HomeMy WebLinkAbout3821; Calavera Lake Creek & Agua Hedionda Creek; Calavera Lake Creek Sediment Study; 1989-08-01SEDIMENT STUDY
FOR CALAA/ERA LAKE CREEK
IN CARLSBAD
for Sediment Detention Basins at Rancho Carlsbad
ENGINEERING DEPT. LIBRARY
City of Carlsbad
2075 Las Palmas Drive
Carlsbadl CA 92009-4859
9v prepared by
Howard H. Chang
Ph.D., P.E.
August 1989
TABLE OF CONTENTS
I. INTRODUCTION 1
II. SCS METHOD FOR SEDIMENT PRODUCTION AND YIELD 5
SCS Method by Flaxman 5
Revised SCS Method 6
Procedure of Computation 7
III. SEDIMENT YIELDS UNDER EXISTING CONDITIONS 8
Comparison with Sediment Deposition in Channel 9
VI. SEDIMENT YIELDS UNDER ULTIMATE CONDITIONS 9
V. CAPACITIES OF SEDIMENT DETENTION BASINS 11
VI. MAINTENANCE REQUIREMENTS 12
REFERENCES 13
APPENDIX A. COMPUTER PROGRAM FOR SCS METHOD
APPENDIX B. PAGES FROM EROSION AND SEDIMENT CONTROL HANDBOOK
PERTAINING TO SEDIMENT DETENTION BASINS
SEDIMENT STUDY FOR CALAVERA LAKE CREEK IN CARLSBAD
for Sediment Detention Basins at Rancho Carlsbad
I. INTRODUCTION
The present stream channel for Calavera Lake Creek, see Fig. 1, through
Rancho Carlsbad Mobile Home Park has been silted and plans are being made to
dredge the channel in order to restore its flow capacity. The purpose of
sediment control for this channel is to avoid its future si1tation. The stream
channel has two branches which converge at the mobile home park. For effective
control of sediment inflow, two sediment detention basins - one for the north
branch and the other for the east branch - are needed. These two basins may also
be interconnected if they are located at the entrance to the mobile home park.
Sediment production refers to the detachment of sediment from the ground
surface. Sediment yield (or delivery) is the amount of sediment delivered to
a certain downstream point during a certain period of time or during a flood
event. Sediment yield may be different from the production because of sediment
storage in the stream valley and on hillsides. A stream channel usually
undergoes changes in channel boundary as some sediment is deposited or removed
from the stream valley and channel boundary. This is particularly evident in
semi-arid areas, where sediment delivery can be substantially different from the
production. For small drainage basins, the production and yield of sediment
may be assumed to be the same if the drainage basin exists in an approximate
state of equilibrium.
Sediment transport and delivery may only be determined for the bed-material,
which is the sediment normally found in the stream bed and small ponds. Wash
load or fine sediment load consists of clay and silt (diameter of particle less
than 0.062 mm) which seldom settle in the stream bed and small ponds. The
transport quantity for wash load depends on the supply and it is not correlated
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to the flow condition. For this reason, the delivery of wash load may not be
computed.
An important task of this study is to determine the volume of storage for
the sediment detention basins. This design consideration shall be based on the
total drainage area lying upstream and the future use of such lands. The storage
capacity for a basin is the volume below the spillway crest. The selection of
the volume shall consider periodic cleanout in order to maintain the capacity
requirements. A reasonable cleanout period shall be once a year or after each
major storm when the maximum level of deposition has been exceeded.
In the study, the sediment yield for Calavera Lake Creek at the entrance
of the Rancho Carlsbad Mobil Home Park will be determined. The Soil Conservation
Service (SCS) method by Flaxman is used to calculate the mean annual sediment
yield collected at a basin. Then the volume of capacity for a basin is
determined based on the sediment yield for a period of say 10 years. This
approach is conservative if the cleanout is going to be on a yearly basis. The
maintenance requirements for the basins are also outlined.
Calavera Lake Creek is a fairly small stream that drains water and sediment
from a semi-arid watershed which has an area of 5.9 square miles at the entrance
of the mobile home park. Of this area, 4.54 square miles is drained by the main
branch (or north branch) and 1.36 square miles is by the east branch. Calavera
Lake controls 3.61 square miles of the main branch. Since the lake traps nearly
all the coarse sediment, only the uncontrolled area of 0.93 square miles supplies
sediment to the creek at the mobile home park. The contributing subbasins for
sediment are shown in Fig. 1. The subbasin areas of the main branch are tabulated
below:
Subbasin Area in square miles
A10 0.36
All 0.23
A12H 0.23
A12L 0.11
Total area 0.93
In the above table, the subbasin A12H refers to the hillslope portion of
subbasins A12, and A12L refers to the flatter and cultivated portion of A12.
