1. Introduction. BALANCE HYDROLOGICS, Inc. Memorandum

Transcription

1 BALANCE HYDROLOGICS, Inc. Memorandum To: Georgia McDaniel, CSW ST2 cc: John Wynne, Skywalker Properties, Ltd. From: Scott Brown, and Barry Hecht Date: May 30, 2011 Subject: Pre- & post-project comparative analysis of sediment transport in Miller Creek and effects on downstream reaches and San Pablo Bay, Grady Ranch, Marin County, California 1. Introduction Skywalker Properties has proposed a stream and valley restoration plan for a portion of Miller Creek and its tributaries within Grady Ranch as part of a planned facility within the Grady Creek subwatershed to be used primarily for advanced, digital technology based entertainment production. The proposed project is outlined in the Precise Development Plan (CSW/ST2, 2009a and 2009b), which is tiered off of the Grady Ranch Master Plan. The Master Plan included a basic restoration program developed in 1993 by Vandivere (1994), and environmental impacts of the Master Plan (including the restoration program) were assessed by Nichols Berman (1996). This memorandum describes the expected changes in sediment transport as a result of the proposed restoration within Grady Ranch and the potential effects to downstream reaches of Miller Creek as a result of these changes. Previous documents prepared by Balance describe the current sediment transport trends at the project site (Richmond and others, 2011), the current condition of the channel to be restored (Brown and Hecht, 2011b), and the style and potential benefits of the proposed restoration (Brown and Hecht, 2010; Owens and others, 2011 and 2008). Projections in this memorandum use these earlier analyses, as well as the regional literature, to quantify the magnitude of sediment transport within the upper Miller Creek watershed 1. Part of the analysis rests on a sediment rating curve analysis, comparing with the nearby San Geronimo/Lagunitas watershed, where similar observations have been conducted beginning in While San Geronimo Creek has experienced past episodes of incision, bank erosion, and corresponding sedimentation (Hecht 1981; 1983), recent observations of sediment transport within the watershed suggest that it is currently under meta stable conditions (Hecht and others, 2010; Owens and Hecht, 2010). 1 We present our findings within the context of previous discussions of post-project channel stability (Owens and others, 2008, for example). Detailed analysis of the stability of the proposed channel is outside the scope of this memorandum, and will be further documented by sediment transport modeling, currently underway as part of the restoration design process Grady Ranch sediment memo (ID33) doc 1

2 Balance Hydrologics, Inc. This memorandum assumes that the reader has some familiarity with the project site, naming and condition of the tributaries and reaches, and the primary issues to be addressed by the proposed restoration. Additional background on these and other issues can be found in a number of previous reports and memoranda (see CSW ST2, 2009a and 2009b; Brown and Hecht, 2010; Brown and Hecht, 2011; Brown and others, 2008; Owens and others, 2008; Owens and others, 2011; Richmond and others, 2011; and Woyshner and others, 2011). 2. Pre- and Post-project sediment comparison Balance Hydrologics has conducted streamflow and sediment transport monitoring at Grady Ranch since fall 2009, continuing through the end of water year 2011 (WY2011) 2. Results for WY2010 monitoring were reported by Richmond and others (2011). Monitoring results for WY2011 have not yet been fully finalized. However, we present some of the preliminary results of the WY2011 monitoring in this memorandum, and use them in the rating curve analysis. 2.1 Existing sediment transport During both WY2010 and WY2011, Balance Hydrologics measured streamflow as well as bedload and suspended sediment transport at six different gaging stations within Grady Ranch, including two stations on the upper Miller Creek mainstem and four stations on tributaries draining to Miller Creek. Complete results of this monitoring have been/will be presented in separate reports (Richmond and others, 2011; Richmond and others, in prep.). For the purposes of this memorandum, we primarily consider sediment transport measured within the mainstem of Miller Creek, as the primary purpose of the memorandum is to compare preand anticipated post project sediment transport related to potential downstream effects. At this time, data are too sparse to broadly project significant trends within the smaller tributary drainages, which tend to exhibit much greater variability in sediment transport, particularly in the Marin County landscape (c.f., Hecht and Enkeboll, 1980). Figure 1 shows the suspended and bedload sediment rating curves for the two mainstem Miller Creek monitoring stations within Grady Ranch ( above Grady Bridge and at downstream property line ). Most prominent on this graph is that the rate of bedload transport is similar to or higher than the rate of suspended sediment transport, resulting in bedload comprising between 40 and 75 percent of the total sediment load in the stream for WYs 2010 and 2011 (Table 1). This near overlapping of the bedload and suspended load rating curves is a surprising and abnormal condition, at the high end of the spectrum compared to many other central and northern California coastal streams. Table 2 shows that bedload sediment transport in upper Miller Creek is more similar to southern California streams (which typically transport 40 to 60 percent of total load as bedload) than other northern California Coast Range streams (commonly 5 to 25 percent bedload; c.f., Knudsen and others, 1992). 2 Most hydrologic and geomorphic monitoring occurs for a period defined as a water year, which begins on October 1 and ends on September 30 of the named year. For example, water year 2010 (WY2010) began on Oct. 1, 2009, and concluded on September 30, Grady Ranch sediment memo (ID33) doc 2

