Storm runoff from urbanized portions of the watershed adversely affects both the water quality and hydrologic regime of Strawberry Creek. An increase in trace metal, suspended solids, turbidity, COD, organic nitrogen, phosphorus, and bacterial concentrations occurred during wet weather. High peak storm flows accelerate streambank erosion and streambed scouring on the central campus as well. Emphasis for non-point controls should be placed initially on non-structural methods that do not require capital expenditures. These amount to no more than good "urban housekeeping" practices and management techniques to prevent pollution and mitigate the effects of urbanization. The following Best Management Practices (BMP's) should be considered for their applicability and implementation in the upper Strawberry Creek watershed.
I. Non-Structural Techniques
1. Improve street sweeping: Sweeping is the most common method used to reduce pollutant loading from streets and parking lots. Vacuum sweeping has proven to be much more effective than conventional broom sweeping for collecting fine particulates which contain a high percentage of pollutants. The effectiveness of present street sweeping practices should be evaluated. Guidelines should then be established and sweeping frequency, scheduling, or protocols,modified as necessary. Special parking regulations can be instituted to allow sweeper access to curb and gutter areas. Operator training alone can significantly increase sweeping effectiveness. Emphasis should be placed on collecting fine curb-side materials, proper broom positioning, and operating speeds. Efficient sweeping can significantly reduce contamination by heavy metals, nutrients, organics, and bacteria. Typical removal rates are: 30-50% of total solids, 25-40% of BOD, 25-40% of TKN, 8-20% of phosphates, and 25-60% of heavy metals (ABAG, 1977).
2. Clean storm drain system: If cleaned regularly, catch basins are effective in removing approximately 56% of the total solids and 40% of the BOD from stormwater (EPA, 1977). However, if not cleaned on a regular basis, they fill with decomposing debris and become sources of nutrients, oil and grease, heavy metals, and bacteria Catch basins should be cleaned with vacuum apparatus rather than flushing or other methods that purge solids down-gradient into the creek. Flushing lines in this manner also increases the probability of clogging storm lines. Preferably, catch basins and storm drain lines should be cleaned twice a year or at least once a year prior to the rainy season.
3. Preserve and enhance riparian areas: Riparian vegetation serves to uptake and filter out pollutants found in stormwater. Vegetation also slows the velocity of streamflow and runoff. Natural floodplain areas should be preserved because they act as natural stormwater storage reservoirs. A buffer strip surrounding the riparian corridors should be maintained.
4. Floodplain management: This technique includes all measures for planning and action which are needed to determine, implement, or revise plans for land use management of floodplain areas. Land use controls include implementation of design measures, floodplain regulations or acquisition, subdivision regulations, and the control of water or sewer extensions. Encroachment upon the floodplain should not be allowed Riparian buffer strip requirements and performance standards for development in sensitive areas can be established Efforts should be taken to control the amount of impervious surface over the entire watershed or to mitigate the impacts through structural techniques. Planning which controls development in the watershed is more effective than mitigation measures taken after development has occurred.
5. Erosion control: The following are non-structural methods for controlling erosion:
A. Protective coverings and mulches: Substances used include straw/hay, wood products, chemicals, and other materials which prevent soil erosion by reducing the effects of rainfall impact and runoff as well as providing a suitable environment for the development of vegetative cover. These should be used until grass or other ground cover is firmly established. Specific materials include:
- Polyethylene sheets
- Straw and hay
- Punched straw
- Net-anchored straw
- Tack:ifiers with straw
- Wood chips or sawdust
- Gravel and stones
- Mulch blanket
- Wood fiber
- Washed dairy waste
- Chemical mulch ( organic or plastic) Wood excelsior mat
- Fiberglass
- Jute netting
- Sod
- Building blocks
B. Land grading practices: Where grading is necessary for construction purposes, grading practices should minimize erosion potential and facilitate vegetative establishment. Development should fit existing topography as much as possible to minimize land disturbance. Slope gradients and lengths should be kept to a minimum and terraces should be installed on long slopes. A terrace should be graded back towards the slope and drain with a gentle gradient to a stable outlet. The surfaces of cut and fill slopes should be left rough or be serrated so that they hold seeds well and allow for good vegetative establishment.
C. Establishing vegetation: This practice consists of planting trees, shrubs, vines, grasses, or legumes on critical areas. These areas are commonly severely eroded sediment-producing sites that require special management to establish and maintain vegetation in order to stabilize soil conditions. Perm.anent vegetative cover should be maintained on slopes. Fast-growing vegetation such as grasses are usually planted to initially stabilize disturbed areas. Factors to consider in plant selection include: erosion control effectiveness (fast growth, complete ground coverage, fibrous root mat); availability; drought tolerance{migation requirements; fire hazard; fertilizer requirements; application and maintenance costs.
