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HomeMy WebLinkAboutHydrologic Analysis - PLN General - 5/30/2002 Hydrologic Analysis Alderbrook & Dalby Creeks 7�177) Hydrologic Analysis Alderbrook & Dalby Creeks For Pat McCullough ESA Engineering By Douglas L. Johnson, P.E. Consulting Engineer May, 2002 DRAFT Hydrologic Analysis Alderbrook Creek and Dalby Creek Watershed This report summarizes engineering analyses performed to determine hydrologic parameters for the Alderbrook Creek and Dalby Creek Watersheds, located on the south side of Hood Canal near Union, Washington. The purpose of this study was to determine the 2-year, 5-year, 10-year, 25-year and 100- year flood peak flows for the watersheds. These flows will be utilized in a salmon restoration project at the Alderbrook resort, which is situated at the mouth of the creeks on Hood Canal. Synthetic Storm Development Since no stream gaging data was available for these watersheds, it was necessary to utilize a synthetic rainfall-runoff model to estimate the magnitude of the 2-year to 100-year flood events. The first step in the hydrologic analysis was to select synthetic storm events of the desired recurrence interval to determine the corresponding flood event. The 24-hour storm volumes were determined from NOAA Atlas 2, while the rainfall hyetographs were based on data contained in analyses of extreme storms in the region' and in Washington State Dam Safety Guidelines2. The type of storm selected was a long duration, 72-hour event, occurring during the fall and winter season. This 72-hour storm contains the 24-hour design event embedded within. This type of storm produces a sustained flood peak, and a large runoff volume, and is typical of flood-producing storms in the fall and winter. The depth-duration characteristics of the storms were based on data for typical events in Washington State'. Table 1 shows the depth-duration characteristics of the storms for a given recurrence interval. Table 1: Storm Depth-Duration Characteristics Storm Event 2-hour Depth 6-hour Depth 24-hour Depth 72-hour Depth (in) (in) (in) (in) 2-year 0.74 1.69 4.50 5.49 5-year 0.90 2.07 5.50 6.71 10-year 0.99 2.25 6.00 7.32 25-year 1.15 2.63 7.00 8.54 100-year 1 1.32 1 3.01 1 8.00 1 9.76 Rainfall-Runoff Model The hydrologic analysis utilized the U.S. Army Corps of Engineers HEC-1 program to analyze the runoff characteristics of the different storms. HEC-1 is a single event model capable of simulating direct runoff from the land surface, channel routing in the creeks, as well as lake elevations and discharges. Inputs to the HEC-1 model include precipitation, soil infiltration rates, and hydraulic characteristics of the streams and lakes. A major limitation with HEC-1 is that it cannot simulate subsurface discharge(interflow)to creeks or lakes. Studies of small watersheds in glaciated areas of the Puget Sound region by Dinicola' have shown that interflow is the dominant runoff mechanism during floods up to the 100-year event. Classical Hortan overland flow does not generally occur in undisturbed, forested areas. This is due to the relatively high permeabilities of the glacial soils compared to the moderate and low rainfall intensities that characterize most storms. A modeling scheme was developed for use with the design Dalby and Alderbrook Creek Watersheds 1 rx � ","• -� t:ISAPASx9GEBARRIFrk .•.roi`4lo» � �'. =;t FISH P.LCSAGE BARRIER FISH Pi SSA GE 4 R4RRIER i,- ,T Dalby Creek Watershed Area= 163 acres :Jv.�•i { .11derbrook Creek Watershed Area top res Upper Watershed, Subbasins Al and A2 _ i, ze � w_ 1 inch rquAI,L(hit)rev Iz— ALDERBROOK CREEK PROFILE 500, 400' 300' 200' t i r 0. 