IX. EROSION AND SEDIMENTATION
This chapter provides guidelines for evaluating erosion potential, sediment yield, and sediment retention.
1. Sheet Erosion Sheet erosion is the wearing away of a thin layer of the land surface. The Universal Soil Loss Equation was developed to model sheet erosion.
E = RKLSCP [9-1]
E = the average annual loss, in tons per acre;
R = a factor expressing the erosion potential of average annual rainfall;
K = the soil erodibility factor and represents the average soil loss, in tons per acre per unit of rainfall factors, R, from a soil in cultivated continuous fallow, with a standard plot length and % slope arbitrary selected as 72.6 feet and 9% respectively.
S and L = the topographic factors for adjusting the estimate of soil loss for specific land gradient (S) and length of slope (L).
C = the cropping management factor and represents the ration of the soil quantities eroded from land that is cropped under specified conditions to that which is eroded from clean-tilled fallow under identical slope and rainfall conditions.
P = the supporting consideration practice factor (strip cropping, contouring, etc.). For straignt-row farming P = 1.0.
Values for these parameters and a detailed description of the Universal Soil Equation is provided in the paper by Wischmeier, W.H., and Smith, D.D., "A Universal Soil Loss Equation to Guide Conservation Farm Planning," 7th Internatioan Congress of Soil Science, Madison, 1960.
2. Gully Erosion Gully development is usually associated with severe climatic events, improper land use, or changes in stream base levels. During the significant bully activity, the sediment produced and delivered to downstream locations is found in regions of moderate to steep topography having thick soil mantles. The total sediment outflow from eroding gullies, though large, is usually less than that produced by sheet erosion.
A field study of gully activity in several locations throughout the United States has resulted in the tentative relationship.
R = 0.15A.49
R = is the average annual gully head advance, in feet
A = is drainage area, in acres
S = is the slope of approach channel, in %
P = is annual summation of rainfall, in inches, from rains equal to or greater than 0.5 inches/24 hours
E = is the clay content of eroding soil profile, as a percentage of weight.
The United States Department of Agriculture and Soil Conservation Services has devised an equation for the soliton of field design problems involving gullies. The equation is:
R = 1.5A .46
R and A are defined in Equation 9-2 and
P = the summation of 24-hour rainfall totals of 0.5 inch or more occurring during the time period, connected to an average annual basis, in inches.
3. Channel Erosion Stream bank erosion is often caused by the clearing of protective cover from banks, from the straightening and realigning of channels, and changes in flow regimes.
Streambed erosion can initiate downcutting cycles in tributary channels and gullies because of the lowering of the base level.
a. Evaluation For channels in noncohesive sediments, Lane's relationship can be used qualitatively to predict the erosive channel conditions.
QS = Gs
Q = is stream discharge,
S = is longitudinal slope of stream channel
= is bed sediment discharge
= is particle diameter of bed material
Quantative estimate of channel erosion or deposition rates are obtained from the time sequence comparisons of surveyed cross sections, from maps, aerial photographs, and historical records. Predictions of channel changes are based on erosion or deposition rates when future changes in the flow regime are expected of scow or fill can be obtained from sediment discharge formulas (Lane and Borland, 1951; Einstein, 1950; Cotley and Hembree, 1955), the use of principles of fluvical morphology (Leopold and Maddoci, 1953). The use of Regime theory (Blench, 1957) or other methods that consider the forces exerted on the stream boundaries (Lane, 1955).
b. Control Alternatives
(1) Ordinary Riprap Riprap is a layer of loose rock or aggregate placed over an erodible surface to protect the soil surface from the erosive forces of water.
Riprap is placed at soil-water interfaces where soil conditions , water turbulence and velocity, expected vegetataive cover and groundwater conditions may cause erosion at design flow conditions. Locations that may require riprap are storm drain outlets, channel banks, and/or bottoms, roadside ditches, drop structures and shorelines.