The east branch has three subbasins; their respective areas are given below.
Subbasin Area in square miles
LI 0.49
L2 0.48
L3 0.39
Total area 1.36
The climate of the watershed is semi-arid with warm summer and mild winter
temperatures. Precipitation occurs as rainfall, mostly from December to March
with the passage of low pressure systems and associated cold fronts.
Sediment yield for Calavera Lake Creek is analyzed for the two following
conditions:
(1) Existing conditions: This refers to the stream valley in its present
state before future developments.
(2) ultimate conditions: This refers to the conditions when future
developments are completed.
II. SCS METHOD FOR SEDIMENT PRODUCTION AND YIELD
There exist several methods for estimating the potential long-term yield
of a drainage basin (e.g., SCS method by Flaxman, 1973; Pacific Southwest Inter-
Agency Committee, 1974; Brownlie and Taylor, 1981). The Universal Soil Loss
Equation (USLE) has also been modified to suit western watersheds (Soil
Conservation Service, 1972).
The production of sediment in San Diego County has been estimated by numerous
investigators. Such estimates are for the natural conditions of the watershed,
under which the production and yield are assumed to have reached an approximate
equilibriun.
Several methods for estimating the sediment yield from natural watersheds
were considered for use in this study. Of these, the revised SCS method by
Elliott M. Flaxman was found to be the most useful because it was developed from
data compiled from 11 western states: Arizona, California, Colorado, Idaho,
Montana, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming. In addition,
the Pacific Southwest interagency Committee (1974) recommended the use of
Flaxman 's revised method, based on the result of a test which showed close
agreement between actual field data and computations from the revised SCS formula
by Flaxman. The sediment yield computed using the revised SCS method compared
favorably with field data collected from 1919 to 1948 in the Lake Hodges
watershed of San Diego County. Field data (Agricultural Research Service, 1973)
indicated that 500 tons of sediment had been produced per square mile per year;
the computed data predicted 370 tons per square mile (Department of Water
Resources, 1977) .
The following section describes the original SCS method by Flaxman and its
revision.
SCS Method by Flaxman - The SCS method (Flaxman, 1973; California Department
of Conservation, 1978) , a multiple regression equation, relates sediment yield
to watershed characteristics by using field data from small reservoirs and stock
ponds to estimate the potential for sedimentation. Since wash load (sediment
smaller than 0.062 am in size) does not settle in small ponds in large
quantities, the yield computed using this method consists of primarily coarse
sediments (sand and gravel) .
Flaxman determined statistically that climate, watershed slope, soil
texture, and soil aggregation or dispersion cause most of the variance in
sediment yield. These independent variables are identified as X-^ through X4 in
the equation, which is in the form:
log(Y + 100) = 6.21301 - 2.19113 log^ + 100)
+ 0.06034 log(X2 +100)
- 0.01644 log(X3 +100)
+ 0.04250 log(X4 + 100) (1)
in which Y is the mean annual sediment yield expressed in acre-feet per square
mile. X^ is the ratio of average annual precipitation (in inches) to average
annual temperature (in degrees F), an indirect expression of the natural response
of vegetation to climate. A higher value of X-^ indicates a better vegetation
cover and therefore a lower sediment yield from the drainage area. In the case
of cultivated area, the ground may be stripped of vegetation and this may be
expressed by a very small value of X^. The parameter ^ is the weighted average
slope of the watershed, expressed as a percentage. X3 is the percentage of soil
particles coarser than 1.0 mm; it reflects their resistance to erosion and
transport. X^ shows the aggregation or dispersion characteristics of clay-size
particles 2/1(2.0 x 10"^.) or finer, expressed as a percentage. A soil's pH
value is used in defining this variable. Soils with pH values of 7.0 or lower
(acidic) are generally associated with high precipitation and plentiful
vegetation cover. Because these soils are usually well-aggregated, a negative
value is assigned to the X4 variable. Soils with pH values higher than 7.0
(alkaline) are generally associated with lower precipitation and sparser
vegetative cover. Because they are poorly aggregated, a positive value is
assigned to the x^ variable. If more than 25 percent of the particles are
coarser than 1.0 mm, the variable X4 is assigned a value of zero in Flaxman's
multiple regression equation. This is based on the theory that coarse particles
dominate the erosion characteristics, regardless of aggregation or dispersion
tendencies. Flaxman emphasized that the watershed characteristics he described
excluded the effect of substantial gully and stream channel erosion. Sediment
from these sources would have to be added to the amounts determined by means of
his equation. in addition, this method is for natural watersheds whose
characteristics have not been altered by human activities, such as sand and
gravel mining.
