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Hydromodification Management Plan

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Hydromodification Management Plan HYDROMODIFICATION MANAGEMENT PLAN San Mateo Countywide Stormwater Pollution Prevention Program May 12, 2005 May 12, 2005 Mr. Habte Kifle San Francisco Bay Regional Water Quality Control Board 1400 Clay Street, Suite 1400 Oakland, CA 94612 Subject: Submittal of Hydromodification Management Plan Dear Habte: This transmits two copies of the San Mateo Countywide Stormwater Pollution Prevention Program’s (STOPPP) Hydromodification Management Plan (HMP) as required by Provision C.3.f. of the permit amendment. The Plan has been totally revamped based on the Water Board staff’s comments on the November 12, 2004 draft HMP. The HMP responds to the January 25, 2005 letter prepared by Keith Lichten and yourself on the draft Plan and the direction provided by Water Board staff at our meeting on February 23, 2005. STOPPP may need to amend or modify this HMP based on the new information or other circumstances created by the still evolving nature of hydromodification control. Some modifications in STOPPP’s HMP may be appropriate based on the results of the current process to adopt an interim HMP for the SCVURPPP. There is also the possibility that the HMPs that have been prepared by other municipal stormwater programs may contain useful ideas for incorporation into STOPPP’s HMP. In addition, it is possible that additional information may become available on the locations of hardened channels or other factors that would affect the areas proposed as exempt from hydromodification management requirements or that affect other HMP issues. If you have any questions or comments on the HMP, please contact me. Very truly yours, Robert Davidson TAC Coordinator Enc: Hydromodification Management Plan Table of Contents Table of Contents ........................................................................................................................... i 1. INTRODUCTION 1.1 Problem Statement............................................................................................... 1-1 1.2 Permit Requirements Overview........................................................................... 1-2 1.3 Work Completed Previously................................................................................ 1-2 1.4 Background on San Mateo County Watersheds .................................................. 1-4 1.5 Anticipated Future Development......................................................................... 1-5 1.6 Organization of HMP Report............................................................................... 1-6 2. HMP REQUIREMENTS FOR PROJECTS 2.1 Coordination with Other Countywide Programs ................................................. 2-1 2.2 HMP’s Management Objective and Performance Criteria .................................. 2-1 2.3 Water Board’s HMP Approval Process ............................................................... 2-5 2.4 Implementation Timeline..................................................................................... 2-5 3. AREAS SUBJECT TO HMP 3.1 Background.......................................................................................................... 3-1 3.2 Assessment Approach.......................................................................................... 3-1 3.3 Exempt Areas....................................................................................................... 3-2 4. HMP IMPLEMENTATION GUIDANCE 4.1 Applicability and Requirements .......................................................................... 4-1 4.2 Incorporating HMP Requirements into Local Approval Process ........................ 4-2 4.3 Flow Duration Control Design............................................................................. 4-4 4.4 Inspection and Maintenance Requirements ......................................................... 4-7 4.5 Monitoring and Reporting Requirements ............................................................ 4-8 F:\Sm4x\Sm45\Sm45-03\HMP Report\HMP Report Final\0TOC_HMP.DOC i LIST OF FIGURES 3-1 Watershed Areas ................................................................................................ 3-12 4-1 HMP Applicability and Requirements Flow Chart............................................ 4-10 4-2 Typical Development Review Process Flow Chart for Provision C.3 Incorporating HMP Requirements..................................................................... 4-12 LIST OF TABLES 1-1 Watershed Imperviousness and Percent Unmodified Creek Channels................ 1-5 3-1 Information Resources for Determining Location of Channel Segments Continuously Hardened Downstream to their Outfall in the Bay........................ 3-4 3-2 ABAG 2000 Land Uses Occurring in San Mateo County That Are Designated As Developed Land Use Types Using Best Professional Judgment.................... 