Appendix VI: Illustrative example

Transcription

1 Central Valley Hydrology Study (CVHS) Appendix VI: Illustrative example November 5, 2009 US Army Corps of Engineers, Sacramento District Prepared by: David Ford Consulting Engineers, Inc.

2 Table of contents Project overview... 3 Motivation... 3 Focus... 3 Illustrative analysis... 5 Watershed description... 5 Analysis tasks summary... 5 Task 1.0. Data collection and augmentation... 7 Task 2.0. Model acceptance Task 3.0. Unregulated flow time series development Task 4.0. Flood flow-frequency analysis Task 5.0. Development of regulated flow time series Task 6.0. Development of flow transform curves Task 7.0. Development of stage-flow relationships Study products at analysis points Alternative analysis List of references Attachment A. Illustrative example of event selection process

3 Project overview Motivation The California Department of Water Resources (DWR) has contracted with the US Army Corps of Engineers, Sacramento District (Corps) to complete a hydrologic analysis of the Sacramento and San Joaquin river basins. This analysis is hereinafter referred to as the Central Valley Hydrology Study (CVHS). The goal of this hydrologic analysis is to estimate peak flows and hydrographs of various annual exceedence probabilities to describe flood exposure throughout the basins. This hydrologic analysis will be of sufficient detail to permit DWR to prepare floodplain maps of areas protected by state-federal levees in the basin, consistent with the requirements of the National Flood Insurance Program (NFIP). The locations at which frequency curves will be developed are referred to herein as analysis points. Floodplain maps are to be developed for the following events: p=0.10 (10- year), p=0.02 (50-year), p=0.01 (100-year), p=0.005 (200-year), p=0.002 (500-year), and 2 additional events yet to be determined by DWR. Due to the expected uses of the results of the CVHS for a range of DWR activities, a committee composed of individuals directly involved in the DWR CVFED program as well as outside e familiar with CV hydrology has been formed. This committee is referred to as the Hydrologic Advisory Committee (HAC). To facilitate communication with the HAC and other project stakeholders, a series of documents describe the study procedures. The first document was the Sacramento and San Joaquin river basins: Procedure for hydrologic analysis, referred to herein as Procedures document. This document described the analysis approach and key tasks for completion of the CVHS. The Procedures document has been reviewed by the HAC and a response to comments prepared. To provide additional detail for review, a series of technical appendices is prepared. These are in the following procedure areas: Unregulated time series development Regulated time series development Flow frequency analysis Development of flow and stage transforms Ungaged watershed analysis A separate technical appendix addresses each of these. In addition, a CVHS product uses document was prepared in May 2009 to illustrate how the study products could be used to meet various floodplain analysis goals. Focus This technical appendix illustrates by example the steps in the Procedures document, consistent with details in the 5 technical appendices. The intent is to provide additional insight into the study steps and to supplement the CVHS product uses document. 3

4 Although an actual watershed from the CVHS is used for this illustrative example, the products, values, and results shown are not intended for any other uses rather than this illustration. In some cases, the actual data were modified or simplifications were made to illustrate the steps of the study. Further, the watershed models are still under development, thus interim models were used for simulations. To avoid confusion and potential misinterpretations, some figures included in this appendix have scales removed. 4

5 Illustrative analysis This appendix is included for illustration of the CVHS procedures only. Data, models, and results have been modified, as well as simplifications made, to illustrate key concepts for the study. Results shown are not final products of the CVHS. Watershed description In the section below, we illustrate analysis of the Yuba-Feather system following the steps in the Procedure document. We also summarize the study products available at the analysis points. Finally, we describe how the study products can be used to evaluate system flood management alternatives. The Yuba-Feather river watershed is illustrated in Figure 1. The total watershed size is 6,264 miles to the confluence with the Sutter Bypass. This includes 5,365 miles at the Feather-Yuba river confluence, our area of interest. The 2 flood control reservoirs in the system are Oroville Reservoir and New Bullards Bar Reservoir. Oroville Reservoir includes 750,000 ac-ft of flood control storage. New Bullards Bar Reservoir includes 170,000 ac-ft of flood control storage. From a flood management perspective, approximately 2/3 of the Yuba River watershed is unregulated. Oroville Reservoir regulates the majority of the Feather River watershed to its confluence with the Yuba River. On the Feather River, the federal-state levee system starts at approximately river mile 61 near Thermalito Dam and extends downstream to the confluence with the Sutter Bypass. On the Yuba River, the federal-state levee system starts at approximately river mile 61 near the Thermalito Dam and extends downstream to the confluence with the Feather River. For this example, we focus on 1 analysis points: Feather River below the confluence with the Yuba River. Figure 2 is a schematic of the Yuba-Feather river watershed used in the example description. This analysis point is labeled as FR+YR Junction. Analysis tasks summary The Procedure document identifies the following tasks to develop the required frequency curves and hydrographs: 1. Collect data and augment as necessary. 2. Accept study models for unregulated and regulated conditions. 3. Develop unregulated flow time series. 4. Complete flood flow-frequency analysis. 5. Convert unregulated flow time series to regulated flow time series. 6. Develop unregulated to regulated flow transform. 7. Develop flow to stage transform. The sections that follow illustrate each of these in turn for the Yuba-Feather example. 5

