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Catchments & Creeks guidelines
Level Spreaders
DRAINAGE CONTROL TECHNIQUE
Low Gradient ✔ Velocity Control Short Term ✔
Steep Gradient [1] Channel Lining Medium-Long Term ✔
Outlet Control ✔ Soil Treatment Permanent ✔
[1] Level spreaders can release sheet flow down steep slopes, but the level spreader itself must be
constructed across a level gradient.
Symbol
Photo 1 – Diversion drains (centre) collect Photo 2 – Level spreader established to
stormwater from roadside table drains, discharge stormwater from a diversion
then releases the water as sheet flow via a drain into the roadside property
level spreader
Key Principles
1. Flow must be released from the level spreader as sheet flow.
2. Flow must be released over a stable, well-grassed surface that will maintain suitable flow
conditions down the slope.
3. Critical design parameter is the length of the outlet sill.
4. Critical operational parameter is the level construction of the outlet sill.
Design Information
The length of the outlet sill (weir) of the level spreader is governed by the design discharge, and
the allowable flow velocity of the down-slope area.
Allowable flow velocity for grassed surfaces can be determined from Table 1.
Minimum dimension can be determined from Tables 2 and 3.
Minimum sill length is 4 metres.
Maximum sill length is 25 metres. If a longer sill length is required, then the inflow must be spilt
and released through more than one level spreader.
Up-slope channel grade should not exceed 1% for the last 6 metres before entering the level
spreader.
Discharge must release evenly along a level surface (sill) of 0% cross gradient.
Caution the use of a design discharge exceeding 0.85 m 3/s.
Caution the release of water onto grass slopes steeper than 10%.
© Catchments & Creeks Pty Ltd V2 February 2010 Page 1
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Table 1 – Allowable flow velocity (m/s) for grassed surfaces [1]
Percentage Gradient of grass surface (%)
grass cover 1 2 3 4 5 6 8 10 15 20
[2]
70% 2.0 1.8 1.7 1.6 1.6 1.5 1.5 1.4 1.3 1.3
[3]
100% 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.9 1.8 1.7
[3]
Poor soils 1.5 1.4 1.3 1.2 1.2 1.1 1.1 1.1 1.0 0.9
[1] Maximum allowable flow velocity limited to 2.0m/s due to shallow water flow and resulting high shear
stress. 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 is
sufficient plant establishment time.
[3] “Poor soils” refers to the soil’s high erosion potential, such as dispersive clays (Emerson Class 1 and
2) such as sodic, yellow and red soils. Unstable, dispersible clayey sands and sandy clays, such as
yellow and grey massive earths formed on sandstones and some granites. Highly erodible soils may
include: lithosols, alluvials, podzols, siliceous sands, soloths, solodized solonetz, grey podzolics,
some black earths, fine surface texture-contrast soils, and Soil Groups ML and CL.
Table 2 – Level spreader sill length B metres per unit discharge (m per m;/s) [1]
Land Allowable down-slope velocity over well grassed surface (m/s)
slope (%) 1.0 1.2 1.5 1.8 2.0 2.2 2.5
1.0 3.5* 2.5* 1.6* 1.1* 0.9* 0.8* 0.6*
2.0 5.2 3.8* 2.5* 1.8* 1.4* 1.2* 0.9*
3.0 6.6 4.8 3.2* 2.3* 1.8* 1.5* 1.2*
4.0 7.7 5.6 3.8* 2.7* 2.2* 1.8* 1.4*
5.0 8.7 6.3 4.3* 3.1* 2.5* 2.1* 1.6*
6.0 9.5 7.0 4.7 3.4* 2.8* 2.3* 1.8*
7.0 10.3 7.6 5.2 3.7* 3.1* 2.6* 2.0*
8.0 11.0 8.2 5.6 4.0* 3.3* 2.8* 2.2*
9.0 11.8 8.7 6.0 4.3* 3.5* 3.0* 2.4*
10.0 12.4 9.2 6.3 4.6* 3.8* 3.2* 2.5*
Caution the release of water onto grass slopes steeper than 10%.
15.0 15.2 11.3 7.8 5.7 4.8 4.0* 3.2*
20.0 17.4 13.1 9.1 6.7 5.6 4.7 3.7*
25.0 19.4 14.6 10.3 7.6 6.3 5.3 4.3*
33.3 22.1 16.8 11.9 8.8 7.4 6.2 5.0
50.0 26.6 20.3 14.5 10.8 9.1 7.8 6.3
3
* Sill length limited to minimum 4m for discharges less than 0.85m /s.
Design example:
Design a level spreader to release a flow rate of 0.5m3/s down a 10% slope containing a good
(70%) grass cover on moderately erodible soil.
Solution:
From Table 1, choose a maximum flow velocity of 1.4m/s as best representative of a good grass
cover on a moderately erodible soil.
From Table 2, select a sill width per unit flow rate of 7.3m/m3/s.
Therefore, the sill length would need to be 0.5 x 7.3 = 3.65m < 4m (minimum).
Conclusion, specify a sill length of 4m.
