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HomeMy WebLinkAbout02/29/2002, B-2 - SAN LUIS OBISPO PHASE II WATERWAY MANAGEMENT PLAN Council Agenda Report- San Luis Obispo Phase lI Waterway Management Plan Page 4 duration of flood events and the fact that the 10 year flows would continue through the culvert throughout the course of the floodevent, thus maintaining a relatively strong flow. One home on Fox Hollow Road would be affected by the project and would need to be relocated out of the flood zone. San Luis Obispo Creek Mid-Higuera Vegetation Management and Minor Channel Excavation (Figure 2) This is a measured, environmentally sensitive channel maintenance program and minor channel excavation on the east side of the creek just below the Marsh Street Bridge. This program and project was included in the adopted Mid Higuera Plan. Similar programs could be carried out throughout the creek system but particularly in areas such as the mid-Higuera where the flow restrictions are so great. The work involves reducing channel roughness of the creek banks by thinning_ and limbing up the willows, and inter planting taller growing,single trunked native trees on the creek banks such as sycamores and cottonwoods. These taller trees would eventually shade out the shorter willows. In the short term, the lower branches on existing willows would be thinned during an annual maintenance visit. Any large gaps in the canopy would be inter-planted with tall, straight, tree forming species. Work would focus on willows along the lower channel banks. The process would require continual environmental monitoring to ensure little or no negative impacts. The initial goal is that channel capacity can be increased by reducing channel flow resistance to flow over a 5-7 year period. The channel excavation proposed is on the east side of the creek just south of the Marsh Street Bridge. Engineering Analysis This project will allow the getaway from the Marsh Street Bridge to be improved and should improve the ability of the bridge to carry high flows. If not done carefully it could lead to erosion of banks, as velocities in the area would increase. The excavation part of the project is particularly important to allow the water to escape under the Marsh Street Bridge and to redirect water back into the creek that does manage to enter Higuera Street. The excavation would occur after the property at the southwest corner of Higuera and Marsh Streets has been acquired and the improvements removed by the City as proposed in the Mid-Higuera Street Plan. Environmental Evaluation Vegetation removal would be undertaken annually to prevent vegetation buildups that could lead to storm flow blockages. This would be done by thinning lower branches that can catch debris, but not eliminate existing shade. No manipulation of the low or normal flows is anticipated. The channel excavation is designed to allow flows around the Marsh Street Bridge to immediately return to the creek. Excavation would occur on a slope from Higuera Street to the "back side" of the riparian zone. The riparian area itself, and the low flow channel would not be a- �f Council Agenda Report- San Luis Obispo Phase II Waterway Management Plan Page 5 affected. Excavation would occur on currently developed land or on the grassy portion of the Mathews property. The environmental impact of this work on the creek is therefore expected to be minimal. San Luis Obispo Creek Mid-Higuera Flood Plain Excavation (Figure 3) This is the overflow by-pass channel that the Council has already approved as a part of the Mid- Higuera Plan. The project runs from Marsh Street to Madonna Road and creates a by-pass channel along the west side of the existing creek. The only points where existing creek banks will be disturbed are at the places where creek water is allowed to enter the by-pass channel and rejoin the creek. Engineering Analysis The only way to significantly increase the amount of water carried in the creek and thus reduce the level of flooding adjacent to the creek is to provide additional area of creek for the water to flow in. This project provides a channel to one side of the existing creek, with a bottom higher than the existing_ creek that will have water flowing in it only when stream flow exceeds a 2-5 year storm. Environmental Evaluation The parallel channel would have minor impacts on the low flow channel at the "take out" and "put in" points of the parallel channel. This would include riparian vegetation removal at those points, although selected vegetation may be able to remain, or new vegetation to restore or maintain shading of the creek would be emplaced. The low flow channel itself would not be touched. The parallel channel would lie outside of the existing riparian vegetation with a few minor exceptions. The sides of the channel would be vegetated with riparian