Tss 330 - Stormwater Management

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HYDROLOGIC CYCLE

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THE GLOBAL PROCESS OF THE EARTH'S WATER MOVEMENT

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PRECIPITATION OCCURS AS...

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RAINFALL, SNOWFALL OR MIXTURES OF EACH.

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TRUE/FALSE: SNOW AND SNOW MELT ARE NOT TYPICALLY MAJOR FACTORS IN SMALL WATERSHEDS. FOR SITE PLANNING AND DESIGN, STORMWATER MANAGEMENT FOCUSES ON THE ESTIMATION OF RUNOFF FROM RAINFALL.

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TRUE

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TRUE/FALSE: THE AMOUNT OF RUNOFF WATER THAT FLOWS TO A PARTICULAR POINT IN THE LANDSCAPE IS DEPENDENT UPON THE SIZE OF THE WATERSHED, THE PORTION OF THE LANDSCAPE THAT CONTRIBUTES OR DRAINS RUNOFF TO THAT POINT.

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TRUE

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TRUE/FALSE: THE WATER SHED HAS A TOPOGRAPHICALLY DETERMINED BOUNDARY, CONSISTING OF A LINE OF RIDGES AND SADDLE POINTS THAT DIVIDES THE LAND CONTRIBUTING RUNOFF WATER TO THE PARTICULAR POINT OF CONCERN, FROM AREAS THAT CONTRIBUTE RUNOFF ELSEWHERE.

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TRUE

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TRUE/FALSE: THE CHARACTER OF LAND COVER AFFECTS INTERCEPTION OF RAINFALL

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TRUE

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TRUE/FALSE: LAND COVER WITH GREATER COMPLEXITY WILL INTERCEPT MORE PRECIPITATION. THE MOST COMPLEX LAND COVERS ARE HIGHLY LAYERED PLANT COMMUNITIES WITH VAST AMOUNTS OF LEAF AREA THAT MUST BE WETTED BEFORE RUNOFF IS SHED

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TRUE

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TRUE/FALSE: ONE OF THE EFFECTS OF URBANIZATION IS THE SIMPLIFICATION OF SURFACES.

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TRUE

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TRUE/FALSE: ARTIFICIAL SURFACES TEND TO BE SUBSTANTIALLY LESS COMPLEX THAN NATIVE SURFACES, AND INTERCEPT CONSIDERABLY LESS RAINFALL.

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TRUE

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WHAT IS THE PRIMARY CAUSE OF RAINFALL LOSS

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INFILTRATION INTO THE GROUND

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TRUE/FALSE: SOIL TYPE IS THE PRINCIPLE DETERMINANT OF INFILTRATION.

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TRUE - SANDY SOILS INFILTRATE MORE WATER AT A FASTER RATE THAN CLAY SOILS. PAVED SURFACES NEGATE THE INFILTRATION CAPACITY OF SOILS. URBAN SOILS CAN ACT VERY MUCH LIKE PAVEMENT BECAUSE THE POROUS STRUCTURE OF THE SOIL HAS BEEN DESTROYED BY COMPACTION.

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TRUE/FALSE: A MAJOR IMPACT OF URBANIZATION ON STORMWATER IS THE ESTABLISHMENT OF LARGE AREAS OF IMPERVIOUS SURFACES. IMPERVIOUSNESS RADICALLY ALTERS THE WATER BALANCE OF A SITE BY INCREASING RUNOFF IN TERMS OF BOTH VOLUME AND PEAK DISCHARGE.

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TRUE

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TRUE/FALSE: WHERE RUNOFF DRAINS DIRECTLY FORM IMPERVIOUS SURFACES INTO DRAINAGEWAYS, RUNOFF IS MAXIMIZED. WHERE RUNOFF DRAINS FROM IMPERVIOUS SURFACES AS SHEETFLOW ONTO PERVIOUS SURFACES, RUNOFF IS MINIMIZED.

