V. HYDROLOGY
A. Purpose
This chapter prescribes procedures for estimating runoff rates and volumes and presents related policy and criteria. The procedures and criteria presented are based on generally accepted principles and practice but are as specific as possible to Placer County.
B. General Principles and Policies
- A consideration of risk is appropriate.Hydrology is not a precise science, and very little data is available on Placer County streamflows. Estimates of flows and the actual flow subsequently experienced in the event have often been quite dissimilar. Planning and design of drainage facilities and delineation of areas subject to flooding should consider the risk involved if the estimate is too high or low as appropriate.
- A relationship is assumed between precipitation frequency and flow frequency.Relatively extensive information on precipitation exists and allows reasonable estimation of precipitation frequencies. An assumption is made that the precipitation of a given frequency will result in runoff peaks and volumes of the same frequency, and all durations of an event have the same frequency of occurrence.
- The approach used shall be consistent with the appropriate Basin Plan Master Model.In accordance with policies explained in Chapters II and III, master planning models may exist for any given watershed. Assumptions and parameters used in evaluating a portion of the watershed shall be consistent with those used in the master planning model. Where appropriate, estimates made by the master planning model should be used as input to the results, and of both levels of models should be reasonably consistent.
C. Precipitation
Precipitation results from widespread, general rainstorms which originate in the Pacific Ocean.
Orographic lifting when storms encounter the Sierra Nevada range results in a long-term precipitation pattern which increases with elevation up to the crest of the range. East of the crest, however, orographic lifting does not occur, and the region is markedly drier.
Cloudbursts occurring within general rainstorms are generally the cause of floods on watersheds of a few hundred square miles or less in area and elevations below 4000 feet on western slopes in the foothill areas. A cloudburst is a severe thunderstorm with very intense short-duration rainfall, often with hail, strong winds or tornadoes. It is most likely to occur inland at lower elevations, in winter or early spring and in association with subtropical moisture sources. In this region, the cloudburst usually covers an area of less than 300 square miles and lasts less than two hours.
From 3000 feet to 5000 feet, cloudburst effects diminish rapidly. Above 5000 feet, the portion of precipitation falling as rain diminishes and the portion falling as snow increases.
1. Mean Annual Precipitation The relationship between elevation of a location and its mean annual precipitation (MAP) reflects the orographic nature of regional precipitation. For slopes west of the Sierra Nevada crest, MAP ranges from 20 inches at the southwest corner of the county to almost 70 inches near the crest.
2. Depths and Intensities The criteria presented are based on the records of regional gages both within and near Placer County and relationships developed through the analysis of long-term gages in the region (3). These criteria reflect the strong differences in precipitation with elevation and exposure exhibited in the data. For elevations greater than 3000 feet, significant precipitation occurs during the year as snow but does not directly contribute to peak flows from small watersheds. For these elevations, the criteria reflect only the amount falling as rain. Equation 5-2 below presents a relationship between depth, and elevation. Related coefficients for various durations and frequencies are presented in Appendix V-A.
D = mE + b [5-1]
where
D = depth, inches
E = elevation, feet
m,b are from Table 5-A-1, Appendix V-A
Precipitation depths and intensities for selected durations, return periods and elevations at a point are presented in Tables 5-A-2 and 5-A-3 in Appendix V-A.
3. Design Storms The criteria for design storms include both temporal and spatial distributions of precipitation intensities. The criteria and examples are discussed briefly below and in more detail in Appendix V-B. A computer program for generating specific design storm data for use with HEC-1 is available from the District for use on personal computers. Under certain circumstances, the conventional design storms specified in this manual may be inappropriate. Those circumstances include watersheds greater than 200 square miles in area and design of storage basins which store water for more than a day. District staff should be consulted in these or other potential exceptional cases.
a. Temporal Distribution The design storm pattern centers the most intense precipitation from the shortest duration and incorporates depths (the depth-duration-frequency data) for all successive durations from within the overall duration of the storm. The result is a pattern that tapers from the center in both directions.
b. Spatial Distribution The spatial distribution is generally significant for watersheds greater than one (1) mile in area.