Revised SCS Method - The original SCS method was revised by Flaxman as
described in the report of Pacific Southwest inter-Agency Committee (1974). It
includes an additional factor (X^) - 50 percent chance peak discharge expressed
in cubic feet per second per square mile (CSM). This variable, along with
variable XT, reflects the effects of vegetative cover. The value of X^ is an
inverse function of the watershed area; the revised SCS equation gives a lower
yield per unit watershed area as the area increases. This equation thus accounts
for the natural sediment storage in watersheds, or the delivery loss to the
watershed exit, which normally increases with the watershed size.
The revised equation also converts sediment yield (Y) from acre-feet to tons
per square mile. Other variables, X-^ through X4, remain unchanged in the revised
formula. The revised SCS equation has the form:
log(Y + 100) = 524.37231 - 270.65625 log^ + 100)
+ 6.41730 log(X2 +100)
- 1.70177 log(X3 +100)
+ 4.03317 log(X4 +100)
+ 0.99248 log(X5) (2)
Procedure of Computation - The following steps were followed in using the
revised SCS method:
(1) The watershed was divided into subbasins whose areas are measured from the
map.
(2) The precipitation-temperature ratio (X^) was determined for each sub-basin,
based upon the mean annual precipitation and the mean annual temperature. The
mean temperature of 59°F and the mean annual precipitation of 10 inches are used.
(3) The weighted average slope of the watershed (X2) was determined from the
200-scale topographic maps. To accomplish this step, the investigator
determined :
(a) The area between every fifth contour.
(b) The mid-contour length of the area.
(c) The average width between contours (by dividing the area by the
contour length) .
(d) The weighted average slope (by weighing the slopes according to
the area, multiplying the area of the interval by the slope,
adding the results for the entire watershed, and then dividing by
the entire area) .
(4) The percentage of soil particles coarser than 1.0 mm (X3) was determined
by drawing gradation curves for each subarea from data published in the soil
survey of different regions, by the Soil Conservation Service (SCS) , U.S.
Department of Agriculture.
*W (5) Gradation curves were plotted for the predominant soil types in the study
area, using the SCS soil maps and tables. Because these curves indicated that
the soils were mostly coarse in nature, the soil aggregation index (X*) was taken
as zero. This was done on the theory that the coarse sizes dominate the erosion
characteristics, regardless of the dispersion or aggregation tendencies of any
fraction less than 2 (2.0 x 10~^m).
(6) the 50 percent chance peak discharge in CSM (X^) was computed using the
following regression equation,
Q « 30.67 A0*557 (3)
30.67
A07443thus X5 = ----- (4)
where A = area in square miles and Q = discharge in cfs. This equation was
derived from frequency curves used by the California Department of Water
Resources (1976).
(7) The mean annual sediment yield (Y) was computed using the revised SCS
equation, then multiplied by the number of square miles within the subarea to
determine the total yield.
(8) The yield in tons per year is converted into cubic yards per year using the
conversion factor that one cubic yard of deposited sediment weights 1.38 tons.
That is, one cubic foot of deposited sediment weights about 102 pounds.
The computer program for the revised SCS method is included in Appendix A
of this report.
III. SEDIMENT YIELDS UNDER EXISTING CONDITIONS
The measured and computed parameters for the uncontrolled basins of the
north branch (below Lake Calavera) are tabulated below.
Subbasin Xl JC2 JK3 X4_ _X5 Yield Area Total Yield
tons/mi ^ mi^ tons
A10 0.17 14.30 17.50 0 48.22 483.53 0.36 174.07
All 0.17 12.80 17.50 0 58.81 474.10 0.23 109.04
A12H 0.17 14.20 17.50 0 58.81 521.39 0.23 119.92
A12L 0.17 3.30 12.00 0 81.54 536.70 0.11 59.04
Total 462.07
For the subbasin of the east branch, the results are tabulated as follows.