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TRUE

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CONNECTED IMPERVIOUSNESS

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WHERE RUNOFF DRAINS DIRECTLY FORM IMPERVIOUS SURFACES INTO DRAINAGEWAYS, RUNOFF IS MAXIMIZED

BECAUSE IT IS DIRECTLY CONNECTED TO THE DRAINAGE SYSTEM

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DISCONNECTED IMPERVIOUS

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WHERE RUNOFF DRAINS FROM IMPERVIOUS SURFACES AS SHEETFLOW ONTO PERVIOUS SURFACES, RUNOFF IS MINIMIZED.

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TRUE/FALSE: SITE DESIGN THAT SEEKS TO MAXIMIZE DISCONNECTED IMPERVIOUSNESS WILL RESULT IN LESS STORMWATER RUNOFF AND BETTER WATER QUALITY

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TRUE

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FLOOD PROTECTION IS TYPICALLY DEFINED BY 2 GENERAL CATEGORIES:

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1) MAJOR FLOODING: PUTS LIVES AND SIGNIFICANT STRUCTURES AT RISK

2) MINOR FLOODING: PUTS CONVENIENCE AND MINOR STRUCTURES AT RISK

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MINOR SYSTEM

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THE MINOR SYSTEM MINIMIZES THE INCONVENIENCES ASSOCIATED WITH FREQUENTLY OCCURRING STORMS. TYPICAL EX ARE STORM SEWERS OR ROADSIDE OR BACKYARD SWALES. THESE SYSTEMS ARE USUALLY DESIGNED TO ACCOMODATE THE 2,5, 10 YEAR STORM.

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MAJOR SYSTEM

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IS USED WHENEVER THE MINOR SYSTEM CAPACITY IS EXCEEDED. THIS IS CAUSED BY AN INFREQUENT EVENT SUCH AS THE 25, 50, 100 YEAR STORM OR MAXIMUM PROBABLE RAINFALL EVENT.

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TRUE/FALSE: THE EROSION AND SEDIMENTATION PROCESS DELIVERS THE LARGEST LOAD OF POLLUTANTS INTO WATER BODIES THAT RECEIVE RUNOFF.

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TRUE

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TRUE/FALSE: THE MOST COMMON CAUSE OF DEPLETION OF DISSOLVED OXYGEN IS EXCESSIVE NUTRIENT LOADS DELIVERED TO THE WATER BODY.

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TRUE

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TRUE/FALSE: WATER QUALITY PROTECTION FORM NON-POINT SOURCE POLLUTION BEGINS BY CONTROLLING SEDIMENT, THE LARGEST CONTRIBUTOR OF POLLUTANTS INTO WATER BODIES. CONTAMINANTS INCLUDE NUTRIENT LOADING, HEAVY METALS, CHEMICALS AND PATHOGENS

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TRUE

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SCHEMATIC DESIGN STRATEGIES

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1) REPRODUCING PREDEVELOPMENT HYDROLOGICAL CONDITIONS

2) PLACE DEVELOPMENT IN LEAST CRITICAL AREAS

3) FIT DEVELOPMENT TO TERRAIN

4) UTILIZE NATURAL DRAINAGE SYSTEM

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WATER QUALITY PROTECTION SYSTEM

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TRAPS SEDIMENT, FILTERS AND INFILTRATES RUNOFF TO REMOVE CONTAMINANTS. TYPICALLY DESIGNED TO TREAT THE VOLUME OF RUNOFF FROM A 1.25 IN RAINFALL AND PROTECT AGAINST EROSION FORM 2 YEAR RAINFALLS. THESE SYSTEMS SHOULD BE DESIGNED USING SMALL STORM HYDROLOGY METHODS.