D. Irrigation water management: This entails the use and management of irrigation water to prevent oversaturation of soils and to reduce erosion. Quantity should be determined by soil moisture holding capacity and the needs of the vegetation. Irrigation water should not be applied in excess of plant needs to prevent oversaturation of soils. Water should be applied efficiently to prevent erosion and excessive runoff which degrade the water quality of Strawberry Creek. At the present time, excessive runoff from the central campus turf areas is commonplace. The existing irrigation system is antiquated and almost completely manually controlled. High volume sprinkler heads and compacted soil conditions along with the lack of close supervision of the system's operation all combine to create excessive runoff from the turf areas. The irrigation system will be updated in the future as funds permit. Automation of the system would alleviate the need for manual operation by the overburdened gardeners. Aeration of the turf areas should be done to the extent that staffing permits. Vegetative cover other than turf should be considered to reduce the possibility of soil saturation by over-irrigation.
II. Structural Techiques
1. Diversion: Stormwater can be diverted to retention/detention areas or rerouted to bypass critical areas such as the central campus. The feasibility of rerouting storm water from both forks, especially the North Fork, to the city storm drain system downstream of the central campus should be investigated. This would help mitigate the water quality and erosion problems on the central campus, but would require substantial capital expenditures. On a smaller scale, portions of the storm drain network in North Berkeley and on the central campus could be diverted to accomplish the same goals.
2. Detention: Temporary storage of runoff can reduce peak flows and subsequently ameliorate erosion and water quality problems. Many options exist for the construction of storage facilities by simply modifying existing sites and facilities. Detention facilities may be located on-site, upstream of highly developed areas, in the storm drain system itself, dispersed throughout the watershed, or even downstream of intense development. Here are some of the options:
A. Rooftop storage: This is often economically achieved because roofs are usually designed to support the required loads. Maximum depth of rooftop ponding is usually less than or equal to 4 inches which applies a load of about 2 0 lb/ft2 (Sheaffer et al, 1982). Structural engineers should verify roof loading capacities of existing buildings. A simple perforated ring placed around existing roof drains can easily regulate the flow from the roof.
B. Detention ponds: Usually located on-site, these are normally dry except during storm events when runoff is detained and slowly released. These ponds can have aesthetic value and can be incorporated into recreational and open space elements. Ponds may also be coupled with filtration systems to remove particulates. Natural or artificial mediums such as sand filters and gravel berms can be employed in this capacity. A possible location for such a pond is along the North Fork immediately below the LBL complex outfall.
C. Retention ponds: These are more permanent ponds that act as infiltration basins rather than just detaining and releasing stormwater. They probably have limited applicability in the watershed due to the relatively impermeable soil conditions.
D. Parking lot storage: Significant amounts of highly polluted runoff can be detained through grading and/or controlled outlets such as undersized catch basins and curb cuts. Often particulates will settle out and can be cleaned up later by conventional sweeping practices. If properly designed, conflict with lot users is not a significant problem as appreciable storage would occur on the average of several times every ten years and maximum storage would occur for only a short time about once every 100 years.
E. Driveway/street storage: As with parking lots, runoff can be collected in depressed areas and outlets could regulate the discharge as desired.
F. Cistern storage: Underground tanks with appropriate release outlets can be placed in-line or on-site. These may discharge into the storm drain system or infiltrate into the ground.
G. Open space storage: Storm runoff can be diverted or held in bermed open areas such as lawns, median strips, grass swales, or playing fields. These areas would probably require installation of french drains (subsurface drains) due to the generally impermeable soils in the watershed. These areas would be compatible with recreational/open space/parks uses. Greenbelt areas can also be constructed around parking lots to receive runoff. Buffer strips should be maintained around developed sites and urbanized areas to receive runoff. A typical greenbelt strip would consist of a layer of topsoil underlain by a layer of sand on top of a thick bed of gravel or other porous fill. Aesthetic benefits could help defray the cost of construction and maintenance.
3. Porous pavement: Pervious surfaces produce less runoff and attenuate adverse water quality impacts. Porous asphalt, concrete block-type materials interspersed with grass, and unpaved gravel surfaces are alternatives to conventional asphalt. The performance, cost, practicality, and durability of these surfaces varies. They have limited applications because of these constraints.
4. Catch basin modifications: A variety of methods to reduce pollutant loading from catch basins are available. Leaching catch basins can be installed instead of conventional catch basins where soil conditions permit. The bottom of leaching basins are underlain by gravel or porous fill that allows percolation of stormwater into the ground to facilitate natural soil filtration and attenuation of various pollutants. Oil/water separators or oil sorption/trap systems can be installed on catch basins to reduce oil contamination. These systems would require that debris be removed from the catch basins on a regular basis. Gooseneck outlets install¢ above the bottom of catch basins can prevent accumulated debris and pollutants from being flushed out into the creek.
5. Erosion control: Many structural methods exist to control runoff and reduce the amount of
sediment and other pollutants carried by runoff. Stream channel erosion control and stabilization techniques are discussed separately in Section 5 .1.2. Erosion control methods include:
A. Diversion dike: A dike is a ridge of soil or non-erodable material constructed at the top of cut or fill slopes or around the perimeter of a disturbed area to divert overland flow from small areas and prevent runoff from leaving the disturbed area. Diversions are commonly used at the head of slopes if runoff from higher areas is a problem. Daily construction of temporary dikes during construction will greatly reduce erosion from unexpected storm events or other sources of runoff. Dikes remain in place until permanent drainage facilities are installed and/or slopes are stabilized.