species and the net effect on the terrestrial environment would be to create a wider creek corridor, similar to that which exists at the Water Reclamation Facility. Therefore, it is concluded that the environmental impacts of flood control activities in this reach would be minimal. San Luis Obispo Creek Moderate Channel Improvements-Mausoleum to Prado Road (Figure 4) The project envisioned here is an increase of creek capacity created by construction of a by-pass channel with some minor levees outside of the existing riparian area along the existing creek. Areas of existing bank will be disturbed at points where the water is allowed to enter the by-pass channel and rejoin the existing creek. Within a 35-meter wide right-of-way (r/w), a separate trapezoidal channel would be excavated at the bottom with an elevation about 1 meter above the existing flow line. This work runs from about the restrictions at the Mortuary downstream for about 1400 meters to just south of the Prado Road Bridge. Council Agenda Report- San Luis Obispo Phase H Waterway Management Plan Page 6 Engineering Analysis The capacity of the waterway could be increased to contain a 25-year flow. This will require a 20-meter wide bypass channel (within the 35 meter r/w). When the capacity is exceeded overland flow would leave the creek beginning at various points below Marsh Street and flow across Highway 101. The alternative of containing all of the design flows within the existing creek bed by construction of levees along the-low side of the creek was investigated. The impact on up stream property was substantial because the levees raised the water level in the creek causing a backwater effect and increased flooding that would be felt up to the Madonna Road Bridge. Environmental Evaluation This portion of the project would have impacts similar to those outlined for the Mid-Higuera parallel channel. There would be vegetation removal at the `-`take out" and "put in" points, but the existing riparian corridor would be left between those points. The low flow channel itself would not be touched. In this reach the two "islands"left by construction of the parallel channels would contain high value habitat, which with control of exotics could be considerably enhanced. The overall effect would therefore again is the creation of a much wider creek corridor. Therefore, it is concluded that the environmental impacts of flood control activities in this reach would also be minimal. Prefumo Creek and San Luis Obispo Creek (Figure 5) Prefumo/San Luis Obispo Creek Project This is actually two projects under one title. The first project is a culvert/bridge project to improve the flow characteristics of Perfumo Creek from the west side of the freeway to its convergence with San Luis Creek on the east side. The second project would require creation of a widened flood plain adjacent to the existing channel both above and below Los Osos Valley Road (LOUR). The project would begin at the south end of the Wastewater Reclamation Facility where the natural creek swings towards the freeway. There is a cut off channel that goes from this point directly towards Los Osos Valley Road. The consultant feels this cut off channel needs to be deepened to become more effective. In addition the project would widen the east bank below LOVR about 30 meters from the existing bridge to about the existing City Limits, a distance of about 150 meters. The San Luis Creek structure under LOVR would not need modification. Engineering Analysis This project would increase the capacity of the Perfumo Creek culvert under the freeway to handle a 100-year flood event. In this case the 100-year event is a combination of the flows from Perfumo Creek itself and the overflows from San Luis Creek that occurred upstream beginning at the Mausoleum. Replacement of the freeway culvert would be required for the area to handle these flows and would most likely occur with the LOVR interchange improvement project. Council Agenda Report - San Luis Obispo Phase H Waterway Management Plan Page 7 The improvements to the open channel. flow characteristics of San Luis Creek from its convergence with Perfumo Creek to Higuera Street would take place to allow capacity to carry a 100-year event assuming that all other improvements discussed have been implemented. This means that a 100-year event will be defined as assuming that all improvements upstream on San Luis Creek, Perfumo Creek and Stenner Creek have been implemented. Environmental Evaluation This portion of the project is the most difficult because of the problems caused by the confluence of the two creeks and the proximity of the freeway and Los Osos Valley Road and the possible future improvements to the LOVR interchange. Above LOVR an old flood bypass channel, which has silted in and been colonized by willows, would be re-excavated to a