3 Balance Hydrologics, Inc. The relatively high rates of bedload transport within the upper Miller Creek watershed match the geomorphic condition of the stream as observed in the field. Brown and others (2011b) attribute high rates of sediment contribution to the stream as a result of tributary incision and collapse of stream banks and adjacent terrace deposits (Figure 2). These deposits are composed primarily of older alluvial sediments that, when re worked, seem to produce a high percentage of bedload sized material. Other potential sources of sediment at the watershed scale (such as landslides, rilling, or creep and colluvial delivery) are not likely to consistently deliver material that is so coarse, and thus suggests that the primary source of the high sediment loads in upper Miller Creek is the result of erosion of terrace material. 3 In addition, the mainstem of Miller Creek presently stores a large volume of sediment within the channel that is easily re worked during stormflow events (Figure 3). Another notable aspect of sediment transport within the upper Miller Creek watershed is the relatively low flow threshold of sediment transport initiation. Measureable bedload is transported at flows less than 20 cubic feet per second (cfs) in the mainstem of upper Miller Creek, and at flows as little as 2 to 3 cfs on the tributaries. By comparison, this threshold is about 60 cfs under existing conditions in San Geronimo Creek. This low threshold of bedload movement is a likely reflection of abundant, easily mobilized in channel supply (storage) as well as abundant coarse sand and gravel sized particles that are easily transported as bedload at low flows. Our data (Table 3) show that bedload sediment in Miller Creek and tributaries are predominantly medium and coarse sand, plus some fine gravel. These data are significant because: a. they indicate that all streams are transporting similar sized sediment (predominately less than mm), despite different energetics. This suggests that they originate from a common source, likely the alluvium of the Grady Ranch valley floor. b. the bedload sediment is of a size that is generally too small to provide favorable spawning or winter shelter conditions for anadromous fish. While Lewis and others (2010) note that the locations of spawning in Miller Creek are not known, the coarser particles that provide favorable habitat for steelhead and other aquatic species are in very limited supply in the material emanating from Grady Ranch. Further, sands fill and impair pools in reaches downstream from the project, reducing habitat value for other aquatic species (Lewis and others, 2010). It is important to note that the sediment rating curves for the upper Miller Creek watershed are based on only two years of data, and measurements do not encompass the upper end of the flow range experienced in those years. The rating curves presented herein are subject to change 3 For further discussion bearing on bedload vs. suspended sediment supply and transport in nearby Marin County streams, see Hecht and others (2010), Haible (1976), Stillwater Sciences (2008). For perspective, measurements during the low-recurrence January 1982 storm in four Lagunitas Creek watersheds led to calculated estimates of a 17 to 26 percent bedload fraction (Table 9 in Hecht, 1983). Hillslope failures were widespread during that storm Grady Ranch sediment memo (ID33) doc 3