(a) Design Discharge The minimum design storm discharge for channels and diversions shall be the peak discharge from a 10 year frequency rainfall event based upon ultimate development of the watershed.
The design stone size is the d50
or median stone diameter which is defined as the stone size which is exceeded by the 50 percent of the mixture by weight. Diameter of the largest stone shall be 1.5 times the design stone size, d50
If the rip rap size, d50
, computed for bends is less than 10 percent greater than the rip rap size d50
for straight channels, use the straight channel size. If greater than 10 percent use larger size riprap d50
, in the bends. Use no more than two sizes on any channel.
(b) Size The following procedure determines a design stone size that is stable under design flow conditions. It is from the National Cooperative Highway Research Program Report No. 108, entitled "Tentative Design Procedure for Riprap-Lined Channels." It is based on the tractive force method and covers the design of riprap in two basic channel shapes, trapezoidal and triangular.
The procedure is for the uniform flow in channels and is not to be used for design of riprap deenergizing devices immediately downstream from such high velocity devices as pipes and culverts.
The procedure assumes that the channel is already designed and the remaining problem is to determine the riprap size tht would be stable in the channel. The n value for design is estimated by estimating a riprap size and then determining the corresponding n value for the riprapped channel from Figure 9-1.
Figure 9-1 is based on the following equation:
n = 0.0225 (d50
n = Mannings roughness coefficient
= Median stone diameter, inches
When the channel dimensions are known the riprap can be designed (or an already completed design may be checked) as follows:
- Compute the ration of the wetted perimeter to hydrualic radius, P/R.
- Use Figure 9-2 with Sb , Q, and P/R to find median riprap diameter, d50, for straight channels.
- Use Figure 9-1 to find the actual n value corresponding to the d50 from Step 2. If the estimated and actual n values are not in reasonable agreement, another trial must be made.
- For channels with bends, calculate the ratio Bs/Ro, where Bs is the channel surface width and Ro is the radius of the bend. Use Figure 9-4 and find the bend factor, FB. Multiply the d50 for straight channels by the bend factor to determine riprap size to be used in bends. If the d50 for the bend is less than 1.1 time the d50 for the straight channel, then the size for the straight channel may be used in the bend, otherwise the larger stone size calculated for the bend shall be used. The riprap shall extend across the full channel section and shall extend upstream and downstream from the ends of the curve a distance equal to five times the bottom width.
- Use figure 9-5 to determine maximum stable side slope of riprap surface.
- Enter Figure 9-3 with Sb, Q, and Z and find the median riprap diameter, d50 for straight channels.
- Enter Figure 9-1 to find the actual n value. If the estimated and actual n values are not in reasonable agreement another trial must be made.
- For channels with bends, see Step 4 under Trapezoidal channels.
The riprap size to be specified on the plans shall be the maximum stone size in the mixture which shall be 1.5 times the d50
. The thickness of the riprap layer is 1.5 times the maximum stone size, but not less than six inches. Freeboard shall be added to the channel depth and shall be not less than 0.2 times the depth of flow or 0.3 feet, whichever is greater.
(c) Limits or Riprap The upstream limit is from the point of curvature a distance equal to 5 times the channel bottom width.
Downstream, the limit shall be from the point of tangency a distance equal to 5 times the channel bottom width.
The limit shall also be from the toe of the bank up to the maximum high water elevation or to a point where vegetation can be established. Where there is no paving or riprap on the bottom of the channel, the riprap shall extend at least 1.5 times the maximum stone size or minimum of 1.0 foot below the channel bottom.
(d) Thickness The riprap layer shall be a minimum of 1.5 times the maximum stone size but not less than six inches.
(e) Gradation Riprap shall be well graded down to the one-inch size particle. Well graded is herein defined as a mixture composed of primarily the larger stone but with a mixture of other sizes to fill the progressively smaller voids.
