Transcript Sandy Run Watershed Flood Mitigation Final Report Team #4 Alison Hafer

Sandy Run Watershed Flood
Mitigation Final Report
Team #4
Alison Hafer
Mike Shinton
Chris Naab
John Zollers
Steve Welsh
Location
Problem Background
 Development
Residential
Commercial
 Highly Developed in Past 20 Years
Location
 Causes of flooding
Combined discharge
Three watersheds
Channel path
Problem Background
 Several floods have occurred in this area
 Some prominent examples
Hurricane Floyd
Overtopping of the Loch Alsh Reservoir
Problem Background
Tropical Storm Allison
Resulted in washout of a SEPTA bridge over Sandy Run
on the R5 Lansdale line
Introduction
 Three independent watersheds
Analyzed individually
 TR-55 Analysis
Development
Soil types
Topography
 Hydrograph
Hydrographs
10 yr Composite Runoff Hydrograph at Wissahickon Creek
3500
Total
Pine Run
Rapp Run
Sandy Run
3000
Flow (cfs)
2500
2000
1500
1000
500
0
10
15
20
25
Time (hours)
30
35
Introduction
 Criteria for success
Reduces the risk of flooding and damage
Does so economically
“Do no harm”
 Two approaches
Water detention
Increase channel velocity
Introduction
 Alternatives evaluated
Detention basins
Impoundments
Rain barrels and gardens
Channel Improvements
 Best Practices
Introduction
 Detention basins
Temporarily store runoff
Two types
Surface
Subsurface
 Impoundments
Mini in-stream dam
Runoff is stored upstream
Introduction
 Rain barrels
Store roof runoff
Varying sizes and applications
 Rain gardens
Vegetative area that promotes infiltration
Public and private
Detention Basins
 How do they
function
Reduce peak flow
Release storage
Detention Basins
 Basin selection
 Develop local
hydrograph
 Size basin
Outflow vs Elevation
 Modified Puls routing
Detention Basins
 Final design
Two surface basins
Two subsurface
 Results
5% peak reduction
Detention Basins
 Other Issues
Localized flooding
Do no harm
 Not economically
feasible
Detention Basins
 Cause of minimal impact
Service area too large
Inadequate storage capacity
 Feasibility
Many small surface and subsurface basins
Allow slow release
Impoundments – In-Stream Detention
 Designed to retard the movement of runoff through the watershed
 Flow is restricted by a concrete structure (mini-dam)
 Can fail-safe through overtopping
 Behaves like a quickly filling dam
Impoundments
 Two suitable locations
were identified
 Site characteristics
estimated using
topographic maps
 Maximum height of water
limited to prevent local
flooding
Impoundments
Water Height (ft)
Outflow vs Height
10
8
6
4
2
0
0
200
400
Outflow (cfs)
600
Impoundments
 Performance is limited by the storage area the
location is able to provide
 The size of the opening is adjusted to dictate the
rate at which the impoundment fills
 The impoundment reduces the peak outflow by
transferring inflow to storage
IDEAL IMPOUNDMENT PERFORMANCE
180.0
8.0
INFLOW HYDROGRAPH
OUTFLOW HYDROGRAPH
7.0
WATER LEVEL IN RESERVOIR
140.0
6.0
120.0
Q (cfs)
5.0
100.0
4.0
80.0
3.0
60.0
2.0
40.0
1.0
20.0
0.0
0.00
5.00
10.00
TIME (hours)
15.00
0.0
20.00
WATER LEVEL IN RESERVOIR
(ft)
160.0
RAPP RUN IMPOUNDMENT PERFORMANCE
9.0
200.0
INFLOW HYDROGRAPH
OUTFLOW HYDROGRAPH
WATER LEVEL IN RESERVOIR
160.0
140.0
8.0
7.0
6.0
Q (cfs)
120.0
5.0
100.0
4.0
80.0
3.0
60.0
2.0
40.0
20.0
1.0
0.0
0.0
20.00
0.00
5.00
10.00
TIME (hours)
15.00
WATER LEVEL IN RESERVOIR
(feet)
180.0
Impoundments
 In-stream impoundments are not feasible in this
watershed
Available storage areas are far too small to handle the
amount of runoff generated
Homes are situated too close to the flood plains which
reduces the storage area
Available impoundment locations are too far
downstream
Estimated cost of $43,000 each
Rain Barrels
 Rain barrel system
design
54 gallon rain barrel
Includes all necessary
installation equipment
Capacity doubled to
108 gallons by stacking
two rain barrels
Rain Gardens
 Vegetated areas that store runoff and promote
infiltration and transpiration
Rain Gardens
 Small scale applications
Typical residential rain gardens
Surface area = 50 ft2
Depth = 0.5 ft
Plants
• Water-tolerant trees
• Shrubs
• Herbaceous plants
 Large scale applications
Rain Barrels and Rain Gardens
 Best case scenario: 0.013 inches of runoff held
 Density of 10 houses/acre would hold 0.1 inches
Subsurface Runoff Storage
 Stores runoff on-site
 Intended for commercial sites
 Underneath parking or other open area
 Feasible for other locations as well
 Stored water ideally infiltrates into soil
 Most local soil infiltrates too slowly
 Detained water will be drained into streams
 Slow drainage rate effectively removes stored runoff from the
hydrograph
Subsurface Runoff Storage
 Three possible options explored
Subsurface gravel storage bed
Water stored between voids in gravel
Subsurface storage structure
Water stored in underground pipes or structures