4 1.3x 0 mi D.1 D mi. 0.20 mi. 0.30 mi. 0.40 mi. 0.50 mi. 0.60 mi. 0.70 mi. 0.86 mi. 0.90 mi 1.00 mi. 1.10 mi. 1.20 tni o:1.13 mi.,472' Gain:+475'-3'=+472'4J Printed fromTOPO1 01998 Wildflower Productions(www.topo.com) ALDERBROOK SITE AND WATERSHED SOILS Soil Designation: Eh: Everett Gravelly Sandy Loam - extremely porus gravelly, sandy soil on 5-15% slopes. Ac: Alderwood Gravelly Sandy Loam on 15 to 30% slopes. Hardpan with the top two to three feet of soil being weathered and fractured silty sands and gravels. vi AbAd it 71/1 1f//J� /i J Ac( 4 7 Ek -�' 1�� Ab Ab / oa %Ab IlAc Ad n - ( Nf / Ad Ac Ad Eh Ad /, %Ab Ed fJt C N E j - E 2c, t� SPbrl Eh I ;\`` Ee k Ek �r s ear Tahuyai ,F fares r4 � r- - h u `�•J.s° urX l 6+, Aycy d c - Ice I ahuya Sisters -_ Pt O^ � .Rc m t q CD 32 3 C Aa o Ac 6 AbCf •) c`� Ab Alderbrook.-•y: Aa Ab / E� rimAb Ad Ac Ab Ab� i Jf ll/�__i�J �• X u 5 4 Ab 1 i/ „ !r Ab Lal-e , DRAFT storm that approximates an interflow component for infiltrated precipitation,as well as the standard surface runoff component for disturbed, non-forested areas of the basins. Drainage Basin—Alderbrook and Dalby Creeks are located on the south side of Hood Canal. The basins are mostly forested,with the exception of the golf course at the headwaters of Alderbrook Creek. Both creeks discharge into Hood Canal at the Alderbrook Resort. For purposes of runoff modeling,the Alderbrook basin was divided into two smaller subbasins:the 32 acres that include the golf course at the south end, and the 108 acres in the steep sloped, forested basin at the north end of the watershed. The Dalby basin is much simpler, as it is mostly undeveloped forested land, so it was modeled as a single watershed. The total drainage area of the Alderbrook Creek Watershed was 140 acres, while the area of the Dalby Creek watershed was 163 acres. See Figure 1. Soils and Infiltration—Based on USDA Soil Conservation Service mapping of the Mason County Area, the soils in the basin are almost entirely from the Alderwood Series. This soil consists of gravelly, sandy loams derived from gravelly glacial till dominated by acid igneous rock. A nearly impervious layer of cemented till underlies this soil at depths from 22 to 60 inches. The permeability of the upper layers of this soil range from 0.6 to 2.0 inches/hour, while the cemented till has a permeability of 0.06 in/hr. These soils are quite similar to the interflow-dominated watersheds studied by Dinicola. For the range of storms considered in this study,the maximum precipitation intensities were mostly less than the 0.6 to 2.0 inches/hour range, so little surface runoff occurs,and shallow groundwater flow(interflow) is likely the dominant runoff mechanism. The only exception to this is the golf course area,where native soils have been disturbed, and Hortan overland flow may occur during large storms. The soil moisture conditions antecedent to the beginning of the storms were assumed to be typical of late fall to mid winter,when basin soils are near saturation. Thus, soils were assumed to be at field capacity, and no soil moisture deficit was present. Based on the preceding information, only interflow modeling was performed for the forested, undeveloped subbasins, using a deep percolation rate of 0.06 in/hr. For the golf course area, surface runoff was assumed to occur during large storms, and the SCS Loss Rate method was used to determine runoff. Runoff Modeling Approach Undisturbed Areas- The long duration storms considered contain relatively low precipitation intensities,and for storms up to the 100-year event, little or no surface runoff is produced. Nearly all precipitation is infiltrated into the soil. The runoff is thus dominated by shallow subsurface flow called interflow. Interflow