Patent Publication Number: US-8973325-B1

Title: Method for roof drainage

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for roof drainage for reducing urban waterlogging. 
     2. Description of the Related Art 
     Roofs of modern buildings are equipped with drainage devices. Drainage exits of the drainage devices are arranged on low-laying areas on the roof for draining the accumulated water, particularly the accumulated water formed within a short period during a cloudburst, out of the building. Typical methods for roof drainage target at accumulating and draining the water out of the building as soon as possible, which facilitates the accumulation of rainwater on the ground of the city. However, with the enlargement of areas impervious to water, such as concrete constructions, asphalt pavements, and parking lots, water flow and the peak flow thereof on the ground surface of the city increase, thereby resulting in more and more urban waterlogging. Disadvantages of conventional methods for roof drainage lie in that the rainwater on various underlying surface is drained simultaneously and the peak flow of the surface runoff is too high, which easily results in urban waterlogging in case of frequent and heavy rainfalls. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems, it is one objective of the invention to provide a method for roof drainage for reducing urban waterlogging and lowering peak flow on the ground surface in the city. 
     To achieve the above objective, in accordance with one embodiment of the invention, there is provided a method for roof drainage for reducing urban waterlogging. The method comprises: arranging a drainage device on a roof, the drainage device comprising a drainage exit, a drainage pipe comprising a wall, and a water outlet hole; increasing a height of the drainage exit to allow the drainage exit to be between 5 and 10 cm higher than the roof; and arranging the water outlet hole having a certain drainage capacity on the wall of the drainage pipe at a position that has the same height as the roof or is lower than the roof. 
     Because the arrangement of the drainage exit is higher than the roof, the water level of drainage of the roof increases. In case of cloudbursts, a certain amount of rainwater can be stored by the roof, so that the method of the invention is capable of prolonging the drainage of the rainwater out of the roof, decreasing the peak flow of the rainwater on the ground surface of the city, and reducing the hazards of the urban waterlogging. Furthermore, as the water outlet hole is arranged on the wall of the drainage pipe, the drainage device has a certain drainage capacity for draining off the accumulated rainwater from the roof in case of light rain, moderate rain, and cloudburst. Advantages of the invention are summarized as follows: the method of the invention is capable of prolonging the drainage of the rainwater out of the roof, decreasing the peak flow of the rainwater on the ground surface of the city, and reducing the hazards of the urban waterlogging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described hereinbelow with reference to accompanying drawings, in which the sole FIGURE is an assembly diagram of a drainage device in accordance with one embodiment of the invention. 
       In the drawings, the following reference numbers are used:  1 . Drainage device;  2 . Roof;  3 . Drainage exit;  4 . Drainage pipe;  5 . Water outlet hole;  6 . Siphon; and  7 . Ball cock mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     For further illustrating the invention, experiments detailing a method for roof drainage for reducing urban waterlogging are described below. It should be noted that the following examples are intended to describe and not to limit the invention. 
     Conception of the method of the invention is that: the rainwater is retained contemporarily on the roof for prolonging the drainage to lower the peak flow of the rainwater drained from the roof, stagger the peak flow of the rainwater drained from the roof and the peak flow of rainwater drained from other underlying surfaces, thereby reducing the peak flow of the surface runoff, and increasing the rainwater storage capacity of the city pipelines. 
     As shown in  FIG. 1 , a method for roof drainage for reducing urban waterlogging comprises: arranging a drainage device  1  on a roof  2 , the drainage device  1  comprising a drainage exit  3 , a drainage pipe  4  comprising a wall, and a water outlet hole  5 ; increasing a height of the drainage exit  3  to allow the drainage exit  3  to be between 5 and 10 cm higher than the roof  2 ; and arranging the water outlet hole  5  having a certain drainage capacity on the wall of the drainage pipe  4  at a position that has the same height as the roof or is lower than the roof  2 . 
     Because the arrangement of the drainage exit  3  is higher than the roof  2 , the water level of drainage of the roof  2  increases. In case of cloudbursts, a certain amount of rainwater can be stored by the roof  2 , so that the method of the invention is capable of prolonging the drainage of the rainwater out of the roof  2 , decreasing the peak flow of the rainwater on the ground surface of the city, and reducing the hazards of the urban waterlogging. Furthermore, as the water outlet hole  5  is arranged on the wall of the drainage pipe  4 , the drainage device  1  has a certain drainage capacity for draining off the accumulated rainwater from the roof in case of light rain, moderate rain, and cloudburst. 
     The height difference between the drainage exit  3  and the roof  2  is determined by the rainfall amount precipitating in different regions and the requirement of the roof load according to Load Code for the Design of Building Structure (National Standard of the People&#39;s Republic of China), and a preferable height difference is within the range of between 5 and 10 cm. 
     Because the water outlet hole  5  has a limited drainage capacity, a siphon  6  is disposed on an upper part of the drainage pipe  4  in order to facilitate the water drainage out of the roof  2  after the cloudburst. The siphon  6  comprises an inlet and an outlet; the inlet of the siphon  6  faces the roof  2 ; and the outlet of the siphon  6  extends inside the drainage pipe  4 . 
     Different rainfall amounts impose different requirements on the water outlet hole  5  of the water drainage device  1 . In general, the water outlet hole  5  is required to have a complete drainage capacity. In case of cloudburst, the drainage capacity of the water outlet hole  5  is required to be reduced, or even to be closed, to realize the retention of the rainwater. In the invention, a ball cock mechanism  7  controls opening and closing of the water outlet hole. The water outlet hole is often in an open state. The water outlet hole is closed by the ball cock mechanism  7  if a water level on the roof reaches a preset height. 
     For example, a demonstration area of green building in Chongqing has an area of 3 ha, in which, a building area is 0.63 ha, accounting for 21% of the total; a public green area is 0.81 ha, accounting for 27% of the total; a waterscape area is 0.33 ha, accounting for 11% of the total; a road area is 0.27 ha, accounting for 9% of the total; and a hard-surface area is 0.96 ha, accounting for 32% of the total. Residential buildings include high floors and have a plurality of floors; and an availability of the building roof is 100%. 
     1. Designed rainwater flow Q in conventional rainwater drainage system in the residential area 
     A formula of the designed rainwater flow Q is as follows:
 