B. Interceptor ditch: These are permanent structures located on top of a cut slope. They divert drainage away from the slope of the cut to prevent erosion.
C. Slope drain: Down drains are temporary or permanent conduits which convey drainage from slopes to stable points of discharge down-gradient Drains may be a pipe drop, flume (chute), or flexible down drain. These should be installed immediately after completion of the cut or fill and before revegetation of the slopes.
D. Diversion: This is a temporary or permanent structure consisting of a channel or ditch and a ridge constructed acros_s a sloping land surface along a contour or with predetermined grades to intercept and divert surface runoff before it gains sufficient volume and velocity to create erosion. The runoff is then collected and conveyed laterally along the diversion at slow velocity and discharged onto a protected area or into an outlet channel.
E. Check dam: This is a small temporary dam constructed across a swale, drainage ditch, or other low flow area. It reduces the velocity of concentrated stormwater flows and reduces erosion of drainageways. Check dams also trap small amounts of sediment flowing in the channel but should not be used as a sediment trapping device. These temporary dams are used in small open channels that drain ten acres or less. Check dams may be constructed of stone, sand bags, hay bales, logs, staked brush, or other materials.
F. Grade stabilization structure: These structures stabilize a grade or control the eroding heads of gullies or stream channels cutting back upstream. Such structures serve to reduce environmental and pollution problems by reducing or eliminating erosion and subsequent sedimentation in the specific area being controlled. Structures range from simple dams using brush and wire, staked brush, riprap, gravel, hay bales, or prefabricated materials to major structures of reinforced concrete, masonry, compacted fill, or similar materials. Some of the minor structures may be temporary until stabilizing vegetation is established.
G. Erosion checks (stops): These consist of flexible porous long-lived mat-like materials such as fiberglass, plastic, or jute. These mats are installed in a slit trench perpendicular to the direction of flow in a swale or ditch. They provide positive grade control in shallow unlined drainageways. Erosion checks prevent the formation of rills and gullies by permitting subsurface water migration without removing soil particulates. These may also be used on critical slopes where severe sheet flow problems may occur.
H. Water ladder: These are wooden structures similar to a sta:ir:,vay that convey water across a steep slope while preventing downcutting. They serve the same purpose as half-round culverts, sluiceways, or swales that rout water over steep slopes onto vegetated and/or slash-covered areas. Water ladders are essentially energy dissipation devices that can effectively handle concentrated runoff. They work well in conjunction with check darns or at the downstream end of cross-road drains.
I. Sediment traps: These are structural or vegetative measures which trap sediment from disturbed areas to prevent clogging of drainage control structures and reduce sediment runoff. Traps can be installed in a drainageway, at a storm drain inlet, or at points of discharge from disturbed areas. Examples of traps include filter berms, sandbag or hay bale barriers, filter inlets, vegetative filter strips, culvert risers, small temporary basins, and silt fences.
J. Sediment detention basin: This is a reservoir which retains peak flows sufficiently to cause deposition of suspended sediment. Basins may be either temporary or permanent structures which prevent off-site transportation of sediment generated from construction activities or other land disturbance. Basins can be excavated or formed by a combination of earthen fill dam and excavation. An ungated pipe through the dam with a perforated riser within the basin above the sediment storage level permits the release of runoff waters. Some basins may serve a dual function by storing both debris and stormwater. A number of small basins are more effective than one large basin.
K. Subsurface drain: This is a conduit such as pipe, tile, or tubing that is installed beneath the ground to collect and/or convey water. In areas with a high water table where the benefits of lowering or or controlling groundwater or surface runoff justify installing a subdrain, the following benfits can be realized: improvement of soil conditions; interception and prevention of water movement into undesirable areas; removal of surface runoff; and relief of artesian pressure. Subdrains can also serve as outlets for other drains and as dewatering devices for detention basins. Subdrains may also take the form of french drains which are trenches filled with stones or other porous materials.
L. Heavy use area protection: This method consists of installing applicable structures, surfacing with suitable materials such as gravel, stone, and mulch, or establishing vegetative cover to protect heavily used areas. It applies to urban, recreational, or essential facility areas which are subjected to heavy use by people, vehicles, or animals.
M. Drip irrigation systems: These systems consist of facilities that are installed for efficient application of water directly to the root zone of plants by means of applicators (orifices, emitters, porous tubing, perforated pipe, etc.) operated under relatively low pressure. Use is adapted to steep slopes where other irrigation methods would cause excessive erosion and to the irrigation of gardens, flowers, and shrubs in urban settings. Advantages include low flow rates over long periods of time, the use of small diameter pipe, and a reported savings in water compared to other irrigation methcxls.