is a complex flow mechanism composed of both unsaturated and saturated flow through the soil6. The HEC-1 program is primarily a surface runoff model, and was not designed to handle interflow directly.Normally, a two-step approach is used to address this problem,the first step determining the surface runoff component, and the second step simulating the interflow component, with a longer travel time. However, since the rainfall intensities for the storms used in assessing the spillway adequacy are less than the surface permeabilities of the Alderbrook soils, only the interflow component was modeled for the undisturbed portion of the basins. For the HEC-1 interflow model, all infiltrated precipitation was assumed to be available for interflow. A deep percolation rate of 0.06 in/hr was used for the undisturbed subbasins to simulate the transfer of water to deep ground water. This moisture was assumed to be unavailable for generating a runoff response. The Incomplete Gamma Distribution?was used to model the unit hydrographs to simulate DST the subsurface lag time for travel to the mouth of the creeks. The soils in the Tahuya lake basin are similar hydrolgically to the surface soil and glacial till profile described and modeled by Dincola5. Based on work by Barkers,the interflow response for steep, forested glacial till slopes (>15%)has a basin lag time ranging from 2.7 to 7.5 hours, with an average of 5.3 hours . Based on a previous study of the nearby Wooten Lake and Haven Lake basins by the author9,the lag times for the undisturbed areas were selected to be 6 hours in the Alderbrook/Darby basins.. Disturbed Areas—The 32 acres at the north end of the Alderbrook basin have been disturbed by construction of a golf course, roads,and scattered homes, with less than half the area still forested. Thus, overland flow is likely the dominant runoff mechanism in this area. The soils in this subbasin are of the Alderbrook Series,they fall under SCS Hydrologic Group C. The SCS Loss rate equation was used to model infiltration and runoff in this subbasin, and the average Curve Number was determined to be 79.5. The basin lag time was determined using methods outlined in TR-5510 to be 45 minutes, and the Gamma Distribution was again used to model the unit hydrograph. Flood Flow Estimates The runoff modeling approach described above was used to compute the flood hydrographs as generated by the 2-year through 100-year storms. The results of this analysis are shown in Table 2. Table 2—Flood Routing Summary Storm Event 24-Hour Alderbrook Alderbrook Dalby Creek Dalby Creek Rainfall Basin Runoff Basin Peak Flow Basin Runoff Peak Flow (in) (in) (efs) (in) (cfs) 2-year 4.5 3.6 24 3.4 23 5-year 5.5 4.7 31 4.5 30 10-year 6.0 5.2 34 5.0 34 25-year 7.0 6.3 42 6.1 41 100-year 8.0 7.4 49 7.2 48 As can be seen by the data, the flood estimates for Dalby Creek were actually lower than Alderbrook Creek, even though the Dalby Creek basin is larger. This is due to the higher runoff rates generated by the developed land within the Alderbrook Creek Watershed. Since no stream gaging data was available to verify the rainfall runoff modeling results, it was decided to instead compare the data to estimated peak discharges computed from the USGS regression equations contained in the WSDOT Highway Hydraulics Manualll. Peak flow estimates were determined for the area of each basin for the 2, 10, 25 and 100-year floods,then were compared to the peak discharges from the HEC-1 model. This comparison indicated that the HEC-1 flood peaks were well within the standard error of the USGS estimates. Based on this check, the computed flood peaks should be suitable for design of drainage features and salmon enhancements at the Alderbrook Resort. DRAFT REFERENCES ' Schaefer, Melvin G.,Regional Analyses of Annual Precipitation Maxima in Washington State, Water Resources Research,Vol 26,No. 1,pp. 119-132,January 1990. 