Rain garden with volume storage bed
Combines storage space and rain garden to hold water
and promote infiltration and transpiration
Subsurface Runoff Storage
 Sample designs
Designed for site
pictured at right
Hold runoff from a
2-year storm that
falls over the entire
site
Drain storage area
within 2-3 days
Subsurface Storage: Option 1
 Gravel bed with 40% void space stores water in
voids
 Runoff routed into gravel bed via porous
pavement and/or edge drains
 Geotextile protects gravel from contamination by
fine-grained soils
 Perforated geopipe drains gravel at a rate of ~0.3
ft3/s
Subsurface Storage: Option 1
Subsurface Storage: Option 2
 Underground storage which utilizes some
structure (often pipe) to store water
More efficient use of space than gravel
 Routing of runoff into storage and drainage after
rainfall unchanged from gravel storage
 Has potential to store water from larger storms or
from adjacent land areas
Subsurface Storage: Option 2
Subsurface Storage: Option 3
 Rain garden with storage area
Sand storage area beneath a planting bed
Combines volume storage with infiltration and
transpiration-promoting plants
Requires considerable open space but also includes
aesthetic and environmental benefits
 Most effective in locations that already have soil
that supports some infiltration
Subsurface Storage: Option 3
Subsurface Runoff Storage
Best Practices
 Runoff controls are already required for all new
construction
Runoff controls should additionally be required for
renovation and reconstruction of existing facilities
 To encourage runoff detention efforts, subsidies
or tax credits should be offered to landowners to
control runoff beyond the minimum requirements
Best Practices
 Newly constructed housing should include rain
barrels and rain gardens to supplement local
detention basins
 Adds less than 1% to cost of most new homes
 Their installation can be encouraged in at
existing homes with tax credits or subsidies
Channel Improvements
 Current channel path causes slow down in
stream flow
“Question mark” shape
 It is narrow which doesn’t allow a larger flow
during storms
 Bridge area also affects the flow
 Make changes to improve flow and protect
commercial areas adjacent to stream
Channel Improvements
Channel Improvements
Channel Improvements
 Improvements include excavation and
construction of a high flood wall
 Three options
Short area near railroad bridge
Long area
Long area with stream widening
Channel Improvements
Channel Improvements
 Short area
 Excavation of islands and small areas along
parking lots
 Construction of flood wall adjacent to parking
lots and railroad
Channel Improvements
Channel Improvements
 Long area
 Excavation of islands and small areas adjacent
to commercial areas
 Construction of flood wall from railroad bridge to
Route 309 & PA Turnpike ramps
Channel Improvements
Channel Improvements
 Long area with stream widening
 Excavation of islands and large unused areas to
widen stream channel
 Construction of flood wall from railroad bridge to
Route 309 & PA Turnpike ramps
Channel Improvements
Channel Improvements
 Estimates
 Most costs associated with excavation and flood wall
construction
 Short Area
 $1,404,000
 Long Area
 $1,639,000
 Combination
 $2,800,000
Channel Improvements
 Cost Benefit
 Local commercial areas have suffered flood
damages in the past
R & S Imports had approx. $1.4 million in damages in
a storm a few years ago
 Using at least one of the options would probably
help in preventing or lessening flood damage.
Channel Improvements
 Future work/considerations
Hydrologic study to determine effects of
improvements
Make use of a physical model
Use of parking lots to make a uniform stream width
Analyze downstream effects if improvements are
implemented
Channel Improvements
Channel Improvements
Engineering Budget
 Updated time
allocation for
certain areas
of the project
and
overhead
costs
 Total
estimate:
$147,000
Rain Barrel Estimate
 Double unit system costs
Rain Barrel Solution Estimate (Double Unit)
 Rain barrel
 Shipping
 Miscellaneous
installation
Object
Quantity
Cost
Total
54 Gallon
Rain Barrel
2
$290.00
$290.00
Shipping Cost
per barrel
2
$21.00
$42.00
Installation
1
$50.00
$50.00
Subtotal
$382.00
 Overhead
 Total Estimate: $460
 Residential Rain Gardens
 Estimate: $300
Overhead
$75.00
Total Estimate
$457.00
SAY
$460.00
Subsurface Detention
 Subsurface Gravel Bed
 Excavation
 Gravel and asphalt
 Geotextile
 Total Estimate: $950,000
 Rain Garden
 Excavation
 Mulch and topsoil
 Vegetation
 Volume storage sand
 Total Estimate: $1,200,000
Schedule
Conclusions
 Final solution encompasses entire watershed
 There is no immediate short-term solution
Channel Improvements have potential
Require more research to design
 Long-term solution requires changes in design
practices