level lower than it was originally. A habitat "island" containing the cypress trees that can be seen from LOVR would remain unless an engineering study shows that it must be removed to permit storm flows in the area to escape downstream. Reestablishing the old bypass or parallel channel would result in the removal of a fairly large stand of willows that have established there over time. The low flow channel of San Luis Obispo Creek would not be touched. Below LOVR the recommendation would be to again create a design with a "take out" point, now occupied by some large eucalyptus trees. From that point there would be a parallel channel outside of the riparian corridor and extending downstream to a point where the flows can be safely returned to the creek. Two things should be pointed out: a) the next downstream property has a flood easement overlying it, which is held by the County; and b) the LOVR interchange improvements may significantly affect statements made here; until alignments have been chosen a final creek design is impossible to foretell. However, the LOVR interchange project will receive its own EIR and the City will be pushing for two goals simultaneously: good traffic design and minimal environmental impacts on the creek system. Given the caveats above, the impacts of this phase of the project will be similar in nature, though greater in scope, to the upstream impacts. However, if an engineering study shows that the habitat island above LOUR must be removed, the environmental impact of this portion of the project will be increased. This can be mitigated, but will be more direct in the form of shade loss that will take some years to restore. The overall assessment of this portion of the project is that there are environmental impacts to the riparian corridor that are unavoidable, but they can be mitigated by new riparian plantings alongside the new channels. Stenner Creek Stenner Creek Bridge Replacements (Figure 6) Foothill Bridge, Murray Bridge, and Santa Rosa Bridge The Foothill Bridge is currently undergoing replacement. The Murray Bridge, near the intersection of Murray and Santa Rosa, is a single span structure that cannot handle a 100-year storm. The creek banks all along Stenner Creek can carry a 100-year storm. The replacement of i Council Agenda Report - San Luis Obispo Phase II Waterway Management Plan Page 8 these two bridges with new structures that can pass a 100-year storm will allow the creek to cant' the 100-year flood flows. Currently, when the water gets out of the banks at these two bridges, it bypasses down to Old Garden Creek and the neighborhoods between Stenner Creek and North Broad Street. The Murray Bridge is old, narrow and poorly impacts the intersection of Murray and Santa Rosa Street. Replacement will benefit both traffic circulation and flood flows. Santa Rosa Bridge is on State Highway 1. The structure is old and has been modified by several extensions making computer flow modeling difficult. The consultant has recommended that the City conduct additional studies in cooperation with CalTrans before coming to any conclusions about whether this bridge should be replaced. Engineering analysis The existing creek can handle a 100-year flow all the way to its confluence with San Luis Creek. The only restrictions in the system are the three bridges. The bridges, when replaced, should be designed for the ultimate 100-year flow. The best order of bridge replacement should be further studied as there are valid arguments concerning liability issues with only a partial implementation of the three bridge replacements. On one hand starting upstream immediately decreases flood damage at that area but in some amount, will increase flooding downstream. While implementing flow restrictions upstream until all improvements are in results in flooding areas of town unnecessarily. The order in which the capacity enhancements should take place is not a part of this policy recommendation but is mentioned because it has relevance to the upcoming Foothill Bridge replacement project. Environmental Evaluation Replacing the current bridges over Stenner Creek with clear spans is considered a beneficial impact. 