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Item 4KCITY OF SOUTHLAKE MEMORANDUM (November 3, 2015) To: Shana Yelverton, City Manager From: Robert H. Price, P.E., Director of Public Works Item 4K Subject: Ordinance No. 1131, 1st Reading, Adopt ordinance to abandon a permanent drainage easement in the Zelda Office development Action Requested: Ordinance No. 1131, 1st Reading, Adopt ordinance to abandon a permanent drainage easement as recorded in instrument D213130362 of the Deed Records of Tarrant County, Texas, situated in the O.W. Knight Survey, Abstract No. 899, in the City of Southlake, Tarrant County, Texas and being more particularly described as the Zelda Office development. (If approved, the public hearing will be held November 17, 2015.) Background Information: City staff is recommending abandonment of a drainage easement that was originally dedicated to the city to provide for a temporary detention basin during the construction of the first building in the Zelda Office development. The easement was recorded in the County records when it was intended to be a temporary measure during the development of Phase I of the development. Zena Rucker, the developer of the Zelda Office development has requested the abandonment of a permanent drainage easement that has been proven to be not necessary. The permanent drainage easement to be abandoned is on property described as a tract of land deeded to William R. & Zena Rucker as recorded in Instrument Volume 12069, Page 1671 of Deed Records of Tarrant County, Texas. The applicant has provided a signed and sealed letter by a licensed Professional Engineer in the State of Texas which provides the justification for this development to forego the detention requirement. This drainage easement must be abandoned since the applicant currently has a second building under construction over the drainage easement. This will also facilitate future property transactions. Item 4K Financial Considerations: The filing fee for recording the Quitclaim Deed in Tarrant County records will be the only cost associated with the abandonment of the easement. Strategic Link: The easement abandonment links to the City's strategy map relative to Corporate Focus Area of Quality Development. Citizen Input/ Board Review: None Legal Review: The ordinance has been reviewed by the City Attorney's office. Alternatives: The City Council may approve Ordinance No. 1131, amend or deny it. Staff Recommendation: Staff recommends approval of Ordinance No. 1131, 1st Reading, Adopt ordinance to abandon a permanent drainage easement as recorded in instrument D213130362 of the Deed Records of Tarrant County, Texas, situated in the O.W. Knight Survey, Abstract No. 899, in the City of Southlake, Tarrant County, Texas and being more particularly described as the Zelda Office development. (If approved, the public hearing will be held November 17, 2015.) Supporting Documents: Location Map Engineers Letter Document Instrument No. D213130362 Ordinance No. 1131 Ordinance No. 1131 Quitclaim Deed Staff Contact: Robert H. Price, P.E., Public Works Director Alejandra Ayala, P.E., Civil Engineer _ 1- f + GAT SHE�AD GT "" '� a � - .. i:ll LLi 1131.1 i.i11. •.. E SOUTHL-AKE-BLVD— �~Or ! Approximate Easement Location ifx a r.htIL p .I{\46 - ;z YY Q7_47 z + ❑' IV Zelda Office { °� - Development i._.yiL� _ �'� .•fid '.l �� ut: . � � • T _ - .- .. _ .ice _ . Ordinance No. 1131 ORDINANCE NO. 1131 AN ORDINANCE VACATING AND ABANDONING A DRAINAGE EASEMENT PREVIOUSLY GRANTED TO THE CITY BY ZENA RUCKER, WHICH EASEMENT IS FILED AS INSTRUMENT NO. D213130362 AND RECORDED IN THE DEED RECORDS OF TARRANT; DECLARING THAT SUCH EASEMENT IS UNNECESSARY FOR USE BY THE PUBLIC; AUTHORIZING THE MAYOR TO EXECUTE A QUITCLAIM DEED RELEASING AND ASSIGNING THIS EASEMENT TO THE OWNERS OF THE FEE ESTATE; PROVIDING A SEVERABILTY CLAUSE; AND PROVIDING AN EFFECTIVE DATE. WHEREAS, the City of Southlake, Texas ("the City") is a home rule city acting under its power adopted by the electorate pursuant to Article XI, Section 