6 Figure 1. Yuba-Feather System (adapted from Sacramento and San Joaquin Comprehensive Study documentation) 6

7 Figure 2. Yuba-Feather system modeling schematic Task 1.0. Data collection and augmentation Task 1.0 calls for collecting the gage and watershed data required for the analysis. A large data collection effort was completed as part of the Sacramento and San Joaquin Comprehensive Study (Comp Study) and that effort is built on for the CVHS Identify gages The first step is to identify the locations and types of gages in the watershed. This includes, for example, identifying reservoir level and streamgages in both the headwaters portion of the watershed as well as the valley floor. After reviewing DWR, Corps, and USGS databases, the available gages were identified in the watershed and compiled in a study database. These gages are listed in Table 1. This investigation focused on streamgage and reservoir elevation gages. 7

8 Table 1. Available gages in the Yuba-Feather watershed ID (1) USGS gage ID (2) Location (2) Thermalito afterbay release to Feather River Feather River at Oroville, CA Feather River near Gridley, CA North Honcut Creek near Bangor, CA South Honcut Creek near Bangor, CA Feather River a Yuba City, CA 20 Yuba River below Englebright Dam near Smartville, CA Deer Creek near Smartville, CA Yuba River at Smartville, CA Yuba River near Marysville, CA 54 Record length (4) 1.2. Filter gage list by record length, collect data In development of the study database for the step above, we identified the location of each gage, the record length, and the recorded values. The database was configured following the conventions defined in the Sacramento-San Joaquin hydrologic analysis in support of DWR floodplain mapping study, Data management plan (2007), referred to as Data management plan. Here, we filter this list to those gages that will be useful for the study, specifically those with a long record length. For the CVHS we will use gages with at least 25 years of record through the 2008 water year. The critical common period of record in this example is the 80 years between 1928 and The gages available for the illustrative example are shown in Figure 3. For this analysis, we collect data from which we can complete statistical analyses and derive flow-frequency curves. Thus, we require the annual maximum flows at each of the analysis points. Because of the size of the system, the annual maximums may not occur at each analysis point during the same event. Therefore, when the annual maximum flow occurs at one analysis point, corresponding flows are also needed for the other analysis points. Thus, for many years, several historical event data are required. This dataset is referred to as the floods-of-record events. 8

9 Figure 3. Available gages in the Yuba-Feather watershed (show in red is the analysis point) 1.3. Filter gage list by unregulated watershed, collect data Subsequent study tasks will require regression analyses and regional skew studies. For these, streamgage data that are not affected by regulation, or data time series where the values have been adjusted to remove the effects of regulation, will be required. At this step, this subset is identified Augment gage data The gages within the Yuba-Feather watershed each have a different record length. Thus, a common long record length is not available. Regression analysis techniques, as described in EM (USACE, 1993) and EM (USACE, 1997), can be used to extend individual record lengths and thus create a longer common record length. These techniques are described in more detail in the technical appendix on development of the unregulated flow time series. We extend record lengths using methods to preserve the variance in the dataset to develop the common record length. At the end, we have an extended common record length for simulation. (In the technical appendix on frequency analysis, we describe the time period used for completing statistical analysis at each analysis point. For that, we use the entire period of record available at the gage or reach, not just the common period used to develop the regulated and unregulated flow time series, especially on the tributary reaches.) 9

