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NSD Frequently Asked Questions

Updated: 01/31/2013

Q1: What is the New Stream-reach Development (NSD) hydropower resource potential?
Q2: How do you classify different sources of hydropower potential?
Q3: What is purpose of this hydropower resource assessment?
Q4: What is new for this hydropower resource assessment?
Q5: What is a NSD stream-reach?
Q6: How are the stream-reaches identified?
Q7: What are Hydrologic Region (HUC02), Subregion (HUC04), Basin (HUC06), Subbasin (HUC08), Watershed (HUC10), and Subwatershed (HUC12)?
Q8: What is the rationale behind H*Q*S?
Q9: How is the hydraulic head estimated?
Q10: How is the hydraulic capacity (flow) estimated?
Q11: How is the potential capacity (MW) estimated?
Q12: How is the potential energy (MWh) estimated?
Q13: What is the difference between greater than 1MW and less than 1MW stream reaches?
Q14: What other attributes are also available in this assessment?
Q15: How can I download the NSD data?
Q16: Some data and maps are marked as "PROVISIONAL". What do you mean?
Q17: What is the cost of the data?
Q18: Will the NSD hydropower resource potential be available for Alaska and Hawaii?
Q19: I am a developer and I am interested to find a potential site for development. What should I be aware of before using the NSD data?
Q20: Will there be future NSD assessments and data released to the public?
Q21: I want to know more details beyond what have been explained in the methodology report. How can I get them?
Q22: What do you mean by "Environmental Attribution"?
Q23: What environmental attributes are available in your study?
Q24: Will you provide more environmental related information in the future?
Q25: Why don't you use environmental attributes to identify potential locations for future development?

Q1: What is the New Stream-reach Development (NSD) hydropower resource potential?
This research considers NSD as the additional new hydropower that could be available from undeveloped stream-reaches. A primary intent of the NSD project is to estimate the potential capacity, mean monthly energy generation, as well as inundated areas, reservoir volumes, approximate hydraulic head and flow, and overlapping environmental attributes for hypothetical development in undeveloped stream-reaches; thereby collectively termed NSD hydropower resource potential.

Q2: How do you classify different sources of hydropower potential?
The U.S. Department of Energy (DOE) Water Power Program classifies hydropower potential into multiple resource classes, including (1) upgrades to existing facilities, (2) expansion of existing facilities, (3) powering of non-powered dams, (4) development of new (undeveloped) stream-reaches, and (5) energy recovery in constructed waterways. In addition, although it does not yield a net production of energy, pumped-storage hydropower is recognized as a valuable resource for grid flexibility and energy storage.

Q3: What is purpose of this hydropower resource assessment?
The main purpose of this NSD resource assessment is to assist the DOE Water Power Program to reasonably account for the total national undeveloped hydropower resource potential. Although trying to evaluate NSD potential in the units of undeveloped stream-reaches, this assessment does not recommend nor imply hydropower may be developed at a specific location. The designed methodology is at the reconnaissance-level and does not produce estimates of capacity, production, cost, or impacts of sufficient accuracy to determine absolute economic feasibility or to justify financial investments in individual site development.

Q4: What is new for this hydropower resource assessment?
With the rapid development of multiple national geospatial datasets on topography, hydrology, and environmental characteristics in the recent decade, new opportunity arises for the refinement of hydropower resource potential from undeveloped stream-reaches. This assessment tried to utilize the most state-of-the-art geospatial datasets to evaluate the national hydropower potential, with the method designed specifically for smaller and more environmental-feasible run-of-river hydropower development. Given the rich data inputs, more hydropower attributes, including potential monthly energy production, capacity factor based on flow availability, surface inundation, potential storage, and plenty of environmental-related attributes, are now provided in this new assessment.

Q5: What is a NSD stream-reach?
A NSD stream-reach is derived from the flowlines defined by the National Hydrography Dataset Plus (NHDPlus). Each NHDPlus flowline is discretized into 150-meter long sub-segments for potential site identification. During the optimization process, multiple neighboring segments may be combined as a stream-reach to estimate the NSD potential.

Q6: How are the stream-reaches identified?
The NSD stream-reaches are identified through a site identification process. The assessment is performed for each of the Hydrologic Subregions (HUC04). The hydraulic head (H), flow (Q), and slope (S) are estimated for each sub-segment (discretized from NHDPlus flowlines). The neighboring sub-segments with a higher product of H*Q*S are selected as potential NSD stream-reaches through an optimization process.