4 Balance Hydrologics, Inc. as more data are collected. Nonetheless, the WY2010 and 2011 data indicate relatively high sediment transport rates and low flow thresholds of bedload movement. 2.2 Comparison to nearby San Geronimo Creek The nearest stream with a sustained record of streamflow and sediment transport not regulated by dams is San Geronimo Creek. The San Geronimo watershed has a similar sheared sandstone and shale ( Franciscan ) geology to the upper Miller Creek watershed. A streamflow gage has been operated on San Geronimo Creek at Lagunitas Road, near its mouth, since 1979; sediment transport has been measured at the gage since 1995 (and previously in the early 1980s). 4 The gaging station is approximately 5 miles west southwest of the Grady Ranch project site. San Geronimo Creek, a tributary to Lagunitas Creek, drains westward within an alluvium filled valley with small tributary watersheds draining toward the middle of the valley, similar to the upper Miller Creek watershed. Though the watersheds above the gaging stations differ (8.7 square miles for San Geronimo, versus 2.8 for Miller Creek at downstream property line), the valley slopes are somewhat similar (just under 1% for San Geronimo, and just over 1% for upper Miller Creek). Figure 4 shows a comparison of the sediment transport rates for San Geronimo Creek and the upper Miller Creek watershed. The graph highlights manual measurements of suspended sediment and bedload transport in both the San Geronimo and upper Miller Creek watersheds. Points plotting toward the left and top of the graph represent higher rates of sediment transport. For the purposes of illustration, data from all six monitoring stations within the Miller Creek watershed are plotted as compiled bedload and suspended load datasets; generally, the tributaries exhibit the highest rates of sediment transport (toward the upper left on the graph) within the full scatter of data. Figure 4 clearly shows that the San Geronimo and Miller Creek data plot in separate areas of the chart, with nearly all of the Miller Creek watershed data plotting toward the higher sediment transport side of the graph. Despite scatter in each of the datasets, there is very little overlap. Also of note is the fact that the 2011 bedload curve for San Geronimo Creek is within the lower portion of the 16 year cloud of data, further accentuating the lack of overlap. For a given discharge, sediment transport in the upper Miller Creek watershed is on the order of 1 to 1.5 orders of magnitude higher than in San Geronimo Creek. Table 1 compares the total sediment loads for the San Geronimo Creek and Miller Creek mainstem stations for WY2010 and WY2011. San Geronimo Creek transported a total of 1,200 and 1,900 tons of sediment past the gaging station in WY2010 and 2011, respectively, with 23% and 13% transported as bedload. The bedload fraction at the San Geronimo station is much lower than that measured within the Miller Creek watershed (see section 2.1), which we interpret as at least partly due to active channel incision into the alluvium of the valley floor deposits at Grady Ranch, and the related lower threshold of bedload movement in Miller Creek. 4 Sponsored by the Marin Municipal Water District (MMWD), this gage has been operated by Balance Hydrologics staff since inception Grady Ranch sediment memo (ID33) doc 4

5 Balance Hydrologics, Inc. Because of the differences in watershed area and corresponding flow scaling between the two watersheds, it is instructive to compare sediment transport volumes as a function of flow within each watershed (annual average sediment concentration, in tons/ac ft.; Table 1). Based on the data from WY2010 and 2011, upper Miller Creek appears to be transporting similar to somewhat higher concentrations of suspended sediment, but bedload concentrations between 10 and 27 times the concentrations in the San Geronimo watershed. While some of this difference may simply be statistical or related to the brief period of record at Grady Ranch relative to San Geronimo, some of the difference is certainly due to the lower entrainment thresholds within Miller Creek and highlights the magnitude of excess sediment being transported within the watershed. We suggest, based on field evaluations, that this difference is largely due to high sediment inputs from tributary incision, and bank/terrace erosion along the main stem. 2.3 Estimated post-project sediment transport As discussed above, upper Miller Creek is experiencing much higher rates of sediment transport (especially bedload) than the rates that have been documented in San Geronimo Creek, a stream in a similar (though larger) watershed that is currently under meta stable conditions 5. Field characterization of stream geomorphology (Brown and Hecht, 2011b) suggests that the high transport rates in upper Miller Creek are related to one or more incisional episodes working their way up valley and into the tributaries, along with associated collapse and erosion of stream banks and adjacent terrace deposits, resulting in abundant sediment supply. In addition, streamflow velocities are high due to low channel complexity (low hydraulic roughness) and channel confinement due to vertical separation of the channels from associated floodplain areas. The proposed restoration project within the upper Miller Creek watershed includes the following elements: 1. Raising the base level of Miller Creek and portions of its tributaries using compacted fill sourced within the project site. This will allow for re activation of floodplain area that has been abandoned by channel down cutting (providing flow and sediment attenuation), and reduced height of eroding banks. 2. Laying back stream banks in selected locations. This will reduce sediment contribution to Miller Creek from slumping of terrace material, allow for additional floodplain area for flow attenuation, and help reconnect the stream corridor with the rest of the valley floor. 3. Installing boulder weirs and step pool sequences. This will provide stability to the system and establish a channel grade that is in sync with the flow and sediment transport regime of the watershed. 5 San Geronimo Creek has, itself, experienced episodes of erosion and incision. Recent trends in monitoring data indicate that the stream is currently within the lower range of the historic sediment transport regime, and may represent a meta-stable geomorphic condition Grady Ranch sediment memo (ID33) doc 5