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VELOCITY (V)

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IS THE DISTANCE TRAVELED OVER A GIVEN TIME. IT IS THE CRITICAL FACTOR FOR ESTIMATING AND UNDERSTANDING RUNOFF MOVEMENT. RUNOFF VELOCITIES ARE TYPICALLY EXPRESSED IN METERS PER SECOND OR FEET PER SECOND

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DISCHARGE (Q)

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IS RATE OF RUNOFF FLOW, OR VOLUME TRAVELLING AT A PARTICULAR VELOCITY. DISCHARGE IS EXPRESSED IN CUBIC METERS PER SECOND OR CUBIC FEET PER SECOND. RUNOFF DISCHARGE FOR CROSS SECTIONAL SHAPES SUCH AS CHANNEL AND PIPE SECTIONS IS EQUAL TO VELOCITY TIMES AREA OF FLOW (a) [Q=Va]

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VOLUME OF FLOW (Qvol)

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AS DISCHARGE CONTUES THROUGH A CROSS SECTION FOR A PERIOD OF TIME, THE DISCHARGE CAN BE MULTIPLIED BY THE LENGTH OF TIME TO ARRIVE AT A TOTAL VOLUME OF FLOW (Qvol)

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HYDROGRAPH

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IS A SUMMARY OF STORMWATER FLOWS. IT IS A RECORD OF FLOW RATES AT A SPECIFIC LOCATION OVER A GIVEN PERIOD OF TIME.

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PEAK RATE OF FLOW

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THE PEAK OF THE HYDROGRAPH IS THE MAXIMUM RATE OF FLOW.

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TIME OF CONCENTRATION (tc)

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THE TIME WATER TAKES TO FLOW FROM THE MOST DISTANT POINT IN A WATERSHED TO ITS OUTLET

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TRAVEL TIME (tt)

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THE AVERAGE TIME FOR WATER TO FLOW THROUGH A REACH OR OTHER STREAM OR VALLEY SEGMENT THAT IS LESS THAN THE TOTAL LENGTH

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STORM FLOWS

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ARE LARGE, INFREQUENT FLOWS OF RUNOFF CHARACTERIZED BY HIGH PEAK DISCHARGES

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BASE FLOWS

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ARE THE STEADY FLOWS THAT CONTINUE TO OCCUR AFTER THE PULSE OF FLOW FROM A STORM HAS SUBSIDED

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DESIGN STORMS

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OR RAINFALL EVENTS ARE STATISTICAL ABSTRACTIONS DRAWN FROM RAINFALL RECORDS. THEY REPRESENT PROBABILITY ESTIMATES OF EXPECTED RAINFALL AMOUNTS IN TERMS OF INTENSITY, DURATION AND FREQUENCY.

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DURATION

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THE LENGTH OF TIME OVER WHICH HISTORICAL RAINFALL DEPTHS ARE DISTRIBUTED FOR PEUPOSES OF ANALYSIS, TYPICALLY EXPRESSED IN HOURS.

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FREQUENCY

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IT IS THE PROBABILITY OF RECURRENCE OF AN EVENT THAT PRODUCES A RAINFALL DEPTH, TYPICALLY EXPRESSED IN YEARS.

SOMETIMES CALLED THE RETURN PERIOD.

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INTENSITY

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THE RATE AT WHICH THE RAIN FALLS, EXPRESSED IN MILLIMETERS PER HOUR.

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STEEL FORMULA

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INTEGRATES STATISTICAL DATA OR RAINFALL INTENSITY, DURATION AND FREQUENCY TO PROVIDE A MEANS OF DIRECTLY COMPUTING RAINFALL INTENSITY GIVEN THE STORM DURATION RETURN PERIOD, AND THE REGION OF THE US.

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RATIONAL FORMULA

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ESTIMATES PEAK DISCHARGES OF RUNOFF FROM RAINFALL IN URBAN AREAS

Q=CIA

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RATIONAL FORMULA: Q=CIA

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Q: PEAK DISCHARGE OF RUNOFF

K: CONSTANT

C: RUNOFF COEFFICIENT

I: RAINFALL INTENSITY AT TIME OF CONCENTRATION

A: WATERSHED AREA

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ASSUMPTIONS INTRINSIC TO THE RATIONAL METHOD:

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1) RAINFALL INTENSITY IS UNIFORM THROUGH-OUT OF THE STORM AND AREA OF THE WATERSHED.

