Sediment Characteristics of Managed Flood Control Channels in Southern San Francisco Bay Authored by: David Gluchowski*, Sarah Pearce** and Lester McKee*** *[email protected] **[email protected] ***[email protected] Methods Results Abstract In order to maintain floodwater capacity, manag- SITE DESCRIPTION In general, San Lorenzo Creek had the coarsest gate appears to cause preferential sorting and sediment deposited in the flood control channel, deposition of coarser sediment on its upstream ers must de-silt the channels, which can be very 6 Although the three watersheds are all with an average D50 of 3.4mm, the Alameda side. In SLC, the finest sediment is present in the 1 expensive and require difficult-to-obtain permits. located in Alameda County, and all Creek Flood Control Channel had finer sediment tidal section, with slightly coarser sediment just 2 In addition, removing the sediment from the chan- enter the southeast portion of the with an average D50 of 2.5mm (Figure 6), and downstream of the stilling basin, and the coars- nels prevents that sediment from ever reaching San Francisco Bay, the watersheds vary Old Alameda Creek had the finest sediment with est sediment in the downstream section of the greatly (Figure 1). Alameda Creek is an average D50 of 0.014mm (Table 1). Many concrete channel. Sediment in the Alameda the Bay margin, and potentially being reworked the largest local tributary to the San factors such as channel gradient and geometry, Creek Flood Control Channel is driven by supply and deposited in tidal flats or marshes. Data on Francisco Bay (1,682 km2) draining up- along with upland sediment erosion and supply from its large watershed; deposited sediment dis- the in-channel deposited sediment (volume, loca- land, interior valley, and alluvial plain influence sedimentation in these types of chan- plays an overall downstream-fining trend tion, grain size) is lacking in the Bay Area. This areas, through a 19 km long large nel systems. Sedimentation in each channel re- (Figure 7a) reflecting distance downstream earthen trapezoidal flood control flects watershed size, topography, geology, tec- from the apex of the alluvial fan. In contrast, the study represents a first step of data collection on channel (Figure 2). San Lorenzo tonics, land use, sediment sources, management, grain size distribution in Old Alameda Creek and a regional scale to better understand sediment in Creek is a 124 km2 watershed that tidal prism, and climatic variability, among other San Lorenzo Creek appears to be less driven by managed flood control channels. The data from drains upland and alluvial plain areas, factors. Engineered structures such as the tide source characteristics, but instead controlled by through a 7.5 km long narrow con- gate in Old Alameda Creek (Table 2), as well as the in-channel engineered structures (Figures this study could be used to support numerical 7a crete box and earthen trapezoidal 35 the stilling basin and concrete channel in San Lo- 7b and 7c). Throughout the reaches in OAC modeling of in-channel processes, explore alterna- 3 4 D85 flood control channel (Figure 3). Old 30 renzo Creek (Table 3) seem to have an effect on and SLC there were some small variations in D50 tives to de-silting or more generally evolving man- Alameda Creek is a 57 km2 watershed grain size distribution. In OAC, the coarsest sedi- grain size, possibly caused by tributary channel D15 agement methods, or perhaps even to help man- that only drains the alluvial plain area 25 ment was found upstream of the tide gate. The inputs or channel geometry. agers consider alternative applications for the re- (receiving some overflow water and 20 sediment from ACFCC), through an Figure 6: Sediment sample collected in Old 15 moved sediment, such as wetland restoration or earthen trapezoidal flood control Grainsize (mm) Alameda Creek. D85 (mm) D50 (mm) D15 (mm) % gravel % sand % silt % clay Figure 7: Grain size distribution (D85, D50, beach nourishment. channel (Figure 4). The tidal reaches 10 San Lorenzo Creek 16.1 3.44 0.311 38.9 56.8 2.33 1.97 and D15) for each sample plotted with channel 1 of all three channels are constrained distance (meters) upstream from the Bay Old Alameda Creek 0.064 0.014 0.002 0.096 8.62 43.7 47.6 5 margin. Along the x axis, zero is the mouth of Alameda Creek Flood Control Channel 7.91 2.5 0.45 20.9 43.5 26.1 9.53 by maintained levees. (a) Alameda Creek, (b) Old Alameda Creek, and 0 (c) San Lorenzo Creek. 