VII. STORAGE FACILITIES

A. Introduction

Storage can be an effective tool for the management of stormwater runoff. Storage lends itself to multiple land uses and is often the least costly alternative for achieving a particular flood control objective. Temporarily detaining a few acre feet of runoff can significantly reduce downstream flows and pipe and channel sizes, especially when the flood hydrograph has a rapid rise and fall. Storage can also remove sediment and debris which reduces sediment and pollutant loading on receiving waters.

The use of storage to reduce flood peaks is also potentially detrimental to flooding conditions, however. Storage facilities must be adequately maintained in order to function properly. Further, storage may potentially worsen downstream conditions for events larger or smaller than a single design event, and storage provided at some locations in a basin can actually increase total watershed peak flows by causing runoff peaks to coincide with peaks from other parts of the basin.

This section is concerned with the planning and design of facilities and features for providing storage in drainage systems. An emphasis is placed on use of storage for flood control.

B. Concepts and Definitions

1. Detention and Retention

Detention storage temporarily delays a portion of the inflow so that the maximum outflow is less than the maximum inflow. The storage of runoff is temporary: water stored is released soon after the maximum inflow has occurred.

Retention storage functions similarly to detention storage except water is stored for a significantly longer period. Water may be released from retention storage after a storm has ended or it may be retained for a much longer period for other uses such as recreation, surface water supply, or groundwater recharge.

2. Instream Storage Instream storage involves the construction of an embankment across a channel so that a storage pond is formed. Instream storage affects the timing of the flood peak moving downstream. Instream storage can trap sediment and thereby improve water quality.

Spillways are important with in-stream storage to pass large floods exceeding the design runoff without threatening the integrity of the embankment.

3. Off-stream Storage With off-stream storage, flows are diverted to a storage area adjacent to the stream such as an open depression, reservoir, or low lying recreational field. Flow into the off stream storage is controlled by a side channel spillway or conduit from the main channel. Water is returned to the stream at a reduced rate. Off-stream storage ponds may not remove as much sediment and, therefore, may require less maintenance. On the other hand, off-stream storage provides no benefits from sediment and debris collection.

Offstream storage requirements for flood control are typically less than those required with instream storage since only the upper portion of the instream hydrograph needs to be stored to achieve the desired effect.

4. Regional vs Local Storage Basins A distinction is made between regional and local storage basins. A regional storage basin is typically larger in scale. It is often identified in watershed master plans with the aid of a hydrologic model of the total watershed. It can be incorporated into the Flood Control District's existing or proposed drainage system. It is owned and operated by the Flood Control District, although it may be joint use.

A regional basin will reduce the downstream peak flow rate and the necessary downstream conveyance capacities.

A local detention basin, on the other hand, is one which will not be incorporated into the Flood Control District's existing or proposed drainage system, and is owned by an individual or organization other than the Flood Control District. Local basins will reduce the downstream peak flow rate, but may not be credited with downstream flow reduction.

5. Joint Use Storage Basin A joint use storage basin has uses in addition to flood control such as a football field, parking lot, golf course or lake.

6. Temporary Storage Basin A temporary storage basin is a local basin used to reduce downstream peak flow rates until ultimate capacities can be provided as part of a phased development. Generally the life of a temporary basin does not exceed 10 years.

7. On-site Storage The storage of water close to the points of rainfall occurrence is on-site storage. On-site storage is typically small scale and includes ponding in parking lots, property line swales, small ponds in green areas, underground tanks and infiltration trenches.

8. Channel Storage Natural stream channels provide significant storage of runoff in wide overbank areas. Additional channel storage may be induced by providing obstacles to flow and maintaining channel roughness so that the flow is deeper and possibly wider.

C. Principles and Policies

1. Scale and Mitigation Storage is often an acceptable and effective alternative for mitigating the effects of development.

The most desirable use of storage for mitigation is frequently the design and construction of a regional storage facility. Regional facilities tend to be more effective and can be constructed, operated and maintained at lower costs.