2) PEAK DISCHARGE OCCURS AT THE TIME OF CONCENTRATION

3) DURATION OF THE RAINFALL IS EQUAL TO THE TIME OF CONCENTRATION

4) TIME OF CONCENTRATION INCLUDES TIME FOR SATISFACTION OF INITIAL ABSTRACTIONS, AND SHOULD NEVER BE CONSIDERED TO BE LESS THAN SIX MINUTES.

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TRUE/FALSE: SMALL STORM HYDROLOGY -

DESIGN FOR WATER QUALITY MANAGEMENT FOCUSES ON CAPTURING AND TREATING THE VOLUME OF WATER RATHER THAN THE PEAK DISCHARGE

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TRUE

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THE FUNDAMENTAL CONVEYANCE

TECHNIQUES ARE:

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CHANNELS, SWALES, CULVERTS, AND STORM SEWERS

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TRUE/FALSE: WITH THE EXCEPTION OF CULVERTS, STANDARDS DESIGN OF CONVEYANCE STRUCTURES ASSUMES STEADY UNIFORM OPEN CHANNEL FLOW CONDITIONS, NOT PRESSURE FLOW CONDITIONS. FLOW IS CAUSED BY GRAVITY. UNIFORM FLOW MEANS DEPTH, SLOPE, VELOCITY AND CROSS SECTION REMAIN CONSTANT OVER THE LENGTH OF THE CHANNEL

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TRUE

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WETTED PERIMETER

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THE PERIMETER OF THE CONTAINING CROSS SECTION IN CONTACT WITH WATER AT THE DESIGN DEPTH OF FLOW.

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TRUE/FALSE: THE FLOW IN OPEN CHANNELS IS A FUNCTION OF VELOCITY AND THE CROSS SECTIONAL AREA OF FLOW. VELOCITY IS A FUNCTION OF SLOPE, SURFACE ROUGHNESS AND CROSS SECTIONAL SHAPE. THE MANNING FORMULA IS USED TO COMPUTE VELOCITY.

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TRUE

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CULVERTS

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ARE DESIGNED TO WORK UNDER PRESSURE FLOW CONDITIONS AS THEY CAUSE WATER TO BACK UP AT THE INLET END OF THE PIPE

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TRUE/FALSE: STORM SEWERS ARE USUALLY DESIGNED USING THE RATIONAL METHOD FOR THE 10 YEAR RAIN FALL EVENT

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TRUE

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TRUE/FALSE: MAXIMUM DISCHARGE THROUGH CIRCULAR PIPES OCCURS WHEN DEPTH OF FLOW IS ABOUT 90 PERCENT OF PIPE DIAMETER

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TRUE

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TRUE/FALSE: DETENTION FOR FLOOD PROTECTION IS BEST PROVIDED ON A REGIONAL BASIS RATHER THAN ON AN INDIVIDUAL SITE BASIS

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TRUE

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STORAGE IS USED TO

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REDUCE THE PEAK DISCHARGE FROM THE

DEVELOPED CONDITIONS TO THE

PREDEVELOPMENT LEVEL

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DRY DETENTION PONDS

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ARE TYPICALLY DESIGNED TO DRAIN WITHIN 72 HOURS AFTER A RAINSTORM. DISCHARGE RATE IS CONTROLLED AT OR BELOW PRE-DEVELOPMENT PEAK DISCHARGE. THE DESIGN VOLUME FOR SMALL PONDS MAY BE DETERMINED BY EITHER THE SCS METHOD OR THE RATIONAL MASS INFLOW METHOD

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DRY DETENTION PONDS - OUTLETS:

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SHOULD HAVE AT LEAST 2:

1) A PRINCIPAL SPILLWAY

2) AN EMERGENCY OR OVERFLOW SPILLWAY

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WHAT IS THE PRINCIPAL SPILLWAY

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AN OUTLET SIZED TO A RELEASE RATE FOR THE DESIGNED POND VOLUME

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WHAT IS THE EMERGENCY SPILLWAY

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PROVIDES AN ALTERNATE RELEASE PATH FOR CASES WHEN THE PRINCIPAL IS BLOCKED OR THE CAPACITY OF THE POND IS EXCEEDED

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WET DETENTION PONDS

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ARE DESIGNED TO HAVE A PERMANENT POOL OF WATER WITH ADDITIONAL VOLUME ABOVE THE WATER SURFACE FOR HANDLING RUNOFF PILSES. WET DETENTION HAS BECOME A STANDARD REQUIREMENT FOR REMOVAL OF SEDIMENT FROM RUNOFF.