Stone for riprap shall be field stone or uneven quarry stone of approximately rectangular shape. The stone shall be hard and angular. Individual stones shall have a specific gravity of at least 2.5. The stone will not disintegrate on exposure to water or weathering.
(f) Filter Layer A filter layer of material shall be placed between the channel bottom and riprap to prevent soil movement into and through the riprap when either of the following conditins exist:
- Riprap is not well graded down to the one inch size particle.
- Soil layer below riprap is sand-sized or finer with a plasticity index, PI, less than 10.
Filter shall be a layer of plastic filter cloth or properly graded layer of sand, gravel, or stone. The plastic filter cloth shall be woven of polypropylene monofilment yarns or equal cloth manufactured expressly for this use.
Aggregate filter shall conform to the following criteria:
d15 = size of particle which 15% is finer by weight
d85 = size of particle which 85% is finer by weight
base = soil under filter
(g) Placement Subgrade for riprap or filter shall be prepared to the lines and grades shown. Fills shall be compacted to the density of adjacent undisturbed material.
Stone for filter or riprap may be placed to the required thickness and limits by equipment or hand. Placement of riprap or filter shall be in one full operation to the full course thickness. Avoid displacement of underlying materials.
If a filter cloth is to be used, riprap that is 12 inches or larger shall not be dumped directly onto the cloth. A 4 inch minimum thickness of gravel shall be placed on the cloth prior to placement of the riprap. Any rips or holes in the cloth shall be repaired by placing another piece of cloth over the tear or hole. Overlap ends of cloth a minimum of one foot.
(h) Maintenance Inspect riprap periodically expecially after heavy storms. Note loss or displacement or riprap and replace as necessary. Check for sediment buildup in riprap indicating a tear in filter and make all necessary repairs.
(2) Grouted Riprap Grout may be used to tie the individual rock pieces together, providing a monolithic mass. It also permits the use of smaller sized rock. The group may be a weak mix with a 28-day strength of at least 2,000 psi. The group should penetrate into the riprap mass. A veneer within the top few inches of the riprap should be avoided. It is generally more effective to have a rough surface with portions of the rock particles projecting out from the grout surface. The projecting rocks should be cleaned with a wet broom after completing the placement of grout.
Cracking of the grouted riprap will occur with settlement and frost; however, this does not affect appearance or function.
Attention to aesthetics should be given with grouting of riprap in developed areas to insure a reasonably acceptable appearance.
(3) Gabions Side slopes of 1:1 are satisfactory for channel banks. The gabions should be keyed into both banks for drops to prevent flanking, and downstream cutting should be considered.
The hydraulic roughness of gabions is usually about 0.035; however, for use in drops larger stones may be used at the surface to increase the roughness to dissipate additional hydraulic energy.
(4) Vegetation Vegetation may be used to protect streambank from erosion. Use of grass is discussed in Chapter VIII. References 2, 3, 6, 9,. 10 and 11 discuss other alternatives.
C. Sedimentation and Sediment Retention Structures
1. Purpose - Sediment retention basins are needed to preserve the capacity of downstream waterways and to protect underground storm drainage facilities. The basins are designed to collect and store portions of the sediments or debris being carried by runoff. They work by slowing the velocity of the runoff and allowing suspended soil particles to settle by gravity.
2. Policy - Sediment basins will be used to trap sediment originating from construction sites where physical conditions or land ownership preclude adequate treatment of the sediment source by the installation of permanent erosion control measures. Sediment basins will be required where natural streams enter underground storm drainage systems or improved channel reaches. The minimum design trap efficiency shall be 60 percent.