10 1.5. Collect reservoir properties and rule curves The 2 flood control reservoirs in the system are New Bullards Bar and Oroville reservoirs. The effective water control manuals, which include the reservoir properties and rule curves, are: New Bullards Bar Reservoir North Yuba River, California. Reservoir regulation for flood control, June Oroville Dam and Reservoir Feather River, California. Reservoir regulation for flood control, August These water control manuals serve as the basis of the reservoir simulation Collect properties of stream networks and channels For the system, information about stream networks and channel geometry are stored in current channel models of the system. Task 2.0. Model acceptance Here, we identify and adopt the required hydrologic and hydraulic models and computer programs necessary to complete the example Accept reservoir simulation models For this simulation, we accept an HEC-ResSim model currently being used for operational studies in the watershed Accept hydraulic models For this simulation, we accept an HEC-RAS model developed by the Corps for studies in the watershed. Task 3.0. Unregulated flow time series development Here, we develop an unregulated flow time series at each of the analysis points. This unregulated flow time series will be used (1) as the basis of the unregulated flow-frequency analysis, and (2) as the input to our regulatedcondition simulations Develop reservoir inflows As shown in Figure 2, reservoir inflows are needed for 2 locations: Oroville Reservoir and New Bullards Bar Reservoir. To develop the time series of reservoir inflows, historical records of reservoir outflows and reservoir elevations were used. The unregulated reservoir inflows were inferred based on data of reservoir releases and storages. This calculation includes an accounting of headwater reservoir storage and the impact that has on the unregulated flow time series. These time series are stored in HEC-DSS following the Data management plan conventions Develop flows from ungaged watersheds In subtask 3.1, we develop the upstream boundary conditions for the unregulated flow time series. In subtask 3.2, we develop the internal boundary conditions, also known as the local flow contributions. The local flow 10

11 contributions are flows from the ungaged watersheds downstream of reservoirs or between gages. A stream gage measures the total flow in the channel from the watershed above it. Thus, flow from these incremental portions of the watershed is ungaged. For this illustrative example, 3 local flow watersheds have been identified, as shown in Figure 2. These include flow from the contribution of the watershed between the following areas: Oroville Reservoir to Yuba City, called Yuba local. New Bullards Bar Reservoir to Englebright Reservoir, called Englebright local. Englebright Reservoir to Marysville, called Marysville local. The preferred method for estimating local flows is to route observed flows at an upstream point to a downstream point, and then compare the routed flows to observed flows. The difference in the 2 sets of flows at the downstream location is the local flow contribution. However, the specific method used to estimate these historical flows for each local watershed will vary based on the availability of stream flow data or other measured data, channel models, and watershed characteristics. These methods are described in the Procedures document and the technical appendices. In this example, we focus on the local contributing area at Yuba City for a single hypothetical event. For this local area, we use releases from Oroville Reservoir and the corresponding streamgage data at Yuba City to estimate the local flow contribution. Figure 4 illustrates this. The green line is the total flow measured by the gage. This includes outflow from the reservoir and other inflow to the channel between the reservoir and the gage. The latter is the local flow. In the figure, the red line is the Oroville outflow; routed to the gage to account for translation and attenuation in the channel. The reservoir outflow is included in the historical dataset. Routing is accomplished with the channel model. The blue line in the figure is the difference between the total flow gaged and the routed reservoir release the incremental local flow. This process of estimating local flows is repeated for each local watershed for each flood of record, given the availability of models and data to do so. Once estimated, the local flows calculated from these local watersheds will be reviewed based on knowledge of the watersheds and historical events. For this example, we may compare the calculated local flows to ratios of flow per square mile of drainage area or flows recorded by nearby streamgages. 11

12 Gaged streamflow Flow Routed reservoir outflow Ungaged local flow Time Figure 4. Illustration of local flow contribution 3.3. Complete unregulated flow time series After development of the upstream and internal boundary conditions, we complete the unregulated flow time series. To do this, we: 1. Compute the flow contribution from upstream that would have occurred without the reservoir regulation. To do so, we use the inferred inflow series from subtask 3.1, routing that instead of record releases to the analysis point. 2. Add the local flows from subtask 3.2 that contribute above the analysis point, distributing that flow appropriately into the channel. The completing of the unregulated flow time series is illustrated in Figure 5. This process is repeated such that all local flows are integrated and the unregulated flow time series is constructed at each of the analysis points. Error! Reference source not found. Note that the same local flow used here will also be used when developing the regulated flow time series in subsequent tasks. In general, the local flow will be added to the total flow in the same manner for both the regulated and unregulated cases. More specifically, in terms of hydraulic modeling terminology, the flow will be added as a lateral inflow hydrograph or a uniform lateral hydrograph. For some cases where the local flow is restricted from entering the channel due to levees or other obstructions, appropriate modifications will be made. 12