The design and construction of interim local facilities is an alternative where regional facilities are not planned. The avoidance of detrimental effects is an important concern for this alternative, however.

2. Uncertainty and Mitigation Estimates of flows and volumes are subject to significant uncertainty. For mitigation, the risk of overestimating preproject conditions is greater than the risk of underestimating. Therefore, when storage (regional, local, temporary or joint use) is to be used to mitigate downstream impacts due to increased flows generated by development of a site, the storage capacity and outlet size shall be such that the estimated pre-development peak flow rate from the site for all frequency storms up to and including the 100-year shall be discounted by an appropriate amount specified in Section D 1 a.

3. Avoiding Detrimental Effects No storage facility shall worsen conditions downstream. Any storage facility, especially a detention basin, has a potential for creating worse conditions downstream by altering the timing of peak flows in the stream and its tributaries. In order to avoid detrimental effects, the following alternative measures are suggested.

- A hydrologic study of the watershed in which the basin would be sited. The downstream limit of the study would be the point beyond which changes in peak flows would not be measurable. Where they exist, watershed models supported by the local jurisdiction or the District should be used.

- Construction of storage basins which limit outflows to the 2-year pre-development peak flow rate.

- Construction of in-stream detention basins which result in reasonably the same outflow hydrographs as previously existed for the 2-, 10-, 25- and 100-year events.

4. On-site Storage Local jurisdictions may require on-site storage for mitigation of increased runoff. Project features which can be used for incidental storage include parking lots, parks, and other common areas. However, such facilities may not be credited with the reduction in flows downstream in regional facilities.

5. Road Culverts and Embankments Road embankments shall be used to provide storage where appropriate. Road embankments may create incidental storage which is effective in reducing peak flows downstream. The ponding function of embankments and culverts should be considered with their planning and design and coordinated with master drainage plans.

6. Multiple Uses Storage facilities typically offer opportunities for combined use of resources. Therefore, plans for storage facilities shall routinely consider other needs, including recreation facilities, water quality and sedimentation, and wildlife habitat.

7. Maintenance Assurance Approval of interim, local storage facilities shall be conditional to assurance that the facility would be maintained in an effective condition.

Cash trusts held by a responsible public entity may be required to ensure maintenance of interim local detention basins, including joint use.

A maintenance district using a Community Facilities District or other acceptable public financing shall be established to operate and maintain the joint use facility.

In addition, local basin can be incorporated into other features of a development as a joint use as a means of providing adequate maintenance.

D. Hydrologic Evaluation

The evaluation of the effects of storage on flows is the same regardless of the scale of the storage facility.

The objectives of the hydrologic evaluation to determine the required storage capacity and to verify the effectiveness of the outlet design in achieving objective flows.

The required capacity of a storage basin is a function of the objective outflows, design inflows, and required freeboard. Carryover and multi-purpose storage are also factors when retention is involved.

A routing of the design storm inflows is required to determine the capacity for storage basins. The outflows used in the detailed routing shall be based upon hydraulic rating curves for the outlet works proposed for the basin.

Note that if the actual storage capacity is limited by topography or costs of land acquisition and construction, it may be necessary to reformulate the objective outflows.

1. Objective Outflows Objective outflows may be clearly designated in a stormwater management plan or may be a matter of discretion for the regulating jurisdiction. They may be based on reduction of flows at the site or at downstream locations. In any case, however, the determination of objective outflows should at least consider downstream channel capacities, interactions with other storage basins, water quality, and erosion and sedimentation. Sections C.1 and C.2 (Principles and Policies) above are especially relevant to the determination of objective outflows.