Subbasin XI X2 X3 X4 X5 Yield Area Total Yield
tons/mi^ mi2 tons
LI 0.17 8.00 18.00 0 42.07 286.04 0.49 140.16
L2 0.17 7.80 18.00 0 42.45 282.50 0.48 135.60
L3 0.17 14.20 20.00 0 46.54 453.54 0.39 176.88
Total 452.64
The total yields given in tons per year are converted into 334,8 cubic yards per
year for the north branch, and 328 cubic yards per year for the east branch.
The sum of these two branches is 662.8 cubic yards per year.
Conparison with Sediment Deposition in Channel - The computed mean annual
sediment yield of 662.8 cubic yards per year is compared with the mean annual
sediment deposition in the channel through the mobile home park. Based on the
channel bed level, the total volume of deposition in the 3,200-foot channel is
estimated to be about 10,500 cubic yards in bulk volume. Since the channel has
been in existence for about 20 years, the mean annual deposition is therefore
525 cubic yards. This figure should be somewhat less than the mean annul
sediment inflow from the drainage basin because a part of the sediment inflow
has been transported out of the channel reach to reach the lagoon. The wash load
is not a part of the estimate. It can be seen that the computed yield compares
favorably with the observed sediment deposition in the channel.
VI. SEDIMENT YIELDS UNDER ULTIMATE CONDITIONS
Development plans for the drainage basin of Calavera Lake Creek have been
devised. Under the developed conditions, sediment production and yield will be
significantly different from those of the existing conditions because of
buildings, roads, pavements, ground cover, drainage facilities, and so on.
C
Generally speaking, development increases the runoff discharge of water due to
the increase in runoff coefficient and the decrease in the time of concentration.
The effects of development on sediment production, on the other hand, are just
the opposite. The reduction of sediment production is attributed to buildings,
roads, and pavements that reduce the land area for sediment production. Also,
landscaping, maintained by irrigation, protects soil surface against erosion.
The effects of development on sediment yield are accounted for in the SCS
method through adjustments of the following parameters:
a. Drainage basin area - Paved areas in a drainage basin are excluded in
sediment computation. Thus the net area contributing to sediment production is
reduced. The percentage of reduction in surface area is estimated to be 15%
for subbasins A10, All, and A12H; 10% for subbasins LI, L2, and L3; and 65% for
A12L. These percentage reductions are on the basis of future land use.
b. Effective rainfall - irrigation water is equivalent to rainfall since it
improves the ground cover and thus reduces soil erosion. The effective rainfall
under the planned developments is estimated to double the natural annual
precipitation.
Based on the adjustments described above, the computed sediment yields under
the developed conditions are obtained and summarized in the following tables.
For the north branch, we have
Subbasin XI X2 X3 X4 X5 Yield Net Area Total Yield
tons/mi2 mi2
A10
All
A12H
A12L
0.34
0.34
0.34
0.34
14
12
14
3
.30
.80
.20
.30
17.
17.
17.
12.
50
50
50
00
0
0
0
0
52
64
64
127
.28
.01
.01
.64
279
275
305
220
.30
.09
.99
.40
tons
83.79
52.27
58.14
8.82
Total 203.02
The summary for the east branch is given below.
10
Subbasin XI X2 X3 X4 X5 Yield Net Area Total yield
tons/mi ^ mi^ tons
LI
L2
L3
0
0
0
.34
.34
.34
8.00
7.80
14.20
18
18
20
.00
.00
.00
0
0
0
44.12
44.12
48.83
147
144
255
.80
.87
.71
65
63
89
.03
.74
.50
Total 218.27
The total yield for the north branch is 147 cubic yards per year, and for the
east branch, it is 158 cubic yards per year.
•V. CAPACITIES OF SEDIMENT DETENTION BASINS
Guidelines for the design of sediment detention basins are given in Erosion
and Sediment Control Handbook published by the California Department of
Conservation, Resources Agency, May 1978. Most of the guidelines given in this
manual have also been adopted by the City of San Diego (1984). Pertinent pages
of the handbook are reproduced and included in Appendix B of this report.
Storage capacity of a sediment detention basin is the volume below the pipe
spillway crest or the emergency spillway crest. Selection of the capacity
depends on the period of cleanout. For a selected capacity, the maximum
allowable level of deposited sediment before cleanout should also be determined
and provided in the design data.
