Difference between revisions of "Overflow"

From LID SWM Planning and Design Guide
Jump to: navigation, search
m (Overflow elevation)
(Overflow elevation)
 
(17 intermediate revisions by 4 users not shown)
Line 1: Line 1:
 
[[File:Routing.png|thumb|Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the cell via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.]]
 
[[File:Routing.png|thumb|Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the cell via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.]]
==Routing==
+
===Routing===
 
*Infiltration facilities can be designed to be inline or offline from the drainage system. See [[Inlets]]
 
*Infiltration facilities can be designed to be inline or offline from the drainage system. See [[Inlets]]
 
*Inline facilities accept all of the flow from a drainage area and convey larger event flows through an overflow outlet. The overflow must be sized to safely convey larger storm events out of the facility.  
 
*Inline facilities accept all of the flow from a drainage area and convey larger event flows through an overflow outlet. The overflow must be sized to safely convey larger storm events out of the facility.  
:
 
 
   
 
   
*The overflow must be situated at the far end of the facility to prevent any localised ponding to cause bypassing of the infiltration facility.  
+
*The overflow must be situated at the maximum surface ponding elevation or furthest downgradient end of the facility to limit surface ponding during periods of flow in excess of the facility storage capacity.  
*Offline facilities use flow splitters or bypass channels that only allow the required water quality storage volume to enter the facility.  
+
*Offline facilities use flow splitters or bypass channels that only allow the design storm runoff storage volume to enter the facility. Higher flows are conveyed to a downstream storm sewer or other BMP by a flow splitting manhole weir or pipe, or when the maximum surface ponding depth has been reached, by by-passing the curb opening and flowing into a downstream catchbasin connected to a storm sewer.
:Higher flows are diverted and do not enter the infiltration practice. A pipe can by used for this, but a weir or [[curb cuts|curb cut]] minimizes clogging and reduces the maintenance frequency.
 
  
==Overflow elevation==
+
===Overflow elevation===
The invert of the overflow should be placed at the maximum water surface elevation of the practice. i.e. the maximum ponding depth. A good starting point is around 300 mm over the surface of the practice. However, consideration should be given to public safety and time for the ponded water to drain. See [[Bioretention: Sizing#Additional step for system without underdrain|Bioretention]]
+
The invert of the overflow should be placed at the maximum water surface elevation of the practice (i.e. the maximum surface ponding level).  
 +
A good starting point is 150 to 350 mm above the surface of the mulch cover. However, consideration should be given to public safety, whether or not an underdrain is included, the time required for ponded water to drain through the filter bed surface, and if no underdrain is present, into the underlying native soil (must drain within 48 hours).  
 +
See [[Bioretention: Sizing]] and [[Stormwater planters]] for more details.
  
==Options==
+
===Freeboard===
 +
{{:freeboard}}
 +
 
 +
===Overflow outlet options===
 
Metal grates are recommended (over plastic) in all situations.  
 
Metal grates are recommended (over plastic) in all situations.  
  
Line 47: Line 50:
 
|}
 
|}
  
==Gallery==
+
===Gallery===
 
{{:Overflow:Gallery}}
 
{{:Overflow:Gallery}}

Latest revision as of 01:03, 14 July 2020

Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the cell via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.

Routing

  • Infiltration facilities can be designed to be inlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. or offlineRefers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed. from the drainage systemA system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006. See Inlets
  • InlineRefers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet. facilities accept all of the flow from a drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. and convey larger event flows through an overflow outlet. The overflow must be sized to safely convey larger storm events out of the facility.
  • The overflow must be situated at the maximum surface ponding elevation or furthest downgradient end of the facility to limit surface ponding during periods of flow in excess of the facility storage capacity.
  • OfflineRefers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed. facilities use flow splitters or bypass channels that only allow the design storm runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. storage volume to enter the facility. Higher flows are conveyed to a downstream storm sewer or other BMPBest management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls. by a flow splitting manhole weir or pipe, or when the maximum surface ponding depth has been reached, by by-passing the curb opening and flowing into a downstream catchbasinBox like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement. connected to a storm sewer.

Overflow elevation

The invert of the overflow should be placed at the maximum water surface elevation of the practice (i.e. the maximum surface ponding level). A good starting point is 150 to 350 mm above the surface of the mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. cover. However, consideration should be given to public safety, whether or not an underdrainA perforated pipe used to assist the draining of soils. is included, the time required for ponded water to drain through the filter bed surface, and if no underdrainA perforated pipe used to assist the draining of soils. is present, into the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. (must drain within 48 hours). See Bioretention: Sizing and Stormwater planters for more details.

Freeboard

  • In swales conveying flowing water a freeboard of 300 mm is generally accepted as a good starting point.
  • In bioretention the freeboard is the difference between the invert elevation of the overflow structure and the inlet. 150 mm will suffice, so long as the inlet will not become inundated during design storm conditions.
  • In above grade stormwater planters, the equivalent dimension would be the difference between the invert elevation of the overflow structure and the lip of the planter (150 mm minimum)

Overflow outlet options

Metal grates are recommended (over plastic) in all situations.

Feature Anti Vandalism/Robust Lower Cost Option Self cleaning
Dome grate x
Flat grate x
Catch basinGround depression acting as a flow control and water treatment structure, that is normally dry. x
DitchA long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line. inlet catch basinGround depression acting as a flow control and water treatment structure, that is normally dry. x x
Curb cut x x x

Gallery