The cloudburst storm is limited in areal extent and exhibits a decrease in rainfall intensities from the maximum at the center to background intensities at the edges of the storm for the one-hour period of greatest intensities. Outside the edges of the cloudburst precipitation and for times outside the most intense hour, the distribution is uniform throughout the watershed.
Above the cloudburst region (ie, higher than 4000 feet), a uniform distribution may be assumed over the entire watershed.
The distribution of cloudburst precipitation takes an elliptical shape with a 2:1 ratio of axes.
The alignment of the long axis of the storm ellipse is restricted to a zone extending from 350° Northwest to 60° Northeast. (Bearing relative to North, measured positive clockwise).
The centering which produces the greatest precipitation within maximum peak flow from the watershed is the appropriate centering for estimating flows of the same return period as the design storm.
Table 5-1 presents factors for use in distributing precipitation within the elliptical shape of the cloudburst storm.
The precipitation depths at the center of the ellipse are the point values for the recurrence interval (return period) of the desired design storm.
Table 5-1 is used to determine intensities away from the center for the most intense one-hour period. Table 5-1 values are ratios of isohyetal values to center (point) values for 1-hour depths at the edge of an ellipse enclosing the area shown in the far left column.
TABLE 5-1
POINT GAGE AND AREA SPATIAL INTENSITY RELATIONSHIPS |
Area
(mi2) | Ishoyet to Center Ratios for 1-Hour Depths | Minor Axis
(mi) | Major Axis
(mi) |
| 2 | 5 | 10 | 25 | 50 | 100 | 200 | 500 |
0.1
0.5
1
2
3
4
5
10
15
20
30
40
50
75
100
150
200
300
400 | 1.00
0.96
0.87
0.74
0.66
0.59
0.55
0.42
0.36
0.32
0.29
0.27
0.25
0.24
0.23
0.22
0.21
0.21
0.21 | 1.00
1.00
0.98
0.92
0.87
0.82
0.78
0.64
0.55
0.50
0.42
0.38
0.35
0.31
0.28
0.26
0.24
0.23
0.22 | 1.00
1.00
1.00
0.97
0.94
0.91
0.88
0.76
0.68
0.62
0.54
0.48
0.44
0.38
0.34
0.30
0.28
0.25
0.24 | 1.00
1.00
1.00
1.00
0.99
0.97
0.95
0.88
0.81
0.76
0.68
0.62
0.57
0.49
0.44
0.38
0.34
0.30
0.28 | 1.00
1.00
1.00
1.00
1.00
0.99
0.98
0.92
0.87
0.82
0.75
0.69
0.64
0.55
0.50
0.42
0.38
0.33
0.30 | 1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.95
0.91
0.88
0.82
0.76
0.72
0.63
0.57
0.49
0.44
0.38
0.34 | 1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.98
0.95
0.92
0.87
0.82
0.78
0.70
0.64
0.55
0.50
0.42
0.38 | 1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.98
0.96
0.92
0.88
0.85
0.79
0.73
0.65
0.59
0.50
0.45 | 0.25
0.56
0.80
1.13
1.38
1.60
1.78
2.52
3.09
3.57
4.37
5.05
5.64
6.91
7.98
9.77
11.3
13.8
16.0 | .050
1.03
1.60
2.26
2.76
3.19
3.57
5.05
6.18
7.14
8.74
10.1
11.3
13.8
16.0
19.5
22.6
27.6
31.9 |
| Table 5-2 SNOWMELT RATES |
| Location | Melt Rate (inches/hour) |
Elevation
(feet) | Direction
From
Crest | 10-
Year | 25-
Year | 100-
Year |
4000
5000
6000
7000
and above
5000 | West
East or
West
East | 0.09
0.08
0.06
0.04
0.06 | 0.10
0.09
0.07
0.05
0.06 | 0.11
0.10
0.07
0.05
0.06 |
4. Snowmelt The snowmelt rates shown in Table 5-2 will be used for planning and design involving small watersheds. These amounts are included in unit peak flows in the method for estimating peak flows from small watersheds (see section E 2 below). They may also be used as base flow with HEC-1 for evaluating watersheds of less than 10 square miles. District staff should be consulted concerning the evaluation of larger watersheds.