0 5000 10000 15000 20000 Table 1: Comparison of grain size in San Meters upstream from mouth of Alameda Creek Introduction Lorenzo Creek, Old Alameda Creek, and Alameda Creek Flood Control Channel. Reach of Old Alameda Creek D85 (mm) D50 (mm) D15 (mm) % gravel % sand % silt % clay FIELD METHODS 2 Upstream of Tide Gate 0.13 0.039 0.0016 0.38 21.5 45.0 33.1 0.4 Construction of flood control channels on many of Figure 1: Location of San Lorenzo Creek, Old Table 2: Average D85, D50, D15, and the Downstream of Tide Gate 0.04 0.005 <0.0012 0.00 4.32 43.2 52.5 Samples were collected from both tidal and fluvial Alameda Creek and the Alameda Creek Flood D85 7b percentage of material in each size category the rivers and creeks draining to San Francisco Bay 5 Control Channel. The reaches sampled are 0.35 D50 for the sections upstream and downstream of reaches of each channel, and for OAC, samples were outlined in black. D15 the tide gate in Old Alameda Creek. (California, USA) was prompted by the combination Reach of San Lorenzo Creek D85 (mm) D50 (mm) D15 (mm) % gravel % sand % silt % clay taken in the north and south channels, as well as on 0.3 of the rapidly urbanizing Bay Area and the series of Figure 2: The view upstream of the third Table 3: Average D85, D50, D15, and the 3 Downstream of stilling basin 13.1 1.91 0.27 33.37 62.83 1.87 1.93 the bar between the two channels. Aerial photo- rubber dam in the Alameda Creek Flood Control percentage of material in each size category Straight concrete 26.8 6.03 0.39 53.80 43.13 1.83 1.23 large regional devastating floods in the 1950s. Con- Channel. 0.25 for the three sections downstream of the Tidal channel 10.3 2.65 0.29 31.85 62.55 3.05 2.55 graphs showing the proposed sample locations were stilling basin in San Lorenzo Creek. sequently, due to the constructed dimensions (width, Figure 3: The view looking upstream at the used alongside a handheld Garmin Etrex Legend 0.2 depth, gradient, planform), these channels have concrete channel and stilling basin in San GPS unit, to navigate the field team as close as pos- Lorenzo Creek. Grainsize (mm) been filling with sediment. Flood control channel 0.15 sible to the proposed sample locations. Exact sample construction in Alameda County began in the 1950s, Figure 4: The view looking downstream in the locations were selected based upon field conditions. tidal section of old Alameda Creek. 0.1 primarily for routing floodwaters to the Bay. Con- At sample locations located in the tidal portion of Figure 5: Deployment of the petite Ponar over struction often overlooked transport of sediment as 0.05 Conclusion the system, a boat was used to navigate to sample the edge of the boat. This photo was taken in well as other beneficial uses such as salmonid fish Old Alameda Creek and strictly shows the petite locations. A small, hand deployed petite Ponar Ponar methodology. 0 migration and wildlife habitat for birds and other dredge sampler was used to excavate sediment 0 1000 2000 3000 4000 5000 6000 7000 8000 This work represents a first step towards characterizing and understanding sediment de- species. In addition, both the Alameda Creek Flood Meters upstream from mouth of Old Alameda Creek samples from the channel bed (Figure 5). The pe- posited in flood control channels across the San Francisco Bay area. Grain size data as Control Channel (ACFFC) and San Lorenzo Creek tite Ponar sampler was lowered over the edge of LAB METHODS (SLC) have been aggrading since their construction. presented here can be used to support numerical modeling efforts, explore alternatives the boat collecting sediment from approximately They have each been de-silted several times; SLC was Samples from ACFCC were analyzed by Consolidated Engineering Laborato- 30 to the de-silting process, and find alternative applications for the removed sediment such the upper 5 cm of a 15 cm x 15 cm footprint in the D85 most recently de-silted in 2004, ACFCC is scheduled ries and dry-sieved using 3”, 1.5”, ¾”, 3/8”, #4, #10, #20, #40, #60, #100, and D50 as beach nourishment and wetland restoration. This information is not only applicable to bed. The sediment sample was then transferred 25 7c for de-silting this year, and Old Alameda Creek #200 sieves. Samples from OAC and SLC were analyzed by ENGEO Inc. and D15 from the Ponar sampler to a stainless steel bowl management of each flood control channel, but also District-wide and region-wide man- (OAC) has likely been de-silted in the past as well. dry-sieved using the same size sieves, as well as a hydrometer that deter- where it was mixed with a stainless steel spoon until 20 Data on sediment grain size is important for model- mined grain size down to 0.0012 mm.