# Presentation: 035005

## Shades Creek Sediment TMDL

- Cahaba River Basin
- Birmingham, AL

## Sediment Problem on Shades Creek

- Included on 303(d) List as not supporting designated use of Fish and Wildlife due to siltation, turbidity and habitat alteration
- Siltation implies deposition of fine-grained materials on channel bed hampers oxygenation of coarser bed material, creating poor habitat
- Turbidity implies excessive amounts of fine-grained materials transported in water column

## TMDL Approach

- In absence of a numerical target, loads and bed material composition measured in Shades Creek are compared to unimpaired or “reference” streams in Ecoregion 67 (Ridge and Valley)
- Available TSS data collected in early 1980’s; TSS load about 52.6 T/yr/km2
- Numerical modeling using AnnAGNPS and CONCEPTS used to confirm existing loads

## Slide 4

## Expressing the TMDL

- 40 CFR 130.2 allows TMDL to be expressed in units other than mass per time
- TMDL expressed as mean annual yield in units of T/yr/km2
- Calculate percent reduction necessary to achieve loading conditions in reference streams
- Embeddedness target: 13.4% bed materials finer than 2 mm

## Sediment Sources in Shades Creek

- Point Sources – NPDES Regulated Facilities
- WWTPs located on tributaries – given WLA based on permit limits for TSS and design flow
- MS4 area – given WLA based on mean annual yield
- Contruction
- Nonpoint Sources
- Runoff from watershed
- Instream processes (primarily bank sloughing and not bed erosion due to bedrock)

## Calculating Loads

- Load, L = 0.0864 C Q
- Where:  C = concentration, mg/l; Q = flow, m3/s; 0.0864 converts seconds to days and from mg to tonnes; L is in units of T/d
- Calculated at gages with mean-daily flow records and suspended sediment data
- Loads are normalized by drainage area to compare reference conditions to loads on Shades Creek
- For TMDL, daily values are converted to mean annual sediment yield

**Notes:**
Daily load values are summed by calendar year and divided by drainage area to obtain the annual suspended sediment yield (T/yr/km2) for each year of flow record at the gage.  Mean annual suspended sediment yields were calculated by dividing the number of years of complete flow record

## Reference Loads

- In Ecoregion 67, sufficient flow data and suspended sediment samples were available at 56 USGS gauging stations to calculate daily and average annual loads
- Mean annual suspended sediment load 24.7 T/y/ km2

## Sediment Load Targetfrom ARS Oxford, Ms Research Lab, Dr. Simon

- Sediment Load Targetfrom ARS Oxford, Ms Research Lab, Dr. Simon

## Numerical Modeling Approach

- National Sedimentation Laboratory study on Shades Creek in Winter 2003
- Completed RGAs at 107 cross-sections corresponding to locations from FEMA study (’76)
- AnnAGNPS provided load from upland areas and tribs
- CONCEPTS routed upland loads through channel; analyzed bank and bed stability

## Why Do We Use CONCEPTS?

- Most of our work is in the Mid-Continental USA
- Highly erodible soils
- Channelized stream systems
- Unstable stream systems
- How can we reduce impact or stabilize these stream systems

## Hydraulics – Flow Variable Calculation

- Flow area is divided into sections affected only by left floodplain, left bank, bed, right bank, and right floodplain
- Flow variables are calculated for each section

## Sediment Transport – Modeling

- Mass conservation of sediment by size fraction

*C* = sediment mass (m2)
*c* = sediment concentration (ppm)
*D* = deposition rate (m2/s)
*E* = entrainment rate (m2/s)
*k* = size class
*q**s* = sediment inflow rate (m2/s)
- *s* = soil unit weight (N/m3)

## Sediment Transport – Bed Adjustment (cont.)

- erosion partly wetted bed

- deposition partly wetted bed

- erosion fully wetted bed

- deposition fully wetted bed

## Streambank Erosion

## Streambank Erosion – Streambank Stability

- Weight is the primary force tending to move the failure block
- Mobilized shear at the slip surface is the primary resisting force
- Shear strength is affected by
- Pore-water pressures
- Vegetation
- Mechanical reinforcement
- Reduction of soil moisture through canopy interception and evapotranspiration
- Increase soil moisture through stem flow

## AGricultural NonPoint Source model (AGNPS).

- AGNPS = suite of models developed to assist in quantifying impacts of agricultural non-point source pollution on water quality and environment.
- Models concerned with runoff modeling are AGNPS and AnnAGNPS.
- Both utilize TR-55 (USSCS, 1986) to simulate runoff generation.
- Uses distributed parameter inputs and operates on a cell basis.
- AGNPS data-intensive but contains tools such as a geographic information system and a database management system.

## Modeling Results

- 67% of sediment load is from instream processes; 33% from watershed
- CONCEPTS predicts sediment yield at gage of 42.9 T/yr/km2 (within 20% of measured value)
- Results used to identified stable and unstable reaches based on Simon’s Channel Evolution Model
- AnnAGNPS and CONCEPTS implementation tools

## TMDL Components

-   Percent reduction applies to LA and MS4 Components

## What’s Next

- Need for biological data
- Relate imbeddedness (percent of fines embedded in coarse grained bed materials) to impairment of biological communities
- SWMA’s role and responsibility
- Monitor during both wet and dry periods; track effectiveness of BMPs
- ADEM’s roles and responsibility
- Enforcement of construction activities through the NPDES permit program