D. Infiltration and Other Losses
An accounting of losses of precipitation is important to estimates of basin runoff. In general, losses include interception, ponding in small depressions, and infiltration, and these may vary over time. However, soils in the region are generally shallow, relatively impervious and readily saturated. High groundwater conditions also occur in many areas. Therefore, rates of infiltration are relatively low and are assumed constant. Further, since soils in the region are easily saturated and long periods of precipitation often precede intense, flood-producing precipitation, initial losses are generally assumed negligible.
1. Infiltration The infiltration rates specified below are based on evaluations of runoff under various soil, vegetative cover, and antecedent moisture developed by the Soil Conservation Service.
Three key factors affecting infiltration are identified: soil characteristics, soil cover or vegetation type, and antecedent moisture conditions.
a. Soil Characteristics The Soil Conservation Service (SCS) classifies soils into four hydrologic soils groups:
Group A Low runoff potential. Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well to excessively drained sands or gravel. These soils have a high rate of water transmission.
Group B Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep-to-deep, moderately well-to-well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission.
Group C Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine texture. These soils have a slow rate of water transmission.
Group D High runoff potential. Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils have a very slow rate of water transmission.
Soils maps and soil surveys of the western portion of the County are available for inspection in local libraries, at the Placer County Resource Conservation District, and the Flood Control District.
b. Soil Cover Type The type of vegetation or ground cover on a watershed and the quality or density of that cover have a major impact on the infiltration capacity of a given soil. The SCS classifications of cover type, shown below, will be used. These apply to both natural and urban areas.
Poor Heavily grazed or regularly burned areas. Less than 50 percent of the ground surface is protected by plant cover or brush and tree canopy.
Fair Moderate cover with 50 percent to 75 percent of the ground surface protected.
Good Heavy or dense cover with more than 75 percent of the ground surface protected.
c. Infiltration Rates Constant infiltration rates shall be used for pervious areas. Impervious areas shall be assigned an infiltration rate of zero.
Table 5-3 presents recommended infiltration rates for various combinations of soil and cover types. The values in Table 5-3 are based on Soil Conservation Service cumulative loss "curve numbers" for each soil-cover complex adjusted for saturated ground conditions. If a particular soil-cover complex is not specifically identified in Table 5-3, one for a similar area should be used.
| TABLE 5-3 |
| CONSTANT INFILTRATION RATES1 FOR HYDROLOGIC SOIL-COVER COMPLEXES |
| Cover Type | Quality of
Cover2 | Soil Group |
| A | B | C | D |
| NATURAL COVERS - | | | | | |
| Bare - Rockland, eroded and | | .10 | .02 | .01 | .01 |
| newly-graded areas | | | | | |
| | | | | | |
| Grass,Annual or Perennial | Poor | .16 | .09 | .06 | .04 |
| Fair | .31 | .16 | .09 | .07 |
| Good | .41 | .22 | .12 | .09 |
| | | | | | |
| Meadows - Areas with seasonally | Poor | .20 | .11 | .06 | .05 |
| high water table, principal | Fair | .30 | .15 | .09 | .07 |
| vegetation is sod-forming grass | Good | .50 | .24 | .17 | .14 |
| | | | | | |
| Chaparral, Broadleaf | Poor | .28 | .15 | .09 | .06 |
