The New York City Department of Environmental Protection is undertaking a four-year detailed inspection and study of six of its largest water supply dam and reservoir facilities. Peter H Baril reports

The New York City Department of Environmental Protection (NYCDEP) is embarking on an ambitious four-year detailed engineering investigation of its key water supply dams located in the Catskill and Delaware watersheds in upstate New York, US. The city has appointed GZA GeoEnvironmental of New York (GZA) to oversee and execute all engineering aspects of the US$3.4M contract (CAT-146).

The focus of the work is the performance of detailed safety inspections of six of the city’s largest water supply reservoirs, which provide 90% of New York’s current demand of 5.5B litres per day. The NYC system is one of the largest municipal surface water storage and supply complexes in the world.

The goal of the project is to provide the city with recommendations for modifications and repairs needed to upgrade these critical water supply reservoirs to a first-class working condition.

Project work began in the summer of 1998 and some of the key ongoing investigatory tasks for the project include:

  • Detailed physical inspections and records review for existing facilities.

  • Geotechnical, structural, hydraulic, mechanical and electrical engineering assessment.

  • Subsurface explorations and installation of embankment monitoring instrumentation.

  • Underwater inspection of associated inlet/outlet works.

  • Hydrologic/hydraulic evaluation of spillway capacities under PMF conditions.

  • Reservoir dredging feasibility studies.

  • Emergency action plans, dam breach modelling studies and inundation mapping using state-of-the art digital mapping technology.

NYC water supply system

The New York City water supply consists of an amazing system of reservoirs and lakes, aqueducts, tunnels and water mains which distribute water daily to nearly 9M people. The system had its infancy during the Dutch colonisation of Manhattan in the early 1600s and consisted of a series of dug wells and small holding ponds within the city limits. These early, privately owned and operated water storage and distribution systems were plagued with quality and quantity problems and were eventually supplanted by the Croton system, constructed in the mid-1800s as the city’s first successful public water supply system.

The Croton system, located east of the Hudson river in Westchester County, provided abundant water which allowed the city’s industrial and population base to grow significantly throughout the 19th century. The city’s population in 1830 was 202,600 and by the 20th century that number had swelled to 1.8M. This nine-fold increase over 70 years was primarily due to the effective and efficient growth of the water supply system. By the 1880s, the Croton system was further enlarged with a new dam (New Croton dam) and new aqueduct. The present Croton system, constructed between 1885 and 1911, provides about 10% of the daily water supply demand.

By 1900, the city’s insatiable thirst required New York City leaders to extend their reach northward to the pristine watersheds beyond metropolitan New York. State legislation allowed the city to purchase land in the Catskill mountains and to dam streams and create a series of new reservoirs which became the Catskill and Delaware systems, the so-called ‘West of Hudson system’.

The focus of the current CAT-146 project is the inspection of the dams and reservoirs in the Catskill and Delaware systems, located about 161km northwest of New York. The Catskill system includes Ashokan reservoir, completed in 1917, and Schoharie Reservoir, which together supply up to 40% of the city’s daily needs. The newer Delaware system provides about 50% of the system demand and includes reservoirs built between 1951 and 1964 (Rondout, Neversink, Pepacton and Cannonsville). The watersheds of these two main systems cover an area of 4144km2 and the reservoirs have a combined storage capacity of approximately 2137B litres.

Up close and personal

Due to forethought, planning and engineering, New York is blessed with a water source of exceptional quality that is almost entirely supplied by gravity. Most of the system’s infrastructure improvements over the past few decades have focused on major distribution system design and construction. This has primarily included city tunnel no 3, a 96.5km, 7.3m diameter deep aqueduct, which when completed in 2020, will be the largest capital construction project in the history of New York. Now the Department of Environmental Protection, the agency responsible for the operation and upkeep of the water supply system, has turned its attention to the ageing dams. Most of these have not had a thorough evaluation since the US Army Corps of Engineers’ (USACE) phase 1 inspection programme which was conducted in the late 1970s.

Most of the dams in the West of Hudson system are in good condition, benefiting from continued routine maintenance. However, through the CAT-146 project, these massive earth embankments and stone masonry structures are now getting close attention that routine maintenance cannot typically address. In addition, none of the impoundments have instrumentation. This makes periodic monitoring and identification of seepage, settlement, displacement, etc difficult to carry out, if not impossible to detect. Also, engineering methods in evaluating such key safety criteria as embankment stability and spillway capacity have been enhanced over the intervening years since most of these dams were built. Re-analysis is therefore required under this contract.

Not surprisingly, the visual inspection of the dams and appurtenant structures required a myriad of engineering expertise. The multi-disciplined team, including sub-consultants to GZA, involves people with skills in civil, geotechnical, hydrologic, hydraulic, mechanical, structural and electrical engineering. Other related specialties integral to the project include: aerial photogrammetry, conventional field surveys, underwater diving and rock climbing.

