Technical Memorandum No Basis of Comparison

Transcription

1 Pajaro River Watershed Study in association with Technical Memorandum No Basis of Comparison Task: Basis of Comparison To: PRWFPA Staff Working Group Prepared by: J. Schaaf Reviewed by: R. Raines Date: October 8, 2001 Introduction The purpose of this Technical Memorandum (TM) is to establish the basis upon which Pajaro River watershed conditions will be compared. The basis could be peak discharge or volume of discharge. The staff working group of the Pajaro River Watershed Flood Prevention Authority will be the selector of the basis of comparison for this project. The basis of comparison will allow decision makers to determine which course or courses of action to pursue to improve the level of flood protection to the residents of the Pajaro River valley. Project Scope and Background The Pajaro River Watershed Flood Prevention Authority was formed to develop flood protection strategies in the Pajaro River Watershed. The first phase in developing the strategies is to construct a streamflow model. The model shall address a number of key issues, including the following: What are the causes of flooding on the Pajaro River? Has rainfall runoff increased downstream with increasing development upstream? Has the improvement and/or maintenance of streams affected flooding? Has erosion or sedimentation in the streams affected flooding? Have upstream retention basins reduced or mitigated the degree of flooding? How will future conditions change the degree of flooding? BASIS OF COMPARISON -1- October 8, 2001

2 Answering these and other related questions regarding Pajaro River flooding requires the development of hydrologic and sediment models for the Pajaro River and its tributaries. Setting The Pajaro River drains an area of approximately 1,300 square miles of the coastal plains and mountains of Central California. A tributary of Monterey Bay, the watershed drains portions of Santa Cruz, Monterey, Santa Clara and San Benito Counties. As shown in Figure 1, the watershed is somewhat elongated toward the southeast. The lower portions of the Pajaro River from Murphy s Crossing to the Pacific Ocean are protected by a Corps of Engineers levee project constructed between 1949 and Four miles above this federal project is the USGS stream gage Pajaro River at Chittenden, CA. This gage has been in continuous operation since the 1939 water year. The drainage area at this gage is 1,186 square miles. Two miles above the Chittenden gage site, the San Benito River is confluent to the Pajaro. At this point the San Benito River drains 661 square miles - slightly more than half the drainage area at the Chittenden gage. The Pajaro River at the outlet to Soap Lake a low-lying area of Santa Clara and San Benito Counties has a drainage area of approximately 500 square miles. Previous Hydrologic Reports Two federal agencies, the US Army Corps of Engineers (USACE) and the Federal Emergency Management Agency (FEMA), have been responsible for all previous hydrologic reports. The USACE, San Francisco District, has authored: Flood Control Survey Report for the Pajaro River, 1942, Office Report on Standard Project Flood, Pajaro River Basin, 1961 Interim Report for Flood Control, Pajaro River Basin, 1963 Flood Plain Information Report Uvas-Carnadero Creek, 1973 Flood Plain Information Report - San Felipe Lake and Pacheco Creek, 1973 Flood Plain Information Report - San Benito River, 1974 Flood Plain Information Report San Felipe Lake Unit 2 FEMA has prepared the following Flood Insurance Study reports in which peak discharges are presented for the Pajaro River and tributary watercourses: Santa Clara County, 1981 Santa Cruz County, 1986 San Benito County, 1991 BASIS OF COMPARISON -2- October 8, 2001

3 The FEMA discharges listed for the 100-year flood have particular meaning for a number of federal agencies and agencies insured or guaranteed by the federal government. These discharges are to be used in planning facilities that use federal monies or for projects that are insured or backed by federal monies. While USACE discharge values art the ones that agency will utilize for analysis, design and construction of flood control projects done under its jurisdiction, the FEMA discharges take on a wider area of jurisdiction because of federal regulations. Basis of Comparison The Pajaro River watershed is large and the land uses are varied from dense urban to intensive agricultural to grazing lands to unused acreage. Changes in land use and management plans can affect watershed behavior. To be sure the hydrologic model will address the needs of decision makers and planners, three questions must be addressed: what hydrologic parameters are necessary for comparison, where in the watershed should these parameters be predicted, and at what exceedence frequencies should these parameters be predicted. Parameters to be used The most widespread parameter used for comparing changes to watersheds is the annual instantaneous maximum peak discharge. This is the discharge (rate of flow) in a stream channel and adjoining overbanks that is the greatest value at any time during a water year no matter how long the discharge lasts. A water year is the year ending September 30 and beginning the previous October 1. It is assigned the calendar year corresponding to the September 30 date. The second most prevalent hydrologic parameter is the volume of flow in the stream. Generally the annual maximum 1-day average discharge value or 3-day average discharge is used in highlighting differences in runoff. For the Pajaro River watershed the annual maximum 3-day average discharge is recommended because the watersheds are generally large and the 1-day average discharge is often reflective of the instantaneous peak discharge. Two parameters are recommended instantaneous peak discharge and 3-day average discharge. Both parameters are to be annual maximum values. Parameters to be predicted Shown in Table 1 are annual instantaneous maximum peak discharges from two longterm stream gages one on the San Benito River near the City of Hollister and one on the Pajaro River at Chittenden just upstream of the end of the Corps of Engineers Flood Control project. BASIS OF COMPARISON -3- October 8, 2001