| (Manzanita and scrub oak) | Fair | .40 | .20 | .12 | .08 |
| Good | .49 | .25 | .14 | .10 |
| | | | | | |
| Open Brush - Softwood shrubs, | Poor | .21 | .11 | .07 | .05 |
| buckwheat, sage, etc. | Fair | .34 | .18 | .11 | .07 |
| Good | .39 | .20 | .12 | .08 |
| | | | | | |
| Woodland - Coniferous or broadleaf | Poor | .35 | .18 | .11 | .07 |
| trees predominate. Canopy density | Fair | .44 | .22 | .13 | .09 |
| is at least 50%) | Good | .53 | .26 | .15 | .11 |
| | | | | | |
| Woodland, Grass | Poor | .25 | .13 | .08 | .06 |
| (Coniferous or broadleaf trees | Fair | .36 | .18 | .11 | .08 |
| with canopy density from 20 to 50%) | Good | .47 | .24 | .14 | .09 |
| | | | | | |
| URBAN COVERS - | | | | | |
| Residential or Commercial | Good | .48 | .25 | .16 | .12 |
| Landscaping (Lawn, shrubs, etc.) | | | | | |
| | | | | | |
| Open Space | | | | | |
| (grass cover < 50%) | Poor | .26 | .09 | .06 | .04 |
| (grass cover 50-75%) | Fair | .31 | .16 | .09 | .07 |
| (grass cover > 75%) | Good | .41 | .22 | .12 | .09 |
| | | | | | |
| Streets and Roads | | | | | |
| Paved with open ditches, incl. right-of-way | | .07 | .06 | .03 | .02 |
| Gravel, incl. right-of-way | | .11 | .06 | .04 | .03 |
| Dirt, incl. right-of-way | | .14 | .08 | .05 | .04 |
| | | | | | |
| AGRICULTURAL COVERS - | | | | | |
| Fallow | | .11 | .06 | .04 | .03 |
| (Land plowed but not tilled or seeded) | | | | | |
| | | | | | |
| Legumes, Close Seeded | Poor | .18 | .11 | .02 | .01 |
| (Alfalfa, sweetclover, timothy, etc.) | Good | .24 | .14 | .08 | .06 |
| | | | | | |
| Orchards, Deciduous | See Note 2 | |
| (Apples, apricots, pears, walnuts, etc.) | | | | | |
| | | | | | |
| Orchards, Evergreen | Poor | .25 | .13 | .08 | .06 |
| (Citrus, avocados, etc.) | Fair | .36 | .18 | .11 | .08 |
| Good | .47 | .24 | .14 | .09 |
| | | | | | |
| Pasture, Dryland | Poor | .16 | .09 | .06 | .04 |
| (Annual grasses) | Fair | .31 | .16 | .09 | .07 |
| Good | .41 | .22 | .12 | .09 |
| | | | | | |
| Pasture, Irrigated | Poor | .24 | .12 | .07 | .05 |
| (Legumes and perennial grass) | Fair | .36 | .18 | .11 | .08 |
| Good | .47 | .25 | .14 | .09 |
| | | | | | |
| Small Grain | Poor | .18 | .11 | .07 | .05 |
| (Wheat, oats, barley, etc.) | Good | .20 | .12 | .07 | .05 |
| | | | | | |
| Vineyard | | See Note 2 | | |
|
| 1. Loss rates in inches/hour. | | | | | |
| 2. Use appropriate ground cover designation. | | | | | |
| | | | | | |
If several soil-cover complexes are present and well-distributed in a watershed, an area-weighted average value may be used. Otherwise, it may be more appropriate to represent highly differentiated areas with separate watersheds.
d. Impervious Areas Impervious areas are assigned an infiltration rate of zero.
e. Connected and Unconnected Impervious Areas Representation of impervious areas depends upon whether the areas are connected or unconnected and on the method used to compute runoff. An impervious area is connected if runoff from it flows directly into a concentrated flow, such as a swale or gutter system, and unconnected if it flows over a pervious area as sheet flow before becoming concentrated. Commercial, industrial, and high density residential areas are typical cases where impervious areas are connected. Impervious areas in low density residential areas are typically unconnected.
Specific adjustments for imperviousness are described with each method for computing runoff.
2. Snow-Covered Areas Snow covered areas are assumed impervious since the ground beneath is likely to be saturated and could also be frozen. The portion of the watershed covered with snow depends on elevation and location relative to the Sierra Nevada crest as shown in Table 5-4.