Decisions and policies related to objective outflows shall be coordinated with the Flood Control District.

a. Uncertainty in Pre-Development Flows When storage is to be used to mitigate downstream impacts due to increased flows generated by development of a site, the objective flow shall be taken as the estimated pre-development peak flow rate less 10% of the difference between the estimated pre-development and post-development peak flow rates from the site for all standard design storms ranging in frequency from the 2-year and up to and including 100-year. In no case, however, shall the objective flow be less than 90 percent of the estimated pre-development flow. Figure 7-1 presents this criterion graphically.



b. Use of Downstream Channel Capacity If a non-damaging downstream channel capacity can be shown to exist, then objective flows at 90% of the downstream channel capacity may be used to reduce the storage required for mitigation. For mitigation of development, it must be demonstrated that no damages occur with the proposed channel capacity. If flood control projects have been developed downstream, the design flows of these projects may be considered in determining channel capacities. If 100-year floodplains have been designated, the 100-year flow may be considered as a basis.

c. Downstream Erosion Where downstream erosion is a concern the duration of erosive flow velocities for all frequency storms shall not be substantially increased unless other forms of mitigation are provided. This can be accomplished by reducing the peak outflow rate further than that required above.

d. Spillway Outflow The spillway design shall attempt to decrease outflows as much as possible with the surcharge storage capacity available at the site.

e. Outflow Control In most cases, it is desirable to design the storage to operate under hydraulic control: i.e. the hydraulics of the outlet control the outflow rates. For large regional facilities, however, it may be desirable to incorporate manual control of the outlets to achieve more effective operation when resources are limited. In no case, however, shall the spillway be manually operated.

2. Design Inflows When available, the appropriate portions of a basin master planning model will be used to model the effects of the storage facility. If a basin master planning model does not exist, the local jurisdiction or flood control district will be consulted regarding the inflow hydrograph to be used.

a. Frequency The design of the outlets will be based on achieving the 2-, 10-, 25- and 100- objective outflows. The event equal to 125% of the 100-year flood will be used for spillway design where a spillway is necessary.

b. Duration The duration of flood routings shall be sufficiently long for streamflows and storage levels to return to initial conditions. Runoff from storms of increasing durations shall be routed through the storage basin to determine the maximum volume required considering carryover from one period of high runoff to the next. These storms shall be based on the design storm criteria contained in the chapter on Hydrology for durations less than 24 hours. For durations greater than 24 hours, a historical storm distribution shall be used.

3. Initial Conditions

a. Spillway Design Flood All storage basins shall be assumed 100% full at the beginning of the spillway design portion of flood routing.

b. Other Floods The storage dedicated to flood control may be assumed empty at the beginning of design flood routings except for the spillway design flood, as indicated above. Any joint-use storage shall be assumed full.

E. General Criteria

This section provides general criteria and guidelines for the planning and design of local and regional detention and retention basins for flood control. For simplicity, the term "storage basin" shall apply below to both detention and retention basins unless otherwise noted.

The criteria described below apply generally to regional and local basins except as noted.

1. Dam Safety Jurisdiction Storage basins may be subjected to the approval and inspection of California State Division of Dam Safety. The two criteria which determine their jurisdiction are the height of the dam and the total capacity of the reservoir formed by the dam. Figure 7-2 indicates the sizes of basins which are subject to the jurisdiction of California State Division of Dam Safety. If Dam Safety criteria conflict with the criteria in this chapter, the more restrictive criteria will apply. If the criteria are contradictory, then the Dam Safety Criteria apply and the contra- diction shall be noted on the appropriate plans.

2. Site A combination of topography, economics, and effectiveness generally determines the location of storage basins. Effectiveness means that the basin would achieve flood reduction goals and, further not contribute to flooding at any location during any size of flood event.

Sites for regional storage basins are generally determined by the Flood Control District and presented in the Basin Master Drainage Plans. Sites may also be designated by policy in a specific area in the Basin Master Drainage Plans. In the absence of Basin Master plans, sites for all storage facilities of any size shall be coordinated with the Flood Control District.

3. Freeboard

a. Local and temporary basins shall have a minimum 1-foot of freeboard above the spillway design flood HWL on the emergency spillway or 2-feet of freeboard above the 100-year HWL in the basin, whichever is more stringent.

b. Regional basins shall have a minimum of 2 feet of freeboard above the spillway design flood HWL on the emergency spillway. For basins with larger surface areas the freeboard shall be increased due to possible wave action. Also, a Seismic Seiche analysis may be required to determine necessary freeboard.

c. Joint use basins shall conform to the applicable local or regional freeboard requirements. For small basins such as parking lot and tennis court basins, the freeboard requirement may be reduced.

d. Initial Conditions

(1) Spillway Design Flood All storage basins shall be assumed 100% full at the beginning of the spillway design portion of flood routing.