5 of the Texas Constitution and Chapter 9 of the Local Government Code; and WHEREAS, on or about May 22, 2013, Zena Rucker, granted a permanent drainage easement ("the easement'') to the City, which easement is filed of record as Instrument No. D213130362 and recorded in the Deed Records Of Tarrant County, Texas; and WHEREAS, the City Council of the City of Southlake, after careful study and consideration, has determined that a portion of the easement is not being used by, nor useful or convenient to the public in general, and that the public would be better served and benefited by its vacation and abandonment; and WHEREAS, in order to remove any question as to the continued interest in the easement, the City desires to execute a quitclaim deed releasing and assigning all title and control in said easement to the owners of the fee estate on which the easement is located. NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF SOUTHLAKE, TEXAS: SECTION 1 The City Council finds that a portion of the permanent drainage easement granted to the City on or about May 22, 2013 by Zena Rucker, which easement is filed of record as Instrument No. D213130362 and recorded at The Deed Records of Tarrant County, Texas, and which is described in the attached Exhibit B and shown on the attached Exhibit BI, is not being used by, nor useful or convenient to the public in general and that the public would be better served and benefited by the vacation and abandonment of the easement. 1 Ordinance No. 1131 SECTION 2 The Mayor of the City of Southlake, Texas, is hereby authorized and empowered to execute a quitclaim deed to the above-described easement to the current owner of the fee estate on which the easement is located, specifically, Zena Rucker. SECTION 3 After the quitclaim deed is executed, the City Secretary is authorized and instructed to file it in the deed records of Tarrant County, Texas. SECTION 4 It is hereby declared to be the intention of the City Council that the phrases, clauses, sentences, paragraphs and sections of this ordinance are severable, and if any phrase, clause, sentence, paragraph or section of this ordinance shall be declared unconstitutional by the valid judgment or decree of any court of competent jurisdiction, such unconstitutionality shall not affect any of the remaining phrases, clauses, sentences, paragraphs and sections of this ordinance, since the same would have been enacted by the City Council without the incorporation in this ordinance of any such unconstitutional phrase, clause, sentence, paragraph or section. SECTION 5 This Ordinance shall be in full force and effect from and its passage, and it so ordained. PASSED AND APPROVED ON FIRST READING ON THIS 3RD DAY OF NOVEMBER, 2015. THE HONORABLE LAURA HILL, MAYOR ATTEST: CITY SECRETARY 2 Ordinance No. 1131 PASSED AND APPROVED ON SECOND READING ON THIS 17 TH DAY OF NOVEMBER, 2015. THE HONORABLE LAURA HILL, MAYOR ATTEST: CITY SECRETARY EFFECTIVE: APPROVED AS TO FORM AND LEGALITY: CITY ATTORNEY EXHIBIT B METES AND BOUNDS DESCRIPTION 0.2534 ACRES (11,040 SQUARE FEET) DRAINAGE EASEMENT 2.730 ACRES IN THE O.W. KNIGHT SURVEY, A-899 CITY OF SOUTHLAKE, TARRANT COUNTY, TEXAS All that certain 0.2534 acres (11,040 square feet) of land, which is a portion of the 2.950 acres of land described in the deed to William R. and Zena Rucker recorded in Volume 12069, Page 1671 in the Deed Records of Tarrant County, Texas (D.R.T.C.T.), in the O.W. Knight Survey, A-899, in the City of Southlake, Tarrant County, Texas and more particularly described by metes and bounds as follows: (all bearings shown hereon are based on Texas State Plane Coordinate System, North Central Zone); COMENCING at a 3/4" iron rod found for the southeast corner of said 2.730 acres, common to the southwest corner of the deed described to Billy W. Hayes, recorded in Volume 6551, Page 938, in the D.R.T.C.T., THENCE along the common lot line of said 2.730 acres and deed to Billy W. Hayes North 010 14' 45" West — 476.99' to the POINT OF BEGINNING and southeast