For the sediment detention basins proposed in this case, the annual cleanout
is considered desirable. On the basis of this yearly cleanout out, the storage
capacity equal to the ten-year accumulated sediment yield at the site is
recommended. Thus, the storage volume for the north branch is 3,350 cubic yards,
and for the east branch 3,300 cubic yards.
At the 100-year flood, the velocity in the downstream part of the basin
shall be maintained at about 2 feet per second or less. This velocity is
considered to be the threshold for bed load transport. With such a velocity,
most of the bed load sediment will be trapped in the basin.
11
Although the storage capacity for each sediment basin is equivalent to ten
years of sediment yield, a basin may still be silted in a much shorter period
of time, or even in one flood event. This is because sediment yield is not
uniformly distributed as it varies significantly from time to time. Accelerated
sediment yield may be caused by grading or forest fires in the watershed. The
maximum level of sediment deposition is recommended to be one half of the total
storage volume. If at any time, this maximum level is exceeded, then the basin
must be cleaned even if it is not due for a maintenance cleanout.
The sediment detention basins may be totally silted during a major flood
event, such as the 100-year flood. However, the amount of sediment trapped
during a 100-year flood is so substantial that one should not expect any
objectionable deposition in the downstream channel. It should also be pointed
out that siltation in the existing downstream channel has occurred primarily
during smaller storm events but not major events. The channel is less likely
to be silted during major floods because a large portion of the total flood
discharge for major floods comes from the drainage basin above Calavera Lake.
The flood discharge through the lake is depleted of its bed load; therefore it
tends to pick up sediment from the downstream channel to satisfy its capacity
for sediment transport. in that process, it reduces the siltation in the
downstream channel.
IV. MAINTENANCE REQUIREMENTS
1. The sediment detention basins are large enough so that annual cleanout
should be adequate in ordinary times.
2. Grading, forest fires or other development may increase the sediment
yield considerably. The stored sediment in a basin should be removed whenever
the capacity is reduced to unsafe, improperly functioning levels. The maximum
level of sediment deposition is recommended to be one half of the total storage
volume. If at any time, this maximum level is exceeded, then the basins must
be cleaned even if it is not due for a maintenance cleanout.
3. The dredged sediment should be disposed of in such a manner that will
prevent its return to the basin or movement into downstream areas during
12
subsequent runoff.
4. Trash racks on top of the standpipe or the spillway should be cleaned
whenever debris starts to accumulate,
stopped
REFERENCES
Agricultural Research Service (1973). "Summary of reservoir sediment deposition
survey made in the United States through 1970", Misc. Pub. No. 1266.
Brownlie, W. R. and Taylor, B. D. (1981). "Coastal sediment delivery by major
rivers in Southern California", EQL Rept. No. 17-C, California institute of
Technology.
California Department of Conservation (1978). "Erosion and sediment control
handbook", Resources Agency, State of California, EPA 440/3-78-003.
California Department of Water Resources (1976). "Upper San Diego River flood
control investigation", State of California, Bulletin No. 182.
City of San Diego (1984). "Drainage Manual"
Flaxman, E. M. (1973). "Predicting sediment yield in Western United States,"
Journal of the Hydraulics Division, ASCE, 98(HY12).
Pacific Southwest Inter-Agency Committee (1974). "Erosion and sediment yield
methods", Report of the Water Management Subcommittee./
Soil Conservation Service (1972). National Engineering Handbook, USDA.
13
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APPENDIX A.
(2 ******************************************************
C * *
C * SCS METHOD FOR SEDIMENT YIELD BY FLAXMAN *
C * *
Q ******************************************************
CHARACTER NAME*8,FILENAME*30
DATA IR,IW/1,3/
C*** STATEMENTS FOR PC
CALL TERPO(1,1)
CALL TERCL(2)
CALL TERMOUT(-1,1,' SEDIMENT YIELD BY @',1)
CALL TERMOUT(-1,2,' FLAXMAN METHOD @',1)
5 CALL TERMIO(10,10,'ENTER INPUT FILE: ',0,30,FILE$NAME,DUM,STATUS)
IF(STATUS.LE.O) STOP
OPEN(FILE=FILE$NAME,UNIT=IR,STATUS=IOLD1,ERR=9999,IOSTAT=IOERR)
10 CALL TERMIO(10,12,'ENTER OUTPUT FILE:',0,30,FILE$NAME,DUM,STATUS)
IF(STATUS.LE.O) THEN
CLOSE(IR)
GO TO 5
ENDIF
CALL OPEN$NEW(IW,FILE$NAME,'SEQI,0,STATUS)
IF(STATUS.NE.O) GO TO 10
WRITE(IW,93)
93 FORMAT(///5X,'Subbasin XI X2 X3 X4 X5 Yield
1 Total Yield1/)
C
C*** INPUT PARAMETERS
C
100 READ(IR,101,END=999) NAME,PRECP,TEMP,X2,X3,AREA
WRITE(IW,102) NAME,PRECP,TEMP,X2,X3,AREA
101 FORMAT(A8,7F8.2)
102 FORMAT(2X,A8,5F8.2,F10.2,F12.1)
C
C*** COMPUTATION
C
X1=PRECP/TEMP
X4=0.