(2) Other Floods The storage dedicated to flood control may be assumed empty at the beginning of design flood routings except for the spillway design flood, as indicated above. Any joint-use storage shall be assumed full.

4. Outlet Works

a. Hydraulic Grade Line Pressure flow closed conduits shall be designed such that the hydraulic grade line is 6 inches below the ground or street surface. In those reaches where no surface flow will be intercepted (now or in the future), a hydraulic grade line which encroaches on or is slightly higher than the ground or street surface will be acceptable.

b. Partially-Full Flow Non-pressure flow closed conduit capacities shall be based on a flow depth less than 0.8 times the conduits diameter or height to avoid instable flow regimes.

Note:

Height is normally measured from the natural streambed elevation at the downstream toe of the embankment to the maximum possible water surface elevation. However, other criteria may be used at the discretion of the Division of Dam Safety.

c. Diameter The outlet pipe for all basins except temporary basins shall be a minimum 24" RCP for local basins and a minimum 36" RCP for regional basins. The outlet pipe or conduit shall be encased with cutoff collars designed per "Section 242. Cut and Cover Conduit Detail" of the Bureau of Reclamation's publication Design of Small Dams (8).

d. Outlet Control A metered outlet structure may be necessary to provide the necessary flow attenuation for all frequency storms. "V" shaped weirs and notched weirs are preferred over other alternates because they do not plug with debris and trash as easily as other designs.

e. Emergency Drawdown All storage facilities, including multipurpose facilities, shall draw down within 72-hours. To the extent feasible, this shall be accomplished with an ungated, gravity outlet. If this is not feasible, the facility will be equipped with an additional manually operated outlet that permits full drawdown within this time.

f. Trash Racks Trash racks shall be provided at the inlet to the basin outlet structures(s). Trash racks shall be sufficiently large that partial plugging will not adversely restrict flows reaching the outlet control. Computations shall verify that outflow is not restricted if 90% of the trash rack is effectively blocked.

Trash racks shall be designed to be readily accessible by hand or serviceable by a piece of medium sized equipment. Access must be from above the trash rack so the loose material can be removed.

g. Anti-Vortex Devices Anti-vortex devices shall be provided where warranted.

h. Depth Gage A depth gage shall be provided on the basin outlet structure in order to monitor debris deposition and basin operation.

5. Inlet Structures

a. Energy Dissipator Where storm drains enter the basin, energy dissipators and/or erosion protection shall be provided. Plans must be approved by the Flood Control District before plan approval if the basin is to be operated and maintained by the Flood Control District.

Energy dissipators may be required when the inletting flow velocities exceed 5 fps.

b. Invert Stabilization Where natural drainage courses or channels enter the basin some form of invert stabilization such as a reinforced concrete spillway shall be provided.

6. Emergency Spillway A spillway shall be required only when the exceedence of the design flood would result in damage to the structure or downstream damages or risk of injury or loss of life.

a. Requirement An emergency spillway is required for all regional storage facilities, and all facilities storing more than 2 acre-feet of water at maximum where an embankment is used.

b. Capacity All storage basin spillways shall be designed to pass a design flood equal to 125% of the 100-year flood required by the State's Division of Safety of Dams, whichever is greater.

c. Point of DischargeSpillway outflows shall be adequately conveyed to a storm drain, drainage channel, street or an established watercourse.

d. Materials Generally, all spillway structures shall be constructed of reinforced concrete. For temporary storage basins with an expected life less than 10 years the spillway may be constructed with grouted rock or other forms of approved protection designed to resist maximum design velocities. When the spillway crest is more that 3 feet above the flowline of the facility the spillway outlets into, the spillway shall be constructed of reinforced concrete.

e. Crest Elevation Generally the spillway crest shall be at or above the basin's design 100 year high water line.

