 9A and 9B , in a second general aspect of the present invention the barrier is assembled from individual, water-inflatable modules  900  that include attachment mechanisms  902  such as ties, hook-and-loop, or some other attachment mechanism known in the art. In the embodiment of  FIGS. 9A and 9B , the modules have a triangular cross-sectional shape. As illustrated in  FIG. 9B , this enables them to be assembled to form a barrier having a desired overall shape, such as a pyramid. While the base of the barrier is only slightly wider than the height in  FIG. 9B , in other embodiments the base is at least six times as wide as the height. 
     In the embodiment of  FIG. 9B , the sloping shape of the water-facing surface causes the water pressure to press the barrier against the ground and thereby increases friction and helps the barrier to resist being shifted horizontally by the water. The embodiment of  FIG. 9B  further includes an anchoring sheet  904  that is attached to the barrier and extends in front of the barrier, where it is pressed against the ground by the water  906  in front of the barrier, so that there is a high friction between the anchoring sheet  904  and the ground that further inhibits lateral movement of the barrier by the water  906 . 
     The anchoring sheet in the embodiment of  FIG. 9B  is wrapped around the forward-located modules of the barrier, thereby attaching the anchoring sheet  904  to the barrier. In similar embodiments, the anchoring sheet  904  is wrapped around the entire barrier, or is attached to the barrier by some other means known in the art. 
     In some embodiments, the anchoring sheet  904  is sufficiently flexible to allow it to conform closely with the underlying shape of the water-facing surface. And in some of these embodiments, the anchoring sheet  904  is made from a material that naturally clings to the water-facing surface of the barrier due to static electrical attraction. 
     In embodiments, the flexible material of the barrier allows the base of the barrier to form a seal with ground even if the ground is rough. The embodiment of  FIG. 9B  further includes a flexible underlying sheet  908  that increases resistance to puncture of the barrier from beneath, and which forms a seal with the ground so as to further resist penetration of water beneath the barrier. In some of these embodiments, the underlying sheet  908  includes a cushioning layer such as foam or a puncture-proof air bag that enables the underlying sheet to form a seal with very rough ground, and also further helps to avoid puncture of the barrier from beneath. In certain of these embodiments, the underlying sheet  908  is filled with dry sand, foam or some other compliant material that will not get wet from the flood water. 
     With reference to  FIG. 10 , some embodiments include steps  1000  that provides a convenient means for crossing the barrier  300 . The steps  1000  are configured to be free-standing, but to conform somewhat closely to the outer shape of the barrier  300 , so as to provide additional structural support to the barrier  300  by inhibiting changes to the barrier's shape. In the embodiment of  FIG. 10 , the steps  1000  further provide vertical support to the barrier  300  by including coupling features  1002  on the steps  1000  that can be attached to complementary coupling features  1004  provided on the top of the barrier  300 . 
       FIG. 11  is a cross-sectional view of the embodiment of  FIG. 10 , where the relationship between the steps  1000  and the barrier  300  can be more clearly seen. A vertical offset between the steps  1000  and the barrier  300  is included in  FIG. 11 , which simplifies the illustration of the coupling mechanisms  1002 ,  1004 . In other embodiments, such as the embodiment of  FIG. 13 , the steps  1000  include little or no vertical offset from the top of the barrier  300 , and in some of these embodiments the steps apply a small vertically downward pressure to the top of the barrier  300 . 
       FIG. 12  is a close-up view of the top of the embodiment of  FIG. 11 , wherein the coupling features  1002 ,  1004  can be more clearly seen. In  FIGS. 10-12 , a strap  1004  is attached to the top of the barrier  300 , and is looped through and buckled to a rigid loop  1002  that extends from the side of the steps  1000 . While  FIGS. 10-12  present a specific example of coupling features, it will be understood that the scope of the invention includes all coupling mechanisms known in the art, such as hooks, clamps, bolted brackets, nuts and horseshoe bolts, and such like. With reference to  FIG. 13 , it will also be understood that some embodiments do not include coupling of the steps  1000  to the barrier  300 . 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.