13 Flow (1,000 cfs) Date Figure 5. Unregulated time series at analysis point Task 4.0. Flood flow-frequency analysis In task 3, we develop an unregulated flow time series at each analysis point. Here, we fit an unregulated flow-frequency curve at each point. In addition, we develop unregulated flow-frequency curves for various durations (volumes). For the flow-frequency analysis, we use the standard-of-practice procedures. At present, those are described in Bulletin 17B (Interagency Advisory Committee on Water Data 1982) and EM (USACE 1993) Coordinate with USGS on regional frequency analysis study The standard-of-practice for flood flow-frequency analysis calls for fitting a log Pearson type III probability distribution to a series of unregulated flow annual maximums. Bulletin 17B calls for estimating parameters of the frequency distribution using mean and standard deviation computed with unregulated flow series and skew coefficient computed as a weighted value of skew of the sample and regional skew. As described in the Procedure document, the CVHS makes use of results of the 2009 USGS regional frequency study. That study yielded, amongst other products, new regional skew estimates for California. Those are used here as recommended in Bulletin 17B and Corps guidance. 13

14 4.2. Complete unregulated flow-frequency analysis To complete the unregulated flow-frequency analysis, we extract the annual maximums from the unregulated flow time series at the two example analysis points. Following Bulletin 17B procedures, we fit unregulated flow-frequency curves to the annual peaks. Following procedures in EM , we also computed frequency curves for 3-, 7-, 15-, and 30-day annual maximum volumes. These frequency curves are illustrated in Figure 6. Figure 6. Illustration of unregulated flow-frequency curve at Feather-Yuba confluence Task 5.0. Development of regulated flow time series A regulated floods-of-record series is required to transform the unregulated frequency curves to the regulated curves required for mapping. Here, a regulated time series is needed that is based on the unregulated flow time series from Task 3.0, a consistent set of regulation assumptions, and a consistent set of routing models. Thus, this consistent time series is used as opposed to actual regulated historical releases which are routed through the system. In Task 3, we developed an unregulated flow time series for the analysis points in the study. Here, we assess the effect of regulation of the system on that time series. To do this, we develop the regulated flow time series using a 14

15 reservoir simulation model, regulated-condition channel model, and unregulated flow time series. At the completion of this task, we have a regulated flow time series at each location for which we had an unregulated flow time series. The regulated flow time series is used in development of unregulated to regulated flow transform curves and stage-flow transform curves. With these, the unregulated flow-frequency curves are transformed into regulated flow-frequency curves needed for floodplain mapping Route flows with reservoir simulation model The first step in developing the regulated flow time series is to simulate reservoir operations using the floods-of-record. As input flows to the reservoir simulation model, we use the unregulated flow time series, including the local flows from subtask 3.2. The result of the simulation is a time series of reservoir releases. Thus, this is similar to the routing showed in Figure 5 except that the Oroville inflows first are routed through Oroville Reservoir, and then releases are then routed downstream. To complete this task, the HEC-ResSim model of the Yuba-Feather system is configured to use the inputs developed from Task 3. In addition, a consistent set of reservoir simulation assumptions are established and configured in the model. For example, a consistent set of assumptions on starting pool elevations is established for headwater reservoirs. This same set of assumptions is used for routing all of the floods of record Route flows through channel model (regulated condition) The next step is to route flows through the adopted regulated condition channel model. Here, we use the results from subtask 5.1, the reservoir regulated flow time series, as the upstream inflow hydrographs to the channel model. Again, the ungaged local flows from subtask 3.2 are added in the channel model as appropriate. This channel model simulates the effects of hydraulic structures and operable weirs and diversions as well as the effects of levees. For the CVHS, the routing will be completed based on the agreed upon and coordinated assumption of levee failures. For this illustrative example, levees are assumed not to fail during the simulations. The result of this simulation is a regulated flow time series at each of the analysis points. This regulated flow time series for a given event is illustrated in Figure 7. The red line in the figure represents the regulated flow. For comparison, the unregulated flow, the blue line, is shown as well. This regulated time series is conditional, based on the assumptions made during the routing process. For example, a different levee failure assumption would produce an alternative set of regulated flows. 15

16 Unregulated flow Flow Regulated flow Time Figure 7. Illustration of regulated flow time series and unregulated time series Task 6.0. Development of flow transform curves Here, we develop a relationship of unregulated flow to regulated flow for the analysis points, using the unregulated and regulated time series developed previously. The purpose of this step is to develop a relationship that captures the effects of: Instream storage (reservoirs). Offstream storage, due to capacity exceedence or levee failure. Reservoir operation. Historic storm patterns and timing. Impact of water control features such as channels, levees, weirs, and bypasses. The result of this task is a relationship, based on common historical events, of unregulated flow and regulated flow. This relationship is used to transform the unregulated flow-frequency curve from Task 4 to a regulated flowfrequency curve Identify regulated and unregulated event maxima A key step in development of the transform is to identify the critical duration at each analysis point. The critical duration answers the question: What duration of the inflow hydrographs has the greatest impact on outflow peak? 16