X5=30.67/AREA**0.443
Tl=524.37231-270.65625*ALOG10(Xl+100,)
T2=6.41730*ALOG10(X2+100.)-1.70177*ALOG10(X3+100.)
T3=4.03317*ALOG10(X4+100.)+0.99248*ALOG10(X5+100.)
T4=T1+T2+T3
Y=10.**T4-100.
TY=Y*AREA
C
C*** OUTPUT PARAMETERS
C
WRITE(IW,121) NAME,X1,X2,X3,X4,X5,Y,TY
121 PORMAT(2X,A8,7F8.2)
GO TO 100
999 CLOSE(IR)
CLOSE(IW)
STOP
9999 CALL FERROR(IOERR)
GO TO 5
END
C
CHANG
EPA 440/3-78-003
APPENDIX B. PAGES FROM EROSION AND SEDIMENT CONTROL HANDBOOK
PERTAINING TO SEDIMENT DETENTION BASINS
EROSION AND SEDIMENT
CONTROL HANDBOOK
PERRY Y. AMIMOTO, ENGINEERING GEOLOGIST
DIVISION OF MINES AND GEOLOGY
Department of Conservation
The handbook was prepared under the direction of the Department of Conservation with the concurrence
of the California Association of Resource Conservation Districts and County Supervisors Association
of California and in consultation with the Environmental Quality Committee of the County Engineers
Association. It was published with the financial assistance provided by the U.S. Environmental Protection
Agency, Water Planning Division, Washington, D.C.
152
c. Sandbags exceeding two bags in
height may require anchoring with
steel rods, rebars, etc.
91.30 FILTER INLET [57]
1. General
a. A filter inlet is a temporary sediment
trap consisting of gravel or crushed
rock placed at storm sewer curb in-
let structures. See Figure 35.
b. Filter inlets retain sediment on-site
by retarding and filtering storm run-
off before it enters the storm or sew-
er system.
c. Trapped sediment should be
removed and the clogged filter
material cleaned out or replaced af-
ter each storm.
2. Specifications
/
a. Concrete building blocks placed in
throat of inlet. Filter material placed
between blocks and street in the gut-
ter section. See Figure 35.
b. All filter material should be coarse
(3/4* to 3"), well graded gravel or
crushed rock. Fines less than five
percent.
91.40 VEGETATIVE FILTER STRIP
1. General
a. A vegetative filur strip is a tempo-
rary or permanent sediment trap
which consists of an area of vegeta-
tive cover through which storm wa-
ter must flow before it enters
streams, storm sewers, conduits, etc.
b. As the water containing suspended
solids flows through the vegetative
filter strip, some of the sediment is
removed by "filtering" and by depo-
sition as the flow velocity is reduced.
c. Vegetative filter strips are naturally
occurring or man-made.
d. Tall, dense stands of grasses form
the best sediment traps.
2. Specifications
a. Naturally occurring vegetation may
suffice. Light fertilizing may en-
hance the growth.
b. Man-made grasses may be provided
by sod or by planting.
c. Minimum width of vegetative filter
strios:
Above Diversions: 15' plus 1/2 of
channel width. [21]
Along Live Streams: 100' minimum
(Recommended by California De-
partment of Fish and Game in log-
ging areas.)
91.50 CULVERT RISER
1. The culvert riser is described in the
chapter on culverts.
2. The chapter on sediment detention ba-
sins should help in the safe storage de-
sign of culvert risers.
92.00 SEDIMENT DETENTION BASIN
A sediment detention basin is a reservoir
which retains high flows sufficiently to cause
deposition of transported sediment. Sedi-
ment basins may be either temporary or per-
manent structures which prevent off-site
transportation of sediment generated from
construction activities. See Photos 45,46 and
Figure 36.