2 Schaefer,M.G.,Dam Safety Guidelines,Technical Note 3: Design Storm Construction, Washington State Dept. of Ecology,Publication 92-55G,April 1993. 3 Schaefer, M.G.,Characteristics of Extreme Precipitation Events in Washington State,Department of Ecology, Water Resources Program,Publication No. 89-51,Olympia, WA, 1989. 4 U.S. Army Corps of Engineers,HEC-1 Flood Bydrograph Packag,e,Hydrologic Engineering Center Davis California, 1990. 5 Dinicola,R.S.,Characterization and Simulation of Rainfall-Runoff Relations for Headwater Basins in Western King and Snohomish Counties, Washington, U.S. Geological Survey, Water Resources Investigations Report 894052.,Tacoma,WA, 1990. 6 Freeze,R. Allen and Cherry,John A.,Ground Water,Prentice Hall,pp.217-221, 1979. Schaefer,M.G. and Stevens,G.T. Flood Hydrograph Construction using the Incomplete Gamma Distribution, Hydrologic Series Bulletin,University of Missouri-Rolla,May 1977. 8 Barker,Bruce,Modifications to the Santa Barbara Unit Hydrograph Method to Improve Detention Pond Performance,King County Surface Water Management Division,April 1992. 9 Johnson,Douglas L.,Hydrologic Analysis of Wooten-Haven Lakes Watershed,a report for ESA Engineering, Lakewood, Washington, 1999. io Natural Resources Conservation Service,TR-55—Urban Runoff for Small Watersheds,USDA, 1986. " Washington State Department of Transportation,Hi hway Hydraulics Manual, Olympia, WA, 1998 ALDERBROOK CREEK FISH PASSAGE FLOWS - BASED ON FISH PASSAGE DESIGN FLOWS FOR UNGAGED CATCHMENTS IN WASHINGTON 5/24/02 Hydrologic Analysis-Flood Flows and Fisheries Design Flows Basin:ALDERBROOK BEACH DR Cuhvd: AND SR 106. Region: 2 Basin Area: 0.219 sq.mi. Mean Annual Precipitation: 75 inches 2-year 24-hour Precipitation: 4.5 inches Discharge Discharge -SE +SE Description/Quantile a b c S.E. (cfs) (curs) (cfs) (cfs) Proportion of 0100 Peak Flood Flows 1 2-year 0.090 0.877 1.51 0.56 16 L5 7 25 3.19 0.31 10-year 0.129 0.868 1.57 0.53 30 2.8 14 46 1.69 0.59 25-year 0.148 0.864 1.59 0.53 39 3.5 18 58 1.34 0.74 50-year 0.161 0.862 1.61 0.53 45 4.2 21 69 1.13 0.89 100-year 0.174 0.861 1.62 0.54 51 4.8 24 79 1.00 1.00 Fish Passage Design Flows 2-region 2 (use just for region 2) Winter 0.13 0.930 1.15 0.49 4.36 0.4 2 7 Spring 0.00 1.090 2.07 0.75 1 0.1 0 3 Fish Passage Design Flows 2-region 1 (use just for region 1) Winter 6.99 0.95 1.01 0.257 8 6 9 Spring 2.25 0.85 0.95 0.306 3 2 3 Default Fish Passage Flow (0.18+Q2+36) 16 1.5 7 25 Approx.Bankful Width 5 ft Sources: 1 Magnitude and Frequency of Floods in Washington,USGS Water Resources Investigations report 974277,USGS,Tacoma,1998 2 Hydrauhcs Manual,Washvtgton State Department of Transportation, January 1997(updated May 1997) !' .,.n�sR .;���� �►tLFO f%1 �:lti1L�►L>>�.'I�S�.. �,� +r 1 V�.� AO "���'�I7�����\\\tom Pot em SK 1 ��r 7 ; �� �� - - //,o1 1I NN go q do. OR .1, van FORM lei �.__�. ��'� �. ��1r/t�� sum�•�� '�»i�11.� �� � v .��r : � NO � ,_ � �..a��_.►..�y+.,�—=�/ Cat�.�.. MCI `a'' ��iinrom /fi r: 111 : u WASHINGTON i,pin FRIDAY HARBOR 9 le RN NO, fir rQ Loam szm i ME g Kill Ij 11.. Nil t tomill 2-YEAR 24-H UR PRECIPITATION 1-1 L PRECIPITATIONX OF AN INCH ��� • NNUAL 1 Figure 1 Dalby and Alderbrook Creek Watersheds FISH PASSAGE BARRIER , P. FISH PASSAGE BARRIER ' ;4r `r FISH PASSAGE t BARRIER v A K* '� �iC '� S P rF •/� .1i > ii 11 1. f :�_" \ �� �j.�y+�•,t ,��r III*}S�(/lf n P ( '�./ t i + � sa.� �� •`Pam' .r � s , r'�•M�j :� � r((� , L � ..ri .r��i��".ii5�41i�.1'4 dam.'