ODOT_Water_Quantity_Performance_Standard_Final

Oregon Theodore R Kulongoski, Governor Department of Transportation Geo-Environmental Section Transportation Building 355 Capitol St NE, Rm 301 Salem, OR 97301-3871 Date: October 22, 2008 Subject: Water Quantity (Flow Control) Design Storm Performance Standard FILE CODE: Executive Summary The Oregon Department of Transportation (ODOT) has developed a Stormwater Management Program (Program) to streamline the stormwater permitting process, while continuing to meet its mission as a transportation agency Under the Program, ODOT’s goal is to reduce the amount of runoff generated to the extent practicable before relying on engineered stormwater facilities to meet water quantity and water quality requirements Through this process, ODOT worked internally with management, project delivery and maintenance teams, as well as with representatives from regulatory agencies The key outcomes from the Program include: Revised water quality (treatment facility) design storms Revised water quantity (flow control) design storms Stormwater treatment guidance: Best Management Practice (BMP) Selection Tool and User’s Guide (for water quality facilities) Contributing Impervious Area Guidance Endangered Species Act (ESA) guidance and pilot ESA consultations Guidance for situations where stormwater cannot be treated to the extent required by regulation This memorandum documents the basis for the revised water quantity design storm for sizing ODOT stormwater flow control facilities The driver for re-evaluating ODOT’s water quantity design storm was to provide certainty and technical basis for its definition relative to guidance for consultations under the National Marine Fisheries Service (NMFS) HCD Stormwater Online Guidance (March 2003) The water quantity design storm has two components corresponding with the lower discharge and upper discharge end-points as listed in Table ES-1 below Deviations from this definition of the water quantity design storm would require additional justification and analysis A sitespecific hydrologic analysis can be conducted to justify a different design storm Table ES-1 Water Quantity Design Storm Performance Standard (1) Lower Discharge End-Point Upper Discharge End-Point • 10-year event, for incised • West Region(2): 42 percent of the 2-year event streams or • East Region(2): • Event corresponding to the bank o Southeast, Northeast, North Central overtopping recurrence interval for minimally incised Regions: 48 percent of the 2-year event streams o Eastern Cascades Region: 56 percent of the 2-year event (1) This standard does not supersede local regulations that may require detention of larger events (2) As defined by the USGS flood frequency zones Certain projects are excluded from application of the water quantity design storm performance standard as follows: (i) projects that discharge into major water bodies, such as mainstem rivers and large lakes and reservoirs; (ii) projects in local jurisdictions that have more conservative discharge thresholds; and (iii) the uncontrolled peak post-construction runoff rate from the new impervious surface area is less than 0.5 cubic feet per second during the 10-year, 24-hour storm event from the total proposed contributing area In addition to applying the water quantity design storm, use of low impact development (LID) should be considered to reduce the volume of water for detention Water Quantity Design Storm Evaluation ODOT re-evaluated the water quantity design storm that had been used for hydrologic modification of highway runoff Approaches based solely on peak discharge control (e.g., using detention facilities) are not adequate to address the range of impacts associated with urban runoff, including stream channel stability, because detention ponds can discharge at flow rates near the peak discharge for much longer times than in the predevelopment state Regardless of a detention facility’s ability to limit or reduce the peak discharge runoff from a given site, the overall runoff volume will more than likely increase compared to pre-project conditions, no matter the size of the project (impervious area) This increased volume released over a longer period of time may contribute to stream bed and bank impacts to the receiving stream Therefore, the new ODOT water quantity design storm performance standard is for no increase in sediment transporting flows over a defined range of flows That is, the performance standard would meet the goal of matching pre-project hydrology from a defined low discharge, high-frequency event to a defined high discharge, low-frequency event This memorandum presents a definition for the low discharge, high-frequency events (lower discharge end-point) for different regions across the state, and a redefinition for the high discharge, low frequency events (upper discharge end-point) 1.0 Lower Discharge End-Point Generally, the