Level Spreaders - Catchments and Creeks

1 Catchments & Creeks Pty LtdV2 February 2010 Page 1 Level Spreaders DRAINAGE CONTROL TECHNIQUELow Gradient Velocity ControlShort Term Steep Gradient[1]Channel LiningMedium-Long Term Outlet Control Soil TreatmentPermanent [1] Level Spreaders can release sheet flow down steep slopes, but the Level spreader itself must beconstructed across a Level 1 Diversion drains (centre) collectstormwater from roadside table drains,then releases the water as sheet flow via alevel spreaderPhoto 2 Level spreader established todischarge stormwater from a diversiondrain into the roadside propertyKey Principles1.

2 Flow must be released from the Level spreader as sheet Flow must be released over a stable, well-grassed surface that will maintain suitable flowconditions down the Critical design parameter is the length of the outlet Critical operational parameter is the Level construction of the outlet InformationThe length of the outlet sill (weir) of the Level spreader is governed by the design discharge, andthe allowable flow velocity of the down-slope flow velocity for grassed surfaces can be determined from Table dimension can be determined from Tables 2 and sill length is 4 sill length is 25 metres.

3 If a longer sill length is required, then the inflow must be spiltand released through more than one Level channel grade should not exceed 1% for the last 6 metres before entering the must release evenly along a Level surface (sill) of 0% cross the use of a design discharge exceeding m 3 the release of water onto grass slopes steeper than 10%. Catchments & Creeks Pty LtdV2 February 2010 Page 2 Table 1 Allowable flow velocity (m/s) for grassed surfaces [1]Gradient of grass surface (%)Percentagegrass cover123456810152070% [2] [3] soils [3] [1]Maximum allowable flow velocity limited to due to shallow water flow and resulting high shearstress.

4 High flow velocities are allowable on reinforced grass.[2]70% cover would be typical for most grasses recently established by seed, but only when there issufficient plant establishment time.[3] Poor soils refers to the soil s high erosion potential, such as dispersive clays (Emerson Class 1 and2) such as sodic, yellow and red soils. Unstable, dispersible clayey sands and sandy clays, such asyellow and grey massive earths formed on sandstones and some granites. Highly erodible soils mayinclude: lithosols, alluvials, podzols, siliceous sands, soloths, solodized solonetz, grey podzolics,some black earths, fine surface texture-contrast soils, and Soil Groups ML and 2 Level spreader sill length B metres per unit discharge (m per m.)

5 /s) [1]Allowable down-slope velocity over well grassed surface (m/s)Landslope (%) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *Caution the release of water onto grass slopes steeper than 10%. * * * * * Sill length limited to minimum 4m for discharges less than example:Design a Level spreader to release a flow rate of down a 10% slope containing a good(70%) grass cover on moderately erodible :From Table 1, choose a maximum flow velocity of as best representative of a good grasscover on a moderately erodible Table 2, select a sill width per unit flow rate of , the sill length would need to be x = < 4m (minimum).

6 Conclusion, specify a sill length of 4m. Catchments & Creeks Pty LtdV2 February 2010 Page 3 The minimum sill lengths presented in Table 2 have been determined assuming a Manning sroughness for 50-150mm (Class D) grassed surfaces based on Equation 1. The sill length issensitive to the selection of Manning s roughness. Variations between Table 2 and otherpublished design tables for is due to variations in the assumed Manning s roughness, which ishighly variable depending on the type and length of grass, and local growing D roughness.

7 (Eqn 1)Table 3 Minimum dimension of Level spreaderDischarge (m3/s)Entrance width (m)Depth (m)End width (m)0 to to to of a Level spreader may require formation of flow control banks as shown inFigures 1 to 1 Example of a Level spreader used for flow diversion around a soil disturbanceFigure 2 Typical layout of Level spreadernRRS=+1610140 4512420 77/..log(.) Catchments & Creeks Pty LtdV2 February 2010 Page 4 DescriptionLevel Spreaders consist of a Level , grassed,side-flow weir ( water discharges at 90degrees to the inflow direction)

8 Constructedalong the to allow concentrated inflow to bereleased as sheet flow down a stable,vegetated be used as an outlet for Catch Drainsand Flow Diversion Spreaders are commonly used in ruralareas to discharge stormwater fromroadside table drains into an adjacentproperty (Photos 1 & 2).LimitationsMinimum sill length of sill length of discharge of around m3 only be used where the outflow canbe discharged to an undisturbed, stable,grassed traffic should be prohibitedfrom the area of the Level suitable for highly erosive soils,dispersive soils.

9 Or soils with poorvegetation to construct and be difficult to construct the outlet sill tothe required require a considerable width ofundisturbed require the land to be free of trees,shrubs and other surface irregularities toavoid local erosion ProblemsThe most common problems result fromdamage to the outlet sill either from erosion,sedimentation, or problems can result from water flowconcentrating below the Level spreader dueto the existence of a concave surface,vehicular tracks, or uneven RequirementsOutlet area must be free of depressionsthat may concentrate the erosion protection using jute mesh,Erosion Control Mats, turf, rock etc.

10 May berequired at the sill (Figure 4).Generally constructed by bozers no largerthan D5 or caution must be exercised whenattempting to discharge sheet flow down asteep gradient (>10%) to ensure that thesedimentation or damage to the outlet silldoes not concentrate the InspectionCheck for sediment build-up on the sill, orthe concentration of for erosion down-slope of the Refer to approved plans for location,dimensions and construction details. Ifthere are questions or problems withthe location, dimensions, or method ofinstallation contact the engineer orresponsible on-site officer Wherever practical, locate the levelspreader on undisturbed, stable Ensure flow discharging from the levelspreader will disperse across a properlystabilised slope not exceeding 10:1(H.)