��•{5 k���r '�•�> .., ,y!a y2,� �*t^��` f /�y�(�`r "�' Dalby Creek Watershed Area = 163 acresUO Alderbrook Creek Watershed I{ Area = 140 acres .a�.m.�.s��-•`-t�'4�y�a+.- •�f �.�`• _ `r.�. - �'S� :A : N,,,�. 4 r ,.rr y . �+� upper Watershed ''PAR, . �t� Subbasins Al & A2 P r�v`�•' '4, Y ire �s t.• Yy' y +• ` T�_7 •�� 1 1' LT 1 inch equals 1,000 feet � , tmp#1.txt Culvert Calculator Entered Data: Shape ........................... Circular Number of Barrels ............... 1 Solving for ..................... Headwater Chart Number .................... 1 Scale Number .................... 1 Chart Description ............... CONCRETE PIPE CULVERT; NO BEVELED RING ENTRAN CE Scale Decsription ............... SQUARE EDGE ENTRANCE WITH HEADWALL Overtopping ..................... Off Flowrate ........................ 1.0000 cfs Manning's n ..................... 0.0130 Roadway Elevation ............... 29.8000 ft Inlet Elevation ................. 26.5000 ft Outlet Elevation ................ 25.5000 ft Diameter ........................ 2.5000 ft Length .......................... 50.0000 ft Entrance Loss ................... 0.0000 Tailwater ....................... 0.1000 ft Computed Results: Headwater ....................... 26.9109 ft Inlet Control Slope ........................... 0.0200 ft/ft Velocity ........................ 4.4765 fps Messages: Inlet head > Outlet head. Computing Inlet Control headwater. Solving Inlet Equation 26. Solving Inlet Equation 28. Headwater: 26.9109 ft Page 1 tmp#2.txt Culvert Calculator Entered Data: Shape ........................... Circular Number of Barrels ............... 1 Solving for ..................... Headwater Chart Number .................... 3 Scale Number .................... 1 Chart Description ............... CONCRETE PIPE CULVERT; BEVELED RING ENTRANCE Scale Decsription ............... (A) SMALL BEVEL = 0.042D Overtopping ..................... Off Flowrate ........................ 4.0000 cfs Manning's n ..................... 0.0130 Roadway Elevation ............... 29.8000 ft Inlet Elevation ................. 26.5000 ft Outlet Elevation ................ 25.5000 ft Diameter ........................ 2.5000 ft Length .......................... 50.0000 ft Entrance Loss ................... 0.2000 Tailwater ....................... 0.1000 ft Computed Results: Headwater ....................... 27.3674 ft Inlet Control Slope ........................... 0.0200 ft/ft Velocity ........................ 6.7775 fps Messages: Computing Inlet Control headwater. Solving Inlet Equation 26. Solving Inlet Equation 28. Headwater: 27.3674 ft Page 1 STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN Figure III-3.11 Parking Lot Perimeter Trench Design (Reproduced with permission from Schueler (16) ) + �I Top View Side View Dripliine of Tree Should Not Extend Over Trench Berm (Grassed) Slope of Parking Lot 08.0• O.c o'. Slotted Curbs Actr .0 0?.' as a Level Spreader Cars ii �>�' >r7TII1L1RlC Trench 0. 0 Filter Strip o.. Directly Abuts a c.. Protective Filter Pavement .. �Q Cloth Layer • ' Q Sand Filter Storm Drain Slotted Curb Spacers (It Partial Exfiltration) III-3-37 FEBRUARY, 1992 STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN Figure III-3.10 Median Strip Trench Design (Reprod*cmd With permission from Schueler (16) ) Top View Side View =� Inflow :,Grass'Filter 20'Grass Filter Stflp Tr nch. - u e i0 Grass:.", °.: Sides Lined with Permeable Filter Fabric -'o Filter? Permeable Filter o.. 0 ' Fabric One Foot Clean Washed Stone or Gravel Below Surface. o' c° (1.5.3.0Inch) °4'0 Traps Debris 6-12 Inch Sand Filter or Permeable Filter Screened Overflow Pipe Cloth Lines Bottom Outflow III-3-36 FEBRUARY, 1992 STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN Figure III-3.12 Oversized Pipe Trench Design (Reproduced with permission from Schueler (16) ) Side View Impermeable Overflow Pipe Filter Fabric Observation Well �Ir . rlr r.'f�'N` • .1 1 1 t I I 11 1 Standard ACCMP Pipe(Temporarily� i Stores Runoff) - CurbInlet Alk °p.o : 0,1 • b' 0•'.Ol le p. !. QO.O' ..,O eOo 00 !