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HomeMy WebLinkAboutStaff Report 266-08TO: City of Palo Alto City Manager’s Report HONORABLE CITY COUNCIL FROM:CITY MANAGER DEPARTMENT: PUBLIC WORKS DATE:JUNE 9, 2008 CMR:266:08 SUBJECT: BASEMENT CONSTRUCTION AND DEWATERING IMPACTS This is an informational report and no Council action is required. BACKGROUND Residential and commercial basements and underground parking garages are constructed throughout Palo Alto, except where they are disallowed in the flood zones. If a basement or underground garage site has high groundwater, the contractor will need to dewater the site so they can construct the basement or garage without groundwater filling the excavation. Accordingly, the contractor prepares and submits a dewatering plan to Public Works. The plan typically includes pumping water from the shallow aquifer below the site to a settlement tank and then via a pipe or hose to the closest storm drain inlet in the street. Public Works reviews and approves the dewatering plan, charges a dewatering fee and issues a street work permit. Public Works inspectors confirm the dewatering is done per approved plans and with minimal impact to the community. Public Works currently issues 5-10 dewatering permits for residential basements annually. Recently, a number of citizens have voiced their concerns to the Public Works Department that dewatering has many negative impacts on the community and should potentially be disallowed, especially in residential areas. The concerns have been about the discharge of large volumes of water into the storm drain system, pump noise, land subsidence, tree impacts, groundwater impacts and contaminated groundwater migration. DISCUSSION Public Works and Planning Division staff have been aware of construction dewatering impacts and concerns for a number of years. They have conducted research and sought the advice of experts to address these concerns. In 2004, the Planning & Transportation Commission raised some of the same concerns about dewatering that citizens recently have. Consequently, the Planning Division retained an environmental consultant, EIP Associates, to research and report on these concerns. In 2004, EIP prepared the attached report titled, "Draft Technical Memorandum: Correlation between New Basement Construction and the Groundwater Regime in Palo Alto, California." Further, Public Works Engineering staff has consulted with representatives of the Santa Clara Valley Water District (SCVWD), the California Regional Water Quality Control Board (CRWQCB), the United States Geological Survey (USGS), dewatering contractors, basement contractors, architects, geotechnical engineers, and staff from Public Works’ Environmental Compliance Division and-the Planning and Community Environment’s Planning and Building Divisions about dewatering impacts and concerns. CMR:266:08 Page I of 5 To assist Council in understanding the differences between shallow and deep aquifers (described more completely in EIP’s attached report), staff provides the following descriptions. Shallow aquifers are formed by rain seeping through the ground and pooling close to the ground surface. The top surface of the shallow aquifer is called the water table and is typically 10-30 feet below the ground surface in most areas of Palo Alto other than the hills. This is the aquifer that basement excavations may extend into, necessitating dewatering. Shallow aquifer water is nonpotable as it does not meet drinking water standards. Deep aquifers are separated from the shallow aquifers by impermeable sediment layers, like rock or clay, called aquicludes that prevent shallow aquifer water from reaching the deep aquifers. In Palo Alto, the deep aquifers are approximately 200 feet below the ground surface. Dewatering basement excavations has virtually no effect on the deep aquifers. Certain layers of permeable sediment, like sand or gravel, may trap and hold pockets of groundwater temporarily between shallow and deep aquifers, but these are typically not affected by basement dewatering operations. Below is a brief summary of the above research organized by community key concerns. Discharge Volume A soils report is required for all projects with basements or underground garages. This report determines the depth to the shallow aquifer below the ground surface. If a contractor believes the excavation will go into the groundwater, they will typically submit a drawdown well dewatering plan to Public Works. Drawdown wells are typically installed around the perimeter of the excavation and pump water out of the shallow aquifer to draw down the level of the groundwater so the basement can be constructed without groundwater filling the excavation. These drawdown well systems pump approximately 30-50 gallons per minute into the storm drain system non-stop for 3-6 months while the contractor constructs the basement. The volume of water pumped into the storm drain system from a drawdown well dewatering operation is substantial, typically a few million gallons. It could be used as landscaping water, but it is too large a volume for individual use and too impractical to capture and reuse for other use. The water pumped out of the ground is discharged into the storm drains, which