lower discharge end-point is defined in terms of when substantial bed load transport begins to occur from the channel bed and banks As discussed in Booth (1997), three basic criteria have been used to characterize threshold discharges of when sediment transport begins to occur: • • • A specified fraction of the bankfull discharge; Discharge associated with a particular flow duration (i.e., discharge that equaled or exceeded some specified fraction of the year); and A specified fraction of the discharge associated with the flood of a particular recurrence Based on Booth (1997), “the threshold of significant bedload movement… is probably best represented by the central value of 50 percent of the 2-year discharge.” Studies have shown that two-thirds (2/3) of the bankfull discharge generally represents when substantial bedload transport begins in sand-bedded streams (Soar and Thome, 2001) The 1.2-year event and 1.5-year event correspond to the return periods for the average bankfull discharge events in the western and eastern regions of the state, respectively (Castro, 2001) Based on this information, ODOT used this as the basis to define the lower discharge endpoint for the performance standard as follows: • • 2/3 of the 1.5-year storm for Eastern Oregon 2/3 of the 1.2-year storm for Western Oregon However, flood flow data for the 2-year event is generally the more commonly available recurrence period The question then becomes how the proposed lower discharge end-points compare to the 2-year discharge, as well as the 50 percent of the 2-year discharge identified by Booth (1997) as an acceptable central value for threshold of bedload movement This comparison is summarized below Method of Analysis - Lower Discharge End-Point Analysis The analysis compares the two proposed lower discharge endpoints to the 2-yr event (and 50 percent of the 2-yr event) The resulting data tables from the analysis display each of the scenarios Conclusions are made regarding how the performance standard for the lower discharge endpoint could be applied for different regions across the state The new U.S Army Corps of Engineers Hydrologic Engineering Center software program, Statistical Software Package (HEC-SSP 1.0), evaluates flood flow frequencies based on the U.S Geological Survey (USGS) Bulletin 17B, “Guidelines for Determining Flood Flow Frequency” (1982) For this analysis, 64 gages were selected, eight from each of the eight flood frequency regions defined by USGS The flood zones are shown in Attachment A and the gage locations are shown in Attachment B Nearly all gages had periods of record longer than 30 years and did not appear to be on a regulated stream Gages with less than 30 years were selected when necessary to fill geographical gaps in the analysis To evaluate whether or not the stream was controlled, the gages were mapped in ArcGIS and compared with the locations of Oregon dams over 10 feet (current through 1998) as regulated by the Oregon Water Resources Department The dam geographical information was downloaded from the Oregon Geospatial Clearinghouse (http://www.oregon.gov/DAS/EISPD/ GEO/alphalist.shtml) The HEC-SSP program downloaded peak flow information for the selected gages from the USGS website The program performed a Bulletin 17B analysis customized to evaluate the 1.2and 1.5-year flood events in addition to the normal flood frequency events Sample output graphs from the HEC-SSP program are shown in Attachment C In an MS Excel spreadsheet the discharge at 66 percent (2/3) of the 1.2- and 1.5-year events were evaluated against the 2-year discharge as shown in the Table Summary statistics (mean, median, standard deviation and variance) were calculated for the ratio of the bankfull events (2/3 of the 1.2-yr and 1.5-yr) to the 2-year event values The statistics were calculated for each region including all of the gages and for the case where the high and low values were omitted The summary statistics are shown in Table In addition, an analysis of variance test (ANOVA) was used to compare the means of the ratios among the different flood zones to determine whether the ratios can be considered the same for all of the zones or whether they are statistically different Output from the ANOVA analysis is shown in Attachment D Results for Lower Discharge End-Point Analysis For Western Oregon, 2/3 of the 1.2-year flow is on average 42 percent of the 2-year flow for the gages considered The mean and median for the ratio of 2/3 of 1.2-yr to 2-yr is 0.42 and 0.41, respectively; while standard deviation including all data points is 0.078 For Eastern Oregon, 2/3 of the 1.5-year flow is on average 50 percent of the 2-year flow for the gages considered For all eastern flood zones combined, the mean and median for the ratio of 2/3 of 1.5-yr to 2-yr is 0.50; while standard deviation including all data points is 0.062 For the eastern cascades region; however, 2/3 of the 