6 Balance Hydrologics, Inc. 3. Re introducing woody debris to the system. This will provide additional channel complexity (slowing flows) and, where added as secure structures, will help dissipate flow energy and increase bank stability. These elements have been described in the Precise Development Plan documentation (CSW ST2, 2009a) and further clarified in subsequent documentation (CSW ST2, 2009b; Owens and others, 2011; and Brown and Hecht, 2010). In sum, the proposed restoration is expected to arrest incision, significantly reduce sediment inputs from erosion of banks and terrace deposits, and increase sediment buffering capacity of the channel. With slower flows, less bank erosion, more roughness, and additional bed and floodplain storage for sediment, we expect reduced sediment transport to downstream reaches of Miller Creek as a result of the restoration. We would expect sediment reduction to occur over a period of five to ten years after restoration, allowing for some (expected) adjustment of the designed channel to occur and for full establishment of stabilizing plantings and other vegetation to take hold. Balance is currently conducting sediment transport modeling to confirm the stability of the proposed channel design and to be used to adapt design specifics as full design drawings of the restoration are prepared. Findings of this modeling effort will be documented in a subsequent memorandum. As discussed previously, sediment transport rates (as a function of flow) are over 10 times higher than in the nearby San Geronimo watershed. Re establishing a stable channel within an equilibrium valley system will reduce the sediment load. We note that not all of the high sediment load will be attenuated, as some is coming from outside the Grady Ranch project area and will not likely be directly reduced as a result of the proposed restoration. In addition, differences in drainage area can also explain some of the differences in the sediment transport rating curves and corresponding concentrations (Barry and others, 2004) 6. Though expansion of the flood prone area within the restoration reach will provide some additional sediment storage area, and will help attenuate sediment inputs that are coming in from further up in the watershed, it is unlikely that sediment transport can be reduced to levels seen in the San Geronimo watershed. It is reasonable to assume, however, given the proposed restoration elements described above, that rates of sediment transport (especially bedload transport) within the Miller Creek watershed will shift toward those seen in the more stable San Geronimo watershed. Using an estimate of a 50% shift toward the difference between the two watersheds, this would result in a decrease (at the Grady Ranch property line) of over 1,200 tons of bedload and over 250 tons of suspended load relative to WY2011 numbers. That would translate to at least 1,000 fewer cubic yards of sediment transported to downstream reaches of Miller Creek in a single water year 7. In wet years, this reduction would be significantly larger, assuming no additional watershed disturbance (natural or human induced). 6 For two monitoring stations of differing watershed areas, rates of sediment transport at a given flow will tend to be higher for the smaller watershed, all other factors being equal, even though total sediment transport within the larger watershed will be higher due to higher flow volumes. 7 Assumes 108 lbs/ft 3 of sediment Grady Ranch sediment memo (ID33) doc 6