typically discharge into the creeks. San Francisquito Creek is a losing creek, meaning that water is lost by seeping through the creek bed and into the shallow aquifers. So, in this case, water pumped out of the shallow aquifers is added back to it. For water pumped into lined creeks, the water flows to the bay and is lost to the aquifer. The volume of groundwater pumped out of an excavation site is a small fraction of the total volume of the aquifer and does not deplete or lower the aquifer, except, of course, in the immediate vicinity of the excavation. The USGS reports that due to natural (rain) and manmade (irrigation, leaking sewer pipes, and the SCVWD’s groundwater recharge program) methods, more water is recharged into the shallow aquifers than is pumped out of it by all pumping in the Santa Clara Valley. The EIP report also confirmed that the water table is only drawn down CMR:266:08 Page 2 of 5 locally (within tens of feet of the excavation) and reestablishes itself quickly after dewatering ceases. Therefore, the cumulative effect of dewatering on the shallow aquifers is negligible. Pump Noise Dewatering pumps can make excessive noise if installed improperly, and this is a concern for neighboring residents since the pumps run 24 hours a day. Public Works is tightening the requirements for pump operation to eliminate this problem. Water Table Impacts While the City currently prohibits basements in flood zones, there is no blanket prohibition against construction in areas with shallow aquifers. Basements are not typically constructed so deep that they actually go into the water table, but they do in some cases. In other cases, the water table might rise up, as at the end of a particularly wet winter, and surround a basement. However, in these cases, the water table level and the flow of the groundwater are not changed due to the presence of basements, as reported by EIP. Subsidence Land settlement, or subsidence, caused by temporary (such as 6 months) construction dewatering is negligible, as reported by EIP and USGSo For subsidence to occur, dewatering needs to occur over a number of years. Tree Impacts Relative to Water Table Changes The Planning Division arborist reports that in most of the developed areas of Palo Alto the preponderance of absorbing tree roots are not found in lower soil horizon levels below seven feet. Therefore, the majority of temporary dewatering projects are not expected to impact trees. If a tree’s roots are however deep enough and have been determined, on the basis of a certified arborist report or other qualified assessment, to be dependent on the water table, then the mitigation would be for the contractor to provide separate irrigation for the tree(s) during the dewatering period. Contaminated Groundwater Migration Citizens have expressed a concern that large volumes of groundwater being pumped out of the aquifers might cause nearby contaminated groundwater plumes to migrate towards the pumping site. When an application is submitted, staff checks dewatering sites against known contaminated groundwater plume maps. If a site is within a certain proximity to a known plume, staff requires the water to be tested for contaminants prior to and during discharge. The contractor must retain an independent testing service, test for the contaminants Public Works specifies, and submit those results to Public Works. If the water is contaminated, as it was in one case near the Stanford Research Park superfund site, it must be treated before it can be released or discharged to the sanitary sewer under permit from Public Works. The CRWQCB is drafting requirements for contractors to test groundwater discharged to the storm drain system. Staff awaits the adopted version of these requirements, scheduled for this summer, and will implement them at that time. To date, there has been no evidence that contaminated groundwater has been discharged into the storm drain system or that contaminated groundwater plumes have migrated. CMR:266:08 Page 3 of 5 Discharge of Groundwater after Basement Construction A few years ago, Public Works allowed the use of perforated drain pipes to be installed behind basement walls and under basement slabs when the geotechnical engineer reported that groundwater would not rise to the level of these pipes. The pipes are installed to capture rainwater that filters through the ground and collects behind basement walls in order to minimize the chance of the water leaking through the walls. The pipes drain to a sump where a pump then pumps the water to the street gutter. Unfortunately, after some wet winters, groundwater did rise up to these pipes and was then pumped continuously into the street gutter for long periods of time, creating a number of public nuisance and safety concerns. Accordingly, Public Works adopted a policy two years ago that prohibits the use of perforated drain pipes for basements in areas of the City with relatively high groundwater (east of Foothill Expressway) to eliminate these potential nuisances. Public Works also recommends that applicants for new basement projects retain a waterproofing consultant to ensure the basement does not leak. Older basements that were permitted with perforated drain pipes still may occasionally