Q7: What are Hydrologic Region (HUC02), Subregion (HUC04), Basin (HUC06), Subbasin (HUC08), Watershed (HUC10), and Subwatershed (HUC12)?
Six levels of hydrologic units, labeled by different digits of hydrologic unit code (HUC): Region (2-digit HUC or HUC02), Subregion (4-digit HUC or HUC04), Basin (6-digit HUC or HUC06), Subbasin (8-digit HUC or HUC08), Watershed (10-digit HUC or HUC10), and Subwatershed (12-digit HUC or HUC12), are defined by the Watershed Boundary Dataset (WBD).

Q8: What is the rationale behind H*Q*S?
Since hydropower is proportional to the product of H*Q, higher H*Q is preferable for further investment. However, higher H will also lead to larger inundation and may increase the (environmental) difficulty for development. Therefore, the average slope S is introduced jointly for site identification. Generally speaking, a stream-reach with larger S (steeper) tends to result in smaller inundation than a smoother stream-reach. The usage of H*Q*S is intended to find a balance between hydropower production and potential surface inundation.

Q9: How is the hydraulic head estimated?
A reference height Href is defined in this NSD assessment as the surrogate of hydraulic head. Href is the elevation difference between a NSD stream-reach to the nearest Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) 100-year flood line. Although the purpose of FEMA FIS is unrelated to hydropower, the existing flood zones may be used to infer the selection of NSD stream-reaches. Due to the higher insurance rate and other regulatory consideration, there are usually fewer existing residencies or civil structures in the FEMA 100-year flood zones (i.e., relatively empty) and, hence, the FEMA 100-year flood line can be regarded as an invisible boundary of the existing civil development. In other words, if the NSD inundation is limited to the regions within FEMA 100-year flood zones, there is more likely a chance that the new hydro development will affect fewer existing structures and could potentially be less costly.
However, it is important to note that Href is not necessarily the most profitable or environmentally feasible dam height. It is possible that the final design head can be either higher or lower than the reference height, depending on the results of site-specific engineering design and economic/environmental evaluation. After a particular stream-reach is chosen for serious consideration of new hydropower development, detailed analysis would need to be conducted to refine the design dam height and assess all economic and environmental issues.

Q10: How is the hydraulic capacity (flow) estimated?
For each HUC04 Subregion, all U.S. Geological Survey (USGS) National Water Information System (NWIS) gauge stations with complete recent 20-year (1989–2008) daily observations are identified. The 30 percent daily exceedance flow (Q30) is then computed at each gauge station from the 1989–2008 observation. At the same location as the USGS gauge station, the corresponding NHDPlus annual mean flow QNHDPlus is also identified for comparison. Given that a strong linear relationship is typical between Q30 and QNHDPlus, a conversion ratio is estimated to calculate Q30 based on QNHDPlus, so that the plant hydraulic capacity can be estimated at each NHDPlus sub-segment.

Q11: How is the potential capacity (MW) estimated?
The potential capacity PNSD is computed by c*γ*η*Href*Q30, in which c=(0.3048)^4 is the unit conversion factor, γ = 9800 N/m3 is the specific weight of water, η=0.85 is the generating efficiency, Href (ft) is the reference height, and Q30 (cfs) is the plant hydraulic capacity.

Q12: How is the potential energy (MWh) estimated?
Since the daily or sub-daily resolution streamflow time series are unavailable at most of the ungauged locations, the monthly streamflow time series synthesized from the USGS WaterWatch runoff are used in this NSD assessment as an alternative to calculate ENSD. Within each month, the part of streamflow higher than Q30 is considered spilled and not used for hydropower generation. By summing all monthly energy from January 1989 to December 2008, and dividing by 20 years, the potential mean annual energy production ENSD is estimated. The ENSD will serve as the baseline estimate of energy, and can be improved in the future studies by increasing the resolution and accuracy of the synthesized streamflow time series.

Q13: What is the difference between greater than 1MW and less than 1MW stream reaches?
A NSD stream-reach may have estimated installed capacity greater 1MW or less than 1MW. However, given the limited resources; environmental attribution, estimation of storage and inundation, and quality-control are only performed for those greater than 1MW reaches.