3-6 3-3 ABAG 2000 Land Uses Occurring in San Mateo County that Are Designated As Undeveloped Land Use Types Using Best Professional Judgment................ 3-8 3-4 Estimated Percent Imperviousness and Percent Developed Area in Lower Subwatershed Areas of Bayside Watersheds in San Mateo County.................... 3-9 3-5 Location of Streets or Highway Delineating Subwatersheds Areas of Bayside Watersheds in San Mateo County........................................................................ 3-9 F:\Sm4x\Sm45\Sm45-03\HMP Report\HMP Report Final\0TOC_HMP.DOC ii 1. INTRODUCTION 1.1 Problem Statement As land becomes urbanized natural watershed and creek processes become altered by the construction of impervious surfaces, such as roof tops, roads, and parking lots. In addition, creeks are altered by installing drainage and flood control structures that modify creek channels. One of the most important changes is that creeks are typically isolated from their flood plains. In alluvial areas creek channels are sized by natural conditions to carry up to about a 1.5 to 2 year recurrence interval storm flows with larger storm flows overtopping the creek banks. Restricting storm flows that previously overtopped creek banks so that they remain in the creek channels destabilizes the channels as erosion and downcutting occurs to accommodate the larger flows. The hydrologic changes caused by paving over watersheds and installing storm drain systems that increase the efficiency with which flows reach creeks, lead to many creek impacts. These include increased frequency and magnitude of bankfull and subbankfull flows, larger sediment loads, creek bed down cutting and bank erosion as channels enlarge, and other changes in channel morphology. All of these factors lead to loss of creek habitat and aquatic diversity and adversely impact the beneficial uses of creeks. The San Francisco Bay Regional Water Quality Control Board (Water Board) is requiring municipal stormwater programs to develop and implement hydromodification management plans (HMPs). While many factors affect hydromodification, this HMP focuses on minimizing additional increases in the flow rates and durations of flows caused by increases in impervious surfaces and the construction of storm drainage systems that drain runoff faster than occurred under pre-project conditions. San Pedro Creek in the City of Pacifica provides an example of the complex connection between historic land use changes and creek channel downcutting and bank erosion. Since 1960 one section of this creek’s bed has incised vertically 15 feet. Some of the factors that are believed to be contributing to the creek channel’s instability include the loss of access to its former flood plain, the loss of connection to a former lake, straightening and reducing the channel’s length by one mile, constructing grade control structures in the channel, and the increasingly flashy stormwater runoff caused by urbanization. Further, a plan developed for the creek states that the creek is still not in dynamic equilibrium with the flashy stormwater runoff it receives, and it likely will continue to erode. 1.2 Permit Requirements Overview The Water Board is interested in protecting creeks from excessive erosion and sedimentation caused by increases in flows associated with new development and redevelopment. On February 19, 2003, the Water Board adopted an amendment to the San Mateo Countywide Stormwater Pollution Prevention Program (Program) NPDES Permit, Order No. 99-059, to incorporate F:\Sm4x\Sm45\Sm45-03\HMP Report\HMP Report Final\1 Introduction.doc 1 - 1 May 12, 2005 specific new development and redevelopment requirements, including requirements for an HMP. The requirements apply to development projects that exceed certain thresholds of impervious surface area. Provision C.3.f of the NPDES permit, “Limitation on Increase of Peak Stormwater Runoff Discharge Rates,” describes the HMP requirements. “The HMP will be implemented so that post-project runoff shall not exceed estimated pre-project rates and/or durations …” Runoff controls are not required for projects where the potential for erosion, or other impacts to beneficial uses, is minimal. Such situations may include: discharges into creeks that are concrete- lined or significantly hardened (e.g., with rip-rap, sack concrete, etc.) downstream to their outfall in San Francisco Bay; underground storm drains discharging to the Bay; and construction of infill projects in highly developed watersheds, where the potential for single-project and/or cumulative impacts is minimal (Provision C.3.f.ii). It is important to recognize that the HMP does not address how to correct existing creek channel erosion problems. Rather,
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