17 Without regulation, the regulated peak equals the unregulated peak, and the critical duration is irrelevant. As upstream storage increase, the critical duration lengthens because reservoir storage attenuates the inflow peaks, and thus causing inflow volume therefore has a greater effect on downstream flow. The critical duration is unique to each analysis point and configuration of the regulated system. For the CVHS, we found the critical duration by analysis point by routing various historical events. This process is described in the technical appendices. Once the critical duration is identified, we extract the appropriate event maximum unregulated and regulated flows. For this example, we find that the Feather River below the confluence has a 3-day critical duration. Thus, for each of the historical events, we extract the 3-day unregulated flow and the peak regulated flow. These values are plotted in Figure Regulated peak flow (1,000 cfs) Unregulated 3-day volume (1,000 cfs) Historical events Figure 8. Unregulated and regulated flow pairs for the Feather River below the confluence with the Yuba River 6.2. Extend the unregulated-regulated flow transform For this study, we require estimates of the p=0.002 (500-year) regulated flow. Based on frequency analysis of unregulated peaks and values for the Feather-Yuba confluence, we see that the 3-day flow value is approximately 340,000 cfs. Comparing this to Figure 8, we see that data are not available in this range. Without these data, we are not able to define accurately a relationship between unregulated and regulated flows. Thus, to define this 17

18 extreme part of the curve, we rely on simulation of scaled historical events, consistent with the guidance in EM To complete this task, we follow the steps below along with the procedures called for in the technical appendices: 1. Identify need for extending the unregulated-regulated flow transform using flow-frequency curves. 2. Identify events and scaling factors for extending unregulated-regulated transforms. Selection of the historical events to scale and what factors to use is a significant task. The selection of the historical events and the scaling factor is discussed in technical appendix IX entitled Development of flow and stage transforms. Here, we selected the largest system events and factors of 1.5, 2.0, and Scale historical unregulated event hydrographs. 4. Route scaled historical unregulated hydrographs through reservoir simulation model. 5. Route regulated scaled event hydrographs using hydraulic channel models (unregulated and regulated conditions). 6. Extract data event maxima. 7. Add scaled data points to unscaled data sample. Figure 9 illustrates the results of this step Regulated peak flow (1,000 cfs) Unregulated 3-day volume (1,000 cfs) Historical events Scaled events Figure 9. Historical and scaled historical flow pairs for the analysis point 18

19 6.3. Fit flow-transform curve DRAFT The paired values of event unregulated and regulated flows, from the floodsof-record time series and the scaled time series, will be used to fit an unregulated-regulated flow transform curve. The process to fit the flow transform curve through the data points is described in technical appendix IX. The unregulated-regulated flow transform curve for the Feather-Yuba river confluence is illustrated in Figure 10. We fitted this as a polynomial, with procedures described in the technical appendices Regulated peak flow (1,000 cfs) Unregulated 3-day volume (1,000 cfs) Historical events Scaled events Flow transform Figure 10. Illustration of unregulated-regulated flow transform curve at the analysis point 6.4. Apply flow-transform curve The flow-transform curve is combined with the unregulated flow-frequency curve from subtask 4.2 to develop a regulated flow-frequency curve at each analysis point. This process is illustrated conceptually Figure 11. The result for this illustration is shown in Figure

20 Figure 11. Illustration of applying the flow transform curve Figure 12. Illustration of applying the flow transform curve at the analysis point 6.5. Develop expected hydrographs The CVHS product uses document describes the expected (design) hydrograph. This hydrograph represents a time series with a specified peak flow quantile and a consistent, coincident volume based on actual historical regulated hydrographs. To develop the expected volumes (hydrographs), we: 20

21 1. Extract from the entire regulated flow time series (from Task 5) the 1-day flow, 3-day flow, 7-day flow, and so on. 2. Pair and plot the peak flow for each flow duration. 3. Fit a most-likely curve though each set following the procedures described in technical appendix IX. Figure 13 illustrates these characteristic curves. Using these characteristic curves, we find the volumes associated with the p=0.01 (100-year) peak flow. These are shown in Table 2. For some applications, a system-wide hydrograph is needed, not just an expected (design) hydrograph at an analysis point, as described in the CVHS product uses document. The October version of the CVHS product uses document includes an illustrative example of how a single historical event that would be consistent with the expected (design) hydrograph could be selected. For completeness, that description is repeated here in Attachment A. Table 2. Expected volumes (hydrographs) for p=0.01 event Duration (day) (1) Flow (cfs) (2) Peak 296, , , , , , ,000 21