discharge groundwater into the street gutter. Public Works addresses these cases by working with the homeowners to eliminate the discharge, typically accomplished by having the homeowner raise the pump in the sump above the level of the groundwater. Basement Excavation Some residents have expressed a concern that the excavation pit for a basement comes too close to adjacent properties, potentially jeopardizing the stability of these properties. Although this strictly does not relate to dewatering, staff recognizes it as a legitimate concern. As previously mentioned, the Building Division requires geotechnical reports for al! projects that involve basements or underground structures. A standard feature of these reports is recommendations and requirements from the geotechnical engineer that specify measures to stabilize the excavation during construction. The Building Division inspects all basement construction to ensure conformance with the geotechnical report and to verify all recommended stabilization measures are implemented. In addition, Building Inspectors will require the contractor to install extra precautionary measures before work can continue. Storm Drain Capacity Staff is concerned that dewatering basement excavations may take up too much capacity in the City’s storm drain pipes, minimizing the system’s ability to accommodate storm water and potentially causing or exacerbating flooding. This is not a concern raised by citizens, nor has there been any incidents where dewatering has caused flooding, but staff is developing some guidelines for wintertime dewatering in an effort to avoid a problem. The draft guidelines currently disallow dewatering during the winter unless an exemption is granted by the Director of Public Works. CONCLUSION Staff has researched and analyzed each of the concerns about dewatering raised by citizens. Based on that research, staff believes that the cumulative effects of dewatering basement excavations has minimal impacts on the City and that the practice should be allowed to continue. The attached EIP report essentially comes to the same conclusion. The number of residential basements permitted in the City has increased from approximately 20 a year at the start of the decade to approximately 30 a year currently. However, Public Works only issues about 5-10 CMR:266:08 Page 4 of 5 dewatering permits a year. So, most basements are built without requiring dewatering. Public Works will continue to monitor dewatering activities to ensure the City’s procedures remain sound and protective of Palo Alto. POLICY IMPLICATIONS Staff is currently updating dewatering requirements to ensure that dewatering has minimal impacts to the community. Limitations beyond those discussed in this report would likely result in a wholesale prohibition of basements where groundwater is present, which would be a major policy issue to be decided by Council. Per direction from Council at the May 12, 2008 council meeting, staff will prepare a report on the array of basement construction impacts and issues, including dewatering, and present it to the Planning and Transportation Commission in the near future. ATTACHMENTS Attachment A: Draft Technical Memorandum: Correlation between New Basement Construction and the Groundwater Regime in Palo Alto, California PREPARED BY: DEPARTMENT HEAD: CITY MANAGER APPROVAL: Senior Project Manager GLENN S. ROBERTS Director of Public Works gTEVE EMSLJ~/K~LY ~4ORARIU Deputy City Managers CMR:266:08 Page 5 of 5 ATTACHMENT A Draft Technical Memorandum: Correlation between New Basement Construction and the Groundwater R~gime in Paio Alto, California Statement of the Planning and Transportation Commission’s concerns. At the 14 January 2004 Commission meeting, the plarmmg staff presented a number of proposed changes to the existing regulation of basements in the R-1 zones. Duxing the ensuing discussion, several Cornrmssion members expressed concerns about the impact of basement construction on groundwater levels and flow dkections. Eight specific, interrelated issues were identified. Is groundwater pumping causing or contmibutmg to land subsidence? What are the effects of pumping for months to dewater a basement constx’uction site? Are basements being pemaitted in some inappropriate areas [where the xvater table is only a few feet below the ground surface], creating the need for continuous pumping? What groundwater effects occur if water is xvithdrawn from the water table and pumped into the sewers or creeks? What groundwater diversion effects occur i_f basement wails are built along creeks and/or perforate aquifers? What are the effects on landowners adjacent to, and down gradient from, pumping sties? What are the cumulative effects of basements on the groundwater r4gime? Can basement regulations be crafted to address the hydro-geology of specific building sites? The general concern underlying these issues was expxessed by Commissioner Annette Bailson: the Commission does not have the information needed to identify whether these are issues of concern, or to make informed decisions on the issues. The remainder of this technical memorandum seeks to respond to that underlying concern by provide some backg~:ound information about the listed issues and about groundwater