Introduction
Study area
Paleogeographic evolution
Hydrogeology
Historic development
Methods
Hydrogeologic reconstruction
Compaction estimates
Subsidence vulnerability map
Results
Subsurface composition
North of Metairie-Gentilly Ridge
Metairie-Gentilly Ridge
South of Metairie-Gentilly Ridge
Peat compaction
Area | Borehole No. | Depth range (m) | LOI (%) | Measured dry bulk density (g/cm3) | Estimated uncompacted bulk density (g/cm3) | h0 (cm) | Calculated compaction (%) |
---|---|---|---|---|---|---|---|
South of Metairie-Gentilly Ridge | 018 | 103–108 | 48 | 0.36 | 0.19 | 9.24 | 46 |
027 | 251–256 | 52 | 0.28 | 0.17 | 7.91 | 37 | |
027 | 340–345 | 58 | 0.24 | 0.15 | 7.94 | 37 | |
027 | 375–380 | 56 | 0.23 | 0.16 | 7.24 | 31 | |
031 | 336–341 | 24 | 0.46 | 0.33 | 6.89 | 27 | |
032 | 252–257 | 45 | 0.26 | 0.20 | 6.43 | 22 | |
032 | 295–300 | 29 | 0.32 | 0.29 | 5.47 | 9 | |
034 | 257–262 | 22 | 0.44 | 0.34 | 6.34 | 21 | |
041 | 290–295 | 53 | 0.24 | 0.17 | 7.09 | 30 | |
042 | 275–280 | 38 | 0.29 | 0.24 | 6.01 | 17 | |
042 | 160–165 | 27 | 0.36 | 0.31 | 5.74 | 13 | |
045 | 250–255 | 23 | 0.41 | 0.34 | 6.04 | 17 | |
054 | 285–290 | 50 | 0.25 | 0.18 | 6.93 | 28 | |
North of Metairie-Gentilly Ridge | 005 | 139–144 | 28 | 0.45 | 0.32 | 7.07 | 29 |
005 | 103–108 | 65 | 0.30 | 0.13 | 11.76 | 57 | |
050 | 172–177 | 41 | 0.59 | 0.22 | 13.17 | 62 | |
051 | 75–80 | 26 | 0.58 | 0.31 | 9.23 | 46 | |
051 | 113–118 | 44 | 0.24 | 0.21 | 5.81 | 14 | |
051 | 183–188 | 36 | 0.33 | 0.25 | 6.54 | 24 | |
056 | 175–180 | 35 | 0.33 | 0.26 | 6.29 | 20 | |
057 | 127–132 | 23 | 0.43 | 0.34 | 6.39 | 22 | |
058 | 190–195 | 20 | 0.50 | 0.36 | 6.92 | 28 | |
064 | 115–120 | 51 | 0.43 | 0.18 | 12.25 | 59 | |
069 | 115–120 | 20 | 0.64 | 0.36 | 8.94 | 44 | |
072 | 175–180 | 67 | 0.24 | 0.12 | 9.62 | 48 | |
072 | 195–200 | 29 | 0.40 | 0.29 | 6.77 | 26 |
Peat above LGL (cm) | LOI | LOI minimum (%) | LOIlost (%) | Subsidence (cm) |
---|---|---|---|---|
10 | 20 | 10 | 10 | 1 |
30 | 20 | 10 | 10 | 3 |
50 | 20 | 10 | 10 | 5 |
70 | 20 | 10 | 10 | 7 |
10 | 40 | 10 | 30 | 3 |
30 | 40 | 10 | 30 | 9 |
50 | 40 | 10 | 30 | 15 |
70 | 40 | 10 | 30 | 21 |
10 | 65 | 10 | 55 | 6 |
30 | 65 | 10 | 55 | 17 |
50 | 65 | 10 | 55 | 28 |
70 | 65 | 10 | 55 | 39 |
A | B | C | D | E | F |
---|---|---|---|---|---|
Peat below LGL (cm) | Current compaction (%) | Initial thickness (cm) | Max potential compaction (%) | Thickness after 70% compaction (cm) | Potential subsidence (cm) |
10 | 10 | 11 | 70 | 3 | 7 |
50 | 10 | 56 | 70 | 17 | 33 |
100 | 10 | 111 | 70 | 33 | 67 |
250 | 10 | 278 | 70 | 83 | 167 |
10 | 30 | 14 | 70 | 4 | 6 |
50 | 30 | 71 | 70 | 21 | 29 |
100 | 30 | 143 | 70 | 43 | 57 |
250 | 30 | 357 | 70 | 107 | 143 |
10 | 60 | 25 | 70 | 8 | 3 |
50 | 60 | 125 | 70 | 38 | 13 |
100 | 60 | 250 | 70 | 75 | 25 |
250 | 60 | 625 | 70 | 188 | 63 |
Lithological sequence types
Litho-sequence No. | Lithological sequence type | Description | Subsidence vulnerability |
---|---|---|---|
1 (red) | Clay, organic clay, (decomposed) peat on sand/loam. 1a. sand > 150 cm below surface 1b. sand < 150 cm below surface | Floodplain deposits (mainly C–SiC–SiCL) that are partly organic, intercalated with gyttja and peat layers that are partly decomposed (crumbly, no fibers), on top of sandy and/or loamy deposits of the Pine Barrier Island. The clay is often soft. Decomposed peat thickness <70 cm (within 160 cm below surface), total peat thickness <100 cm | Vulnerable to subsidence due to oxidation and compaction (in general, the maximum subsidence potential is less for 1a compared to 1b, due to a thinner layer of the soft/organic material above the sand) |
2 (red) | Clay, organic clay and peat, intercalated | Floodplain deposits (C–SiC–SiCL–CL) that are partly organic, intercalated with peat and gyttja layers. Clay layers are often soft. Peat occurs in most cases deeper than 150 cm below surface, with a maximum total thickness of 160 cm | Vulnerable to subsidence due to compaction |