corner of the herein described easement; THENCE South 88° 45' 15" West — 120.00' to the southwest corner of the herein described easement; THENCE North 01 ° 14' 45" West — 92.00' to the northwest corner of the herein described easement; THENCE North 88° 45' 15" East — 120.00' to a 1/2" iron rod with a cap stamped "SPRY" found in the said common lot line of said 2.730 acres and deed to Billy W. Hayes, for the northeast corner of the herein described easement; THENCE along the common lot line of said 2.730 acres and deed to Billy W. Hayes South 010 14' 45" East — 92.00' to the POINT OF BEGINING and containing 0.2534 acres (11,040 square feet) of land. THIS METES AND BOUNDS DESCRIPTION IS ISSUED IN CONJUNCTION WITH THE SURVEY PREPARED BY SPRY SURVEYORS, HEREBY REFERENCED AS EXHIBIT A. THIS METES AND BOUNDS DESCRIPTION IS EXHIBIT B. 0000800000000000O0000000ao C °°DAVID00 ° CARLTON° LE °.0 5647 Np uo \ Oj w l,9 ° �F0 °` °SS. yF. sU� D b ,NINE 17, 2012 i CERTIFlCATE DATE vi I EXHIBIT 131 14' COMMON ACCESS ESMT. /CAB. A, SLIDE 4650, P.R.T.C.T. I 1 12' UTIt1TY & ACCESS ESMT VOL 13378,PG 164, D.R.T.C.T. .1� 12' UTILITY & ACCESS ESMT I -,--VOL 13279,PG 557, D.R.T.C.T. SOUTH LINE OF PROPOSED LOT 1, BLOCK A, RUCKER ADDITION N 8845'15" E 120.G /DRAINAGE EASEMENT 0.2534 ACRES 11,040 SQ. FT. S 8845 15" IN 120• 2.730 ACRES REMAINDER OF 2.95 ACRES WILLIAM R. & ZENA RUCKER VOL. 12069, PG. 1671 D.R.T.C.T. I certify that this survey was made on the ground, that this plat correctly represents the facts found at the time of survey. 4 6 DAVID CARLTON LEWIS, R.P.L.S. NO. 5647 1/2" IRF w/CAP "SPRY" J Q j a) 0 U0 CWS L>_.I LU a��� LO =��U �wi-)ci� U� J�cDp LO JOJ m„o P.O. B. z O I .p P.O.C. 3/4” IRF This survey Is Issued In con)unctlon with the Mates and Bounds Description by Spry Surveyyaarraa hereby referenced as Exhibit B2. This Survey is Exhibit 81. 11,040 SQUARE FOOT DRAINAGE EASEMENT 2.95 ACRES WILLIAM R. & ZENA RUCKER O.W. KNIGHT SURVEY, ABSTRACT 899 CITY OF SOUTHLAKE, TARRANT COUNTY, TEXAS ISSUE DATE: 04/17/2013 1 SCALE: i' - 40' 1 PROJECT NO.: 034-053-50 SPRY SURVEYORS 8241 Mid—Cities Blvd., Suite 102 • North Richland Hills, TX 76182 Ph 817.896.6150 • Fax 817284.8408 • www.sprysurveyors.com NOTICE OF CONFIDENTIALITY RIGHTS: IF YOU ARE A NATURAL PERSON, YOU MAY REMOVE OR STRIKE ANY OR ALL OF THE FOLLOWING INFORMATION FROM ANY INSTRUMENT THAT TRANSFERS AN INTEREST IN REAL PROPERTY BEFORE IT IS FILED FOR RECORD IN THE PUBLIC RECORDS: YOUR SOCIAL SECURITY NUMBER OR YOUR DRIVER'S LICENSE NUMBER. QUITCLAIM DEED STATE OF TEXAS COUNTY OF TARRANT KNOW ALL MEN BY THESE PRESENTS: That the City Council of the City of Southlake, County of Tarrant, State of Texas, after careful study and consideration, has determined that a portion of an existing permanent drainage easement granted to the City by Zena Rucker, filed with the County Clerk of Tarrant County, Texas as Document No. D213130362, and recorded in the deed records of Tarrant County, Texas, is not being used by, nor useful or convenient to the public in general, and therefore constitutes a public charge without a corresponding public benefit, and the public would be better served and benefited by its vacation and abandonment. Therefore, for and in consideration of the above findings and for other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the City of Southlake, Grantor, does hereby assign and convey and forever quitclaim unto the current owner of the fee estate on which such easement is located, specifically, Zena Rucker, Grantee, and its successors or assigns, all of the City's right title, and interest, if any, in and to the real property described in the attached Exhibit B and shown on the attached Exhibit B 1. In accordance with governing law, the undersigned has been authorized to execute this conveyance by Ordinance No. 1131, passed by the City Council of the City of Southlake on , a copy of which ordinance is attached hereto as Exhibit C and incorporated herein by reference. This deed shall convey any right, title and interest, if any, owned