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The minimum sill lengths presented in Table 2 have been determined assuming a Manning’s
roughness for 50-150mm (Class D) grassed surfaces based on Equation 1. The sill length is
sensitive to the selection of Manning’s roughness. Variations between Table 2 and other
published design tables for is due to variations in the assumed Manning’s roughness, which is
highly variable depending on the type and length of grass, and local growing conditions.
R1/ 6
Class D roughness: n= (Eqn 1)
. + 20.77 log10 (R1.4 . S0.4 )
5124
Table 3 – Minimum dimension of level spreader
Discharge (m3/s) Entrance width (m) Depth (m) End width (m)
0 to 0.28 3.0 0.15 0.9
0.29 to 0.57 4.9 0.18 0.9
0.58 to 0.85 7.3 0.21 0.9
Construction of a level spreader may require formation of flow control banks as shown in
Figures 1 to 3.
Figure 1 – Example of a level spreader used for flow diversion around a soil disturbance
Figure 2 – Typical layout of level spreader
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Description Special Requirements
Level spreaders consist of a level, grassed, Outlet area must be free of depressions
side-flow weir (i.e. water discharges at 90 that may concentrate the outflow.
degrees to the inflow direction) constructed
Extra erosion protection using jute mesh,
along the contour.
Erosion Control Mats, turf, rock etc. may be
required at the sill (Figure 4).
Purpose
Generally constructed by bozers no larger
Used to allow concentrated inflow to be
than D5 or equivalent.
released as sheet flow down a stable,
vegetated slope. Extreme caution must be exercised when
attempting to discharge sheet flow down a
Can be used as an outlet for Catch Drains
steep gradient (>10%) to ensure that the
and Flow Diversion Banks.
sedimentation or damage to the outlet sill
Level spreaders are commonly used in rural does not concentrate the outflow.
areas to discharge stormwater from
roadside table drains into an adjacent Site Inspection
property (Photos 1 & 2).
Check for sediment build-up on the sill, or
the concentration of outflow.
Limitations
Check for erosion down-slope of the sill.
Minimum sill length of 4m.
Maximum sill length of 25m. Installation
3
Maximum discharge of around 0.85 m /s. 1. Refer to approved plans for location,
dimensions and construction details. If
Must only be used where the outflow can
there are questions or problems with
be discharged to an undisturbed, stable,
the location, dimensions, or method of
grassed surface.
installation contact the engineer or
Construction traffic should be prohibited responsible on-site officer for
from the area of the level spreader. assistance.
Not suitable for highly erosive soils, 2. Wherever practical, locate the level
dispersive soils, or soils with poor spreader on undisturbed, stable soil.
vegetation cover.
3. Ensure flow discharging from the level
spreader will disperse across a properly
Advantages
stabilised slope not exceeding 10:1
Inexpensive to construct and maintain. (H:V) and sufficiently even in grade
across the slope to avoid concentrating
Disadvantages the outflow.
Can be difficult to construct the outlet sill to 4. The outlet sill of the spreader should be
the required precision. protected with erosion control matting
to prevent erosion during the
May require a considerable width of establishment of vegetation. The
undisturbed land. matting should be a minimum of
May require the land to be free of trees, 1200mm wide extending at least
shrubs and other surface irregularities to 300mm upstream of the edge of the
avoid local erosion problems. outlet crest and buried at least 150mm
in a vertical trench. The downstream
Common Problems edge should be securely held in place
with closely spaced heavy-duty wire
The most common problems result from staples at least 150mm long.
damage to the outlet sill either from erosion,
sedimentation, or stock. 5. Ensure that the outlet sill (crest) is level
for the specified length.
Other problems can result from water flow
concentrating below the level spreader due 6. Immediately after construction, turf, or
to the existence of a concave surface, seed and mulch where appropriate, the
vehicular tracks, or uneven vegetation level spreader.
cover.
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Maintenance
7. Grass height should be maintained at a
1. Inspect the level spreader after every minimum 50mm blade length within the
rainfall event until vegetation is level spreader and downstream
established. discharge area, and a maximum blade
length no greater than adjacent
2. After establishment of vegetation over
grasses.
the level spreader, inspections should
be made on a regular basis and after
Removal
runoff-producing rainfall.
1. Temporary level spreaders should be
3. Ensure that there is no soil erosion and decommissioned only after an
that sediment deposition is not causing alternative stable outlet is operational,
the concentration of flow. or when the inflow channel is
4. Ensure that there is no soil erosion or decommissioned.
channel damage upstream of the level 2. Remove collected sediment and
spreader, or soil erosion or vegetation dispose of in a suitable manner that will
damage downstream of the level not cause an erosion or pollution
spreader. hazard.
5. Investigate the source of any excessive 3. Remove and appropriately dispose of
sedimentation. any exposed geotextile.
6. Maintain grass in a health condition 4. Grade the area and smooth it out in
with no less than 90% cover unless preparation for stabilisation.
current weather conditions require
otherwise. 5. Stabilise the area as specified on the
approved plan.
Figure 3 – Alternative level spreader layout
Figure 4 – Cross-sectional profile of end sill
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