Presentation on theme: "WATERSHED – BAY INTERACTIONS: ANTHROPOGENIC INFLUENCES ON SEDIMENT SUPPLY, TRANSPORT, AND STORAGE Laurel Collins, Watershed Sciences, Berkeley, CA"— Presentation transcript:
WATERSHED – BAY INTERACTIONS: ANTHROPOGENIC INFLUENCES ON SEDIMENT SUPPLY, TRANSPORT, AND STORAGE Laurel Collins, Watershed Sciences, Berkeley, CA
Watersheds adjacent to the San Francisco Bay proper that provide sediment directly to the bay and tidal wetlands comprise the Bayshed. Sediment routing to the Bay has been significantly altered in every watershed and no two are alike. I will compare two watersheds and start with the Sonoma watershed, which is comprised of four significant parts.
2005 SONOMA TIDAL MARSHLANDS SEDIMENT TRAP ANALYSIS CONDUCTED TO COMPARE TO THE SEDIMENT SOURCE ANALYSIS 2005 UPLAND SEDIMENT SOURCE ANALYSIS WAS CONDUCTED FOR A TMDL (Total Maximum Daily Load) 127 sq mi (not including marshlands) Carneros 6.9 sq mi Schell 21.0 sq mi Former extent of historical Sonoma Marsh prior to reclamation: 23 sq mi
The intent of the Sediment Source Analysis was to identify the major types of sediment sources in the watershed, estimate rates of sediment supply for each source between the years 1800 and 2005, and report on the results as the basis for a sediment reduction implementation plan to be developed by the California Regional Water Quality Control Board (RWQCB). Four approaches were taken for the source analysis.
(1) An historical ecology assessment coupled with a Revised Universal Soil Loss Equation (RUSLE) model was used to predict erosion from land surfaces throughout the watershed both pre- and post-European impact (Sonoma Ecology Center, Talon Associates LLC, and Tessera Consulting). (2) Field measurement of erosional voids within channels and adjacent bank sources (hillside dry ravelling, soil creep, landslides) was conducted to determine long-term sediment supply. Supply rates were stratified by different geomorphic units and correlated to drainage network length to predict erosion from unmeasured sites. Stereo aerial photos were also analyzed for landslides and channel migration (Watershed Sciences). (3) A SEDMODL analysis of sediment contributions was conducted to determine seidment supply from roads (Martin Trso, P.G.). (4) An empirical check was conducted through suspended sediment and turbidity sampling in mainstem Sonoma Creek and tributaries (Sonoma Ecology Center).
Morphologic Units Map Morphologic units were based on geology and geomorphological characteristics of the valley Stream network length was drawn in detail from aerial photos and cumulatively measured at each channel node, at each geomorphic unit, and at stream crossings for the individual sediment supply correlation analysis. A critical element was mapping of ditches (shown in pink) and storm drains (shown in orange). As a result of land use, drainage density (channel length per unit area) increased by: 10% Sonoma drainage 13% Carneros drainage area 47% Schell drainage area. The significance is that the mainstream channel must adjust its hydraulic geometry (width, depth) to accommodate flashier runoff. This means more sediment production from the channel as its widens and /or incises. Most Bay Area channels have done this as they have adjusted to modern and legacy land use.
Good correlations of sediment supply to channel network length for most of the geomorphic units predicted 68,650 tons/year of sediment from channel beds, banks, and adjacent hillsides in the greater Sonoma watershed (includes Schell, and Carneros watersheds). The channels were the biggest suppliers of sediment to the watershed. Cumulative erosion of streambed and banks due to anthropogenic causes comprised about 66 percent of the sediment load in the greater Sonoma watershed, an additional 5-10 percent is from anthropogenic causes associated with roads and agricultural fields.
The pre 1800 surface erosion rate was determined to be 169,000 tons/year, of which 5 percent was estimated to be delivered to the stream (8,400 tons/year). The modern 2005 surface erosion rate is 313,000 tons/year, also with a 5 percent delivery ratio (15,900 tons/year). Sediment delivery from roads due to road tread cut and fill slope erosion supplies 5,600 tons/year, or 5 tons per mile of road. Suspended sediment and turbidity sampling ranged from 1 to 2 tons/ac/year, or 106,239 to 212,479 tons/year.
Adding all the individual sediment sources, total sediment loading to Sonoma Creek and tributaries equals 111,750 tons/year. Results indicate the estimated total sediment supply is more than twice that of natural background, predicted to be about 32,000 to 55,000 tons/year. Notice that this is overbank sedimentation at the upland tidal transition zone. The deposits contain abundant organics and sediment up to coarse sand size. The overbank deposits form natural levees that help transport coarse bedload into the sloughs. Strong ebb currents used to move the bedload through the system before the tidelands were diked. The proposed waste-load allocation called for by the Regional Board for channel and upland sediment sources is 8,100 tons per year, a called- for load reduction of 35,200 tons/year from anthropogenic-related sediment sources.
Today, the tidal sloughs have lost significant capacity. The tidal channels have narrowed and shallowed. There is frequent backwater flooding when high tides coincide with upland floods. By the 1890’s dredging of the main Sonoma Slough was needed on a daily basis, every day of the year to keep the tidal channel open for shallow draft scows to navigating tidal sloughs to bring hay to San Francisco. By the 1920’s, dredging for navigation was no longer considered useful because commerce relied upon trains and automobiles. Levee construction started in the 1860’s along the tidal sloughs in Sonoma Marsh. By the 1880’s nearly all the tidal marshlands were removed from tidal processes (reclaimed). 1850s map incomplete of tideland details in Sonoma Marsh The analysis of fine sediment deposition in downstream marshes provided a check of both predicted and measured sources of sediment.