by Grantor in and to the above described property unto the said Grantee, so that Grantee, and its heirs and assigns, shall have and hold such property conveyed, if any, forever, and so that neither Grantor nor Grantor's successors or assigns shall have, claim or demand any right or title to the aforesaid property, premises or appurtenances or any part thereof. Executed this the day of , 2015. CITY OF SOUTHLAKE, Grantor The Honorable Laura Hill, Mayor ATTEST: Lori Payne, City Secretary STATE OF TEXAS COUNTY OF TARRANT BEFORE ME, the undersigned authority, on this day personally appeared Laura Hill, Mayor of the City of Southlake, known to me to be the person whose name is subscribed to the foregoing instrument, and acknowledged to me that he executed the same for the purposes and consideration therein expressed. GIVEN UNDER MY HAND AND SEAL OF OFFICE this day of , 2015. Notary Public in and for the State of Texas My Commission expires: After filing please return to: City Secretary City of Southlake 1400 Main Street Southlake, Texas 76092 EXHIBIT B METES AND BOUNDS DESCRIPTION 0.2534 ACRES (11,040 SQUARE FEET) DRAINAGE EASEMENT 2.730 ACRES IN THE O.W. KNIGHT SURVEY, A-899 CITY OF SOUTHLAKE, TARRANT COUNTY, TEXAS All that certain 0.2534 acres (11,040 square feet) of land, which is a portion of the 2.950 acres of land described in the deed to William R. and Zena Rucker recorded in Volume 12069, Page 1671 in the Deed Records of Tarrant County, Texas (D.R.T.C.T.), in the O.W. Knight Survey, A-899, in the City of Southlake, Tarrant County, Texas and more particularly described by metes and bounds as follows: (all bearings shown hereon are based on Texas State Plane Coordinate System, North Central Zone); COMENCING at a 3/4" iron rod found for the southeast corner of said 2.730 acres, common to the southwest corner of the deed described to Billy W. Hayes, recorded in Volume 6551, Page 938, in the D.R.T.C.T., THENCE along the common lot line of said 2.730 acres and deed to Billy W. Hayes North 010 14' 45" West — 476.99' to the POINT OF BEGINNING and southeast corner of the herein described easement; THENCE South 88° 45' 15" West — 120.00' to the southwest corner of the herein described easement; THENCE North 01 ° 14' 45" West — 92.00' to the northwest corner of the herein described easement; THENCE North 88° 45' 15" East — 120.00' to a 1/2" iron rod with a cap stamped "SPRY" found in the said common lot line of said 2.730 acres and deed to Billy W. Hayes, for the northeast corner of the herein described easement; THENCE along the common lot line of said 2.730 acres and deed to Billy W. Hayes South 010 14' 45" East — 92.00' to the POINT OF BEGINING and containing 0.2534 acres (11,040 square feet) of land. THIS METES AND BOUNDS DESCRIPTION IS ISSUED IN CONJUNCTION WITH THE SURVEY PREPARED BY SPRY SURVEYORS, HEREBY REFERENCED AS EXHIBIT A. THIS METES AND BOUNDS DESCRIPTION IS EXHIBIT B. 0000800000000000O0000000ao C °°DAVID00 ° CARLTON° LE °.0 5647 Np uo \ Oj w l,9 ° �F0 °` °SS. yF. sU� D b ,NINE 17, 2012 i CERTIFlCATE DATE vi I EXHIBIT 131 14' COMMON ACCESS ESMT. /CAB. A, SLIDE 4650, P.R.T.C.T. I 1 12' UTIt1TY & ACCESS ESMT VOL 13378,PG 164, D.R.T.C.T. .1� 12' UTILITY & ACCESS ESMT I -,--VOL 13279,PG 557, D.R.T.C.T. SOUTH LINE OF PROPOSED LOT 1, BLOCK A, RUCKER ADDITION N 8845'15" E 120.G /DRAINAGE EASEMENT 0.2534 ACRES 11,040 SQ. FT. S 8845 15" IN 120• 2.730 ACRES REMAINDER OF 2.95 ACRES WILLIAM R. & ZENA RUCKER VOL. 12069, PG. 1671 D.R.T.C.T. I certify that this survey was made on the ground, that this plat correctly represents the facts found at the time of survey. 4 6 DAVID CARLTON LEWIS, R.P.L.S. NO. 5647 1/2" IRF w/CAP "SPRY" J Q j a) 0 U0 CWS L>_.I LU a��� LO =��U �wi-)ci� U� J�cDp LO JOJ m„o P.O. B. z O I .p P.O.C. 3/4” IRF This survey Is Issued In con)unctlon with the Mates and Bounds Description by Spry Surveyyaarraa hereby referenced as Exhibit B2. This Survey is Exhibit 81. 