TABLE 5-4
SNOW-COVERED AREAS |
| Location | Percent
Snow-
Covered |
| Elevation (feet) | Direction
From Crest |
3000
4000
5000
6000
7000 and above
5000 | West
East or West
East | 0
30
60
90
100
60 |
E. Runoff Computation This section addresses methods for estimating runoff peaks and volumes in response to precipitation. Alternative approaches and related criteria and guidelines are briefly described.
The alternative methods described below are as consistent as possible with each other in terms of underlying principles and concepts.
1. General Concepts The following basic concepts underlie use of any of the alternative approaches to basin response.
a. Representation of Watershed Characteristics The alternative approaches to runoff require abstraction and simplification of the watershed. Diverse physical characteristics must often be represented by a single parameter It is important, then, that relatively homogeneous watersheds are chosen so that they may be reasonably represented by simple parameters.
b. Subdivision of Large Watersheds When the watershed is relatively large, it is often appropriate to create sub-divisions of the watershed and to compute total watershed outflow as the sum of all subbasin outflows routed to the watershed outlet. Such sub-divisions may be required from or based on areas with homogeneous characteristics, level of detail required, tributary confluences, and controlling watershed features such as road crossings.
c. Controlling Features In many watersheds, one or more features may substantially control watershed outflow. Road and railroad bridges and culverts are typical structures which attenuate peak flows creating ponding. The effects of controlling storage and conveyance features of a watershed should be reflected in watershed response provided they are reasonably permanent. Where possible, they should be explicitly and directly represented.
d. Consistent Framework The same basic framework shall be used to evaluate conditions with and without a proposed watershed change. A consistent framework is necessary to eliminate the effects of a change in the framework itself on the result. Further, the method used must be capable of reasonably reflecting the change.
2. Peak Flows From Small Watersheds
a. Application The method described in this section allows an evaluation of the peak flow from a small watershed without extensive effort. It may be used to estimate the peak runoff from basins of up to 200 acres in areas in which no significant ponding occurs. If it is believed that significant ponding due to an obstruction such as an undersized road culvert or due to natural channel overbank flows would significantly reduce peak flows under all reasonably foreseeable future conditions, then an HEC-1 analysis should be used to evaluate the effect of the obstruction.
HEC-1 should also be used if it is necessary to route and combine subbasins or to produce a hydrograph of flow, such as needed for evaluating a detention basin, for example.
The method is based on a relationship between the characteristic watershed response time and peak flow per unit area from precipitation patterns typical for the region. The relationship was developed using HEC-1 with a range of possible watershed configurations.
b. Criteria Peak flow is a product of watershed area and peak discharge per unit area which, in turn, is a function of a computed response time.
Q
p = qA [5-2]
where
Q
p = peak discharge, cfs
q = unit peak discharge, cfs/acre
A = area, acres
(1) Response Time Response time t
r is an indication of the response of the watershed to intense precipitation. It is determined as the sum of separate response times for a path consisting of the initial, overland (sheet) flow and succeeding collector flows from the most hydraulically remote location in the watershed to the watershed outlet.
(a) Overland Flow Overland flow includes flow over planar surfaces such as roofs, streets, lawns, parking lots and fields.
The overland flow length is not always well defined in natural areas, but it usually becomes concentrated in shallow rivulets or swales within 600 feet. In areas with development, the point at which overland flow is concentrated in a collector, such as a gutter or pipe, is usually identifiable.
In developed areas, two overland flow surfaces with different response characteristics, but sharing a common collector, are often present. The surfaces involved with a typical 1/4 acre single family residence, for example, include roof, lawn, driveway, and street surfaces, with the street gutter serving as collector. It is appropriate to represent these surfaces with two overland elements: a smooth one for directly connected impervious areas, such as roofs, driveways and streets, and a rough one for landscaped and unconnected impervious areas.