7 Balance Hydrologics, Inc. The stability of the fill material and boulder structures that are proposed as part of the restoration project has been discussed in previous reports and memoranda (Owens and others, 2008; CSW ST2, 2009b; Brown and Hecht, 2011a), and the estimated amount of fill to be placed was presented as part of the PDP submittal (CSW ST2, 2009a). The restoration project would use approximately 50,000 cubic yards of fill to raise the bed of the Miller Creek maintstem 8, with approximately 20 boulder structures (or sets of structures) to reinforce channel stability. Conceptually, in the unlikely event of a complete failure of a boulder structure during a veryhigh flow event, some scour of the fill material behind the structure may occur, but this scour is estimated not to exceed a volume on the order of 2,500 cubic yards 9. Effects of partial failure would be more limited. Thus, an increased amount of sediment available for transport within Miller Creek as a result of a boulder structure failure would only be, at most, about 2.5 times the anticipated reduction in sediment transported in a year of similar magnitude to WY2011 as a result of the project (see discussion above) 10. Or, stated another way, as long as the rate of structure failure is less than one every 2 3 years (which would be an unrealistically high rate of failure given the low recurrence of the very large events that might cause such a failure), the project would result in a net reduction in sediment transport. Long term, we suggest that the value of reduced sediment transport as a result of restoration greatly outweighs the potential sediment increase associated with the unlikely failure of one of the boulder structures. 3. Downstream effects As discussed above, upper Miller Creek is experiencing high rates of sediment transport. This sediment is transported to other reaches of Miller Creek downstream of Grady Ranch. Field evidence of stream aggradation is apparent within the Lucas Valley Estates and Baylands reaches of lower Miller Creek (Woyshner and others, 2011). In addition, Lewis and others (2010) have described increased flooding problems due to channel aggradation in lower Miller Creek, as well as degradation of spawning gravels and decreased pool volumes due to the persistent and elevated sediment influx. Given the constrained existing conditions due to sediment deposition within the downstream reaches, reduction in sediment transport from Grady Ranch would reduce the potential for (or decrease the rate of) channel aggradation and loss of flow capacity in flood prone reaches downstream. Also, the sediment reduction has the potential to greatly enhance downstream aquatic resources. Hecht and others (2008; 2010) have shown that, for the nearby Lagunitas watershed, when sediment transport decreases, habitat generally improves. 8 In this memorandum, for the purposes of these calculations, we consider only the fill and structures planned for the Miller Creek mainstem. Total fill for the entire restoration effort is estimated to be approximately 68,000 cubic yards. Due to steeper grades, there is less fill per structure within the tributary watersheds. 9 50,000 cubic yards of fill divided by 20 structures. 10 At San Geronimo Creek, sediment transport in WY2005, the most recent very-wet year, was over 17 times that reported for WY2011. Assuming a similar ratio applies to upper Miller Creek, total sediment transport in WY2005 may have been on the order of 70,000 tons, or 48,000 cubic yards. A 5.2-percent decrease in sediment transport in such a year would equal the estimated maximum volume sediment released as a result of a structure failure Grady Ranch sediment memo (ID33) doc 7

8 Balance Hydrologics, Inc. Another important aspect to consider is the relationship of the low threshold of bedload movement to downstream aquatic resources. As discussed above (section 2.1), bedload is entrained at relatively low flow thresholds within the upper Miller Creek watershed, flows that are common during late winter storms. High sediment influx to downstream reaches during storms within this time period have a high potential to bury recent redds of spawning salmonids. 4. Summary Total sediment transport under existing conditions is very high relative to the neighboring San Geronimo Creek watershed, which is considered to be under metastable conditions. Rates of suspended sediment transport in Miller Creek and its tributaries are similar to or slightly higher than those in the San Geronimo Creek watershed. Rates of bedload transport are on the order of 10 to 20 times higher than in the San Geronimo watershed. Field observations of collapsing banks and terrace deposits, incising tributaries, and high in channel sediment supply are consistent with high rates of bedload transport as measured at Grady Ranch. We believe these to be a predominant source of the additional bedload sediment, which appears to be impairing bed habitat on Grady Ranch and at locations downstream. The bedload fraction of upper Miller Creek is high in sand and fine gravel sized sediment, smaller than that typically considered beneficial to salmonid spawning habitat. We estimate that the proposed restoration effort is likely to reduce bedload sediment generation and transport by about 1,200 tons (about 45% of pre project bedload transport) in a year similar to WY2011. Suspended sediment is likely to be reduced by about 280 tons (15% of pre project suspended load) in a WY2011 type year. The reduced sediment transport would result in improved spawning conditions for anadromous fish in downstream reaches of Miller Creek. In addition, reduced sediment delivery would also reduce the flooding risk downstream, as there would be a reduced threat of channel capacity loss due to sedimentation in low gradient reaches Grady Ranch sediment memo (ID33) doc 8