11,040 SQUARE FOOT DRAINAGE EASEMENT 2.95 ACRES WILLIAM R. & ZENA RUCKER O.W. KNIGHT SURVEY, ABSTRACT 899 CITY OF SOUTHLAKE, TARRANT COUNTY, TEXAS ISSUE DATE: 04/17/2013 1 SCALE: i' - 40' 1 PROJECT NO.: 034-053-50 SPRY SURVEYORS 8241 Mid—Cities Blvd., Suite 102 • North Richland Hills, TX 76182 Ph 817.896.6150 • Fax 817284.8408 • www.sprysurveyors.com By MARY LOUISE GARCIA COUNTY CLERK CITY OF SOUTHLAKE 1400 MAIN ST STE 310 SOUTHLAKE, TX 76092 Submitter: CITY OF SOUTHLAKE 100 West Weatherford Fort Worth, TX 76196-0401 PHONE (817) 884-1195 DO NOT DESTROY WARNING - THIS IS PART OF THE OFFICIAL RECORD. Filed For Registration: 5/22/2013 4:35 PM Instrument #: D213130362 E D213130362 6 PGS $32.00 ANY PROVISION WHICH RESTRICTS THE SALE, RENTAL OR USE OF THE DESCRIBED REAL PROPERTY BECAUSE OF COLOR OR RACE IS INVALID AND UNENFORCEABLE UNDER FEDERAL LAW. ®D i CIVIL ENGINEERING j �. 0 TT 1 `.. INC.. TFNAS FIRM No, F-3116 August 12, 2014 Ms. Alex Ayala, P.E. City of Southlake 1400 Main Street Southlake, Texas 76092 Re: Rucker Commercial on Southlake Blvd. Detention Requirement Dear Ms. Ayala: Thank you for meeting with me on Friday, July 251' regarding the detention requirement for the above referenced property. The letter is a follow up to that meeting and a request for confirmation that detention for the 3 -acre commercial tract owned by Ms. Rucker will not need detention due to its location in the watershed and current drainage policy as reflected in the Integrated Storm Water Management System manual published by the North Central Texas Council of Governments, Section 2.1.9, "Downstream Hydrologic Assessment" on page 2.1-47 thru 2.1-49. (copy attached) Attached is a drainage area map showing the drainage area contributing to the flow in the tributary adjacent to Ms. Rucker's property. The tributary crosses under FM 1709 a few hundred feet west of Ms. Rucker's 3 -acre parcel and flows in a south by southeast direction affecting the subject tract near its southerly end. According to available data and as shown on the attached drainage area map, the overall drainage area is 236.2 acres to the point that the subject tract drains into it. Since the subject tract is only a total of 3 acres in size, it is much, much less than the 10% threshold of the 10 percent rule" as defined by NCTCOG's iSWM (also attached). It's actual contribution is 1.27% of the watershed and therefore will have no significant impact on the flow of the creek whether detention is provided or not. Therefore, I request that the detention requirement be waived for the subject 3 acre commercial tract owned by Ms. Rucker. We do recognize that a more detailed study will be required to ascertain the limits of flow of the 100 -year (1%) storm in the tributary near the southern portion of the property as further commercial improvements are constructed to ensure that the buildings are not in the floodplain. This is required irrespective of the detention requirement. Please call if you have any questions and thank you for your assistance. Sincerely, DeOtte, Inc. W.1 (b�r Richard W. DeOtte, P.E., CFM ..RICHARD W. DeOTTE 74232 _.. ,i ►:t�p: _��(s r PHONE (817) 337-8899 420 JOHNSON ROAD; SUITE 303 - KELLER, TEXAs 76248 FAx (817) 337-5133 %vmv.DE0T7S.COM January 2006 2.1.9 Downstream Hydrologic Assessment 2.1.9 Downstream Hydrologic Assessment The purpose of the Streambank Protection and Flood Control criteria is to protect downstream properties from flood and erosion impacts due to upstream development. Some Local Criteria require the designer to control peak flow at the outlet of a site such that post -development peak discharge equals pre - development peak discharge. It has been shown that in certain cases this does not always provide effective water quantity control downstream from the site and may actually exacerbate flooding problems downstream. The reasons for this have to do with (1) the timing of the flow peaks, and (2) the total increase in volume of runoff. Due to a site's location within a watershed, there may be very little reason for requiring flood control from a particular site. In certain circumstances where detention is