hydrology of the City relative to the construction of basements. Page 1 of 7 2.Differences between shallow (surface) and deep (confined) groundwater aquifers. Defining the Aquifers An aquifer is a body of geologic material, usually rock or some mixture of gravel, sand, silt and clay, that is sufficiently permeable to conduct groundxvater. Some definitions include the stipulation that the body produce an economically significant flow of xvater before it may be considered an aquifer. For the put’poses of this technical memorandum, the broader definition is applied to alloxv for easier discussion of the xvater-beafing formations underlying the City. Of the various types of aquifers, two are of particular interest in this discussion: the shallow or surface aquifer, and the deep or confined aquifer. The relative tern-is ’shalloxv’ and ’deep’ refer to the depth of the aquifer below the surface of the ground (usually expressed as ’number of feet bgs’ in hydrologT studies). A surface aquifer is so named because it is open to the surface of the ground. Rain falling on the ground surface seeps through the soil (infiltration) to some depth where it pools to form a more or less continuous body of water occupying the spaces between sediment particles or rock fragments (groundwater). The top of this body of groundwater is the water table. In the Santa Clara Plain, which forms the lowlands of Palo Alto, the xvater table occurs at depths of as little as ten feet below the ground surface. Being open to the surface of the ground, the surface aquifer is subject to the influences of overlying land cover and land uses. Modem stream channels, such as the numerous reaches of San Francisquito Creek, intersect or overlie the surface aquifer, extracting water from it or adding xvater to it. Paving and construction create artificially impermeable surfaces that prevent local direct infiltration to the surface aquifer. Chemical constituents in urban and agricultural runoff enter the surface aquifer tt~rough infiltration from channels or detention basins, loxvering the quality of the groundxvater. Leaking landfill cells, lea’king underground storage tanks, and liquid spills also contribute to the reduction of xvater quality in the surface aquifer. Although current stewardship has sloxved water quality deterioration, the surface aquifer still cannot be used as a source of potable xvater. A confined aquifer is one that is separated hydrologically from the overlying and underlying sediments and rock and from other aquifers. Usually the separating agent (called an aquiclude) is formed by a layer of impermeable sediment, such as clay, or by impermeable rock, such as unfractured granite. The confined aquifer is not connected directly to the overlying ground surface and is separated from the surface aquifer by an aquiclude. It is, in effect, a separate hydrologic system, gaining water from some distant source (i.e., not local Page 2 of 7 rainfall) and transmitting it to some other relatively distant discharge area. Because the confined aquifer is beloxv, and hydrologically separated from, the surface aquifer, it is, by definition, a deep aquifer, irrespective of the number of feet it is below the ground surface. Several aquifers may underlie each other. This is the case beneath the Santa Clara Plain xvhere geologically recent stream-laid (alluvial) gravel, sand, silt, and clay form a sequence of deposits nearly 1500 feet thick bet~veen the foothills of the Coast Ranges and San Francisco Bay. Channels of ancient rivers depositing this material have been cut off and filled by succeeding intersecting channels, xvhich, in turn, have been buried by the deposits of more modern channels. In this xvay a complex series of sediment layers of unconsolidated (loose), partially consolidated (dense), and consolidated (very dense) material has been built up as the Santa Clara Plain. The layers are discontinuous and of greater or lesser permeabilitT, depending on their density and clay of silt content. A complicating factor in examining such a series of aquifers is that often they are not completely confined. The aquicludes separating the aquifers may not be tota!ly impermeable (in xvhich case they are called aquitards) alloxving water to seep from one aquifer to another. The aquifers may be connected xvithin or outside the local area, arising from a common source or floxving to a common discharge area. The aquifers may be connected artificia!ly through leaks in wells or along pilings passing through the aquifers. Beneath the portion of the Santa Clara Plain in Palo Alto, there is a confining clay layer that separates the surface aquifer from the deeper aquifers, but, on a regional level, this separation attenuates and, eventually, disappears farther south in San Jose. Being separated from the surface aquifer in this part of the Santa Clara Plain, the confined aquifers beneath the City are not subject to the direct influences previously described for land cover and land uses above the surface aquifer. To the extent that groundxvater migrates from the southern part of the Santa Clara Plain groundxvater basin to the northern part, the effects of similar land cover and land uses in areas toxvard San Jose may affect water quality in the deep