•I1 '.l V�•�J' .O: 000 e.�ei 0• r � 0 0•000'0. VV oV �00 0• � o"O'�...�o. • O. 0� S,. •O. e • ''Q• O o Permeable v0••o .o o ° e�':o. p�. 00 �oDo a o7�0:0 o0,•.Q.'o�OpO. 1� l/�7'QO.Q' •.Q.: • O' vTJ e•. .O o. �tT•oQ.•'� � O•'o o• e•o�, O•• Pretreatment Filter •o .p: o•.o.•. o'...o�.,� �•• o oQ�.• O O• o• o. •'.O� : 0. :�°' i•.• c o••'O o'o0�e •O• eo Faculty Fabric •..o � ..e.D.... ..e�'0'•? �o'•. .O:O .'.O,•:p�':o'O�.o�o�.. I Top View Observation Well IRMOop Holes Drilled in Underside of Pipe n Overflow ••0 Modified Pipe . e -. t�Qo". 0. "o Two chamber Inlet 1.5-3.0 Inch Clean Stone III-3-38 FEBRUARY, 1992 STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN Figure III-3.9 Schematic of an Infiltration Trench (Reproduced with permission from Schueler (16) ) Wellcap Observation Well <i b- Emergency Overflow Berm • °'••:�::o'o.�•. �� ;' '•o�rt•d•. •�,,,�••o p ;Runoff Filters Through �r�'. h" '- Pg ;20 Foot Wide Grass Buller Strip.' �.Q•o;.�:1rJQ.A;00:.•• I •:.o:• a 1: ..po: Q;.o Protective Layer of Filter Fabric CJ .°.•Q `? ':b �?�•� 9�00 Filter Fabric Lines Sides to .��jo�•Trench °• Prevent Soil Contamination / 0:0••3-8 Feet iaao:e Deep Filled '•°�• OOQ:::o;with 1.5-2.5 i40�o �04+�••Inch Diameter eQ°' Clean Stone ' 00 .e.;- .o•o .•ate". Sand Filter(6 12) Inches Deep or Fabric Equivalent Runoff Exfiltrates Through Undisturbed Subsoils with a Minimum fc of 0.5 Inches/Hour III-3-35 FEBRUARY, 1992 F 4TAr.. ` i. s y, �F y STATE OF WASHINGTON DEPARTMENT OF ECOLOGY Mail Stop PV-11 • Olympia, Washington 98504-8711 • (200) 459-6000 Editor's Note Many thanks to those of you who reviewed and commented on the previous drafts of the manual. Much of the continued improvement of the manual has been, and will continue to be, the result of reviewers' comments and constructive criticisms. The final version of the manual has been prepared following review of the June, 1991 "Public Review Draft" that was distributed to some 900 reviewers last summer. Technical workshops were held in Everett, Bellevue, Tacoma, and Olympia during August and September, 1991. Written comments were received from 55 reviewers. Changes in the Puget Sound Stormwater Management Program The Puget Sound Stormwater Management Program mandated by the Puget Sound Water Quality Management Plan (PSWQMP) has changed substantially since the release of the 1991 draft manual. i:eLe are no longer two "companion" rules. The language requiring the adoption of local stormwate: _ management programs has been placed in the PSWQMP instead of being contained in rule form. The Puget Sound Water Quality Management Plan states that local stormwater management programs are to be consistent with the Plan, as well as Ecology's guidance, model ordinances and Stormwater Management Manual. Ecology is to monitor compliance with the Plan requirements, reviewing the status of each local program every two years. Information on implementation schedules and procedures for reviewing local programs are provided in the "Stormwater Program Guidance Manual for the Puget Sound Basin" (hereafter referred to as the Guidance Manual), a companion to this manual. Copies of either this manual or the Guidance Manual can be obtained> y calling (206) 438- 7116 or (206) 438-7059. The cost is not known for certain as we go to press, but should be approximately $40.00 (U.S.) for either manual. Do not send us funds, as a bill will be sent with the manual. Changes Made