in place or a master drainage plan has been adopted, a development may receive or plan to receive less that ultimate developed flow conditions from upstream. This might be considered in the detention needed and its influence on the downstream assessment. Any consideration in such an event would be with the approval of the local authority. This section outlines a suggested procedure for determining the impacts of post -development storm water peak flows and volumes that a community may require as part of a developer's storm water management site plan. 2.1.9.1 Reasons for Downstream Problems Flow Timing If water quantity control (detention) structures are indiscriminately placed in a watershed and changes to the flow timing are not considered, the structural control may actually increase the peak discharge downstream. The reason for this may be seen in Figure 2.1.9-1. The peak flow from the site is reduced appropriately, but the timing of the flow is such that the combined detained peak flow (the larger dashed triangle) is actually higher than if no detention were required. Flow } Peak flow increase Total flow with detention Figure 2.1.94 Detention Timing Example Time In this case, the shifting of flows to a later time brought about by the detention pond actually makes the downstream flooding worse than if the post -development flows were not detained. This is most likely to happen if detention is placed on tributaries towards the bottom of the watershed, holding back peak flows and adding them as the peak from the upper reaches of the watershed arrives. Increased Volume ISWW1 Design Manual for Slte Development 2.1-47 2.1.9 Downstream Hydrologic Assessment January 2006 f An important impact of new development is an increase in the total runoff volume of flow. Thus, even if the peak flow is effectively attenuated, the longer duration of higher flows due to the increased volume may combine with downstream tributaries to increase the downstream peak flows. Figure 2.1.9-2 illustrates this concept. The figure shows the pre- and post -development hydrographs from a development site (Tributary 1). The post -development runoff hydrograph meets the flood protection criteria (i.e., the post -development peak flow is equal to the pre -development peak flow at the outlet from the site). However, the post -development combined flow at the first downstream tributary (Tributary 2) is higher than pre -development combined flow. This is because the increased volume and timing of runoff from the developed site increases the combined flow and flooding downstream. In this case, the detention volume would have to have been increased to account for the downstream timing of the combined hydrographs to mitigate the impact of the increased runoff volume. Figure 2.1.9-2 Effect of Increased Post -Development Runoff Volume with Detention on a Downstream Hydrograph 2.1.9.2 Methods for Downstream Evaluation The downstream assessment is a tool by which the impacts of development on storm water peak flows and velocities are evaluated downstream. The assessment extends from an outfall of a development to a point downstream, determined by one of two methods: • Zone of Influence — Point downstream where the discharge from a proposed development no longer has a significant impact upon the receiving stream or storm drainage system • Adequate Outfall — Location of acceptable outfall that does not create adverse flooding or erosion conditions downstream These methods recognize the fact that a structural control providing detention has a "zone of influence" downstream where its effectiveness can be felt. Beyond this zone of influence the storm water effects of a structural control become relatively small and insignificant compared to the runoff from the total drainage area at that point. Based on studies and master planning results for a large number of sites, a general rule of thumb is that the zone of influence can be considered to be the point where the drainage area controlled by the detention or storage facility comprises 10% of the total drainage area. This is known as the 10% Rule. As an example, if a structural control drains 10 acres, the zone of influence ends at the point where the total drainage area is 100 acres or greater. Typical steps in a downstream assessment include: 2.1-48 ISWMTM Design Manual for Site Development January 2004 2.1.9 Downstream Hydrologle Assessment 1. Determine the outfall location of the site and the pre- and post -development site conditions. 