3 (orange) | Clay on organic clay and peat | Floodplain deposits (>200 cm thick, consisting of C–SiC–SiCL–CL) on organic clay and peat. Total peat thickness <40 cm | Vulnerable to subsidence due to compaction. Less vulnerable than sequence type 2 because the peat is generally thinner and more compacted already |
4 (orange) | Clay and silt on organic clay | Natural levee deposits (>100 cm, C–SiC–SiCL–CL–L) on organic floodplain deposits (C) that may be soft and may include thick (~10 cm) wood fragments, as well as thin (cm) peaty layers | Vulnerable to subsidence due to compaction. Less vulnerable than sequence type 3 because only thin peat layers occur |
5 (orange) | Clay on silty/sandy deposits on organic clay | Floodplain deposits (100–200 cm; C–SiC–SiCL–C) on levee/crevasse deposits (LS–SL–S) on clay (C–SiC–SiCL) that is partly organic (thin layers of peaty clay/gyttja may occur) | Vulnerable to subsidence due to compaction. Somewhat less susceptible than sequence type 4 because the overburden is heavier (including sandy layers), hence more compaction has already occurred |
6 (orange) | Clay | Floodplain deposits (C–SiC–SiCL–CL). Not organic, include both soft and firm clay intervals | Vulnerable to subsidence due to compaction of soft clay layers |
7 (green) | Clay, silt and sand mixtures/intercalations | Natural levee and/or crevasse splay deposits (SiC–SiCL–CL–SiL–SL–L–LS). Predominantly loamy deposits. Organic clay intervals may occur | Some clay intervals may be vulnerable to subsidence due to compaction |
8 (green) | Clay and silt on sand/loam | Natural levee and/or floodplain deposits (C–SiC–SiCL–CL–SiL–L) on Pine Barrier Island sand or other sandy loamy deposits (e.g. top of channel deposits). Depth of sand varies between 50 and 470 cm below surface | Low vulnerability to shallow subsidence |
9 (green) | Fill | Anthropogenic fill consists mainly of sand but may also include clayey to silty layers | Low vulnerability to shallow subsidence |
Hydrogeology
Zone no. (recharge/discharge area) | Zone description | General characteristics | (Ground-)water flow |
---|---|---|---|
1 (recharge) | Mississippi River | The navigable channel is 12–15 m deep, with local depths of up to 60 m. The water level at New Orleans city is on average 3.5 m above mean sea level. The lowest and highest river water levels measured are ~0.3 and 6 m NGVD (National Geodetic Vertical Datum of 1929; RiverGages.com). These water levels are always higher than the groundwater levels in the adjacent urban zones | The sandy Mississippi channel body is an important (freshwater) infiltration area for the urban shallow and deeper groundwater system of New Orleans. The relative high river level determines the hydraulic head in the silty and clayey natural levee zone along the river |
2 (recharge) | Mississippi natural levee zone | The natural levee zone adjacent to the river is elevated up to ~5 m above mean sea level and is ~1.5–3 km wide and generally covered by a (silty) clay layer. Northwards, the surface level gradually falls. | Local infiltrated rainfall is drained by the leaking sewer and storm drainage pipes. Perforation of the covering clay layer that occurred during construction activities may cause groundwater flooding, especially during high river water level (only close to the relatively high Mississippi level) |
3 (discharge) | Lower basin-shaped (‘the Bowl’) area between Mississippi levee and Metairie-Gentilly ridge | This zone is partly below mean sea level. The soil consists mainly of peaty freshwater marsh deposits. The ‘bowl’ morphology has increased through time due to subsidence, causing this zone to be vulnerable to urban flooding | This is mostly a groundwater discharge area. Infiltrated rainwater, drinking water loss, and groundwater discharge are drained by leaking wastewater and storm drainage pipes |
4 (recharge) | The Metairie-Gentilly Ridge | An abandoned natural river channel that is visible as a long ridge in the landscape but forms a barrier for surface-water drainage of the area to the south | This zone acts as a (shallow) groundwater infiltration area. Infiltrated rain (and drinking water) flows largely in the northern direction and exfiltrates in the southern part of zone 5 |