7. Pool Elevation and Depth

a. Local Basins

(1) 100-year Pool Elevation When feasible, the 100 year design pool elevation should be at or below existing natural ground. Generally no more than 50% of the basin's 100 year storage depth should be above existing ground (i.e., 50% or more of the 100-year minimum storage depth must be below the lowest ground outside basin).

(2) 2-year Pool Elevation The necessary storage depth for debris plus the two year flow attenuation shall be below existing ground.

(3) 100-year Depth The basin's maximum water depth for 100-year design should be 6 feet or less.

(4) Exceptions When site conditions warrant and safety can be assured, the above depth requirements may be modified if the basin embankment is constructed of material, or has a solid core, which does not allow seepage or piping to occur.

b. Regional Basins

(1) Depth Depths shall be as approved by the Flood Control District.

(2) Embankment Height Basins with heights greater than or equal to 25 feet and capacity greater than or equal to 15 Ac.ft., or a capacity greater than or equal to 50 ac. ft. and a height greater than or equal to 6 feet, shall be reviewed and approved by the State's Division of Safety of Dams. (see Figure 7-1)

c. Joint Use Basins Depths should be shallow and compatible with the secondary use. The allowable depth in most cases will be site specific and shall be approved by all agencies involved.

8. Embankments

a. Top width The minimum top width shall be 8 feet.

b. Side Slopes Basin side slopes should be of 3H:1V or flatter on the wet side and 2H:1V or flatter on the dry side. Steeper slopes may be acceptable on a case by case basis if rock lined and recommended in a soils and geotechnical report.

c. Soils and Geotechnical Analysis For design of the embankment abutments and adjacent slopes, a soils and geotechnical report may be required by local jurisdictions. The report shall be prepared by an engineer with a demonstrated expertise in earth fill dam design. The report shall be reviewed and approved by the Flood Control District for regional facilities. The contents of the report are specified in Appendix B of this chapter.

9. Drainage

a. Low Flow Channel A low flow channel shall be provided from the basin inlet(s) to the basin outlet. The low flow channel shall be designed to carry .01 cfs per acre of upstream watershed.

(1) Lining Where basin slopes exceed 2% or produce erosive flow velocities the low flow channel should be protected from erosion with reinforced concrete, rock lining or other form of approved erosion protection.

(2) Joint Use basins

(a) A low flow channel or conduit should be provided to conduct minor flows around the joint use facilities wherever possible. Low flow channels may not be necessary for parking lot basins or other similar joint uses.

(b) The flow channel may be grass lined if there exists a maintenance program which includes mowing and maintenance of turf in good condition and velocities of flow through the various stages of discharge are low enough to be non-erosive.

b. Earth Basin Floors Earth basin floors shall slope at a minimum 1.0% grade to the low flow channel and shall have a minimum grade of 1.0% from the inlet to the outlet.

c. High Water Table In case the water table is higher than the bottom elevation of the basin, plans should consider the disposition of the water. Either a system of finger drains and a low flow channel can be used, or some provision for a permanent water pool can be made. The contents of the permanent water pool should not be considered part of the capacity of the storage basin.

10. Sedimentation Provisions for storage of the projected sediment shall be included in the freeboard & maintenance requirements.

11. Fencing If a public safety concern exists, fencing or other protective barriers shall be provided to the satisfaction of the local jurisdiction. Similar barriers may be required for an area with a vertical drop of 36" or more. Barriers for regional facilities shall be approved by the Flood Control District. Joint use basin fencing will be site specific and must meet the need of all agencies utilizing the basin.

12. Maintenance Strip A maintenance strip is required for regional storage facilities. Maintenance of the basin in general is usually done from the bottom of the basin.

The outlet structure for the basin must be readily accessible throughout the year. Maintenance strips for access to the outlet structure are located on the sides or immediate perimeter of the basin. The ideal position of the maintenance strip is along the sides of the basin at an elevation above the outlet pipe, usually 2 to 4 feet above the bottom of the basin. The maintenance strip should be a minimum of 15 feet wide with no slopes greater than 10%. No special paving is required for the maintenance strip. Any other roadway is considered an access road.

