Class Results - Mystery Soil #1 14 12 10 8 6 4 2 0 Sandy Clay Loam Sandy Loam Silty Loam Class Results - Mystery Soil #2 6 5 4 3 2 1 0 Silty Clay Clay Loam Sandy Sandy Silty Sandy Silty Loamy Loam Clay Loam Clay Clay Sand Loam Loam URBAN SOILS & SEATTLE EXAMPLES ESRM 311 / SEFS 507 Week 4 1
Urban Soils Outline What are urban soils? How they differ from unmodified soils Physical properties Hydrologic cycle Hazards Denny re-grade Earthquake hazards Example of Urban Soil Magnuson Park Soil Pits How would you define an Urban Soil? 2
Urban Soil Bockheim (1974) gives an appropriate and useful definition of urban soil: A soil material having a non-agricultural, manmade surface layer more than 50 cm thick, that has been produced by mixing, filling, or by contamination of land surfaces in urban and suburban areas Soil Physical Properties Horizons Structure Texture Color Moisture Temperature Granular Blocky Prismatic Columnar Platy Single Grained 3
Idealized Soil Profile Layered characteristics Example forest O horizon - Litter layer A horizon Organisms or gravity Plant root growth E horizon Organic acids through leaching B horizon Clays accumulate C horizon Parent material Urban Soil Characteristics Urban Soil Great vertical and spatial variability Modified soil structure: compaction Waste materials and other contaminants Restricted aeration and water drainage Interrupted nutrient cycling and modified soil organism activity Surface crust; usually hydrophobic Modified temperature High ph 4
Soil Mixing Slope that was back filled Original profile covered Organic layers covered (13) Fill materials used Refuse, Clayey fill Would you build a house on the fill? Cold weather Organic matter Structure and Compaction Disturbed, displaced or compacted Reduce pore space and increase bulk density Low organic matter Soil organisms activity less Low frequency wet-dry or freeze-thaw Less aggregation and structure formation Compressive forces Vehicle, water, human or paving Reduced vegetative cover Bare soil, erosion, crust formation 5
Crusting and ph Crusting Precipitation on bare or exposed soil sorts the finer matter and fills small pores Impervious surface Barrier for plant growth ph Ca increase soil ph Sources: De-icing compounds Weathering building rubble Calcium-rich water in gardens Water movement and Aeration Urban environment limits aeration and infiltration Water movement restriction Mixing of layers Impervious surface 6
Hydrologic Cycle in Urban Areas Relationship between impervious cover and surface runoff. Impervious cover in a watershed results in increased surface runoff. As little as 10 percent impervious cover in a watershed can result in stream degradation. 30% evapotranspiration 40% evapotranspiration 55% runoff 10% runoff 10% shallow infiltration 75-100% Impervious Cover 5% deep infiltration 25% shallow infiltration Natural Ground Cover 25% deep infiltration http://water.epa.gov/polwaste/nps/urban_facts.cfm Lack of OM & Decomposition Water movement is restricted Little aeration Limited root zone Less favorable for organisms 7
Denny Re-grade Removal of Denny Hill Denny Re-grade 8
Denny Re-grade Hazards Earthquake hazards Liquefaction: Water-saturated sandy soils that lose strength during earth shaking 9
Hazards Earthquake hazards Site Class National Earthquake Hazards Reduction Program (NEHRP), site class where soils amplify ground shaking Urban environment affects soil? Covering of soil Disturbed water cycle Nutrient and decomposition limited Horizon mixing 10
Example Urban Soil Magnuson Park 1855: surveyed alder grove, Douglas fir forest with trees 2-6 feet, Cedar, streams and wetlands Mid 1930 s: conversion to military base, trucked in hundreds of loads of fill, Mud lake, the marsh and Pontiac Bay covered Current Day: restoration of the park Magnuson Park Site Characteristics 1953 1927 2011 11
Soil Pits Dug three soil pits 1. Forested Area 2. Cottonwoods 3. Grassland Characteristics Horizons Bulk density ph Structure Sites and Pits Site 1 Site 2 Site 3 12
Survey Results Site Horizon Type Horizon Depth (inches) Texture Moisture Content ph Structure 1 A 29-25.5 Clay Loam 29.7% 5.41 Crumb Structure (small rock or shot Fe) 1 Bts 25.5 14 Sandy Clay 21.2% 5.00 Clay Crumb 1 Bts2 14 6 Clay Loam 82.1% 4.75 Saturated Clay (plastic limit) 2 A 28 25 Silt Clay 46.0% 5.32 Organic with rocks 2 B1 25 17 Sandy Loam 25.2% 5.45 Fill with Fingering 2 B2 17 12 Silt Clay 26.6% 5.80 Clay with Moddles 2 B3 12 0 Loamy Sand 28.0% 6.07 Sand with Organic Matter 3 A 23 17 Clay Loam 31.0% 5.37 Crumb Structure 3 Bts 17-0 Silt Clay 27.9% 5.95 Blocky Bulk Density Site Horizon Moisture Volume Content (cm 3 ) Bulk Density (g/cm 3 ) 1 Bts 26.5% 177.11 1.39 2 B1 13.1% 68.70 2.45 3 Bts 16.2% 68.70 2.10 Magnuson Park Vegetation indicator of underlying soils Horizon development Natural horizon, moved? (site 1) Water logged (site 2) Hill area (site 3) Poor drainage, site 2 water table 2 feet High bulk density Trees dying off around site 2 13
Urban Soils Urban soils vs. idealized profile Characteristics Variable horizons Modified soil structure Compaction Waste materials mixed in Interrupted nutrient cycling Low soil organism activity High ph Local examples Denny Hill re-grade Magnuson Park 14