Gully / inlet spacing¶
HDAT contains road-drainage logic to estimate spread, propose gully spacing, and compute bypass progression, including sag checks.
Segment runoff contribution (legacy)¶
For a road segment length \(L_{seg}\) and corridor width \(W_{corr}\):
\[
Q_{seg} = \frac{C\cdot I \cdot (L_{seg}\cdot W_{corr})}{3600000}
\]
Total segment flow:
\[
Q_{tot} = Q_{seg} + Q_{carry}
\]
Spread estimation (legacy)¶
\[
Spread = \left(\frac{Q_{tot}\cdot n}{0.376 \cdot S_x^{5/3}\cdot |S_l|^{0.5}}\right)^{3/8}
\]
Where:
- \(S_x\): cross slope
- \(S_l\): longitudinal slope
- \(n\): roughness
Capture requirement and efficiency¶
Required length:
\[
L_{req} = 0.817 \cdot Q_{tot}^{0.42}\cdot |S_l|^{0.3}\cdot \left(\frac{1}{nS_x}\right)^{0.6}
\]
Efficiency:
- if \(L_{gully} \ge L_{req}\): \(Eff=100\%\)
- else:
\[
Eff = \left(1-\left(1-\frac{L_{gully}}{L_{req}}\right)^{1.8}\right)\cdot 100
\]
Bypass:
\[
Q_{bypass} = \left(1-\frac{Eff}{100}\right)\cdot Q_{tot}
\]
Sag inlet count (legacy)¶
\[
N_{inlet}=\left\lceil \frac{Q_{sag,total}}{1.656\cdot L_{sag}\cdot (S_x\cdot SpreadAllow)^{1.5}} \right\rceil
\]
Engineering notes¶
- These equations are preserved from the legacy behavior; confirm governing authority for inlet capacity/spread constraints per project.
- Record the constraint used for “allowable spread” and whether the location is grade vs sag.