2. Using a topographic map determine a preliminary lower limit of the zone of influence (approximately 10% point). 3. Using a hydrologic model determine the pre -development peak flows and velocities at each junction beginning at the development outfall and ending at the next junction beyond the 10% point. Undeveloped off-site areas are modeled as "full build -out" for both the pre- and post -development analyses. The discharges and velocities are evaluated for three storms: "Streambank Protection" storm, either the 1- or 2- year, 24-hour event "Conveyance" storm of either the 5-, 10-, or 25 -year, 24-hour event 100 -year, 24-hour storm event 4. Change the land use on the site to post -development conditions and rerun the model. 5. Compare the pre- and post -development peak discharges and velocities at the downstream and of the model. If the post -developed flows are higher than the pre -developed flows for the same frequency event, or the post -developed velocities are higher than the allowable velocity of the downstream receiving system, extend the model downstream: Repeat steps 3 and 4 until the post - development flows are less than the pre -developed flows, and the post -developed velocities are below the allowable velocity. Allowable velocities are given in Table 4.4-3 in Chapter 4 of this manual. 6. If shown that no peak flow increases occur downstream, and post -developed velocities are allowable, then the control of the flood protection volume (Qf) can be waived by the local authority. The developer saves the cost of sizing a detention basin for flood control. In this case the downstream assessment saved the construction of an unnecessary structural control facility that would have been detrimental to the watershed flooding problems. In some communities this situation may result in a fee being paid to the local government in lieu of detention. That fee would go toward alleviating downstream flooding or making channel or other conveyance improvements. 7. If peak discharges are increased due to development, or if downstream velocities are erosive, one of the following options are required. • Document that existing downstream conveyance is adequate to convey post -developed storm water discharges (Option 1 for Streambank Protection and Flood Control) • Work with the local government to reduce the flow elevation and/or velocity through channel or flow conveyance structure improvements downstream. (Option 2 for Streambank Protection and Flood Control) • Design an on-site structural control facility such that the post -development flows do not increase the peak flows, and the velocities are not erosive, at the outlet and the determined junction locations. Even if the results of the downstream assessment indicate that no downstream flood or erosion protection is required, the water quality steps of the integrated Design Approach will still need to be addressed. 2.1.9.3 Example Problem Figure 2.1.9-3 illustrates the concept of the ten -percent rule for two sites in a watershed. Discussion Site A is a development of 10 acres, all draining to a wet Extended Detention (ED) storm water pond. The flood portions of the design are going to incorporate the ten -percent rule. Looking downstream at each tributary in tum, it is determined that the analysis should end at the tributary marked "80 acres." The 100-acre (10%) point is in between the 80 -acre and 120 -acre tributary junction points. ISWM m Design Manual far Site Development 2.1-49 - -.-., =r¢�.- .- ye.4 •�j.-i-...-eve+e-.w!.cxw-o-+rve:nw'x-uc�-<sv�rnm�• n:nstxxua^x..