to the Manual �- Changes were made to the 1991 draft manual based both on new information available and comments received. Some things didn't change: for example, we are still emphasizing the use of infiltration where appropriate. A reduced release rate of 50% of the pre-developed discharge rate has been retained for streambank erosion control. The correction factors have been kept but they are only necessary for streambank erosion control BMPs. The 7-day design storm has not been incorporated at this time. Further hydrologic modelling and analysis is planned for this year with the goal of providing more accurate and easy-to-use methods (see "Unfinished Business", below). New Develonment/Redevelopment • The threshold for land disturbing activities between small parcels and large parcels has been increased to I acre (page I-2-2). • The size threshold of 11.5% for redevelopment sites greater than 1 acre in size was dropped. 5,000 square feet is used as the trigger, regardless of the size of the development (page I-2-4). • A "documented water quality problem" is now defined (page I-2-4). Minimum Requirements • Many editorial changes were made to better clarify the content and rationale for the Minimum Requirements. The modifiers "General" and "Special" for Minimum Requirements were dropped, and the requirements arranged in a more logical order. Objectives were added for each Minimum Requirement. • The Erosion and Sediment Control "General Criteria" were moved from Chapter II-4 to Chapter I-2, and consolidated as Minimum Requirement #l, Erosion and Sediment Control. • The off-site analysis requirement (now Minimum Requirement #8, Off-site Analysis and Mitigation) was simplified. • The ability to use an implemented and adopted basin plan to modify the Minimum Requirements was included (see Minimum Requirement #9, Basin Planning). - Remainder of Chapter I-2 • More specific information and two examples of experimental BMPs have been provided. Chapter 1-3 • A Small Parcel BMP checklist which can be used "as-is" or modified by the local government has been added. Completion of this checklist can be considered to have met the Small Parcel Erosion and Sediment Control Requirements. • Large parcels require the development of a "Stormwater Site Plan" which is in turn made up of a Large Parcel Erosion Control Plan (LPESC) and a Permanent Stormwater Quality Control Plan (PSQC). The LPESC addresses erosion and sediment control during the construction phase, and the PSQC plan addresses permanent stormwater management facilities. Chapter 1-4 • The BMP selection process has been revised to more clearly define the type of pollution problem that a BMP is targeted to control. The problem may be water quality (WQ), which addresses either nutrients-or conventional pollutants, streambank erosion control (SBEC), source control or, as is most likely, a combination of these types. 2 `:•r jr'Y/�.T � - �rj'^�•J,c'r.'''.�'•,�� _�T' 01712, t�...,m+Y..y�•,- r .r r , r lby and Alderbrook Creek Watersheds :1..:.?• •.:': �. yia��'���.• •` ��,aty}•�+- •.,�..;,fir; ... . .-fir.+`:� aY�� � •- t .4.. � !•r•w!�.�.•� •' ram ;' _ � ��T� it..� �yl %`Q y . ►�` r 1 r1. ip .i •r ,r��}•t ,p f r 1' � � �+1.}tij r,�l;Y 1."a ik �x., tee.`A r,. ; r " v 1 't• T: If NrtDjarlr �y./'f�/ J/l- t�yd�� t�1 �X�.tti. ,,,�1,�qt�,`•i,�C j. ' ', .a ,. .��� '�+,♦,r p7'.: •?�," +�• . r Dalb �' .34,,+,�y�,r>;�`.t�s'1��,'�`�j1•IF" ��'^�✓�" t.� rF�`+� �%''i�: ..I��{' ''•� I, t••'�[•.�J �. �y,�,� `c . •�,�,'�•i;� i �i� �� .,.;.. ,��� ',a.:.•h it � �O l ,' r YG nk+`:" '�^� 'iF` .t,�}j�C�•�.;^���1 '� •,\cam� '• t i;,' r�• .1�' .yNf� +�rJ.; � r� ,r:.G,,�.h� Yr• 9•.�,�',' a +f �. 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