9 Balance Hydrologics, Inc. 5. Limitations This report is based on field data and measurements of sediment transport. As with all field studies, additional data can add to the representativeness and validity of the conclusions. The sediment transport numbers cited in this memo are preliminary and subject to revision upon further review, and as additional data are collected. Data have been collected in accord with approved methods, and consistent with those used in the nearby streams used for comparison. Anyone with additional observations or information is urged to contact us. Modeling of existing and post project hydrology and hydraulics is presently underway. The findings of this report may warrant refinement once the models have been completed. 6. References Barry, J., Buffington, J., and King, J., 2004, A general power equation for predicting bed load transport rates in gravel bed rivers. Water Resources Research, Vol. 40, W10401, 22p. Brown, S, and Hecht, B, 2011a, Regional Analogs to Proposed Restoration of Miller Creek, Grady Ranch, Marin County, California: Balance Hydrologics technical memorandum prepared for Skywalker Properties, Ltd., 34 p. Brown, S, and Hecht, B, 2011b, Existing channel stability in upper Miller Creek on Grady Ranch, Marin County, California: Balance Hydrologics technical memorandum prepared for Skywalker Properties, Ltd., 41 p. Brown, S, and Hecht, B, 2010, Stream and Valley Floor Restoration Vision: Technical memorandum prepared by Balance Hydrologics for Skywalker Properties, Ltd., 7 p. Brown, S., Owens, J., and Hecht, B., 2010, Annual hydrologic record and sediment yield, Corte Madera Creek, Portola Valley, California: Final data report for WY2009: Consulting report prepared by Balance Hydrologics for Utilities Division, Stanford University, 34p. Brown, S., Owens, J., Woyshner, M., Bartholomaus, T., and Hecht, B., 2008, Geomorphic assessment of upper Miller Creek, Grady Creek, and their tributaries, Grady Ranch, Marin County, California. Consulting report prepared by Balance Hydrologics for Skywalker Properties Ltd, 50p. Collins, L., 1998, Sediment sources and fluvial geomorphic processes of Lower Novato Creek watershed. Report to Marin County Flood Control and Water Conservation District. July CSW ST2, 2009a, Grady Ranch precise development plan (DP 09 7), Assessor s Parcels , 17, and 19, 2828 Lucas Valley Road, San Rafael: Submittal to the Marin County Community Development Agency, April 14, 2009 CSW ST2, 2009b, Response to comments to the Grady Ranch precise development plan (DP 09 7) Notice of Project Status letter (NOPS1) dated May 29, 2009: Submitted to the Marin County Community Development Agency, November 25, Grady Ranch sediment memo (ID33) doc 9

10 Balance Hydrologics, Inc. Hecht, B., 1981, Substrate enhancement/sediment management study Lagunitas Creek, Marin County. Phase IIIA. Sediment transport and substrate conditions, : H. Esmaili & Associates consulting report prepared for the Marin Municipal Water District, 92 p. Hecht, B., 1983, Substrate enhancement/sediment management study Lagunitas Creek, Marin County. Phase IIIb. Sediment transport and substrate conditions, : HEA, A division of J.H. Kleinfelder & Associates consulting report prepared for the Marin Municipal Water District, 172 p. + 6 appendices. Hecht, B., and Enkeboll, R.A., 1980, Substrate enhancement/sediment management study; Lagunitas Creek, Marin County; Phase II: Sediment transport and substrate conditions Report prepared by H. Esmaili & Associates for Marin Municipal Water District, 134p. Hecht, B., Strudley, M. and Brown, S., 2010, Lagunitas Creek sediment and riparian management plan, Marin County, California: streambed monitoring report, : Consulting report prepared by Balance Hydrologics for the Marin Municipal Water District, 125 p., plus 6 appendices. Knudsen, K., Hecht, B., Holmes, D.O., and Flaschka, I., 1992, Hydrologic and geomorphic factors affecting management of the lower Sisquoc River alluvial corridor, Santa Barbara, County, California: Balance Hydrologics, Inc. project number , consulting report prepared for SP Milling Company, 66 p., 7 tables, 16 figs., and 5 appendices Lewis, L, and others, 2010, Miller Creek existing conditions report: Marin County Public Works draft report, 21p. Nichols Berman, 1996, Final environmental impact report: Lucasfilm Ltd., Grady Ranch/Big Rock Ranch Master Plan. FEIR prepared for Marin County Community Development Agency. State Clearinghouse No Multiple volumes. Owens, J., Chartrand, S., and Hecht, B., 2011, Feasibility of restoration concept at Grady Ranch, Marin County, California, California: Balance Hydrologics consulting report prepared for Skywalker Properties, Ltd., 11 p. Owens, J. and Hecht, B., 2010, Streamflow and sediment transport in San Geronimo Creek at Lagunitas Road, WY2010: Balance Hydrologics, Inc. consulting report prepared for the Marin Municipal Water District, 18 p. Owens, J., Brown, S., Woyshner, M., Ballman, E., and Hecht, B., 2008, Hydrologic and geomorphic recommendations for stream conservation areas at Grady Ranch, Marin County, California. Consulting report prepared by Balance Hydrologics for CSWST2. Richmond, S., Woyshner, M., Owens, J., and Hecht, B., in prep., Surface water, groundwater, and sediment transport monitoring, water year 2011, Grady Ranch, Marin County, California: Consulting report prepared by Balance Hydrologics for Skywalker Properties, Ltd Grady Ranch sediment memo (ID33) doc 10