The tidal channel width has changed from historically about 100 ft wide by about 10 ft deep to 20 feet wide by 3 feet deep where a debris jam lodged 7 years ago.
The amount of sediment contained in the levees and the amount of overbank deposition within flooded reclaimed lands that had known levee breaches also had to be determined. We expect that about 80% (+/- 30%) of the total sediment load (mostly suspended and sand load) has been captured. It is unclear what the fate is of most of the coarse bed load. The tides also provide sediment to the remaining marsh and tidal sloughs. Minimal data exists however that pertains to the proportion of sediment that is supplied by the tides versus the terrestrial. For a different project in Alameda Creek Flood Control Channel in the southeast Bay, I determined that the tides provide about 22% of the total sediment supply to the tidal channel segment. In tidal flow it is important to note that a large portion of the tidal supply is simply reworked sediment from the local uplands. For the tidal marsh changes, we assumed that the tidal landscape started to respond to land use in 1841 when the first hydrographic surveys of the channels were conducted. 163 years was used for calculating rates of sediment supply. Other hydrographic and nautical charts, published reports, and aerial photos were used to establish historic channel widths and/or depths and the older levees often demarcate historic channel width.
Measurements of sediment infilling, constructed levees, and sedimentation from levee breaks indicated 105,937 tons/year +/- 30%. This estimate is equivalent to 1.3 tons/ac/year supplied to the marsh by upstream sources in the watershed. The upland sediment source analysis determined 111,750 tons/year. For the Sonoma tidal sloughs, we assumed that 25% of the total sediment was derived from tidal rather than upland sources. This includes re- suspended sediment from adjacent mudflat erosion and mixed suspended sediment from Sonoma and other watersheds. No more than 5% of the trapped tidal sediment is expected to have originated from sources outside of Sonoma watershed. To determine the total load supplied from the Sonoma uplands, 5% of the total deposited amount within the marsh, sloughs, and flooded areas was subtracted from the total amount deposited.
This represents a potential landscape lowering of the greater Sonoma Watershed of 0.16 to 0.29 mm/yr, or an average of 0.22 mm/yr. This is more than the likely average tectonic uplift rate or possible isostatic rebound rate from reduced sea level stand of the geologic past. Bob McLaughlin (USGS), considers the background uplift rate for the northern Sonoma Mountains to be about 0.3 mm/yr. If this was averaged for the entire watershed (especially considering that the northern mountains represent about one third of the watershed, while the southern third of the watershed appears to be subsiding), the overall average uplift rate might be about one third of the northern rate. We assumed about 0.10 mm/yr. Hypothetically, if natural landscape erosion kept pace with average tectonic uplift, our 0.22 mm/yr of landscape-lowering would indicate that sediment supply rate to the Sonoma marshlands exceeds the average tectonic uplift rate by about 120%, +/- 30%. The excess amount is a likely an anthropogenic signal caused by land use impacts. The present proportion of human-related sediment determined by this sediment trap analysis is 54% +/- 30%. This is well within the range of anthropogenic influences reported for other Bay Area watersheds.
Alameda Creek Watershed Historically the drainage area was at least 640 sq mi but now the functional drainage area is 309 sq mi because it has been influenced by large dams.
2. Historical Sediment Distribution and Channel Connectivity Bay Arroyo de la Laguna Watershed Upper Alameda Cr. Watershed Alameda Fan Niles Canyon Toe of Fan Livermore – Amador Valley Calaveras Valley Sunol Valley Tulare Lake Tidal Marshes Sand & Silt on Floodplain and Fans Silt & Clay on Floodplain, Fans & Marshes Coarse Sediment in Valleys and Fans
Sandy delta deposits of Arroyo Honda extent up to 20 ft and crossed the lake prior to the water being drawn down for retrofitting of the Calaveras Reservoir Dam. 2004
Of the 60% of the total sediment load transported past Niles Gage that is deposited in the Alameda Creek Flood Control Channel, 34% has been removed by desilting activities. The remaining 40% of the total load that is not deposited in the Flood Control Channel represents 50,136 cu yd/yr supplied to the bay. Spread over one sq mi, its rate of deposition would be 0.6 in/yr (15 mm/yr).
Old Alameda Creek Alameda Creek Flood Control Channel Petaluma River Sonoma Creek San Lorenzo Creek, Hayward Amount to South Bay is equivalent to 15 mm/yr spread over 1 mi 2. Amount to San Pablo Bay is equivalent to 18 mm/yr spread over 1 mi 2.
San Lorenzo Creek, north of Alameda Creek Significant capture of sediment in most of its tributaries by dams but flood control channel transport most of the sediment; it does not need dredging. As a result, a delta fan of greater significance has formed at its outlet unlike Sonoma and Alameda Creek Drainage area = 11 sq mi
These watershed examples should provide food for thought for future considerations on sediment management. How will TMDL load reductions or large stream restoration projects influence sediment rates to the Baylands? More storm drains, development, ditches = more drainage density = increased mainstem channel adjustments = more sediment supply (for a while), but where will it end up? If flood control channels leading to the Bay margin were redesigned as 4-stage channels that reduce the need for dredging and improve ecological and fluvial processes for different flow stages of low flow, 1.5-yr RI (bankfull), 25-yr RI, and 100-yr RI floods, would increased sediment supply to the Baylands be beneficial or need to be managed? If tidal sloughs were restored by pulling levees, in Sonoma Marsh or the South Bay for example, would the increased tidal prism erode sufficient sediment that has been deposited between the levees to restore build up formerly reclaimed marsh surfaces and help protect upland infrastructure during rising sea level? Removed levee soils could be used to help build marsh surfaces or develop natural shaped levees that would augment bedload transport.