 Q=ψ   m   qF  
 
     in which, ψ m  represents an average runoff coefficient and is calculated by weighted average of different underlying surfaces; q represents an intensity of the cloudburst in Chongqing, and 
               q   =       2822   ⁢     (     1   +     0.775   ⁢   lg   ⁢           ⁢   P       )           (     t   +     12.8   0.076       )     0.77         ,         
where p is 2a, t is 5 min, F is 3 ha, so that Q=693.62 L/s.
 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Calculation table for flow runoff coefficient 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Concrete 
                 Crushed  
                   
                 Average runoff  
                 Average runoff 
               
               
                 Species of 
                   
                   
                 and 
                 stone 
                   
                 coefficient Ψ m ′  
                 coefficient Ψ m   
               
               
                 underlying 
                 Water 
                 Green 
                 asphalt 
                 Road 
                 Hard 
                 (not including 
                 (including the 
               
               
                 surfaces 
                 system  
                 area 
                 road 
                 surface 
                 roof  
                 the roof) 
                 roof) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Runoff 
                 1 
                 0.25 
                 0.9 
                 0.5 
                 1 
                 0.418 
                 0.628 
               
               
                 coefficient Ψ 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Percentage of a  
                 11 
                 27 
                 9 
                 32 
                 21 
                 89 
                 100 
               
               
                 total area 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     2. Determination of time t 1  for the rainwater on the roof to produce runoff according to the increased height H of the drainage exit 
     As a rainfall depth is 
                 H   1     =         ∫   0   T     ⁢     i   ⁢           ⁢     ⅆ   t         =       ∫   0   T     ⁢           A   1     ⁡     (     1   +     c   ⁢           ⁢   lg   ⁢           ⁢   P       )           (     t   +   b     )     n       ⁢           ⁢     ⅆ   t             ,         
thus a rainfall duration is
 
               t   =         (           (       -   n     +   1     )     ⁢   H         Ψ   roof     ⁢       A   1     ⁡     (     1   +     c   ⁢           ⁢   lg   ⁢           ⁢   P       )           +     b       -   n     +   1         )       1     n   -   1         -   b       ,         
so that the relationship of the rainfall depth H 1  and the rainfall duration is calculated, and the rainfall depth for the rainwater on the roof to produce the runoff is equal to the increased height of the drainage exit. Take the increased height of the drainage exit being 50 mm as an example, when the storm return period is 2a and the rainfall duration is t 1 =26.77 min, the rainfall depth is equal to the increased height of the drainage exit, that is, when the increased height of the drainage exit is 50 mm, runoff will not be produced until the rainfall depth on the roof is 26.77 min.
 
     3. Designed rainwater flow Q in improved rainwater drainage system in the residential area 
     a) at t 0 , a designed rainwater flow Q 1  of other underlying surfaces except the roof is Q 1 =ψ m ′qF 
     in which, ψ m ′=0.418, F=2.37 ha, t 0 =5 min, and P=2a, so that Q 1 =362.94 L/s. 
     b) at t 1 +t 0 , a designed rainwater flow Q 2  of the residential area is Q 2 =ψ m qF 
     in which, ψ m =0.628, F=3ha, t 1 =26.77 min, and P=2a, so that Q 2 =346.47 L/s. 
     It is known from the principal of ultimate storm intensity that Q′=Max{Q 1 , Q 2 }, the designed rainwater flow Q′ in improved rainwater drainage system in the residential area is Q′=362.94 L/s. 
     4. Reduction rate of the rainwater peak flow in the residential area and maximum caliber of the drainage pipe 
     a) ΔQ=Q−Q′=693.62−362.94=330.68 L/s, that is, the reduction rate of the rainwater peak flow=ΔQ/Q×100%=47.67%. 
     b) Q=693.62 L/s, Q′=362.94 L/s, i represents 0.002, it is known from the hydraulic 
     Calculation chart that DN=900, DN′=700 mm, that is the maximum caliber of the drainage pipe is 700 mm, and a reduction of the maximum caliber of the drainage pipe is 200 mm. 
     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.