92.10 DESIGN CONSIDERATIONS
1. The design of the sediment basin shall be
based on the total drainage area lying
upstream and on the future use of such
lands.
2. The spillway overflow from a debris ba-
sin should not increase the down stream
sediment loads.
3. Vegetation should be planted on slopes
of embankments composed of erodible
soil.
153
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4. Beyond certain limitations on the height
of the dam and the storage capacity of
the reservoir, the design of the sediment
basin will come under the jurisdiction of
the California Division of Safety of
Dams (See Table 23).
5. For basins which also serve as perma-
nent water storage consideration should
be given to the prevention of "algae
bloom" which is aesthetically unsightly.
92.11 STORAGE [57]
1. The site should be selected to provide
adequate storage.
2. Storage capacity shall be the volume be-
low the pipe spillway crest or emergency
spillway crest.
3. Consideration should be given to plan
for periodic cleanout in order to main-
tain the capacity requirements.
4. The maximum allowable level of depos-
ited sediment before cleanout shall be
determined and given in the design data
as a distance below the top of the riser.
c
92.12 PIPE SPILLWAY [20] [57]
1. The combined capacity of the pipe and
emergency spillways will be designed to
handle the design flood.
2. Runoff will be figured by an acceptable
hydrologic procedure, and should be
based on drainage area conditions ex-
pected to prevail during the anticipated
effective life of the structure.
3. The pipe spillway will consist of a per-
forated vertical pipe or box-type riser
joined to a horizontal pipe conduit
(barrel) which will extend beyond the
downstream toe of the embankment.
4. The horizontal pipe conduit (barrel)
will be a minimum of 12 inches in di-
ameter.
5. The riser is a minimum of 30 inches in
diameter and has a cross-sectional area
of at least 1.5 times the cross-sectional
area of the horizontal pipe conduit.
6. The crest elevation of the riser shall be
such that full flow will be generated
before there is discharge through emer-
gency spillway and at least one foot be-
low crest of emergency spillway.
7. If no emergency spillway is provided,
the crest elevation of riser must be at
least three feet below crest of emer-
gency spillway.
8. The upper 1/2 to 2/3 of the riser shall
be perforated with 1-1/2 to 4 inch
holes, 10 to 12 inches on center and
staggered.
9. The antivortex device can increase vol-
ume of discharge by as much as 50 per-
cent.
10. An approved antivortex device is a
thin, vertical plate normal to the cen-
terline of the dam and firmly attached
to the top of the riser. The plate dimen-
sions are:
Height — diameter of barrel
Length = diameter of riser plus 12
inches
11. The riser shall have a base attached
with a watertight connection and shall
have sufficient weight to prevent flota-
tion of the riser. Three recommended
methods are:
a. A square concrete base 18 inches
thick with the riser embedded six
inches in the base. Each side of base
will be diameter of standpipe plus
24 inches.
b. A 1/4 inch minimum thickness
steel plate welded all around the
base of the riser to form a water-
tight connection. The plate shall be
square with each side equal to two
times the riser diameter. The plate
shall have two feet of stone, gravel,
or tamped earth placed on it to pre-
vent flotation.
c. Properly anchored guy wires may
be substituted for the anchor block.
154
w
12. The trash rack consisting of #4 bars, 6
inches on center shall be welded across
the top of riser.
13. At least one seepage ring is required
and each ring shall be rectangular with
each side a minimum of barrel diameter
plus 24 inches.
92.13 EMERGENCY SPILLWAY [20] [57]
1. The emergency spillway should be de-
signed for 1.5 maximum design flow.
Two recommended designs are:
a. Discharge over top of dam or em-
bankment. Spillway must be lined
with 3 inch thick gunite or 4 inch
concrete reinforced with 6x6-
10/10 wire mesh, extending to a
minimum of 3 feet down the up-
stream face of embankment. Spill-
way will be minimum of 18 inches
deep with 1 1/2:1 side slopes.
b. Earth spillways must be installed on
undisturbed soil (not on fill) by
grading. Side slopes will not be
steeper than 2:1. Embankment and
spillway channel must be protected
by vegetation, rock riprap, etc. The
maximum allowable velocity in exit
channel shall be 6 feet per second.
92.14 FREEBOARD [49]
1. Freeboard is the vertical distance
between the elevation of the water sur-
face in the pond when spillway is dis-
charging at designed depth and the
elevation of the top of the dam after all
settlement has taken place.