Q14: What other attributes are also available in this assessment?
Based on the 10-meter resolution National Elevation Dataset (NED), the potential inundated surface area, reservoir storage and residence time are also estimated for each NSD stream-reach (with capacity greater than 1MW).

Q15: How can I download the NSD data?
The NSD data are summarized in the units of HUC10 polygons. The HUC10 shapefiles and attribute tables can be downloaded through this public website. Users who are interested to use the reach-level data will need to wait until a user agreement is in place. Please check back later for the update.

Q16: Some data and maps are marked as "PROVISIONAL". What do you mean?
PROVISIONAL data are provided as a courtesy to allow early examination right after our technical assessment is completed. However, the results may be further adjusted during the quality-control and peer-review process. The "FINAL" results are expected to be released around the end of 2013.

Q17: What is the cost of the data?
NSD data are free of charge. However, user agreement may be needed for some subsets of data. Please contact us for your further questions.

Q18: Will the NSD hydropower resource potential be available for Alaska and Hawaii?
NSD estimates are currently unavailable for Alaska and Hawaii. Given the considerable differences in available data, energy demand, and local hydrologic characteristics compared to the contiguous U.S.; further reconciliation of data were needed and are currently underway. Please check back later for updates.

Q19: I am a developer and I am interested to find a potential site for development. What should I be aware of before using the NSD data?
Although the stream-reaches are identified as a basis to compute the NSD potential, it only suggests a possible starting point for more detailed site-specific evaluation. All design characteristics, including hydraulic head and flow should be re-evaluated based on more accurate direct measurement. It is again emphasized that it is a reconnaissance-level resource assessment and the main purpose is to provide large-scale screening.

Q20: Will there be future NSD assessments and data released to the public?
Yes, ongoing efforts will aim to improve NSD analyses results and data. In particular, NSD assessments can be updated when more recent and accurate geospatial data have become available to the research team. Please check back later for new information.

Q21: I want to know more details beyond what have been explained in the methodology report. How can I get them?
The NSD Team is currently preparing other journal publications focusing on the specific algorithms used in this study. When available it will be referenced through this web page. Please check back later.

Q22: What do you mean by "Environmental Attribution"?
Environmental attribution is a process where environmental information (available as geospatial data) are assigned to stream-reaches (i.e. those identified as locations for potential hydropower development) based on spatial interaction (i.e. overlap, touching the boundary of, etc.). Buffering procedures were used to "attribute" dams, inundated areas, and tailwaters designated as potential areas for hydropower development. Buffering ensured that environmental attributes were assigned to stream reaches based on spatial proximity, which varied according to the specific environmental attribute.

Q23: What environmental attributes are available in your study?
Environmental attributes within our analysis included the following categories: ecological, geopolitical/protected land, water quality, recreation/aesthetic, water use, and land disturbance concerns. Specifically, the following variables within each category were summarized at each potential stream reach location. Ecological variables included 1) US Fish and Wildlife Critical Habitats (federally listed species), 2) the number of federally listed fish species, 3) the number of fish species of concern, 4) the number of potadromous fish species. Geopolitical/Protected Lands included 1) the presence of protected lands (including National Parks and Wild-and-Scenic Rivers). Water Quality concerns included 1) the number of waterbodies designated under each state's 303d listing. Recreation/Aesthetics included 1) the number of American Whitewater boating runs, 2) the number of fishing access locations, and 3) the number of boat ramps. Water use included 1) surface, and 2) ground water use. Land Disturbance included 1) urban land cover, 2) population density, 3) the number of local dams, 4) the total number of upstream dams, and 5) the localized watershed disturbance index (from National Fish Habitat Action Plan).

It is important to note that environmental attributes are just indicators of potential environmental concerns and not observed or realized concerns. These values are prone to uncertainty based on the best available geospatial information and errors in spatial coverage overlap.

Q24: Will you provide more environmental related information in the future?
Yes, our intent is to include more environmental related data in NHAAP. If you have suggestions of information that should be considered, please contact us.

Q25: Why don't you use environmental attributes to identify potential locations for future development?
Considering there are different levels of data uncertainties across various datasets, in the current study we are not planning to use environmental attributes as hard constraints in the stream-reach identification algorithm. Nevertheless, all related environmental attributes will still be reported to assist future decision making.