22 Regulated peak flow (1,000 cfs) Regulated peak flow (1,000 cfs) Regulated 3-day volume (1,000 cfs) Regulated 3-day volume (1,000 cfs) Historical events Scaled events Flow transform Historical events Scaled events Flow transform Regulated peak flow (1,000 cfs) Regulated peak flow (1,000 cfs) Regulated peak flow (1,000 cfs) Regulated 7-day volume (1,000 cfs) Historical events Scaled events Flow transform Regulated 15-day volume (1,000 cfs) Historical events Scaled events Flow transform Regulated peak flow (1,000 cfs) Regulated 10-day volume (1,000 cfs) Historical events Scaled events Flow transform Regulated 30-day volume (1,000 cfs) Historical events Scaled events Flow transform Figure 13. Illustration of set of regulated peak-regulated flow transform curves for the Feather River below the confluence with the Yuba River Task 7.0. Development of stage-flow relationships Because a channel model is being used for the flow routing, as a by-product, relations of flow and stage can be developed. Although computation of water surface profiles is being completed under a different program, CVFED, these stage-flow relationships are useful for accounting for the coincidence of flows in the system. Conditions in adjacent and downstream channels affect water surface profiles (stages) at the analysis points. For example, the stage at a given cross section upstream from a confluence with a tributary stream may be influenced by the coincident event and timing of flows on that tributary. This effect must be considered when estimating the stage of a given annual exceedence probability at the cross section. 22

23 To account for this downstream effect, EM suggests identifying stage-flow relationships from historical events and fitting a most likely curve. These stage-flow relationships would come from simulation of historical events. We will follow that guidance here. Once developed, this stage-flow relationship can be used in conjunction with the regulated flow-frequency curve from Task 6 to develop a stage-frequency curve. A stage-flow transform from a downstream point can also be used as a model boundary condition Identify regulated and unregulated stage-flow maxima The first step is to extract the stage-flow points from the regulated time series in subtask 5.2. As with development of the flow transform curve, we will select the largest event each year to define 1 point on the stage-flow relationship. If we need to extend the information points for the larger events, we will extract points from the scaled floods-of-record channel routing in subtask Fit stage-flow transform After extracting the stage-flow data points, we establish a relationship between event maximum flows and maximum stages, similar to development of the flow transform curve in subtask 6.3. We fit this curve by examining the stage-flow points, fitting a curve using a statistical analysis procedure, and examining the results. Technical appendix IX describes this process Apply stage-flow transform, if needed If needed, the stage-flow transform can be combined with the regulated flowfrequency curve from subtask 6.4 to develop a stage-frequency curve at each analysis point or can be used as described in the CVHS Product uses document. Study products at analysis points The CVHS products include both the hydrologic analysis output and accompanying technical documentation. A detailed discussion of the study products are described in the CVHS product uses document. In addition to the study products at the analysis points, the set of historical and scaled historical system-wide events are also available for analysis of the system. With these, hydrographs are available at model handoff points and modeling nodes in the reservoir simulation model and the channel simulation models. For example, in this illustration, we focused on the analysis point noted. However, flow hydrographs are available upstream on the Feather River and Yuba River above the analysis point. 23

24 Alternative analysis As described in the CVHS product uses document, the models and information developed in the study are available for alternative analysis. A given alternative, as indicated in Table 3, could change one or more of the following items: unregulated flow-frequency curve, unregulated-regulated flow transform, or stage-flow transform. Table 3. How products could change for alternative analysis Alternative (1) New reservoir Reservoir reoperation Levee setback (channel widening) Levee raising Expected product change (2) Change in the unregulated-regulated flow transforms Change in the unregulated-regulated flow transforms Change in the unregulated-regulated flow transforms and the stage-flow transforms Change in the unregulated-regulated flow transforms and the stage-flow transforms Use 4 of the CVHS product uses document provides more discussion on alternative analyses. Here we consider a non-structural alternative reservoir re-operation. Specifically, we will answer the question, What are the effects of increased flood storage at Oroville Reservoir? For this illustration, we evaluate the impact of an additional 300,000 ac-ft of flood control storage in Oroville. Looking at Figure 2 from the Project Management Plan, we can see that this will affect results from Task 4.0 through Task 7.0. The change in reservoir operation affects the regulated flow hydrographs which in turn result in a change to the unregulated-regulated flow transform. Finally, this alternative results in a modified regulated flow-frequency curve. This process is illustrated conceptually in Figure 16. In Table 4 we describe how each task is affected by the proposed alternative. 24