1.5 year flow is slightly greater, being 56 percent of the 2-year flow ANOVA was conducted on the mean of the ratios for each flood frequency zone to evaluate whether a single lower discharge endpoint value can be used for the entire western or eastern region The output tables are included as Attachment A At the percent significance level, the ANOVA results indicate that the means for the western region can be considered statistically the same for all of the flood frequency zones With the eastern region, there is enough variation in the mean ratios among the zones that, statistically, the ratios cannot be considered the same for the entire eastern region However, the ANOVA results indicate that among the four eastern region zones, the Southeast, Northeast, and North Central Regions can be considered to have the same mean ratios That is, the mean ratio for the eastern cascades region is statistically different than those for the other three eastern region flood frequency zones The average of the mean ratios for these three eastern region zones is 0.48 percent of the 2-year 2.0 Upper Discharge End-Point The upper discharge endpoint can be defined by one of two criteria The first is the discharge and stage at which additional discharge results in little or no additional geomorphic effectiveness This is the channel bank over-topping event Once the channel bank is over-topped, the additional flow occurs in the floodplain portion limiting flow velocity increases within the channel (Janine Castro, U.S Fish and Wildlife Service [USFWS], email communication with ODOT, June and 14, 2007) The other criterion is the event at which the amount of impervious surface area in the watershed begins to have little effect on determining stream discharges Based on discussions with the USFWS geomorphologist, that storm is considered to be the 10-year event (Janine Castro, USFWS, email communication with ODOT, June and 14, 2007) Based on the consideration above, ODOT is defining the upper discharge end-point for the water quantity performance standard as follows: • • Channel bank over-topping event, for minimally incised streams 10-year event, 24-hour storm event, for incised streams Incision (or entrenchment) describes the relationship between a stream and its valley It is a measure of how accessible a floodplain is to the stream The classification for incised or unincised would be based on the Rosgen Stream Classification System (Rosgen, 1996) In particular, incision is defined by an “entrenchment ratio” of the flood-prone width of the valley divided by the bankfull width of the channel A ratio of less than 1.4 is generally considered incised, while a ratio greater than 2.2 is considered slightly or minimally incised A hydrologichydraulic analysis would need to be conducted in the case of minimally incised streams to determine the channel-bank over-topping event It is unique to each stream 3.0 Minimum Criteria for Applying Water Quantity Performance Standard Attempting to define a minimum project size is difficult because the impacts to the stream bed are a function not only of the quantity of runoff from the site, but also the stream bed characteristics, stream flow conditions, other discharges to the stream at upstream locations, as well as the stream’s sensitivity to change Thus, stream variability and incremental impacts make it impractical to define a minimum project size that defines when the criteria above should apply (Janine Castro, USFWS, email communication with ODOT, September 25, 2007) Based on the discussions with USFWS, it was decided that a “pragmatic” approach is most appropriate to define the minimum criteria for applying the water quantity performance standard Initial discussions concluded that the criteria should be to require flow control whenever new conveyance or increased conveyance capacity from existing facilities (i.e., increased pipe diameter) is or would be necessary as a result of project improvements at a site There are two considerations to establishing these criteria: (i) very small increases in flow are adequately managed by water quality BMPs without having to resort to flow control facilities; and (ii) the construction of flow control facilities for very small discharges is not generally practical ODOT currently defines when land owners need to provide flow control before discharging stormwater from new impervious surface area into the ODOT storm drain system These criteria have been in place since the mid-1990s and have not encountered issues with its use from the development community, local jurisdictions, and other resource agencies For these reasons, these same criteria, as defined, provide the “practical” basis for defining the minimum criteria to apply the water quantity performance standard, as described below Certain projects are excluded from application of the water quantity performance standard as follows: (i) projects