Two major aspects of the project deal with the evaluation of current spillway capacity and the preparation of hypothetical dam breach inundation mapping. The main objective of the hydrological studies is to re-evaluate and, in some cases, re-confirm the hydraulic capacity of existing principal spillway structures at each of the six dams under probable maximum flood (PMF) conditions. GZA determined that the spillway capacity of the dams required reassessment, primarily because the unit hydrographs, formerly used to estimate rainfall/runoff relationships, were developed from streamflow records through the late 1970s to early 1980s and did not include more recent severe storms. In most cases, the estimation of the probable maximum precipitation (PMP) was based on methods that have been updated.

Image mapping technology

GZA hydrologists built upon the body of existing hydrological information within the river basins (from previous USACE hydrology reports) to evaluate spillway capacity using current conditions and methods. Detailed unit hydrograph theory was used for the gauged and ungauged streams within these very large contributing watersheds (233-1181km2).

The general sequence of hydrological analyses to assess spillway capacity involved the review of river basin model methodology, input data and supporting calculations, as well as obtaining records from US Geological Survey (USGS) streamflow gauges and National Weather Service rainfall within the study areas for periods since river basin reports were issued (1975 to present).

The original USACE HEC-1 runoff models were ‘re-created’ using historic 1950s-era floods and refined using updated information on key hydrological characteristics, such as initial and constant rainfall losses, baseflow recession, and main dam/reservoir storage and discharge relationships. The HEC-1 models were calibrated and verified with input from more recent rainfall/runoff data within the contributory watersheds of each dam. Finally, the PMP distribution was applied to the HEC-1 models and the resultant PMF was routed through each study reservoir. The results indicate that each dam safely passes its respective PMF without overtopping.

GZA has taken advantage of the latest image mapping technology to develop floodplain cross-section data and present final inundation maps as part of the emergency action plan phase of the project. The results from the dam breach (using the NWS DAMBRK simulation model) analyses are managed in AutoCAD format for electronic development of data, display of information and printing of maps. As a base for display, scanned and geo-referenced images of standard USGS quadrangle maps serve as a familiar base map.

These maps, known as digital raster graphics (DRG) are scanned images of standard USGS series topographic maps and are complete with all full-colour base map information including hydrography, roadways, political boundaries and other cultural features. Topographic information for each base map is ‘layered’ onto the map as a digital elevation model (DEM), consisting of an array of elevations for ground positions at regularly spaced intervals. The extraction of riverine cross-sections (for dam break modelling) and display of resultant water surface inundation areas have been aided through the use of BOSS-RMS (river modelling system) for AutoCAD.

BOSS RMS allows for the simultaneous use of the dam break model (DAMBRK) and DEMs to construct cross-sections quickly and accurately. After a DAMBRK analysis has been performed, the inundation area is automatically overlayed onto the DRG/DEM base map. This method provides a highly effective and cost efficient means of computer visualisation and modelling for the dam break and inundation mapping portion of the project.

Equipment check

The project also involves hands-on inspection and exercising of the various gates and valves used to control water releases from the reservoirs which supply aqueducts, as well as federally mandated minimum releases to downstream rivers and streams. These visual inspections require the normal operation of the water supply delivery system to be modified, involving temporary shutdowns and tunnel dewatering, enabling inspection engineers to view in-line control valves and gate mechanisms.

In certain cases, due to safety concerns or operational issues, intake chambers were viewed underwater by using a remote operating vehicle equipped with a video camera. As part of the work the project engineers, including sub-consultant Hazen and Sawyer, will investigate the feasibility of renovating and modifying some of the dam intake structures to allow for better hydraulic efficiency and flexibility of tunnel discharge operations as well as reducing raw water turbidity.

As part of the project, under a separate contract (CAT-173), the DEP has undertaken a comprehensive sub-surface exploration programme for the Catskill and Delaware system dams. The objective of the work is to determine:

  • The character, thickness and stratification of the dams, dikes and their foundation materials.

  • The density of the granular soils and consistency of cohesive soils.

  • The consistency and condition of concrete, masonry and cyclopean fill in dam related structures (including the taking of soil and core samples where required).

  • The depth to solid rock.

  • The groundwater and piezometric levels within the various structures.

This 16-month programme, begun in the Fall of 2000, includes approximately 503m of drilling and 1067m of rock coring, consisting of over 190 individual bore holes, 111 monitoring wells and 142 piezometers, as well as 145 falling head permeability and 93 packer tests. Other related work includes sub-drain video surveys and geophysical testing using seismic cross-hole, ground penetrating radar, and laboratory seismic velocity testing methods.

New York dams in the Catskill and Delaware systems…