25 Table 4. Summary of impacts of proposed alternative Task (1) Description (2) 1.0 Data collection and augmentation No change. 2.0 Model and computer program selection and acceptance 3.0 Development of unregulated flow time series How results from the specified task change with the proposed alternative (3) The study reservoir simulation model must be modified to reflect the proposed alternative. No change. 4.0 Flood flow-frequency analysis No change. 5.0 Development of regulated flow time series 6.0 Development of flow transform functions 7.0 Development of stage-flow relationships The proposed alternative changes the reservoir releases from Oroville Reservoir. Thus, a new regulated flow time series is required for each analysis point impacted by the alternative. This is illustrated in Figure 15. Because of the change in the regulated flow time series, the unregulated-regulated flow transform curve and the expected (design) hydrographs at downstream analysis points will be affected. Following the study procedures, these outputs need to be updated to reflect the alternative. A change in the unregulated-regulated flow transform results in a change in the regulated flow-frequency curve. Figure 16 illustrates how the flow transform curve may change. The adjusted regulated flow-frequency curve is illustrated in Figure 17. Note that with a change in reservoir operation or configuration, the critical duration may change as well. For alternatives such as this, the critical duration should be re-examined. Changing reservoir operation, and thus changing the regulated flow time series, may impact the coincidences and timing of downstream flows. Thus, the stage-flow transform may need to be modified per the alternative as well. 25

26 Task 3.0 Task 4.0 Task 5.0 (without) Task 6.0 (without) Regulated flow Regulated flow Unregulated flow Unregulated flow Time Probability Change with alt. Regulated flow Time Task 5.0 (with alt.) Unregulated flow Task 6.0 (with alt.) Probability Regulated flow freq. Regulated flow Time Regulated flow Unregulated flow = Without-project condition = With-project (alternative) condition Figure 14. Illustration of task products with and without alternative (from the CVHS product uses document) 26

27 Unregulated flow Flow Original regulated flow Alternative regulated flow Time Figure 15. Comparison of regulated flow time series due to alternative Regulated peak flow (1,000 cfs) Original flow transform Alternative flow transform Unregulated 3-day flow (1,000 cfs) Alternative events Alternative flow transform Scaled events Original flow transform Figure 16. Comparison of unregulated-regulated flow transform curves 27

28 Figure 17. Comparison of flow-frequency curves 28

29 List of references DRAFT Interagency Advisory Committee on Water Data (1982). Guidelines for determining flood flow frequency, Bulletin #17B, US Department of the Interior, Geological Survey. Available at US Army Corps of Engineers (USACE) (1993). Hydrologic frequency analysis, EM , U.S. Army Corps of Engineers, Washington, D.C. USACE (1997). Hydrologic engineering requirements for reservoirs, EM , U.S. Army Corps of Engineers, Washington, D.C. USACE (2002). Technical studies documentation December 2002 for the Sacramento and San Joaquin river basins comprehensive study, Sacramento District, Sacramento, CA. USACE (2007). Data management plan, Sacramento-San Joaquin hydrologic analysis in support of DWR floodplain mapping study. Prepared by David Ford Consulting Engineers, Inc., Sacramento, CA. 29

30 Attachment A. Illustrative example of event selection process This section is a reprint from the modified CVHS product uses document from October 29, It is included herein for completeness. As a part of the CVHS, a large collection of both historical and scaled historical events will be developed and analyzed. Only the historical events will be used as the basis of the unregulated flow-frequency analysis. However, both historical events and scaled historical events will be used as the basis of the various flow transforms. Thus, a large dataset of system-wide events will be available. As described for Use 2 and Use 3, one or more of these system-wide events may be useful for various applications of the CVHS products. For example, an event hydrograph may be used as an inflow boundary condition to an unsteady flow hydraulics model for a floodplain mapping study. Here, we provide an illustrative example of one way to select a historical event for a given application. For this illustration, we presume that a p=0.01 event hydrograph is needed at a given analysis point. Based on results of the CVHS, the critical duration for each analysis point will already have been identified. The critical duration defines which of the unregulated flow-frequency curves best predicts the peak regulated frequency curve at a given analysis point, and is needed for the event selection process. Here, we presume that critical duration is 1 day. To select the event, we: 1. Identify the unregulated flow from the analysis point s flow-durationfrequency curve. For this, we use the specific flow-frequency curve associated with the critical duration for that analysis point. As illustrated in Figure 18, for the p=0.01 event and the 1-day duration, we find the unregulated flow is 412,000 cfs. 2. Identify the regulated peak flow given the unregulated flow from the unregulated-regulated flow transform for the analysis point. This flow transform is constructed based on the critical duration of the analysis point. Thus, it is a relation of 1-day unregulated flow and peak regulated flow. As shown in Figure 10, for this illustration, the peak regulated flow is 308,000 cfs. 3. Identify historical and scaled historical events that locally define the flow transform curve for the region of interest. The identified events will have regulated peak flows approximately equal to 308,000 cfs. The identification process is illustrated in Figure 10. The graph shows the regulated peak flows of scaled and unscaled events. Here, 4 events have regulated peak flows approximately equal to 308,000 cfs. Thus, these 4 represent the candidate events for selection. A magnified section of the graph indicates the ID of each of the 4 events and, in parentheses, the scaling factor. 30

