MLG to MLLW Vertical Datum Conversion

Abstract: The U.S. Army Corps of Engineers, New Orleans District is converting the vertical datum for all coastal navigation projects from Mean Low Gulf (MLG) to Mean Lower Low Water (MLLW) in accordance with USACE (2014), memorandum directing conversion from USACE HQ. This memorandum describes new policy for federal navigation projects where the decision documents supporting project authorization and the project authorization in law do not reference the Mean Lower Low Water (MLLW) datum.

This report documents the calibration / relationship between the localized MLG as used in practice for this project and the NOAA-maintained MLLW that provides for no impact in current project channel maintenance practices. The report provides details on how this relationship was determined and will be applied.

The North American Vertical Datum of 1988 (NAVD88) is a geodetic datum that is defined and maintained by the National Geodetic Survey (NGS). This datum is typically used for surveying (in addition to design and construction) and can be related to other datums as needed, to ensure project datums are referenced as required.

Mean Lower Low Water (MLLW) is a tidal datum that is defined and maintained by the National
Oceanic and Atmospheric Administration (NOAA). This tidal datum is defined as the average of the lowest of the two daily low water heights observed over the National Tidal Datum Epoch (which spans a 19 year period).

Mean Low Gulf (MLG) is a legacy terrestrial datum that was originally defined relative to local
mean sea level as observed at the Biloxi gage in 1899 in the Gulf of Mexico. It has been used as a navigation (and construction) reference datum in coastal waterways such as the Gulf Intracoastal Waterway (GIWW) and the coastal portion of the Mississippi River navigation channel (Reference 2). MLG was intended to represent the low water level of the Gulf of Mexico, and was defined by District memorandum in 1944 as being 0.78 feet below local mean sea level as it was understood at that time. The origin of the 0.78 foot offset between MLG and LMSL is not known precisely; however, this value is half the tidal range at the Biloxi (Cadet Point) Tidegage (NOAA gage 8743735). Therefore, MLG as defined is equal to Mean Low Water (MLW) at Biloxi. Mean Low Water is the average of all low tides, whereas Mean Lower Low Water is the average of only the lower of the two daily low tides. The Gulf of Mexico has diurnal tides (one low tide per day), so the difference between MLW and MLLW is academic. At Biloxi, the two are approximately one tenth  of one foot apart, which is beyond the precision of either dredging or hydrographic
surveying. Consequently, it seems very likely that MLG was intended to represent the average low tide condition in the Gulf of Mexico, so that a given draft in MLG would be, on average, navigable during low tide. Mean Lower Low Water is presently 0.46 ft. below local mean sea level at Pilottown, LA, and 0.6 ft. below local mean sea level at Pilot’s Station East (at the mouth of Southwest Pass).2

Therefore, in theory, MLG and MLLW are essentially equal as they are within 0.2 - 0.4 ft, as related to MSL. The intent and application of MLLW and MLG were and are, also in theory, defined to represent the same water condition; as a tidal datum of a lowest daily water level that will be typically observed for that location. 

LIST OF ACRONYMS
- AHP: Above Head of Passes
- BHP: Below Head of Passes
- CEPD: Comprehensive Evaluation of Project Datums
- EDR: Engineering Documentation Report
- HQ: Headquarters office of the United States Army Corps of Engineers
- IPET: Interagency Performance Evaluation Task Force
- LCA: Local Cooperation Agreement
- LWRP: Low Water Reference Plane
- MLG: Mean Low Gulf datum, as a historic reference
- MLGSWP: In practice usage of Mean Low Gulf as localized for this project
- MLLW: Mean Lower Low Water datum
- MVN: Mississippi Valley New Orleans District
- NAVD88: North American Vertical Datum of 1988
- NOAA: National Oceanic and Atmospheric Administration
- NWLON: National Water Level Observation Network
- OPUS: Online Positioning User Service
- SWP: Mississippi River Southwest Pass
- USACE: United States Army Corps of Engineers
- USACE MVN: United States Army Corps of Engineers New Orleans District

Citation: US Army Corps of Engineers New Orleans District (2016) MLG to MLLW Vertical Datum
Conversion. Engineering Documentation Report EDR-OD-01, Prepared by: US Army Corps of Engineers New Orleans District Engineering
Division, 02 November 2016.

URL: http://www.mvn.usace.army.mil/Portals/56/docs/Navigation/EDRs/EDR-%20SWP-%20EDR-OD-01__2016-11-02.pdf

Rapid attribution of the August 2016 flood-inducing extreme precipitation in south Louisiana to climate change