4 The San Benito River near Hollister gage had a drainage area of 586 square miles, while the current gage located at Highway 156 has a drainage area of 607 square miles. The drainage areas at the two gage locations are within 3.5 percent of one another and the combined record can be considered as one continuous record since The drainage area at the San Benito stream gage is approximately half of that at the Pajaro River at Chittenden gage. Data has been collected on the Pajaro River continuously since The four largest instantaneous peak events shown on Table 1 are in the 1956, 1958, 1995 and 1998 water years. The ratios for the peak discharges at the Chittenden gage divided by the peak discharges at the San Benito River gage for the four major flood years are: YEAR Ratio Because the ratio of the drainage areas at the gages is approximately 2.0, one might expect that the peak discharges maintain about that same ratio. However, the 1956 event, the Christmas 1955 flood, shows much more of the peak discharge attributable to the Soap Lake portion of the Chittenden gage s drainage area. The April 1958 flood was fairly evenly distributed. The two most recent floods, the March 1995 flood and the February 1998 flood, had much more of their peak discharge coming from the San Benito River portion of the overall watershed at the Chittenden gage site. Table 2 shows the average daily discharges on the two rivers for the four largest flood recorded at the Chittenden gage. The ratios of the sum of the average flows for the maximum three consecutive days are shown below: 12/1955 Chittenden 45,300 cfs-days; San Benito 10,040 cfs-days; Ratio = /1958 Chittenden 44,480 cfs-days; San Benito 12,580 cfs-days; Ratio = /1995 Chittenden 41,120 cfs-days; San Benito 19,170 cfs-days; Ratio = /1998 Chittenden 45,800 cfs-days; San Benito 25,790 cfs-days; Ratio = BASIS OF COMPARISON -4- October 8, 2001

5 Interestingly, the maximum consecutive 3-day flow volume was approximately the same for all four major floods on the Pajaro River. The amount of volume contributed by the San Benito River watershed, however, has grown from around a quarter in the 1950 s floods to around a half in the 1990 s floods. This means that the rest of the 1,186 square mile watershed at the Chittenden gage contributed less volume in the 1990 s floods than it did in the 1950 s floods. The instantaneous peak discharge and the maximum average discharge for a three contiguous day period are the two parameters selected as a basis of comparison. Locations of Parameters The stream gage data presented in the preceding section indicate that the San Benito River can be an important predictor of what the peak discharge and the volume of flow will be for the lower Pajaro River (that portion downstream of the Chittenden stream gage location.) Thus there needs to be a comparison point located on the river just upstream of the confluence with the Pajaro River a drainage area of approximately 661 square miles. A comparison point must also be at the Chittenden gage location. With a drainage area of 1,186 square miles, this point is critical because it is the location of a long-term stream gage record. The flow at this point represents the discharge to the upper portions of the Corps flood control project. A final upper watershed comparison location will be on the Pajaro River just upstream of US Highway 101. The discharge at this point represents the flow from a drainage area of approximately 500 square miles. It also represents the outflow from what a significant storage area upstream of US Highway 101 in Soap Lake. A fourth and final comparison point will be on the Pajaro River just downstream of the confluence with Salsipuedes Creek. This flow represents the discharge along the lower portions of the Corps of Engineers flood control project. The drainage area of this point is approximately 1,273 square miles. These four comparison points are shown in Figure 1. Frequencies to be used The frequencies used for comparison purposes should span the hydrologic spectrum of floods. To provide results over this spectrum the following frequencies should be used: 2-, 10-, 25-, 50-, 100- and 200-year return periods. The frequency given in terms of return period is simply the reciprocal of the annual exceedance probability. For example the 50-year flood has a 2 percent chance of being equaled or exceeded in any given water year. A 2-year flood has a 50 percent chance of being equaled or exceeded in any given water year. The annual exceedance probability is BASIS OF COMPARISON -5- October 8, 2001

6 the more correct way to think about risk but the return period concept is more commonly used. The use of frequency curves is preferred over use of the six annual maximum flood events from 1994 to 1999 because the six flood events only represent the rainfall patterns that occurred and may not give a proper accounting for those patterns, which may occur. The frequency curves have an inherent statistically probable rainfall pattern and depthduration relationship. Conclusion The basis for comparison will be done with a series of eight frequency curves. Four curves will be for instantaneous peak discharge and four for average 3-day discharge. There will be two frequency curves, peak and 3-day discharge, at each of four locations: Pajaro River downstream of Salsipuedes Creek near Watsonville; Pajaro River at Chittenden; Pajaro River upstream of US Highway 101; and the San Benito River at the confluence with the Pajaro River. BASIS OF COMPARISON -6- October 8, 2001

7 TABLE 1 Annual Maximum Peak Discharges (in cfs) San Benito San Benito Pajaro River River River Chittenden nr. Hwy 156 YEAR

8 TABLE 2 Average Daily Discharges (in cfs) Pajaro R. San Benito R. Date (Active Gage) 12/22/ /23/ /24/1955 pk 24, /25/ /26/ /1/ /2/ /3/1958 pk 23, /4/ /5/ /10/ /11/1995 pk 21, /12/ /13/ /14/ /1/ /2/ /3/1998 pk 25, /4/ /5/

9 Figure 1 Locations of Comparisons Pajaro River Watershed Locations Pajaro River Downstream of Salsipuedes Creek Pajaro River at Chittenden Pajaro River at US 101 San Benito River Upstream of Confluence with Pajaro River