11 Balance Hydrologics, Inc. Richmond, S., Woyshner, M., Owens, J., and Hecht, B., 2011, Surface water, groundwater, and sedimenttransport monitoring, water year 2010, Grady Ranch, Marin County, California: Consulting report prepared by Balance Hydrologics for Skywalker Properties, Ltd., 23 p., plus appendices. Stillwater Sciences, 2008, Lagunitas limiting factors analysis; limiting factors for coho salmon and steelhead. Final report: Stillwater Sciences consulting report prepared for Marin County Resource Conservation District, 73 p. Woyshner, M., Richmond, S., and Hecht, B., 2011, Baseline channel conditions downstream of Grady Ranch, Marin County, California: Consulting report prepared by Balance Hydrologics for Skywalker Properties, Ltd., 35 p. Vandivere, W.B., Preliminary grading and drainage plan, sheet 7, Plan sheet prepared for the Big Rock Ranch Master Plan, Marin County, California Grady Ranch sediment memo (ID33) doc 11

12 TABLES Balance Hydrologics, Inc.

13 Table 1. Comparison of flow and sediment transport, San Geronimo Creek near Lagunitas and upper Miller Creek at Grady Ranch, Marin County, California. Rainfall Runoff Volume Mean Flow Mean Unit Flow a Sediment Load Sediment Concentration (inches) (ac-ft) (cfs) (cfs/sq.mi.) Suspended Bedload Suspended Bedload (tons) (tons) (tons/ac-ft) (tons/ac-ft) WY2010 Miller Creek above Grady Bridge Water Year Multiples of San Geronimo Creek = San Geronimo Creek at Lagunitas Road Water Year Percent of Miller Creek above Grady Bridge = 144% 56% 26% 10% WY2011 Miller Creek above Grady Bridge Water Year Multiples of San Geronimo Creek = Miller Creek at downstream property line d Water Year Multiples of San Geronimo Creek = San Geronimo Creek at Lagunitas Road Water Year 2011 c Percent of Miller Creek above Grady Bridge = 528% 27% 138% 7% Percent of Miller Creek at property line = 93% 10% 35% 4% Notes: a. Watershed area above the Grady Bridge gaging station is 2.1 square miles; above the property line station is 2.8 square miles; and above the San Geronimo station is 8.7 square miles. b. Values shown with more than 2 or 3 significant figures are the result of electronic calculations and do not imply increased precision. c. Mean flow and runoff volume were calculated based on a partial-year records (through mid-april). Miller Creek is close to drying for the season, whereas San Geronimo will continue to flow through the end of the water year. For San Geronimo, total volume will be higher for WY2011 when the final data are released. Totals for Miller Creek will change as well, but likely not by a significant amount. d. Sediment transport for WY2010 not calculated for the 'property line' station due to damage to the gaging station Miller SanGer sediment compare.xls, intensity duration Data preliminary and subject to revision 2011 Balance Hydrologics, Inc.

14 Table 2. Estimates of bedload as a percent of total sediment load, central coastal California. Miller Creek has a much higher bedload fraction percentage than other Bay Area streams, and is more similar to Southern California channels where caving banks and re-worked bars are common. Stream Bedload as percent of total load San Lorenzo River at Bigtrees 1 5% Pajaro River at Chittenden 1 5% Uvas Creek above Uvas Reservoir 1 5 to 12% San Geronimo Creek at Lagunitas Road 2 20% Salinas River at Santa Margarita Lake 1 26% Santa Clara River at Montalvo 1 28% Corte Madera Creek at Westridge Drive, Portola Valley 3 29% San Geronimo Creek at Woodacre (MMWD WTP) 4 30% Devils Gulch at Sir Francis Drake Hwy 4 30% Santa Maria River at Guadalupe, Santa Maria 1 40% Miller Creek above Grady Bridge (WY2010; 2011) 5 43%; 75% Lopez Creek near Arroyo Grande 1 48 to 50% Santa Clara River at LA/Ventura County line 1 52% Sespe Creek near Fillmore 1 54% San Juan Capistrano Creek 1 59% Miller Creek at downstream property line (WY2011) 5 60% Sources: 1 As compiled by Knudsen and others, Owens and Hecht, Brown and others, Hecht, Richmond and others, 2010; in prep Miller SanGer sediment compare.xls, Bedload percentages 2011 Balance Hydrologics, Inc.