aquifers beneath Palo Alto. Construction-period Dewatering Effects In general, const_t-uction-period dexvatemag effects are limited to the surface aquifer. This would not necessarily be the case for maior high-rise construction where foundations and beloxv-grade levels may extend 100 or more feet beneath the ground surface, increasing the chances of encountering confined aquifers. It is, hoxvever, the case for the type of relatively shalloxv basement construction being considered in the Zoning Ordinance Update. In the Santa Clara Plain portion of Palo Alto, the uppermost sequence of unconsolidated and partia!ly consolidated a!luvium is about 200 feet thick. This sequence contains the Page 3 of 7 surface aquifer, the base of xvhich is the previously mentioned clay aquidude identified by the Santa Clara Valley Water District (SCVWD) in its 2001 Groundwater Management Plan. The general direction of groundwater flow in this area is northeast toxvard the Bay, so the surface aquifer and the deeper, confined aquifers tend to remain separated in Palo Alto until they reach the vicinity of the Bay margin. The removal of groundxvater from an excavation during beloxv-ground-level construction is necessary to provide safety for the construction xvorkers, and is a prerequisite for xvaterproofing the building’s foundation and subsurface floors. One method for accomplishing this is to dig a small pit beloxv the base of the foundation excavation, slope the excavation so groundxvater drains to the pit, and then pump the xvater out of the pit and into the storm drainage system. Another method is to drill temporary wells around the building footprint and pump directly from the groundwater body to the storm drainage system until the local xvater table drops below the base of the excavation. In either case, groundxvater floxving into the area of draxvdown created by the dewatefing process is deflected toxvard the base of the excavation, xvhence it is pumped to the storm drainage system. Groundxvater beyond the influence of the dexvatermg process continues to floxv normally. Dexvatering pumping continues until the foundation and subsurface floors are completed and the excavation is filled. The amount ofxvater deflected depends on the level of the xvater table, the permeability of the material adjacent to the excavation, and the length of dme the excavation needs to be kept open and dry. An increase in aW of these factors increases the amount ofxvater deflected. This amount is small xvhen compared to the total volume of available groundwater directly beneath the Santa Clara Plain (see below). Because the deflection is temporary and very localized, and because groundxvater levels at the sites recover rapidly once pumping has ceased, there appears to be no discemable long-term effect on the surface aquifer. In the areas adjacent to the site being dexvatered, the xvater table xvould be loxvered temporarily by tlae dexvatering process. This effect could extend from several feet to several tens of feet beyond the excavation depending on the method used, the level of the xvater table at the d_me dewatering began, the permeability of the material adjacent to the excavation, and the length of time the excavation needed to be kept open and dry. The possibility ex_ists that adjacent landscaping could be experience deterioration from reduced groundxvater availability. Deflection or Reduction of the rate of Groundwater Flow Although the amount of xvater pumped from an excavation may appear substantial as it Page 4 of 7 A 5; S t} C I A 1 E S flows along a street to a storm drain inlet, it is small compared to the amount of groundwater dkectly beneath the Santa CIara Plain. The SCVWD’s current estimate is that there is more than 350,000 acre-feet of groundwater available in the Santa Clara Subbasin. _An excavation &watering flow of I cubic foot per second would deflect 1.98 acre-feet of water per day. Because groundwater would be pumped out of the excavation faster than could flo~v in, the alteration in groundwater flow rate would be less than the rate of dexvatering. Because the resultant groundwater flow deflection is temporary, small, and very localized, there appears to be no discernable long-term effect on the surface aquifer. Because dewatem-~g for basement construction occurs only in the uppermost portion of the surface aquifer, there would be no effect on the deep aquifer. In a typical 3-month excavation period the 1.98 acre-feet per day dewatefing floxv xvould amount to 0.05% (one-t~ventieth of one percent) of the minimum known groundwater resource in the subbasin. No published information about the subbasin’s water budget has been found, so any to attempt to predict hoxv quickly the xvater would be replaced through recharge xvould be speculative. It is knoxvn, hoxvever, that the importation of potable water and the SCVWD controlled recharge program have assisted groundxvater levels in the subbasin to rise 200 feet during the last 40 years. Most of that rise has been in the surface aquifer. The implication is that the subbasin is being recharged at a rate substantially higher than the rate of withdraxval from all pumping, including dexvatering for basement construction. Consequently, it appears that the amount of floxv