13. Access

a. Access Road Access to any type of storage basin area shall be provided by at least one roadway from a public street or public access to the parcel upon which the basin is constructed.

b. Access Maintenance Access shall be maintained under all weather conditions.

c. Perimeter Road A 15-foot wide roadway shall be provided along the top of embankment, across the spillway and around the basin. The criteria may be modified where it can be shown the recommended top width is not necessary for structural safety and maintenance. Approval will be required by the Flood Control District for regional facilities or by the local jurisdiction for local facilities.

(1) Alternative Turnarounds If access across the spillway is not provided and no other access exists, minimum 40' X 60' turn arounds may be required on both sides of the spillway.

(2) Alternatives for Joint Use If there exists adequate access for maintenance, this requirement may be amended for local, temporary or joint use basins.

d. Access Ramps Access ramps shall be provided to the basin floor. A minimum of one - 15 foot wide ramp is required for local basins. Minimum of two - 15 foot wide ramps are required for regional basins.

e. Slope The maximum roadway or access ramp slope shall be 10%.

f. Turning Radius The minimum access and roadway inside turning radius shall be 35 feet.

14. Rights-of-Way Sufficient rights-of-way shall be provided for the construction and economical maintenance of the basin(s), (including all fill and cut slopes) and shall include sufficient area to provide for an access road from a dedicated public street to the basin.

a. Regional Basins Regional basins shall be dedicated to the District in fee title.

b. Local Basins Local basins shall be covered by an adequate drainage easement or dedicated in fee title to the local jurisdiction.

15. Landscaping A landscaping plan may be required by local jurisdictions. It is recommended that top soil within the basins should be salvaged and spread on the basin slopes and invert to provide a suitable environment for future landscaping and a water supply line for future irrigation should be installed at the basin site.

F. Alternative On-site Storage

The following suggests alternatives to dedicated, on-site flood control basins. In general, criteria have not been developed for effective on-site storage in this region. Proposals for reducing off-site flows with these methods must be reviewed case by case.

1. Hydrologic Criteria The effectiveness of on-site storage shall be evaluated using the basic hydrologic concepts and criteria for storage basins.

2. Parking Lots Storm runoff can be detained on parking lot sites by shallow basins or swales. Arrangement of areas in a parking lot to accept ponding shall be planned so that pedestrians are inconvenienced as little as possible. A 7-inch design depth is allowable for parking locations in the remote areas of lots.

3. Open Space, Parks and Recreation Fields Grassed recreational fields can be utilized for the temporary storage of the storm runoff without adversely affecting their primary function. Positive drainage toward the outflow control structure is important to avoid the prolonged entrapment of water which can cause swampy soil, growth of undesirable plants, and mosquitoes.

4. On-Site Ponds On-site ponds which have recreational uses can provide significant storm runoff storage benefits when properly planned and designed. Overflow spillways must be provided to bypass or discharge flows into floodways on the peripheries of the ponds so that safe water-storage elevations are not exceeded nor banks breached. For extremely large ponds, adequate design precautions should be taken to minimize possible shoreline erosion attributable to wind and wave action. Restricting sediment accumulation and water pollution in large ponds is of particular importance, especially when recreational use of the pond is permitted or planned. The polluting effects of roadside accumulations of salts, copper, and asbestos from brake linings, grease, oil, and heavy metals are significant if allowed to enter a storage pond. Such deleterious material should be screened out of the drainage system by interception and disposition before it reaches stormwater storage ponds. Without such precautions, water quality can deteriorate to a point where a storage pond becomes a distinct detriment to the development.

5. Road Embankments and Culverts Drainage systems are designed to utilize the roadway embankments as dams and control structures so that runoff will pond upstream of these points. The structures will normally pass small flows unimpeded, and ponding will be activated when flow rates approaching a control point exceed the pass-through rate used in the hydraulic design. Roadway embankments at control points should be stabilized and protected to minimize erosional effects of rainfall and stream flows.

