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

1. 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.
   
2. 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.
   
3. 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