Abstract: A stationary low pressure system and elevated levels of precipitable water provided a nearly continuous source of precipitation over Louisiana, United States (U.S.) starting around 10 August, 2016. Precipitation was heaviest in the region broadly encompassing the city of Baton Rouge, with a three-day maximum found at a station in Livingston, LA (east of Baton Rouge) from 12–14 August, 2016 (648.3 mm, 25.5 inches). The intense precipitation was followed by inland flash flooding and river flooding and in subsequent days produced additional backwater flooding. On 16 August, Louisiana officials reported that 30,000 people had been rescued, nearly 10,600 people had slept in shelters on the night of 14 August, and at least 60,600 homes had been impacted to varying degrees. As of 17 August, the floods were reported to have killed at least thirteen people. As the disaster was unfolding, the Red Cross called the flooding the worst natural disaster in the U.S. since Super Storm Sandy made landfall in New Jersey on 24 October, 2012. Before the floodwaters had receded, the media began questioning whether this extreme event was caused by anthropogenic climate change. To provide the necessary analysis to understand the potential role of anthropogenic climate change, a rapid attribution analysis was launched in real-time using the best readily available observational data and high-resolution global climate model simulations. The objective of this study is to show the possibility of performing rapid attribution studies when both observational and model data, and analysis methods are readily available upon the start. It is the authors aspiration that the results be used to guide further studies of the devastating precipitation and flooding event. Here we present a first estimate of how anthropogenic climate change has affected the likelihood of a comparable extreme precipitation event in the Central U.S. Gulf Coast. While the flooding event of interest triggering this study occurred in south Louisiana, for the purposes of our analysis, we have defined an extreme precipitation event by taking the spatial maximum of annual 3-day inland maximum precipitation over the region: 29–31º N, 85–95º W, which we refer to as the Central U.S. Gulf Coast. Using observational data, we find that the observed local return time of the 12–14 August precipitation event in 2016 is about 550 years (95 % confidence interval (C.I.): 450–1450). The probability for an event like this to happen anywhere in the region is presently 1 in 30 years (C.I. 11–110). We estimate that these probabilities and the intensity of extreme precipitation events of this return time have increased since 1900. A Central U.S. Gulf Coast extreme precipitation event has effectively become more likely in 2016 than it was in 1900. The global climate models tell a similar story, with the regional probability of 3-day extreme precipitation increasing due to anthropogenic climate change by a factor of more than a factor 1.4 in the most accurate analyses. The magnitude of the shift in probabilities is greater in the 25 km (higher resolution) climate model than in the 50 km model. The evidence for a relation to El Niño half a year earlier is equivocal, with some analyses showing a positive connection and others none.

Citation: van der Wiel, K., S.B. Kapnick, G.J. van Oldenborgh, K. Whan, S. Philip, G. A. Vecchi, R.K. Singh, J. Arrighi, and H. Cullen (2017) Rapid attribution of the August 2016 flood-inducing
extreme precipitation in south Louisiana to climate change.” Hydrol. Earth Syst. Sci., 21,
897–921.

URL: https://www.hydrol-earth-syst-sci.net/21/897/2017/hess-21-897-2017.pdf

Characterization of peak streamflows and flood inundation of selected areas in Louisiana from the August 2016 flood

Report cover.

Abstract: Heavy rainfall occurred across Louisiana and southwestern Mississippi in August 2016 as a result of a slow-moving area of low pressure and a high amount of atmospheric moisture. The storm caused major flooding in the southern portions of Louisiana including areas surrounding Baton Rouge and Lafayette. Flooding occurred along the rivers such as the Amite, Comite, Tangipahoa, Tickfaw, Vermilion, and Mermentau Rivers. Over 31 inches of rain was reported in the city of Watson, 20 miles northeast of Baton Rouge, La., over the duration of the event. Streamflow-gaging stations operated by the U.S. Geological Survey (USGS) recorded peak streamflows of record at 10 locations, and 7 other locations experienced peak streamflows ranking in the top five for the duration of the period of record. In August 2016, USGS hydrographers made 50 discharge measurements at 21 locations on streams in Louisiana. Many of those discharge measurements were made for the purpose of verifying the accuracy of stage-streamflow relations at gaging stations operated by the USGS. Following the storm event, USGS hydrographers recovered and documented 590 high-water marks, noting location and height of the water above land surface. Many of these high-water marks were used to create 12 flood-inundation maps for selected communities of Louisiana that experienced flooding in August 2016. Digital datasets of the inundation area, modeling boundary, water depth rasters, and final map products are available online.

Citation: Watson, K.M., Storm, J.B., Breaker, B.K., and Rose, C.E., 2017, Characterization of peak streamflows and flood inundation of selected areas in Louisiana from the August 2016 flood: U.S. Geological Survey Scientific Investigations Report 2017–5005, 26 p., https://doi.org/10.3133/sir20175005.

URL: https://pubs.er.usgs.gov/publication/sir20175005

High Water Elevations on the Vermilion River During the Flood of August 2016

Abstract: Flooding from locally heavy rainfall in the Vermilion River Basin of Louisiana in August of 2016 caused extensive property damage. Following the flood, the United States Geological Survey (USGS) in cooperation with the Federal Emergency Management Agency (FEMA) surveyed high water marks in areas across South Louisiana which experienced flooding. This paper presents and discusses analyses of those high water data from the Vermilion River Basin, and provides a number of conclusions.

Acknowledgements: The author gratefully acknowledges the USGS for making available all flood data presented andused here. These data were either downloaded through USGS web sites or taken from USGSreports. The author is, however, solely responsible for all analyses, discussion, conclusions, anderrors presented in this paper.

Citation: Waldon, Michael G. (2018) High Water Elevations on the Vermilion River During the Flood of August 2016. Working paper published February 2018 on ResearchGate.net

URL: https://www.researchgate.net/publication/323108783_High_Water_Elevations_on_the_Vermilion_River_During_the_Flood_of_August_2016