2. Design Manual Policy No Net Fill—a new policy. Attachments 3 and 4 Staff concurs with the consultant's recommendation to adopt a "No Net Fill Policy" for areas in the flood plain not currently developed. The entire area affected .by this policy is shown in Attachment 3. Within the City, this is an area along San Luis Obispo Creek between Elks Lane and Prado Road, areas south of Los Osos Valley Road, and in areas pending annexation such as the Dalidio property and the airport area. "No Net fill" means that when a developer wishes to build in an existing flood plain he must balance the area of cut and fill below the flood plain 100-year water level so that the flood level is not changed. See Attachment 4. The developer would also have to prove, as do all other developers, that their project has no impact on other development. Typically a developer will regrade a site in the flood plain to raise the area where building is to occur so that the foundations are above the high water level. The no net fill policy would allow this, but a compensating amount of material would be taken away to balance the fill and so result in "No Net Fill". The � � V Council Agenda Report - San Luis Obispo Phase II Waterway Management Plan Page 9 volume of material to be balanced would include any part of a building constructed below the flood level. This could occur in commercial development, but not residential. Under current. regulations commercial buildings may be built with the lowest floor below the 100 year flood level providing the floor is flood protected. Residential property on the other hand must be constructed with all occupied floors above the 100 year flood level, flood protection only, is not allowed. When the Gottschalks project was built, the building pads were raised above the 100-year flood level and the adjacent parking lots lowered to compensate for the fill. They still caused floodwaters to rise. This policy would have caused them to lower the parking lots about an additional foot. Within developed areas of the community this policy change would be hard to accomplish because lots are smaller making the opportunity to compensate very difficult. The Airport Area warrants special discussion. The Airport Area Specific Plan contains three facility Master Plans. One of those is a Master Storm Drainage Master Plan, which provides 100-year flood protection standards for all development within the Area. Once the Plan is fully implemented, the No-Net Fill Policy would not apply. However, until areas are so protected or if the Airport Area Plan is not adopted or is adopted and not implemented, the No-Net Fill Policy will prevail. Environmental Evaluation As already stated there will be a full environmental evaluation of the project and project alternatives prior to the Council making any formal adoption of this policy. In addition any project proposed by a developer will under go environmental review. City staff has reviewed the Consultants recommendation and expects that the "No Net Fill" policy will have positive benefits on the environment without significantly impacting the ability of property ownersto reasonably develop their property in the City. Current policy allows filling in the flood plain. This proposed policy does not change that. The new policy will ensure that the level of flooding does not increase at all and do that without increasing erosive velocities in the waterways. In some cases this policy may result in construction of"by-pass" channels or "flood terraces" to allow high water a way to be carried without any effect on the main channel and the normal low flows. There may be increases in riparian zones along the waterways. FISCAL IMPACT All projects in all reaches as well as the new ones identified above will be expensive. However, the city is in the early stages of forming a storm water utility that, overtime, will be able to fund these expenses. The beneficial environmental aspects of the projects cannot be quantified (i.e. building a secondary channel keeps development further away from the creek habitat), etc. Council Agenda Report - San Luis Obispo Phase II Waterway Management Plan Page 10 ALTERNATIVES These are policies that will define the "project" for an upcoming EIR. If the Council is uncomfortable with these policies for the "project defmition" alternative policies must be chosen so as to let the project and its EIR proceed. The Council could adopt policies that implement fewer projects and continue to flood residents and businesses, keep erosion at current rates, and provide no new protected areas for habitat. This alternative will cost the City less but will likely result in criticism by citizens impacted by flooding. The Council could adopt policies that implement more projects, provide more flood protection, spend considerably more money, and require substantial habitat disturbance and repair. More significant projects may also require more property acquisition and business and residential relocations. ATTACHMENTS 1. Existing Flow Capacity and Future Design Goals 2. Capacity That Can Be Achieved as a Result of Various Projects 3. Special Floodplain Management Zones 4. Proposed Regulations: No Net Fill—No Flood Increase 1:car/phase 2 wwmp January cc final for 1-29-02 ATTACHMENT 1 E CD I Z E " " Em ` " J « °®ts 00 k 2 ` 5ƒ . ~ )}« cm « « — m %� J / cq 7\ » C-a 2 ] ) ■# _ m a2 § R0 Co LOC k 9L CL \ . \� LO � \ �� ` m — � ff /± /#e fffm / ) �k� — ] } I} wQ WL)Wu % / . . .. . _ —52 _ � _ Lo In In 04 m � ) . . wLO . / ) § . Q. 0 m ® } { f k2 �\■CL 