15 Table 3. Sediment-size distribution for bedload and bed-core samples collected in the upper Miller Creek watershed, Grady Ranch, Marin County, California. Samples indicate that bedload transport (as sampled during the January 20 and February 25, 2010 events) had a high proportion of medium- to coarse sand (less than 4mm), with some fine gravels (<16mm) but little coarse gravel or cobbles (>16mm). Location Sample date/time D-size 1 D16 D50 D84 Miller Creek above Grady Bridge 1/20/ : Miller Creek above Grady Bridge 2/26/ : Grady Creek 1/20/ : Grady Creek 2/26/ : S4 Tributary 1/20/ : S4 Tributary 2/26/ : S4 Tributary 1/20/ : S4 Tributary 2/26/ : Mean Maximum Minimum Notes: 1 D-size is the size, in millimeters, for which a given percentage of the sample is smaller. For example, a D50 of 4 means that 50% of the sample is less than 4mm; a D84 of 30 means that 84% of the sample is less than 30mm bedload size summary.xlsx, Sheet Balance Hydrologics, Inc.

16 FIGURES Balance Hydrologics, Inc.

17 1000 MCGB bedload-sediment rating curve (WY 2010 and 2011) MCGB suspended-sediment rating curve (WY 2010) MCGB suspended-sediment rating curve (WY 2011) MCPL bedload-sediment rating curve (WY 2011) MCPL suspended-sediment rating curve (WY 2011)??? 100 Watershed area above the stations are: Miller Creek above Grady Bridge (MCGB) = 2.1 sq. mi. Miller Creek at downstream property line (MCPL) = 2.8 sq. mi. Instantaneous sediment discharge (tons/day) 10 1 Higher rates of sediment transport Field data showed a shift toward lower rates of suspendedsediment transport at MCGB in WY The MCGB and MCPL suspendedsediment rating curves were the same in WY Lower rates of sediment transport Instantaneous stream discharge (cfs) Figure 1. Sediment-discharge rating curves, Miller Creek above Grady Bridge and at downstream property line, Grady Ranch, Marin County, California. Curves show that bedload was transported at similar or higher rates than suspended sediment in WYs 2010 and See Figure 4 for data points on which curves were based Sediment log and curves 2011.xls, Sed Trans Memo Fig 2011 Balance Hydrologics, Inc.

18 Upper terrace surface (valley floor) Freshly eroded terrace deposits contributing sediment to Miller Creek Inputs from incising tributaries Figure 2. Sediment contribution to upper Miller Creek, Grady Ranch project site, Marin County, California. High rates of sediment transport (especially bedload; see Figure 1) are likely due to high contributions of sediment from collapsing banks and incising tributaries, among other factors Sediment comparison photo figures.xlsx, Figure Balance Hydrologics, Inc.

19 Upper terrace surface (valley floor) Figure 3. Photograph showing large amounts of mobile bed material in storage within the upper Miller Creek channel, Marin County, California. The material is a mix of poorly-consolidated fine sand to coarse gravel, with a low threshold of bedload entrainment during flow events Sediment comparison photo figures.xlsx, Figure Balance Hydrologics, Inc.

20 Upper Miller Creek watershed: suspended-sediment discharge Upper Miller Creek watershed: bedload-sediment discharge Miller Creek at property line: WY2011 bedload rating curve San Geronimo Creek: suspended-sediment discharge San Geronimo Creek: bedload-sediment discharge San Geronimo Creek: WY2011 bedload rating curve Peak flow for WY2011 in Miller Creek at the downstream? property line was approximately? 400 cfs Higher rates of sediment transport? Instantaneous sediment discharge (tons/day) Miller Creek watershed stations show a very low threshold of bedload movement. Peak flow for WY2011 in San Geronimo Creek at was approximately 685 cfs. Lower rates of sediment transport 0.1 When observations and/or measurements indicate that no bedload is moving, we assign a value of 0.01 tons per day, so the value can be plotted as threshold data Instantaneous stream discharge (cfs) Figure 4. Comparison of sediment discharge in upper Miller Creek watershed to San Geronimo Creek. Miller Creek has significantly higher rates of sediment transport than the nearby San Geronimo Creek watershed. Data for Miller Creek is an aggregate of six stations within the upper watershed within Grady Ranch for WYs 2010 and 2011 (Richmond and others, 2011; in prep.). Data for San Geronimo Creek is from WYs 1995 through 2011 (Owens and others, in prep) Miller-SanGer sediment compare.xls, Sediment comparison 2011 Balance Hydrologics, Inc.