Equation 5-3 is used to estimate the overland flow component of response time (7)(8). Solutions for this equation may be obtained graphically in Figure 5-1.
| tr = | .355(nL)0.6 | [5-3] |
| s0.3 | |
where
t
r= response time, minutes
n = Manning's roughness coefficient (Table 5-5)
L = flow length, feet
s = slope of surface, feet/feet
TABLE 5-5
ROUGHNESS PARAMETERS
FOR OVERLAND FLOW 1 |
| Surface | n2 |
| Smooth surfaces (concrete
asphalt, or bare soil)
Grass:
Short grasses
Dense grasses
Bermuda grass
Poor grass cover on
moderately rough surface
Woods with Underbrush | 0.11
0.15
0.24
0.40
0.40
0.40 - 0.80 |
|
1. Sources (1) and (8).
2. Both surface roughness and vegetation cover affect runoff, but only the portion less than 0.1 foot high.
i) Collector Flow Equation 5-4 below or the Manning equation may be used for estimating velocities of concentrated flow in rivulets, swales, gutters, pipes, and channels. Equation 5-4 applies to an open, triangular channel with no inflow at its upstream end and side inflows uniformly distributed along its length.
This representation is considered adequate for approximating open channels and pipes in most situations. Equation 5-4 may be solved graphically using Figure 5-2.
| tr = | .00735Ln.75 (1+Z2).25 | [5-4] |
| |
s.375 (AcZ).25 | |
where
t
r = response time, minutes
n = Manning's roughness coefficient
See Table 6-3 or Table 8-1
s = slope, feet horizontal/foot vertical
L = length, feet
Z = side slope, feet horizontal/foot vertical
A
c = contributing area, acres
In natural watersheds, it may be appropriate to use higher values of Manning's n for the initial collector where the flow is shallow. Manning's equation may be used to estimate the collector response time when it is felt that Equation 5-4 does not apply. A flow of 2 cfs per acre of contributing watershed should be used to evaluate velocities. The velocity computed for open channel flows using Mannings equation shall be increased by an adjustment factor as follows to account for celerity:
FIGURE 5-1
| Channel Section | Celerity
Factor |
Triangular
Wide Rectangular | 1.33
1.67 |
(2) Unit Peak Discharge Unit peak discharge is determined from t
r and Figure 5-3.
(3) Infiltration Factor The effect of infiltration is reflected in the infiltration factor F
i. F
i is found from the infiltration rate in Equation 5-5 below or by using Figure 5-4.
F
i = I (1+1/(1.3+.0005E)) [5-5]
where
F
i = infiltration factor, cfs/acre
I = infiltration rate, inches/hour
E = Elevation, feet
(4) Adjustment for Infiltration When pervious overland flow areas are present, the estimate of peak flow is computed with Equation 5-6.
Q
p = qA-A
pF
i [5-6]
where
Q
p = peak flow, cfs
A = total watershed area, acres
q = unit peak runoff
A
p = pervious area, acres
F
i = infiltration factor, cfs/acre
(5) Procedure
- Determine the most characteristic flow path with the longest probable response time.
- Determine the cumulative response times for the overland flow element and collectors for the characteristic flow path using Equations 5-3 and 5-4 or Figures 5-1 & 5-2.
- Determine the unit peak discharge for the response time from step 2 using Figure 5-3.
- Determine the pervious infiltration factor using Equation 5-5 or Figure 5-4.
- Compute the peak flow using Equation 5-6.
The computations above can easily be implemented with a generalized spreadsheet program on a personal computer. An example spreadsheet is presented in Figure 5-5.