0 / . � . � 2 \\ / CD \ kk)I= \ § , r f _ — — \ Z k LU 0 — ƒ % e r kk 7 ) � f% f) oM \%%�j20 -00 k \) ® � 2 ■ ))Ui.. J §§ kkk ) } [�\ E \Z § ) �� �� ) ° o ° E ° © �a— = >�=�� _m \) \E . ' ) ( kkkk �! -0= ,a ' 2 ° LL | 7 / 0 810 (L 00 EL § 4� E§ / fk # �. � )Ij k]2 ) I) � 00 � i . . < \j )) JCL § § § § E co ( c ` ) � ■ ; _ _ @ « ® ° ° 5 ® —] » / ) _ CL ; _ § ƒ 5 LL— a ] Eta ƒ% wI § c0 ( � w —® � . - - - o7—// Attachment 2 Level Location Benefit Benefit/Cost> 1 Recommended Project One Mid-Higuera Vegetation 15 year Yes Yes Management One Stenner Creek Bridge 100 year Maybe Replacements Foothill Bridge Yes Murray Bridge Yes Santa Rosa Bridge Requires additional study One Hwy 101 Detention Basin 100 year Maybe Yes Two Moderate Channel 25 year No Yes Improvements—Mausoleum to Prado Two Moderate Widening below 25 year No Yes LOVR Two Mid-Higuera.Flood Plain 20 year No Yes Excavation Three Prefumo/San Luis Obispo 100 year** No Yes Creek Project Note Benefits shown for each level assumes all related projects of lower and the same level have been accomplished. Benefit/Cost relationship is estimated by consultant and is very preliminary. Typically B/C relationship must be greater than 1 to justify expenditure of public funds. * This project was already approved by the Council and is included here for consistency. **100 year capacity is available in both creeks but only if there is no break out of flows on San Luis Obispo Creek above Prado Road. 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' 'Imic Special Floodplain ManagementZflnes Figure scn creek rhm 11 2 Orahege AAawel al1ESFA'ENGINEERMG CORPORATION ,r..rr:•F.fro:+...oa.-se..Cad'Ir•..T_=.=<-►.. 14TL4t ,:.•wi .. _ K.11.9:'•ii.l O Attachment 4 TERRACE 100 YEAR FLOOD ELEVATION It 15 YEAR FLOOD ELEVATION FLOODPLAIN 2 YEAR FLOOD ELEVATION 1. TYPICAL CONDITIONS CREEK NEW 100 YR 1. ............ .............60 You YA............ IMPORTED FILL , TERRACE I.................1VEW.15yw...................... FLOODPLAIN 2. EXISTING CONDITIONS CREEK FILL .........................................................100 YEAR FLOOD ELEVATION TERRACE . 15 YEAR FLOOD ELEVATION FLOODPLAIN CREEK 3. PROPOSED REGULATIONS NO NET FILL - NO FLOOD INCREASE d- 1v Jan 28 02 08: 43a Dave Romero 805-594-1336 P. 1 N ?ING AGENDA DAI E�ITEM #= U�fANCIL ❑ CDD DIR p FIN DIR To: Honorable Mayor and City Council MIACAO p 5FIt CHIEF RNEY W DIR LERK/ORIG ❑ POLICE CHF From: Dave Romero ❑ T EAD ❑ REC DIR ❑ UML DIR Subject: waterway Management Plan ❑ HR DIR I have been a member of Zone 9 since its inception and have actively g2*ticipated on all phases of the current steely, however I will be unable to attend the January 29, 2002 Councilmeeting to provide imput. Although I have some differences with details of the plan, there will be opportunities to air these at subsequent hearings. At this time it is most imortant that the Council provide sufficient direction that a Project can move through the EIR process. This is one more necessary step to enable the City to continue its long term efforts to protect its citizens from flooding. Keep it moving. Thanks RECEIVED JAN 2 8 2002 SLO CITY COUNCIL MF7NG AGENDA DAI, ,!(TEM#® ��Il��������iii►►�III�Pp1�IIII�► council mcmomnaum [City of San Luis Obispo DATE: January 29, 2002 TO: Mayor& Members of the City Council RECEIVED VIA: Ken Hampian, CAO 9'K FROM: Mike McCluskey, Public Works Director SLO CITY COUNCIL SUBJECT: SLO Phase II Waterway Management Plan Attached are Figures 1 through 5 inadvertently left out of, but referenced in, the Council Agenda Report. Staff will also have these figures available as overheads this evening. c: Wendy George, ACAO Dept. Heads C NCIL ❑ CDD DIR The Tribune 2'qA0 ❑ FIN DIR 0 0 FIRE CHIEF ORNEY [2 PW DIR jj E K/ORIG ❑ POLICE CHF P HEAD ❑ REC DIR ❑ UTIL DIR Council Memorandum 1 l 1 I ♦ ' s f � rl � ` Y YL 1 _ ? �lliss 1 * �`-tet; •• -. .� �.I _ + t J � - 1. TyY.% 11� 51FSf � �► I � �"7" ,r�s"-'/ ro'• 't:' f 1� � y1 ���a� Jy�- _-may{\�.+ et'' ♦,ti . 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Yet 4�' � ` � �, \� . �•i "rw.t , , d ��t.Slf . fY-%o s _ rte.` �.'� [ '� ,- +y SJt�+h ..... r40t':.,.�As n `• ' '; t Fit ga• + l Nov. IS J.no..rr.p."il .• • .f� ., ;� .. -� �r .�y-t< °���, it Jr/ J � -Rif t:�• ll� �� � - y ',yam''' l it �' itNl2gYfYf . LEGEND CUT AREA CHANNEL CENTERLINE .. if4 ,� � u. 77]' !>� �_SAY►r`— w�-'_ ) . !� ��• o r,'� *hy�a•Y'>�.�F" '.1 "JJ�F >�' 1/ LLam– � -.. _ - . - � 06have 25-yr M . l '`•"!�.=+ d'qty/ `., '► Existing culverts capacityta • J �l rManagevegetationng y ' creek alo 91 Creek is reduced. 4 per♦ ♦ 100-year �"y�. '�j ✓ .] �> 1! �'/�), M: .. � Y,:7. � .Ls 'y t 7jA�`'/, - 1 � r /•�.! Fi �F}-.'e^^ 1 ':. � W � �.. � fie• �•�. l •rte, r/!v�� �.•��� amp lilt .r Y.,. '- 7 ♦ a r1- l:t A7 e'f$ „fd"P" iL..r CUT AREA CHANNEL CENTERLINEY� S sl' ♦ NHD•'J4 tii. y�� 11Z: yMr. y Zl�t�r_j' 5711 'y .i-