that discharge into major water bodies, such as mainstem rivers and large lakes and reservoirs (ii) projects in local jurisdictions that have more conservative discharge thresholds; and (iii) the uncontrolled peak post-construction runoff rate from the net new impervious surface area is less than 0.5 cubic feet per second during the 10-year, 24hour storm event from the total proposed contributing area The increase of 0.5 cfs for the 10-year, 24-hour storm is the criterion used by ODOT to determine when adjacent land owners need to provide flow control before discharging into the ODOT storm drain system For example, in the North Willamette Valley 0.5 cfs is generated from a proposed development area of approximately 0.25 acre In this case, using the ODOT Hydraulics Manual (Chapter 7), for Zone the rainfall intensity for a 10-year storm is 2.1 inches/hour For an area of 0.25 acre, assuming a 5-minute time of concentration and a runoff coefficient of 0.90 for pavement, using the Rational Method yields a peak flow rate of 0.47 cubic feet per second 4.0 Conclusion For any given stream, the appropriate endpoints for the water quantity performance standard, especially the low end, may vary considerably from the average due to a multitude of factors However, determining what is best for an individual stream is difficult and time consuming The alternative, as proposed here, is to develop a generalized performance standard while allowing project teams to conduct incipient motion studies to demonstrate whether the proposed guidance/requirements are too restrictive for project conditions The water quantity performance standard considered is based strictly on sediment transport considerations Other ecological benefits are not explicitly accounted for, which could result in conflicting basis for performance standards Based on findings from the analysis, the following performance standards are defined for the lower and upper discharge end-points for the water quantity performance standard Lower Discharge End-point: • West Region (as defined by the USGS flood frequency zones): 42 percent of the 2year, 24 hour event • East Region (as defined by the USGS flood frequency zones): o Southeast, Northeast, North Central Regions: 48 percent of the 2-year, 24 hour event o Eastern Cascades Region: 56 percent of the 2-year, 24 hour event Upper Discharge End-point: • 10-year, 24 hour event, for incised streams or • Event corresponding to the bank overtopping recurrence interval for minimally incised streams Minimum Criteria: These performance standards are applied when the uncontrolled peak post-construction runoff rate from the new impervious surface area is greater than 0.5 cubic feet per second during the 10year, 24-hour storm event References: Booth, Derek 1997 Rationale for a “Threshold of Concern” in Stormwater Release Rates Center for Urban Water Resources Management University of Washington Castro, J and P.L Jackson 2001 Bankfull discharge recurrence intervals and regional hydraulic geometry relationships: patterns in the Pacific Northwest, USA Journal of the American Water Resources Association, Vol 37, No 5, pg 1249 1262 Castro J 2007 Email communication with ODOT Re: Stormwater: Request for Review of Water Quantity Design Storm - Low Discharge Endpoint memo June Castro J 2007 Email communication with ODOT Re: Re: Stormwater: Question re: Water Quantity Guidance, September 25 USGS 1982 Guidelines for Determining Flood Flow Frequency Bulletin 17B Soar, P and Colin Thome 2001 Channel Restoration Design for Meandering Rivers United States Corps of Engineers ERDC/CHL CR-01-1 September Comparison of Flood Frequency Flows Water Quantity Design Storm • • Notes: HEC-SSP (Ver 1.0) used to calculate the flood frequency flow (exceedance probability curves) Flood Frequency Zones are based on “Magnitude and Frequency of Floods in Western Oregon” USGS, 1979 and ““Magnitude and Frequency of Floods in Eastern Oregon” USGS, 1982 Summary Statistics for Flood Frequency Ratios Water Quantity Design Storm Attachment A: Flood Frequency Zones 10 11 Attachment B: Locations of Stream Gages Analyzed 12 Attachment C: Sample Output Plots from HEC-SSP Attachment D: Analysis of Variance (ANOVA) ANOVA is a general technique that can be used to test the hypothesis that the means among two or more groups are equal, under the assumption that the sampled populations are normally distributed When the null hypothesis of equal means is true, the two mean squares estimate the same quantity (error variance), and should be of approximately equal magnitude In other words, their ratio should be close to If the null hypothesis is false, MST should be larger than MSE The test statistic, used in testing the equality of treatment means is: F = MST / MSE The critical value is the tabular value of the F distribution, based on the chosen level and the degrees of freedom DFT and DFE In this case, α = 0.05 14 15