31 4. Identify the regulated flow properties of the expected (design) hydrograph for the analysis point. The expected (design) hydrograph is described in the CVHS product uses document. The properties of that hydrograph are based on a statistical analysis of the regulated flow hydrographs analyzed in the CVHS. The properties of the expected (design) hydrograph are referred to herein as characteristic curves and are relationships between peak flow and various flow durations. Figure 13 shows the characteristic curves used for this illustration. Note that these curves are based on an analysis of all the system-wide events used in the CVHS not just the candidate events shown. The properties of the expected (design) hydrograph are listed in the first row of Table 5. Column 2 identifies the peak flow; column 3 identifies the 1-day flow; column 4 identifies the 3-day flow; and column 5 identifies the 7-day flow. 412,000 Figure 18. Identification of unregulated flow for given quantile and duration 31

32 (2.5) 1956 (1.5) 1982 (2.5) Regulated peak flow (1,000 cfs) (1.0) Unregulated 1-day volume (1,000 cfs) Unscaled Events Scaled Events Unregulated=Regulated (1:1 slope) Flow transform Figure 19. Identification of regulated peak flows and events defining the curve Regulated peak flow (1,000 cfs) (1.5) 1997 (1.0) 1982 (2.5) 1974 (2.5) Regulated peak flow (1,000 cfs) (1.5) 1997 (1.0) 1974 (2.5) 1982 (2.5) Regulated 1-day volume (1,000 cfs) Regulated 3-day volume (1,000 cfs) Regulated peak flow (1,000 cfs) (1.5) 1997 (1.0) 1982 (2.5) 1974 (2.5) Regulated 7-day volume (1,000 cfs) Figure 20. Identification of volume characteristics of candidate events 32

33 Table 5. Characteristics of expected hydrograph and candidate events for specific durations Event ID (1) Characteristic curve Peak flow (cfs) (2) 1-day flow (cfs) (3) 3-day flow (cfs) (4) 7-day flow (cfs) (5) 308, , , , (1.5) 300, , , , (2.5) 285, , , , (2.5) 291, , , , (1.0) 249, , , , Extract the regulated flow properties of the candidate events. The properties of the actual events are listed in Table 5 below the properties of the expected hydrograph. 6. Review and select the appropriate event, or multiple events, to use for the application of the CVHS products. For some applications, a single event may be adequate for analysis. For other applications, it may be more appropriate to account for the variability expressed by multiple events. To select the event, we use the method described below. However, other methods and criteria or variations of this method could also be employed. To review which event best matches the expected (design) hydrograph, we compare ratios of characteristics of the event hydrograph to the expected (design) hydrograph. This is illustrated in Table 6. For example, column 2 is the ratio of the peak flow from the expected (design) hydrograph to that of the specified event, and column 3 is the ratio of the 1-day flow from the expected hydrograph to the 1-day flow for the specified event. This comparison is completed for each of the flow durations considered here. Once the ratios are calculated, we inspect the ratios and choose the appropriate event for our specific application. Factors or criteria to be considered in the selection of the event or multiple events, when analyzing the values in Table 6, may include: An event that has a ratio for the peak approximately equal to 1, and an appropriate concurrent volume. An event that has a ratio for specific flow duration (volume) approximately equal to 1. An event that has an average ratio for a range of flow durations (volumes) equal to 1. This is in not an exhaustive list of criteria that can be applied, but is an example of criteria that could be considered. Further, criteria beyond the ratios noted in Table 6 could be used such as site specific and analysis specific criteria for the given application. 33

34 Table 6. Example comparison of historical events to expected (design) hydrograph Event ID (1) Peak curve ratio (2) 1-day flow ratio (3) 3-day flow ratio (4) 7-day flow ratio (5) 1956 (1.5) (2.5) (2.5) (1.0)