2. Minimum freeboard shall be 1.0 foot for
sediment basins where the maximum
length of pond is less than 660 feet.
92.15 EMBANKMENT [49]
1. The embankment shall have top widths
based on the following:
Height Top
of dams width
under 10' 8'
10'-15' 10'
15'-20' 12'
20'-25' 14'
2. Side slopes shall be no steeper than 2:1.
92.20 CONSTRUCTION [23] [20]
92.21 SITE PREPARATION
1. The foundation area reservoir area shall
be cleared of all trees, stumps, roots,
brush, boulders, sod, and debris.
2. All topsoil containing excessive amounts
of organic matter shall be removed.
92.22 BORROW AREAS
1. All borrow areas outside the pool shall
be graded, seeded, and left in such a
manner that they are well drained and
protected from erosion.
92.23 EMBANKMENT
1. The embankment material shall be taken
from borrow areas as stated on plans.
2. The material shall be free of all sod,
roots, woody vegetation, large rock (ex-
ceeding 6 inches in diameter,) and other
debris.
3. The embankment should be constructed
to an elevation which provides for an-
ticipated settlement to design elevation
(allow 10% for settlement).
4. The foundations for embankment shall
be scarified prior to placement of fill.
5. Placement of fill material shall be started
at the lowest point of the foundation and
shall be placed in 6 inch maximum lifts
which are to be' continuous over entire
length of fill and approximately horizon-
tal.
6. The satisfactory compaction is usually
achieved when the entire surface of the
fill is traversed by at least one pass of the
loaded hauling equipment or through
use of a roller.
92.24 PIPE SPILLWAY
1. The barrel shall be placed on a firm
foundation to the lines and grades shown
on the plans.
155
2. Backfill material shall be placed around
the barrel in 4 inch layers and each layer
shall be thoroughly compacted with suit-
able hand-operated equipment to at
least 2 feet above the top of the pipe and
seepage rings before heavy equipment is
operated over it.
92.25 VEGETATIVE PROTECTION
1. A protective vegetative cover shall be es-
tablished on all exposed surfaces of the
embankment, spillway, and borrow area
to the extent practical.
92.26 PROTECTION OF SPILLWAY DIS-
CHARGE AREA
1. All areas subject to discharges from pipe
spillway and emergency spillway must
be protected with vegetation, rock, rip-
rap, etc.
92.30 SEDIMENT CLEANOUT AND DIS-
POSAL
1. The sediment should be removed when-
ever the storage capacity has been re-
duced to unsafe, improperly functioning
levels.
2. The sediment must be disposed of in
such a manner that will prevent its re-
turn to the sediment basin or movement
into downstream areas during subse-
quent runoff.
USD A Soil Conservation Service
A Sediment Trap Would Have Allowed This Storm Sewer System to Collect Runoff Without Excessive Sediment Load.
160
ANTI-VORTEX PLATE
RISER
ENERGY DISSIPATOR
NATURAL1 GROUND
ANTI-SEEP COLLAR GRAVEL CONE
MINIMIM DKSTLCTMG RA8TH STANDARD
/ / -X" '
RIPRAP ,6" KIN. f
SIZE IROCK, ^
' ) '' 1
±
.2"T
*
— I* P|=i* . r
1 >
I m ' /L.- --h
— _. __
/(12" DIA*KIH«)
/
^-__i 1 OFFSETi- — _G-
Q" (UNITE OR If "CONCRETE
6»S-10/10 WIRE MESH
sicnai A-A
OUNITE OR CONCRETE AT THE
OVERFLOW IS TO EXTEND 3' KIN.
DOWN EACH PACE OP THE DIEE.
-8'MIN.r
DISCHARGE TO PAVED
STREET OR APPROVED
DRAINAGE COURSE
IN PAVED CHANN
ft BARS 9 6" C.C. WELDED
ACROSS TOP OP STAVDPIPE
30" DIAMETER 8TANDPIPE
»" PERFORATIONS 12" 0/C STAGOERED
NOTE:
k-12"
SECTION
C-Cc
1) PROPERLY ANCHORED GUY WIRES
MAY BE SUBSTITUTED FOR THE
ANCHOR BLOCK.
2) SEE SECTION 92. 15 FOR
RECOMMENED WIDTH OF
EMBANKMENT.
Figure 36. Schematic Design of Sediment Detention Basins. [20] [56]