FIGURE 5-5
Placer County Flood Control and Water Conservation District
Small Watershed Peak Flow Worksheet |
| Date | February 22, 1994 |
| Engineer | Example |
| Project | Example |
| Watershed | Example |
| Area, acres | 100 | Elevation,
Feet | 100 | Return Period, Years | 10 |
| | Length
(feet) | Slope
(V/H) | Mannings
n | Contributing
Area (Acres) | Side Slope
(ft H per
1 ft V) | Response
Time
(minutes) |
| Overland Flow | 150 | 0.01 | 0.40 | | | 16.49 |
| Collector 1 | 200 | .03 | .08 | 2 | 10 | 1.23 |
| Collector 2 | 800 | .02 | .04 | 20 | 10 | 1.92 |
| Collector 3 | 1700 | 1.00 | .03 | 100 | 10 | 6.77 |
| Total Response Time (minutes) | 26.41 |
| Unit Peak Flow (cfs/acre) | .95 |
| Infiltration Rate (inches/hour) | 0.10 | |
| Infiltration Factor (cfs/acre) | 0.17 |
| Percent Impervious | 30 |
Watershed Peak Flow =
Area x Unit Peak Flow - (1 Percent Impervious) x Area x Infiltration Factor | 83.1 |
3. HEC-1
a. Application HEC-1 shall be used for all basin master planning models and for the analysis of all major detention basins.
HEC-1 is a generalized computer program developed and extensively used by the Corps of Engineers and others. HEC-1 provides flexibility by allowing many combinations of basic hydrologic processes to simulate the response of a watershed.
HEC-1 is supported by the Corps of Engineers, but technical assistance and microcomputer versions of the program outside of the Corps are provided by several sources in the private sector. Copies of the micro-computer version and limited technical support are also available from the Placer County Flood Control District.
b. Criteria The following criteria and guidelines apply to the use of HEC-1 for Placer County hydrology:
(1) Simulation Time Step The simulation time step specified in HEC-1 will be 5 minutes for all watersheds with a response time greater than 10 minutes, and 1 minute for all watersheds with a response time of less than 10 minutes.
(2) Design Storms The precipitation which defines the design storm is specified period by period as input to HEC-1. Design storm criteria are specified under Precipitation above. The following additional criteria apply as well:
(a) Frequency The frequency for design storms is specified in the appropriate sections on planning and design of facilities.
(b) Duration The duration of the design storm should at least exceed four times the response time of the total watershed. When the design of storage facilities is involved, the design storm shall be sufficiently long that runoff and storage volumes return to near their level at the beginning of the simulation.
(3) Infiltration In general, an initial loss of zero and a constant infiltration rate shall be assumed. See the above section on infiltration for appropriate values.
(4) Imperviousness The amount of impervious area is expressed as a fraction of the total area in percent.
(5) Base Flow A base flow of 1.0 cfs per sq. mile should be assumed for major streams.
(6) Runoff Response The kinematic wave method shall be the basic approach to runoff response for developing watersheds. For convenience, a Clark or Snyder unit hydrograph may be used if the parameters can be shown to give a response consistent with a kinematic wave analysis. Since the kinematic wave method responds dynamically to levels of precipitation intensity, this means that the unit graph parameters should also reasonably reflect the intensities expected in the application.
(a) Kinematic Wave
i) Simplification The representation of a watershed with the kinematic wave model requires great simplification and reduction. Inferences about the complex behavior of watershed are made from the behavior of an idealized representation composed of a few overland flow and channel elements.
Parameters chosen for elements are typical of the watershed and do not necessarily represent specific, physical elements.
ii) Separately Connected Overland Flow Areas Separately connected overland flow areas with significantly different characteristics, as often occurs in developed areas, should be represented by two overland flow elements. The surfaces involved with a typical 1/4 acre single family residence, for example, include roof, lawn, driveway, and street surfaces. It is appropriate to represent these surfaces with two overland elements: a smooth one for roofs, driveways and streets, and a rough one for the landscaped and lawn areas. Each element is connected, separately and independently, with a common collector.
iii) Channel Representation The kinematic wave procedure is appropriate for representation of channel routing where channel storage does not attenuate peak flows, such as engineered channels. Other procedures, such as the Muskingum-Cunge procedure, are appropriate for channels where attenuation would occur, such as in natural channels. These procedures are discussed below under "Channel Routing".
iv) Maximum Watershed Size The largest watershed described by one set of kinematic wave parameters should not exceed one (1) square mile in area.