Sediment retention structures may be single or multiple purpose flood control structures and shall contain the following reatures:
a. Excavated basin and/or compacted embankments
b. Inlet channels to allow flows to enter the basin.
c. Baffles to spread runoff throughout basin.
d. Pipe riser as a principle spillway.
e. Basin dewatering device.
f. Emergency spillway.
g. Outlet protection or direct hookup to storm drain system.
h. Access road and ramp for basin clean-out.
i. Fencing and other safety for public safety.
a. Size Distribution of Sediments The particle size distribution of the stream bed soils and principle sediment sources shall be determined by sampling and laboratory analysis.
b. Minimum Basin Storage Capacity Determine minimum storage capacity of reservoir from appropriate curve on Figure 9-1 for the design trap efficiency.
c. Area and Depth of Ponding For particle to be trapped, the basin travel time must be equal to or greater than the particle setting time as shown on Table 9-1.
Unless otherwise required by the State Department of Fish and Game, the minimum sediment basin depth shall be based upon a settling velocity of the median grain size of the sediments or 60 microns, whichever is less.
4. Gross Erosion and Sediment Delivery Ratio The average annual erosion quantity from sloping uplands shall be computed by means of the Universal Soil Loss Equation (USLE). The estimated quantities from stream bank and gully erosion should be added to the USLE value.
The total sediment uield is determined by the equation,
Y = E (DR) [9-5]
Y = Sediment Yield (Tons/unit area/year)
E = Gross erosion, tons/unit area/year
DR = Sediment Delivery Ratio, Figure 9-2.
Basin Clean Out Interval The basin clean out interval shall be based upon basin capacity (C) less the volume of water required to produce the settlement time and is computed as follows:
I = (3630C-AD) / (YWs
I = Clean out interval in years
C = Capacity, Acre-inches
A = Area of Basin, sq. ft.
D = Depth of Flow, ft.
Y = Sediment yield, tons/acre/year
= Submerged sediment volume unit weight, lbs/cu.ft.
|TABLE 9-1 |
SETTLING VELOCITIES OF SELECTED PARTICLES
||Settling Velocity |
Reference: U.S. Soil Conservation Service, National Engineering Handbook, Section 3, Sedimentation
- Chow, Ven Te, Open-Channel Hydraulics, McGraw-Hill, 1959.
- Henderson, Jim E., and F. Douglas Shields, Environmental Features for Streambank Erosion Projects, U.S. Army Corps of Engineer Waterways Experiment Station, Report E-84-11, 1984.
- Heyson, James R., et al, Environmental Features for Streamside Levee Project, US Army Corps of Engineers Waterways Experiment Station, Report E-85-7, 1985.
- High Sierra Resource Conservation and Development Council, Erosion and Sediment Control Guidelines for Developing Areas of the Sierras, 1981.
- Israel Ministry of Agriculture, "Gully Development and Sediment Yield", Research Report No. 13, Soil Conservation Division, Feb., 1966
- Jackson, William, ed., Engineering Considerations in Small Stream Management, American Water Resources Association Bulletin, Vol 22, No. 3, 1986.
- King, Horace W. and Ernest F. Brater, Handbook Hydraulics, McGraw-Hill, 1976.
- Ministry of Environment, Stream Enhancement Guide, Canada, March, 1980.
- Nunally, Nelson R. and F. Douglas Shields, Incorporation of Environmental Features in Flood Control Channel Project, U.S. Army Corps of Engineers Waterways Experiment Station, Report E-85-3, 1987.
- Schiechtl, Hugo, Bioengineering For Land Reclamation and Conservation, University of Alberta Press, 1980.
- Shields, F. Douglas, Environmental Features for Flood Control Channel, U.S. Army Corps of Engineers Waterways Experiment Station, Report E-82-7, 1987.
- Soil Conservation Service, Engineering Division, Design of Open Channels, Technical Release No. 25, 1977.
- Thompson, J.R., "Quantative Effect of Watershed Variables on the Rate of Bully Head Advancements", Transactions, American Society of Agricultura Engineers, Vol 7, No. 1, St. Joseph, Michigan, 1964, pp. 54-55).
- Wischmeier, W.H., and D.D. Smith, "A Universal Soil Loss Equation to Guide Conservation Farm Planning", 7th International Congress of Soil Science, Madison, 1960.