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THE STANDARD COMPONENTS OF A WET

DETENTION POND SEDIMENT REMOVAL ARE:

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1) FOREBAY

2) A STORAGE BASIN

3) AN OUTLET STRUCTURE

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FOREBAY

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SLOWS AND PREADS RUNOFF FLOW USING RIP RAP OR A LEVEL SPREADER OR BOTH.

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LEVEL SPREADER

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USED TO DISTRIBUTE FLOWS EVENLY OVER A WIDE AREA TO MINIMIZE EROSION OR MAXIMIZE FILTERING BY VEGETATED SURFACES

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FILTER STRIPS

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TREAT WATER ACCEPTED AS SHEET FLOW FROM SURFACES THROUGH COMBINED FILTERING AND PONDING

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SAND FILTERS

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ARE SURFACE OR UNDERGROUND FACILITIES USED TO CLEAN RUNOFF WATER IN URBANIZED AREAS WHERE SPACE IS LIMITED.

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SITING CONSIDERATIONS:

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THE BOTTOM OF INFILTRATION BASINS MUST BE AT LEAST 2-4FT ABOVE THE HIGH WATER TABLE AND AT EAST 3FT FROM THE BEDROCK TO PREVENT POLLUTION OF THE GROUNDWATER. THE EDGE OF INFILTRATION BASINS AND BEDS SHOULD BE AT LEAST 100 FT FROM WELLS, AND 10FT FROM BUILDINGS. THE BOTTOM OF THE INFILTRATION ROCK SHOULD BE SET BELOW THE FROST LINE

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INFILTRATION BASINS

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ARE PONDS DESIGNED TO LEAK INTO THE UNDERLYING SOIL. THESE FACILITIES HAVE A HIGH FAILURE RATE AND SHORT LIFE IF NOT PROTECTED BY AN UPSTREAM SEDIMENTATION BASIN.

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RECHARGE TRENCHES

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A MODERN FRENCH DRAIN

LOCATED OFF LINE

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BIORETENTION PONDS

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COMBINES PHYSICAL FILTERING OF RUNOFF WITH BIOLOGICAL PROCESSING OF RUNOFF POLLUTANTS. IT IS SIMILAR TO AN INFILTRATION POND BUT WITH A DEEP LAYER OD PLANTING SOIL MIX AT THE BOTTOM. BIORETENTION PONDS WORK BEST WITH SMALL DRAINAGE AREAS. LOCATED OFF LINE

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INFILTRATION BEDS

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ARE AN UNDERGROUND VERSION OF THE RECHARGE TRENCH. GENERALY PLACED OFFLINE

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INFILTRATION WELLS

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USED TO INFILTRATE RUNOFF FROM ROOF DOWNSPOUTS AND OTHER VERY SMALL DRAINAGE AREAS WITH FAIRLY CLEAN RUNOFF. DESIGN IS ESSENTIALLY THE SAME AS FOR A RECHARGE TRENCH OR BED. A CRITICAL DESIGN FEATURE IS AN ABOVE GROUND OVERFLOW OUTLET TO PROTECT THE WATER FROM BACKING UP DOWNSPOUTS OR DAMAGING THE INFILTRATION WELL ITSELF.

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POROUS ASPHALT PAVEMENT

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BASICALLY A RECHARGE TRENCH WITH A POROUS BITUMINOUS TOP LAYER USED INSTEAD OF GRAVEL. BEST USED WHERE IT ACCEPTS RUNOFF ONLY FROM POROUS SURFACES (GRASSES ETC). IT SHOULD NOT BE USED FOR PARKING AREAS BECAUSE CONTAMINANTS FROM AUTOS MOVE DIRECTLY INTO THE RECHARGE TRENCH.