5 (discharge) | Lakeview, Gentilly, New Orleans NO | Located largely below sea level with peat-like ground and sandy Pine Barrier deposits located mostly below the surface. In predrainage times, most parts of this zone were covered by freshwater cypress swamps. The north part was salt-brackish marsh. The area subsided several meters during urbanization over the last 100 years, especially directly after the onset of drainage | A groundwater discharge area. Along the canals and Lake Pontchartrain this groundwater discharge is brackish-salt. Infiltrated rainwater, drinking water loss, and groundwater discharge is drained by leaking wastewater and storm drainage pipes. In New Orleans East, open canals also support groundwater drainage. This zone is highly vulnerable to pluvial flooding |
6 (recharge) | Lake Pontchartrain, Bayou St. John, Inner Harbor Canal and outfall canals | Lake Pontchartrain is 3–4 m deep on average. It is linked directly to the Gulf of Mexico and thus is brackish/saline (2,500–5,500 mg/L chloride). The city’s (outfall) canals are in open connection with the lake | Lake and canals are infiltrating bodies. Brackish salt groundwater flows towards the city. Most of this water is drained by the leaking pipes below the streets adjacent to the shore and canals and back into the lake |
Discussion
Subsidence and subsidence vulnerability
Area north of Metairie-Gentilly Ridge
Area south of Metairie-Gentilly Ridge
Land subsidence in relation to (geo)hydrology
Perspectives for subsidence-sensitive urban planning
Conclusions
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A substantial part of the surficial peat occurring north of the MG Ridge has been oxidized since the 1950s, driving average subsidence rates of 30 mm/year. The total remaining peat thickness in this area is generally <1 m. Peat above the average lowest groundwater level is largely decomposed but may still have a relatively high organic content. Peat occurs above, at, or just below the average lowest groundwater level, making this area still vulnerable for future subsidence due to peat oxidation and, to a somewhat lesser extent, peat compaction due to loading and/or a drop in pore water pressure following groundwater level lowering.
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In large parts of the area south of the MG Ridge, peat occurs at greater depths below surface. This area is therefore especially vulnerable to subsidence due to peat compaction and is less prone to subsidence due to peat oxidation as compared with the area north of MG Ridge. Here, subsidence rates of ~35 mm/year have occurred over the last 70 years.
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Areas where the subsurface contains abundant soft clays, possibly with intercalated peat layers, remain vulnerable to subsidence due to compaction. Sandier areas are less vulnerable to subsidence and in turn are assigned a lower risk level category.
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In the city of New Orleans, peat has been compacted ~31% on average, with a range of 9–62%. Continued subsidence due to peat compaction is expected.
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During dry periods, average groundwater levels drop to ~150 cm below surface levels and during wet periods, average groundwater levels increase to ~50 cm below surface.
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Past and present water management in the city of New Orleans has been an important factor contributing to shallow land subsidence: drainage for logging and city development, and drainage by (leaking) stormwater drainage, wastewater, and drinking water pipes have resulted in land subsidence. Subsequently, land subsidence affected the hydrological system by reversing the groundwater flow direction and thereby causing salinization of the deeper groundwater level.
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This study is a crucial first step to address urban land subsidence, and provides support for decision makers in dealing with subsidence risks.