6. Underground Tanks This type of on-site detention involves the underground construction of a holding tank or large size pipe as a means of providing controlled runoff from the site. This application should be limited to highly congested areas where surface ponding is prohibited due to the scarcity or high cost of available land, or areas where surface topography is not conducive to aboveground storage. Outflow control devices may consist of small gravity pipes, weirs, or proprietary devices. In some applications, pumping may be required to discharge stored runoff. Overflow capabilities must be provided.

7. Roadside Drainage Swales Drainage swales adjacent to roadways without curbs, or having rolled curb configuration, can be provided to serve as storage areas for excess runoff in low-density residential neighborhoods. Roads having low traffic volumes and speed limits are compatible with this concept, particularly where path and walkway systems are built into the planning scheme to skirt the roadways and provide pedestrian access thereto. Swaling of roadways having good infiltration rates may provide low-cost stormwater storage and retard discharge rates in grass-lined channels. Few, if any, storm drains are required below grade. Swales and roadway shoulders must be designed and constructed with care to keep damage and maintenance to a minimum. They should be carefully designed so as not to constitute a traffic hazard. A 6 to 1 maximum slope when installed on roads with speed limits in excess of 55 kmh (35 mph) is a usual design criterion.

8. Subsurface Disposal Below-grade disposal of runoff through pervious soils may be a viable alternative to aboveground storage. Deep soil sampling should be performed to assess the feasibility of waterloading the various geological strata. Percolation tests, pumping tests, and soil sampling should provide useful data as to depth, size, and locations where subsurface disposal is practical. Levels of groundwater and the impacts, if any, on local water supply wells also effect feasibility.

Dry wells can provide a means of storage as well as a significant discharge or dissipation potential in permeable soils. Provisions should be included in design, construction, and maintenance programs to minimize siltation and clogging of the permeable soil strata to avoid significant impairment of infiltration capacity. Dry well should not only extend to depths where pervious soils are present but should also be deep enough so that possible seepage downhill does not create a problem. In some areas, lateral trenching in permeable soils may be an alternative of excess storm runoff. Perforated drain pipe or an open graded fill of rock or gravel may be used. Filter Cloth or other protection for the drain or trench must be provided to screen out migration of fine materials which can clog the voids and negate the percolation potential of the receiving soil strata. A thorough soil investigation across the site is a primary prerequisite in planning such structures. When placed in operation, underground stormwater disposal facilities can be protected by filter sheets or bags located ahead of the inlet(s) to trap the first flush of debris, silt and pollutants for subsequent removal by maintenance crews. Without adequate and frequent maintenance, failure of such traps may lead to flooding problems.

Disposal based upon infiltration and percolation may not provide a long-term solution in as much as sediment may seal the soil interface and reduce the rate at which water enters the soils.

G. Documentation

A report which documents the planning and design of storage facilities must be submitted for review in addition to information in the normal stormwater management plan.

The contents of the additional documentation are identified in a supplement to this manual.

REFERENCES

1. American Society of Civil Engineers, the National Association of Homebuilders and the Urban Land Institute, Residential Stormwater Management, 1975.
   
2. APWA, Urban Stormwater Management - Special Report No. 49, 1981.
   
3. Chow, Ven Te, Editor-in-Chief, Handbook of Applied Hydrology, McGraw-Hill, 1964.
   
4. De Groot, William, ed., Stormwater Detention Facilities, ASCE, 1982.
   
5. Federal Water Pollution Control Administration, Water Pollution Aspects of Urban Runoff, Washington, D.C., March 1969.
   
6. Linsley, Ray K. and Franzini, Joseph B. Water-Resources Engineering, McGraw-Hill, 1964.
   
7. Task Committee on the Design of Outlet Control Structures, Stormwater Detention Outlet Control Structures, ASCE, 1985.
   
8. US Bureau of Reclamation, Design of Small Dams, USGPO, 1987.
   
9. US Soil Conservation Service, Earth Dams and Reservoirs, Technical Release No. 60, August 1981.