(b) Clark Instantaneous Unit Hydrograph The Clark parameters are tc and R. tc relates mainly to the response time and R to overland storage distributed throughout the watershed. Note the Clark tc is defined differently from and does not have the same values as the tc computed in the simplified graphical method above. More information on the Clark method may be found in the HEC-1 Users Manual.
(c) Snyder Unit Hydrograph The Snyder unit hydrograph parameters are basin lag and a peaking coefficient. More information on the Snyder method may be found in the HEC-1 Users Manual.
(7) Channel Routing Channel routing reflects the time delay and attenuation due to channel and overbank storage of a flood wave as it moves downstream. Constrictions such as road culverts may further significantly attenuate peak runoff. Significant constrictions should be separately represented as a level pond in a Modified-Puls routing if they are not otherwise reflected in a routing scheme for the stream channel.
(a) Muskingum-Cunge Method The Muskingum-Cunge method is appropriate for most channel routing. This method requires specification of a representative, uniform channel cross-section, slope, and Manning's n values. The uniform channel cross-section may be represented as a trapezoid, square or rounded bottom, or an eight-point section. The eight point section should be used for natural streams where the channel and floodplain are clearly differentiated. The method should not be used where a backwater exists from a downstream obstruction. This is most likely to occur over long reaches where slopes are nearly flat.
(b) Modified Puls Method The modified Puls method is appropriate when detailed cross section data are available, such as those used in a backwater analysis by FEMA. The modified Puls method is based on a storage-discharge relationship for a reach. (This relationship may be generated from cross section data with HEC-2).
Subreaches are appropriate for modified Puls so that the travel time (based on celerity) through a subreach is approximately equal to the simulation time interval.
c) Muskingum Method
The Muskingum routing method is a simple method appropriate when detailed cross-section data is not available. It is based on a linear relationship between storage and discharge.
The Muskingum parameters for a single reach:
K the total travel time in hours for the reach.
K is based on the celerity, the rate at which the flood wave propagates downstream.
X, a coefficient which reflects the average channel shape as follows:
| Shape | X |
|
Trapezoidal, no overbank
(no attenuation)
Well-defined channel with
some overbank (typical
of South Placer streams)
Swamps, ponds (max. attenuation) | .5
.2
0 |
N, the number of subreaches to use.
K/N should be approximately equal to the simulation time interval, and must be chosen within the following constraints:
4. Statistical Analyses Since streamflow records virtually do not exist for Placer County streams, statistical analysis of recorded streamflows can only be used at best to supplement the synthesis of relationships between flow and frequency. As streamflow data becomes available over time, however, it will be increasingly useful to the evaluation of hydrology in the region.
Data used for frequency curves will be adjusted, if necessary, to reflect changes in basin conditions, primarily but not limited to development and drainage and flood control improvements.
The log-Pearson Type III method as outlined in Bulletin 17B shall be used for the statistical analysis of runoff records.
REFERENCES
- California Department of Water Resources,
Rainfall Depth-Duration-Frequency for California,
Bulletin 190, Microfiche update.
- Corps of Engineers, Hydrologic Engineering Center,
HEC-1, Flood Hydrograph Package, Users Manual,
September 1990.
- Humphrey, John H.
El Dorado County Hydrology Manual,
Unpublished Draft, August 1991.
- Humphrey, John H. and Wesley H. Blood,
Design Storm Methodology for Placer County,
1989.
- Linsley, Ray K., Max A. Kohler, and Joseph L.H. Paulhus,
Hydrology for Engineers,
McGraw-Hill, 1958.
- Miller, J.F., R.H. Frederick, and R.J. Tracey,
Precipitation-Frequency atlas of the Western United States, Vol. XI: California, NOAA Atlas 2,
National Weather Service, 1973.
- Overton, Donald E. and Michael E. Meadows,
Stormwater Modeling,
Academic Press, 1976.
- Soil Conservation Service,
Technical Release 55, Urban Hydrology for Small Watersheds,
1986.
- Walesh, Stuart G.,
Urban Surface Water Management,
John Wiley & Sons,1989.