from one, or even several, dewatering operations would not have long-term effects on the surface aquifer. In the areas adjacent to the site being dewatered, the rate and floxv directions of the groundxvater xvould be altered temporarily by the dexvatering process. Groundxvater in the influenced area would move toxvard the base of the excavation at a rate loxver than the rate of dewatering discharge. This effect could extend from several feet to several tens of feet beyond the excavation depending on the method used, the level of the xvater table at the time dexvatering began, the permeabilitT of the material adjacent to the excavation, and the length of time the excavation needed to be kept open and dry. Flow directions and rates would revert to near normal xvhen dexvatering ceased. There xvould be some displacement of groundwater flow around the newly constructed basement, depending on the permeability of the surrounding soi! materials. The volume of space displaced by the basement could be several thousand to several tens of thousands of cubic feet, xvhich, although small compared to the volume of the surface aquifer, could be significant locally, especially if there were other similarly sized basements in the immediate vicinity. The flow of groundxvater xvould readjust to this condition, possibly altering the level of the xvater table in the vicinity of the site for several weeks or months, but is unlikely to experience any major permanent change. The groundxvater level in the surface aquifer Page 5 of 7 undergoes more significant changes during the rainy season than xvould be expected from long-term flow deflection caused by basements. Saltwater Intrusion and Subsidence Salt~vater intrusion and subsidence in the Santa Clara Subbasin are documented regional effects of the excessive removal of groundwater from the deep aquifer (overdrafting) over many years. This practice was curtailed in the mid-1960s when the importation of potable water increased substantially. Since then, the SCVWD has been recharging the subbasin thereby raising groundxvater levels, impeding saltxvater infiltration of the surface aquifer, and virtua!ly eliminating further overdraft-related subsidence (the effects of previous subsidence cannot be reversed because pordons of the deep aquifer have been compressed permanendy). Such basin-wide effects could recur only if the deep aquifer became overdrafted again. Because dewatefing for basement construction occurs only in the uppermost portion of the surface aquifer and involves only a sma!l amount of groundwater withdrawal, no effects xvould occur in the deep aquifer. 3. Palo Alto Public Works Department existing regulatory structure. There are a number of policies in place that provide protection for the City’s groundxvater resource and for propertT oxvners in the vicinity of nexv basement construction. The Public Works Department prohibits the long-troTh pumping of groundwater after a basement has been constt~cted. This eliminates the possibility that the water table in the vicinity of the project xvould be lowered permanently. The Public Works Depaxtment requires basements to be xvaterproofed and strengthened structurally beloxv the expected groundxvater level. This eliminates the need for groundwater pumping. The Public Works Department requires permit applicants xvhose projects xvould have basements to prepare a geotechnical investigation and report that would determine, among other information, the expected highest groundxvater level in the local shalloxv aquifer. This a!loxvs the department to make informed decisions about the advisability of basement construction at a particular site and/or to set the conditions under xvhich basement construction may proceed. If dexvatering is necessary for basement construction, the Public Works Department sets the dexvatering permit conditions based on the hydrology of the specific site under consideration. This ensures resource and property protection where it is needed. The Public Works Department a!lows the removal of seepage xvater that collects along basement xvalls above the water table. Normally this removal xvould need only a minimal amount of pumping, but may need to be monitored. Page 6 of 7 4.Recommendation regarding the advisability of codifying groundwater effects in the Zoning Ordinance Update The above-listed Public Works Department policies dealing xvith basement construction and dexvatefi_ng for such construction are intended to prevent substantial impacts to groundxvater, either on an area-xvide basis or in the vicinity of the construction site. Although the policies and their associated construction standards appear to address the issues adequately, it may be advisable for the Public Works Department to increase the community’s axvareness of these issues through an out-reach program. Because these issues are, essentially, engineering concerns that are site-specific and already covered by e~sting regulations, there is no need to modify the zoning ordinance xvith respect to them. Sincerely, George J. Burwasser, EIP Associates EIP ASSOCb\T~q 353 S.\CILau\IENTO STREET SUITE 1000 SAN FtL,~NCISCO, CALIFOILNIA 94111 Tdcphone 415-362-1500 Facsimi!~ 415-362-1954 E-mai! .q@e{t~a.rso~qales.cot~l ~*~v~a:dpassodates.com